Amygdaloid neurons respond to clozapine rather than haloperidol in behaving rats pretreated with intra-amygdaloid amphetamine

BRAIN RESEARCH ELSEVIER

Brain Research 711 (1996) 64-72

Research report

Amygdaloid neurons respond to clozapine rather than haloperidol in behaving rats pretreated with intra-amygdaloid amphetamine Zhongrui Wang, George V. Rebec * Program in Neural Science, Department of Psychology, Indiana Uniuersily, Bloomington, IN 47405, USA Accepted 24 October 1995

Abstract

Single-unit activity was recorded from the amygdaloid complex in freely moving rats during an infusion of amphetamine directly into the recording site. Relative to the quiet resting period prior to the infusion, amphetamine routinely increased neuronal activity within 5-15 min after infusion onset, and this response continued for at least another 30 min. It was generally accompanied by marked increases in sniffing, rearing, locomotion, and grooming as well as by a tendency to turn to the ipsilateral side. Haloperidol and clozapine, typical and atypical antipsychotic drugs, respectively, were then tested in their ability to reverse these neuronal and behavioral effects. Both antipsychotics were administered subcutaneously at behaviorally effective doses within 10 min after termination of the amphetamine infusion. Haloperidol (1.0 mg/kg) failed to reverse the amphetamine-induced increase in amygdaloid neuronal activity and required more than 20 min to exert a partial blockade of the accompanying behavioral activation. Clozapine (10.0 mg/kg), in contrast, blocked the excitatory effects of amphetamine on all tested neurons and also blocked most amphetamine-induced behaviors within 10 min. Taken together, these results, which support other lines of electrophysiological evidence, point to the amygdala as a critical site in the differential behavioral effects of typical and atypical antipsychotic drugs. Keywords: Amphetamine; Amygdala; Clozapine; Haloperidol; Single-unit recording in freely moving rats

1. Introduction

Amphetamine is a widely abused psychomotor stimulant known to enhance dopamine transmission in the neostriatum and nucleus accumbens [22,30]. This effect plays a key role in the behavioral response to this drug, which in rats includes an increase in many aspects of exploratory behavior such as locomotion, rearing, sniffing, head bobbing, and licking [48,62,63]. Indeed, neostriatal and accumbal neurons that increase activity in conjunction with such movements also are excited by amphetamine [23]. This amphetamine-induced neuronal effect is not secondary to any behavioral change but appears to reflect a primary action of this drug on neurons receiving substantial dopaminergic input [23]. Consistent with this view, intra-striatal infusions of amphetamine increase the firing rate of motor-related striatal neurons several minutes before the onset of overt behavioral changes [73]. These

* Corresponding author. Fax: (1) (812) 855-4520. 0006-8993/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved

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neurons also are activated by iontophoretic application of dopamine [43], and this effect as well as the amphetamine-induced excitation are blocked by dopamine antagonists [43,58]. It appears, therefore, that the ability of amphetamine to facilitate dopamine transmission may help shape the behavioral response to this drug by activating striatal motor-related neurons. Haloperidol is a classical antipsychotic (neuroleptic) drug that blocks virtually all of the behavioral effects of amphetamine in animals [34,56,68]. This amphetamine antagonism, which is used as a preclinical screen of antipsychotic efficacy, has been attributed to a blockade of forebrain dopamine receptors [15]. In support of this view, haloperidol blocks the changes in neuronal activity and behavior elicited by intra-striatal amphetamine infusions [73]. This finding supports increasing evidence that striatal neurons are highly sensitive to haloperidol administration [23,72,77]. In fact, because of the role of the striatum in motor control, this sensitivity may contribute to the motor side effects commonly associated with haloperidol administration [6,16,31]. Like the striatum, the amygdaloid complex receives

Z. Wang, G. V. Rebec~Brain Research 711 (1996) 64-72

substantial dopaminergic input [17], but because of its strategic location in the limbic system, the amygdala is less likely to play a role in the motor side effects of antipsychotic drugs and a more direct role in their therapeutic efficacy [1,8,47]. Some evidence for this view has emerged from recordings of amygdaloid neuronal activity in anesthetized, immobilized animals. Using this preparation, we found amygdaloid neurons to be highly sensitive to amphetamine but relatively insensitive to haloperidol in reversing the amphetamine effect [3,46]. These same studies also revealed that clozapine, an atypical antipsychotic drug devoid of many of the motor symptoms of haloperidol, readily blocked amphetamine-induced neuronal changes. No information is available, however, on how such druginduced neuronal changes in the amygdala relate to behavior. In the present series of experiments, we used the freely moving preparation to assess the effects of amphetamine and the antipsychotic drugs in the amygdaloid complex. Neuronal activity was recorded from the amygdala of awake, unrestrained rats that received intra-amygdaloid infusions of amphetamine. Following the infusion, the animals received systemic injections of either haloperidol or clozapine. Our results, which indicate that clozapine is more effective than haloperidol in blocking amphetamine effects in the amygdala, suggest an intriguing role for this brain region in mediating neuroleptic-induced behavioral effects.

2. Materials and methods 2.1. Animals

Male, Sprague-Dawley rats (350-500 g), bred in our animal colony, were housed individually under standard laboratory conditions. Their use was approved by the Institutional Animal Care and Use Committee. 2.2. Surgery

In preparation for subsequent electrophysiological recording, the animals were anesthetized with Chloropent (3.3 ml/kg, i.p.) and secured in a stereotaxic device. A hole was drilled through the skull over the right amygdala approximately 2.8 mm posterior and 4.5 mm lateral to bregma [42]. The hole was covered with a thin layer of silicon film and a plastic hub, designed to mate with a micromanipulator for single-unit recordings and simultaneous intra-amygdaloid infusions [52], was secured over the hole. A metal post was anchored to the skull to serve as ground for electrophysiological recordings. A catheter was implanted subcutaneously for systemic drug injections. Rats were treated with 0.2 ml (25 mg, i.m.) Rocephin to minimize infection and allowed a 6-10 day recovery period.

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2.3. Electrophysiology

An open-field arena (1.3 m2), within view of a videotaping system, served as the recording location. Following a 2-h habituation period, the animal was ready for singleunit recording. Two tungsten microelectrodes, flameformed to a tip and coated with Epoxylite, were prepared to impedances of 1-2 M J2 measured at 1 kHz. Both microelectrodes and a 23-gauge stainless steel infusion cannula were fitted into the micromanipulator, which then was attached to the head-mounted hub. A distance of less than 400 /zm separated the electrode tips and the tip of the infusion cannula. The micromanipulator is equipped with a threaded assembly that allows the entire recording apparatus (electrodes and infusion cannula) to be lowered through the brain. After positioning 6.5 mm below the cortical surface, the recording apparatus was advanced in increments of approximately 30 /xm until spontaneous neuronal discharges were encountered. Neuronal activity was passed from a head-mounted voltage follower to a differential preamplifier (band-pass: 0.3-10.0 kHz) via shielded 33gauge wire in conjunction with an electric swivel, which allowed the animal complete freedom of movement. Polyethylene tubing connected the infusion cannula with a syringe controlled by a minipump. Neuronal activity, isolated to a signal-to-noise ratio of at least 3:1, was counted on-line with a computer system connected to an amplitude-sensitive spike discriminator and audio monitor. For off-line analysis and subsequent synchronization with behavior, neuronal data were stored on an audio channel of the videotape used to record behavioral activity [73]. Rats received 10 /xg//.d o-amphetamine sulfate (free base, Sigma) or physiological saline at a flow rate of 10 /zl/h. Neuronal activity and behavior were monitored throughout the course of the infusion (20-30 min). In some rats, 1.0 m g / k g haloperidol (McNeil) or 10.0 m g / k g clozapine (dissolved in physiological saline containing one drop of glacial ascetic acid/ml, Sandoz) was injected (s.c.) 10 rain after termination of the amphetamine infusion, and neuronal and behavioral data were recorded for another 30 min. 2.4. Behavior

Videotapes were analyzed to assess the effects of the amphetamine infusion on behavior according to procedures described previously [23,73]. Individual behavioral responses, including locomotion, rearing, sniffing, grooming, and head movements, were rated according to duration and intensity to yield a maximum score of 6. Total behavioral scores were obtained by summing each individual score. Ratings were made for 1-min intervals every 5 min. 2.5. Data analysis

Drug-induced changes in single-unit activity were reported relative to baseline firing rates when the animal

Z. Wang, G. V. Rebec/Brain Research 711 (1996) 64-72

66

rested quietly with no overt movement. Both neuronal and behavioral data were analyzed with an analysis of variance (ANOVA) or t-test.

2. 6. Histology When the recording session was completed, each rat received sodium pentobarbital (200 mg/kg, i.p.), and a 3-mA current was passed through the microelectrode to mark the recording site. Following a transcardial perfusion with formosaline, the brain was removed, frozen, sectioned, and stained with cresyl violet. Electrode and cannula placements were identified by microscopic examination.

3. Results Single-unit activity was recorded from 23 amygdaloid neurons in 23 rats. When the animals rested quietly with no overt movement, most cells fired relatively slowly with occasional bursting activity, as shown in Fig. 1. Bursts were usually characterized by clusters of 3-5 spikes occurring within 60 ms or less. The mean (___S.E.M.) discharge rate of all amygdaloid neurons was 1.35 ( + 0.31) spikes/s.

3.1. Effects of intra-amygdaloid amphetamine All infusions began during quiet rest. Between 5 and 15 min after the onset of the amphetamine infusion, a typical

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Table 1 Percentage of animals showing specific behavioral responses during intra-amygdaloid infusions of amphetamine Type of behavior

Percentage of rats

Sniffing Rearing Locomotion Grooming Head bobbing Chewing Rotation

96 70 65 61 56 56 70 (right) 9 (left)

Calculations are based on a total of 23 animals.

pattern of stereotyped movements emerged, including sniffing, rearing, and locomotion. Some animals also showed episodes of grooming, head bobbing, and chewing. Most animals also showed a preference to turn ipsilaterally (toward the right). The prevalence of each of these behaviors during intra-amygdaloid amphetamine is shown in Table 1. During the course of the infusion, behavior increased in intensity in all animals and then continued for at least another 30 min in rats (n = 4) that did not receive subsequent treatment with an antipsychotic drug. Amygdaloid neuronal activity paralleled the behavior change. Virtually all cells (21 of 23) showed a progressive increase in firing rate that peaked at a mean level of 519 ( + 172)% of resting baseline. The scatterplot shown in Fig. 2 presents these data for each neuron. A paired t-test revealed significant pre-and post-amphetamine differences in firing rate ( P < 0 . 0 0 1 ) . Fig. 3 illustrates the amphetamine-induced neuronal change for a representative recording. Note the relatively slow, bursting pattern during

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Fig. 1. Representative examples of amygdaloid neuronal activity recorded from an awake, unrestrained rat during quiet rest (no overt movement). Unit activity is displayed on-line on an oscilloscope screen (top) and off-line via computer (bottom). Note the bursting pattern in each example. Horizontal and vertical calibration bars indicate, respectively, 10 ms and 40 /.LV (top) and 0.1 ms and 100 p.V (bottom).

Fig. 2. Scatterplot of the log baseline firing rate during quiet rest vs. the log firing rate during the peak neuronal response to an intra-amygdaloid amphetamine infusion for all tested neurons. Points above or below the diagonal indicate amphetamine-induced increases or decreases, respectively, in neuronal activity. Animals that will receive a subsequent injection of haloperidol or clozapine are indicated by open circles or filled squares, respectively. No amphetamine differences were evident between these neuroleptic treatment groups.

Z. Wang, G. V. Rebec~Brain Research 711 (1996) 64-72

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Fig. 3. Digitized spike records obtained during quiet resting baseline (top) and approximately 3 min after onset of the amphetamine infusion (bottom). Note the amphetamine-induced increase in unit activity, including bursting. Calibration bars represent 0.1 s (horizontal) and 150 /xV (vertical).

rest and the marked increase in bursting after the onset of the amphetamine infusion. In two separate animals, saline infusions had no effect either on basal neuronal activity or open-field behavior. An amphetamine-induced change in unit activity of at least 20% from the basal rate occurred with a mean latency of 6.79 + 1.57 min from the onset of the infusion, whereas the mean latency of behavioral activation was 7.67 + 1.23 min. Although this difference is not statistically significant, the neuronal increase routinely preceded the increase in behavior.

3.2. Effects of subsequent treatment with haloperidol or clozapine In 19 animals, neuronal and behavioral recording continued after injection (s.c.) of either 1.0 m g / k g haloperidol (n = 12) or 10.0 m g / k g clozapine (n = 7). As shown for an individual unit in Fig. 4, clozapine rapidly reversed the

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Fig. 4. Rate-meter histogram of a representative example of the response of an amygdaloid neurons to a local infusion of amphetamine (onset: A at arrow; offset: at second arrow) followed by injection (s.c.) of 10.0 m g / k g clozapine (C at arrow). Note the clozapine-induced reversal of the amphetamine response. Calibration bars indicate 5 min (horizontal) and 2 spikes/s (vertical).

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Fig. 5. Scatterplot of the log firing rate during the peak neuronal response to intra-amygdaloid amphetamine vs. log firing rate after treatment with haloperidol (open circles) or clozapine (filled squares). Points above or below the diagonal indicate increases or decreases, respectively, in the amphetamine-induced neuronal response. Note in each case that clozapine was more effective than haloperidol in blocking the amphetamine-induced increase.

amphetamine-induced excitation. In fact, the mean firing rate of all amygdaloid neurons 20 min after clozapine injection was significantly different ( P < 0.02, paired ttest) from the rate calculated during the maximum amphetamine response (0.38 _ 0.13 spikes/s after clozapine vs. 3.42 + 1.05 spikes/s during amphetamine). No such effect was noted after haloperidol (4.79 + 1.53 spikes/s after haloperidol vs. 4.24___ 1.40 spikes/s during amphetamine). The response of each amygdaloid neuron to a clozapine or haloperidol challenge after amphetamine infusion is shown in Fig. 5. Note that clozapine reversed the amphetamine-induced excitation in all cases but that haloperidol was largely ineffective. As shown in Fig. 6, clozapine also was more effective than haloperidol in blocking the amphetamine-induced behavioral response. In most clozapine-treated rats, for example, amphetamine-induced sniffing, grooming, rearing,

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Fig. 6. Rats showing individual behavioral responses to intra-amygdaloid amphetamine before and after treatment with either haloperidol or clozapine as indicated in the legend. Relatively few animals continued to display amphetamine-induced behaviors after clozapine treatment.

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Z. Wang, G. V. Rebec~Brain Research 711 (1996) 64-72

major component of the limbic system. As such, it has been implicated in spatial learning and memory, stimulusreward associations, and fear conditioning [12,13,32]. Dysfunctions of the amygdala have been linked to a variety of pathological conditions, including temporal lobe epilepsy [21] and schizophrenia [53]. Our results focus attention on the amygdaloid complex as an important mediator of the open-field behavioral activation induced by amphetamine. This effect is accompanied by marked increases in the firing rate of amygdaloid neurons, especially those in the central and basolateral nuclei. Moreover, clozapine, but not haloperidol, readily reversed the behavioral and neuronal effects of intra-amygdaloid amphetamine, implicating the amygdala as an important site of action of atypical antipsychotic drugs.

Fig. 7. Schematic representation of the distribution of histologically verified infusion/recording sites in the amygdaloid complex. The approximate location of neurons in the amphetamine + haloperidol, amphetamine +clozapine, and amphetamine only groups are indicated by circles, squares, and stars, respectively. Most placements are located in the basolateral and central nuclei. Note the location of smaller symbols (one of each type) in the medial, basomedial, and basolateroventral nuclei indicating a relatively weaker neuronal and behavioral response to amphetamine compared to the other placements. Numbers indicate distances posterior to bregma after Paxinos and Watson [42].

and locomotion was blocked, whereas after haloperidol most animals continued to persist in these behaviors. Clozapine also blocked the behavioral effects of amphetamine significantly more rapidly than haloperidol (11.6 _+ 1.80 vs. 21.4 + 1.7 min after injection, P < 0.01). Interestingly, neither of the antipsychotics blocked the head bobbing and chewing elicited by intra-amygdaloid amphetamine.

3.3. Histology Histological analysis revealed a distribution of infusion/recording sites mainly in the central and lateral amygdaloid nuclei. Overall response strength (strong or weak) based on the total behavioral score and neuronal increase was calculated to make a rough assessment of the relative sensitivity of each infusion site. These results are summarized in Fig. 7. Note that all central and basolateral amygdaloid infusions registered a strong response to amphetamine, whereas the few sites identified in the medial and ventral nuclei were consistently weak. No regional differences emerged in the effects of subsequent injections of clozapine or haloperidol.

4. Discussion

Together with the hippocampal formation, septum, and cingulate gyrus, the amygdaloid complex is considered a

4.1. Amphetamine effects Infusions of amphetamine into the amygdaloid complex elicited the open-field behaviors commonly associated with systemic amphetamine administration. Most animals also displayed intermittent bouts of grooming activity. Many of these same behaviors, including grooming, have been reported after similar infusions into the striatum [73], suggesting some overlap between these areas in mediating amphetamine-induced behavioral effects. It also is possible, in view of the limbic connections of the amygdala [1,13], that the behavioral response to intra-amygdaloid amphetamine reflects a motivational component (e.g., sniffing and rearing as elements of exploratory behavior), while amphetamine in the striatum activates the motor sequences secondary to a change in motivation. Further behavioral studies are underway to assess this possibility [71]. Both areas receive midbrain dopaminergic input [17], and although their descending projections innervate different targets, both striatal and amygdaloid efferents influence brainstem motor nuclei. The dopaminergic input appears critical to the amphetamine behavioral response in both cases in that roughly comparable depletions of either striatal or amygdaloid dopamine have been shown to attenuate many of the same amphetamine-induced behavioral effects [11]. Comparisons of data from our amygdaloid infusions with previous striatal infusions [73] make it unlikely that our amygdaloid results can be explained by a diffusion of amphetamine into the striatum. For one, the behavioral onset time was considerably quicker for amygdaloid than for striatal infusions (7.67 + 1.23 min vs. 11.04 + 1.3 min). In addition, intra-amygdaloid amphetamine typically elicited ipsilateral turning, whereas the rotational behavior after striatal infusions was routinely contralateral. These turning results also suggest that striatal and amygdaloid projections exert different influences on at least some downstream motor systems. Interestingly, dopaminergic function in the amygdala has been reported to play a role

Z. Wang, G. V. Rebec~Brain Research 711 (1996) 64-72

in the tendency of rats to show a left or right open-field turning preference [7]. Relatively little information is available on amygdaloid neuronal activity during behavior. Our data suggest that amygdaloid neurons fire relatively slowly with occasional bursting activity during periods of quiet rest. Unlike striatal cells, which can be classified as motor- or nonmotorrelated based on their ability to change discharge activity in conjunction with spontaneous movement [18,23,76], amygdaloid neurons are difficult to categorize. Although these cells often change activity during movement, the response is inconsistent, and many neurons also change rate without an obvious link to overt behavior. Because the amygdala receives highly processed information from cortical as w e l l as o t h e r l i m b i c s t r u c t u r e s [9,26,27,35,41,57,69], assessing the relationship of amygdaloid unit activity to ongoing behavior appears to be a difficult task. In almost all neurons sampled, however, amphetamine infusions increased spike activity, including bursting, in conjunction with the onset of behavioral activation. Slow rates of spontaneous neuronal activity in the amygdala have been observed in vitro [59,74] as well as in anesthetized or paralyzed animals [3,5,75] Bursting patterns also have been observed in such preparations, especially during activation of glutamatergic afferents [37,44]. Like striatal neurons, most amygdaloid units appear to be projection cells; rapidly firing amygdaloid neurons have been identified as interneurons [45,74]. Iontophoretic application of dopamine has been reported to inhibit amygdaloid activity [4,67], and consistent with this result, amphetamine treatment exerts a prolonged inhibition of amygdaloid neurons [3], although some cells are excited by this drug [75]. It is difficult to relate these findings to the freely moving preparation, however, because as we have shown for striatal neurons, the action of amphetamine [23,51] and even iontophoretic dopamine [43] depends on the responsiveness of individual neurons to ongoing behavior. Thus, whereas striatal cells excited by spontaneous movement are also excited by amphetamine or iontophoretic dopamine, these same substances routinely inhibit nonmovement-related neurons [23,43,73]. Although our present amygdaloid results show a relatively uniform response to amphetamine, the data, nevertheless, contrast markedly with data obtained from the anesthetized preparation. Attempts to generalize to the ambulant animal, therefore, require considerable caution. 4.2. Antipsychotic drugs

Blockade of amphetamine-induced behaviors by antipsychotic drugs is a widely used preclinical screen of therapeutic efficacy [16,31] Both haloperidol and clozapine are effective in this test, though to widely varying extents. Thus, whereas haloperidol blocks virtually all amphetamine-induced behaviors, clozapine is more selective,

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attenuating sniffing and rearing activity and perhaps locomotion [34,56,68]. This selectivity of clozapine has been interpreted as a sign of the relative inability of this drug to elicit extrapyramidal side effects in humans [6,34]. Another indicator is the relatively mild catalepsy produced by clozapine in animals, whereas haloperidol and other typical antipsychotics are potent cataleptogenic agents [78]. Although several different mechanisms have been proposed to explain the unusual actions of clozapine (e.g., [8,14,33,39,54,61]), our results lend support to the view that the amygdala may play a critical role. Electrophysiological data have suggested that the amygdala is unique among forebrain structures in showing a high sensitivity to clozapine while being relatively insensitive to haloperidol. Thus, whereas both neuroleptics exerted comparable effects on neurons in the striatum, including the nucleus accumbens [49,50], clozapine proved much more effective than haloperidol in altering amygdaloid activity [46]. Long-term treatment with these drugs, moreover, exacerbated this difference, enhancing the neuronal response to clozapine but failing to alter the relatively weak response to haloperidol [2]. Behavioral support for a differential action of these drugs in the amygdala emerged from evidence that electrolytic lesions of the central amygdaloid nucleus blocked the ability of clozapine, but not haloperidol, to reverse amphetamine-induced behaviors [10]. The results of our present infusion experiments not only confirm a role for the amygdala, including the central nucleus, in amphetamine-induced behavioral effects, but also support markedly different sensitivities of amygdaloid neurons to clozapine and haloperidol. That the amygdala is an important site of antipsychotic drug action seems evident from the considerable dopaminergic innervation that this structure receives and the presumed role of this system in limbic, cognitive, and neuroendocrine functions [13]. Dopaminergic afferents to the amygdala, moreover, are unique in comparison with accumbal or striatal dopamine-containing fibers. The amygdaloid dopaminergic projection, for example, lacks autoreceptors [29], which may contribute to an unusually high dopamine turnover rate [28]. These neurons also appear to use a greater percentage of intraneuronal dopamine stores for release [19] and to have an unusually low rate of dopamine uptake [20,25]. To the extent that both haloperidol and clozapine block dopamine receptors, it is difficult to imagine how these unique properties of the amygdaloid dopaminergic projection can explain the unusual action of clozapine. This drug, however, has a higher affinity for D~ and a lower affinity for D 2 dopamine receptor subtypes than haloperidol [61], and these receptors appear to have a nonoverlapping distribution in the rat amygdala [60]. It is conceivable, therefore, that our results could be explained by selective recording from neurons expressing the D~ receptor. Alternatively, clozapine has a high affinity for the D 4 subtype, which is expressed more abundantly in amygdaloid than striatal neurons [70], and this effect could

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account for our results. Follow-up studies are needed to assess the contribution of different dopamine receptor subtypes to the differential effects of clozapine and haloperidol on amygdaloid neurons. It also is possible in view of the neurochemical complexity of the amygdala that the unusual actions of clozapine reflect an interaction with other transmitter systems. At least some neuroleptic action in the amygdala, for example, has been attributed to a change in glutamate release [40,64]. Also noteworthy is evidence that serotonin, which is found in high levels in the amygdaloid complex [17], may play a critical role in the therapeutic action of clozapine and related antipsychotic drugs [38]. These and other hypotheses warrant further investigation in the amygdala. Because the amygdala has direct connections to parts of the striatum and nucleus accumbens [24,36,55,66], it seems likely that intra-amygdaloid amphetamine exerts a direct influence on striatal and accumbal neurons involved in motor circuitry. Clozapine and haloperidol, however, were administered systemically, raising the possibility that their differential efficacy in reversing the effects of intraamygdaloid amphetamine occurs downstream in striatal or accumbal circuits. This interpretation seems unlikely in that striatal and accumbal neurons respond similarly to either clozapine or haloperidol [23,49,50]. When injected systemically, moreover, both of these antipsychotic drugs attenuate the behavioral response to systemic amphetamine [68] in direct contrast to our results with intra-amygdaloid amphetamine. Thus, although the amygdala influences forebrain areas known to play a role in the behavioral effects of amphetamine [65], the differential effectiveness of clozapine and haloperidol in reversing intra-amygdaloid amphetamine effects on behavior appear to reflect an amygdaloid site of action. 4.3. Conclusions

The amygdala, like the striatum, appears to play a role in the behavioral activation induced by amphetamine. Amygdaloid neurons, moreover, respond to this drug in awake, unrestrained rats with marked increases in firing rate that closely parallel the behavioral change. Both the behavioral and neuronal effects of intra-amygdaloid amphetamine are readily reversed by clozapine, but not by haloperidol, implicating the amygdala in at least some of the differential behavioral effects associated with these antipsychotic drugs. Further understanding of the amygdaloid mechanisms underlying these neuroleptic differences promises to shed new light on the neuronal systems by which these drugs exert their behavioral and therapeutic effects. Acknowledgements This research was supported by the National Institute on Drug Abuse (DA 02451). The authors also acknowledge

the technical expertise of Paul Langley and the editorial assistance of Faye Caylor.

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