Inhibition of thalamic and hypothalamic somatosensory evoked potentials by stimulation of substantia nigra and its modification by morphine and methotrimeprazine (levomepromazine).

A brief electrical stimulation of the substantia nigra induced a marked and long lasting inhibition of the somatosensory evoked potentials recorded from the centrum medianum of the thalamus (CM) and posterior hypothalamic area (PHA) following sciatic stimulation in unanesthetized rabbits. The nigral inhibitory effect on CM was prolonged by the administration of morphine (4 mg/kg i.v.) but not influenced by that of methotrimeprazine (2-4 mg/kg i.v.). In contrast, the nigral inhibitory effect on PHA was enhanced by the injection of methotrimeprazine (2 mg/kg i.v.), but not changed by that of morphine (4 mg/kg i.v.). These results indicate that the inhibitory system originating from the substantia nigra operates on the somatosensory transmissions from the peripheral nerve to the thalamus and hypothalamus, and that morphine or methotrimeprazine in small doses induced a selective potentiation of the nigral inhibitory influence on the thalamus or hypothalamus, respectively.