Behavioral Neuroscience 1992, Vol. 106, No. 5, 837-845

Copyright 1992 by the American Psychological Association, Inc. 0735-7044/92/S3.00

Effects of Prenatal Exposure to Cocaine on Conditional Discrimination Learning in Adult Rats

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Charles J. Heyser, Norman E. Spear, and Linda Patia Spear Center for Developmental Psychobiology, State University of New York at Binghamton Adult male rats that were gestationally exposed to cocaine and control offspring were trained on an instrumental conditioning task for assessment of the acquisition and reversal of an appetitive conditional discrimination based on olfactory cues. Offspring were derived from Sprague-Dawley dams that had received subcutaneous injections of 40 mg/kg/3 cc cocaine hydrochloride (C40) daily on Gestational Days 8-20, pair-fed (PF) dams that were injected with saline, nutritional control dams (NC) that had received saline injections, and nontreated control dams (LC). There were no differences among the prenatal treatment groups in acquisition of the barpress response or response rate throughout all phases of training. All prenatal treatment groups required approximately the same number of sessions to criterion on the initial odor discrimination. In contrast, adult C40 offspring required more sessions to acquire the reversal of the conditional discrimination than did animals from the other treatment groups (PF, NC, and LC). In addition, even at criterion performance for acquisition of the reversal discrimination, C40 animals exhibited lower accuracy on the first 10 responses and made significantly more errors before the first reward. Taken together with previous results, these findings suggest that gestational exposure to cocaine results in long-lasting alterations in performance on conditioning tasks that are evident early in life and that persist into adulthood.

1989), alters later functioning of the DA system as well as other neural systems. For instance, Dow-Edwards, Freed, and Fico (1990) reported that prenatal exposure to cocaine resulted in long-lasting alterations in brain metabolism, including decreased utilization of glucose in numerous brain regions along with an increase in Dl DA receptor concentration in the substantia nigra in adult male offspring. An increase in striatal D2 DA binding has been reported in weanling offspring that were exposed prenatally to cocaine (Scalzo, AH, Frambes, & Spear, 1990), along with an increased psychopharmacological sensitivity to the D2 agonist quinpirole (Moody, Frambes, & Spear, in press). Clow, Hammer, Kirstein, and Spear (1991) also observed an increase in [3H]naloxone binding in autoradiograms of numerous brain regions in weanling offspring that were gestationally exposed to cocaine. Gestational exposure to cocaine has also been reported to influence offspring behavioral function. For example, alterations in the normal ontogenetic pattern of locomotor activity (Hutchings, Fico, & Dow-Edwards, 1989; Smith, Mattran, Kurkjian, & Kurtz, 1989) and deficits in both appetitive and aversive classical conditioning tasks early in life (Spear, Kirstein, Bell et al., 1989; Spear, Kirstein, & Frambes, 1989) have been reported after prenatal exposure to cocaine. Such exposure also impairs sensory preconditioning in infant rat pups, a pattern of results interpreted to suggest a general deficit in cognitive function in these offspring rather than a delay in cognitive maturation (Heyser, Chen, Miller, Spear, & Spear, 1990). Indeed, conditioning deficits may persist into adulthood, given that alterations in acquisition of a DRL-20 (differential-reinforcement-of-low-rates) task and deficits in water maze performance have been reported in adult offspring that were exposed gestationally to cocaine (Smith et al., 1989). It should be noted, however, that conditioning deficits are

Several reports have indicated that approximately 4.5-18% of pregnant women have used cocaine during pregnancy in recent years (Frank et al., 1988; Neerhof, MacGregor, & Sullivan, 1989). Although negative findings have also been reported (Richardson & Day, 1991), there are numerous reports that human infants prenatally exposed to cocaine exhibit alterations in performance on the Brazelton Neonatal Behavioral Assessment Scale (Brazelton, 1984), including signs of irritability, tremor, increased muscle tone, and a depression of interactive behavior (Chasnoff, 1988; Chasnoff, Burns, Schnoll, & Burns, 1985; Chasnoff, Griffith, MacGregor, Dirkes, & Burns, 1989; Doberczak, Shanzer, & Kandall, 1987). A variety of animal models of gestational exposure to cocaine have been developed (Dow-Edwards, 1989; Fung, Reed, & Lau, 1989; Spear, Kirstein, & Frambes, 1989; for a review see Church et al., 1991). A number of these studies have observed that early chronic exposure to cocaine, which is a dopamine (DA) reuptake inhibitor (Johanson & Fischman,

This research was supported by National Institute on Drug Abuse (NIDA) Grants R01 DA04478 and K02 DA00140 to Linda P. Spear, National Institute of Mental Health Grant R01 MH35219 to Norman E. Spear, and NIDA Predoctoral Fellowship F31 DA05511 to Charles J. Heyser. We gratefully acknowledge the assistance of Norman Richter for his technical assistance in constructing the apparatus, Yolanda Pinzas for her technical support, and Ralph R. Miller for his valuable comments on earlier versions of this article. This research was submitted in partial fulfillment of the requirements for an M.A. in Psychology from the State University of New York at Binghamton to Charles J. Heyser. Correspondence concerning this article should be addressed to Linda Patia Spear, Center for Developmental Psychobiology, Box 6000, State University of New York, Binghamton, New York 139026000. 837

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C. HEYSER, N. SPEAR, AND L. SPEAR

not always evident in adult offspring that were exposed early in life to cocaine. For instance, no differences were observed in adult offspring that were exposed gestationally to cocaine when tested on active avoidance and spatial navigation tasks (Riley & Foss, 1991). As Riley and Foss noted, these tasks may not have been sufficiently challenging to the animals to reveal cognitive deficits in adulthood. Indeed, task complexity may be a critical variable in determining sensitivity of a conditioning task to the effects of a developmental toxicant (e.g., see Stanton & Spear, 1990). To examine further the cognitive performance of adult offspring that were exposed prenatally to cocaine, the present study assessed such offspring during the acquisition and reversal of a conditional discrimination task. The operant conditional discrimination procedure is useful in that it provides information about performance on a series of tasks that can be presumably viewed as progressively more complex (or challenging): operant barpress responding, responding on an intermittent 10-s fixed-ratio (FR 10) schedule of reinforcement, discrimination training, and finally reversal of the discrimination task. Rate of acquisition of the reversal task may prove to be a particularly sensitive measure. Age-related differences in reversal performance, but not initial acquisition of a discrimination task, have been reported by Saperstein, Kucharski, Stanton, and Hall (1989). A number of studies have similarly demonstrated that reversal performance was disrupted by a variety of experimental manipulations, including lesions of specific brain regions (Eichenbaum, Fagan, & Cohen, 1986; Fagan & Olton, 1986) and gestational exposure to ACTH (McGivern et al., 1987). The conditioning parameters of the conditional discrimination that was used in the present study were designed to mimic those used in drug discrimination conditioning (see Overton, 1987) except that external cues (odors) were used rather than internal cues (drug state). Animals were initially trained to discriminate between two odors; each specific odor served as the cue that signaled which of two levers when depressed would result in delivery of food reward within a particular day's session. Once trained to a consistent level of discrimination, the contingency of the initial odor discrimination was reversed. Studies by Williams and Dunn (1991) and Preston, Dickinson, and Mackintosh (1986) indicated that adult rats are capable of acquiring conditional discriminations (see Mackintosh, 1974, 1983). For instance, Preston et al. demonstrated that animals could learn a contextual conditional discrimination for which one odor signaled the availability of reinforcement and another odor signaled a nonreinforcement period. The cocaine treatment procedure chosen for this study involved subcutaneous administration of 40 mg/kg/day from the day of neural tube closure (Gestational Day [GD] 8) until shortly before term (GD 20). This dose and route of administration produce maternal plasma cocaine levels that are in the range of or slightly above those reported for human cocaine users (e.g., Spear, Frambes, & Kirstein, 1989) and has been observed to induce numerous neurobehavioral alterations in exposed offspring (Clow et al., 1991; Heyser et al., 1990; Spear, KJrsrein, BeJf et al, 1989). Because transient anorexic effects of cocaine are typically seen at the onset of maternal treat-

ment, offspring from various nutritional control groups were also examined.

Method Subjects, Breeding, and Chronic Drug Treatments Offspring were generated from Sprague-Dawley (Charles River, Kingston, NY) rats that were bred in our laboratory. Animals were housed in a temperature-controlled colony room on a 12:12-hr light-dark cycle, with the lights on at 7:00 a.m. All dams were housed in pairs and habituated to the colony room for 2 weeks after arrival. Before the onset of mating, dams were handled for 5 min daily for 5 days. Subcutaneous injections of 3 cc/kg 0.9% saline were given the last 2 days of handling. Dams were individually housed in hanging cages before mating. An adult male was placed in each cage daily at 5:00 p.m. and removed the next morning at approximately 9:00 a.m. GD 1 was defined as the day of detection of a copulatory plug. The dams were randomly assigned to the four prenatal treatment groups and individually housed in Plexiglas breeding cages. Dams that were assigned to the cocaine (C40) and lab chow (LC) treatment groups were given ad lib food (powdered lab chow) and water throughout the study. A pair-fed (PF) group and a cellulose nutritional control (NC) group were included to control for the transient weight loss that is typically associated with the onset of cocaine treatment. Dams in the PF and NC groups were given ad lib access to food (powdered lab chow) and water from GD 1 to GD 7. On GD 8, each dam in the PF group was paired according to GD-8 body weight with a dam in the C40 group that was gestationally 24-48 hr more advanced. Each PF dam received the same amount of food (powdered lab chow) and water that was ingested by the C40 dam with which she was paired on the corresponding day of gestation (GD 8-22). On GD 8, each dam in the NC group was given ad lib access to water and a diet that consisted of 60% powdered lab chow and 40% cellulose throughout the remainder of gestation. Previous research in our laboratory has shown that animals placed on a cellulose-containing diet take 3-4 days to adjust their intake to counteract for the presence of this nondigestible fiber. During this time, although NC dams were not explicitly food restricted, their chow intake closely matched that of C40 dams, which typically show a transient reduction in food intake at the onset of drug treatment (Moody, Goodwin, Heyser, & Spear, 1991). Dams were subcutaneously injected daily between 10:00 a.m. and noon with either 40 mg/kg/3 cc cocaine hydrochloride (C40) or an equal volume of 0.9% saline (PF and NC groups) from GD 8 to GD 20. LC dams were not injected. Body weights and intake of food and water were recorded daily from GD 1 to GD 22. Gestational length was recorded for each gravid dam. On Postnatal Day (PD) 1, all pups were weighed, and the total number of pups of each sex was recorded. Each litter was culled to 8-10 pups and fostered to an untreated surrogate dam that had given birth to a litter within the preceding 24-72 hr. The maternal dams were sacrificed for determination of the number of uterine implantation sites. Litters remained intact with their surrogate dams until weaning on PD 21. At weaning, animals were housed in same-sex pairs in wire hanging cages with ad lib food and water available until the onset of food deprivation. Only 2 male offspring from each litter were used in the present study, with the remaining offspring assigned to other research projects. Offspring from at least 10 litters from each treatment group were examined in this study.

Apparatus Four operant chambers that were enclosed in \ight-conUoUed and sound-attenuated boxes were used. The operant chambers (25 x 30 x

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EFFECTS OF PRENATAL EXPOSURE TO COCAINE 30 cm) were constructed of clear Plexiglas and aluminum and had stainless steel grid floors. Two rodent levers (Coulbourn Instrs., Lehigh Valley, PA) were mounted 13 cm apart and 7 cm above the floor. A food tray was located equidistant between the two levers. Food pellets (45 mg; P. J. Noyes Co., Inc, Lancaster, NH) were delivered by a food dispenser (Ralph Gerbrands Co., Arlington, MA) that provided an audible click with each food presentation. A single jeweled light was positioned above each lever, and a central houselight served to illuminate the chamber. Exhaust fans that were located in the light-controlled boxes provided white noise to mask background noise. All operations and data collections were computer (IBM compatible 286; Dell, Inc.) controlled with Med-Associates, Incorporated (Lafayette Instrs., Lafayette, IN), software programming. The odors consisted of 1.5 cc banana extract (BAN; Virginia Dare brand) and 1.5 cc almond extract (ALM; McCormick brand). The odors were applied to the surface of absorbent cotton, which was placed beneath the grid floor in the center of the operant chamber.

Procedure Subjects were 20 adult male (PD 60-80 at the onset of deprivation) Sprague-Dawley rats from each treatment group (LC, C40, PF, and NC). At the onset of the deprivation procedure, all animals were singly housed in wire hanging cages. Each rat was deprived of food and maintained at 85% of its initial body weight. After an initial 24-hr food-deprivation period, animals were weighed daily and given food for 15 min at approximately 5:00 p.m. Once body weight reached 90% of the initial body weight, magazine training was initiated. Each animal was weighed before the daily sessions, which were conducted Monday through Friday, and given food for a 15-min period after completion of daily training. Each animal was weighed daily on the weekend and given food daily for a 15-min period. Animals were permitted to increase body weight by 10 g per month to a maximum of 450 g. Ad lib water was available throughout the study. Initial training. Each rat was placed into an operant chamber for 20 min each day to acquire the lever press response. Initially, each animal was habituated to the chamber, magazine trained, and allowed to collect food rewards from the hopper. After magazine training, each animal was shaped to barpress by successive approximations. Once animals displayed a constant barpress rate, a procedure to avoid lever preference was initiated. This procedure consisted of using only one lever throughout the entire daily training session; the active lever was alternated on a daily basis. Initial training was conducted under a schedule of continuous reinforcement (CRF schedule, for which one response yielded one food pellet). The schedule of reinforcement was increased until responding on an FR 10 schedule was consistent, with the latter being defined as ±10% of the previous day's total number of responses for three consecutive sessions. The overall number of responses was recorded daily for the entire session (20 min). In the present study, an FR schedule of reward was used throughout discrimination training. Williams (1989) has reported that schedules of partial reinforcement lengthen the amount of training necessary to produce reliable discriminations, although the number of reinforcements is comparable across different schedules. The use of an FR schedule, however, allows the use of two different indices of discrimination. The first discrimination index (DI1) was calculated on the number of correct responses in the first 10 responses, and the second discrimination index was calculated on the number of correct responses over the entire session (DI2). The rationale for this approach is that partial reinforcement schedules provide an extinctionlike test (or probe trial) at the start of each session. Under a CRF schedule, the animal's chance of receiving a reward in a two-lever situation is 50%, and hence it is possible that a chance response on the initial lever press may be correct and alter subsequent responding.

839

Initial odor discrimination training. An odor (1.5 cc of BAN or 1.5 cc of ALM) was placed under the grid floor 10 min before the animal was placed into the chamber, which thereby permitted the odor to diffuse throughout the entire chamber. To minimize response strategies, odor presentations were presented such that neither condition occurred on more than 2 consecutive days; an equal number of presentations occurred over a 10-day period (BAN-ALM-BAN-BAN-ALM-BANALM-ALM-BAN-ALM). For half of the animals (1 of the 2 offspring in each litter), BAN was paired with the activation of the right lever, the appropriate number of responses resulted in food, and the left lever was active in the presence of ALM. This contingency was reversed for the other half of the animals. Both levers were present throughout each 20-min daily training session, but only responses to the correct lever were reinforced on an FR 10 schedule. Responses to the incorrect lever were recorded but resulted in no programmed consequences. Two discrimination indexes (DI1 and DI2) were calculated at the end of each session for each animal. DI1 was calculated by determining the number of responses to the odor-appropriate lever in the first 10 responses and dividing by 10, whereas DI2 was calculated by dividing the number of responses to the odor-appropriate lever by the total number of responses to both levers over the entire session. The criterion for the initial odor discrimination was defined as 5 consecutive days of 80% or better responding in the first 10 responses (DI1) and 90% or greater responding throughout the entire session (DI2). Reversal training. Upon completion of the initial odor discrimination as defined by the just-mentioned criterion, each animal then began the training phase of the reversal task (see previously described procedure). Each animal received a daily odor presentation in a pseudorandom order (see previously described procedure). However, the appropriate lever was opposite that paired with each odor during the initial conditional discrimination (i.e., the odor that cued for the right lever in the initial odor discrimination was paired with activation of the left lever in the reversal phase and vice versa). Only responses to the appropriate lever were reinforced on an FR 10 schedule of reinforcement. The DIs were calculated at the end of each session as just outlined. Criterion for discrimination was defined as outlined in the initial odor discrimination: for DI1, >80% correct responding in the first 10 responses, and for DI2, >90% responding to the odor-appropriate lever across the entire session.

Data Analysis Maternal and offspring data. Data for the percentage of body weight gain, gestational length, and the percentage of reabsorptions were analyzed by a one-way analysis of variance (ANOVA). A 4 (prenatal treatment) x 20 (days) repeated measures ANOVA across days was used to analyze daily maternal body weight and food and water intake. A 4 (prenatal treatment) x 2 (sex) ANOVA was used to analyze pup body weight and the number of pups in the litter before culling on PD 1. Offspring operant data. Data for the number of sessions to criterion, first and second discrimination indexes (DI1 and DI2), and the number of errors were analyzed by 4 (prenatal treatment) x 2 (condition; BAN = right lever, ALM = left lever vs. ALM = right lever, BAN = left lever) x 2 (phase; acquisition vs. reversal) ANOVA, with phase as a within-subjects factor. The number of sessions to reach criterion performance on the FR 10 schedule of reward was analyzed by a 4 (prenatal treatment) x 2 (condition; BAN = right lever, ALM = left lever vs. ALM = right lever, BAN = left lever) ANOVA. Response rate was analyzed by a 4 (prenatal treatment) x 2 (condition; BAN = right lever, ALM = left lever vs. ALM = right lever, BAN = left lever) x 3 (phase; FR 10 vs. acquisition vs. reversal) ANOVA, with phase as a within-subjects factor. Tukey's tests (Kirk, 1982) were used to determine the locus of significant main effects or

840

C. HEYSER, N. SPEAR, AND L. SPEAR

interactions. A significance level of p < .05 was used for all statistical analyses.

As can be seen in Table 1, no differences were observed between the three groups of dams in gestational length or fetal reabsorption rates.

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Results Maternal Data

Litter Size, Composition, and Offspring Body Weight

An ANOVA performed on the percentage of body weight gain during pregnancy revealed a significant main effect of prenatal treatment, F(3, 39) = 3.68, p < .05. Tukey's tests indicated that dams in the LC condition gained significantly more weight than did C40 and PF dams, which did not differ from each other. The body weights of NC dams were intermediate between the C40 and LC dams and did not differ significantly from any of the prenatal treatment groups (Table 1). Similarly, a repeated measures ANOVA on daily body weights revealed a significant main effect of days, F(19, 741) = 636.46, p < .01, along with a significant Prenatal Treatment x Days interaction, F(57, 741) = 5.36, p < .01. Subsequent Tukey's tests revealed that both LC and NC dams weighed more than C40 and PF dams, which did not differ from each other, from GD 14 to GD 20. A repeated measures ANOVA revealed only a significant main effect of days for food intake, F(19,741) = 11.57,/> < .01, which reflects an increase in intake during pregnancy. Although the interaction was not significant, a trend toward a reduction in food intake was observed on GD 8 and 9 in PF dams (M ± SE [in grams] = 17.10 ± 2.62 and 17.32 ± 1.35, respectively), NC dams (18.87 ± 1.87 and 19.65 ± 2.24, respectively), and C40 dams (18.36 ± 3.53 and 20.44 ± 2.26, respectively) compared with LC dams (26.08 ± 1.79 and 27.64 ± 2.05, respectively). A similar ANOVA conducted on daily water intake revealed only a significant main effect of prenatal treatment, F(3, 39) = 6.05, p < .05, and days, F(\9, 741) = 134.82, p < .01. Water intake increased during pregnancy, with dams in both the LC and NC control groups consuming significantly more water than did C40 dams, which consumed significantly more water than did PF dams (Table 1).

An ANOVA conducted on the number of male and female pups in each litter failed to reveal any significant differences (Table 1). The ANOVA performed on litter means of male and female body weights at PD 1 revealed only a significant main effect of sex, F(l, 39) = 110.27, p < .01, with male pups weighing more at PD 1 than females did. An analysis of covariance with age as a covariate that was conducted on male body weights at the onset of food deprivation failed to reveal any differences among the four prenatal treatment groups (Table 1).

Discrimination Training and Reversal Rates of acquisition and reversal. The number of sessions to criterion was used as the dependent measure in the analysis of the acquisition of the initial odor discrimination and its subsequent reversal. Before the analysis of sessions to criterion, it was necessary to examine the number of sessions that were required to achieve stable FR 10 performance. An ANOVA that was conducted on the number of training sessions to criterion on the FR 10 schedule of reinforcement failed to reveal any significant differences. The ANOVA that was conducted on the number of sessions to criterion during initial acquisition and reversal revealed a significant main effect of prenatal treatment, F(3, 72) = 5.06,p < .01, along with a significant Prenatal Treatment x Phase interaction, F(3, 72) = 7.10, p < .01. No other main effects or interactions were significant. Subsequent Tukey's tests on the data collapsed across condition failed to reveal any significant differences among the four prenatal treatment groups in the number of sessions required to attain criterion for acquisition

Table 1 Maternal and Litter Summary Data Derived From Litter Means (±SE) for the Four Experimental Groups Group LC

PF

SE

M

C40

NC

SE

M

SE

M

SE

Variable

M

Gestational weight gain (%) Food intake (in grams) Water intake (in milliliters) Gestational length (in days) Litter size Number of male pups in litter Number of female pups in litter Fetal reabsorption rate (%) PD 1 offspring bw (in grams) Males Females PD 60-80 offspring bw (in grams) Males only

40.21 26.38 60.09 22.90 16.30 8.30 8.90 5.89

1.65 1.30 2.01 0.32 0.70 0.79 0.88 2.39

33.81* 24.86 50.40* 22.91 15.72 8.09 7.64 8.34

1.49 1.09 1.92 0.30 0.97 0.56 0.99 2.58

35.23 26.98 63.86 23.00 15.82 8.36 7.00 9.88

2.29 1.30 3.05 0.00 0.52 0.68 0.60 3.78

31.85* 28.31 53.86* 23.00 14.18 7.91 6.18 5.54

1.73 2.11 1.89 0.00 0.99 0.56 0.70 3.68

7.45 7.14

0.14 0.13

7.26 6.85

0.20 0.23

6.71 6.42

0.16 0.14

7.26 6.79

0.24 0.24

462.02

18.42

442.02

17.20

432.25

11.25

450.25

15.20

Note. LC = nontreated control group; PF = pair-fed control group; NC = nutritional control group; C40 = cocaine treatment group; PD = postnatal day; bw = body weight. 'Significantly different from LCgroup,p < .05.

841

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EFFECTS OF PRENATAL EXPOSURE TO COCAINE of the initial odor discrimination. However, as can be seen in Figure 1, C40 animals required significantly more sessions to reach criterion during the reversal phase of the discrimination than did the three control groups, which did not differ from each other. Thus, although C40 animals acquired the initial discrimination in the same number of training sessions, they required more sessions to reach criterion in the reversal phase compared with LC, PF, and NC offspring. It is possible that the overall response rate may play a role in the instrumental performance of the animal. Therefore, an ANOVA was conducted on the mean response rate, collapsed over the last five sessions in each phase of training. This ANOVA revealed only a significant main effect of phase, F(2, 144) = 67.40,;? < .01. Tukey's tests indicated that significantly more responding occurred during the initial odor discrimination and its subsequent reversal than during simple FR 10 responding (Figure 2). Performance during criterion. To further examine the performance of the various treatment groups at the point of reaching criterion in both the initial discrimination and reversal, analyses were conducted on DI1, DI2, and the number of errors (defined as the number of responses to the incorrect lever before receipt of the first reward) on the last 5 days of training during which criterion was reached. An ANOVA that was conducted on DI1 revealed only a significant Prenatal Treatment x Phase interaction, F(3, 72) = 4.39, p < .01. Tukey's tests revealed no differences in DIs among the four prenatal treatment groups during the 5-day period in which criterion was achieved for the initial odor discrimination. In contrast, during the corresponding period of achieving criterion for the reversal, C40 animals exhibited a significantly lower discrimina-

o 55 D LC

u. o

E2 PF

IT UJ

£2en 1 in UI DC

DC Ul

m

Ul

FR10

ACQUISITION

REVERSAL

CONDITION

Figure 2. Mean number (+SE) of responses per 20-min session collapsed over the last 5 days of training on a 10-s fixed-ratio schedule of reinforcement, acquisition, and reversal of the discrimination. (LC = nontreated control group; PF = pair-fed control group; NC = nutritional control group; C40 = cocaine treatment group.)

tion index than did LC, PF, or NC animals, which did not differ among themselves. These data are presented in Figure 3 (top panel). The ANOVA that was conducted on DI2 failed to reveal any significant differences among the four prenatal treatment groups (Figure 3, bottom panel). The ANOVA that was conducted on the number of errors before the first reward revealed a significant main effect of phase, F(l, 72) = 17.28, p < .01, along with a Prenatal Treatment x Phase interaction, F(3, 72) = 9.06, p < .01. Subsequent analysis with Tukey's tests indicated no differences in the number of errors at criterion of the initial odor discrimination; however, C40 animals committed more errors than did LC, PF, or NC groups at criterion of the reversal. These data are presented in Figure 4.

& NC

m

• C40

ACQLHSmON

REVERSAL

CONDITION

Figure 1. Mean number (+SE) of sessions to criterion for acquisition and reversal of the conditional discrimination. (Criterion for both acquisition and reversal was denned as > 80% correct responses in the first 10 responses and > 90% correct responses over the entire session for 5 consecutive days, "p < .05 for these comparisons. LC = nontreated control group; PF = pair-fed control group; NC = nutritional control group; C40 = cocaine treatment group.)

Discussion Gestational exposure to cocaine produced different effects on the acquisition and subsequent reversal of an appetitive conditional discrimination in adult offspring. Adult offspring from all four prenatal treatment groups (LC, PF, NC, and C40) required approximately the same number of sessions to reach criterion on the initial discrimination, findings that are consistent with previous reports of olfactory discrimination learning in similar tasks in adulthood (Eichenbaum, Mathews, & Cohen, 1989; Jennings & Keffer, 1969; Slotnick & Katz, 1974). In contrast, adult offspring that were prenatally exposed to cocaine were retarded in acquisition of the reversal of the conditional discrimination. These findings are reminiscent of previous reports of disrupted performance during reversal not evident during discrimination acquisition (Fagan & Olton, 1986; McGivern et al., 1987). These data also confirm previous

842

C. HEYSER, N. SPEAR, AND L. SPEAR FIRST DISCRIMINATION INDEX (DI1)

o in cr EC O o ILLJ O

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cc

ACQUISITION

REVERSAL

SECOND DISCRIMINATION INDEX (DI2) 100 -i

o

LU CC CC O

o LU O CC UJ CL

ACQUISITION

REVERSAL

the C40 and PF dams compared with LC control dams. This slight reduction in maternal body weight gain was not. however, associated with any alteration in offspring body weights when examined at PD 1 and at PD 60-80. Hence, it is unlikely that the reduction in maternal weight gain in these dams resulted in significant malnutrition of the offspring. Furthermore, given the similar performance of PF, NC, and LC control offspring on both the acquisition and reversal of the conditional discrimination, it is unlikely that the alterations in reversal performance that were observed in C40 offspring were related to nutritional factors alone. It is also unlikely that the altered performance on the reversal task in C40 offspring was related to sensory or motor capabilities or to motivational state. These offspring did not differ from others in acquisition of a barpress response, which provides evidence that offspring prenatally exposed to cocaine are capable of the motor skills necessary to perform a lever press. In addition, adult offspring that were prenatally exposed to cocaine were as capable as others of forming the simple association necessary for magazine training (i.e., an audible click paired with food) and an instrumental response to produce an outcome (barpress resulting in food delivery). Before discrimination training, response rate on an FR 10 schedule of reward did not differ among prenatal treatment groups, which suggests that the motivational states of these animals resulting from the food deprivation were similar across groups. This conclusion is further supported by similar response patterns on the FR 10 throughout acquisition and reversal of the conditional discrimination. Finally, the number of sessions to criterion on the initial odor discrimination was similar for all groups tested. Given that offspring gestationally exposed to cocaine acquired the initial odor discrimination, it

CONDITION

Figure 3. Mean percentage (+SE) of correct responses in the first 10 responses (first discrimination index [DI1]; top panel) and mean percentage (+SE) of correct responses over the entire session (second discrimination index [DI2]; bottom panel) collapsed over the last 5 days of training during which criterion was reached for acquisition and reversal of the conditional discrimination. (*p < .05 for these comparisons. LC = nontreated control group; PF = pair-fed control group; NC = nutritional control group; C40 = cocaine treatment group.)

tn cc o IT CC UJ

n E3

CC HI

NC

m

C40

findings, which were obtained using other types of conditioning tasks, of deficits in learning performance in both developing (Heyser at al., 1990; Spear, Kirstein, Bell et al., 1989) and adult (Smith et al., 1989) offspring that were gestationally exposed to cocaine. Before considering the possible factors that may have contributed to the altered performance of C40 offspring in the reversal phase, a number of observations should be noted. At the onset of drug treatment, dams that were treated with cocaine (C40) exhibited an apparent reduction in food and water intake that lasted 2-3 days; a similar reduction was also observed in PF and NC dams. This was associated with a significant attenuation in body weight gain during pregnancy in

REVERSAL

CONDITION

Figure 4. Mean number (+SE) of errors before the first reward collapsed across the 5 days of training for both acquisition and reversal of the conditional discrimination. (*p < .05 for these comparisons. LC = nontreated control group; PF = pair-fed control group; NC = nutritional control group; C40 = cocaine treatment group.)

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EFFECTS OF PRENATAL EXPOSURE TO COCAINE appears that they have the necessary sensory capabilities to discriminate between the two odors used. Although no differences were observed with respect to prenatal treatment, an increase in response rate was observed in discrimination training (both acquisition and reversal) compared with simply responding on an FR 10 schedule. Although similar results have been reported in drug discrimination studies (e.g., Middaugh & Ayers, 1988), this effect has not been directly examined with other types of conditional discriminations given that few studies aside from drug discrimination studies have used partial reinforcement schedules (Williams, 1989). Although the increase in response rate may be a useful strategy to maintain reinforcement density at the onset of discrimination learning, it is not clear as to why the enhanced rate is maintained after acquisition of the discrimination response. What is clear is that C40 offspring are as sensitive as control offspring to the rate-enhancing effects that are associated with the discrimination training procedure used in this study. The finding that C40 animals required more sessions than did control offspring to reach criterion in the reversal phase may represent an alteration in the way information is processed in the C40 offspring. It is possible that these animals are slower to adapt to changes in rules and thus persist in a particular response strategy. Persistence in a given response strategy might interfere with, and hence retard, acquisition of the reversal task, which is consistent with results obtained in the present study. In this view, gestational exposure to cocaine is assumed to affect the animal's ability to switch strategies when task rules are changed. Results from the DIs and the number of errors before receipt of first reward provide additional evidence for the hypothesis that the reversal deficit in C40 animals is based on interference from learning of the initial discrimination. There were no differences in either DI among the prenatal treatment groups in acquisition of the initial discrimination. However, in the reversal phase, C40 offspring exhibited more errors and a lower accuracy in the first 10 responses as indexed by DI1, even at criterion performance for acquisition of the reversal discrimination. This attenuated performance is not apparent by the end of each session as indexed by DI2. DI1 (and the assessment of errors before the first reward) may be viewed as an extinctionlike probe trial and may provide a more sensitive measure of what is remembered between sessions than performance during the entire session. Although C40 animals committed more errors at the start of a session, they did not continue to make errors throughout a session. Taken together, these data argue against a general memory deficit per se. Indeed, these offspring acquired the initial discrimination in the same number of trials as control offspring did. The major conclusion is that gestational cocaine impairs discrimination reversal, apparently by enhancing the interference that results from acquisition of the initial odor discrimination. This may be important in consideration of future experiments that examine learning or memory in these offspring. Given evidence that the DA system may be altered by gestational exposure to cocaine (Dow-Edwards et al., 1990; Scalzo et al., 1990), it is possible that these neural alterations

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contribute to the observed findings. A number of studies have investigated the effects of a variety of manipulations of the DA system on learning and memory (Crider, Blockel, & Solomon, 1986; Packard & White, 1991; Weiner, 1990). Disruption of discrimination learning has been reported after 6-hydroxydopamine-induced lesions of the caudate-putamen (Robbins, Giardini, Jones, Reading, & Sahakian, 1990). In contrast, acquisition of a conditional discrimination has been reported to be unaffected after 6-hydroxydopamine lesions of the nucleus accumbens, although lesioned animals were slower to extinguish the learned response (Robbins et al., 1990). Similar effects were obtained by attenuating normal DA neurotransmission with a-flupenthixol, a neuroleptic drug that causes acute blockade of DA receptors (Robbins et al., 1990). Although Robbins et al. (1990) did not examine reversal of the conditional discrimination, it would not be surprising to find retarded acquisition of a reversal response, given the slower rate of extinction in the lesioned animals. In this sense, the behavior of these animals with attenuated DA activity is reminiscent of the findings observed in the present study with adult C40 offspring, which suggests that the behavior of these latter animals may also reflect an attenuation in DA function. Indeed, adult offspring that were exposed gestationally to cocaine have recently been found to exhibit fewer spontaneously active DA neurons in the substantia nigra pars compacta (A9) and ventral tegmental area (A10) DA cell body regions than did control offspring (Minabe, Ashby, Heyser, Spear, & Wang, in press). Although weanling C40 offspring also have been observed to exhibit an increased striatal D2 binding that is associated with increased ligand affinity (Scalzo et al., 1990), this increased binding could reflect a compensatory response to an attenuation in DA release associated with the decrease in DA neuronal activity in these animals. Thus, retarded acquisition during reversal of the conditional discrimination could potentially be due to a long-term attenuation in DA functioning in offspring that have been prenatally exposed to cocaine. In this view, acquisition of a conditional discrimination would not be disrupted in animals that have been gestationally exposed to cocaine, although in these animals acquired associations would extinguish more slowly and interfere with learning a reversal of the initial discrimination. The present results, however, indicate that with continued training offspring that have been gestationally exposed to cocaine are capable of acquiring the reversal task. In conclusion, gestational exposure to cocaine had no effect on the acquisition of an olfactory conditional discrimination but did retard acquisition on the reversal task. This confirms previous reports of alterations in performance on conditioning tasks in adult offspring that were prenatally exposed to cocaine (Smith et al., 1989). In addition, these data provide evidence that previously observed conditioning deficits in C40 offspring early in life (Heyser et al., 1990; Spear, Kirstein, Bell et al., 1989) persist into adulthood.

References Brazelton, T. B. (1984). Neonatal Behavioral Assessment Scale (2nd ed.). Philadelphia: Lippincott.

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macological, and neurochemical studies. Annals of the New York Academy of Sciences, 526, 290-307. Stanton, M. E., & Spear, L. P. (1990). Workshop on the qualitative and quantitative comparability of human and animal development neurotoxicity, work group I report: Comparability of measures of development neurotoxicity in humans and laboratory animals. Neurotoxicology and Teratology, 12, 261-267. Weiner, I. (1990). Neural substrates of latent inhibition: The switching model. Psychological Bulletin, 108, 442-461.

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Received December 31,1991 Revision received March 18,1992 Accepted March 20,1992 •

P&C Board Appoints Editor for New Journal: Experimental and Clinical Psychopharmacology In the fall of 1993, APA will begin publishing a new journal, Experimental and Clinical Psychopharmacology. Charles R. Schuster, PhD, has been appointed as editor. Starting immediately, manuscripts should be submitted to Charles R. Schuster, PhD P.O. Box 2795 Kensington, MD 20891 -2795 Experimental and Clinical Psychopharmacology seeks to promote the discipline of Psychopharmacology in its fullest diversity. Psychopharmacology necessarily involves behavioral changes, psychological processes, or their physiological substrates as one central variable and psychopharmacological agents as a second central variable. Such agents will include drugs, medications, and chemicals encountered in the workplace or environment. One goal of Experimental and Clinical Psychopharmacology is to foster basic research and the development of theory in psychopharmacology. Another is to encourage the integration of basic and applied research, the development of better treatments for drug abuse, and more effective pharmacotherapeutics. To this end, the journal publishes original empirical research involving animals or humans that spans from (a) behavioral pharmacology research on social, behavioral, cognitive, emotional, physiological, and neurochemical mechanisms of drug- or chemical-behavior interaction and behavioral toxicity; to (b) descriptive and experimental studies of drug abuse including its etiology, progression, adverse effects, and behavioral and pharmacological treatment; to (c) controlled clinical trials that, in addition to improving the effectiveness, range, or depth of application, will also increase our understanding of psychological functions or their drug modulation. The journal also publishes theoretical and integrative analyses and reviews that promote our understanding and further systematic research in psychopharmacology. Although case studies are not appropriate, occasional small-sample experiments with special populations may be considered. The journal is intended to be informative and useful to both basic and applied researchers and to practitioners operating in varied settings. Experimental and Clinical Psychopharmacology seeks to be the vehicle for the best research and ideas integrating pharmacology and behavior.

Effects of prenatal exposure to cocaine on conditional discrimination learning in adult rats.

Adult male rats that were gestationally exposed to cocaine and control offspring were trained on an instrumental conditioning task for assessment of t...
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