Dopaminergic involvement in the mediation of morphine effects on vocalization and movement reactivity in the rat

To understand the role of dopamine in the mediation of opiate's analgesic and reinforcing activity, the influence of pharmacologically-induced increase (apomorphine in a high dose) and decrease (haloperidol and apomorphine in a low "presynaptic" dose) of dopamine neurotransmission on morphine-induced depression of vocalization and movement reactivity was investigated in freely behaving rats. Strong enhancement of morphine-induced vocalization reactivity depression and a weakening of movement reactivity depression typical to concomitant pharmacological increase of dopamine neurotransmission may represent the enhancement of its reinforcing and analgesic properties and inhibition of its depressive/sedative activity. Enhancement of MOR-induced depression of movement and vocalization reactivity characteristic of drug-induced decrease of dopamine neurotransmission may be the consequence of the blockade of opiate's reinforcing and stimulatory activity and summation of depressive/sedative actions typical to these drugs administered alone.