Effect of hypothalamic cooling on the tonic vibration reflex of gastrocnemius muscle in cats.

The tonic vibration reflex (TVR) in a medial gastrocnemius muscle (MG) was analyzed in 28 cats anesthetized with pentobarbital sodium while the preoptic-anterior hypothalamic region (PO-AH) was locally cooled or warmed in a thermoneutral environment. Cooling of the PO-AH did not produce shivering or changes in rectal and skin temperatures, but it brought about facilitation (16/28 cats) or inhibition (9/28) of the TVR in the MG. A few cats (3/28) showed inconsistent alteration of the TVR. Warming of the PO-AH did not produce any changes in the TVR of the MG. Histological identification of thermode placements revealed that in cats having the facilitatory response of the TVR, the thermodes were localized at regions caudal to A 14.0 mm and most of the thermodes in those having the inhibitory response were rostral to A 14.0 mm, except for two cases. Bilateral microinjection of a small quantity of anesthetic agent into the medial forebrain bundle (MFB) at the level of the mamillary body abolished the facilitatory effect of PO-AH cooling on the TVR. The results suggest that the spinal motor system is influenced by the PO-AH's sensitivity to local temperature and that the MFB constitutes part of the efferent pathway of the hypothalamic influence.