Hypothalamic inhibition of rat dorsal horn neuronal responses to noxious skin heating

The responses of single lumbar dorsal horn neurons to noxious radiant heat stimuli (42-54 degrees C, 10 sec, 1/2 min) applied to glabrous hind paw skin were recorded in rats anesthetized with sodium pentobarbital. Unit responses to 50 or 52 degrees C stimuli were constant over time and were consistently and powerfully inhibited during bipolar stimulation (three 100 msec trains/sec at 100 Hz, 200 microA) in the medial hypothalamus. Inhibition was also evoked by stimulation in medial and ventrobasal thalamic nuclei, lateral hypothalamus and adjacent cerebral peduncle, and amygdala. Inhibition increased with graded increases in intensity of hypothalamic stimulation, with a mean inhibitory threshold of 71 +/- 43 (S.D.) microA for 13 units. The responses of dorsal horn units to graded increases in the temperature of noxious heat stimuli were inhibited during hypothalamic stimulation, such that slopes of the linear temperature-response functions were reduced with no change in response threshold (mean: approximately 44 degrees C). Inhibition was blocked or reduced in 4/7 units following systemic administration of the 5-hydroxytryptamine (5-HT) antagonist methysergide. The results confirm and extend previous work in the cat and are discussed in relation to analgesic mechanisms.