Antinociceptive Effect of Dopaminergic Neurotransmission Evoked by Mesencephalic Stimulation on Spinal Interneuronal Activity in Cats

Abstract

Adult cats, precollicularly decerebrated and immobilized with Flaxedil® (2 mg/kg, i.v.), were used in the study. Recordings of laminae IV and V cell activity elicited by natural stimulation to the left hind limb or electrical stimulation to the left sural nerve exposed in the popliteal fossa was made at the level of L.6 and L7. Brain stimulation consisting of 100 msec trains of rectangular pulses (0.1 msec, 100 Hz, 3/sec for 1 min, 5–10 V intensity) was delivered to substantia nigra, periaqueductal gray and raphe nucleus in the midbrain via concentric bipolar electrodes. Noxious peri pheral input evoked activity ranging from a few spikes to long duration repetitive discharges. Stimulating the three brain sites individually resulted in inhibitory, facilitatory or no effect on the firing pattern of laminae IV and V cells. Cells responded to nonnoxious activation, however, were without change following central stimulation. The inhibitory effect on the dorsal horn cells was abolished by injecting bulbocapnine (20 mg/kg, i.v.) or tetrabenazine (40 mg/kg, i.V.), suggesting a role of dopaminergic control of the midbrain on laminae IV and V cells. This was further substantiated by the finding that administration of L-dopa (20 mg/kg, i.v.) or apomorphine (20 mg/kg, i.v.) reversed the previous drugs effect and reestablished the brain stem inhibition on the spinal transmission of noxious impulses.