Procaine microinjection into the lower midbrain increases brown fat and body temperatures in anesthetized rats

A tonic inhibitory mechanism on heat production was studied by microinjecting procaine into various regions of the brain while recording temperature changes of the interscapular brown adipose tissue (IBAT) and rectum in urethane-anesthetized rats at room temperature of 23-25 degrees C. Procaine microinjected bilaterally (10%, 1.0 mu l/site, 1.5 mm to midline) into the midbrain and the upper- to mid-pontine area of the reticular formation increased temperatures of the IBAT and rectum. The highest temperature rise (1.02 +/- 0.11 degrees C for IBAT, 0.64 +/- 0.06 degrees C for rectum) with the shortest onset latency (1.5 +/- 0.3 min for IBAT, 4.6 +/- 1.1 min for rectum) was observed when procaine was injected into the lower midbrain (the area between 6 and 7 mm posterior to the bregma, and 6.5 to 8.5 mm deep from the cortical surface). These regions include the retrorubral field, pedunculopontine tegmental nucleus, and rubrospinal tract. Procaine-induced IBAT and rectal temperature increases were dose-dependent, and reproduced reliably from the same injection site of the same animal. Intravenous indomethacin, a prostaglandin H synthase inhibitor, did not affect procaine-induced temperature rise, and propranolol, a beta-blocker, completely blocked it. These results suggest that microinjected procaine exerts its local anesthetic effect and release a tonic inhibition resulting in a disinhibition-induced temperature increase through the enhanced central sympathetic outflow. They support the hypothesis that a bilateral tonic inhibitory mechanism on heat production exists in the lower midbrain.