Behavioral Neuroscience 1996, Vol. 110, No. 2, 331-345

Copyright 1996 by the American Psychological Association, Inc. 0735-7044/96/$3.00

Dissociations Between Appetitive and Consummatory Responses by Pharmacological Manipulations of Reward-Relevant Brain Regions Satoshi Ikemoto and Jaak Panksepp Bowling Green State University Appetitive behaviors of rats were monitored in a runway situation following central infusions of neuroactive substances into brain areas implicated in electrical self-stimulation. Microinjections of the dopamine antagonist cis-flupentixol or the cholinergic antagonist atropine into the nucleus accumbens (Acb) severely reduced the approach speed and anticipatory shuttlebox activity while leaving the consumption of the 20% sucrose reward intact. Microinjections of G A B A into the ventral tegmental area (VTA), pedunculopontine tegmental nucleus (PPTg), and oral pontine reticular nucleus (PnO) also severely disrupted approach without decreasing consumption. The highest doses of atropine into the VTA, PPTg, and PnO disrupted both consummatory and approach responses equally. The results indicate that modulation of various neurochemistries along the trajectory of the self-stimulation system has stronger effects on appetitive approach than consummatory motivation. The implications for understanding appetitive-approach motivation in the brain are discussed.

terminal consummatory process (Panksepp, 1981, 1982, 1986; Robinson & Berridge, 1993). The research on dopamine (DA) functions provides the best example of this distinction. D A systems in the brain, especially the meso-accumbens DA system, have been found to play a major role in maintaining rewarding effects of many stimuli such as food (see Beninger, 1983) and brain stimulation (Corbett, 1990; Franklin, 1978; Gallistel & Freyd, 1987; Gallistel & Karras, 1984; Stellar, Kelley, & Corbett, 1983). Moreover, many lines of evidence indicate that pharmacological manipulations that increase the synaptic activity of DA, especially in the nucleus accumbens (Acb), can mediate rewarding effects (see Fibiger & Phillips, 1986; Koob & Bloom, 1988; Le Moal & Simon, 1991; Wise & Bozarth, 1987). However, the idea that these effects resemble the consummatory components of motivated behaviors (Hoebel, 1988; Wise, 1982) is contradicted by many findings. For example, at the doses that systemic DA receptor blockade disrupts operant responses for food and water, the consumption of these rewards is not affected (Blackburn, Phillips, & Fibiger, 1987; Fibiger, Carter, & Phillips, 1976; Ljungberg, 1987; Rolls, et al., 1974). Likewise, systemic injections of DA agonists and antagonists have no detectable effects on palatability reactions to the taste of infused food (Treit & Berridge, 1990). An alternative conceptual framework that can better account for the rewarding effects of brain stimulation has emerged during the past few decades. The biological theory of reinforcement of Glickman and Schiff (1967), the motivation theory of Bindra (1968), the positive incentive theory of Trowill, Panksepp, and Gandelman (1969) the foraging-expectancy theory of Panksepp (1981, 1982, 1986, 1992), and most recently the "wanting" and "liking" distinction of Robinson & Berridge (1993) provide alternative views for understanding the nature of brain self-stimulation phenomena, and each accepts the distinction that goal-directed behaviors have anticipatory or appetitive components and terminal consummatory

Rewarding effects can be induced by many stimuli such as food, water, temperature changes, receptive sexual partners, various psycho-stimulant drugs, and electrical stimulation in certain regions of the brain, especially along the trajectory of the medial forebrain bundle. That is, organisms readily learn to make operant responses in order to obtain these stimuli; in addition, animals exhibit various classically conditioned responses when these "rewards" are used as unconditioned stimuli. Whether all of these rewarding stimuli derive their motivational strength from interacting with a single brain system remains a controversial issue (Hoebei, 1988; Phillips, 1984). One common sense approach for understanding all such rewarding phenomena is that certain stimuli can generate hedonic feelings (Wise, 1982); however, such a unidimensional conceptualization does not appear to be sufficient to explain rewarding effects of either psychostimulants or brain-stimulation (Robinson & Berridge, 1993). Presently it remains likely that there are at least two distinct brain processes underlying rewarding phenomena: (a) the appetitive-approach process and the consummatory-satisfaction process, and (b) there has been a recent trend toward understanding rewarding effects of brain stimulation and psychostimulants more in terms of appetitive-approach process, rather than in terms of the Satoshi Ikemoto and Jaak Panksepp, Department of Psychology, Bowling Green State University. Satoshi Ikemoto is now at the Institute of Psychiatric Research, Department of Psychiatry, Indiana University. This article is based on a dissertation submitted by Satoshi Ikemoto to the Graduate College of Bowling Green State University in partial fulfillment of the requirements for a PhD. Partial support for this work came from N1H grant R15 HD 30387. We would like to acknowledge Andy Wickiser for his excellent technical assistance. Correspondence concerning this article should be addressed to Jaak Panksepp, Department of Psychology, Bowling Green State University, Bowling Green, Ohio 43402. Electronic mail may be sent via lnternet to [email protected].

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components. Our recent work affirms the broad extent of this psychobehavioral control system in the brain (Ikemoto & Panksepp, 1994), and the aim of the present experiment was to determine whether relevant neurochemical manipulations along the trajectory of this system would have distinct effects on normal appetitive and consummatory motivational processes. Although appetitive and consummatory components of goal-directed behavior are a well-established aspect of motivational processes, relatively little empirical work has distinguished them at the central nervous system level. F r o m the perspective that rewarding brain stimulation arouses the appetitive phase of motivated behavior, we anticipated that various neurochemical manipulations along this system would have greater influences on appetitive than consummatory aspects of reward-seeking behaviors. Brain microinjection techniques was employed to modulate neurotransmission of specific sites that have b e e n implicated in brain selfstimulation. An ethologically straightforward runway paradigm was employed to concurrently examine motivated behavior in terms of appetitive and consummatory responses. The locomotor responses toward the reward clearly correspond to processes involved in appetitive motivation. In addition to permitting independent evaluation of appetitive and consummatory behaviors, the pretrial activity within the start box was used to help discriminate between general and goal-directed arousal effects of various experimental manipulations. Previous research has effectively used such a task to monitor the effects of various drugs (e.g., Butter & Campbell, 1960; Ettenberg & Horvitz, 1990; Stellar et al., 1983), and also to discriminate differential appetitive and consummatory response effects induced by drug manipulations (Cox, 1986; Nencini, Graziani, & Valeri, 1991). A specific subaim of the present work was to further evaluate whether D A systems projecting to the Acb are more integrally involved in appetitive motivational rather than consummatory motivational processes; we also evaluated the role of cholinergic and GABAergic systems in the Acb as well as three other self-stimulation relevant brain areas in the control of the behavior of a hungry rat running down a runway for a high-incentive sucrose reward.

135 cm

Runway

General Method

Subjects Subjects were male Long-Evans rats born and raised at the animal colony of the Bowling Green State University Psychology Department. Rats were maintained on a reversed 12-hr light-dark cycle (lights on at 2300 hr) and had free access to standard laboratory chow and tap water prior to experiments. Colony room temperature was maintained at approximately 23 °C. At 2 to 3 months of age, all subjects were housed singly, and experimental manipulations and procedures were initiated. All procedures were approved by the institutional review board and all work followed the APA policy for ethical treatment of animals.

Apparatus A single J-shaped runway, constructed of plywood and painted gray, was used throughout this work. The dimensions of the runway are shown in Figure 1. The guillotine door at the entry point from the start box to the alley was equipped with a switch that activated a timer used to measure start speeds. The start box and runway were also equipped with 5 infrared photo transistor and infrared emitting diode pairs that were controlled by a IBM PC and were used to assess pretrial activity levels and running speeds. In the start box, 3 pairs were situated parallel to the starting door, 13 cm apart from each other and 7 cm up from the floor). The computer was programmed to automatically record the number of beam interruptions, yielding a pretrial activity level for each subject. Starting latency was monitored by a pair of infrared photo transistorinfrared emitting diodes situated in the long arm, 14 cm from the starting door and 4 cm above the floor. When the rat broke that photo beam, the timer activated by the lifting of the starting door was turned off, and another timer was activated to monitor running time and it was terminated by a pair of photo diodes situated just within the goal box, 15 cm from the end of the runway and 2.5 cm above the floor. The first latency was used to calculate starting speed (1 + the latency), and the second to determine running speed (the 166 cm distance between the two pairs of photo diodes + the latency to get to the goal).

Training Procedure Throughout training, subjects had restricted access to food (15 g/day). Maintenance food was given each day immediately after test

P

o

Start Box

Drinking Bottle

I

T

!

Goal Box

|

46 cm 36 cm

~-~

32 cm

~-

Figure 1. The runway apparatus. Large arrows indicate the locations of infrared photo transistorinfrared emitting diode pairs that were used to assess locomotor activity and running speed.

NEURAL BASES OF APPETITIVE MOTIVATION

90 s), starting speed, running speed, and the amount of sucrose solution consumed in the goal box (for 60 s). The latency to leave the start box and the latency to get to the goal box were measured to 0.0l and 0.1 s, respectively, and fluid consumption was measured to 0.1 ml from an appropriately calibrated drinking tube.

sessions were completed. Initial habituation to the runway consisted of taking rats from their home cages and placing them in the start-box of the apparatus. At the goal-box, 20% sucrose (weight to volume) was freely available. Subjects were initially placed in the runway apparatus in pairs (to facilitate habituation to the novel situation) for the durations of 20, 15, and 10 min on the 1st, 2nd, and 3rd days, respectively. On the 4th and 5th days, they were habituated on the apparatus individually for 10 min a day. Following habituation, subjects were trained to traverse the runway for the sucrose solution. For each session, three trials were given. Each trial consisted of the following: A rat was placed in the start box, and the door separating the start box from runway was lifted. When subjects reached the goal box, they were allowed to drink sucrose for 30 s after which they were returned to a circular intertrial holdingchamber (36 cm in height and 30 cm d) situated adjacent to the runway apparatus. The beginning of successive trials were separated by approximately 90 s. Subjects were given one or two training sessions a day and received a total of 10-15 initial acquisition sessions yielding stable asymptotic behavior prior to the initiation of drug testing.

Pilot Study Before examining the effects of drug manipulations, we first verified t h a t appetitive a n d c o n s u m m a t o r y m e a s u r e s used in the p r e s e n t p a r a d i g m can detect the c o n s e q u e n c e s of a well-established motivational manipulation, namely food deprivation. A l t h o u g h food-deprived animals are well-known to exhibit h e i g h t e n e d appetitive and c o n s u m m a t o r y r e s p o n s e s (see Bolles, 1975, p. 116), it was d e e m e d advisable to verify t h a t using o u r specific procedures. Sixteen rats were habituated to the runway a p p a r a t u s a n d were t r a i n e d to r u n for the sucrose solution with the above methodology. They were given additional sessions with t h e test p r o c e d u r e for 6 days: 4 days with a limited access to food (15 g a day) a n d t h e n 2 days without food restriction (ad libitum in the h o m e cage). Following this experience, effects of food availability were evaluated. H a l f of t h e subjects were tested after a 22-hr food deprivation ( F D condition), a n d the o t h e r half were tested without food deprivation ( N o F D condition). T h e next day, a n o t h e r session was r u n with the conditions of food availability reversed. A two-way within-subjects design analysis of v a r i a n c e ( A N O V A ) was p e r f o r m e d on each m e a s u r e with five trials having N o F D a n d five with FD. T h e results are s u m m a r i z e d in Figure 2. T h e r e was an interaction b e t w e e n deprivation

Test Procedure Each test session consisted of 5 trials, with each trial conducted in the following manner: A rat was placed in the start box for a 90-s period during which time activity was automatically monitored. A tone with the duration of 5 s came on after 90 s in the start box, and as soon as the tone ceased, the start box door was lifted. Any subject that did not reach the goal box within 30 s was manually placed in the goal box by the experimenter, and allowed to consume the sucrose reward. Sixty seconds after entering the goal box, all rats were returned to the holding chamber. Thus, the beginning of one trial and the beginning of the next trial were separated by 210 s. For each trial, this procedure provided four dependent measures: activity level in the start box (for

3.I5

333

Intake

70

Running Speed

60

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50

2.5

"- 40 30

1.5 1.0

20 No Food Deprivation Food Deprivation 10

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0. I

1.6 1.4

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Starting Speed

50

1.2 1.0 0.8 0.6

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Trial

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1

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Trial

Figure 2. Mean numbers (with SEM) of responses per trial as a function of food deprivation (n = 16). Main effects of food manipulation were found on consumption and running speed.

334

IKEMOTO AND PANKSEPP

conditions and trials on the amount of sucrose consumed, F(4, 60) = 3.73, p < .01; a simple effect analysis indicated that under the FD condition, subjects consumed more during early trials than later trials, whereas under the NoFD condition, subjects consumed equally throughout the trials. The main effect of consumption was reliable, F(1, 15) = 14.70, p < .01. Mean consumption levels were 1.9 and 2.4 ml/60 s for the NoFD and FD conditions, respectively. An overall effect of trials was also present, F(4, 60) = 2.72,p < .05, indicating that rats decreased intake as trials progressed. Rats also ran faster under the FD condition, F(1, 15) = 14.61,p < .01, with mean running speeds being 24.4 and 37.1 cm/s for the NoFD and FD conditions, respectively. The trial effect was also reliable, F(4, 15) = 7.06,p < .001, indicating a gradual decrease in running speed. No statistically reliable interaction effect between trials and deprivation conditions was apparent on running speed, F(4, 60) = 1.02,p > .1. Starting speed was not affected reliably by the deprivation manipulation, F(1, 15) = 0.16, with mean starting speeds being .82 and .79 s -1 for the NoFD and FD conditions, respectively. A trial effect on starting speed was evident, F(4, 15) = 5.63, p < .001, indicating a gradual decrease of speed, with no groups by trial interaction effect, F(4, 60) = 1.29, p > .1. Similarly, start box activity was not clearly affected by the deprivation manipulation, F(1, 15) = 2.88,p > .1, with mean activity levels being 31.7 and 34.2 beam interruptions/90 s for the NoFD and FD conditions, respectively. However, start box activity did decrease across trials, F(4, 15) = 4.77,p < .01, but again, no interaction effect was present in the activity measure, F(4, 60) = 0.23,p > .1. These results indicated that the chosen paradigm was able to detect changes in appetitive and consummatory motivation. The consumption of sucrose and the running speed were most clearly affected by the deprivation manipulation, with both measures being affected to a similar extent. However, the food availability manipulation did not have appreciable effects on starting speed and activity prior to reward access. Because rats consumed substantial amounts of sucrose solution even under the no food deprivation condition, these results do not necessarily indicate that starting speed and activity cannot reflect appetitive motivation, but they appear to be less sensitive than the other measures for this manipulation, and hence may serve as measures for nonspecific effects of the various neurochemical manipulations used in the main experiment. Because starting speed did not discriminate among the various conditions in the following experiment, only data for the remaining three measures are summarized, even though full details are available elsewhere (Ikemoto, 1993).

Main Experiment The aim of this work was to evaluate the neurochemical participation of various brain areas implicated in brain selfstimulation in the mediation of natural appetitive and consummatory behaviors, with a focus on dopamine terminal fields in the Nucleus Accumbens (Acb). The behavioral role of the Acb in rewarding effects has been studied extensively. The findings include that the disruption of the DA activity at the Acb by DA

antagonists attenuates operant responses for food (Wise, Spindler, deWit, & Gerber, 1978) and for brain stimulation (Stellar & Corbett, 1989; Stellar et al., 1983), and that destruction of DA terminals of the Acb disrupts selfadministration of various psychostimulant drugs (Lyness, Friedle, & Moore, 1979; Pettit, Ettenberg, Boom, & Koob, 1984; Roberts, Corcoran, & Fibiger, 1977). Increased synaptic activity of DA by Acb injections of DA agonists produces rewarding effects that are shown by place preference (Carr & White, 1986a; White, Packard, & Hiroi, 1991) or intracranial self-administration paradigms (Hoebel, Monaco, Hernandez, Aulisi, Stanley, & Lenard, 1983; Phillips, Robbins, & Everitt, 1994). Thus, the Acb appears to play an important role in mediating rewarding effects. One aim of the present experiment was to examine whether the Acb plays a more important role in appetitive than consummatory motivational processes, using conventional rewards in the aforementioned paradigm. Specifically, runway behaviors were studied after the microinjections of the dopamine antagonist cis-flupentixol into the Acb. Moreover, other neurotransmitter systems including acetylcholine and GABA were also examined to see whether they are involved in rewarding phenomena in the Acb. We evaluated the participation of three additional brain regions in this behavior: (a) the ventral tegmental area (VTA), (b) pedunculopontine tegmental nucleus (PPTg), and (c) oral pontine reticular nucleus (PnO). The VTA was studied because the abundant work cited previously clearly implicates the meso-accumbens DA system in rewarding phenomena. The PPTg was included because recent studies have demonstrated that the PPTg is critically involved in rewarding effects of drugs of abuse as well as exploratory behavior. Work by Bechara and van der Kooy (1989, 1992a, 1992b) indicates that the PPTg mediates the rewarding effects of opiates and psychostimulants. Ibotenic lesions of the PPTg can disrupt the conditioning of place preferences with morphine or amphetamine. In addition, the PPTg may also be involved in approach-foraging behavior. Microinjections of glutamate or picrotoxin (a GABA antagonist) into the PPTg enhance locomotor activity (Brudzynski, Houghton, Brownlee, & Mogenson, 1986), and PPTg lesions by neurotoxin or PPTg inactivation by microinjections of procaine eliminated amphetamine-induced and noveltyinduced locomotor activity (Bechara & van der Kooy, 1992c; Brudzynski & Mogenson, 1985; Mogenson, Wu, & Tsai, 1989). In addition, there appear to be direct neural connections between the PPTg and Acb (Swanson, Morgenson, Simerly, & Wu, 1987). These results suggest that the meso-accumbens DA system and the PPTg may both contribute to unitary psychobehavioral function. Finally, the PnO was examined because our recent work (Ikemoto & Panksepp, 1994) suggests that this region may play an important role in brain self-stimulation and exploratory behavior. Neurotransmission in these regions were modulated by microinfusions of the DA receptor antagonist cis-flupentixol, the cholinergic antagonist atropine, and GABA. DA is a key neurotransmitter in the Acb, and acetylcholine and GABA appear to be involved in the neurotransmission of all of the four brain regions (Fonnum & Walaas, 1981; Henderson & Sherriff, 1991; Mugnaini & Oertel, 1985).

NEURAL BASES OF APPETITIVE MOTIVATION

335

Method

Results

With the subjects under Nembutal (50 mg/kg) anesthesia, bilateral stainless steel cannulas (22-gauge) were implanted into four target regions (based on the stereotaxic atlas of Paxinos & Watson, 1986) as follows: the Acb, bregma + 2.7 mm, lateral --_1.3 mm, ventral -5.2 mm from the skull surface; the VTA, interaural +3.8 mm, lateral ±0.6 mm, ventral -6.7 mm; the PPTg, interaural +1.2 mm, lateral ± 1.8 mm, ventral -6.0 ram; and the PnO, interaural + 1.0 mm, lateral ± 1.0 mm, ventral -7.0 mm. Stylets (0-size insect pins) whose lengths were the same as that of cannulas were placed in cannulas to keep them patent. At least 1 week of recovery from surgery was allowed before initiation of training. Subjects were habituated and trained with the procedures described in the General Method. Sessions were conducted during the first half of the dark cycle (i.e., 1100 hr-1800 hr). Fortyeight hours separated test sessions, and subjects were food-deprived at 1800 hr of the previous day and food was returned at 1800 hr of the test day. In order to maintain appetitive and consummatory responses and to minimize possible development of place- or taste-aversions, subjects were allowed, on the intervening days, a single opportunity to approach and consume sucrose solution (for 60 s) in the runway apparatus. Before the start of formal testing, three sessions with the full test procedures were conducted. To habituate rats to the injection procedure, saline vehicle injections were given on the last of these habituation sessions. Subsequently, effects of each drug were evaluated over four successive sessions. For each test session, subjects were treated with 1 of 4 treatments (vehicle or 1 of 3 doses of the drug). Immediately after each microinjection, a test session was initiated. Typically, each subject received 8 injections to evaluate the effects of 2 out of the 3 drugs used. The order of injection treatments and drug treatments was counterbalanced within and across rats to the fullest extent possible.

Before presenting systematic observations, it may be useful to note incidental behavioral observations that preclude meaningful interpretation of some of the data. Most notable of all was the cataleptic effect of atropine at the 50 I~g dose in the VTA. This manipulation induced obvious immobility in most of the rats. The subjects typically remained still within the start box, with eyes half-closed, for as long as they were left alone. However, such rats did retain intact reflex responses, as indicated by normal righting responses when placed upside down. At the brain stem sites, especially the PnO, the highest dose of G A B A or atropine elicited circling in about half the subjects. Although the infusions of flupentixol or atropine into the Acb also reduced general activity, the drug infusions did not have any other obvious behavioral effects. These side effects will be fully considered in the interpretation of the results. Two factor A N O V A s (4 treatments, and 5 trials, all withinsubjects) were performed on all behavioral measures, and they are summarized in Table 1. Newman-Keuls post hoc tests were performed when main effects of drug manipulations were statistically reliable. The histological analysis indicated that cannulas were correctly positioned, and placements are summarized in Figure 3. Figures 4-7 summarize means ++.SEMof consumption, running speed, and activity after drug treatments in the Acb, VTA, PPTg, and PnO, respectively. Statistically reliable results from the post hoc analyses were as follows: Acb (Figure 4), 25 ~g of flupentixol reduced running speed and activity levels; 5 and 100 Ixg of G A B A produced interaction effects on running speed (as indicated by I, see paragraph below); 50 Ixg of atropine reduced running speed and activity and 10 Ixg of atropine also reduced activity are mentioned in the legends of these figures; V T A (Figure 5), 25 Ixg of flupentixol reduced running speed; 100 Ixg of G A B A attenuated running speed and activity levels; 50 Ixg of atropine reduced the consumption of sucrose, running speed and activity; PPTg (Figure 6), 100 ~Lgof G A B A attenuated running speed; 50 Ixg of atropine reduced sucrose intake and running speed; PnO (Figure 7), 100 Ixg of G A B A reduced running speed; 50 ~g of atropine reduced sucrose intake and running speed, and 10 I~g of atropine produced an interaction effect on running speed (as indicated by I). Additional statistical analyses were performed to further clarify interaction effects of G A B A in the Acb and interaction effects of atropine in the PnO. A post hoc analysis revealed that running speeds of subjects that received injections of 100 ~g of G A B A in the Acb were reliably lower (p < .01) at the first trial than those of subjects that received vehicle injections; moreover, running speeds of subjects that received injections of 5 Ixg dose were reliably higher (p < .05) than those of subjects receiving vehicle injections at the fourth trial. Although running speeds of subjects receiving injections of 10 Ixg of atropine in the PnO were not reliably lower than those of subjects receiving injections of vehicle or 1 ~xg with trials collapsed together, the result of the reliable interaction between trials and doses indicates that speeds of subjects

Drug Injections All drugs were dissolved in 0.9% saline and administered in volumes of 0.5 ixl. The three drugs and doses used were as follows: cis(Z)Flupentixol 2HC1 (Research Biochemicals Incorporated, Natick, MA), 1, 5, 25 v.g/site; atropine (Nutritional Biochemicals Corporation, Cleveland, OH), 1, 10, 50 ixg/site; and gamma-amino-n-butyric acid (Sigma, Sigma Chemical Co., St. Louis, MO), 5, 20, 100 o.g/site. Acidity levels of cis-flupentixol were adjusted with I0N NaOH to the range of pH 6.0-7.0. Drugs were injected into brain sites, using a one-microliter Hamilton syringe, by inserting an injection needle (31-gauge stainless steel tubing) that extended 1.5 mm below the tip of the guide cannulas. Drugs were administered over a period of 60 s, and to promote diffusion, the injection needle remained in the brain an additional 30 s before being removed.

HistologicalAnalyses After the behavioral testing described above, the rats were deeply anesthetized with Nembutal and perfused through the heart with 0.9% saline followed by 10% formalin. The brains were kept in 10% formalin for at least 2 days and then in 20% sucrose formalin for a day before cutting into 35-70 ~m transverse sections on a sliding microtome with a freezing stage. Photographs were made by mounting the fresh sections on slides which were projected directly onto photographic paper. The injection sites were determined by reference to the bottom of the injection needle tracks, using the drawings of the rat brain adapted from the atlas of Paxinos and Watson (1986).

336

I K E M O T O AND PANKSEPP Table 1 Summary

Table of Analysis

of Variance

(ANOVAs)

Flupentixol Injection Site

Intake

Acb

GABA

Running Activity Intake n = 6

Atropine

Running Activity

Intake

Running Activity

ns ns

11.88"** 9.69***

n = 8

n = 6

6.52**

5.79**

ns

ns

ns

ns

ns

ns

2.95** n = 6

ns

Drug Drug x Trial VTA

ns ns

Drug Drug x Trial PPTg

ns

Drug Drug × Trial PnO

ns

Drug Drug x Trial

ns

ns

ns

ns

13.93"*

ns

27.29***

ns

ns

ns

ns

ns

ns

ns

n = 6 ns

4.19" ns

ns ns

ns ns

11.86"**

8.14"*

ns

ns

ns ns

ns ns

9.65***

n=6 ns

ns ns

n =8

n = 4

ns

ns

ns

n = 6 9.67*** 12.27"** 7.41"* 2.28* ns ns n =7

ns

18.63"**

17.45"**

ns

ns

ns

ns

ns

n = 4

n = 5 12.12"** 2.34*

ns ns

Acb = nucleus accumbens; VTA = ventral tegmental area; PPTg = pedunculopontine tegmental nucleus; PnO = oral pontine reticular nucleus. *p < .05. **p < .01. ***p < .001.

Note.

receiving i n j e c t i o n s o f 10 ~g at t h e first two o r t h r e e trials m a y b e l o w e r t h a n t h o s e o f s u b j e c t s receiving injections o f vehicle o r 1 ~g. Thus, a n a d d i t i o n a l N e w m a n - K e u l s analysis was d o n e o n d a t a in t e r m s o f trials by doses. T h e r u n n i n g s p e e d s o f s u b j e c t s receiving injections o f 10 txg at t h e first a n d s e c o n d trials w e r e reliably l o w e r t h a n t h o s e o f subjects receiving

injections o f vehicle o r 1 ~g at t h e first trial ( p s < .01) a n d t h e s e c o n d trial ( p s < .05), respectively. Effects of the highest doses of the drugs on the three m e a s u r e s in f o u r b r a i n r e g i o n s a r e s u m m a r i z e d in F i g u r e 8. O n e - w a y A N O V A s w e r e c o n d u c t e d o n p e r c e n t values o f v e h i c l e for t h e t h r e e m e a s u r e s .

Filled circles (with a letter inside) indicating locations of the tip of the injection needle at the level of brain section shown above. Matched letters between the two hemispheres in each section signify the tips of injection needles from a single subject. All other locations of the tip of the injection needles for each brain region were found within 1 mm of the anterior-posterior plain from the section shown above. CP = caudate putamen; DR = dorsal raphe; ML = medial lemniscus; MLF = medial longitudinal fasciculus; MN = mammilary nucleus; M R = median raphe; PAG = periaqueductal gray; SC = superior colliculus; SN = substantia nigra. Acb = nucleus accumbens; PPTg = pedunculopontine tegmental nucleus; VTA = ventral tegmental area; PnO = oral pontine reticular nucleus.

F i g u r e 3.

NEURAL BASES OF APPETITIVE MOTIVATION

337

Figure 4.

Effects of doses of flupentixol, GABA, and atropine in the Acb on three measures, with trials collapsed together. *p < .05, **p < .01. See text for I (interaction effect).

Figure 5.

Effects of doses of flupentixol, GABA, and atropine in the ventral tegmental area (VTA) on three measures, with trials collapsed together. *p < .05, **p < .01.

Figure 6.

Effects of doses of flupentixol, GABA, and atropine in the pendunculopontine tegmental nucleus (PPTg) on three measures, with trials collapsed together. **p < .01.

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IKEMOTO AND PANKSEPP

Figure7. Effects of doses of flupentixol, GABA, and atropine in the pontine reticular nucleus (PnO) on three measures, with trials collapsed together. **p < .01. See text for I (interaction effect).

Discussion The present experiment found that consummatory, approach, and activity responses can be dissociated by pharmacological modulation of neurotransmission in all of the brain regions studied. In the following discussion, the observed effects of the neurochemical manipulations in each region will be contrasted to results from previous studies, and the overall pattern of effects will be interpreted from the appetitive motivation hypothesis of brain self-stimulation. Fairly extensive data exist on the role of DA in the Acb; however, relatively little published data are available on the motivational functions of GABA and acetylcholine in the brain regions studied in the present experiment.

The Appetitive Motivation Hypothesis of Rewarding Phenomena Before discussing the specifics of the present data, we first summarize the appetitive motivation theory of rewarding effects that has been generated from neuroethological analyses of self-stimulation circuitry. As summarized previously, the most recent variant of this theoretical perspective (Ikemoto & Panksepp, 1994) has been developed from theoretical perspectives put forward by Bindra (1968), Glickman & Schiff (1967), and Panksepp (1981, 1982, 1986, 1992). According to this view, self-stimulation along the lateral hypothalamic continuum reflects the arousal of a coherently operating brain process whose function is to permit an organism to seek out and obtain biologically important objects and consequences from the external environment. This is achieved by the activation and coordination of many subprocesses which

include those involved in perception, cognition, movement patterns, as well as learning and memory. This appetitive motivation process ramifies widely throughout the nervous system, with each region presumably having its own specific functions. Such coordinated brain processes produce behavioral outputs such as investigatory-exploratory responses in unpredictable contexts or operant or conditioned responses in predictable contexts. For instance, behavioral responses that had yielded success during the arousal of the appetitive motivation system are more likely to be evoked when organisms encounter similar environmental contexts. Thus, any stimulus or brain manipulation that has an ability to arouse the appetitive motivation system can also serve as a reinforcement in an operant paradigm or as an unconditioned stimulus in a Pavlovian paradigm. More specifically, brain manipulations such as electrical stimulation along the trajectory of this system or psychostimulant drugs have rewarding effects because these stimuli directly activate this "go-and-get" circuitry of the brain, without necessarily producing any sensory "pleasure" associated with consumption of such stimuli. As expressed by Konner (1990) and Robinson and Berridge (1993), the brain has distinct systems for "wanting" and "liking," and the classical self-stimulation system of the lateral hypothalamus generates a state that is more akin to "wanting" than to "liking." Observations that lead to such an appetitive motivation perspective have emerged largely from the fact that direct brain stimuli that produce rewarding effects also typically have the ability to produce exploratory behaviors. Rewarding electrical brain stimulation in the lateral hypothalamus and the VTA unconditionally produces vigorous investigatory-exploratory behaviors: locomotion, rearing (Miliaressis & Le Moal, 1976), and sniffing (Rossi & Panksepp, 1992) in rats. Such close relationships between rewarding effects and exploratory effects of electrical stimulation are found in many related brain regions such as prefrontal cortex and pontine reticular formation (Ikemoto & Panksepp, 1994), including the various brain areas included in the present study. Stimulation thresholds of rewarding effects and those of exploratory effects are highly correlated in various brain regions (Rossi & Panksepp, 1992; Ikemoto & Panksepp, 1994). Directly activated neurons that are responsible for rewarding effects of stimulation and those responsible for locomotion effects in the lateral hypothalamus also have similar refractory periods (Rolls & Kelly, 1972; Rompr6 & Miliaressis, 1980), suggesting that both behavioral effects ultimately emerge from the same circuitry of the brain. Similar relationships between rewarding effects and exploratory effects have been reported after the administration of many drugs. Especially well studied are the effects of psychostimulant drugs. Increased synaptic activity of dopamine in the Acb appears to explain the rewarding effects of psychostimulant drugs such as amphetamine and cocaine (see Fibiger & Phillips, 1986; Koob & Bloom, 1988; Le Moal & Simon, 1991; Wise & Bozarth, 1987), and the same mechanism appears to be responsible for the locomotion-enhancing effect of these drugs (see Beninger, 1983; Koob & Swerdlow, 1988; Mogenson, 1987). It presently seems that the neural mechanisms for such rewarding effects and those for generating

NEURAL BASES OF APPETITIVE MOTIVATION

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Figure8. Summarized effects on intake, running, and activity of the highest dose of the three drugs: 25 I~g FLU (flupentixol), 100 ~g GABA, and 50 ~g ATR (atropine), as a function of brain loci. Effects are shown in mean percentages of vehicle (with SEM), with trials collapsed together. Within-subjects analysis of variance (ANOVA) were conducted among percent values of vehicle (with trials collapsed together) for intake, running, and activity measures. Newman-Keuls post hoc analyses were conducted when the main effect was reliable. *p < .05, **p < .01.

investigatory--exploratory activities are essentially identical. Indeed, both serve the same biological functions of seeking and obtaining what organisms need in the external environment. Within this theoretical perspective, the aim of the presently conducted experiment was to determine how specific neurochemistries in subareas of this system participate in the behavior patterns rats use to obtain a conventional reinforcement.

Nucleus Accumbens Bilateral microinjections of the dopamine antagonist cisflupentixol into the Acb disrupted the appetitive approach response in the present behavioral task while having no

evident effect on the consumption of the sucrose solution. This result supports the appetitive motivation hypothesis of rewarding phenomena that Acb DA is important for recruiting neural resources that allow animals to seek out rewards. In addition, this result suggests that the D A in the Acb is not essential for the processing of sensory pleasure. As mentioned previously, the synaptic activity of D A in the Acb plays an important role in rewarding effects (see Fibiger & Phillips, 1986; Koob & Bloom, 1988; Le Moal & Simon, 1991; Wise & Bozarth, 1987). Microinjections of D A antagonists into the Acb disrupt operant responses for food (Wise et al., 1978) and for brain stimulation (Stellar & Corbett, 1989; Stellar et al., 1983). The destruction of D A terminals in the Acb disrupts self-administration of psycbostimulant drugs (Lyness et al.,

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1979; Pettit et al., 1984; Roberts et al., 1977). Microinjections of DA agonists can produce place-preference conditioning (Carr & White, 1986a; White et al., 1991), and rats readily learn to self-administer amphetamine directly into the Acb (Hoebel et al., 1983; Phillips et al., 1994). These effects of Acb DA manipulations are consistent with the idea that Acb DA modulates the appetitive-approach process whose function is to find and obtain important consequences in the environment. In addition, the appetitive motivation hypothesis is consistent with recent studies by Phillips and his colleagues who found that enhanced activity of DA in the Acb was triggered by the presentation of conditioned stimuli that predicted food (Blackburn, Phillips, Jakubovic, & Fibiger, 1989; Phillips, Atkinson, Blackburn, & Blaha, 1991) or receptive sexual partners (Pfaus, et al., 1990). Again, arousal of the mesoaccumbens DA system is more clearly involved in the appetitive phase than the terminal consummatory phase of motivated behavior. Thus, the appetitive motivation perspective provides a coherent conceptual framework that is consistent with these findings. Although the present results indicated that DA in the Acb plays minimal role in promoting reward consumption, there is some evidence that Acb DA plays some role in controlling consummatory responses. Microinjections of the DA antagonist haloperidol into the Acb increased food intake in fooddeprived rats (Bakshi & Kelley, 1991a), while high doses of the DA agonist amphetamine in the Acb reduced food consumption (Bakshi & Kelley, 1991b; Carr & White, 1986b; Evans & Vaccarino, 1986) as well as water intake (Carr & White, 1986b). From the present view, these effects make sense in that a reduction or enhancement of appetitive approach tendencies could permit an increase or decrease in consummatory behavior, respectively. In other words, the inability to control the appetitive-approach phase of behavior can reasonably be expected to interfere with intake. Indeed, Koob, Riley, Smith, & Robbins (1978) had shown that there is no difference in intake between Acb 6-OHDA-lesioned animals and control animals if food-intake test was conducted in home cages or in a novel place with a long test period, while lesioned animals consumed more than the control in a novel environment with a short test period. Moreover, Acb injections of haloperidol or 6-OHDA lesions resulted in reduced operant responses for novel food and increased the consumption of regular lab chow provided in the operant chamber (Salamone et al., 1991). Two issues need to be addressed regarding the conceptualization of Acb DA functions. The DA system in the Acb has been studied extensively as being involved in controlling locomotor functions (see Beninger, 1983; Koob & Swerdlow, 1988; Mogenson, 1987). Thus, one obvious interpretation of the present results is that Acb DA is simply involved in complex locomotor functions, and thereby it simply compromised locomotor approach responses while leaving the simpler consummatory acts intact. Such an explanation is consistent with many findings on Acb DA functions, and it is a useful complementary account of DA functions. It should be emphasized that the appetitive motivational account does not exclude such a motor account. However, the present view predicts that manipulations that disrupt appetitive motivation processes will specifically interfere with complex motor activities involved in

finding and obtaining important consequences in the environment. Although the motor account alone is a more simple explanation, the appetitive-approach motivation account is more comprehensive because it not only accounts for why increased activity of Acb DA leads to heightened motor activity but also produces positive conditioning effects. Secondly, it should be mentioned that Acb DA has also been implicated in stress and avoidance behavior, which are controlled by aversive stimuli (Blackburn, Pfaus, & Phillips, 1992; Salamone, 1994). Aversive stimuli such as electrical shock, tail pinch, or bodily restraint can increase extracellular DA and its metabolism in the Acb (Abercrombie, Keefe, DiFrischia, & Zigmond, 1989; D'Angio, Serrano, Rivy, & Scatton, 1987; Fada et al., 1978; Imperato et al., 1992; McCullough, Sokolowski, & Salamone, 1993; Robinson, Becker, Young, Akil, & Castenada, 1987; Thierry, Tassin, Blanc, & Glowinski, 1976). DA depletion in the Acb disrupts avoidance response from electrical shock (McCullough et al., 1993). Although it is commonly thought that expectation of positive stimuli such as food and expectation of negative stimuli such as foot shock elicit distinct central states, it is possible that those commonsense distinctions are not applicable at the level of central nervous system; that is, both types of stimuli may activate identical brain circuitry at least at the subcortical level. Namely, in various situations, animals need to seek rewards as well as avenues of escape, and DA may provide a shared seeking impulse in the presence of both positive and negative rewards. Indeed, mildly aversive stimuli such as tail pinching can induce behaviors including eating, gnawing, licking (Antelman, Szechtman, Chin, & Fisher, 1975), and sexual behavior (Caggiula, 1972), and these behaviors are also known to be elicited by rewarding electrical stimulation in the lateral hypothalamus (see Panksepp, 1981; Valenstein, Cox, & Kakolewski, 1970). Further work contrasting goal seeking deficits in the presence of positive and negative incentives is essential before we should conclude that DA is only important in positive appetitive situations (Salamone, 1994). Also, recent work by Wilson, Nomikos, Collu, and Fibiger (1995) highlights the fact that it is still possible that dopamine is more important in the mediation of drive than the appetitive-anticipatory components of goal-directed behavior. Because these important issues cannot be resolved by the present research, we will simply proceed to interpret the remaining results from the perspective of the appetitive motivational theory upon which the present work was initially premised. Although the effects of GABA treatments were comparatively small, 100 ~g of GABA disrupted the approach response, and 5 ~g of GABA marginally facilitated approach responses. GABA injections had no apparent effect on sucrose intake. Similar results have been reported on locomotor activity. Bilateral microinjections of GABA at a low 2.25 Ixg dose enhanced locomotor activity slightly, while higher doses (10 and 33 I~g) reduced activity slightly (Jones, Mogenson, & Wu, 1981). However, in the present study, none of the GABA injections had reliable effect on activity, suggesting that our brief activity measure was insensitive to small arousal effects. In sum though, the present results as well as previous studies suggest that activation of GABA synapses can slightly facilitate

NEURAL BASES OF APPETITIVE MOTIVATION or inhibit the appetitive phase of behavior, depending on the concentration of GABA available in the Acb. Although cholinergic systems of the forebrain have long been implicated in memory and learning, the behavioral functions of the abundant cholinergic input to the Acb have received little attention (Fonnum & Walaas, 1981; Nonaka & Moroji, 1984; Rotter, Birdsall, Field, & Raisman, 1979; Spencer, Horv~ith, & Traber, 1986). In the present work, bilateral microinjections of the muscarinic cholinergic antagonist atropine into the Acb reduced both start box activity and the approach response, while leaving the consummatory response essentially intact. Although previous work has found that application of atropine into the Acb does not decrease DA-enhanced activity, the cholinergic system in the Acb may operate independently of DA activity in this brain region (Jones et al., 1981). The present results implicate cholinergic arousal within the Acb in the control of appetitive motivational processes.

Ventral Tegmental Area Flupentixol injections in the VTA had modest effects on both appetitive and consummatory responses. Higher doses of fiupentixol tended to reduce all responses marginally, with only running speed being reliably reduced by flupentixol at the 25 ~g dose. Similarly, it has been reported that microinjections of fiupentixol disrupt male sexual behavior somewhat, but the effects are variable (Hull, Bazzett, Warner, Eaton, & Thompson, 1990). VTA microinjections of the dopamine agonist apomorphine also disrupt male sexual behaviors, while having no evident effect on choosing the correct goal box containing a receptive female (Hull et al., 1990; Hull et al., 1991); these effects may be mediated by somatodendritic autoreceptors on A10 DA cells (White, 1991). Because application of DA antagonists into the VTA can increase the firing rates of DA cells (White, 1991), it might have been expected that DA antagonists into the VTA would facilitate motivated behaviors. However, the present data as well as work on male sexual behavior (Hull et al., 1990) suggest the opposite: DA antagonists in the VTA tend to disrupt motivated behaviors. GABA had a more selective effect on appetitive and consummatory responses in the VTA, because both activity and approach behavior were diminished, whereas the consummatory response was not. These effects of GABA may be due to GABA's ability to modulate activities of DA A10 cells, because amphetamine-enhanced locomotor activity was effectively reduced by microinjections of the GABA-transaminase inhibitor ethanolamine-o-sulphate (EOS) into the VTA (Stinus, Herman, & Le Moal, 1982). In addition, locomotor activity induced by D-ala-met-enkephalin into the VTA (enkephalin activates DA neurons, Kalivas, Widerl6v, Stanley, Breese, & Prange, 1983) is also blocked by EOS injections (Stinus et al., 1982). Although it is tempting to suppose that GABA activity selectively inhibits the meso-accumbens DA system, it is noteworthy that 6-OHDA lesions of the Acb did not reduce locomotor activity enhanced by the GABA antagonist picrotoxin into the VTA (Stinus et al., 1982). Thus, GABA in the VTA may have influenced behavior observed in this

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experiment by effects on systems other than the DA systems arising from the A10 neurons. Atropine had powerful effects on all behavior measured, and no differential effects were apparent. The highest dose of atropine reduced all responses nonspecifically, as indexed by the aforementioned induction of catalepsy. However, atropine injections at 10 I~g did marginally attenuate the approach response, while having no apparent effect on the consummatory response. Previous studies have also indicated that cholinergic activity in the VTA plays an important role in motivated behaviors. Infusions of acetylcholine into the VTA enhanced self-stimulation responses (Redgrave & Horrell, 1976). Microinjections of atropine (10-60 txg) or scopolamine into the VTA increased thresholds for self-stimulation (Kofman & Yeomans, 1989; Yeomans, Kofman, & McFarlane, 1985), and injections of the cholinergic agonist caI Jachol into the VTA had rewarding effects as shown by the place-preference conditioning (Yeomans et al., 1985). Although appetitive and consummatory responses were not unambiguously dissociated at the VTA in the present study, previous work does suggest that cholinergic activity in the VTA plays an important role in appetitive motivation. A more extensive dose-response analysis of low doses of atropine is warranted.

Pedunculopontine Tegmental Nucleus As expected from the absence of DA terminals in this area, bilateral microinjections of cis-flupentixol into PPTg had no effects on either appetitive or consummatory responses. This finding provides an anatomical control for effects observed with DA blockade in the Acb and VTA. GABA injections into the PPTg, however, did have differential effects on the present measures. Although consumption and activity measures were not clearly influenced by GABA injections, the approach response was severely disrupted by GABA administration. Because past work indicates that bilateral microinjections of GABA or muscimol, a GABA agonist, into this area can reduce locomotor activity, while picrotoxin enhances locomotor activity (Brudzynski et al., 1986; GarciaRill, Skinner, & Fitzgerald, 1985), it is provisionally suggested that those activity effects may reflect brain processes related to appetitive-approach or exploration rather than simple motor arousal. Alternatively, the short activity tests used in the present experiment may have been insufficient to detect general arousal effects. Atropine microinjections into the PPTg disrupted both consummatory and approach responses, while activity was not clearly modified, suggesting that several aspects of motivation were selectively modified by reduction of cholinergic activity in this area. In past work, however, it has been reported that PPTg injections of carbachol also reduce locomotor activity (Brudzynski, Wu, Mogenson, 1988), while increasing thresholds for electrical self-stimulation within the medial forebrain bundle, while PPTg microinjections of scopolamine decrease those thresholds (Yeomans, Mathur, & Tampakeras, 1993). These results do not yield a coherent picture. From those previous results it might have been expected that the induction of muscarinic blockade in the PPTg would have increased approach responses. However, if decreased cholinergic activity

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Table 2

Predictions From the Appetitive Motivation Perspective Effects Manipulations

Site

Serve as reinformcement or UCS

Increased synaptic activity of acetylcholine Decreased synaptic activity of acetylcholine Increased synaptic activity of GABA Decreased synaptic activity of GABA

Acb, PnO Acb, PnO VTA, PPTg, PnO VTA, PPTg, PnO

yes no no yes

Acquisition of conditioning

Maintenance of conditioned responses

Enhance Disrupt Disrupt Enhance

Enhance Disrupt Disrupt Enhance

Note. UCS = unconditioned stimulus; Acb = nucleus accumbens; PnO = pontine reticular nucleus; VTA = ventral tegmental area; PPTg = pedunculopontine tegmental nucleus.

in the PPTg reduces appetite, as indicated by our consummatory measure, it would have been reasonable to expect that approach to the reinforcement would also have been reduced. Thus, further studies are needed to clearly interpret the observed pattern of results.

Oral Pontine Reticular Nucleus cis-Flupentixol in the PnO had no appreciable effects on either appetitive or consummatory responses, which is consistent with the absence of DA receptors and axon terminals in this part of the brain. Just as was the case with the relatively nearby PPTg, bilateral microinjections of G A B A into the PnO disrupted the approach response, but had no clear effects on the consummatory and activity responses. GABAergic nerve terminals do exist in this region (Mugnaini & Oertel, 1985), and even though there is a paucity of work with G A B A manipulation of the PnO, our results suggest that those terminals may specifically dampen appetitive motivation in the brain. Further work is needed to further evaluate this possibility. The highest dose of atropine within the PnO essentially abolished the intake and the approach response while activity levels remained unaffected. The medium dose disrupted the approach response during the first two trials, while other responses were not clearly affected, suggesting some differential motivational sensitivity within the area. In past work, the PnO has been studied most extensively in terms of REM sleep and theta rhythm; REM sleep is enhanced after the microinjections of carbachol into the PnO (Baghdoyan, Rodrigo-Angulo, McCarley, & Hobson, 1984) or an acetylcholinesterase inhibitor, neostigmine (Baghdoyan, Monaco, et al., 1984), and theta rhythm is also elicited by injections of carbachol into the PnO in anesthetized rats (Nufiez, de Andr6s, & Garcia-Austt, 1991). Theta rhythm of the hippocampus may be an indicator of the appetitive motivational processes, because theta is always present during rat's sniffing (Gray, 1971; Komisaruk, 1970; Macrides, 1975; Whishaw & Vanderwolf, 1971), which appears to be the best behavioral hallmark of the appetitivemotivational state in rats (Panksepp, 1981). Because theta rhythm and sniffing rhythm of rats share strikingly similar characteristics (Gray, 1971; Komisaruk, 1970; Macrides; 1975; Whishaw & Vanderwolf, 1971), and the PnO mediates both, we provisionally conclude that cholinergic systems of the PnO may play some role in the control of appetitive-motivation process.

Predictions and Future Investigations The present results, taken in conjunction with the appetitivemotivation perspective, generate a variety of new predictions that remain to be documented (see Table 2). 1 Because cholinergic receptor blockade in the Acb disrupted approach and activity responses, it is predicted that cholinergic receptor activation would arouse the appetitive-motivation process. Thus, microinjections of cholinergic agonists into the Acb may promote appetitive arousal. In addition, such manipulations may enhance rewarding effects of concurrently administered incentive stimuli, and thereby enhance the acquisition and maintenance of conditioned appetitive responses. For the same reason, microinjections of cholinergic antagonists into the Acb should disrupt rewarding effects of stimuli such as brain stimulation. Because G A B A receptor activation in the VTA, PPTg, and PnO resulted in relatively specific disruption of approach responses, increased synaptic activity of G A B A within these regions appears to inhibit the appetitive-motivation processes. Thus, the microinjections of G A B A agonists may decrease the rewarding effects of brain electrical stimulation. Conversely, microinjections of G A B A antagonists into the these brain regions may be rewarding or enhance rewarding effects of many stimuli. Atropine (at the medium dose) within the PnO had a selective effect on approach responses, suggesting the appetitive-motivation process was disrupted. Microinjections of cholinergic antagonists may, thus, disrupt the rewarding effects of many stimuli including brain stimulation, whereas cholinergic agonists into the PnO may enhance rewarding effects of many stimuli as well as arousal effects. In sum, the present work provides further evidence that the appetitive-motivation perspective provides a better conceptualization of neurobehavioral processes elaborated within the Acb and VTA than any simple hedonic or consummatory-

Confirming data for one of the predictions presented in Table 2 was recently obtained (Ikemoto, Murphy, & McBride). The reinforcing effect of GABA antagonists in the VTA was examined using an intracranial self-administration technique. Rats quickly learned to self-administer infusions of picrotoxin (15 pmol/100-nl infusion) or bicuculline (20 pmol/a 100 nl infusion) into the VTA. Part of the results will be presented at the 4th International Brain Research Organization World Congress of Neuroscience in Kyoto, Japan.

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Received May 3, 1995 Revision received July 31, 1995 Accepted August 16, 1995 •