Disinhibition of lower midbrain neurons enhances non-shivering thermogenesis in anesthetized rats

The hypothesis that the lower midbrain, specifically in and around the retrorubral field (RRF) and/or rubrospinal tract (rs), contains a tonic inhibitory mechanism on non-shivering thermogenesis (NST) was examined in urethane-anesthetized rats. It has been proposed that removal of the tonic inhibitory mechanism in the lower midbrain causes body temperature increase through disinhibition-induced activation of the central sympathetic nervous system. The present experiments were carried out to examine whether and where the proposed midbrain region contains cell bodies that tonically inhibit the NST, and if so, whether they receive any influence from the hypothalamus. Male Wistar rats were anesthetized with urethane (1. 0-1.2 g/kg, i.p.), and various agents were microinjected into the RRF and rs areas of one side before and after knife-cut in the other side of the lower midbrain or isolation of the hypothalamus from the midbrain. Changes in interscapular brown adipose tissue (IBAT), rectum, and tail skin temperatures were monitored.

(1) unilateral midbrain procaine increased IBAT and rectal temperatures only after un-injected side of the midbrain had been pre-transected. (2) Effective midbrain sites for procaine to increase IBAT and rectal temperatures was laterally extended. (3) Tetrodotoxin microinjected into the midbrain site where procaine increased IBAT and rectal temperatures also raised both temperatures. (4) l-glutamate decreased IBAT and rectal temperatures when microinjected into one of the most inner midbrain area of procaine-sensitive sites without affecting tail skin temperature. (5) Isolation of the hypothalamus from the lower midbrain did not affect midbrain procaine-induced IBAT and rectal temperature increases. These results suggest that neurons that tonically inhibit the NST are located in the area close to the midline adjacent to the RRF and rs, and that the integrity of the neurons to tonically inhibit the NST is not affected by disconnecting the hypothalamus from the midbrain.