Hypothalamic control of nocireceptive and other neurons in the marginal layer of the dorsal horn of the medulla (trigeminal nucleus caudalis) in the rat.

The effect of electrical stimulation of the preoptic area of the hypothalamus on the discharge of neurones in the marginal layer (lamina I) of the trigeminal nucleus caudalis was studied in the anaesthetised rat. There was a powerful suppression of the discharge evoked by noxious thermal stimuli in 49/49 specific nociceptor driven (nocireceptive) neurones. The inhibitory effect increased with graded increases in the intensity of preoptic stimulation. Stimulation, however, produced only a small reduction in the discharge of 14/17 cold receptive neurones. Thresholds for producing suppression of cold receptive neurones were generally higher than those for nocireceptive neurones. There was no effect on the activity of 12/12 low threshold mechanoreceptive neurones. The inhibitory action generated on the activity of nocireceptive neurones was reduced by electrolytic lesions in the nucleus raphe magnus (NRM) or the nucleus paragigantocellularis lateralis (PGCL) or the dorsolateral and ventrolateral periaqueductal gray matter (PAG). Lesions made in the ventral or dorsal aspect of PAG were, however, ineffective in reducing the suppression. It is suggested that the powerful descending inhibitory control of nociceptive transmission in the trigeminal nucleus caudalis is one of the neuronal mechanisms mediating analgesia from the preoptic area of the hypothalamus.