Amphetamine-induced withdrawal responding: effects of repeated drug administration

The effects of dose and repeated administration on the longer-term hypophagia produced by amphetamine in rats

Rats are hypophagic approximately 1-3 and 13-27h after receiving amphetamine (2.0mg/kg). This study examined how these short- and longer-term phases of hypophagia were affected by repeated administration of different amphetamine doses. Throughout eight five-day tests, the rats could lever press for food pellets for 1-hour periods beginning every three hours. On test day 1, the rats were treated with saline, and on test day 3, they were treated with a dose of amphetamine. Across tests, for one group, treatment on day 3 alternated between 0.0 (saline) and 0.5mg/kg amphetamine; for a second, group treatment on day 3 alternated between 1.0 and 2.0mg/kg amphetamine; and for a third group, treatment on day 3 was always 1.0mg/kg amphetamine. The patterns of food intake following day 1 saline and day 3 treatment were compared. Short-term food intake was abolished by 0.5, 1.0, and 2.0mg/kg amphetamine, and no tolerance was observed to this effect. Longer-term hypophagia was produced by 1.0 and 2.0 but not by 0.5mg/kg. Tolerance to longer-term hypophagia was seen when 1.0mg/kg alone was used as the day 3 treatment, but not when 1.0 and 2.0mg/kg were alternated across tests as the day 3 treatment. Short- and longer-term hypophagia were dissociated by threshold doses for elicitation and by differential tolerance. Occasional receipt of a higher amphetamine dose may sometimes increase the longer-term hypophagia produced by a lower dose.

Time-dependent effects of amphetamine on feeding in rats

Following administration of a moderate dose of amphetamine, rats appear to pass through a sequence of physiological/psychological states, including stimulant and depressant states. The present research evaluated whether these states could be inferred from time-dependent changes in feeding-related measures. Male rats were housed in individual stations (light-dark 12-12 h, free access to water) where, at 3-h intervals, they could respond for food for 1 h. The work requirement was fixed ratio 1, and each lever press produced six 94-mg food pellets. When the pattern of responding for food stabilized across the light-dark cycle, a series of 6 or 7 tests was run. During each test, rats received a saline treatment (1.0 ml/kg, subcutaneously) followed by a 48-h monitoring period, and then they received an amphetamine treatment (2.0 mg/kg, subcutaneously) followed by a 72-h monitoring period. Different groups were treated at either light onset or light offset. Lever presses and head-in-feeding-bin responses were monitored throughout these tests. Administration of amphetamine at light onset and at light offset produced cumulative food intake functions having four regions: post-treatment hours 1-6 (hypophagia), 7-12 (normal intake), 13-27 (hypophagia), and 28 and beyond (normal intake). The sequence, duration, and quality of the amphetamine-induced changes in food intake resembled those formerly seen in cue state and activity, and provided further evidence of a transient withdrawal state 20-24 h post-amphetamine treatment.

An activity indicator of acute withdrawal depends on amphetamine dose in rats

A moderate dose of amphetamine (AMPH) produces hypoactivity around 20 h post-administration. This hypoactivity may be an indicator of an acute withdrawal state. The purpose was to see how AMPH doses affected the expression of this hypoactivity and, by inference, AMPH-induced acute withdrawal. Rats were housed in individual open fields, with free access to food and water. Light-dark cycles were scheduled such that drug-elicited patterns could be readily detected. Animals first received a series of eight control treatments, and then a series of 10 experimental treatments spaced at 33-h intervals. Different experimental treatment groups received saline, 1.0 mg/kg, 2.0 mg/kg, or 4.0 mg/kg AMPH. The effects of these treatments on 33-h patterns of locomotor activity were observed. Control treatments produced no systematic time-dependent changes in activity beyond the first hour post-treatment. All doses of AMPH produced typical short-term effects: They markedly increased locomotion and/or stereotypy during the first 3 to 6 h post-treatment. Acute and chronic administrations of the 2.0 and 4.0 mg/kg doses also produced similar changes in longer term activity patterns: They produced hypoactivity 20 h later, followed by a recovery of activity around hour 25 post-treatment. The timing of amphetamine-induced hypoactivity and acute withdrawal may be independent of dose over a wide range of doses. Time-dependent changes in AMPH-induced state may influence motivation and drug-related assessments. The methodology described here may provide an easy and rapid way to investigate the determinants of AMPH-induced hypoactivity and acute withdrawal.

Persistent anxiogenic effects of a single or repeated doses of cocaine and methamphetamine: interactions with endogenous cannabinoid receptor ligands

As persistent behavioural changes, such as increased anxiety-related behaviours, can be predicted based on the phenomenon of psychostimulant-induced neuronal plasticity, the time course (3-, 5- and 10-day time points) of the effects of both a single and repeated (daily for 7 days) i.p. administrations of cocaine (COC) and methamphetamine (MA) on anxiety-related behavioural symptoms in the elevated plus-maze test were examined in mice. Furthermore, based on the reported interactions between brain dopamine versus cannabinoid (CB) receptors and the contribution of CB receptors to the occurrence of persistent anxiety-related behavioural symptoms, the interactions of the agonist CP 55940 (CP) and the endogenous ligands anandamide (arachidonylethanolamide: AEA), 2-arachidonylglycerol (ARA), N-arachidonyldopamine (NADA), noladin ether (NL), and virodhamine (VA) with the COC- or MA-induced anxiety-related behaviours were also studied. In both an acute experiment using a single COC (30 mg/kg) or MA (4 mg/kg) dose and a chronic experiment using repeated COC (15 mg/kg) or MA (2 mg/kg) doses, anxiety-related behavioural symptoms were observed similarly at 3- and 5-day time points, but disappeared at the 10-day time point. Among the CB ligands, the agonists CP, AEA, ARA, NADA, and NL provided strong protective effects against each parameter at 3- and 5-day time points. Therefore, it was concluded that both COC and MA caused persistent anxiety-related behavioural symptoms following both a single and repeated treatments. Since these anxiogenic effects were attenuated by the endogenous CB agonists, the involvement of brain CB receptors was suspected.

Effects of training dose on amphetamine drug discrimination: dose-response functions and generalization to cocaine

Rats were trained to discriminate one of three doses of amphetamine (AM), 0.5, 1, or 2 mg/kg, from vehicle (VEH) in a two-lever, food-reinforced, drug-discrimination task. The purpose of the study was to investigate the nature of the shift of the dose-response curve and generalization to cocaine (COC) as a function of training dose. In order to preclude potential differences among the groups in stimulus control, the three training-dose groups were required to perform the discrimination at high and equivalent levels of accuracy. The shift of the dose-response functions to the right as a function of increasing training dose was not parallel. The slope decreased as training dose increased. There was a dose-dependent increase in AM lever responding to test doses of COC that tended to be affected by training dose. The results suggest that proper evaluation of training-dose effects requires that groups be trained to equivalent levels of stimulus control.

Reproductive experience modulates dopamine-related behavioral responses

Reproductive experience (RE), i.e., mating, pregnancy, parturition and lactation, has long-term physiological effects. It reduces the basal levels of circulating prolactin in parous women, decreases the intensity of nocturnal and diurnal prolactin surges in multigravid rats during early pregnancy, as well as the hormonal and neurochemical responses to the dopamine receptor antagonists metoclopramide and haloperidol. In the present study, we evaluated the possible influences of RE on some dopaminergic-related behaviors: (1) acute responses to a new environment represented by an open-field arena plus injection stress; (2) modulation of behavior after a short-term withdrawal subsequent to 7 days amphetamine (AMPH) pretreatment; (3) stereotypy elicited by AMPH and apomorphine (APO); and (4) APO-induced hypothermia. In the 3-min open-field test, there was a decrease in locomotor activity as a function of RE. Behavioral depression was mild and AMPH pretreatment revealed RE alterations. APO-induced stereotyped behavior was slightly more intense in primiparous animals, although no significant differences were found in AMPH-induced stereotyped behavior. No differences were observed between intact and ovariectomized primiparous and nulliparous animals in APO-induced hypothermia. Our data suggest that RE modifies some DA-related behavioral responses. The physiological relevance of the phenomenon is discussed.