Pre-exposure of rats to amphetamine sensitizes self-administration of this drug under a progressive ratio schedule

 Two groups of male rats were tested to determine whether pre-exposure to d-amphetamine would enhance the motivation to self-administer the drug under a progressive ratio schedule of reinforcement. In the first phase of the experiment, one group of rats received d-amphetamine (2 mg/kg IP), while a second group received saline on alternate days for a total of ten injections. Following a 21-day drug withdrawal period, behavioral sensitization was confirmed by a significant increase in amphetamine-induced stereotypy in the d-amphetamine-pretreated group, relative to the saline-pretreated group. In the second phase of the study, all rats were implanted with chronic jugular catheters and trained to self-administer d-amphetamine (0.2 mg/kg per infusion) under a fixed-ratio schedule of reinforcement. The progressive ratio paradigm was then imposed for 7 consecutive days; d-amphetamine-pretreated rats attained significantly higher break points than saline-pretreated animals. These data suggest that pre-exposure to d-amphetamine may enhance the motivation to self-administer this drug. Received: 16 July 1997 / Final version: 22 October 1997     

The development of addiction to d-amphetamine in an animal model: same principles as for alcohol and opiate

We have established a rat model that reflects the course of development of alcohol and opiate addiction. The present study with d-amphetamine aimed to define general principles in the development of an addiction. Male rats had a continuous free choice between d-amphetamine solutions (100, 200 and 400 mg/l) and water for 47 weeks. An initial intake of high doses of d-amphetamine during the first weeks of drug choice was followed by an individually stable pattern of drug consumption of moderate drug doses. During this period of controlled consumption (from week 10 to week 40), the voluntary intake of d-amphetamine depended on individual factors (dominant rats: 0.37 ± 0.02 mg/kg per day, subordinate rats: 0.57 ± 0.05 mg/kg per day) and environmental variables (group housing: 0.21 ± 0.02 mg/kg per day, single housing: 0.41 ± 0.03 mg/kg per day). Beginning with week 41, voluntary d-amphetamine consumption progressively increased (1.9 ± 0.2 mg/kg per day in week 47), although the experimental conditions remained unchanged. Drug intake during a retest (free choice as before) after 6 months of drug deprivation revealed that the rats had persistently lost their control over drug intake and were no longer able to adjust drug taking to internal and external conditions. These addicted rats took very high drug doses, even when all d-amphetamine solutions but not water were adulterated with bitter tasting quinine (6.6 ± 0.6 mg/kg per day; age-matched controls: 0.37 ± 0.04 mg/kg per day). Forced intake of d-amphetamine for 47 weeks (7.1 ± 0.3 mg/kg per day) via the drinking fluid caused physical dependence (hyperreactivity during withdrawal) but did not lead to drug addiction (voluntary intake in the retest with adulteration: 0.42 ± 0.04 mg/kg per day). Both the temporal development and the prerequisites of psychostimulant addiction were in principle the same as for alcohol and opiates. Received: 3 April 1998/Final version: 26 August 1998     

Behavioral sensitization to nicotine is associated with behavioral disinhibition; counteraction by citalopram

This study investigated the effects of repeated nicotine treatment on locomotor activity and behavioral inhibition, and the influence of citalopram on the behavioral effects obtained. Male rats received daily subcutaneous injections of vehicle + vehicle (veh + veh), citalopram (5.0 mg/kg) + vehicle (cit + veh), vehicle + nicotine (1.0 mg/kg; veh + nic) or citalopram+nicotine (cit + nic). Acutely, nicotine stimulated locomotor activity, and repeated daily nicotine injections sensitized veh+nic rats to the nicotine-induced locomotor stimulation after 5, 10 and 15 treatment days, whereas in cit+nic rats, the enhancement of nicotine-induced locomotion was suppressed. However, when challenged with nicotine after citalopram withdrawal (−36 h), the cit + nic treated animals were also observed to be sensitized. In the elevated plus-maze, repeated nicotine treatment produced behavioral disinhibition, measured as an increase of time spent in and entries made into open arms (%), and chronic citalopram treatment attenuated the expression of behavioral disinhibition. Moreover, the degree of nicotine sensitization correlated to the behavioral disinhibition observed. In summary, these findings suggest that behavioral sensitization to nicotine is associated with behavioral disinhibition and that chronic citalopram treatment counteracts the expression of both phenomena. Since citalopram is a highly selective serotonin reuptake inhibitor, the effects of citalopram may be due to a facilitation of serotonin neurotransmission caused by the chronic citalopram treatment. Received: 2 December 1997/Final version: 16 August 1998     

Amphetamine pretreatment facilitates appetitive sexual behaviors in the female rat

Rationale

Intermittent treatment of rats with psychomotor stimulants induces behavioral sensitization to their motor-stimulating effects. This sensitization involves an increase in mesolimbic and nigrostriatal dopamine release, and in male rats, facilitates sexual behavior.

Objectives

The aim of this study is to investigate the effect of repeated injections of d-amphetamine on appetitive and consummatory sexual behaviors in female rats.

Materials and methods

Sexually experienced or naïve females were injected with either d-amphetamine (1 mg/kg, i.p.) or saline every other day for three injections each. After each amphetamine injection, females were placed either in a bilevel testing chamber or in their home cages. After saline injections, females were placed in bilevel chambers. Following a 3-week washout period, females were tested for sexual behavior in bilevel chambers in a drug-free state.

Results

Amphetamine pre-exposure facilitated the display of solicitations, hops and darts, and female–male mounting (FMM), regardless of whether the drug was paired with the testing environment.

Conclusion

Intermittent amphetamine pretreatment that induces behavioral sensitization facilitates appetitive sexual behaviors in female rats, as has been shown previously in male rats. This suggests that the physiological substrates that modulate sensitized responses to psychomotor stimulants also mediate sensitized appetitive responses to sexual cues, including solicitation, hops and darts, and FMM. As in male rats, this facilitation was a direct consequence of amphetamine sensitization and not due to conditioned associations between drug and test environment.

     

Post-weaning housing conditions influence the behavioural effects of cocaine and d-amphetamine

Post-weaning social isolation can induce profound and long lasting effects on an animal’s behaviour. The present study investigated the influence of post-weaning housing conditions on the sensitivity of rats to the behavioural effects of d-amphetamine and cocaine. The locomotor stimulant effects of both drugs were compared following acute and chronic administration. The influence of post-weaning housing conditions on the effects of d-amphetamine and cocaine on responding for food and for a conditioned reinforcer were also examined. Isolated rats showed enhanced locomotor activity on exposure to a novel environment. This difference was further exaggerated following administration of d-amphetamine (0.5 mg/kg) and cocaine (5 mg/kg). Isolated, but not enriched, rats exhibited sensitisation to the locomotor activating effects of repeated administration of a dose of 0.5 mg/kg d-amphetamine, whilst both groups sensitised equally to a dose of 1.0 mg/kg d-amphetamine. Rearing conditions did not affect sensitisation to cocaine (5, 10 mg/kg). Isolated rats exhibited a higher rate of responding for a conditioned stimulus and for food on a progressive ratio schedule of reinforcement, both of which were enhanced to a greater extent in isolates following administration of cocaine (5 mg/kg) and d-amphetamine (0.5 mg/kg). These results suggest that isolation rearing induces an enhancement in sensitivity to both the locomotor stimulant and reinforcing properties of amphetamine and cocaine. Received: 12 June 1996 / Final version: 17 October 1996     

Lack of sensitization to the effects of d-amphetamine and apomorphine on sensorimotor gating in rats

 This study assessed whether repeated injections of d-amphetamine or apomorphine could induce sensitization to the disruptive effects of these psychomotor stimulants on sensorimotor gating in rats. In the first experiment, rats were given six pre-exposures to either 2.0 mg/kg d-amphetamine or saline before being tested for the effects of d-amphetamine (0.0, 0.5, 1.0, 2.0 or 4.0 mg/kg, IP) on prepulse inhibition of acoustic startle (PPI) and locomotor activity. The tests for PPI confirmed that sensorimotor gating could be disrupted by a high dose of d-amphetamine (4.0 mg/kg). However, comparison of the dose-response curves for the drug and saline pre-exposed groups did not reveal evidence for sensitization to this d-amphetamine effect in drug-pre-exposed rats, despite indications that sensitization had developed to the locomotor stimulant effects of d-amphetamine. A similar pattern of results was obtained in a second experiment that examined the effects of apomorphine (0.0, 0.1, 0.2, 0.4 and 0.8 mg/kg, SC) on PPI and locomotion in rats pre-exposed to 2.0 mg/kg of this drug or its vehicle. These findings demonstrate that treatments which induce sensitization to the behavioral activating effects of psychomotor stimulants do not necessarily produce sensitization to the disruptive effects of stimulants on sensorimotor gating. The implications of these results for hypotheses linking sensitization-like processes to the etiology of schizophrenia are discussed. Received: 15 May 1997/Final version: 7 July 1997     

Enhanced conditioned inhibition following repeated pretreatment with d-amphetamine

We have shown that prior repeated exposure to d-amphetamine facilitates appetitive Pavlovian conditioning. However, animals sensitised in this manner also show elevated levels of stimulated activity. To investigate whether enhanced conditioning was dependent upon increased activity, a conditioned inhibition task was employed in the present study. Rats received d-amphetamine (2 mg/kg, IP) or vehicle once per day for 7 days. After a 7-day drug-free period, an activity assay confirmed that repeated d-amphetamine treatment markedly elevated the locomotor response to a subsequent challenge with 0.5 mg/kg d-amphetamine. Conditioning began 6 days later. In phase 1, stimulus A+ (light or tone) immediately preceded sucrose availability (excitatory conditioning). In phase 2, sucrose again was presented after A+ alone, but not after presentation of a compound of A+ with a second stimulus (AB−). Sensitisation enhanced the acquisition of conditioned approach behaviour to the excitatory stimulus A+ in phase 1. Furthermore, acquisition of conditioned inhibition to the stimulus compound, AB−, was also facilitated. Thus, sensitised rats showed reduced levels of responding to the stimulus compound far sooner than controls. Finally, a retardation test was carried out in stage 3, in which the inhibitory stimulus B- was paired alone with sucrose reward. Sensitised rats initially showed retarded acquisition of excitatory conditioned responding relative to controls, suggesting that B possessed stronger inhibitory associations in these animals. However, sensitised animals again exhibited higher levels of responding in later sessions, consistent with the enhanced excitatory conditioning shown in phase 1. These findings suggest that prior repeated d-amphetamine enhanced the acquisition of inhibitory and excitatory Pavlovian associations; a propensity not readily attributable to stimulated locomotor hyperactivity. Received: 29 December 1997/Final version: 21 July 1998     

Effect of d-amphetamine on prepulse inhibition of the startle reflex in humans

Rationale: Prepulse inhibition of the startle reflex (PPI) is attenuated in animals after administration of d-amphetamine and other drugs that stimulate mesolimbic dopamine activity. Objective: The aim of the present study was to evaluate the effects of d-amphetamine (20 mg) on a variety of psychophysiological and subjective measures, including PPI, in humans. Method: Thirty-six participants (18 women) participated in a double-blind, placebo controlled, repeated measures study. In one session, participants received d-amphetamine (20 mg) orally, and in the other session, participants received an identical appearing placebo. Participants were assessed at 60, 90, and 120 min after ingestion with a 5-min block of startle trials (six control trials and six prepulse trials) followed by subjective measures of stimulation and mood. Results: d-Amphetamine increased subjective measures of stimulation and euphoria, attenuated PPI, and increased heart rate, relative to placebo treatment. Conclusions: The effect of d-amphetamine on the subjective measures was substantial and consistent over time, while the effect on PPI was only observed at 90 min after ingestion, and the effect on heart rate was limited to 90 and 120 min after ingestion. Received: 22 June 1998/Final version: 23 November 1998     

Enhanced responsiveness to intraventricular infusion of amphetamine following its repeated systemic administration

Previous studies have demonstrated that long-term administration of d-amphetamine produces a progressive augmentation of behavior. In the present experiment, rats receiving repeated systemic injections responded to an intraventricular infusion of d-amphetamine with an augmented increase in locomotor activity. These results indicate that central mechanisms, rather than peripheral dispositional factors, subserve the enhanced behavioral response to repeated amphetamine administration.     

Super-reactivity to amphetamine toxicity induced by schedule of reinforcement

The chronic exposure of rats to a schedule of operant water reinforcement coupled with chronically restricted access to water sensitized the animals to intermittentd-amphetamine injections (0.31–2.5 mg/kg with intervals of 12–23 days between any two injections) in such a way that this drug came to produce catastrophic losses of body weight (32.4% of control levels). In the sessions whend-amphetamine was administered, the rats were also given a total of 12 brief electric shocks. Loss of body weight was unaccompanied by parallel changes in operant behavior performance, or in food or water intake. Remarkably, in other studies with the same interventions (sham schedule sessions, water deprivation, and foot shocks), with the exception that reinforcers were never delivered,d-amphetamine did not produce catastrophic falls in body weight. This super-reactivity tod-amphetamine toxicity may be mediated by a possible stressor action of the schedule of reinforcement. Its mechanism might be analogous to the known sensitization produced by classical experimental stressor stimuli to the repeated administration ofd-amphetamine.     

On the role of noradrenaline in psychostimulant-induced psychomotor activity and sensitization

Rationale Psychostimulant drugs exert their behavioral effects primarily through enhancement of monoaminergic neurotransmission. Augmented dopamine activity is thought to play a critical role in the psychomotor stimulant effects of amphetamine and cocaine, as well as in the development of long-term behavioral sensitization evoked by repeated exposure to amphetamine. However, despite the fact that brain dopamine and noradrenaline systems are closely interconnected, the extent to which noradrenergic transmission contributes to these behavioral effects of psychostimulants is a relatively unexplored issue. Objectives By inhibiting noradrenergic neurotransmission with the α₂-adrenoceptor agonist clonidine, the α₁-antagonist prazosin and the β-antagonist propranolol, we investigated the involvement of noradrenaline neurotransmission in the psychomotor stimulant and long-term sensitizing effects of d-amphetamine and cocaine in rats. Methods Clonidine (0.003–0.1 mg/kg), prazosin (0.1–3.0 mg/kg) and propranolol (1.0–3.0 mg/kg) were administered prior to d-amphetamine (1.0 mg/kg), cocaine (15 mg/kg) or apomorphine (1.0 mg/kg) and psychomotor activity was measured. In separate studies, clonidine (0.03 mg/kg), prazosin (1.0 mg/kg) or propranolol (3.0 mg/kg) were co-administered with d-amphetamine (2.5 mg/kg) or cocaine (30 mg/kg) for 5 days, and locomotor sensitization was assessed 3 weeks post-treatment. Results The psychomotor stimulant effect of d-amphetamine, but not that of cocaine or apomorphine, was dose-dependently inhibited by clonidine and prazosin, and enhanced by propranolol. Clonidine, prazosin, and propranolol did not influence the induction of sensitization by amphetamine or cocaine. Conclusions Enhancement of synaptic noradrenaline concentrations contributes to the psychomotor stimulant effect of d-amphetamine, but not cocaine or apomorphine. In addition, noradrenergic neurotransmission is not critically involved in the induction of psychostimulant sensitization.     

Activation of 5-HT1B receptors in the nucleus accumbens reduces amphetamine-induced enhancement of responding for conditioned reward

Previously, we have demonstrated that 5-hydroxytryptamine (5-HT) injected into the nucleus accumbens attenuates the potentiating effects of d-amphetamine on responding for conditioned reward (CR). The present studies examined the 5-HT receptor involved in this effect by investigating the effects of 5-HT agonists with differing affinities for 5-HT₁ and 5-HT₂ receptors on d-amphetamine-induced potentiation of responding for CR. Rats were trained to associate a light/tone stimulus (subsequently the CR) with water delivery. In a test phase, they were allowed access to a lever delivering the CR, and an inactive (NCR) lever. Responding on the CR lever was greater than responding on the NCR lever, indicating that the light/tone stimulus functioned as a CR. Responding for the CR was selectively potentiated by injections of d-amphetamine (10 μg) into the nucleus accumbens. This effect was reduced by injections into the nucleus accumbens of 5-CT (0.5 and 1 μg), RU24969 (10 μg), CP93,129 (1.25 and 2.5 μg) but not by DOI (10 μg) or 8-OH-DPAT (5 μg). The lower doses of 5-CT and CP93,129 did not reduce baseline responding for CR, or responding for water in a separate group of animals, indicating that the effects of these drugs were behaviourally selective. The higher doses abolished the CR effect, and in the case of 5-CT and RU24969 also reduced responding for water. All of the effective drugs share in common the ability to stimulate 5-HT_1B receptors, albeit with differing selectivities. The effect of CP93,129, the most selective of the 5-HT_1B agonists, to inhibit the response-potentiating effect of d-amphetamine was reversed by the5-HT_1B/1D antagonist GR127935 (3 mg/kg). The results indicate that activation of 5-HT_1B receptors within the nucleus accumbens attenuates the effects of a dopamine-dependent behaviour, and that activation of these receptors can oppose the behavioural effects of elevated mesolimbic dopamine transmission. Received: 22 April 1998/Final version: 28 July 1998     

Stimulating property of Turnera diffusa and Pfaffia paniculata extracts on the sexual behavior of male rats

Sexually potent and sexually sluggish/impotent male rats were treated orally with different amounts of Turnera diffusa and Pfaffia paniculata fluid extracts (0.25, 0.50, 1.0 ml/kg). While having no effect on the copulatory behavior of sexually potent rats, both plant extracts – singly or in combination – improved the copulatory performance of sexually sluggish/impotent rats. The highest dose of either extract (1 ml/kg) (as well as the combination of 0.5 ml/kg of each extract) increased the percentage of rats achieving ejaculation and significantly reduced mount, intromission and ejaculation latencies, post-ejaculatory interval and intercopulatory interval. Neither extract affected locomotor activity. These results seem to support the folk reputation of Turnera diffusa and Pfaffia paniculata as sexual stimulants. Received: 13 January 1998/Final version: 15 September 1998     

Effects of drugs of abuse and scopolamine on memory in rats: delayed spatial alternation and matching to position

 Drugs of abuse produce amnestic effects in humans and laboratory animals in a variety of tasks. Generally, only a few compounds have been examined in any particular procedure. It was the goal of the present studies to examine drugs of abuse of different pharmacological classes in rats responding under two behavioral schedules historically employed as experimental models of memory: spatial alternation and matching to position. One group of rats responded under a single-response spatial-alternation baseline with a 10-s delay and another group responded under a matching-to-position baseline with delay values of 3, 10 and 30 s. Performance under the spatial-alternation baseline was characterized by low variability and >90% accuracy. Under the matching-to-position baseline, saline control percent accuracy was >95% at 3 s, >85% at 10 s and >70% at 30 s. Under spatial alternation cocaine, d-amphetamine, pentobarbital, diazepam, phencyclidine, scopolamine and methscopolamine produced significant (P<0.05) effects on accuracy, whereas only cocaine, d-amphetamine, pentobarbital and phencyclidine disrupted accuracy under the matching-to-position baseline. These results suggest that spatial alternation may be a more sensitive baseline for determining drug effects on working memory in the rat. Received: 16 April 1997 / Final version: 25 November 1997     

Blockade of cocaine sensitization and tolerance by the co-administration of odansetron, a 5-HT3 receptor antagonist, and cocaine

The present experiment evaluated the ability of the 5-HT₃ antagonist, ondansetron, administered during chronic cocaine administration, to block the development of sensitization and tolerance induced by the intermittent or continuous administration of cocaine, respectively. Rats were pretreated with 40 mg/kg per day cocaine for 14 days by either SC injections or osmotic minipumps, or 0.9% saline, administered by SC injection. During this chronic (cocaine) treatment, all rats received a daily SC injection of 0–1.0 mg/kg ondansetron. The rats were then withdrawn from the pretreatment regimen for 7 days. On day 7 of withdrawal from the cocaine pretreatment all subjects received a 15.0 mg/kg IP cocaine challenge, and their behavior was then rated according to the modified Ellinwood and Balster scale for 60 min. The results indicated that daily injections of ondansetron had no consistent or significant effect on the subsequent behavioral response to cocaine in the saline control subjects. In contrast, daily injections of ondansetron with cocaine significantly blocked the development of sensitization with an inverted U-shape dose-response curve. In the continuous cocaine group ondansetron injections also attenuated the development of behavioral tolerance. The results therefore indicate that 5-HT₃ receptor stimulation during continuous and intermittent cocaine administration is an important link in the development of behavioral tolerance and sensitization. Received: 1 July 1996/Final version: 25 September 1996     

Evidence for dissociable mechanisms of amphetamine-and stress-induced behavioral sensitization: effects of MK-801 and haloperidol pretreatment

The present study examined the ability of pretreatment with MK-801 or haloperidol to block the induction of behavioral sensitization to amphetamine challenge by repeated immobilization stress in male Sprague-Dawley rats. Fifteen minutes before each of ten 30-min restraint sessions, rats were administered saline, MK-801 (0.01, 0.10 or 0.25 mg/kg IP) or haloperidol (0.10 or 0.25 mg/kg IP). Control rats received the same injection regimen without restraint. An additional experiment examined the ability of MK-801 to block the induction of sensitization by repeatedd-amphetamine. In this experiment, rats were administered saline or MK-801 (0.25 mg/kg IP) 15 min before each of ten amphetamine injections (1.0 mg/kg IP, administered under the same regimen as immobilization stress). Four days after the final immobilization or amphetamine injection, rats were tested for locomotor response to novelty, saline andd-amphetamine (1.5 mg/kg IP). Exposure to repeated immobilization stress significantly enhanced the locomotor response to amphetamine challenge but not to saline challenge whether rats were pretreated with saline, MK-801 or haloperidol. Secondary analysis of dose effects in each pretreatment group revealed that at 0.25 mg/kg, repeated MK-801 in itself induced sensitization to the response to amphetamine in control rats and potentiated stress-induced sensitization in restrained rats. In contrast, the sensitization induced by repeated amphetamine was attenuated by MK-801 pretreatment. Neither dose of haloperidol affected the locomotor response to saline or amphetamine in control or stressed rats. These results indicate that the effects of MK-801 on the induction of sensitization are complex and suggest that amphetamine-and stress-induced behavioral sensitization may arise through different mechanisms.     

Blockade of the expression of sensitization and tolerance by ondansetron, a 5-HT3 receptor antagonist, administered during withdrawal from intermittent and continuous cocaine

 The present experiment evaluated the ability of the 5-HT₃ antagonist, ondansetron, administered during withdrawal from chronic cocaine administration, to block the expression of sensitization and tolerance induced by the intermittent or continuous administration of cocaine, respectively. Rats were pretreated with 40 mg/kg/per day cocaine for 14 days by either SC injections or osmotic minipumps, or 0.9% saline, administered via osmotic minipump. During the first 5 days of withdrawal from this pretreatment regimen, all rats received a daily SC injection of 0–1.0 mg/kg ondansetron. On day seven of with-drawal from the cocaine pretreatment (2 days after the final ondansetron injection) all subjects received a 15.0 mg/kg IP cocaine challenge. Their behavior was then rated according to the Ellinwood and Balster (1974) scale for 60 min. The results indicated that daily injections of ondansetron, on days 1–5 of withdrawal from the pretreatment regimen, had no significant effect on the subsequent behavioral response to cocaine in the saline control subjects. In contrast, daily injections of ondansetron, on days 1–5 of withdrawal from intermittent cocaine administration, significantly blocked the expression of sensitization. In the continuous cocaine group, ondansetron injections, on days 1–5 of withdrawal from continuous cocaine administration, also blocked the expression of behavioral tolerance. The results therefore indicate that changes in 5-HT₃ receptor function are associated with the expression of tolerance and sensitization, respectively. Received: 1 April 1997/Final version: 28 July 1997     

Effect of d-amphetamine repeated administration on rat antioxidant defences

The purpose of this study was to evaluate rat tissue antioxidant status after repeated administration of d-amphetamine. Three groups of four rats each were used: control, d-amphetamine sulphate dosed (s.c., 20 mg/kg per day), and pair-fed. After 14 days of d-amphetamine daily administration, superoxide dismutase (CuZnSOD and MnSOD), catalase, glutathione peroxidase (GPx), glutathione reductase (GRed), glutathione-S-transferase (GST), glutathione (GSH), cysteine and thiobarbituric acid reactive substances (TBARS) were measured in liver, kidney, and heart. Various serum and urine parameters were also analysed. d-Amphetamine treatment induced an increase of liver GSH, as well as a decrease of cysteine and MnSOD levels in this organ. A small increase in serum transaminases was also observed in comparison to the pair-fed group. Hepatic levels of TBARS, GPx, GRed and CuZnSOD were found to be similar among the three groups of rats. d-Amphetamine treatment induced an increase of kidney GST, GRed and catalase levels, and an elevation of N-acetyl-β-d-glucosaminidase efflux to the urine, accompanied by a decrease in urinary creatinine, compared to the pair-fed group. In d-amphetamine treated animals, heart cysteine levels were significantly depleted when compared to the pair-fed group, but all three groups of rats were found to have similar heart antioxidant enzyme levels. These results indicate that repeated administration of d-amphetamine caused a certain degree of stress in liver and kidney, which was followed by adaptations of antioxidant defences. The mechanisms involved in d-amphetamine-induced toxicity may explain the different adaptations observed for the studied organs. Received: 19 October 1998 / Accepted: 11 January 1999     

Acamprosate suppresses the expression of morphine-induced sensitization in rats but does not affect heroin self-administration or relapse induced by heroin or stress

Acamprosate (calcium-acetyl homotaurinate) is a new compound used in the treatment of alcohol abuse. Because of the putative link between alcoholism and the endogenous opioid systems in both humans and laboratory animals, we tested in rats the effects of acamprosate on behavioral and neurochemical effects of opioid drugs related to their abuse potential. These included sensitization to the behavioral effects of morphine, morphine-induced dopamine (DA) release in the nucleus accumbens (NAS), intravenous (IV) heroin self-administration and relapse to heroin seeking in drug-free rats. In experiment 1, rats were injected daily with either morphine (10 mg/kg, SC) or saline for 14 days. Three days later in a test for the expression of sensitization, an injection of morphine (10 mg/kg) resulted in increased locomotor activity and enhanced DA release in the NAS in rats previously exposed to morphine. Acamprosate (two injections of 200 mg/kg; 12 h apart; IP) suppressed the expression of the sensitized responses, but did not alter the effects of morphine in drug-naive control rats. In experiment 2, it was found that acamprosate (two injections of 50–200 mg/ kg; IP) had no consistent effects on IV heroin self-administration (50–100 μg/kg per infusion) and, in experiment 3, that acamprosate (100–200 mg/ kg, IP) did not alter reinstatement of drug seeking induced by priming injections of heroin (0.25 mg/kg, SC) or a footshock stressor (15 min; 0.5 mA) after a 5- to 8-day period of extinction. Thus, although acamprosate attenuated the expression of sensitized locomotor activity and DA release in the NAS, it did not have any consistent effect on either the intake of heroin during the maintenance phase or the relapse to heroin seeking in a drug-free state. Thus, to the extent that the self-administration and the reinstatement procedures provide valid preclinical models for drug use and relapse in humans, our data suggest that acamprosate may not be effective in altering drug-taking behavior in heroin users. Received: 4 November 1997/Final version: 25 January 1998     

Effects of histamine agents on methamphetamine-induced stereotyped behavior and behavioral sensitization in rats

In this study, effects of histamine (HA) agents on methamphetamine (METH)-induced stereotyped behavior and behavioral sensitization were examined in rats. Pretreatment with a precursor of HA, L-histidine (750 mg/kg), significantly inhibited the METH (3 mg/kg)-induced stereotyped behavior, whereas pretreatment with an inhibitor of HA synthesis, α-fluoromethylhistidine (FMH) (100 mg/kg), an H₁ antagonist pyrilamine (5 mg/kg) or an H₂ antagonist zolantidine (5 mg/kg) enhanced it. The inhibitory effect of L-histidine on METH-induced stereotyped behavior was significantly blocked by coadministration of pyrilamine and zolantidine, indicating that the effect is mediated through H₁ and H₂ receptors. Moreover, chronic treatment with METH (3 mg/kg) significantly enhanced stereotyped behavior at the rechallenge with METH (1 mg/kg). Chronic treatment with L-histidine (750 mg/kg) plus METH inhibited the METH-induced argumentation of stereotyped behavior, while that with FMH (100 mg/kg), pyrilamine (5 mg/kg) or zolantidine (5 mg/kg) potentiated it. These findings suggest that the HA neuron system has an inhibitory role in METH-induced stereotyped behavior and behavioral sensitization. Received: 1 July 1996/Final version: 26 November 1996