Behavioural effects of intracerebral amphetamine in the marmoset

Chronic cocaine but not chronic amphetamine use is associated with perseverative responding in humans

RATIONALE

Chronic drug use has been associated with increased impulsivity and maladaptive behaviour, but the underlying mechanisms of this impairment remain unclear. We investigated the ability to adapt behaviour according to changes in reward contingencies, using a probabilistic reversal-learning task, in chronic drug users and controls.

MATERIALS AND METHODS

Five groups were compared: chronic amphetamine users (n = 30); chronic cocaine users (n = 27); chronic opiate users (n = 42); former drug users of psychostimulants and opiates (n = 26); and healthy non-drug-taking control volunteers (n = 25). Participants had to make a forced choice between two alternative stimuli on each trial to acquire a stimulus-reward association on the basis of degraded feedback and subsequently to reverse their responses when the reward contingencies changed.

RESULTS

Chronic cocaine users demonstrated little behavioural change in response to the change in reward contingencies, as reflected by perseverative responding to the previously rewarded stimulus. Perseverative responding was observed in cocaine users regardless of whether they completed the reversal stage successfully. Task performance in chronic users of amphetamines and opiates, as well as in former drug users, was not measurably impaired.

CONCLUSION

Our findings provide convincing evidence for response perseveration in cocaine users during probabilistic reversal-learning. Pharmacological differences between amphetamine and cocaine, in particular their respective effects on the 5-HT system, may account for the divergent task performance between the two psychostimulant user groups. The inability to reverse responses according to changes in reinforcement contingencies may underlie the maladaptive behaviour patterns observed in chronic cocaine users but not in chronic users of amphetamines or opiates.

A rodent model of spontaneous stereotypy: initial characterization of developmental, environmental, and neurobiological factors

Stereotypies are patterns of motor behavior that are repetitive, excessive, topographically invariant, and that lack any obvious function or purpose. In humans, stereotyped behaviors are associated with psychiatric, neurological, and developmental disorders. In animals, stereotypy has been frequently associated with adverse environmental circumstances and often related to alterations in striatal dopamine. To assess the development of stereotyped behaviors and to test the hypothesis that these behaviors are associated with environmental restriction, deer mice were housed in either standard laboratory cages or larger, enriched cages, and the development of stereotypy was followed from weaning over a 17-week period. Standard-caged deer mice engaged in stereotyped behaviors at a higher rate and developed these behaviors more quickly when compared to animals in enriched caging. Additionally, enriched caging was associated with higher rates of patterned running, whereas jumping and backward somersaulting were typically observed in standard cages. In addition, there was a significant effect of litter, but no effect of sex or cage, on the time to develop stereotypy. No differences were found in the density of either striatal D1 or D2 dopamine receptors or the concentration of striatal dopamine or its metabolites as a function of rearing condition or as a function of whether the animals developed stereotypy. These results characterize the development of stereotypies in this species, demonstrate the importance of environmental conditions in the genesis of stereotypy, and suggest that alterations in striatal dopamine content or dopamine receptor density do not account for the expression of stereotyped behaviors in this model.

Effects of low doses of cholinesterase inhibitors on behavioral performance of robot-tested marmosets

To investigate at which dose levels undesirable effects started, behavioural performance and several physiological parameters were measured in marmosets (Callithrix jacchus) after soman (1.75 and 3.5 micrograms/kg), sarin (3 and 6 micrograms/kg), physostigmine (10 and 20 micrograms/kg), and pyridostigmine (200 and 400 micrograms/kg). Effects on performance were investigated with a discrete-trial, two-choice visual discrimination task and a hand-eye coordination task. The former test appeared more sensitive to disruption than the hand-eye coordination task. "Motor speed" was not disrupted by any of the four compounds. However, "choice time" as well as "no attempts" increased and were clearly more disturbed by soman and physostigmine than by sarin and pyridostigmine. All effects had disappeared after 24 h. Except for a small effect of sarin on heart rate and blood pressure, none of the cholinesterase (ChE) inhibitors affected a number of physiological parameters at behavioural effective does that caused a profound ChE inhibition in blood. Take together, these results strongly suggest that both soman and physostigmine may interfere with higher CNS functions at low dose levels. These effects may go undetected because physical signs are absent.

Microinjections of dopamine agonists in the nucleus accumbens increase ethanol-reinforced responding

Long-Evans rats (N = 3) were trained to lever press on a fixed-ratio 4 (FR 4) schedule with ethanol (10% v/v) presented as the reinforcer. Each rat received a total of six bilateral nucleus accumbens microinjections, one per week. They were tested with one physiological saline control, three 20.0-microgram/brain d-amphetamine, and two 6.0-microgram/brain quinpirole injections given 10 min prior to operant sessions. Ethanol-reinforced responding terminated after approximately 10 min during control sessions. Microinjections of the D2 agonist quinpirole and the nonspecific dopamine (DA) agonist d-amphetamine increased total responding but produced slowed response rates that continued for 45-60 min. The slowed response rate produced by d-amphetamine resulted in a peak increase in interresponse times (IRTs) between 8-10 s, whereas quinpirole increased IRTs in the 14- to 16-s range, indicating that nonspecific DA activation resulted in higher rates of ethanol-reinforced responding than specific D2 activation although both drugs decreased local response rates. These data indicate that the amount and temporal extent of ethanol-reinforced responding are increased by microinjections of DA agonists in the nucleus accumbens and support the hypothesis that DA activity in this region is involved in the regulation of ethanol-reinforced responding.

An RSPCA/FRAME Survey of the Use of Non-human Primates as Laboratory Animals in Great Britain, 1984–1988

A literature-based survey of the use of non-human primates as laboratory animals in Great Britain in 1984–1988 was carried out as a background to extending debate about the ethical and practical issues involved. The 289 publications considered were grouped in 15 subject areas and reviewed in terms of scientific purpose, methods employed, numbers and species of animals used, and their source, care and ultimate fate. In addition, the Association of the British Pharmaceutical Industry provided a comment on the use of non-human primates by pharmaceutical companies. Specific causes for concern were identified, and future prospects considered.

Social withdrawal following amphetamine administration to marmosets

Approaches and leaves from social encounters by marmosets which had received amphetamine injected either intramuscularly or into the nucleus accumbens or caudate nucleus were recorded and used to determine whether social behaviour was disrupted as a result of behavioural competition or more active social withdrawal. The social isolation observed after the marmosets had received an IM injection of amphetamine (2 mg/kg) was not due to drug-induced increases in alternative behaviours. Drugged animals immediately withdrew from social encounters, interrupting their stereotypies in order to do so, whenever they were approached by an undrugged animal. In contrast, the reduced time spent in social encounters following amphetamine injections into the nucleus accumbens (10, 20 or 40 micrograms) appeared to be a direct consequence of the concurrent increase in locomotion. Animals continued to initiate social encounters despite being hyperactive. Amphetamine injections into the caudate nucleus were without effect on any of the social or individual behavioural measures.

Dominant social status facilitates the behavioral effects of serotonergic agonists

The effects of dominance rank on the behavioral responses to drugs that enhance central serotonergic function were examined in 45 adult male vervet monkeys living in 15 stable social groups. Each group contained 3 adult males, 3 adult females, and their immature offspring. Dominance rank was assessed by measuring success in intermale agonistic encounters. In every group one male was clearly the dominant, or alpha male, and the other two males were subordinate. Males from 5 groups received 3 doses of the serotonin reuptake inhibitor fluoxetine (0.5, 1.0 and 2.0 mg/kg/day); those from a second set of 5 groups received 3 doses of the receptor agonist quipazine (0.25, 0.50 and 1.0 mg/kg/day); those from a third set of 5 groups received the serotonin precursor tryptophan (10, 20 and 40 mg/kg/day). The 3 drug treatments produced strikingly similar behavioral effects. Each produced dose-dependent increases in approaching, grooming, resting and eating and decreases in locomoting, avoiding, being vigilant and being solitary. Dominant males were significantly more responsive behaviorally to all 3 drugs than were subordinate males: the increase or decrease in each behavioral measure was larger in dominant than in subordinate males. In combination with previous studies, these data suggest that dominant and subordinate males differ in the drug sensitivity of their serotonergic systems.