Thyrotropin-releasing hormone action in the preoptic/anterior hypothalamus decreases thermoregulatory set point in ground squirrels

Abstract

Earlier work has shown that thyrotropin releasing hormone (TRH) produces dose-dependent decreases in body temperature ( T b ) and metabolic rate when microinjected into the dorsal hippocampus (HPC) or preoptic/anterior hypothalamus (PO/AH) of awake ground squirrels. This study employed a behavioral paradigm to investigate the possibility that TRH-induced hypothermia is associated with a decrease in thermoregulatory set point. Six animals were successfully trained to press a bar for radiant heat escape and cool air reinforcement in order to obtain a cooler ambient temperature ( T a ). During experimental testing, the animals were microinjected remotely with TRH (10–1000 ng/μl) or a control solution (sterile saline or TRH-OH) into the PO/AH. The micro-injections were delivered via bilateral injection cannulae inserted through chronic bilateral cannula guides that had been stereotaxically implanted under pentobarbital anesthesia. Cumulative and time-integrated bar presses were obtained on a computer generated display. T b , measured in the brain via a bead-type thermistor, and chamber T a were recorded continuously. Following TRH administration, a significant increase in mean bar-press rate was observed during the period in which T b was falling, when compared to a comparable time period just prior to the microinjection. These findings complement results obtained from four animals that were trained to press a bar for heat reinforcement in a cold (−10 °C) environment. In this alternative behavioral paradigm, microinjection of TRH into the PO/AH or HPC induced a decrease in mean bar-press rate as T b was falling. The results support the hypothesis that TRH-induced hypothermia in golden-mantled ground squirrels is achieved by lowering thermoregulatory set point.