LACK OF EVIDENCE FOR AN INVOLVEMENT OF NUCLEUS-ACCUMBENS DOPAMINE D-1 RECEPTORS IN THE INITIATION OF HEROIN SELF-ADMINISTRATION IN THE RAT

The involvement of dopamine D-1 receptor systems in the reinforcing properties of opiate reward was studied by examining the effect of the dopamine D-1 antagonist SCH23390 on the initiation of heroin self-administration in rats. The D-1 antagonist was administered daily systemically or locally in the nucleus accumbens (NAC), after which the animals were allowed to self-administer heroin (IV) in a 3-h session for 5 consecutive days. Systemic treatment with SCH23390 (0.17 and 0.5 mg.kg(-1)) significantly decreased heroin intake during initiation of heroin self-administration, while a dose of 0.06 mg.kg(-1) was not effective. Local administration of SCH23390 (0.5 and 2.5 mu g/site) in the NAC did not affect heroin intake. Both systemic and intra-accumbal administration of SCH23390 dose dependently decreased motor behavior measured in a small open field. The attenuation of heroin intake during initiation of heroin self-administration by blockade of dopamine D-1 receptor systems may be due to a decrease in the reinforcing effects of heroin or more likely to a reduction in non-reinforcement-related behavior. The dopamine D-1 receptors present in the NAC are probably not involved in opiate reward.