Inhibition of spinal dorsal horn neuronal responses to noxious skin heating by lateral hypothalamic stimulation of the cat

The responses of single lumbar dorsal horn units to noxious radiant heating (50°C, 10 s) of glabrous foot pad skin were recorded in cats anesthetized with sodium pentobarbital and 70% N₂O. The heat-evoked responses of each of 38 units were markedly reduced during electrical stimulation (100-ms trains at 100 Hz, 3/s, 25-300 μA) in the lateral hypothalamic area (LH). LH sites at which stimulation inhibited dorsal horn unit heat-evoked responses were mapped by systematically varying the position of the stimulating electrode. Inhibition was generated at posterior through anterior hypothalamic levels in a region extending laterally from the periventricular gray (PVG) to the cerebral peduncles on both sides and ventrally to the base of the brain. The magnitude of inhibition increased with graded increases in LH stimulation intensity. Respective mean current intensities at threshold for generating inhibition were 27.6 ± 17.4 (SD) μA for contralateral and 30.1 ± 23.7 μA for ipsilateral LH stimulation. Dorsal horn unit responses to a series of graded noxious heat stimuli generally increased linearly from threshold (38-45°C) to 52°C. The slopes of such linear temperature-response functions were reduced, with no significant change in threshold, when the temperature series was repeated during concomitant ipsilateral LH stimulation. Contralateral LH stimulation produced similar slope reductions but additionally produced a significant mean threshold increase of 1.7°C. The inhibitory effect of LH stimulation was significantly reduced in nine units from a mean of 28 ± 18% of control to a mean of 59 ± 18% following systemic administration of the serotonin (5-hydroxytryptamine, 5-HT) antagonist, methysergide (0.3-1 mg/kg). Possible functional relationships of LH with brain stem inhibitory systems and its role in analgesic mechanisms are discussed.