Abstract
We report on the effect of environment on amphetamine sensitization in rats with a unilateral 6-hydroxydopamine lesion of the mesostriatal dopamine system. The rats were either housed in the test environment (HOME) or exposed to it only during the treatments (NOVEL). In experiment 1, the rats received seven consecutive i.p. injections of either saline or 2 mg/kg amphetamine. After 1 wk withdrawal the rotational response to 2 mg/kg amphetamine i.p. (i.e.,amphetamine challenge) was compared in saline- vs.amphetamine-pretreated animals. Although both HOME and NOVEL groups sensitized, the magnitude of sensitization was greater in the NOVEL group. In the NOVEL group there was also a greater conditioned response to drug-related cues. In experiment 2 a dose-effect curve (0.75, 1.5, 3.0 and 6.0 mg/kg amphetamine i.p.) was determined before and after six i.p. injections of 4.0 mg/kg amphetamine. Sensitization was indicated by a parallel shift to the left of the dose-effect curve in both groups, but this shift was 2.6 times greater in the NOVEL group than in the HOME group. Finally, in experiment 3, we found that environment- and sensitization-dependent differences in the psychomotor response to amphetamine were not accompanied by differences in the concentration of amphetamine in the plasma or in the striatum.
Animals that are given repeated administrations of psychostimulant drugs, such as amphetamine and cocaine, develop a progressive and persistent hypersensitivity (sensitization) to their psychomotor activating effects (Robinson and Becker, 1986; Stewart and Badiani, 1993). Although the mechanism(s) responsible for this phenomenon is not known, there is evidence that behavioral sensitization is accompanied by long-lasting neuroadaptations, including adaptations in the mesolimbic dopamine system (for reviews, see Kalivas and Stewart, 1991; Robinson and Becker, 1986). It has been hypothesized that these sensitization-related neuroadaptations may contribute to the development of compulsive patterns of drug-seeking and drug-taking behavior in addicts (Robinson and Berridge, 1993). Therefore, the identification of the factors that influence sensitization in animals may provide some insight into the sources of variability in the susceptibility to addiction.
The neuropharmacological actions of amphetamine and cocaine have been relatively well characterized. These drugs increase the synaptic concentration of monoamines in a dose-dependent manner because of their actions on monoamine transporters (for reviews, see Dunwiddie and Brodie, 1993; Seiden et al., 1993; White and Bunney, 1993). It is tempting, therefore, to view the psychomotor response to such drugs as simply a consequence of their direct neuropharmacological actions on specific neural substrates, and, similarly, behavioral sensitization as the result of adaptations in these neural substrates due to their repeated activation. There is evidence, however, that drug-induced behavior is a function of the interaction between the drug and the environmental stimuli associated with drug administration. For example, in a recent series of papers we have reported that both the acute psychomotor response to amphetamine and the rate of amphetamine and cocaine sensitization are greater for rats treated in a “novel” (NOVEL) test environment than for rats treated in a physically identical environment in which the animals live (HOME) (Badianiet al., 1995a, b, c). Indeed, if the cues associated with i.p. injections are eliminated by giving unsignaled i.v. infusions of amphetamine, the acute response to amphetamine becomes negligible and sensitization may not occur (Crombag et al., 1996); for a review, see Robinson et al., 1997).
In our earlier studies the effect of environment on sensitization was quantified using a within-subject design, that is, by the rate of increase in amphetamine-induced psychomotor activation over consecutive test sessions (rate of sensitization). Amphetamine sensitization can be quantified, however, in two different ways as well: 1) as the difference in drug effect between drug- and vehicle-pretreated animals (between-subject design) when both groups are given a “challenge” injection of the same dose of the drug (this design is most appropriate to study the expression of sensitization following a period of withdrawal); and 2) as a shift to the left in a dose-effect curve. It is not known whether these different measures simply capture different aspects of the same phenomenon, or whether they correspond to different sensitization-related phenomena that may or may not share the same neurobiological substrates.
The goal of this study was 3-fold. The first aim was to assess whether exposure to a NOVEL environment would enhance the induction of sensitization when animals were tested following withdrawal from amphetamine (experiment 1). The second aim was to determine whether an intermittent amphetamine treatment given under the NOVEL condition would produce a greater shift to the left in the dose-effect curve for the psychomotor activating effects of amphetamine, relative to the HOME condition (experiment 2). A third aim was to determine if environment- and/or sensitization-dependent differences in amphetamine response were accompanied by differences in the concentration of amphetamine in the plasma or in the brain (experiment 3).
Materials and Methods
Animals
A total of 120 male Sprague-Dawley rats (Harlan Sprague Dawley Inc., Indianapolis, IN), weighing 175 to 250 g at the beginning of the experiments, were used. The rats were housed individually in a room with a 14/10 hr light/dark cycle (lights on from 06:00 to 20:00 hr), and had ad libitum access to food and water. The rats were habituated to the colony room for 1 wk before any experimental manipulation.
6-OHDA Lesion and Rotational Behavior
Rotational behavior in rats with a unilateral 6-OHDA lesion of the mesostriatal dopamine system was used as an index of the psychomotor activating effects of amphetamine. The rationale for using this preparation has been discussed in detail elsewhere (Badianiet al., 1995 c). Briefly, there are three main reasons for studying rotational behavior in rats with a unilateral 6-OHDA lesion rather than the more common measurement of locomotor activity in rats without a lesion. First, the dose-effect curve for amphetamine-induced rotational behavior is linear over a wide range of doses (Ungerstedt and Arbuthnott, 1970), whereas the dose-effect curve for amphetamine-induced locomotor activity in rats without a 6-OHDA lesion is not. Second, the progressive increase in drug effect during sensitization results in a progressive increase in rotational behavior, whereas this is not necessarily the case with locomotor activity (Segal and Schuckit, 1983). Third, exposure to the NOVEL environment in saline-injected rats produces negligible rotational behavior, whereas this induces a marked increase in locomotor activity (Badiani et al., 1995a, b).
The rats were pretreated with desipramine to protect noradrenergic terminals (Breese and Traylor, 1971), anaesthetized with sodium pentobarbital, and then given a unilateral 6-OHDA lesion of the medial forebrain bundle, using procedures similar to those described previously (Robinson, 1984). Briefly, 8 μg of 6-OHDA were infused in 4 μl 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 10 days before they were given 0.05 mg/kg of apomorphine s.c., to assess the development of dopamine receptor supersensitivity. This test is a good indicator of the size of the lesion because a robust rotational response to such a low dose of apomorphine occurs only after at least 90 to 95% of striatal dopamine terminals are destroyed (Hefti et al., 1980a, b). Rotational behavior was quantified 10 min after apomorphine and animals that made less than five rotations/min were excluded from the study.
Procedures
Experiment 1 (effect of environment on the expression of amphetamine sensitization following withdrawal).
Fifty rats were used. Half of them (HOME 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 (NOVEL 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 wk of habituation to these housing conditions NOVEL rats were transferred every day (12:00-13:00 hr) from their home cages in the animal colony to the testing room and placed in plastic buckets identical to those in which HOME rats lived, including the presence of ground corn cob bedding, food and water. They then received an i.p. injection of either saline (SAL-NOVEL; n = 12) or 2.0 mg/kg of amphetamine (AMPH-NOVEL; n = 13). This procedure was repeated on seven consecutive days. At the same time HOME rats received an i.p. injection of either saline (SAL-HOME;n = 12) or 2.0 mg/kg of amphetamine (AMPH-HOME;n = 13) in their home cages. Thus, it must be emphasized that the cages in which HOME and NOVEL rats received amphetamine were physically identical, but this was a “novel” environment for one group and the home environment for the other group. The day after the last injection of amphetamine, all groups were given an i.p. injection of saline (Saline Challenge), under the same conditions in which they received the repeated treatments, to assess for the presence of conditioned hyperactivity. One week after the saline challenge, all groups received an i.p. injection of 2.0 mg/kg of amphetamine (Amphetamine Challenge), to test for the expression of amphetamine sensitization.
All test sessions lasted 90 min, after which time NOVEL rats were returned to their hanging cages in the animal colony. The behavior of the animals was videotaped during the first, third, fifth, seventh test sessions, the Saline Challenge and the Amphetamine Challenge. Rotational behavior was quantified by viewing the videotapes. One rotation was defined as a complete 360° turn.
Experiment 2 (effect of environment on sensitization-induced shift in the dose-effect curve).
Thirty-eight rats were were housed and tested under either HOME (n = 17) or NOVEL (n = 21) conditions, as described for experiment 1. The treatment schedule consisted of three phases, separated by 72 hr during which the animals were left undisturbed. In the first phase (day 1) both HOME and NOVEL groups were divided into four subgroups that received (12:00-13:00 hr) one of four doses (0.75, 1.5, 3.0 or 6.0 mg/kg, i.p.) of amphetamine, producing an ACUTE dose-effect curve. During the second phase (days 4-16) all rats received six i.p. injections of 4.0 mg/kg amphetamine, one every other day. The dose of 4.0 mg/kg was chosen because it was roughly intermediate between the three doses (1.5, 3.0 and 6.0 mg/kg) producing a reliable psychomotor response in the HOME group (see “Results”). During the last phase (day 19) each rat received the same dose it received on day 1 of testing, producing a SENSITIZED dose-effect curve. Rotational behavior was quantified as in experiment 1.
Experiment 3 (plasma and striatal amphetamine concentrations).
Experiment 3 was conducted to determine if the results obtained in experiment 1 (see “Results”) could be attributed to differences in amphetamine pharmacokinetics. This was done by quantifying the amphetamine concentrations in the plasma and in the striatum of HOME and NOVEL rats after either a single injection (HOME-ACUTE and NOVEL-ACUTE) or the last of seven consecutive injections (HOME-SENSITIZED and NOVEL-SENSITIZED) of 2.0 mg/kg amphetamine. The rats were killed 50 min after drug administration, because in experiment 1 this was the point at which there was maximal rotational behavior. The rats were removed from the cages, transported to an adjacent room and quickly (<1 min after removal from the cage) decapitated with a guillotine. Trunk blood was collected in heparinized tubes and after centrifugation the plasma was stored at -20°C. The brains were quickly dissected and the entire striatum was dissected, weighed, homogenized and then stored at -20°C until assayed. Standards were prepared by adding amphetamine to plasma and brain samples of nontreated animals. Plasma and the tissue samples were assayed at the Center for Human Toxicology, University of Utah, using gas chromatography coupled with mass spectrometry.
Drugs.
6-OHDA hydrobromide was freshly dissolved (2 mg/ml) in a cold solution of 0.9 mg/ml NaCl (saline) and 0.1 mg/mll-ascorbic acid. Desipramine hydrochloride was dissolved (15 mg/ml, weight of the salt) in distilled water and given i.p. (15 mg/kg). Apomorphine hydrochloride was freshly dissolved (0.1 mg/ml, weight of the salt) in saline and 0.01 mg/ml of l-ascorbic acid and injected s.c. in the nape of the neck (0.05 mg/kg).D-Amphetamine sulfate was dissolved (1 mg/ml, weight of the salt) in saline and administered i.p.. Sodium pentobarbital, dissolved (64.8 mg/ml) in a 10% ethanol solution (The Butler Company, Columbus, OH), was given i.p. (52 mg/kg).
Data Analysis and Statistics
HOME vs. NOVEL differences in the acute response to amphetamine (experiment 1) were assessed with a planned one-tail Student’s t test on the data from the first test session. The data for the AMPH-HOME and AMPH-NOVEL groups on the first, third, fifth and seventh test session were analyzed with a two-way ANOVA with repeated measures (test environment, two levels, HOME and NOVEL; test session, four levels). The data from the first 5 min after the first and seventh amphetamine injections were analyzed with a two-way ANOVA with repeated measures (test environment, two levels, HOME and NOVEL; test session, two levels, first and seventh). Linear regression analyses were performed for each individual rat and for each treatment group to quantify the rate of sensitization (a positive slope indicates sensitization and a high slope coefficient indicates rapid sensitization). A Student’s t test was used to determine if there was a significant difference between the two groups in the slope coefficients calculated for individual rats (i.e., if there was a difference in the rate of sensitization).
The data from the Saline Challenge and the Amphetamine Challenge tests (experiment 1) were analyzed with two-way ANOVAs (pretreatment, two levels, SAL and AMPH; test environment, two levels), followed by Fisher’s PLSD post-hoc comparisons.
The data from experiment 2 were analyzed with a repeated measure analysis of covariance (grouping factor: test environment, two levels, HOME and NOVEL; covariate: dose, four levels; repeated measures on sensitization, two levels, ACUTE and SENSITIZED), after testing the homogeneity of slopes assumption. The magnitude of the shift in the dose-effect curve induced by the amphetamine pretreatment was quantified as the difference between the dose of amphetamine required to produce the same effect in the ACUTE and in the SENSITIZED condition. Usually this is done for the dose that produce 50% of the maximal drug effect (ED50). The observed dose-effect curves, however, did not allow us to identify a maximal effect. Therefore, we compared the doses that produced 50% of the response to 6.0 mg/kg amphetamine in the SENSITIZED condition.
The data from experiment 3 were analyzed with two-way ANOVAs (sensitization, two levels, ACUTE and SENSITIZED; test environment, two levels, HOME and NOVEL).
The results of the statistical analyses are reported in the figure legends to make the following section more readable. However, all statements made in “Results” are supported by significant statistical tests (P ≤ .05).
Results
Experiment 1.
Figure 1A shows the effects of seven consecutive daily i.p. injections of saline or 2.0 mg/kg amphetamine on rotational behavior for rats in the HOME and NOVEL groups. Rotational behavior was negligible in both the SAL-HOME and the SAL-NOVEL group in all test sessions. In both AMPH-HOME and AMPH-NOVEL groups the first injection of amphetamine induced a robust rotational response that sensitized over repeated test sessions (as indicated by positive slope coefficients). There were, however, significant HOMEvs. NOVEL differences in the magnitude of these effects. In the AMPH-NOVEL group there was a greater acute response to amphetamine and a greater rate of sensitization, as indicated by significant group differences in the mean slope coefficients.
Figure 2 (top panels) shows the time-course of rotations after the first and the seventh injection of saline in the SAL-HOME (left panel) and the SAL-NOVEL group (right panel). In both groups rotational behavior was maximal during the first 5-min time bin and then declined to negligible levels.
Figure 2 (bottom panels) shows the time-course of rotations after the first and the seventh injection of amphetamine in the AMPH-HOME (left panel) and the AMPH-NOVEL group (right panel). In both groups there were two peaks in rotational behavior. There was a high level of rotations during the first 5-min time bin after drug administration, followed by a decline in activity, and then (approximately 20 min after amphetamine) by a second increase in rotational behavior. The early peak, and subsequent decline, in rotational behavior paralleled the changes in rotational behavior seen in the SAL-HOME and SAL-NOVEL groups, suggesting that it was partially due to the nonspecific behavioral activation induced by treatment-associated manipulations (e.g., handling and injection). An ANOVA limited to the first 5-min interval indicates that this early peak in rotational behavior was significantly greater after the seventh vs. the first amphetamine injection. This suggests that treatment-associated manipulations predicting amphetamine administration acquired CS properties (see below). The magnitude of this putative conditioned response was significantly greater in the NOVEL than in the HOME environment, as indicated by an environment by test sessioninteraction.
Figure 2 (bottom panels) shows that a second and larger peak in rotational behavior occurred 30 to 50 min after amphetamine and that both environment-related (NOVEL vs. HOME group) and sensitization-related (first vs. seventh injection) differences in the overall response to amphetamine were accompanied by comparable differences in the peak drug effect.
Figure 1B shows the effects of the Saline Challenge. The rotational response to saline in the AMPH-pretreated groups was greater than in the SAL-pretreated groups, confirming the development of conditioned hyperactivity in response to drug-associated cues. The magnitude of the CR to treatment-associated cues was much greater in the NOVEL than in the HOME environment, as indicated by a pretreatment by environment interaction. The time course of rotations in response to the Saline Challenge (fig. 3, top panels) shows that the CR was maximal during the first 5 min after the injection and then declined rapidly.
Figure 1C shows the effects of an amphetamine injection (Amphetamine Challenge) given to all rats after a 7-day withdrawal period. Amphetamine-induced activity was greater in the AMPH-pretreated group than in the SAL-pretreated group (i.e., there was sensitization) in both the HOME and NOVEL environments. The magnitude of this sensitized response, however, was greater in the NOVEL than in the HOME environment, as indicated by a significant pretreatment by environment interaction. Again, the time-course of rotations during the Amphetamine Challenge (fig. 3, bottom panels) indicates that environment-related and sensitization-related overall differences in amphetamine response were associated with differences in the peak drug effect in both the HOME and NOVEL groups.
Experiment 2.
Figure 4 shows the effect of 0.75, 1.5, 3.0 and 6.0 mg/kg of amphetamine i.p., before (ACUTE) and after (SENSITIZED) intermittent amphetamine treatment with 4.0 mg/kg of amphetamine i.p.. In both the HOME (fig. 4, left panel) and in the NOVEL (fig. 4, middle panel) groups intermittent amphetamine resulted in a shift to the left in the dose-effect curve (i.e., produced sensitization). The magnitude of this shift, however, was significantly greater in the NOVEL group than in the HOME group, as indicated by an environment by sensitization interaction (analysis of covariance). In the HOME group sensitization was characterized by a shift to the left by about 1.1 mg/kg (see “Data Analysis”), and by about 2.9 mg/kg in the NOVEL group. Therefore, the shift for the NOVEL group was 2.6 times that for the HOME group.
Interestingly, exposure to a NOVEL environment appeared to enhance the acute response to amphetamine, relative to HOME environment, at the doses of 0.75 (74.6 ± 20.0 vs. 35.2 ± 18.5) and 1.5 mg/kg (220.2 ± 81.8 versus 89.0 ± 38.3), but not at the doses of 3.0 (276.0 ± 44.7 vs. 284.2 ± 38.9) or 6.0 mg/kg (677.2 ± 178.7 vs. 605.8 ± 53.0). However, it is important to stress that the aim of experiment 2 was to compare the dose-effect curves before and after sensitization using a within-subjects experimental design with a relatively small number of animals in each group (N = 4-5). The hypothesis that the effect of environment on the acute psychomotor response to amphetamine are limited to low doses amphetamine should be further investigated with a more appropriate experimental design.
Experiment 3.
Figure 5 shows the concentrations of amphetamine in blood plasma and in the striatum of HOME and NOVEL groups, 50 min after either a single injection (HOME-AMPH 1 and NOVEL-AMPH 1 groups) or the last of seven consecutive daily injections of 2.0 mg/kg amphetamine (HOME-AMPH 7 and NOVEL-AMPH 7 groups). There were no significant differences between the concentrations of amphetamine produced by the first vs. the seventh injection of amphetamine, nor between the HOME vs.NOVEL conditions (all P > .114).
Discussion
The following major findings are reported in the present study. 1) Repeated amphetamine treatments paired with exposure to a NOVEL environment result in greater rate of sensitization (experiment 1), a greater sensitization after drug withdrawal (experiment 1), and a greater sensitization-induced shift in the dose-effect curve (experiment 2), relative to the same treatments given to rats tested in a HOME environment. 2) The conditioned rotational response to drug-associated stimuli in animals that had previously received amphetamine in the NOVEL environment was greater than in those that received the drug in the HOME environment (experiment 1). 3) HOMEvs. NOVEL differences in peak amphetamine effects do not depend on differences in plasma or striatal amphetamine concentrations (experiment 3).
Environmental modulation of the acute psychomotor response to amphetamine.
We have previously reported that the acute psychomotor response to amphetamine is greater when the drug is administered in a NOVEL vs. a HOME environment (Badianiet al., 1995a, b, c). Although the results of experiment 1 confirm these previous findings, the results of experiment 2 suggest that this effect of environment on the acute response to amphetamine is seen only with low doses (<3.0 mg/kg). It is quite possible that the exposure to a NOVEL environment could not increase further the already robust behavioral response induced by higher doses of amphetamine. This hypothesis requires, however, further investigation (see also Results section). Nevertheless, the fact that the NOVEL environment failed to increase the acute effects of 3.0 and 6.0 mg/kg of amphetamine, whereas it enhanced the sensitized response to the same doses (see below) should not be considered surprising. We have reported previously that the effect of a NOVEL environment on sensitization to psychostimulants appears to be independent of its effect on the acute drug response (Badiani et al., 1995a, b). Indeed, there is some evidence that different mechanisms may be involved in amphetamine-induced increases of NAcc dopamine in sensitized vs. nonsensitized animals. For example, the increase in NAcc dopamine response to acute amphetamine is a calcium-independent process, whereas the sensitization-related enhancement of this response appears to be calcium-dependent (Warburton et al., 1996).
It is not known what is responsible for the effects of a HOMEvs. a NOVEL environment on the acute psychomotor response to amphetamine. The results of experiment 3 suggest, however, that this phenomenon is not due to HOME vs. NOVEL differences in the absorption, bioavailability or distribution of amphetamine to the brain, because there were no group differences in the concentration of amphetamine in either plasma or striatum. Thus, the same brain concentrations of amphetamine appears to be more effective in activating behavior when the animals are exposed to a NOVEL environment. One possibility is that exposure to a NOVEL environment potentiates the acute effects of amphetamine because of its stress-inducing properties (Badiani et al., 1995c; Friedman and Ader, 1967; Hennessy et al., 1977). Indeed, it is has been reported that exposure to stressors can potentiate the acute effects of amphetamine on stereotyped behavior (Anisman et al., 1985; Williams and Barber, 1989). In an initial test of this hypothesis we found that although exposure to a NOVEL environment can activate the HPA axis and increase plasma corticosterone, HOMEvs. NOVEL differences in the acute effects of amphetamine on rotational behavior are not dependent on a stress- (novelty) induced release of adrenal hormones, because they are not eliminated by adrenalectomy (Badiani et al., 1995 c). Thus, if the enhancing effects of a NOVEL environment are due to its stress-inducing properties, other effects of stress, independent of the adrenal response, must be involved. For example, there is evidence that CRH mechanisms participate in the stress response independently of their activating effects on the HPA axis (Berridge and Dunn, 1989;Britton et al., 1986; Cador et al., 1993). The hypothesis that CRH contributes to the enhancement in the acute effect of amphetamine seen in the NOVEL environment remains to be tested.
In a preliminary study we also investigated whether the enhancement in the acute response to amphetamine seen in the NOVEL environment was coupled with a correspondent enhancement of dopamine transmission in the dorso-lateral striatum or accumbens, but no differences in amphetamine-induced dopamine release were found (Robinson and Badiani, 1994). This suggests that the modulatory action of environment could be mediated by changes taking place downstream from the dopaminergic terminals in the striatum (e.g., by changes in the activity of glutamatergic afferents from the prefrontal cortex, amygdala, thalamus or hippocampus).
Environmental modulation of amphetamine sensitization.
Our results confirm and extend our previous reports that repeated amphetamine administrations result in greater sensitization when the treatments are given in a NOVEL vs. a HOME environment (Badiani et al., 1995a, b, c). In these earlier studies a single dose of amphetamine was used and the expression of sensitization after a long period of drug withdrawal was not assessed. We report now that the enhancement in amphetamine sensitization observed in a NOVELvs. a HOME environment is an enduring phenomenon, being evident when an amphetamine challenge was administered after 1 wk of withdrawal from the drug. This is important because there is some evidence that the neuroadaptations responsible for the development of sensitization after brief periods of withdrawal from the drug may be different from those involved in its expression after longer periods withdrawal (Ackerman and White, 1992; Kalivas and Duffy, 1993; Kalivaset al., 1993; Robinson et al., 1988; Segal and Kuczenski, 1992a, b). Furthermore, we found that sensitization resulted in a parallel shift to the left in the dose-response curve to amphetamine for a range of doses from 0.75 to 6.0 m/kg (consistent with the idea that sensitization is accompanied by an increase in drug potency), and that the shift was greater when the treatments were given in the NOVEL environment.
We do not know what mechanisms might account for these effects of environment on amphetamine sensitization. Consistent with the reports of others (for a review see Robinson and Becker, 1986) we found no differences in the concentration of amphetamine in either plasma or striatum after the seventh vs. the first amphetamine injection (experiment 3) in either the HOME or the NOVEL group. This indicates that dispositional changes did not contribute significantly to the development of amphetamine sensitization or to its modulation by environmental factors.
As for the acute effects of amphetamine, the enhancing effects of a NOVEL environment on amphetamine sensitization could be due the actions of a novel environment as a stressor. Indeed, there is evidence that animals exposed to intermittent stress develop sensitization to the psychomotor activating effects of amphetamine (Antelman et al., 1980; Badiani et al., 1992; Robinson et al., 1985). However, we have found that adrenalectomy does not diminish HOME vs. NOVEL differences in amphetamine sensitization (Badiani et al., 1995 c), suggesting that if stress-related changes are involved at all, they must be independent of the adrenal response to stress. Alternatively, the stress associated with exposure to a NOVEL environment may act by impinging more directly on the neural substrates of amphetamine sensitization. One such substrate might involve the mesolimbic dopamine system, which has been shown to undergo sensitization after repeated amphetamine.
It may also be possible to think about HOME vs. NOVEL differences in amphetamine sensitization within the framework of associative learning theory, because the expression of behavioral sensitization can be gated by stimuli associated with drug treatments (Anagnostaras and Robinson, 1996; Post et al., 1981; Tilson and Rech, 1973; Vezina and Stewart, 1984). It has been argued, for example, that sensitization may be due in part to an enhancement in drug response produced by the ability of amphetamine-associated stimuli to acquire CS properties, thereby producing a CR (Hinson and Poulos, 1981; Pert et al., 1990; Tilson and Rech, 1973) and/or modifying the drug response (Anagnostaras and Robinson, 1996; Badiani and Robinson, 1994; Stewart, 1992; Stewart and Badiani, 1993). Indeed, the response to a Saline Challenge in experiment 1 (figs. 1B and 4) indicates that there was a CR to treatment-associated cues under both environmental conditions, and that this conditioned hyperactivity was greater in the AMPH-NOVEL group than in the AMPH-HOME group. It is not clear what kind of cue(s) elicited this CR and produced group differences in its magnitude. Amphetamine-associated cues can be divided in three types. The first type of cue is represented by the injection procedures, such as handling and needle prick, and is common to both environmental conditions. The second type of cue is represented by amphetamine-induced interoceptive cues, and, presumably, is also common to both environmental conditions. (It is possible, however, that both interoceptive cues and injection-related cues had different salience under the HOME vs. the NOVEL conditions because of the different level of arousal or stress experienced by the animals.) The third type of cue is unique to the NOVEL group and includes transportation to the testing room and exposure to a novel test cage. Whatever cue(s) is involved, an analysis of the magnitude and time-course of the conditioned hyperactivity seen in amphetamine-pretreated animals during the Saline Challenge, indicates this CR is not sufficient to account for the phenomenon of amphetamine sensitization. First, the magnitude of the CR during the Saline Challenge (AMPH-pretreated minus SAL-pretreated saline-induced activity) was negligible in comparison with the magnitude of sensitization during the Amphetamine Challenge (AMPH-pretreated minus SAL-pretreated amphetamine-induced activity). There were CRs of 10 and 37 rotations in the AMPH-HOME and AMPH-NOVEL groups, respectively,vs. sensitization-induced increases of 168 and 295 rotations, in the same groups. Second, the CR was maximal in the five minute interval immediately after the Saline Challenge, whereas the sensitization- and environment-related differences in drug effect were maximal between 30 and 50 min after the Amphetamine Challenge. Thus, if the CR elicited by amphetamine-associated cues cannot entirely account for the expression of amphetamine sensitization and for the environmental modulation of sensitization, it appears that amphetamine-associated cues must have acted by modifying the magnitude of the drug response.
Conclusions.
The present data indicate that environmental stimuli associated with amphetamine treatment have an enduring and powerful influence on the magnitude of amphetamine sensitization. This environmental control cannot be accounted for by changes in amphetamine pharmacokinetics. We have discussed how these effects of environment on amphetamine sensitization are possibly related to its actions as a stressor and/or to its ability to facilitate associative learning.
Acknowledgments
The authors thank Drs. David E. Moody and Rodger L. Foltz, and their coworkers at the Center for Human Toxicology of University of Utah for conducting the plasma and striatum assays. We thank also Amy Braunsteiner, Steve Simmerville, and Donna De Jonghe for the assistance in conducting the experiments.
Footnotes
-
Send reprint requests to: Dr. Aldo Badiani, Department of Psychology, The University of Michigan, 525 E. University St., Ann Arbor, MI 48109-1109.
-
↵1 This research was supported by Grant 02494 from the NIDA.
- Abbreviations:
- 6-OHDA
- six-hydroxydopamine
- SAL
- 0.9% NaCl solution
- AMPH
- d-amphetamine
- CS
- conditioned stimulus
- CR
- conditioned response
- NAcc
- nucleus accumbens
- HPA axis
- hypothalamo-pituitary-adrenal axis
- CRH
- corticotropin-releasing-hormone.
- Received January 7, 1997.
- Accepted April 21, 1997.
- The American Society for Pharmacology and Experimental Therapeutics