The involvement of dopaminergic neurotransmission in the inhibitory effect of morphine on caudate neurone activities

Experiments were performed on lightly pentobarbital anaesthetized Sprague-Dawley rats. Intracarotid injection of morphine (3-6 mg/kgL) was found to inhibit the spontaneous unitary discharges in the caudate nucleus while enhance those in some neurones of the substantia nigra. The inhibitory action of morphine on caudate neurones was readily reversed by the morphine antagonist, naloxone (0.3-0.6 mg/kg). and was prevented by the pretreatment with halopcridol (0.5 mg/kg) and pimozide (0.5 mg/kg). It is postulated that morphine may exert its suppressive effect on caudate activities through the activation of specific populations of nigral cells, resulting in the release of dopamine at the nigrostriatal fibre terminals.