Repeated d-amphetamine enhances stimulated mesoamygdaloid dopamine transmission

Abstract

The mesoaccumbens dopamine pathway exhibits an enhanced dopaminergic response to a challenge injection of d-amphetamine or cocaine after repeated intermittent exposure to that drug. Much research has focused on the potential role of this sensitised response in the enhanced propensity of drug-associated stimuli to elicit relapse. However, the amygdala is acknowledged to play a critical role in stimulus-reward learning, and recent work suggests that the mesoamygdaloid dopamine pathway exerts a significant influence upon amygdala function. In the present study, rats were administered d-amphetamine (1 mg/kg, IP) or vehicle once per day, for 14 days. After 11 untreated days, a locomotor assay showed that prior repeated administration of d-amphetamine led to a markedly enhanced locomotor response to 0.5 mg/kg d-amphetamine. There was no effect of d-amphetamine pretreatment upon the response to a novel environment, or to injection with vehicle. Following a total of 14 days in the home cage, subjects were implanted with microdialysis probes within the amygdala, and for comparison also within the nucleus accumbens. Baseline and d-amphetamine-stimulated (0.5 mg/kg) levels of extracellular dopamine were assessed for each brain region. Results showed that baseline levels of dopamine were very similar in sensitised and control animals. By contrast, prior treatment with d-amphetamine enhanced dopamine overflow in response to a challenge with d-amphetamine both in the nucleus accumbens and amygdala. These results indicate that changes in the pattern of dopamine transmission both in the nucleus accumbens, and the amygdala, accompany the behavioural sensitisation observed after repeated exposure to d-amphetamine. Hence, an enhanced propensity of drug-associated stimuli to elicit relapse may not depend solely upon changes relating to the mesoaccumbens dopamine projection.