Rapid decline of stereotyped behavior in rats during constant one week administration of amphetamine via implanted ALZET osmotic minipumps

Acquisition of nicotine self-administration in amphetamine and phencyclidine models of schizophrenia: A role for stress?

Nicotine use and dependence is very high in patients with schizophrenia. One possible reason is that altered dopamine or glutamate activity in schizophrenia enhances the reinforcing effectiveness of nicotine. We used animal models to test the hypothesis that a hyperdopaminergic state (induced by repeated intermittent injections of amphetamine) or altered glutamate function (subchronic injection of phencyclidine, PCP) facilitates spontaneous acquisition of nicotine self-administration in rats. In Experiment 1 animals in an amphetamine-induced sensitized state (AISS) did not differ from saline-injected controls in their acquisition and maintenance of nicotine self-administration. This effect was replicated in experiment 2, but it was also found that AISS rats and saline-injected controls showed higher rates of nicotine self-administration compared to uninjected controls. This difference was maintained across several fixed ratio and progressive ratio schedules of reinforcement. In Experiment 3 PCP treated rats and their saline-injected controls did not differ in nicotine self-administration. However, both groups showed consistently increased responding for nicotine on FR and PR schedules compared to an uninjected control group. Injection-stress appeared to influence the outcomes of these experiments in two ways. Firstly, injection stress potentially masked the impact of the AISS and PCP treatment on nicotine self-administration. Secondly, injection stress itself may have been sufficient to induce plastic changes in dopamine and glutamate systems, and these changes enhanced the acquisition and maintenance of nicotine self-administration. Further investigation is needed into the role of stress in the development of nicotine use and dependence, in the aetiology of schizophrenia and in their co-morbidity.

Differential sensitivity to amphetamine's effect on open field behavior of psychosocially stressed male rats

RATIONALE

Studies of socially housed rodents have provided significant information regarding the mechanisms of stress and of stress-related disorders.

OBJECTIVE

Since psychosocial stress is known to alter the functional activity of dopaminergic system, we employed amphetamine (AMP) to evaluate the involvement dopamine in mediating the behavioral consequences of psychosocial stress.

METHODS

Male rats housed two per cage were designated as dominant (DOM) or subdominant (Sdom) based on initial evaluations of agonistic behaviors and body weight changes. Diad-housed rats and a group of single-housed (SiH) rats were tested in an open field after injections of saline or amphetamine (0.9 or 2.7 mg/kg IP) prior to and again while diad-housing.

RESULTS

Compared to future DOM rats, saline-injected future Sdom rats entered the open field center less frequently, spent less time in rearing behavior and groomed less. At the pre-diad test AMP treatment elevated locomotor activity of all rats, while stimulation of center entries was more marked in future DOM rats. At the diad test, AMP's locomotor stimulant effect was evident in all experimental groups with DOM rats showing higher effects compared to Sdom and SiH rats. Amphetamine's stimulation of center entries in DOM rats was similar to the pre-diad test, but it was diminished in Sdom rats, while stimulation of rearing behavior was most evident in diad-housed rats.

CONCLUSION

The dopaminergic system modulates the psychosocial stress-induced differences in explorative and emotional behaviors. Furthermore, behavioral traits like frequency of grooming behavior and of center entries were predictive of future hierarchical status.

Decreased reinforcing effects of cocaine following 2 weeks of continuous D-amphetamine treatment in rats

RATIONALE

Recent studies have investigated D-amphetamine as a potential agonist medication for cocaine dependence. In rats, a 14-day continuous infusion of D: -amphetamine via osmotic mini-pump has been shown to decrease cocaine-reinforced responding under a progressive ratio (PR) schedule of reinforcement.

OBJECTIVES

This study was designed to assess the influences of the D-amphetamine treatment dose and self-administered cocaine dose on the magnitude of this effect.

MATERIALS AND METHODS

Experiment 1: rats were trained to self-administer 1.5 mg/kg/inj cocaine under a PR schedule, then implanted with D-amphetamine mini-pumps for 14 days (days 1-7, 5 mg/kg/day; days 8-14, 7.5 mg/kg/day). Breakpoints were evaluated throughout the treatment period and 14 days post-treatment. Experiment 2: rats were trained to self-administer cocaine under a PR schedule and initial dose-response curves were determined before implantation of D-amphetamine mini-pumps. During the 14-day D-amphetamine (5 mg/kg/day) treatment period, rats self-administered one of four cocaine doses (0.19, 0.38, 0.75, or 1.5 mg/kg/inj). A post-treatment PR dose-response curve and responding under a fixed ratio 1 (FR1) schedule were evaluated after mini-pump removal.

RESULTS

Experiment 1: breakpoints for 1.5 mg/kg/inj cocaine were unchanged by the increasing dose of D-amphetamine. Experiment 2: the PR dose-response curve was shifted downward after the treatment period in rats that had self-administered 0.19 and 0.38 mg/kg/inj cocaine. In contrast, rats in the 0.75 and 1.5 mg/kg/inj groups demonstrated increased rates of cocaine intake under an FR1 schedule after the treatment period.

CONCLUSIONS

These data suggest that continuous D-amphetamine treatment attenuates the reinforcing effects of cocaine.

Effects of anorectic drugs on food intake under progressive-ratio and free-access conditions in rats

The effects of two anorectic drugs, dexfenfluramine and phentermine, on food intake under different food-access conditions were examined. Experiment 1 compared the effects of these drugs on food intake under a progressive-ratio (PR) schedule and free-access conditions. Dexfenfluramine decreased food intake under both conditions, but the doses required to decrease intake under free-access conditions were higher than those required to reduce intake under the PR condition. Intermediate doses of phentermine sometimes increased breaking points, and higher doses decreased them. Phentermine decreased food intake at the same doses under both access conditions. Thus the potency of dexfenfluramine, but not phentermine, to decrease food-maintained behavior depended upon the food-access condition. Experiment 2 used a novel mixed progressive-ratio schedule of food delivery to study the duration of drug effects. Sessions consisted of five components separated by 3-hr timeouts. The ratio requirement reset at the beginning of each component and a new breaking point was obtained. Both dexfenfluramine and phentermine dose-dependently decreased breaking points early in the session. In some rats, compensatory increases in breaking point were observed. That is, breaking points later in the session increased over control levels, resulting in no change in the total number of food pellets earned for the session compared to control. The present findings suggest that the effects of some anorectic drugs depend upon the access conditions for food; increasing the effort to obtain food may enhance their ability to decrease food-maintained behavior.

Acute and chronic continuous methamphetamine have different long-term behavioral and neurochemical consequences

We compared two different methamphetamine dosing regimens and found distinct long-term behavioral and neurochemical changes. Adult rats were treated with 1-day methamphetamine injection (3x5 mg/kg s.c., 3 h apart) or 7-day methamphetamine minipump (20 mg/kg/day s.c.). The minipump regimen models the sustained methamphetamine plasma levels in some human bingers whereas the 1-day regimen models a naive user overdose. On withdrawal days 7 and 28, rats were acutely challenged with cocaine to test for behavioral sensitization and subsequently sacrificed for caudate and accumbens dopamine tissue content. Other rats were analyzed on withdrawal days 3, 7 or 28 using voltammetry in caudate slices. On withdrawal days 7 and 28, the methamphetamine injection but not the minipump rats showed behavioral cross-sensitization to cocaine. There was no change in baseline dopamine release, reuptake or sensitivity to quinpirole in any treatment group on either withdrawal day. However, consistent with the behavioral sensitization, cocaine had a greater effect in potentiating dopamine release and in blocking dopamine reuptake in methamphetamine injection versus saline irrespective of withdrawal day. The minipump group showed tolerance to the dopamine releasing effect of cocaine on withdrawal day 28 and had lower dopamine tissue content in the caudate versus the methamphetamine injection group. Dopamine turnover as measured by the DOPAC/dopamine ratio tended to be higher in the minipump-treated rats. These data suggest that the behavioral cross-sensitization seen in the methamphetamine injection rats could be in part due to the increased potency of cocaine in blocking dopamine reuptake and in increasing dopamine release. The decreased potency of cocaine in the caudate slices from the minipump-treated group may be related to decreased dopamine tissue content.

Optimisation of treatment by applying programmable rate-controlled drug delivery technology

A number of programmable rate-controlled drug delivery technologies have been developed during the last two decades with the aim of regulating the rate of drug delivery, sustaining the duration of therapeutic action and/or targeting the delivery of drug to a specific tissue. As a result, several therapeutically beneficial outcomes can be achieved, such as: (i) controlled delivery of a therapeutic dose at a desirable rate of delivery; (ii) maintenance of drug concentrations within an optimal therapeutic range for prolonged duration of treatment; (iii) maximisation of efficacy-dose relationship; (iv) reduction of adverse effects; (v) minimisation of the need for frequent dose intake; and (vi) enhancement of patient compliance. The treatment of illness can thus be optimised. To gain a better understanding of how to optimise the treatment of illnesses by applying programmable rate-controlled drug delivery technologies, this article reviews the scientific concepts and technical principles behind the development of various programmable rate-controlled drug delivery systems that have been marketed or are under active development. Finally, the roles of these technologies in optimising therapeutic outcomes in nine therapeutic areas are discussed.

Augmented methamphetamine-induced overflow of striatal dopamine 1 day after GDNF administration

Glial cell line-derived neurotrophic factor (GDNF) can attenuate the dopamine (DA)-depleting effects of neurotoxic doses of methamphetamine (METH) when given 1 day prior to the METH. The neurotoxic effects of METH may be due, in part, to sustained increases in extracellular levels of DA. It is therefore possible that GDNF may be altering the effects of METH by influencing extracellular levels of DA during the METH treatment. The purpose of the present study was to determine if GDNF has effects on extracellular levels of DA in the striatum by 24-h post-administration. GDNF (10 microgram in 2 microliter vehicle) or vehicle was injected into the right striatum or substantia nigra of anesthetized male rats. The next day the animals were anesthetized again and dialysis probes were positioned in both the right and left striata and perfused with artificial cerebrospinal fluid. Following the collection of baseline samples the rats were administered METH (5 mg/kg, s.c.). The METH injections dramatically increased extracellular DA levels on both sides of the brain. However, levels on the GDNF injected side were significantly greater than levels on the contralateral side. Basal levels of DA were not significantly different between the two sides, but levels of DA metabolites were elevated on the GDNF side. Post-mortem tissue levels of DA metabolites, but not DA, were also elevated in the striatum and substantia nigra. These results indicate that GDNF has significant effects on DA neuron functioning within 24 h of administration and that GDNF can augment DA overflow while inhibiting the neurotoxic effects of METH.

Nimodipine and haloperidol attenuate behavioural sensitization to cocaine but only nimodipine blocks the establishment of conditioned locomotion induced by cocaine

The classical conditioning of the behavioural effects of cocaine has been shown to contribute to behavioural sensitization. In the present experiments, it was demonstrated that the effects of cocaine in rats can be conditioned to contextual stimuli. Furthermore, sensitization to cocaine's locomotor effects were demonstrated, and shown to be context specific. Nimodipine (10 mg/kg, SC), an L-type dihydropyridine Ca2+ channel antagonist, appeared to completely block the establishment of conditioning of cocaine's effects, but only partially blocked sensitization to cocaine. Haloperidol (0.05 mg/kg, IP), a relatively specific D2 dopamine receptor antagonist, attenuated behavioral sensitization but had no influence on the establishment of the conditioned component of cocaine. These results indicate that the sensitization to, and the development of classical conditioning of, cocaine's behavioural effects can be pharmacologically dissociated, but that a non-associative process involved in sensitization is normally overridden by conditioning factors.

Dopamine receptor occupancy in vivo: behavioral correlates using NNC-112, NNC-687 and NNC-756, new selective dopamine D1 receptor antagonists

The ability of dopamine D2, mixed D1/D2 and selective D1 receptor antagonists, including NNC-112, NNC-687, NNC-756, to inhibit the in vivo binding of [3H]SCH 23390 or [3H]raclopride to dopamine receptors was studied in mice and rats. Furthermore, the dopamine-antagonistic effects of these drugs were also studied in various behavioral models. Significant levels of in vivo receptor blockade were required for antagonism of typical dopamine agonist-mediated behaviors. However, fewer D1 than D2 receptors had to be blocked to produce similar antagonistic effects. Thus, there may be a greater receptor reserve for D2 receptors than for D1 receptors.

A novel device for chronic intracranial drug delivery via microdialysis

A system is described for chronic intracranial drug administration in the rat using a modified in vivo microdialysis probe coupled to an Alzet model 2002 osmotic minipump. The results presented demonstrate that this system can be used for the chronic administration of quinolinic acid with minimal non-specific damage. Each pump delivered approximately 225 microliters of solution over a period of 19-20 days when tested in vitro. The dialysis units were uniform in function, delivering greater than 93% of the [3H]quinolinic acid initially loaded into the minipump. For in vivo analysis of this apparatus the dose of quinolinic acid tested produced extensive destruction of the striatum. The present system allows reliable drug diffusion over a relatively large area without pressure injection variability. In conclusion, we have developed a simple and inexpensive technique for administration of drugs into brain parenchyma with substantial advantages over previously used techniques.

Animal models of amphetamine psychosis: neurotransmitter release from rat brain slices

As animal models of psychosis, adult rats were exposed to different types of treatment with amphetamine: (1) subcutaneous implantation with osmotic minipumps or (2) daily intraperitoneal injections for 7 days and (3) one single amphetamine injection. Continuous administration caused tolerance, intermittent injections sensitization and the acute injection a behavioral arousal with stereotypes. All amphetamine treatments diminished the potassium-stimulated (50 mM) release of preloaded labelled dopamine from superfused striatal and frontal cortical slices in vitro. Only acute amphetamine enhanced the release of D-aspartate, an analogue of L-glutamate from frontal cortical slices. The results indicate that amphetamine profoundly affects the brain dopaminergic systems but less the glutamatergic systems. Amphetamine systematically enhanced the release of gamma-aminobutyrate (GABA) from striatal slices, but otherwise the GABAergic systems seem to be more sensitive to handling stress than to the administration of amphetamine. Besides dopaminergic mechanisms other interacting systems must thus be taken into account to explain behavioral responses to amphetamine.

Parker and Radow test of drug withdrawal aversion: opposite effect in rats chronically infused with sufentanil or amphetamine

In rats, cessation of periodic injections of morphine reduces a preference for a palatable saccharin solution presented in a choice with water, and this has been interpreted to reflect withdrawal malaise. We confirmed and examined this "Parker and Radow Model" using subcutaneously implanted osmotic minipumps as the means of drug delivery and the opiate, sufentanil, and the psychostimulant, amphetamine, as the treatment drugs; surgical removal of the pumps was used to initiate withdrawal. Thus, rats withdrawn after 2 weeks exposure to a sufentanil-delivering pump (0.25 microgram/hr) showed a decreased preference for the saccharin and animals exposed to an amphetamine pump (68 micrograms/hr) showed an increased preference, as compared to placebo-exposed controls. This pattern of effects was systematically replicated in new subjects using 4 weeks of treatment and 136 micrograms/hr amphetamine. Since the locomotor increasing and body weight decreasing effects of amphetamine were also demonstrated and the doses of amphetamine and sufentanil were in comparable dose ranges, it was concluded that the Parker and Radow procedure may be a reliable measure of opiate withdrawal, but under similar test and treatment conditions other processes may be operative in amphetamine-treated animals. Problems of measuring motivation of withdrawal, particularly of spontaneous withdrawal, were noted.

Enhanced behavioral stereotypies elicited by intrastriatal injection D1 and D2 dopamine agonists in intact rats

Five components of behavior elicited by dopamine (DA) agonists (locomotor hyperactivity, sniffing, oral activity, grooming and paw nibbling) were evaluated after bilateral infusion of the selective D1 agonist fenoldopam (SKF 82526; 2.5-10 micrograms), the selective D2 agonist quinpirole (LY 171555; 5-40 micrograms) and the muscarinic cholinergic antagonist scopolamine (5-20 micrograms) into the ventral striatum of awake, unrestrained rats. Simultaneous bilateral infusion of various dose combinations of fenoldopam (2.5-10 micrograms) and quinpirole (5-20 micrograms) elicited dramatic increases in stereotyped behaviors relative to the effects produced by corresponding doses of each drug alone. Stereotyped sniffing and paw nibbling (self-directed oral activity) were markedly enhanced, whereas conventional oral behaviors (licking, chewing and/or biting) were either slightly or not at all increased. These potentiated responses were reduced or blocked by concomitant infusion of either the selective D1 antagonist SCH 23390 (1 and 5 micrograms) or the selective D2 antagonist sulpiride (0.15 microgram). Scopolamine (10 micrograms) only slightly increased the effects of quinpirole (5 micrograms) on both sniffing and oral behaviors, whereas it dramatically potentiated the effects of fenoldopam (2.5 micrograms) on oral activity; sniffing was only slightly increased. The effects of both drug combinations were almost completely antagonized by infusion of either SCH 23390 (1 microgram) or sulpiride (0.1 microgram). The results demonstrate that the synergistic effects of co-activation of D1 and D2 receptors observed after systemic administration are mediated at least in part by an interaction at the level of the striatum. Differences and similarities between the behaviors expressed after various treatments are discussed.

Day and night locomotor activity effects during administration of (+)-amphetamine

Rats were given continuous subcutaneous amphetamine infusions (0, 2, 6, 10 and 20 mg/kg/day) via osmotic minipumps. The effects of these treatments on the locomotor activity of rats were determined over both light and dark phases of a 12-hr light/dark cycle for 336 consecutive hours. It was observed that tolerance to the locomotor stimulant actions of (+)-amphetamine is both dose- and light/dark cycle-dependent. Locomotor stimulation induced by the two highest doses remained high during both day and night throughout the period of treatment, except for the first few days and nights with the highest dose. Tolerance developed only to the effects of the two lower doses, and only during the day. Effects of the low doses on locomotor activity and on circadian patterns of locomotor activity are roughly similar to those previously observed with continuous administration of a selective dopamine D2 agonist. This behavioral similarity suggests that dopamine released by continuous administration of low doses of (+)-amphetamine may be producing its effects via selective actions on DA D2 receptors in vivo.

Behavior, striatal and nucleus accumbens field potential patterns and dopamine levels in rats given amphetamine continuously

Behavioral, electrophysiological (field potential recordings, analyzed by computer) and biochemical techniques were employed before, during and after 7 days of continuous administration of amphetamine to rats. Significant changes were observed using all three protocols. Behavioral alterations were greatest on the second day of treatment and progressively normalized during the remainder of treatment. Electrophysiological changes in the striatum were significant only on the second day of treatment, while electrophysiological alterations in the nucleus accumbens were significant on all treatment and recording days, and the magnitude of the changes paralleled the pattern of overt behavioral changes. Levels of DA in the striatum progressively decreased from normal throughout the treatment, declining to less than half of the control level by the sixth day of treatment. In contrast, DA levels in the nucleus accumbens were augmented on the second day of treatment and progressively approached the control level as treatment continued, again paralleling behavior and changes in the electrophysiology of the nucleus accumbens. These results indicate that continuous administration of amphetamine in rats differentially affects electrical activity in the striatum and nucleus accumbens as well as concentrations of DA. In addition, these results have implications for the study of paranoid schizophrenia.

Amphetamine discrimination: effects of dopamine receptor agonists

Dopamine (DA) D-1 and D-2 receptor agonists and antagonists were characterized in receptor binding and adenylate cyclase assays with respect to affinity, selectivity and efficacy. The ability of the ligands to interact with the discriminative stimulus effects of d-amphetamine (AMPH) was then assessed. The D-2 agonists, quinpirole, pergolide and CH 29-717, substituted completely for AMPH while neither partial (SKF 38393 and SKF 75670) nor full D-1 receptor agonists (SKF 89626 and SKF 81297) substituted. On the other hand, the selective D-1 and D-2 antagonists all blocked AMPH. The substitution for AMPH by pergolide was blocked by raclopride but not by SCH 23390, indicating D-2 mediation. In contrast, the motor effects of pergolide were blocked by both raclopride and SCH 23390, indicating mixed D-1/D-2 receptor involvement. These results suggest that D-1 and D-2 are equally involved in the expression of functional effects in the DAergic motor systems. Conversely, D-2 receptors may play a primary role in the DA systems involved in the AMPH cue; furthermore, the D-1 and D-2 receptors in the systems are relatively uncoupled.

Central nervous system stimulants: neuropharmacological mechanisms

The mechanisms underlying CNS-stimulant drug discrimination are discussed. Although different doses of CNS stimulants may produce qualitatively different cues, it appears that a relatively low dose of d-amphetamine (e.g., 1 mg/kg) elicits a "general" CNS-stimulant cue. Presynaptically, this cue may primarily depend on release of endogenous dopamine whereas inhibition of dopamine reuptake, per se, is insufficient to elicit the cue. Postsynaptically, the involvement of both dopamine D-1 and D-2 receptors is implicated. Furthermore, in the drug discrimination situation, D-1/D-2 receptors may be coupled differently than in dopamine-dependent locomotor activation. Anatomically, CNS-stimulant drug discrimination may depend primarily on mesolimbic dopamine systems.

Effect of chronic subcutaneous minipump infusion of lisuride upon locomotor activity of rats

Male Wistar rats were infused continuously for 14 days with lisuride 0.25 mg/kg/day or with vehicle via subcutaneously implanted osmotic minipumps. Locomotor activity was measured at 5 hours, 1, 7 and 14 days after implantation. Thereafter the minipumps were removed and 1, 7 and 21 days later the locomotor activity was recorded after a subcutaneous challenge dose of lisuride 0.1 mg/kg. In the course of continuous infusion the lisuride-treated rats showed a persistent stimulation of locomotor activity which remained almost constant throughout the whole period of exposure. At all intervals after removal of the minipumps lisuride challenge produced a less pronounced locomotor stimulation in lisuride-infused rats compared to vehicle-infused animals. This observation contrasts with the findings after chronic subcutaneous bolus treatment of rats with 0.25 mg/kg lisuride once daily for 29 days which resulted (1) in a progressive enhancement of the locomotor stimulatory effect and (2) in a longlasting hyperresponsiveness towards a subcutaneous challenge dose of lisuride 0.025 mg/kg. These results are discussed with respect to the advantage of the constant availability of lisuride at central dopamine receptors for the management of patients with advanced Parkinson's disease showing fluctuations in motor performance probably related to the kinetics of conventional oral therapy.

An analysis of the 'tolerance' which develops to analgetic electrical stimulation of the midbrain periaqueductal grey in freely moving rats

Electrical stimulation of the ventral midbrain periaqueductal grey (PAG) elicits an opioidergic antinociception against noxious heat and pressure in freely moving rats. Recurrent stimulation was associated with a gradual decline and eventual loss of this stimulation-produced antinociception (SPA). This could be reinstated by an increase in current intensity and this reinstatement was preventable by naloxone. The current intensity--antinociception (dose--response) curve was shifted to the right in recurrently stimulated rats and parallel to that in naive animals. The loss of SPA upon repetitive simulation did not represent a conditioning phenomenon. Thus, tolerant rats exposed to all cues which accompanied stimulation revealed no (compensatory) hyperalgesic response--but rather a slight antinociception. Further, SPA recovered spontaneously in tolerant rats. Moreover, 'extinction' by repeated exposure to all cues accompanying stimulation did not restore or accelerate the recovery of SPA in tolerant animals. Tolerant rats showed no depletion in midbrain PAG or other CNS or hypophyseal pools of beta-endorphin, Met-enkephalin or dynorphin indicating that a depletion of endogenous opioid peptides does not underlie the tolerance which develops to stimulation. In fact recurrently stimulated rats did not show any of the pronounced effects upon CNS pools of opioid peptides which are seen with long-term stress. Moreover, repetitively stimulated rats revealed no indications of stress as judged by a diversity of stress-sensitive parameters; basal nociceptive threshold, core temperature, ingestive behaviour, body weight, adrenal weight and hypophyseal secretion of beta-endorphin and prolactin. The data offer two major conclusions. Firstly, the gradual loss of analgesia upon recurrent stimulation of the midbrain PAG does not reflect a generalized debilitation or stress and neither a conditioning phenomenon nor a depletion of pools of endogenous opioid peptides. Rather it closely corresponds to the pharmacological definition of tolerance and may reflect a process occurring at the level of the opioid receptor and coupled processes. This finding explains the cross-tolerance which we observe recurrently stimulated rats to display to morphine. Secondly, this SPA is not a form of stress-induced analgesia and rats undergoing recurrent stimulation reveal no indications of stress as judged by biochemical, physiological and behavioural parameters.

Environment-specific conditioning and sensitization with (+)-amphetamine

Learning variables have an important role in determining the behavioral effects of some pharmacological treatments. Environmental control of sensitization and conditioning of the stimulant effects of (+)-amphetamine (AMPH) were studied in two experiments. Rats were given 6 1-hr habituation sessions in automated activity chambers conducted every second day. Two days later the 12 rats in the paired group in each study received AMPH (2.0 mg/kg) followed immediately by placement in the chambers for 1 hr whereas rats in the unpaired groups received saline. All rats were injected the following day and left in their home cages afterwards. At this time the paired groups received saline and the unpaired groups received AMPH. Three days later a second pairing and subsequent home cage injection was administered, using the same procedure. Immediately prior to the test session (4 days after the last pairing session) all rats in the sensitization experiment received AMPH and those in the conditioning study received saline. During pairing sessions AMPH treated rats exhibited more vertical activity than controls. On the saline test session in the conditioning study there was still a significant group difference demonstrating environment-specific conditioning. There was no evidence of sensitization on vertical activity; however, a significant difference in horizontal activity was seen on the AMPH test session. Results suggest that these two phenomena can be dissociated behaviorally and may not follow the same time-course.

Biochemical and behavioral changes in rats during and after chronic d-amphetamine exposure

Two groups of rats were implanted with ALZET minipumps to deliver vehicle or a theoretical amount of 1 mg/kg per h of d-amphetamine (A) for 12 days. After 3 days of A-exposure, motor movements and stereotypic behavior were markedly increased. Subsequent testing during A-exposure showed that motor movements and stereotypic behavior remained significantly increased but declined. After removal of the pumps, these effects disappeared and no differences at rest, during stress or A challenge, were apparent in either group. Animals sacrificed after 3 days of drug exposure, showed a drastic decrease in cardiac, but not adrenal, catecholamine levels. In the brain, norepinephrine (NE) levels were markedly decreased in the frontal cortex, hypothalamus, caudate, pons-medulla and cerebellum. Epinephrine (E) levels were unaffected and dopamine (DA) levels were decreased in most areas without reaching statistical significance. Plasma corticosterone levels were similar in both groups. Animals in both groups sacrificed about 25 days after pump removal were biochemically similar. Under our conditions, A-exposure produced marked behavioral and biochemical changes but there was no evidence of residual abnormalities after cessation of drug treatment.

Reduction of 3H-spiroperidol binding in rat striatum and frontal cortex by chronic amphetamine: dose response, time course and role of sustained dopamine release

After 5 days of continuous treatment with d-amphetamine base in doses greater than 0.5 mg/kg/h maintained by subcutaneously implanted osmotic minipumps, specific binding of 3H-spiroperidol was reduced in rat striatum and frontal cortex as previously reported. These effects were dose-dependent at lower doses of amphetamine, whereas with higher doses an apparent ceiling for the reduction in binding was reached at approximately 70% of control values. Similarly, increasing the exposure time to amphetamine for up to 14 days only slightly augmented the reduction in 3H-spiroperidol binding already present after 5 days of treatment. In rats treated for 5 days with amphetamine, concomitant treatment with the dopamine (DA) synthesis inhibitor alpha-methyl-p-tyrosine prevented the decrease in 3H-binding in corpus striatum, and attenuated the decrease in frontal cortex. Furthermore, in rats with unilateral 6-hydroxydopamine lesions of the nigro-striatal DA tract, 5 days of chronic amphetamine had no significant effect on 3H-spiroperidol binding in the denervated striatal tissue. Since a major effect of amphetamine is to release DA from nerve terminals, these results indicate that the reduction of DA receptors by chronic amphetamine in the striatum is mediated by sustained release of DA.