Circulating adrenal hormones are not necessary for the development of sensitization to the psychomotor activating effects of amphetamine

BRAIN RESEARCH ELSEVIER

Brain Research 673 (1995) 13-24

Research report

Circulating adrenal hormones are not necessary for the development of sensitization to the psychomotor activating effects of amphetamine Aldo Badiani a,., M. In~s Morano h, Huda Akil u, Terry E. Robinson a a Department of Psychology and Neuroscience Program, Neuroscience Laboratory Building, The University of Michigan, 1103 East Huron St., Ann Arbor, MI 48104-1687, USA b Department of Psychiatry and Mental Health Research Institute, The University of Michigan, Ann Arbor, MI 48104-1687, USA Accepted 16 November 1994

Abstract

We reported previously that when amphetamine is given in NOVEL test cages both its acute psychomotor activating effects (rotational behavior and locomotor activity) and the degree of sensitization are greater than when amphetamine is given in HOME cages that are physically identical to the NOVEL test cages. Since exposure to the NOVEL environment increases plasma corticosterone levels (Experiment 1) it is possible that the enhancement in the effects of amphetamine in the NOVEL condition is mediated by corticosterone. If this hypothesis is correct adrenalectomy (ADX) should abolish the difference between the HOME and NOVEL groups. This was tested in three independent experiments, in which the response (rotational behavior in Experiments 2 and 3; locomotor activity and rearing behavior in Experiment 4) to repeated injections of amphetamine was assessed in rats that underwent adrenalectomy (ADX) or a sham operation (SHAM). ADX animals received either no corticosterone replacement or one of two corticosterone replacement treatments. Adrenalectomy, with or without corticosterone replacement treatment, had no significant effect on the development of amphetamine sensitization, either in the HOME or the NOVEL environment. By contrast, the effects of adrenalectomy on the acute response to amphetamine varied depending on the behavioral measure and possibly on the dose of amphetamine (2.0 mg/kg, 3.0 m g / k g and 1.5 m g / k g IP, in Experiments 2, 3 and 4, respectively). We conclude that: (i) a stress-induced secretion of adrenal hormones is not responsible for the enhancement in sensitization to amphetamine seen in animals tested in a NOVEL environment; (ii) circulating adrenal hormones are not necessary for development of sensitization to the psychomotor activating effects of amphetamine.

Keywords: Stress; Corticosterone; Hypothalamic-pituitary-adrenai axis; Novel environment; Novelty; Sensitization; Contextspecific sensitization; Environment-specific sensitization; Rotational behavior; Locomotor activity; Rearing behavior; Rat

I. Introduction Animals given repeated injections of addictive drugs, such as amphetamine, cocaine or morphine, develop a progressive and persistent hypersensitivity (sensitization) to their psychomotor activating (for reviews, see [30,47]) and incentive motivational effects [38,60]. Although the neurobiological basis of sensitization is not known, there is evidence that behavioral sensitization is accompanied by long-lasting changes in a number of neural systems, including mesotelencephalic dopamine systems [30,47]. In recent years there has been increas-

* Corresponding author. Fax: (1) (313) 936-2690. 0006-8993/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 0 0 6 - 8 9 9 3 ( 9 4 ) 0 1 3 6 5 - 9

ing interest in the p h e n o m e n o n of sensitization both as a model of neuroplasticity and because of the potential relevance of sensitization-related neuroadaptations to the development of addictive behavior [45,48,51]. Drug sensitization is not a simple pharmacological phenomenon, but is modulated by a number of factors and in particular, environmental factors (for a recent review, see [52]). For example, the physical characteristics of the test environment [21,27,59], the acquisition of conditioned stimulus (CS) properties by environmental stimuli paired with drug administration [26,40,54,57] and past exposure to stress [3,6,46], all influence or gate the development and expression of sensitization. Furthermore, we reported recently [4,5] that both the acute psychomotor activating effects of

14

A. Badiani et al./ Brain Research 673 (1995) 13-24

a m p h e t a m i n e and the d e v e l o p m e n t of sensitization to a m p h e t a m i n e a n d cocaine are greater for rats treated in a ' n o v e l ' ( N O V E L ) test e n v i r o n m e n t than for rats t r e a t e d in a physically identical e n v i r o n m e n t in which the animals live ( H O M E ) . It is not clear how exposure to a N O V E L e n v i r o n m e n t e n h a n c e s either the acute psychomotor activating effects of a m p h e t a m i n e or amp h e t a m i n e and cocaine sensitization. O n e possible exp l a n a t i o n is related to the ability of a novel environm e n t to act as a stressor. I n d e e d , exposure to a novel e n v i r o n m e n t produces a n u m b e r of n e u r o e n d o e r i n e effects similar to those p r o d u c e d by foot-shock or restraint-stress, including activation of the hypothalam i c - p i t u i t a r y - a d r e n a l ( H P A ) axis [22,25] and corticotropin-re]easing h o r m o n e ( C R H ) - d e p e n d e n t hypertension, tachicardia a n d h y p e r t h e r m i a [35]. Since the acute psychomotor response to a m p h e t a m i n e (stereotyped behavior) is p o t e n t i a t e d by stressors such as foot-shock or exposure to cat odors [2,58], it is possible that the N O V E L e n v i r o n m e n t acts similarly. The enh a n c e m e n t of sensitization observed in the N O V E L e n v i r o n m e n t may be explained in terms of stressa m p h e t a m i n e interactions as well. For example, it has b e e n r e p o r t e d recently that sensitization of the locom o t o r response to m o r p h i n e is e n h a n c e d when restraint-stress is given immediately prior to m o r p h i n e t r e a t m e n t s [50]. T h e mechanism(s) by which stress may e n h a n c e the psychomotor activating effects of a m p h e t a m i n e a n d a m p h e t a m i n e sensitization is not known. Exposure to stressors produces m a n y n e u r o e n d o c r i n e changes that could potentially affect the actions of a m p h e t a m i n e [36,56]. O f particular relevance to the p r e s e n t p a p e r are the reports that a d r e n a l e c t o m y reduces the acute effects of a m p h e t a m i n e a n d cocaine on locomotion [13,33] a n d prevents the d e v e l o p m e n t of a m p h e t a m i n e s e n s i t i z a t i o n p r o d u c e d by e i t h e r r e p e a t e d a m p h e t a m i n e t r e a t m e n t s [43] or r e p e a t e d exposure to stress [17-19]. F u r t h e r m o r e , i n t e r m i t t e n t administration of corticosterone sensitizes to the psychomotor activating effects of a m p h e t a m i n e [20]. These reports suggest that activation of the H P A axis a n d the conseq u e n t release of corticosterone p r o d u c e d by exposure to a N O V E L e n v i r o n m e n t could c o n t r i b u t e to both the e n h a n c e m e n t in the acute behavioral activating effects of a m p h e t a m i n e a n d / o r the d e v e l o p m e n t of sensitization. If this hypothesis is correct t h e n a d r e n a l e c t o m y should abolish the difference b e t w e e n H O M E a n d N O V E L groups r e p o r t e d by B a d i a n i a n d colleagues [4,5]. F u r t h e r m o r e , if a stress ( N O V E L e n v i r o n m e n t ) i n d u c e d elevation in plasma corticosterone is required, the effects of a d r e n a l e c t o m y should not be reversed by corticosterone replacement treatments providing plasma corticosterone levels c o m p a r a b l e to those of n o n - s t r e s s e d animals, because in such p r e p a r a t i o n s stress has no effect on circulating corticosterone. The

main p u r p o s e of the e x p e r i m e n t s r e p o r t e d here, therefore, was to test this hypothesis. This was d o n e by c o m p a r i n g the psychomotor activating effects of rep e a t e d injections of a m p h e t a m i n e in rats that u n d e r went a d r e n a l e c t o m y with those in rats that u n d e r w e n t sham surgery. In addition, some a d r e n a l e c t o m i z e d animals received corticosterone r e p l a c e m e n t t r e a t m e n t s designed to m a i n t a i n either n o r m a l d i u r n a l plasma corticosterone levels or n o r m a l circadian oscillations in plasma corticosterone levels.

2. Materials and methods 2.1. Animals

A total of 135 male Sprague-Dawley rats (Harlan Sprague-DawIcy. Indianapolis, IN), weighing 175 250 g at the beginning of the experiments, were used. The rats were housed individually in a room with a 14/10 h light/dark cycle (lights on 06.00-20.00 h) and had ad libitum access to food and water. The rats were habituated to the colony room for 1 week prior to any experimental manipulation.

2.2. 6-Hydroxydopamine (6-OHDA) lesion and rotational behavior (Experiments 2 and 3)

Rotational behavior in rats with a unilateral lesion of the mesostriatal dopamine system [55] was used as an index of the psychomotor activating effects of amphetamine in Experiments 2 and 3. The quantification of the rotational behavior offers a number of advantages over more traditional measures, such as locomotor activity or stereotyped behavior. (i) The dose-effect for amphetamine-induced rotational behavior in rats with a unilateral 6-OHDA lesion is linear over a wide range of doses [55] and the progressive increase in drug effect during sensitization results in a progressive increase in rotational behavior [4,44]. By contrast, the dose-effect curve for amphetamine-induced locomotor activity in rats without a 6-OHDA lesion is very complex and a progressive increase in drug effect is not necessarily characterized by a progressive increase in locomotor activity [49]. (ii) Rotational behavior is easily quantifiable and the measure constitutes an interval scale, rather than the less desirable ordinal scale associated with activityrating scales. The latter makes it difficult to estimate of the magnitude of changes in behavior (sensitization in this case), because the distances between points on the scale are not equal. (iii) The neural basis of amphetamine-induced rotational behavior is well understood, relative to most other druginduced behaviors (e.g., [42]). (iv) Exposure to a novel environment in the absence of a drug injection produces negligible rotational behavior, whereas this induces a marked increase in locomotor activity [4,5]. Therefore, rotational behavior is particularly suitable for studying the interaction between the effects of amphetamine and novelty. (v) In previous studies we established that exposure to a novel environment has similar effects on both amphetamine-induced rotational behavior and amphetamine-induced locomotor activity. which establishes that this effect is not unique to rotational behavior or to rats with a unilateral 6-OHDA lesion [4]. After 1 week of acclimation to the laboratory facilities the rats were pretreated with desipramine to protect noradrenergic terminals [9]. anaesthetized with sodium pentobarbital and then given a unilateral injection of 6-OHDA in medial forebrain bundle, using procedures similar to those described previously [44]. Briefly, 8 Izg of

A. Badiani et al./Brain Research 673 (1995) 13-24 6 - O H D A were infused in 4/.d of a saline-ascorbate solution over an 8-min period, via a 29 gauge stainless steel cannula. The animals were then allowed to recover from surgery for at least 2 weeks before they were given 0.05 m g / k g of apomorphine, to assess the development of dopamine receptor supersensitivity (denervation supersensitivity). Denervation supersensitivity is a good indicator of the size of the lesion because vigorous contraversive rotation occurs only after 9 0 - 9 5 % of dopamine terminals are destroyed [23,24]. Animals that made less than 50 rotations over a 30-min test were excluded from the study.

2.3. Adrenalectomy and corticosterone treatments Within 1 week after the apomorphine screen the rats were placed in one of two groups. Rats in one group had their the adrenal glands removed bilaterally through a dorsal midline incision, u n d e r methoxyflurane or sodium pentobarbital anesthesia. Adrenalectomies ( A D X ) were performed during the first half of the light cycle, 1 week before animals received their first injection of a m p h e t a m i n e or saline (see below). Adrenalectomized animals were given ad libitum access to a 0.9 g / l NaCI solution. Rats in the second group received s h a m (SHAM) surgery at the same time, consisting of a dorsal midline incision. In addition, the A D X rats received, at the time of the surgery, either: (i) (Experiments 2, 3 and 4; see below) no corticosterone replacement ( A D X / N R ) , (ii) (only in Experiments 2 and 3) a subcutaneous implant of a corticosterone pellet intended to produce plasma corticosterone levels similar to those at the nadir of the cycle in S H A M animals ( A D X + P) or (iii) (only in Experiment 3) a subcutaneous implant of a corticosterone pellet plus nocturnal (07.00-23.00 h) access to corticosterone dissolved in their drinking water ( A D X + P + B). This latter procedure was intended to produce circadian oscillations in plasma corticosterone levels similar to those in S H A M animals. U p o n completion of the experiments blood samples were obtained to insure that adrenalectomy and the manipulations of plasma corticosterone levels had the desired effects. Animals were removed from their home cages between 13.00 h and 14.00 h and quickly placed in plastic restrainers. An initial blood sample was obtained immediately (baseline) by clipping the dorsal vein of the tail. T h e animals were then restrained for 15 min before a second sample (15' Stress) was obtained. The blood was collected in heparinized microhematocrit capillary tubes and after centrifugation the plasma was frozen in dry ice and then stored at - 8 0 ° C . A D X rats that had detectable ( > 0.2 txg/100 ml) plasma corticosterone levels (either at baseline or after stress) were excluded from the study. Furthermore, in Experiment 3 the nocturnal levels of corticosterone (22.00-23.00 h) were m e a s u r e d in the S H A M - N O V E L and A D X + P + B groups.

2. 4. Testing procedures In this section only the experimental procedures are described. T h e rationale for the experimental design and the statistical procedures are discussed in another section (Experimental Hypotheses and Statistics).

15

decapitated with a guillotine. The other seven rats (group N O V E L ) were exposed for 30 min to the N O V E L environment before decapitation. T h e blood drained from the trunks was collected in heparinized tubes and after centrifugation the plasma was frozen in dry ice and then stored at - 8 0 ° C .

2.4.2. Experiment 2 Sixty three rats were used. Some of them (the H O M E groups) were housed in a testing room in cylindrical (25 cm diameter, 36 cm high), plastic buckets equipped with drinking tubes. The floors of these buckets were covered with ground corn cob bedding. The other rats (the N O V E L groups) were housed in stainless steel hanging cages located in the main animal colony room. The waste trays below these cages were covered with pine wood shavings. After 1 week of habituation to these housing conditions (and adrenalectomy), N O V E L rats were transferred every day from their home cages in the animal colony to the testing room and placed in plastic buckets identical to those in which H O M E rats lived, including the presence of ground corn cob bedding, food and water. They then received an IP injection of 3.0 m g / k g of amphetamine. This procedure was repeated on 7 consecutive days. At the same time H O M E rats received 3.0 m g / k g of a m p h e t a m i n e in their home cages. Therefore, the environments in which H O M E and N O V E L rats received amphetamine were physically identical, but this was a 'novel' environment for one group and the home environment for the other group. The H O M E and N O V E L groups were further subdivided into those groups that received bilateral adrenalectomy ( A D X - H O M E , N = 15 and A D X - N O V E L , N = 17) and those that received a s h a m operation ( S H A M - H O M E , N = 14 and S H A M - N O V E L , N = 17). The A D X groups were further subdivided into groups that received a corticosterone pellet ( A D X + P) and those that received no corticosterone replacement ( A D X / N R ) . In Experiment 1 there were, therefore, a total of six groups: (i) S H A M - H O M E ( N = 14), (ii) A D X / N R - H O M E ( N = 5 ) , (iii) A D X + P - H O M E ( N = 10), (iv) S H A M N O V E L ( N = 1 7 ) , (v) A D X / N R - N O V E L ( N = 7), (vi) A D X + P N O V E L ( N = 10). All test sessions lasted 90 min, after which time N O V E L rats were returned to their hanging cages in the animal colony. The behavior of the animals was videotaped during the 1st and 7th test sessions and rotational behavior was quantified by viewing the videotapes. O n e rotation was defined as a complete 360 ° turn.

2.4.3. Experiment 3 The results of Experiment 2 (see Results) were in contrast with a study by Rivet and colleagues [43]. Therefore, we conducted Experiment 3 in an attempt to replicate the finding of Experiment 2. Thirty-eight rats were used. T h e testing procedures were identical to those for Experiment 2, with two minor changes. First, the rats received 2 m g / k g a m p h e t a m i n e to assess if the negative findings of Experiment 1 were the results of using too high a dose. Second, there were a total of five groups, one H O M E group and four N O V E L groups: (i) S H A M - H O M E ( N = 9); (ii) S H A M - N O V E L ( N = 8), (iii) A D X / N R - N O V E L ( N = 7), (iv) A D X + P - N O V E L ( N = 7) and (v) A D X + P + B - N O V E L ( N = 7). Rotational behavior was quantified as in Experiment 2.

2.4.1. Experiment 1 It has been reported that exposure to a novel environment activates the H P A axis [22,25], producing increased plasma corticosterone levels. Experiment 1 was conducted to assess if exposure to the specific N O V E L environment used in Experiments 2 and 3 (a cylindrical plastic bucket) produces a similar effect on plasma corticosterone levels. Thirteen rats were used. Six of them (group H O M E ) were removed from their home cages (11.30-12.00 h), transported to an adjacent room, and quickly ( < 1 min after removal from the cage)

2.4.4. Experiment 4 Experiment 4 was conducted to determine if the negative results obtained in Experiments 2 and 3 (see Results) were unique to rotational behavior or to rats with a unilateral 6 - O H D A lesion. Twenty one naive (i.e., no lesion) rats were assigned to either a S H A M ( N = 11) or a A D X / N R ( N = 10) group. T h e testing procedures were similar to those of N O V E L animals in Experiments 1 and 2, except for the following differences. (i) The N O V E L environment was an ellipsoidal (45 cm x 27 cm, 28 cm high) activity test cage, with

16

A. Badiani et al. /Brain Research 673 (1995) 13-24

two sets of photobeams interfaced with a Commodore 64 microcomputer for the collection of the data. The first set (designed to record movements along the length of the cage: crossovers) consisted of two photobeams placed 23.5 cm apart (on the long side of the cage) and 4.5 cm from the cage floor. The computer was programmed such that after one of these photobeams was interrupted and a single count registered, no more counts could be registered until the other photobeam had been interrupted. The other set (designed to record rearings against the wall) consisted of four photobeams placed 13.5 cm from the cage floor (two photobeams 38.5 cm apart on the long side; the other two photobeams 23 cm apart on the short side). Each interruption of these photobeams was recorded as a count. (it) The treatments consisted of five IP injections of 1.5 m g / k g of amphetamine given on alternate days. 2.5. Plasma corticosterone assay Plasma corticosterone levels were quantified using radio immunoassay (RIA) procedures described elsewhere [34]. Briefly, plasma samples (5 /zl) from each animal were extracted with dichloromethane to eliminate the endogenous transcortin and after evaporation of the solvent, the samples were resuspended in a R I A buffer (50 m M sodium-phosphate, 2.5% bovine serum albumin, pH 7.5). Corticosterone was assayed by R1A using rabbit antiserum (B3a), raised against B-21-hemisuccinate:BSA. This antiserum, used at a final titer of 1:4,000, has negligible cross-reactivity ( < 2%) with cortisol, deoxycortieosterone, progesterone, estradiol, testosterone and aldosterone. [3H]corticosterone was used as a tracer. The detection limit of the R I A was 1 pg of corticosterone. The intra- and inter-assay coefficient of variation were 2% and 3%, respectively. 2.6. Drags 6 - O H D A (2,4,5-trihydroxyphenethylamine) hydrobromide was freshly dissolved (2 m g / m l ) in a cold solution of 0.9 m g / m l NaCI (saline) and 0.1 m g / m l L-ascorbic acid. Desipramine hydrochloride was dissolved (15 m g / m l ) in distilled water and given IP (15 mg/kg). Apomorphine hydrochloride was freshly dissolved (0.1 m g / m l ) in saline and 0.01 m g / m l of 1.-ascorbic acid and injected subcutaneously in the neck (0.05 mg/kg), v - A m p h e t a m i n e sulfate was dissolved (1 m g / m l ) in saline and administered IP (3.0 m g / k g , weight of the salt in Experiment 1, 2.0 m g / k g in Experiment 2 and 1.5 m g / k g in Experiment 3). Corticosterone 21-sulfate was dissolved (50 p~g/ml) in the saline used as drinking water for A D X animals. All these drugs were purchased from Sigma Chemical Company (St. Louis, MO). Corticosterone pellets (50 m g / p e t l e t ; Innovative Research of America, Toledo, OH) were implanted under the skin of the neck. Sodium pentobarbital, dissolved (64.8 m g / m l ) in a 10% ethanol solution (The Butler Company, Columbus, OH), was given IP (52 mg/kg). 2. 7. Experimental hypotheses and statistics Group differences in Experiment I were assessed with a Student's t-test. Experiments 2 and 3 were designed to determine if adrenal hormones contribute to the increase in amphetamine's psychomotor activating effects seen when the drug is given in a N O V E L (versus H O M E ) environment. Two specific hypotheses were tested initially. (i) The first hypothesis was that the e n h a n c e m e n t in the acute response to a m p h e t a m i n e seen in a N O V E L environment requires a stress ( N O V E L environment)-induced increase in corticosterone secretion. None of the A D X animals, regardless of their corticosterone treatment condition, should show a stress-induced increase in corticosterone secretion and therefore, the various A D X groups were

pooled to test this hypothesis. In Experiment 2 this was done by analyzing the data from the first test session with a two-way A N O V A (test ent~ironment, two levels, H O M E and NOVEL; by surgery, two levels, S H A M and ADX). In Experiment 3 a one-way A N O V A was used (group, three levels, S H A M - H O M E , S H A M - N O V E L and ADX-NOVEL). Fisher's PLSD tests were used for post-hoc pair-wise comparisons. (it) The second hypothesis was that the e n h a n c e m e n t of a m p h e t a m i n e sensitization (as defined by the increase in amphetamine-induced rotational behavior from the first to the seventh test session) seen in a N O V E L environment requires a stress ( N O V E L environment)-induced increase in corticosterone secretion. Again, the A D X animals were grouped together irrespective of their corticosterone treatment, for the reason given above. The data from Experiment 2 were analyzed with a three-way A N O V A with repeated measures on one factor (test environment, two levels, H O M E and NOVEL; by surgery, two levels, S H A M and ADX; by test session, two levels, 1st and 7th). The data from Experiment 3 were analyzed first with a two-way A N O V A with repeated measures on one factor (group, three levels, S H A M - H O M E , S H A M - N O V E L and A D X - N O V E L ; by test session, two levels, 1st and 7th); then the S H A M - N O V E L and A D X - N O V E L groups were individually compared to group S H A M - H O M E with 'post-hoc' one-way A N O V A s with repeated measures (test environment, two levels, H O M E and NOVEL; by test session). In addition, two other hypotheses were tested. These concerned the role of basal levels o f corticosterone on the acute psychomotor response to amphetamine and in the development of sensitization. (iii) The third hypothesis was that the acute response to amphetamine (regardless of the test environment) is reduced in the absence of basal levels of circulating corticosterone. If this hypothesis is correct then A D X without corticosterone replacement should reduce the acute response to amphetamine both in the H O M E and in the N O V E L environment and this effect should be reversed by corticosterone replacement treatments. To test this hypothesis the data from the first test session of Experiment 2 were analyzed with a two-way A N O V A (test environment, two levels, H O M E and NOVEL; by group, three levels, SHAM, A D X / N R , A D X + P). In Experiment 3 the data obtained on the first test session for the N O V E L groups were analyzed with a one-way A N O V A (SHAM, A D X / N R , A D X + P and A D X + P + B ) . Fisher's PLSD tests were used for post-hoc pair-wise comparisons. (iv) The fourth hypothesis was that amphetamine sensitization (regardless of the test environment) is reduced in the absence of basal levels of circulating corticosterone. The data from Experiment 2 were analyzed with a three-way A N O V A with repeated measures on one factor (test environment, two levels, H O M E and NOVEL; by group, three levels, SHAM, A D X / N R and A D X + P ; by test session, two levels, 1st and 7th). In Experiment 3 the data from the N O V E L groups was assessed with a two-way A N O V A with repeated measures on one factor (group, four levels, SHAM, A D X / N R , A D X + P and A D X + P + B ; by test session, two levels. 1st and 7th). Finally, Experiment 4 was conducted to determine if the negative results obtained in Experiments 2 and 3 (see Results) were unique to rotational behavior or to rats with a unilateral 6 - O H D A lesion. The data from the first test session were analyzed with a Student's t-test and the data from the five test sessions were analyzed with a two-way A N O V A with repeated measures on one factor (surgery, two levels, S H A M and A D X / N R ; by test session, two levels, 1st to 5th). The development of sensitization over test sessions was indicated by a significant positive linear regression coefficient of the m e a n activity scores over test sessions. Linear regression coefficients of activity scores over test sessions were calculated for individual rats and a Student's t-test was used to determine if S H A M and A D X / N R groups were significantly different in the mean regression coefficient (i.e., if there was a difference in the rate at which sensitization developed). The details of the statistical analyses are reported in the figure

17

A. Badiani et al. / Brain Research 673 (1995) 1 3 - 2 4

legends to make the results section more readable. No statements are made in the results section regarding drug effects or group differences unless they are supported by statistically significant analyses ( P < 0.05).

3. Results 3.1. Experiment 1

Exposure to the NOVEL environment for 30 min produced a large increase in plasma corticosterone levels (11.4 + 1.6 /zg/100 ml in group NOVEL versus 1.4 + 0.5 /zg/100 ml in group HOME; t = 5.62, P = 0.0002; see also Fig. 2). 3.2. Experiment 2

Fig. 1 shows the effects of the first and seventh injection of amphetamine (3 mg/kg, IP) on rotational behavior, in sham-operated and adrenalectomized rats tested in either the HOME or the NOVEL environment. It illustrates five major findings. (i) The acute response to amphetamine was significantly greater in

SHAM-HOME ADX-HOME SHAM-NOVEL

1000 900-

•

800o U o t'v

"6 ..Q

E

•

animals tested in the NOVEL environment, relative to animals tested in the HOME environment (left panel, 1st test session). (ii) The suppression of stress-induced corticosterone secretion (ADX) had no effect on the increase in the acute response to amphetamine seen in the NOVEL environment. (iii) The magnitude of amphetamine sensitization was significantly greater when amphetamine was given in the NOVEL environment than when it was given in the HOME environment (as indicated by a test environment by test session interaction; left panel). (iv) The suppression of stress-induced corticosterone secretion (ADX) had no effect on the enhancement of amphetamine sensitization seen in the NOVEL environment (left panel). (v) Adrenalectomy, with (ADX + P) or without ( A D X / N R ) corticosterone pellet replacement, had no effect on the acute rotational response to amphetamine or the development of sensitization, in either the HOME (middle panel) or the NOVEL environment (right panel). Fig. 2 shows the mean plasma corticosterone levels, measured in the middle of the light phase (13.00-13.30 h) before (Baseline) and after 15 min of restraint-stress (15' Stress), for groups SHAM, A D X / N R and ADX + P. Adrenalectomy and corticosterone replacement

SHAM ADX/NR ADX+P

I ---<)~ SHAM ADX/NR

700600500400300-

Z

NOVEL

0 2

3

4

5

6

2345

3456

Test Session Fig. 1. T h e m e a n n u m b e r of full rotations ( + S.E.) during the 1st and the 7th test sessions for animals that received a m p h e t a m i n e (3 m g / k g , IP) either in their H O M E cage or in a N O V E L environment. Left panel: the rotational response to a m p h e t a m i n e of animals that received sham surgery ( S H A M - H O M E and S H A M - N O V E L ) was compared to that of animals that underwent adrenalectomy ( A D X - H O M E and A D X - N O V E L ) using a three-way A N O V A with repeated measures for one factor. There was a significant effect of test environment (F1,59 = 41.95, P < 0.0001), test session (F1.59 = 97.04, P < 0.0001), and a test environment by test session interaction (F1,59 = 11.12, P < 0.001), indicating that there was greater sensitization in the N O V E L environment than in the H O M E environment. The main effect of surgery and the other interactions were not significant ( F < 0.6 for all values, P > 0.4 for all values), indicating that there was no effect of A D X on sensitization in either test environment. A two-way A N O V A on just the data from the 1st test session also resulted in a significant effect of test environment (F1,59 = 22.32, P < 0.0001), but there was no significant effect of surgery or test environment by surgery interaction (F1,59 < 0.2, P > 0.66 for all values). Middle and right panels: the rotational response to a m p h e t a m i n e is shown in animals tested either in the H O M E or the N O V E L environments, respectively, after the A D X groups were subdivided in two groups that received either no corticosterone replacement ( A D X / N R ) or a corticosterone pellet ( A D X + P). A three w a y - A N O V A with repeated measures for one factor resulted in no significant effect of group (F2,57 = 0.33, P > 0.72), nor into a test environment by group, group by test session, test environment by group by test session interaction ( F < 1.76 for all values, P > 0.18 for all values). A two-way A N O V A on just the data from the 1st test session resulted in a significant effect of test environment (F2,57 = 21.63, P < 0.0001), but no significant effect of group or a group by test session interaction ( F < 0.75 for all values, P > 0.48 for all values).

A. Badiani et aL / Brain Research 673 (1995) 13-24

18 ~,

14-

8

12-

t

[]

Baseline

•

15' Stress

10-

o

8-

o o

6

O

© 0

g

4-

2O-

SHAM ADX/NR ADX + P Fig. 2. The mean (_+ S.E.) plasma corticosterone levels measured in the middle of the light phase (13.00-13.30 h) the day after the last amphetamine injection for the animals whose behavior is shown in Fig. l (open and closed bars). The data from H O M E and NOVEL animals were pooled because there were no group differences. Two blood samples were taken by clipping the dorsal vein of the tail. An initial blood sample (Baseline) was taken immediately after the animals were placed in plastic restrainer and a second blood sample (15' Stress) after 15 min of restraint-stress. Note that: (i) A D X / N R animals had undetectable levels of corticosterone ( < 0.2 ~ g / 1 0 0 ml); (ii) baseline corticosterone levels in ADX + P group were identical to those of SHAM surgery; and (iii) in both A D X / N R and ADX + P groups there was a complete suppression of stress-induced corticosterone secretion. The asterisk shows the mean ( _+S.E.) plasma corticosterone levels after 30 min of N O V E L environment-stress (see Experiment 1).

placement, had no effect on the acute rotational response to amphetamine or the development of sensitization, in the N O V E L environment. However, on the first test session A D X / N R rats did tend to have lower rotational scores than SHAM rats and they were significantly less active than adrenalectomized rats given a corticosterone pellet plus nocturnal access to corticosterone in the drinking water (group A D X + P + B). Fig. 4 shows the mean plasma corticosterone levels, measured in the middle of the light phase (13.00-13.30 h) before (Baseline) and after 15 min of restraint-stress

700 SHAM-HOME SHAM-NOVEL

600-

Co

:,,:

---O---

ADX-NOVEL

ADX+P+~i~

500-

O 400k*-o 300.Q

Z 100-

SHAM ADX/NR ADX+ P

f NOVEL

produced the desired effects. (i) Adrenalectomized rats without pellet replacement ( A D X / N R ) had undetectable ( < 0.2 /zg/100 ml) levels of corticosterone both before and after 15 min of restraint-stress. (ii) Adrenalectomized rats with pellet replacement (ADX + P) had corticosterone levels comparable to those at the nadir of the cycle in sham-operated rats and stress-induced corticosterone secretion was completely suppressed.

3.3. Experiment 3 Fig. 3 shows the effect of the first and the seventh injection of amphetamine (2 m g / k g , IP) on rotational behavior in sham-operated rats tested in the H O M E or N O V E L environments and in ADX rats tested in the N O V E L environment. The results of Experiment 3 confirmed the findings in Experiment 2. (i) The acute response to amphetamine and the magnitude of amphetamine sensitization was significantly greater in animals tested in the N O V E L environment, relative to those tested in the H O M E environment (left panel). (ii) The suppression of stress-induced corticosterone secretion (group A D X - N O V E L ) had no effect on the enhancement in the acute response to amphetamine or amphetamine sensitization seen in the N O V E L environment (left panel). (iii) Adrenalectomy, with (ADX + P) or without ( A D X / N R ) corticosterone pellet re-

2

3

4

5

6

2 3 4 5

Test Session Fig..3. The mean number of full rotations (_+S.E.) during the 1st and the 7th test sessions for animals that received amphetamine (2 mg/kg, IP) either in their H O M E cage or in a NOVEL environment. Left panel: the rotational response to amphetamine of sham operated animals tested in their H O M E environment (SHAM-HOME) is compared to that of animals tested in a NOVEL environment, after receiving either a sham operation (SHAM-NOVEL) or adrenalectomy (ADX-NOVEL). A two-way A N O V A with repeated measures for one factor resulted in a significant group by test session interaction (F2.35=8.35, P = 0 . 0 2 ) . There was a group by test session interaction even when the groups SHAM-NOVEL and ADXNOVEL were individually compared to group SHAM-HOME (Ft,15 = 6.78, P = 0.02 and Fl.2s = 14.16, P = 0.001, respectively). In addition, on the 1st test session the rotational scores of groups SHAMNOVEL and A D X - N O V E L were virtually identical and both were significantly higher than that of group SHAM-HOME (one-way ANOVA: F2,35 = 6.49, P = 0.01; Fisher's PLSD test: P < 0.01 for all values). Right panel: the rotational response to amphetamine is shown in animals tested in the N O V E L environment, after the ADX group was subdivided in three groups that received either no corticosterone replacement ( A D X / N R ) , a corticosterone pellet ( A D X + P), or a corticosterone pellet plus noctural access to corticosterone in the drinking water ( A D X + P + B). The two-way A N O V A with repeated measures for one factor resulted in no significant effect of group (F3,25 = 2.71, P = 0.066), and there was no group by test session interaction (F3,2s = 0.66, P > 0.58). By contrast, there were significant group differences on the 1st test session (F3,25=3.9, P = 0.02) and group ADX + P + B was different from groups A D X / NR (Fisher's PLSD test: P = 0.002) and A D X + P ( P < 0.05), but not from group SHAM ( P = 0 . 0 6 6 ) . The difference between groups SHAM and A D X / N R was not significant ( P > 0.13).

A. Badiani et aL / Brain Research 673 (1995) 13-24

(15' Stress), for groups S H A M - H O M E , A D X / N R N O V E L , A D X + P - N O V E L and A D X + P + BNOVEL. In addition, the nocturnal plasma corticosterone levels (10.00-11.00 h ) w e r e measured in groups S H A M - N O V E L and A D X + P + B-NOVEL. Again, adrenalectomy and corticosterone replacements produced the desired effects. (i) Adrenalectomized rats without pellet replacement ( A D X / N R ) had undetectable ( < 0.2 /~g/100 ml) levels of corticosterone both before and after stress. (ii) Adrenalectomized rats with pellet replacement (ADX + P and A D X + P + B) had nadir corticosterone levels comparable to those of sham operated rats and the restraint-stress-induced corticosterone secretion was completely suppressed. (iii) Adrenalectomized rats with pellet replacement plus nocturnal access to corticosterone in the drinking water (ADX + P + B) had nocturnal corticosterone levels comparable to those of sham operated rats. 3.4. Experiment 4

Fig. 5 shows the effects of five IP injections of 1.5 m g / k g of amphetamine on the total number of photobeam counts (total activity), on the number of crossovers and on rearing behavior of SHAM and A D X / N R rats. Fig. 5 illustrates three main findings.

12 ]

~"

[] Baseline

10

•

15' Stress

"~

~

8

[]

Night

g0

4

o

0

SHAM

HOME[

SHAM

ADX/NR

ADX + P

NOVEL

ADX + P + B

]

Fig. 4. The mean (+S.E.) plasma corticosterone levels for the animals whose rotational behavior is shown in Fig. 3. Three blood samples were taken by clipping the dorsal vein of the tail. An initial blood sample (Baseline) was taken in the middle of the light phase (13.00-14.00 h), immediately after the animals were placed in plastic restrainer and a second blood sample (15' Stress) after 15 min or restraint stress. In addition, a third blood sample was taken 2-3 h after the onset of the dark phase (22.00-23.00 h) in the SHAMNOVEL and A D X + P + B - N O V E L groups. Note that: (i) ADX/NR-NOVEL animals had undetectable levels of corticosterone ( < 0.2 /zg/100 ml); (ii) baseline corticosterone levels in ADX + P-NOVEL and ADX + P + B-NOVEL groups were identical to those of the SHAM groups; (iii) in all ADX surgerys there was a complete suppression of stress-induced corticosterone secretion; and (iv) the nocturnal levels of corticosterone in the A D X + P + B NOVEL group were not different from those of SHAM-NOVEL group.

19

(i) On the first test session adrenalectomy significantly reduced total activity and rearing behavior, but not crossovers. (ii) As expected (see Materials and Methods), the changes in amphetamine-induced crossovers over test sessions were complex and the number of crossovers did not change linearly over test sessions. We reported previously [4] that exposure to the N O V E L environment alone, in the absence of drug, produces a marked increase in crossovers which habituates over the first 3 - 4 test sessions and that a parallel decrease was seen for amphetamine-induced crossovers when the treatments were given in the N O V E L environment. This probably accounts for the decrease in crossovers seen over the first few test sessions here as well. A two-way A N O V A resulted in a significant effect of surgery, but no significant effect of test session, nor surgery by test session interaction. (iii) Amphetamine-induced total activity and rearing behavior increased linearly over test sessions (i.e., sensitized) in both groups and there were no group differences in the rate of sensitization (as indicated by no group differences in slope coefficients).

4. Discussion

Three major findings are reported here. (i) Both the acute psychomotor activating effects of amphetamine (rotational behavior in rats a unilateral 6-OHDA lesion) and the magnitude of amphetamine sensitization are greater in a N O V E L test environment than in a H O M E environment (Experiments 2 and 3). (ii) The secretion of adrenal hormones produced by exposure to a N O V E L environment is not required for this N O V E L environment-associated enhancement in either the acute effects of amphetamine or in amphetamine sensitization (Experiments 2 and 3). (iii) Circulating adrenal hormones are not required for the development of sensitization to the effects of amphetamine on rotational behavior in either H O M E or N O V E L environments (Experiments 2 and 3) or on rearing behavior in a N O V E L environment (Experiment 4). These results confirm previous reports that both the acute behavioral activating effects of amphetamine and the development of sensitization to these effects are enhanced when drug treatments are given in a N O V E L test environment [4,5]. These phenomena are not unique to rotational behavior in rats with a unilateral 6 - O H D A lesion because similar results are obtained if locomotor activity is quantified in rats without a 6O H D A lesion [4]. In addition, it should be noted that the effect of a N O V E L environment on the acute response to amphetamine appears to be independent of its effect on amphetamine sensitization [4,5].

20

A. Badiani et al. / B r a i n Research 673 (1995) 1 3 - 2 4

TOTAL ACTIVITY

CROSSOVERS 3OO

6OO

REARING BEHAVIOR 30O

SHAM

260 -

•

ADX/NR

E 200_

o

o ~d

250-

200-

1

150-

~

100-

~

-

@

150- -'"

..Q

E

200-

100-

z 50-

1000

i

i

i

~

O

SHAM

•

ADX/NR

0

i

1 2 3 4 5

Test Session Fig. 5. The mean number of photobeam counts ( + S.E.) during five test sessions for animals that received amphetamine (1.5 m g / k g , IP) in NOVEL test cages. Left panel: amphetamine-induced activity (total number of counts) in animals that received sham surgery (SHAM) or adrenalectomy ( A D X / N R ) . A two-way A N O V A with repeated measures for one factor resulted in a significant effect of surgery (F]j~ = 8.09, P = 0.01) and test session (F4,76 = 6.07, P > 0.001), but no surgery by test session interaction (F4.76 = 0.4, P > 0.76). The dashed lines represent the linear regression of the mean number of counts over test sessions for group SHAM (mean slope coefficient: 28.0 _+ 13.8, P < 0.05; r 2 = 0.82) and group A D X / N R (mean slope coefficient: 34.6 _+ 17.5, P < 0.05; r 2 = 0.88). There was no group difference in the slope coefficients (t = 0.30, P > 0.76). The asterisk indicates a significant effect of surgery on the first test session (Student's t-test, t = 3.08, P < 0.01). Middle panel: amphetamine-induced crossovers. A two-way ANOVA with repeated measures for one factor resulted in a significant effect of surgery (Fl.19 = 5.16, P < 0.05), but no effect of test session (F4,76 = 1.77, P > 0.14), nor a surgery by test session interaction (F4.76 = 0.97, P > 0.42). A Student's t-test on just the data from the 1st test session resulted in no significant effect of surgery (t = 1.4, P > 0.17). Right panel: amphetamine-induced rearing behavior. A two-way A N O V A with repeated measures for one factor resulted in a significant effect of surgery (FI.19 = 5.92, P < 0.05) and test session (F4.7~, = 7.14, P > 0.0001), but no surgery by test session interaction (F4,76 = 0.2, P > 0.93). The dashed lines represent the linear regression of the mean number of counts over test sessions for group SHAM (slope coefficient: 22.5 ± 5.6, P < 0.05; r 2 = 0.77) and group A D X / N R (slope coefficient: 30.8 _4_4.2, P < 0.01; r 2 = 0.93). There was no group difference in the slope coefficients (t = 0.47, P > 0.64). The asterisk indicates a significant effect of surgery on the first test session (Student's t-test, t = 3.38, P < 0.01).

4.1. The role of stress-induced corticosterone secretion in the effects of a N O V E L test environment There is considerable evidence showing that exposure to a novel environment produces a 'stress response', including an increase in plasma corticosterone levels [22,25] and this effect was replicated under the conditions used here. Exposure to stress enhances the acute psychomotor response to amphetamine [2,58] and can produce sensitization (stress-induced sensitization) to the action of psychostimulant drugs [3,6,44]. Furthermore, adrenalectomy is reported to block stress-induced sensitization of the locomotor response to amphetamine [17,18,19] and amphetamine sensitization [43]. Therefore, we hypothesized that exposure to a N O V E L environment may enhance the acute psychomotor activating effects of amphetamine a n d / o r the development of amphetamine sensitization because of the actions of a N O V E L environment as a stressor and in particular, because of a stress-induced increase in the synthesis and secretion of corticosterone [4,5]. However, the present results indicate that this hypothesis is untenable: the suppression of stress ( N O V E L

environment)-induced corticosterone secretion by adrenalectomy had no effect on either the enhancement in the acute response to amphetamine or the magnitude of sensitization seen in a N O V E L (versus H O M E ) environment. Consistent with this, Badiani and colleagues [5] have found that chronic intermittent exposure to restraint-stress produces a progressive increase in food- and water-intake (i.e., sensitization to the prophagic and polydipsic effects of stress) and that this effect is not blocked by adrenalectomy either. The present results establish that the secretion of adrenal hormones induced by exposure to a N O V E L environment does not mediate the enhancement in either the acute actions of amphetamine or amphetamine sensitization seen in a N O V E L environment. However, it is possible that other neuroendocrine responses to stress (exposure to a N O V E L environment in this case), independent of the secretion of adrenal hormones, are involved. For example, extrahypothalamic CRH-containing neurons are activated by stress [14,39] and local injections of C R H into the ventral tegmental area, locus coeruleus, hippocampus or amygdala potentiate some behavioral and neu-

A. Badiani et al. /Brain Research 673 (1995) 13-24 rochemical effects of stress [11,29,31,32]. There is also evidence that ICV injections of C R H produce stresslike 'anxiogenic' responses in animals with surgical or chemical suppression of the H P A axis [8,10]. It is possible, therefore, that exposure to a N O V E L environment enhances the effects of amphetamine by activating some C R H mechanism(s) independent of the H P A axis. In support of this hypothesis, it has been reported that ICV injections of C R H antiserum attenuate the expression of stress-induced amphetamine sensitization [15] and that repeated ICV infusions of C R H induce long-term sensitization to the locomotor activating effects amphetamine [12]. Furthermore, these effects of C R H on amphetamine sensitization appear to be independent of the H P A axis, because subcutaneous treatment with C R H was ineffective [12]. If extrahypothalamic CRH-containing systems do contribute to the difference in amphetamine sensitization seen in the N O V E L (versus H O M E ) environment, pretreatment with C R H antagonists should attenuate the effect. This hypothesis remains to be tested. Alternatively, the stress associated with exposure to a N O V E L environment may have enhanced both the acute response to amphetamine a n d / o r the development of sensitization by impinging more directly on the neural substrate underlying amphetamine-induced behavioral activation a n d / o r amphetamine sensitization. For example, stressors such as restraint or footshock activate the mesostriatal dopaminergic system [1,41,53] and, of particular importance here, this effect is independent of circulating corticosterone [28]. If exposure to a N O V E L environment activates the mesotelencephalic dopamine system, the administration of amphetamine in a N O V E L environment may be equivalent to treatment with a higher dose of amphetamine. This would be expected to produce an enhancement in the acute response to amphetamine and, with repeated treatments, greater sensitization. A similar case may be made for the effects of stress on noradrenergic activity. These hypotheses remain to be tested. 4.2. The role of adrenal hormones in the acute psychomotor stimulating effects of amphetamine It is clear from the above that a stress-induced release of adrenal hormones is not responsible for the enhancement in the acute effects of amphetamine on rotational behavior seen in the N O V E L environment. In addition, adrenalectomy without pellet replacement, regardless of environmental condition, had no significant effect on the acute rotational response to amphetamine. By contrast, adrenalectomy produced a modest reduction in amphetamine-induced crossovers and a large decrease in amphetamine-induced rearing behavior. There are contrasting reports in the litera-

21

ture on the effects of adrenalectomy on amphetamineinduced psychomotor activity. Some authors reported that adrenalectomy reduces the acute effects of systemic [13] or intra-accumbens [13,16] amphetamine on crossovers, whereas others reported no effect of adrenalectomy on amphetamine-induced locomotor activity [37,43]. There are a number of possible reasons for this discrepancy. First, differences in exactly how locomotor activity is quantified could have a large effect on the outcome. For example, Rivet and colleagues [43] found no effect of adrenalectomy on the acute locomotor response to amphetamine when the behavior was quantified in small square test cages (25 × 25 × 35 cm), whereas authors from the same laboratory [13] reported that adrenalectomy produced a modest decrease in the locomotor response to amphetamine when behavior was quantified in circular corridors (170 cm long and 10 cm wide). Furthermore, the findings in Experiment 4 indicate that the position of the photobeams in the monitoring apparatus (i.e., the behavioral patterns actually quantified) can have a significant effect on the outcome of such experiments. It is possible, therefore, that some of the discrepancies in the reported effects of adrenalectomy on amphetamine-induced locomotor activity are due, at least in part, to differences in how behavior is quantified. Another possibility is that the effect of adrenalectomy on the acute psychomotor response to amphetamine differs depending on the dose of amphetamine used. For example, Pauly and colleagues [37] reported that adrenalectomy reduced the locomotor response to 5 m g / k g of amphetamine in mice, whereas it did not affect the response to 10 m g / k g . Although no attempt was made here to assess the effects of adrenalectomy on the dose-effect curve for amphetamine-induced rotational behavior, it is interesting that in Experiment 2 adrenalectomy was completely without effect on the rotational response to 3 m g / k g of amphetamine, whereas in Experiment 3, where the dose of amphetamine was only 2 m g / k g , A D X / N R rats did tend to have lower rotational scores than SHAM rats (although this difference was still non-significant). In summary, it appears that under some conditions adrenalectomy produces a modest decrease in the psychomotor activating effects of amphetamine, but this effect is dependent on a number of variables that are not well characterized. 4.3. The role of adrenal hormones in amphetamine sensitization It was somewhat surprising that adrenalectomy, with or without corticosterone replacement, had no effect whatsoever on amphetamine sensitization in either H O M E or N O V E L environments. This is because Rivet

22

A. Badiani et al. /Brain Research 673 (1995) 13-24

a n d c o l l e a g u e s [43] r e p o r t e d previously t h a t a d r e n a l e c t o m y blocks the d e v e l o p m e n t of sensitization to the l o c o m o t o r activating effects of a m p h e t a m i n e , ' w h e r e a s d e x a m e t h a s o n e c o m p l e t e l y r e s t o r e d a n d even p o t e n t i a t e d sensitization to a m p h e t a m i n e ' . T h e y suggested, t h e r e f o r e , that basal levels o f a d r e n a l h o r m o n e s a r e necessary for the d e v e l o p m e n t of sensitization. W e do not know w h a t accounts for the d i s c r e p a n c y b e t w e e n the p r e s e n t study a n d that of Rivet a n d c o l l e a g u e s [43]. O n e obvious possibility is r e l a t e d to the d i f f e r e n t behaviors m e a s u r e d in the two studies: r o t a t i o n a l behavior in rats with a u n i l a t e r a l 6 - O H D A lesion in Experim e n t s 2 a n d 3 h e r e and l o c o m o t o r activity by Rivet a n d c o l l e a g u e s [43]. P e r h a p s a d r e n a l e c t o m y blocks the dev e l o p m e n t of sensitization to the effects of amp h e t a m i n e on l o c o m o t o r activity in naive rats, but not sensitization o f r o t a t i o n a l b e h a v i o r in rats with a unil a t e r a l 6 - O H D A lesion. This is unlikely, however, because in E x p e r i m e n t 4 t h e r e was no effect of a d r e n a l e c t o m y on sensitization of overall activity or r e a r i n g b e h a v i o r in rats w i t h o u t a u n i l a t e r a l 6 - O H D A lesion. F u r t h e r m o r e , Pauly a n d c o l l e a g u e s [37], who q u a n t i f i e d l o c o m o t o r activity in mice, also f o u n d that a d r e n a l e c t o m y had no effect on the d e v e l o p m e n t of a m p h e t a m i n e sensitization. In conclusion, the e x p e r i m e n t s r e p o r t e d h e r e establish t h a t the e n h a n c e m e n t in the a c u t e p s y c h o m o t o r r e s p o n s e to a m p h e t a m i n e a n d a m p h e t a m i n e sensitization seen in a N O V E L test e n v i r o n m e n t (relative to a H O M E e n v i r o n m e n t ) d o e s not d e p e n d on a stress-ind u c e d s e c r e t i o n of a d r e n a l h o r m o n e s a n d that circulating a d r e n a l h o r m o n e s a r e not r e q u i r e d for amp h e t a m i n e sensitization. O f course, it is still possible that the activation o t h e r n e u r o e n d o c r i n e o r n e u r a l systems involved in the s t r e s s - r e s p o n s e may be r e s p o n sible for the facilitating effects o f a N O V E L environment. A l t e r n a t i v e l y , the effects of a N O V E L environm e n t on a m p h e t a m i n e sensitization m a y d e p e n d on a d i f f e r e n t class of p h e n o m e n a , such as a d i f f e r e n t i a l acquisition of CS p r o p e r t i e s by c o n t e x t u a l a n d interoceptive stimuli in the H O M E versus the N O V E L condition. This hypothesis is b a s e d on the a s s u m p t i o n that sensitization is p r o d u c e d (or at least greatly e n h a n c e d ) by a progressive increase in the effects of CSs on the drug r e s p o n s e (see [4,5] for a full discussion of this hypothesis). W h a t e v e r the mechanism(s), these experim e n t s e m p h a s i z e the extent to which the d e v e l o p m e n t o f sensitization is i n f l u e n c e d by e n v i r o n m e n t a l factors a n d that to u n d e r s t a n d sensitization it will be necessary to e x p l o r e how the e n v i r o n m e n t exerts these effects.

Acknowledgements This r e s e a r c h was s u p p o r t e d by a G r a n t from the N I D A (No. 02494) to T . E . R a n d by a G r a n t from the

N I M H (No. 42251) to H.A. W e t h a n k L a n Bui a n d T a n y a Cvetkovski for the assistance in c o n d u c t i n g t h e experiments.

References [1] Abercrombie, E.D., Keefe, K.A., DiFrischia, D.S. and Zigmond, M.J., Differential effect of stress on in vivo dopamine release in striatum, nucleus accumbens and medial frontal cortex, J. Neurochem., 52 (1989) 1655-8. [2] Anisman, H., Hahn, B., Hoffman, D. and Zacharko, R.M., Stressor invoked exacerbation of amphetamine-elicited perseveration, Pharmacol. Biochem. Behav., 23 (1985) 173-183. [3] Antelman, S.M., Eichler, A.J., Black, C.A. and Kocan, D., lnterchangeability of stress and amphetamine in sensitization, Science, 207 (1980) 329-31. [4] Badiani, A., Anagnostaras, S.G. and Robinson, T.E., The development of sensitization to the psychomotor stimulant effects of amphetamine is enhanced in a novel environment, Psychopharmacology, in press. [5] Badiani, A., Browman, K.E. and Robinson, T.E., Influence of novel versus home environments on sensitization to the psychomotor stimulant effects of cocaine and amphetamine, Brain Res., in press. [6] Badiani, A., Cabib, S. and Puglisi-Allegra, S., Chronic stress induces strain-dependent sensitization to the behavioral effects of amphetamine in the mouse, Pharmacol. Biochem. Behal., 43 (1992) 53-60. [7] Reference deleted. [8] Berridge, C.W. and Dunn, A.J., CRF and restraint-stress decrease exploratory behavior in hypophysectomized mice, Pharmacol. Biochem. BehaL,., 34 (1989) 517-519. [9] Breese, G.R. and Traylor, T.D., Depletion of brain noradrenaline and dopamine by 6-hydroxydopamine, Br. J. Pharmacol., 42 (1971) 88-99. [10] Britton, K., Lee, G., Dana, R., Risch, S. and Koob, G.F., Activating and 'anxiogenic' effects of corticotropin releasing factor are not inhibited by blockade of the pituitary-adrenal system with dexamethasone, Life Sci., 39 (1986) 1281-1286. [11] Butler, P.D., Weiss, J.M., Stout, J.C. and Nemeroff, C.B., Corticotropin-releasing factor produces fear-enhancing and behavioral activating effects following infusion into the locus coeruleus, J. Neurosci., 10 (1990) 176-183. [12] Cador, M., Cole, B.J., Koob, G.F., Stinus, L. and Le Moal, M., Central administration of corticotropin releasing factor induces long-term sensitization to D-amphetamine, Brain Res., 606 (1993) 181-186. [13] Cador, M., Dulluc, J. and Mormede, P., Modulation of the locomotor response to amphetamine by corticosterone, Neuroscience, 56 (1993) 981-988. [14] Chappell, P.B., Smith, M.A., Kilts, C.D., Bissette, G., Richtie, J., Anderson, C. and Nemeroff, C.B., Alterations in corticotropin-releasing factor-like immunoreactivity in discrete brain regions after acute and chronic stress, J. Neurosci., 6 (1986) 2908-2914. [15] Cole, B.J., Cador, M., Stinus, L., Rivier, J., Vale, W., Koob, G.F. and Le Moal, M., Central administration of a CRF antagonist blocks the development of stress-induced behavioral sensitization, Brain Res., 512 (1990) 343-6. [16] Cools, A.R., Differential role of mineralocorticoid and glucocorticoid receptors in the genesis of dexamphetamine-induced sensitization of mesolimbic, alpha 1 adrenergic receptors in the ventral striatum, Neuroscience, 43 (1991) 419-28.

A. Badiani et al. / Brain Research 673 (1995) 13-24 [17] Deroche, V., Piazza, P.V., Casolini, P., Le Moal, M. and Simon, H., Sensitization to the psychomotor effects of amphetamine and morphine induced by food restriction depends on corticosterone secretion, Brain Res., 611 (1993) 352-356. [18] Deroche, V., Piazza, P.V., Casolini, P., Maccari, S., LeMoal, M. and Simon, H., Stress-induced sensitization to amphetamine and morphine psychomotor effects depend on stress-induced corticosterone secretion, Brain Res., 598 (1992) 343-8. [19] Deroche, V., Piazza, P.V., Le Moal, M. and Simon, H., Social isolation-induced enhancement of the psychomotor effects of morphine depends on corticosterone secretion, Brain Res., 640 (1994) 136-139. [20] Deroche, V., Piazza, P.V., Maccari, S., Le Moal, M. and Simon, H., Repeated corticosterone administration sensitizes the locomotor response to amphetamine, Brain Res., 584 (1992) 309-313. [21] Einat, H. and Szechtman, H., Environmental modulation of both locomotor response and locomotor sensitization to the dopamine agonist quinpirole, Behav. Pharmacol., 4 (1993) 399403. [22] Friedman, S.B. and Ader, R., Adrenocortical response to novelty and noxious stimulation, Neuroendocrinology, 2 (1967) 209212. [23] Hefti, F., Melamed, E., Sahakian, B.J. and Wurtman, R.J., Circling behavior in rats with partial, unilateral nigro-striatal lesions: effect of amphetamine, apomorphine and DOPA, Pharmacol. Biochem. Behav., 12 (1980) 185-188. [24] Hefti, F., Melamed, E. and Wurtman, R.J., Partial lesions of the dopaminergic nigrostriatal system in rat brain: biochemical characterization, Brain Res., 195 (1980) 123-37. [25] Hennessy, J.W., Levin, R. and Levine, S., Influence of experiential factors and gonadal hormones on pituitary-adrenal response of the mouse to novelty and electric shock, J. Comp. Physiol. Psychol., 91 (1977) 770-777. [26] Hinson, R.E. and Poulos, C.X., Sensitization to the behavioral effects of cocaine: modification by Pavlovian conditioning, Pharmacol. Biochem. Behav., 15 (1981) 559-62. [27] Hirabayashi, M. and Alam, M.R., Enhancing effect of methamphetamine on ambulatory activity produced by repeated administration in mice, Pharmacol. Biochem. Behav., 15 (1981) 925932. [28] Imperato, A., Puglisi-Allegra, S., Casolini, P. and Angelucci, L., Changes in brain dopamine and acetylcholine release during and following stress are independent of the activity of the pituitary-adrenocortical axis, Brain Res., 538 (1991) 111-117. [29] Kalivas, P.W., Duffy, P. and Latimer, L.G., Neurochemical and behavioral effects of corticotropin-releasing factor in the ventral tegmental are of the rat, J. Pharmacol. Exp. Ther., 242 (1987) 757-763. [30] Kalivas, P.W. and Stewart, J., Dopamine transmission in the initiation and expression of drug- and stress-induced sensitization of motor activity, Brain Res. Rev., 16 (1991) 223-244. [31] Lee, E.H.Y. and Tsai, M.J., The hyppocampus and the amygdala mediate the locomotor stimulating effects of corticotropinreleasing factor in mice, Behav. Neural. Biol., 51 (1989) 412-413. [32] Liang, K.C. and Lee, E.H.Y., Intra-Amygdala injections of corticotropin-releasing factor facilitate inhibitory avoidance learning and reduce exploratory behavior in rats, Psychopharmacology, 96 (1988) 232-236. [33] Marinelli, M., Piazza, P.V., Deroche, V., Maccari, S., Le Moal, M. and Simon, H., Corticosterone circadian secretion differentially facilitates dopamine-mediated psychomotor effect of cocaine and morphine, J. Neurosci., (1994) 2728-2731. [34] Morano, M.I., V~quez, D.M. and Akil, H., The role of the hippocampal mineralcorticoid and glucocorticoid receptors in hypothalamo-pituitary-adrenal axis of the aged Fisher rat, Mol. Cell Neurosci., 5 (1994)

23

[35] Morimoto, A., Nakamori, T., Morimoto, K., Tan, N. and Murakami, N., The central role of corticotrophin-releasing factor (CRF-41) in psychological stress in rats, J. PhysioL, 460 (1993) 221-229. [36] NATO Advanced Research Workshop on Psychobiology of Stress, Psychobiology of Stress, Kluwer, Dordrecht, 1990. [37] Pauly, J.R., Robinson, S.F. and Collins, A.C., Chronic corticosterone administration enhances behavioral sensitization to amphetamine in mice, Brain Res., 620 (1993) 195-202. [38] Piazza, P.V., Demini~re, J.M., Le Moal, M. and Simon, H., Factors that predict individual vulnerability to amphetamine self-administration, Science, 245 (1989) 1511-3. [39] Plotsky, P.M., Cunningham, E.T., Jr. and Widmaier, E.P., Catecholaminergic modulation of corticotropin-releasing factor and adrenocorticotropin secretion, Endocrine Rev., 10 (1989) 437458. [40] Post, R.M., Lockfeld, A., Squillace, K.M. and Contel, N.R., Drug-environment interaction: context dependency of cocaineinduced behavioral sensitization, Life Sci., 28 (1981) 755-60. [41] Puglisi-Allegra, S., Imperato, A., Angelucci, L. and Cabib, S., Acute stress induces time-dependent responses in dopamine mesolimbic system, Brain Res., 554 (1991) 217-222. [42] Pycock, C.J., Turning behaviour in animals, Neuroscience, 5 (1980) 461-514. [43] Rivet, J.M., Stinus, L., Le Moal, M. and Mormede, P., Behavioral sensitization to amphetamine is dependent on corticosteroid receptor activation, Brain Res., 498 (1989) 149-53. [44] Robinson, T.E., Behavioral sensitization: characterization of enduring changes in rotational behavior produced by intermittent injections of amphetamine in male and female rats, Psychopharmacology, 84 (1984) 466-475. [45] Robinson, T.E., Persistent sensitizing effects of drugs on brain dopamine systems and behavior: implications for addiction and relapse. In S.G. Korenman and J.D. Barchas (Eds.), The Biological Basis of Substance Abuse, Oxford University Press, New York, 1993, pp. 373-402. [46] Robinson, T.E., Angus, A.L. and Becker, J.B., Sensitization to stress: the enduring effects of prior stress on amphetamine-induced rotational behavior, Life Sci., 37 (1985) 1039-42. [47] Robinson, T.E. and Becker, J.B., Enduring changes in brain and behavior produced by chronic amphetamine administration: a review and evaluation of animal models of amphetamine psychosis, Brain Res. Rev., 396 (1986) 157-98. [48] Robinson, T.E. and Berridge, K.C., The neural basis of drug craving: an incentive-sensitization theory of addiction, Brain Res. Rev., 18 (1993) 247-291. [49] Segal, D.S. and Schuckit, M.A., Animal models of stimulant-induced psychosis. In I. Creese (Ed.), Stimulants: Neurochemical, Behavioral and Clinical Perspectives, Raven Press, New York, 1983, pp. 131-67. [50] Shaham, Y., Kelsey, J.E. and Stewart, J., Temporal factors in the effei:t of restraint stress on morphine-induced behavioral sensitization in the rat, Psychopharmacology, in press. [51] Stewart, J., Neurobiology of conditioning to drugs of abuse, Proc. N Y Acad. Sci., 654 (1992) 335-346. [52] Stewart, J. and Badiani, A., Tolerance and sensitization to the behavioral effects of drugs, Behav. Pharmacol., 4 (1993) 289-312. [53] Thierry, A.M., Tassin, J.P., Blanc, G. and Glowinski, J., Selective activation of the mesocortical DA system by stress, Nature, 263 (1976) 242-243. [54] Tilson, H.A. and Rech, R.A., Conditioned drug effects and absence of tolerance to d-amphetamine induced motor activity, Pharmacol. Biochem. Behav., 1 (1973) 149-153. [55] Ungerstedt, U. and Arbuthnott, G.W., Quantitative recording of rotational behavior in rats after 6-hydroxy-dopamine lesions of the nigrostriatal dopamine system, Brain Res., 24 (1970) 485-93.

24

A. Badiani et al. / Brain Research 673 (1995) 13-24

[56] Ursin, H., Baade, E. and Levine, S., P~ychobiology ~f Stress, Academic Press, New York, 1978. [57] Vezina, P. and Stewart, J., Conditioning and place-specific sensitization of increases in activity induced by morphine in the VTA, Pharmacol. Biochem. Behav., 20 (1984) 925-34. [58] Williams, J.L. and Barber, R.G., Effects of cat exposure and cat odors on subsequent amphetamine-induced stereotypy, Pharmacol. Biochem. Behac., 36 (1989) 375-380.

[59] Willner, P., Papp, M., Cheeta, S. and Muscat, R., Environmental influences on behavioural sensitization to the dopamine agonist quinpirole, Beha~'. Pharmacol., 3 (1992) 43-50. [60] Woolverton, W.L., Cervo, L. and Johanson, C.E., Effects of repeated methamphetamine administration on methamphetamine self-administration in rhesus monkeys, PharmacoL Biochem. Behae., 21 (1984) 737-41.