- Email: [email protected]
Effects of dopamine receptor blockade on avoidance performance: assessment of effects on cue—shock and response—outcome associations
BEHAVIORAL AND NEURAL BIOLOGY
36, 280-290 (1982)
Effects of Dopamine Receptor Blockade on Avoidance Performance: Assessment of Effects on Cue-Shock and Response-Outcome Associations 1 HYMIE ANISMAN, JILL IRWIN, ROBERT M. ZACHARKO, AND TOM N . TOMBAUGH
Department of Psychology, Unit for Behavioral Medicine and Pharmacology, Carleton University, Ottawa, Ontario K1S 5B6, Canada Treatment with pimozide, a neuroleptic drug which blocks postsynaptic dopamine receptors, retarded active avoidance performance among naive mice. The performance deficit was eliminated among mice that were previously trained in the avoidance task. Like avoidance training, prior CS-shock pairings enhanced avoidance performance, but this treatment did not reduce the effects of pimozide on later avoidance performance. Likewise, treatment with pimozide prior to CSshock pairings did not influence the performance ehancements evident in a later nondrug test of avoidance performance. Moreover, the drug did not influence the extinction of avoidance induced by response prevention (flooding). Together, these data suggest that pimozide does not influence S-S associations, but affects avoidance performance by disrupting response initiation processes.
It has been demonstrated repeatedly that treatment with neuroleptics will disrupt acquisition of an active avoidance response (Fibiger, Zis, & Phillips, 1975; Posluns, 1962) and may retard response latencies in some types of escape tasks (Anisman, Remington, & Sklar, 1979). It has been suggested that in tasks involving aversive motivation dopamine receptor blockers do not impair acquisition of the cue-shock relationship, i.e., the S - S associations (Beninger, MacLennan, & Pinel, 1980a; Beninger, Mason, Phillips, & Fibiger, 1980b), but rather induce deficits in response initiation and maintenance (Fibiger et al., 1975). Provided animals are initially trained in either an avoidance or escape task, performance deficits ordinarily engendered by dopamine receptor blockers will be largely reduced or eliminated (Anisman et al., 1979; Fibiger, et al., 1975). Moreover, when animals are trained in an avoidance task in the drug state, avoidance deficits may not be evident upon subsequent drug-free retests (Beninger Supported by Grants A9845, U0058, and A7074 from the Natural Science and Engineering Research Council of Canada. 280 0163-1047/82 $2.00 Copyright @ 1982 by Academic Press, Inc. All fights of reproduction in any form reserved.
DOPAMINE AND AVOIDANCE
281
et al., 1980a; Fibiger et al., 1975). Thus it appears that if animals are initially trained in the drug condition the S-S association will be established, resulting in positive transfer in tasks requiring animals to perform a highly prepared response (Beninger et al., 1980a), or where the response contingencies can be readily established (Fibiger et al., 1975). In addition, these processes may be augmented by the response requirement being partially established during initial training (i.e., running in response to, rather than in anticipation of, shock thus positively influencing subsequent avoidance performance). The prophylactic effects of prior escape or avoidance training on subsequent performance may be attributable to the establishment of requisite motor responding or to the establishment of the cue-shock relationship (fear or expectancy of shock) which increases the motivation to escape or avoid. In respect to the latter, it may be further speculated that a dopamine receptor blocker influences performance either by blunting the negative reinforcement derived from appropriate responding or by reducing motivation. It wilt be recognized that this viewpoint represents an extrapolation from the anhedonia hypothesis (Wise, 1982; Wise, Spindler, DeWit, and Gerber, 1978) which posits that dopamine neuronal activity is fundamental to reward processes in tasks involving appetitive motivation. In the aversive paradigm prior establishment of an avoidance response could potentially increase motivation to avoid or escape from the aversive stimulus, thus eliminating the potential effects of the drug treatment. In any case, the present series of experiments were undertaken to evaluate the effects of the dopamine receptor blocker, pimozide, on avoidance performance with particular attention devoted to the analysis of the drug's effect on the establishment of S-S associations and to the contribution of motoric changes to the induction of the performance disturbances. EXPERIMENTS 1 AND 2
Experiment 1 was designed to evaluate the effects of pimozide on active avoidance performance, and to determine whether the disruptive effects of the drug treatment would be eliminated or reduced among mice previously trained in an avoidance task. Since such an outcome could result from the prior establishment of a particular response or the establishment of the cue-shock contingency, Experiment 2 assessed the effects of prior classically conditioned CS-shock pairings (i.e., no instrumental response requirement) on subsequent performance deficits normally induced by pimozide. It has been demonstrated on several occasions that pairing an explicit cue with short duration shock facilitates subsequent avoidance behavior, provided that shock associated immobility can be suppressed, e.g., in a one-way avoidance task or in a shuttle task involving mild shock intensities (Anisman, 1973). Presumably, CS-shock pairings result in the establishment of the expectancy of shock, reduce
282
ANISMAN ET AL.
the S-S associative requirements in the yet-to-be-learned avoidance response, and enhance performance. If initial training (Experiment 1) inhibits pimozide-induced performance deficits as a function of establishing an S-S association, then prior CS-shock pairings should also be prophylactic in this respect. Conversely, if the effects of initially training animals in the avoidance task are related to the establishment of an active motor response, then prior classically conditioned CS-shock pairings should be ineffective in reducing the performance deficits provoked by treatment with pimozide. Method
Subjects Experiments 1 and 2 involved 48 and 60 male CD-I mice procured from the Canadian Breeding Laboratories, Laprairie, Quebec, Canada, at 50-55 days of age. Mice were housed in groups of 5 in standard polypropylene cages and acclimated to the laboratory for at least 7 days prior to being used as experimental subjects.
Apparatus Avoidance training was conducted in four identical black, Plexiglas shuttle-boxes which measured 29.2 x 8.9 x 16.5 cm. Opaque disks, 2.54 cm in diameter, centered 2.54 cm from the top of the end walls, could be illuminated by bulbs. Additionally, a speaker was centered on the red Plexiglas roof of each chamber. The light and tone were used as a compound CS. The floor of each chamber consisted of 0.32-cm stainless-steel bars spaced 1.0 cm apart (center to center), connected in series by neon bulbs. Shock of 150/~A (60 Hz, AC) could be delivered to the grid floor through a 3000-V source, thereby providing relatively constant current. Each shuttle-box was divided into two equal-sized compartments by a stainless-steel wall partially made up of a solenoid controlled horizontally movable gate. When the gate was open a 5.2 x 6.1-cm space situated above a 1.27-cm hurdle permitted access to the adjacent compartment. The hurdle was made of thick cork and was lined on either side by thin stainless-steel plates which were connected in series by a neon bulb, thereby preventing mice from avoiding or escaping shock by sitting on the hurdle. Situated 1.1 cm on either side of the hurdle were two infrared photodetectors 1.27 and 2.54 cm above the grid floor. The photocells were wired so that the cells would not trigger when both sides of the hurdle were crossed simultaneously (e.g., when the mouse was half-way across the hurdle). Only when the beam on the nonshock side alone was crossed would the cell be activated. The photocells were also designed with a delay such that very brief photocell interruption (e.g., the tail of a mouse crossing the hurdle) would not trigger the
DOPAMINE AND AVOIDANCE
283
photocell. A third set of photocells were located 2.54 cm from the end walls of each compartment. In the event that the mouse jumped over the beams next to the hurdle, the photocell at the end of each compartment was invariably triggered. The shuttle-boxes were housed in sound attenuated chambers, and were controlled by a microcomputer system. Procedure Mice of Experiment 1 received either 50 avoidance training trials or no treatment on Day 1. Avoidance training consisted of animals being individually placed in the shuttle-boxes, 60 sec after which training commenced with presentation of the compound CS (light plus tone) and opening of the gate separating the compartments. If the mouse crossed into the adjacent chamber within 10 sec of CS onset the trial terminated and shock was withheld. If an avoidance response was not made within 10 sec of CS onset, shock (150 tzA) was presented until an escape response was made, after which the CS and shock terminated and the gate closed. The intertrial interval was 60 sec in duration. Following avoidance training mice were returned to their home cages until the time of testing which was conducted 72 hr later. On test day mice of the pretrained and nontrained groups were subdivided and injected intraperitoneally (ip) with either pimozide (0.2 or 0.4 rag/ kg, in a volume of 10 ml/kg) or its vehicle (n = 8/group). These doses were previously shown to retard escape performance in some types of tasks and to disrupt active avoidance performance in mice (Anisman et al., 1979, Corradini, Tombaugh, Zacharko, & Anisman, 1982). Three hours after drug administration mice received 50 avoidance trials using the procedures employed during initial training. Pimozide was dissolved in acetic acid and warm dextrose (5.5%) was added to make up the final volume. In Experiment 2 mice were placed in one side of the shuttle-box and given either no treatment, l0 or 30 CS-shock pairings. On each trial the CS (light plus tone) was presented for 10 sec followed by presentation of a 2-sec shock (150 /~A), after which the CS and shock terminated. The interval between trials was 60 sec in duration. Mice that were not shocked were placed in the shuttle-boxes for the same duration as mice in the 30 CS-shock group, but neither the CS nor the shock was presented. The number of CS-shock pairings employed was predicated on preliminary experiments which indicated that 30 CS-shock pairings (in which shock was of 2-sec duration) facilitated performance to the same extent as did 50 avoidance training trials. After the initial training session mice were returned to their home cages until the time of testing conducted 72 hr later. On test day, mice of each of the preshock groups were subdivided and injected ip with either pimozide (0.4 mg/kg) or its vehicle, and tested 3 hr later in the shuttle avoidance task as described in Experiment 1.
284
A N I S M A N E T AL.
Results and Discussion Figure ] shows the mean percentage avoidance responses for each of the groups of Experiment 1. Analysis of variance of the avoidance scores revealed a significant Pretraining × Drug Treatment interaction, F(8, 168) = 1.98, p = .052. Newman-Keuls multiple comparisons (a = .05) revealed that among naive mice treated with the vehicle or pimozide, performance for the 0.2 mg/kg condition improved over blocks of trials. In contrast, among mice treated with the 0.4 mg/kg dose of pimozide, performance deteriorated between the first and second trial blocks, possibly as a result of diminution of pseudoconditioned responses, and remained depressed thereafter (see Fig. 1). Throughout the last four trial blocks performance was inferior to that of the vehicle and 0.2 mg/kg pimozide groups. Predictably, performance was enhanced among mice that had previously received avoidance training. In contrast to the effects of pimozide among naive animals, neither dose of the drug retarded performance significantly among mice that had previously received avoidance training. As reported elsewhere (Fibiger et al., 1975) proficient avoidance performance was evident in these animals regardless of the drug treatment received. Of course, with a sufficiently high dosage it can reasonably be expected that the disruptive effects of the drug would still be evident among mice that had previously received avoidance training (Anisman et al., 1979). The effects of pimozide on performance of mice that received CSshock pairings (Experiment 2) was clearly distinguishable from that of mice that received avoidance training prior to test (Experiment I). Specifically, CS-shock pairing was found to influence later avoidance per-
LU
~7 100
,VEHICLE o PIMOZIDE (0.2mg/kg) o PIMOZlDE (0.4 mg/kg)
t,U Z
~ 6o 6 ~ 40 Z W
~ 2o n
B L O C K S OF T R I A L S
FIG. 1. M e a n (+- SEM) percentage avoidance r e s p o n s e s emitted over blocks of 10 trials as a function of pimozide t r e a t m e n t a m o n g naive mice (--) and mice previously trained to avoid (--).
DOPAMINE
AND AVOIDANCE
28.5
rn z ,o 80 Y a
IJJ 60 0 f g 40 P $ 20 i f
0 CS-SHOCK
30
10 PAIRINGS
FIG. 2. Mean (*SEMI percentage avoidance responses emitted over 50 trials as a function of pimozide treatment among mice that previously received 0: 10, or 30 classical conditioning CS-shock pairings.
formance, F(2, 54) = 3.29, p < .05. Newman-Keuls multiple comparisons indicated that both 10 and 30 pairings of the CS and shock enhanced subsequent avoidance performance (see Fig. 2). Regardless of prior training, pimozide (0.4 mg/kg) disrupted avoidance performance, F(1, 54) = 9.89, p < .Ol. It should be noted that among vehicle-treated animals the 30 CS-shock pairings enhanced performance to about the same extent as did prior avoidance training in Experiment 1. Accordingly, the differential effect of the drug in the two experiments cannot be attributed to different rates of acquisition of the avoidance response. Parenthetically, it has previously been demonstrated that even a small number of CS-shock pairings would appreciably enhance later avoidance performance. In fact, even a single CS-UCS conditioning trial augmented performance to a degree equivalent to that of 10 to 20 avoidance trials (Anisman, 1978). If the prophylactic effects of prior avoidance training were due to establishment of fear or expectancy of shock (Experiment I), then CSshock pairings (Experiment 2) likewise should have had equivalent effects. The fact that this was not the case suggests that the elimination of the pimozide-induced avoidance deficits by prior training in Experiment 1 was due to the establishment of a goal-directed motor response compatible with that necessary for successful avoidance in the subsequent test session. EXPERIMENT
3
Although treatment with a neuroleptic may retard avoidance performance, high levels of performance are typically noted upon retest in the absence of the drug (Beninger et al., 1980a; Fibiger et al., 1975). It could be argued that the performance enhancement may be due to the establishment of a running response during initial drug-free training (i.e.. on avoidance and escape trials) which positively influences later avoidance
286
ANISMAN ET AL.
performance. Alternatively, an S-S association may have been established during initial avoidance training despite the presence of the drug, thus resulting in a performance enhancement upon subsequent testing in the drug-free condition. Experiment 3 was conducted to determine whether the facilitative effects of CS-shock pairings on later avoidance performance would be influenced by pimozide administered prior to initial training. Method
Subjects and Apparatus Forty experimentally naive CD-I mice served as subjects. The subject characteristics and the apparatus specifications were the same as those of Experiment 1.
Procedure Mice received either an ip injection of pimozide (0.4 mg/kg) or its vehicle followed 3 hr later by exposure to either 10 CS-shock pairing or no shock (n = 10/group) as described in Experiment 2. Mice were returned to their home cages and 72 hr afterward were tested in the avoidance task in the drug-free condition. The avoidance procedure was identical to that of Experiment 1. Results and Discussion The percentage avoidance responses for each of the groups is shown in Table 1. An analysis of variance revealed that prior CS-shock pairings enhanced subsequent avoidance performance, F(1, 36) = 18.63, p < .01, while the pimozide treatment was without effect (see Table 1). It is clear that despite the pimozide treatment, pairing a CS with shock effectively enhanced subsequent avoidance performance, suggesting that the drug did not disturb the formation of the S-S association.
TABLE 1 Mean (-2_SEM) Percentage A v o i d a n c e R e s p o n s e s as a Function of P r e s h o c k and Pimozide Condition during Pretraining Drug t r e a t m e n t
Preshock 10 CS plus shock N o shock
Vehicle
Pimozide (0.4 mg/kg)
63.40 - 8.02* 38.0 _ 4.86
55.20 - 4.26* 33.40 --- 3.64
* p < .05 relative to n o n s h o c k e d controls.
DOPAMINE AND AVOIDANCE
287
EXPERIMENT 4
The previous experiments show that pimozide, at least under the conditions employed, probably does not alter the processes underlying formation of S - S associations. That is, pimozide did not influence the ability of the animal to learn and utilize environmental cues (CSs) predicting the occurrence of an impending aversive event. If this general formulation is correct, then pimozide should not be expected to significantly alter processes governing the "unlearning" of the predictive relationship when the role of the CS is reversed and it is correlated with no shock (extinction). One technique particularly appropriate for this determination is response prevention (flooding), where the subject is forced to remain in the presence of the CS alone (UCS not presented) prior to the administration of a standard extinction procedure. Previous experiments (Baum, 1969) have shown that this procedure produces faster extinction, presumably because the previous " C S - U C S " association had been replaced with a " C S - n o UCS" association. The results from the first three experiments suggest that the administration of pimozide during response prevention should not influence performance on the extinction test compared with the behavior of vehicle controls. Method
Subjects and Apparatus Thirty-six naive male CD-1 mice, as described in Experiment 1, were used as subjects. The apparatus was the same as that of the preceding experiment.
Procedure Mice received two sessions of 100 avoidance trials on two consecutive days. Subjects which displayed fewer than 80% avoidance responses during the last 50 trials were rejected from the experiment. On the third day mice received ip injections of either pimozide (0.4 mg/kg) or its vehicle. Mice in each condition were subdivided and 3 hr later exposed either to 25 response prevention trials or handled but given no further treatment. Response prevention consisted of mice being placed in one chamber of the shuttle-box, and the CS presented for 10 sec at 60-sec intervals. The gate separating the compartments did not open thus preventing escape from the CS. Mice were returned to their home cages and 72 hr afterward the response to the CS was assessed. Mice were placed in the shuttle boxes and given 25 trials in which the CS was presented and the gate opened. If the mouse crossed the hurdle the trial was terminated. If no response was emitted within 10 sec the trial was likewise terminated. Under no condition was shock presented during test.
288
ANISMAN ET AL.
Results and Discussion
During initial training the groups did not differ in the number of avoidance responses made during the last 50 trials (88.2%). Table 2 displays the percentage of responses emitted upon CS presentation during the test session. As reported previously, the response prevention procedure effectively reduced the number of responses emitted in extinction, F(I, 32) = 7.32, p < .05. The pimozide treatment was not found to modify the responsivity to the CS. Thus, it appears that treatment with the neuroleptic did not alter the S-S association involved in extinction. This effect is seen as particularly significant since it shows that the failure of pimozide to alter S-S associations in aversively motivated tasks is not restricted to CS-UCS pairings but has parallel applications when the CS predicts the absence of the UCS. General Discussion
As previously reported (Anisman et al., 1979; Fibiger et al., 1975; Posluns, 1962) treatment with a DA receptor blocker was found to produce deficits in the acquisition of active avoidance performance. However, if mice were initially trained in the avoidance task, the deficits ordinarily engendered by pimozide were eliminated. In contrast, among mice that were initially exposed to classically conditioned CS-shock pairings, thereby enhancing subsequent avoidance performance to the same extent as noted among mice that had previously received avoidance training, the disruptive effects of pimozide were still evident. In fact, when prior avoidance training preestablished both the CS-shock (S-S) and response-outcome contingencies, the performance deficits that might otherwise be induced by pimozide were prevented. However, establishment of just the S-S association (Experiment 2) was ineffective in this respect. Of course, it cannot be assumed that in the present investigation the S-S association was equally well established in the two paradigms, nor that the motivation to avoid/escape was comparable. Nevertheless, these data provisionally TABLE 2 Mean ( ± SEM) Percentage Responses in Extinction as a Function of the Response Prevention and Drug Conditions Drug treatment
Response prevention No treatment
Vehicle
Pimozide (0.4 mg/kg)
35.55 ± 6.81" 56.00 ± 5.32
30.66 ± 8.72* 48.44 ± 6.94
* p < .05 relative to nontreated animals.
DOPAMINE AND AVOIDANCE
289
suggest that the disruptive effects of pimozide on avoidance performance stem from deficits in response initiation of goal directed behavior, and the effectiveness of prior training in mitigating the drug-induced performance deficits is due to the establishment of the instrumental component of the avoidance response rather than merely the preestablishment of shock expectancy or conditioned fear. It was demonstrated previously that although neuroleptics disrupt active avoidance performance (Anisman et al., 1979; Fibiger et al., 1975) or responding in a defensive burying task (Beninger et al., 1980a), upon retest in the nondrug state performance was comparable to that of animals that had not been trained in the drug condition. Thus, the hypothesis was entertained that the disturbances induced by neuroleptics in these tasks were due to motoric factors, while the association between CS and shock was unimpaired. The results of the present investigation are consistent with this position, in that the avoidance enhancement introduced by explicit pairings of a CS with shock, where shock offset was independent of responses, was not affected among animals treated with pimozide prior to the CS-shock pairings. Likewise, the findings that pimozide did not influence the effects of response prevention on a later extinction test suggest that the drug did not alter the associative or motivational processes that subserve the extinction of the avoidance response. It might be added at this juncture that we recently demonstrated (Corradini et al., 1982) that in a discriminated Y-maze avoidance task the disruption of avoidance responding was not accompanied by a reduction in the accuracy of responding on the discrimination component of the task (i.e., entering the safe arm on avoidance and escape trials). Indeed, this was the case in both a cue-discrimination and in a response-choice (always turn in a predetermined direction) task. Coupled with the results of the present investigation, these data suggest that pimozide not only is without effect on associations between cues and shock onset, but likewise is without consequence in affecting the association between cues or responses and shock offset. Moreover, comparison of these results with those obtained using appetitive discrimination problems (Szostak & Tombaugh, 1981; Tombaugh, 1981; Tombaugh, Ritch, & Shepherd, 1980) supports a previously advanced hypothesis (Szostak & Tombaugh, 1981) that dopamine does not play an important role in the mediation of S-S learning regardless of the appetitive or aversive nature of the task. It is, however, unclear at the present time the manner in which DA mediates S - R learning, and is involved in motivational processes which underlie the activation and maintenance of behavior. REFERENCES Anisman, H. (1973). Effects of pretraining compatible and incompatible responses on subsequent one-way and shuttle-avoidance performance in rats. Journal of Comparative and Physiological Psychology, 82, 95-104.
290
ANISMAN ET AL.
Anisman, H. (1978). Aversively motivated behavior as a tool in psychopharmacologic analysis. In H. Anisman & G. Bignami (Eds.), Psychopharmacology of Aversively Motivated Behavior. Plenum: New York. Anisman, H., Remington, G., & Sklar, L. S. (1979). Effects of inescapable shock on subsequent escape performance: Catecholaminergic and cholinergic mediation of response initiation and maintenance. Psychopharmacology. 61, 107-124. Baum, M. (1969). Extinction of an avoidance response following response prevention: Some parametric investigations. Canadian Journal of Psychology, 23, 1-10. Beninger, R. J., MacLennan, A. J., & Pinel, J. P. J. (1980). The use of conditioned defensive burying to test the effects of pimozide on associative learning. Pharmacology, Biochemistl:v and Behavior, 12, 445-448. (a) Beninger, R. J., Mason, S. T., Phillips, A. G., & Fibiger, H. C. (1980). The use of conditioned suppression to evaluate the nature of neuroleptic-induced avoidance deficits. Journal of Pharmacology and Experimental Therapeutics, 213, 623-627. (b) Corradini, A., Tombaugh, T. N., Zacharko, R. M., & Anisman, H. (1982). Discriminated avoidance and escape performance after pimozide treatment. Paper presented at the 43rd Annual Meeting of the Canadian Psychological Association, Montreal, June 1982. Fibiger, H. C., Zis, A, P., & Phillips, A. G. (1975). Haloperidol-induced disruption of conditioned avoidance responding: Attenuation by prior training or by anticholinergic drugs. European Journal of Pharmacology, 30, 309-314. Posluns, D. (1962). An analysis of chlorpromazine-induced suppression of the avoidance response. Psychopharmacology, 3, 361-373. Szostak, C., & Tombaugh, T. N. (1981). Use of a fixed consecutive number schedule of reinforcement to investigate the effects of pimozide on behavior controlled by internal and external stimuli. Pharmacology, Biochemistry and Behavior, 15, 609-617. Tombaugh, T. N. (1981). Effects of pimozide on nondiscriminated and discriminated performance in the pigeon. Psychopharmacology, 73, 137-141. Tombaugh, T. N., Ritch, M. A., & Shepherd, D. T. (1980). Effects of pimozide on accuracy of performance and distribution of correct responding on a simultaneous discrimination task in the rat. Pharmacology, Biochemistry and Behavior, 13, 859-862. Wise, R. A. (1982). Neuroleptics and operant behavior: The anhedonia hypothesis. Behavioral and Brain Sciences, 5, 39-87. Wise, R. A., Spindler, J., DeWit, H., & Gerber, G. (1978). Neuroleptic-induced "anhedonia" in rats: Pimozide blocks the reward quality of food. Science, 201,262-264.
36, 280-290 (1982)
Effects of Dopamine Receptor Blockade on Avoidance Performance: Assessment of Effects on Cue-Shock and Response-Outcome Associations 1 HYMIE ANISMAN, JILL IRWIN, ROBERT M. ZACHARKO, AND TOM N . TOMBAUGH
Department of Psychology, Unit for Behavioral Medicine and Pharmacology, Carleton University, Ottawa, Ontario K1S 5B6, Canada Treatment with pimozide, a neuroleptic drug which blocks postsynaptic dopamine receptors, retarded active avoidance performance among naive mice. The performance deficit was eliminated among mice that were previously trained in the avoidance task. Like avoidance training, prior CS-shock pairings enhanced avoidance performance, but this treatment did not reduce the effects of pimozide on later avoidance performance. Likewise, treatment with pimozide prior to CSshock pairings did not influence the performance ehancements evident in a later nondrug test of avoidance performance. Moreover, the drug did not influence the extinction of avoidance induced by response prevention (flooding). Together, these data suggest that pimozide does not influence S-S associations, but affects avoidance performance by disrupting response initiation processes.
It has been demonstrated repeatedly that treatment with neuroleptics will disrupt acquisition of an active avoidance response (Fibiger, Zis, & Phillips, 1975; Posluns, 1962) and may retard response latencies in some types of escape tasks (Anisman, Remington, & Sklar, 1979). It has been suggested that in tasks involving aversive motivation dopamine receptor blockers do not impair acquisition of the cue-shock relationship, i.e., the S - S associations (Beninger, MacLennan, & Pinel, 1980a; Beninger, Mason, Phillips, & Fibiger, 1980b), but rather induce deficits in response initiation and maintenance (Fibiger et al., 1975). Provided animals are initially trained in either an avoidance or escape task, performance deficits ordinarily engendered by dopamine receptor blockers will be largely reduced or eliminated (Anisman et al., 1979; Fibiger, et al., 1975). Moreover, when animals are trained in an avoidance task in the drug state, avoidance deficits may not be evident upon subsequent drug-free retests (Beninger Supported by Grants A9845, U0058, and A7074 from the Natural Science and Engineering Research Council of Canada. 280 0163-1047/82 $2.00 Copyright @ 1982 by Academic Press, Inc. All fights of reproduction in any form reserved.
DOPAMINE AND AVOIDANCE
281
et al., 1980a; Fibiger et al., 1975). Thus it appears that if animals are initially trained in the drug condition the S-S association will be established, resulting in positive transfer in tasks requiring animals to perform a highly prepared response (Beninger et al., 1980a), or where the response contingencies can be readily established (Fibiger et al., 1975). In addition, these processes may be augmented by the response requirement being partially established during initial training (i.e., running in response to, rather than in anticipation of, shock thus positively influencing subsequent avoidance performance). The prophylactic effects of prior escape or avoidance training on subsequent performance may be attributable to the establishment of requisite motor responding or to the establishment of the cue-shock relationship (fear or expectancy of shock) which increases the motivation to escape or avoid. In respect to the latter, it may be further speculated that a dopamine receptor blocker influences performance either by blunting the negative reinforcement derived from appropriate responding or by reducing motivation. It wilt be recognized that this viewpoint represents an extrapolation from the anhedonia hypothesis (Wise, 1982; Wise, Spindler, DeWit, and Gerber, 1978) which posits that dopamine neuronal activity is fundamental to reward processes in tasks involving appetitive motivation. In the aversive paradigm prior establishment of an avoidance response could potentially increase motivation to avoid or escape from the aversive stimulus, thus eliminating the potential effects of the drug treatment. In any case, the present series of experiments were undertaken to evaluate the effects of the dopamine receptor blocker, pimozide, on avoidance performance with particular attention devoted to the analysis of the drug's effect on the establishment of S-S associations and to the contribution of motoric changes to the induction of the performance disturbances. EXPERIMENTS 1 AND 2
Experiment 1 was designed to evaluate the effects of pimozide on active avoidance performance, and to determine whether the disruptive effects of the drug treatment would be eliminated or reduced among mice previously trained in an avoidance task. Since such an outcome could result from the prior establishment of a particular response or the establishment of the cue-shock contingency, Experiment 2 assessed the effects of prior classically conditioned CS-shock pairings (i.e., no instrumental response requirement) on subsequent performance deficits normally induced by pimozide. It has been demonstrated on several occasions that pairing an explicit cue with short duration shock facilitates subsequent avoidance behavior, provided that shock associated immobility can be suppressed, e.g., in a one-way avoidance task or in a shuttle task involving mild shock intensities (Anisman, 1973). Presumably, CS-shock pairings result in the establishment of the expectancy of shock, reduce
282
ANISMAN ET AL.
the S-S associative requirements in the yet-to-be-learned avoidance response, and enhance performance. If initial training (Experiment 1) inhibits pimozide-induced performance deficits as a function of establishing an S-S association, then prior CS-shock pairings should also be prophylactic in this respect. Conversely, if the effects of initially training animals in the avoidance task are related to the establishment of an active motor response, then prior classically conditioned CS-shock pairings should be ineffective in reducing the performance deficits provoked by treatment with pimozide. Method
Subjects Experiments 1 and 2 involved 48 and 60 male CD-I mice procured from the Canadian Breeding Laboratories, Laprairie, Quebec, Canada, at 50-55 days of age. Mice were housed in groups of 5 in standard polypropylene cages and acclimated to the laboratory for at least 7 days prior to being used as experimental subjects.
Apparatus Avoidance training was conducted in four identical black, Plexiglas shuttle-boxes which measured 29.2 x 8.9 x 16.5 cm. Opaque disks, 2.54 cm in diameter, centered 2.54 cm from the top of the end walls, could be illuminated by bulbs. Additionally, a speaker was centered on the red Plexiglas roof of each chamber. The light and tone were used as a compound CS. The floor of each chamber consisted of 0.32-cm stainless-steel bars spaced 1.0 cm apart (center to center), connected in series by neon bulbs. Shock of 150/~A (60 Hz, AC) could be delivered to the grid floor through a 3000-V source, thereby providing relatively constant current. Each shuttle-box was divided into two equal-sized compartments by a stainless-steel wall partially made up of a solenoid controlled horizontally movable gate. When the gate was open a 5.2 x 6.1-cm space situated above a 1.27-cm hurdle permitted access to the adjacent compartment. The hurdle was made of thick cork and was lined on either side by thin stainless-steel plates which were connected in series by a neon bulb, thereby preventing mice from avoiding or escaping shock by sitting on the hurdle. Situated 1.1 cm on either side of the hurdle were two infrared photodetectors 1.27 and 2.54 cm above the grid floor. The photocells were wired so that the cells would not trigger when both sides of the hurdle were crossed simultaneously (e.g., when the mouse was half-way across the hurdle). Only when the beam on the nonshock side alone was crossed would the cell be activated. The photocells were also designed with a delay such that very brief photocell interruption (e.g., the tail of a mouse crossing the hurdle) would not trigger the
DOPAMINE AND AVOIDANCE
283
photocell. A third set of photocells were located 2.54 cm from the end walls of each compartment. In the event that the mouse jumped over the beams next to the hurdle, the photocell at the end of each compartment was invariably triggered. The shuttle-boxes were housed in sound attenuated chambers, and were controlled by a microcomputer system. Procedure Mice of Experiment 1 received either 50 avoidance training trials or no treatment on Day 1. Avoidance training consisted of animals being individually placed in the shuttle-boxes, 60 sec after which training commenced with presentation of the compound CS (light plus tone) and opening of the gate separating the compartments. If the mouse crossed into the adjacent chamber within 10 sec of CS onset the trial terminated and shock was withheld. If an avoidance response was not made within 10 sec of CS onset, shock (150 tzA) was presented until an escape response was made, after which the CS and shock terminated and the gate closed. The intertrial interval was 60 sec in duration. Following avoidance training mice were returned to their home cages until the time of testing which was conducted 72 hr later. On test day mice of the pretrained and nontrained groups were subdivided and injected intraperitoneally (ip) with either pimozide (0.2 or 0.4 rag/ kg, in a volume of 10 ml/kg) or its vehicle (n = 8/group). These doses were previously shown to retard escape performance in some types of tasks and to disrupt active avoidance performance in mice (Anisman et al., 1979, Corradini, Tombaugh, Zacharko, & Anisman, 1982). Three hours after drug administration mice received 50 avoidance trials using the procedures employed during initial training. Pimozide was dissolved in acetic acid and warm dextrose (5.5%) was added to make up the final volume. In Experiment 2 mice were placed in one side of the shuttle-box and given either no treatment, l0 or 30 CS-shock pairings. On each trial the CS (light plus tone) was presented for 10 sec followed by presentation of a 2-sec shock (150 /~A), after which the CS and shock terminated. The interval between trials was 60 sec in duration. Mice that were not shocked were placed in the shuttle-boxes for the same duration as mice in the 30 CS-shock group, but neither the CS nor the shock was presented. The number of CS-shock pairings employed was predicated on preliminary experiments which indicated that 30 CS-shock pairings (in which shock was of 2-sec duration) facilitated performance to the same extent as did 50 avoidance training trials. After the initial training session mice were returned to their home cages until the time of testing conducted 72 hr later. On test day, mice of each of the preshock groups were subdivided and injected ip with either pimozide (0.4 mg/kg) or its vehicle, and tested 3 hr later in the shuttle avoidance task as described in Experiment 1.
284
A N I S M A N E T AL.
Results and Discussion Figure ] shows the mean percentage avoidance responses for each of the groups of Experiment 1. Analysis of variance of the avoidance scores revealed a significant Pretraining × Drug Treatment interaction, F(8, 168) = 1.98, p = .052. Newman-Keuls multiple comparisons (a = .05) revealed that among naive mice treated with the vehicle or pimozide, performance for the 0.2 mg/kg condition improved over blocks of trials. In contrast, among mice treated with the 0.4 mg/kg dose of pimozide, performance deteriorated between the first and second trial blocks, possibly as a result of diminution of pseudoconditioned responses, and remained depressed thereafter (see Fig. 1). Throughout the last four trial blocks performance was inferior to that of the vehicle and 0.2 mg/kg pimozide groups. Predictably, performance was enhanced among mice that had previously received avoidance training. In contrast to the effects of pimozide among naive animals, neither dose of the drug retarded performance significantly among mice that had previously received avoidance training. As reported elsewhere (Fibiger et al., 1975) proficient avoidance performance was evident in these animals regardless of the drug treatment received. Of course, with a sufficiently high dosage it can reasonably be expected that the disruptive effects of the drug would still be evident among mice that had previously received avoidance training (Anisman et al., 1979). The effects of pimozide on performance of mice that received CSshock pairings (Experiment 2) was clearly distinguishable from that of mice that received avoidance training prior to test (Experiment I). Specifically, CS-shock pairing was found to influence later avoidance per-
LU
~7 100
,VEHICLE o PIMOZIDE (0.2mg/kg) o PIMOZlDE (0.4 mg/kg)
t,U Z
~ 6o 6 ~ 40 Z W
~ 2o n
B L O C K S OF T R I A L S
FIG. 1. M e a n (+- SEM) percentage avoidance r e s p o n s e s emitted over blocks of 10 trials as a function of pimozide t r e a t m e n t a m o n g naive mice (--) and mice previously trained to avoid (--).
DOPAMINE
AND AVOIDANCE
28.5
rn z ,o 80 Y a
IJJ 60 0 f g 40 P $ 20 i f
0 CS-SHOCK
30
10 PAIRINGS
FIG. 2. Mean (*SEMI percentage avoidance responses emitted over 50 trials as a function of pimozide treatment among mice that previously received 0: 10, or 30 classical conditioning CS-shock pairings.
formance, F(2, 54) = 3.29, p < .05. Newman-Keuls multiple comparisons indicated that both 10 and 30 pairings of the CS and shock enhanced subsequent avoidance performance (see Fig. 2). Regardless of prior training, pimozide (0.4 mg/kg) disrupted avoidance performance, F(1, 54) = 9.89, p < .Ol. It should be noted that among vehicle-treated animals the 30 CS-shock pairings enhanced performance to about the same extent as did prior avoidance training in Experiment 1. Accordingly, the differential effect of the drug in the two experiments cannot be attributed to different rates of acquisition of the avoidance response. Parenthetically, it has previously been demonstrated that even a small number of CS-shock pairings would appreciably enhance later avoidance performance. In fact, even a single CS-UCS conditioning trial augmented performance to a degree equivalent to that of 10 to 20 avoidance trials (Anisman, 1978). If the prophylactic effects of prior avoidance training were due to establishment of fear or expectancy of shock (Experiment I), then CSshock pairings (Experiment 2) likewise should have had equivalent effects. The fact that this was not the case suggests that the elimination of the pimozide-induced avoidance deficits by prior training in Experiment 1 was due to the establishment of a goal-directed motor response compatible with that necessary for successful avoidance in the subsequent test session. EXPERIMENT
3
Although treatment with a neuroleptic may retard avoidance performance, high levels of performance are typically noted upon retest in the absence of the drug (Beninger et al., 1980a; Fibiger et al., 1975). It could be argued that the performance enhancement may be due to the establishment of a running response during initial drug-free training (i.e.. on avoidance and escape trials) which positively influences later avoidance
286
ANISMAN ET AL.
performance. Alternatively, an S-S association may have been established during initial avoidance training despite the presence of the drug, thus resulting in a performance enhancement upon subsequent testing in the drug-free condition. Experiment 3 was conducted to determine whether the facilitative effects of CS-shock pairings on later avoidance performance would be influenced by pimozide administered prior to initial training. Method
Subjects and Apparatus Forty experimentally naive CD-I mice served as subjects. The subject characteristics and the apparatus specifications were the same as those of Experiment 1.
Procedure Mice received either an ip injection of pimozide (0.4 mg/kg) or its vehicle followed 3 hr later by exposure to either 10 CS-shock pairing or no shock (n = 10/group) as described in Experiment 2. Mice were returned to their home cages and 72 hr afterward were tested in the avoidance task in the drug-free condition. The avoidance procedure was identical to that of Experiment 1. Results and Discussion The percentage avoidance responses for each of the groups is shown in Table 1. An analysis of variance revealed that prior CS-shock pairings enhanced subsequent avoidance performance, F(1, 36) = 18.63, p < .01, while the pimozide treatment was without effect (see Table 1). It is clear that despite the pimozide treatment, pairing a CS with shock effectively enhanced subsequent avoidance performance, suggesting that the drug did not disturb the formation of the S-S association.
TABLE 1 Mean (-2_SEM) Percentage A v o i d a n c e R e s p o n s e s as a Function of P r e s h o c k and Pimozide Condition during Pretraining Drug t r e a t m e n t
Preshock 10 CS plus shock N o shock
Vehicle
Pimozide (0.4 mg/kg)
63.40 - 8.02* 38.0 _ 4.86
55.20 - 4.26* 33.40 --- 3.64
* p < .05 relative to n o n s h o c k e d controls.
DOPAMINE AND AVOIDANCE
287
EXPERIMENT 4
The previous experiments show that pimozide, at least under the conditions employed, probably does not alter the processes underlying formation of S - S associations. That is, pimozide did not influence the ability of the animal to learn and utilize environmental cues (CSs) predicting the occurrence of an impending aversive event. If this general formulation is correct, then pimozide should not be expected to significantly alter processes governing the "unlearning" of the predictive relationship when the role of the CS is reversed and it is correlated with no shock (extinction). One technique particularly appropriate for this determination is response prevention (flooding), where the subject is forced to remain in the presence of the CS alone (UCS not presented) prior to the administration of a standard extinction procedure. Previous experiments (Baum, 1969) have shown that this procedure produces faster extinction, presumably because the previous " C S - U C S " association had been replaced with a " C S - n o UCS" association. The results from the first three experiments suggest that the administration of pimozide during response prevention should not influence performance on the extinction test compared with the behavior of vehicle controls. Method
Subjects and Apparatus Thirty-six naive male CD-1 mice, as described in Experiment 1, were used as subjects. The apparatus was the same as that of the preceding experiment.
Procedure Mice received two sessions of 100 avoidance trials on two consecutive days. Subjects which displayed fewer than 80% avoidance responses during the last 50 trials were rejected from the experiment. On the third day mice received ip injections of either pimozide (0.4 mg/kg) or its vehicle. Mice in each condition were subdivided and 3 hr later exposed either to 25 response prevention trials or handled but given no further treatment. Response prevention consisted of mice being placed in one chamber of the shuttle-box, and the CS presented for 10 sec at 60-sec intervals. The gate separating the compartments did not open thus preventing escape from the CS. Mice were returned to their home cages and 72 hr afterward the response to the CS was assessed. Mice were placed in the shuttle boxes and given 25 trials in which the CS was presented and the gate opened. If the mouse crossed the hurdle the trial was terminated. If no response was emitted within 10 sec the trial was likewise terminated. Under no condition was shock presented during test.
288
ANISMAN ET AL.
Results and Discussion
During initial training the groups did not differ in the number of avoidance responses made during the last 50 trials (88.2%). Table 2 displays the percentage of responses emitted upon CS presentation during the test session. As reported previously, the response prevention procedure effectively reduced the number of responses emitted in extinction, F(I, 32) = 7.32, p < .05. The pimozide treatment was not found to modify the responsivity to the CS. Thus, it appears that treatment with the neuroleptic did not alter the S-S association involved in extinction. This effect is seen as particularly significant since it shows that the failure of pimozide to alter S-S associations in aversively motivated tasks is not restricted to CS-UCS pairings but has parallel applications when the CS predicts the absence of the UCS. General Discussion
As previously reported (Anisman et al., 1979; Fibiger et al., 1975; Posluns, 1962) treatment with a DA receptor blocker was found to produce deficits in the acquisition of active avoidance performance. However, if mice were initially trained in the avoidance task, the deficits ordinarily engendered by pimozide were eliminated. In contrast, among mice that were initially exposed to classically conditioned CS-shock pairings, thereby enhancing subsequent avoidance performance to the same extent as noted among mice that had previously received avoidance training, the disruptive effects of pimozide were still evident. In fact, when prior avoidance training preestablished both the CS-shock (S-S) and response-outcome contingencies, the performance deficits that might otherwise be induced by pimozide were prevented. However, establishment of just the S-S association (Experiment 2) was ineffective in this respect. Of course, it cannot be assumed that in the present investigation the S-S association was equally well established in the two paradigms, nor that the motivation to avoid/escape was comparable. Nevertheless, these data provisionally TABLE 2 Mean ( ± SEM) Percentage Responses in Extinction as a Function of the Response Prevention and Drug Conditions Drug treatment
Response prevention No treatment
Vehicle
Pimozide (0.4 mg/kg)
35.55 ± 6.81" 56.00 ± 5.32
30.66 ± 8.72* 48.44 ± 6.94
* p < .05 relative to nontreated animals.
DOPAMINE AND AVOIDANCE
289
suggest that the disruptive effects of pimozide on avoidance performance stem from deficits in response initiation of goal directed behavior, and the effectiveness of prior training in mitigating the drug-induced performance deficits is due to the establishment of the instrumental component of the avoidance response rather than merely the preestablishment of shock expectancy or conditioned fear. It was demonstrated previously that although neuroleptics disrupt active avoidance performance (Anisman et al., 1979; Fibiger et al., 1975) or responding in a defensive burying task (Beninger et al., 1980a), upon retest in the nondrug state performance was comparable to that of animals that had not been trained in the drug condition. Thus, the hypothesis was entertained that the disturbances induced by neuroleptics in these tasks were due to motoric factors, while the association between CS and shock was unimpaired. The results of the present investigation are consistent with this position, in that the avoidance enhancement introduced by explicit pairings of a CS with shock, where shock offset was independent of responses, was not affected among animals treated with pimozide prior to the CS-shock pairings. Likewise, the findings that pimozide did not influence the effects of response prevention on a later extinction test suggest that the drug did not alter the associative or motivational processes that subserve the extinction of the avoidance response. It might be added at this juncture that we recently demonstrated (Corradini et al., 1982) that in a discriminated Y-maze avoidance task the disruption of avoidance responding was not accompanied by a reduction in the accuracy of responding on the discrimination component of the task (i.e., entering the safe arm on avoidance and escape trials). Indeed, this was the case in both a cue-discrimination and in a response-choice (always turn in a predetermined direction) task. Coupled with the results of the present investigation, these data suggest that pimozide not only is without effect on associations between cues and shock onset, but likewise is without consequence in affecting the association between cues or responses and shock offset. Moreover, comparison of these results with those obtained using appetitive discrimination problems (Szostak & Tombaugh, 1981; Tombaugh, 1981; Tombaugh, Ritch, & Shepherd, 1980) supports a previously advanced hypothesis (Szostak & Tombaugh, 1981) that dopamine does not play an important role in the mediation of S-S learning regardless of the appetitive or aversive nature of the task. It is, however, unclear at the present time the manner in which DA mediates S - R learning, and is involved in motivational processes which underlie the activation and maintenance of behavior. REFERENCES Anisman, H. (1973). Effects of pretraining compatible and incompatible responses on subsequent one-way and shuttle-avoidance performance in rats. Journal of Comparative and Physiological Psychology, 82, 95-104.
290
ANISMAN ET AL.
Anisman, H. (1978). Aversively motivated behavior as a tool in psychopharmacologic analysis. In H. Anisman & G. Bignami (Eds.), Psychopharmacology of Aversively Motivated Behavior. Plenum: New York. Anisman, H., Remington, G., & Sklar, L. S. (1979). Effects of inescapable shock on subsequent escape performance: Catecholaminergic and cholinergic mediation of response initiation and maintenance. Psychopharmacology. 61, 107-124. Baum, M. (1969). Extinction of an avoidance response following response prevention: Some parametric investigations. Canadian Journal of Psychology, 23, 1-10. Beninger, R. J., MacLennan, A. J., & Pinel, J. P. J. (1980). The use of conditioned defensive burying to test the effects of pimozide on associative learning. Pharmacology, Biochemistl:v and Behavior, 12, 445-448. (a) Beninger, R. J., Mason, S. T., Phillips, A. G., & Fibiger, H. C. (1980). The use of conditioned suppression to evaluate the nature of neuroleptic-induced avoidance deficits. Journal of Pharmacology and Experimental Therapeutics, 213, 623-627. (b) Corradini, A., Tombaugh, T. N., Zacharko, R. M., & Anisman, H. (1982). Discriminated avoidance and escape performance after pimozide treatment. Paper presented at the 43rd Annual Meeting of the Canadian Psychological Association, Montreal, June 1982. Fibiger, H. C., Zis, A, P., & Phillips, A. G. (1975). Haloperidol-induced disruption of conditioned avoidance responding: Attenuation by prior training or by anticholinergic drugs. European Journal of Pharmacology, 30, 309-314. Posluns, D. (1962). An analysis of chlorpromazine-induced suppression of the avoidance response. Psychopharmacology, 3, 361-373. Szostak, C., & Tombaugh, T. N. (1981). Use of a fixed consecutive number schedule of reinforcement to investigate the effects of pimozide on behavior controlled by internal and external stimuli. Pharmacology, Biochemistry and Behavior, 15, 609-617. Tombaugh, T. N. (1981). Effects of pimozide on nondiscriminated and discriminated performance in the pigeon. Psychopharmacology, 73, 137-141. Tombaugh, T. N., Ritch, M. A., & Shepherd, D. T. (1980). Effects of pimozide on accuracy of performance and distribution of correct responding on a simultaneous discrimination task in the rat. Pharmacology, Biochemistry and Behavior, 13, 859-862. Wise, R. A. (1982). Neuroleptics and operant behavior: The anhedonia hypothesis. Behavioral and Brain Sciences, 5, 39-87. Wise, R. A., Spindler, J., DeWit, H., & Gerber, G. (1978). Neuroleptic-induced "anhedonia" in rats: Pimozide blocks the reward quality of food. Science, 201,262-264.