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

Abstract

The responses of single lumbar dorsal horn units to noxious radiant heating (50.degree. C, 10 s) of footpad skin were recorded in cats anesthetized with sodium pentobarbital and 70% N2O. All units'' heat-evoked responses were markedly reduced during concomitant electrical stimulation (100-ms trains at 100 Hz, 3/s, 50-400 .mu.A) of the medial hypothalamus. Sites at which stimulation inhibited dorsal horn unit heat-evoked responses were mapped by systematically varying the depth of the stimulating electrode in tracks at different anteroposterior levels of the median diencephalon. The most effective inhibitory sites were in the periventricular gray (PVG) region at posterior through anterior hypothalamic levels. The magnitude of inhibition of unit heat-evoked responses increased with graded increases in PVG stimulus intensity. For 15 units, the mean current threshold to generate inhibition was 67.4 .+-. 67 (SD) .mu.A. Dorsal horn unit responses to a series of graded noxious heat stimuli increased linearly from threshold (40-45.degree. C) to 52.degree. C. The slope of such linear temperature-response functions was significantly reduced, without a change in response threshold, when the temperature series was repeated during concomitant PVG stimulation. The possible relationship of the PVG to inhibitory systems in the midbrain periaqueductal gray and medullary nucleus raphe magnus and its possible role in analgesia are discussed.