Catecholamines and endogenous opioids in ventral tegmental self-stimulation reward.

Midbrain dopaminergic pathways and opioid receptor systems have been implicated in the reward experienced in electrical intracranial self-stimulation behavior. In the present experiment, the influence of graded doses of the dopamine antagonist haloperidol and of the agonist cocaine were investigated on electrical self-stimulation reward, elicited by electrodes located in the ventral tegmental area. A threshold method, which is rather insensitive for aspecific motor effects, was applied to determine the reward of self-stimulation. The method allowed to determine simultaneously the rate of lever pressing for self-stimulation. All doses of haloperidol and cocaine were administered with and without the opioid antagonist naloxone, in order to investigate the interaction between dopaminergic and opioid modulation of reward. Haloperidol lowered and cocaine tended to increase the response rate, whereas cocaine but also haloperidol lowered the self-stimulation threshold. The effects appear to be dose-dependent. Naloxone did not interact with the effect of the drugs on threshold and it lowered the response rate, but in the haloperidol-treated rats only. It is concluded that dopamine is involved in the reward of electrical self-stimulation elicited from the ventral tegmental area and that this involvement is independent of endorphin systems, suggesting the existence of separate catecholamine and opioid mechanisms modulating brain reward.