Posterior hypothalamic stimulation of anesthetized normothermic and hypothermic rats evokes shivering thermogenesis

Normothermic (37 degrees C), anesthetized Long Evans rats given unilateral electrical stimulation (0.5 ms monophasic pulses of 100-300 microA at 50 Hz for 30 s) of the posterior hypothalamus (PH) had graded, sustained increases in EMG electrical activity of the gastrocnemius muscle (i.e. shivering). In a current-related manner, gastrocnemius muscle temperatures (Tm) immediately increased following PH stimulation, surface temperatures (Tt) did not change and colonic (core, Tc) temperatures initially fell, then subsequently rose after the applied stimulus. A biphasic pressor response occurred after PH electrical stimulation associated with tachycardia. PH electrical stimulation (0.5 ms pulses at 50 Hz for 30 s of only 40 microA) induced shivering in anaesthetized, hypothermic Long Evans rats undergoing acute cold exposure. When these same hypothermic rats were cooled further to cause shivering, PH electrical stimulation (0.5 ms pulses at 50 Hz for 30 s of only 40 microA) induced further increases in the shivering response (increases EMG area of gastrocnemius muscle) from the shivering response before PH stimulation. Results indicate that electrical stimulation of the PH can evoke shivering in anesthetized normothermic rats. Stimulation of the PH with lower current intensity can induce or increase shivering of hypothermic rats previously exposed to the cold.