In vivo amygdala dopamine levels modulate cocaine self-administration behaviour in the rat: D1 dopamine receptor involvement.

Nucleus accumbens dopamine is often hypothesized as the critical factor for modulating cocaine self-administration. In the current study we examined the extent to which dopamine in the amygdala could contribute to cocaine intake behaviour and modify nucleus accumbens dopamine levels. Rats were trained to self-administer intravenous cocaine (1.5 mg/kg/injection) under a fixed-ratio reinforcement schedule in daily 3 h operant training sessions. In the first in vivo microdialysis experiment, extracellular dopamine levels were found to be increased 200% of baseline in the amygdala and by 400% in the nucleus accumbens. Although cocaine induced similar profiles of dopamine overflow in the two mesolimbic areas, in the nucleus accumbens the latency of the dopaminergic response was shorter (three- to four-fold) during both initiation and termination of the cocaine self-administration session than in the amygdala. Despite achieving a stable self-regulated pattern of cocaine intake and high dopamine concentrations in the nucleus accumbens, a unilateral injection of the D1 receptor antagonist SCH 23390 (0.5 or 1.5 microg) into the amygdala was still able to increase the rate of cocaine intake. This behavioural effect was accompanied by a dose-dependent increase in nucleus accumbens dopamine levels; at the highest SCH 23390 concentration cocaine intake was increased by 400% and dopamine levels were potentiated by an additional 400%. In vivo autoradiography using [3H]SCH 23390 showed that D1 receptor sites contributing to the behavioural and subsequent neurochemical effects were predominantly localized to the amygdala and not the nucleus accumbens. Altogether these results point to a significant contribution of in vivo amygdala D1 dopamine transmission to cocaine self-administration behaviour.