Analysis of the thermolytic action of ICV neurotensin in the rat at different ambient temperatures.

Intracerebroventricular (ICV) cannulae were implanted chronically in adult female Sprague Dawley rats for central injection of neurotensin (NT) or artificial CSF vehicle. During experiments carried out at 48 hr intervals, the body temperature of each rat was monitored continuously by a thermistor probe inserted in the colon. At an ambient temperature of 22 degrees C, NT infused ICV in a threshold dose of 1.5 micrograms caused a mean maximum fall of 0.9 degrees C in the rats' body temperature. This decline in core temperature was enhanced further to 1.4 degrees C when the rats were exposed to an ambient temperature of 4 degrees C for 1.0 hr immediately after the NT infusion. Similarly, when the rats were exposed to 34 degrees , 36 degrees or 40 degrees C, again for 1.0 hr directly after NT administration, core temperature increased to a maximum of 0.3 degrees to 0.6 degrees during this interval. The maximum hyperthermic response following the control ICV infusion of CSF in rats exposed to the same elevated ambient temperatures was slightly greater (0.2-0.3 degrees C) than that after infusion of NT, because of the initial thermolytic effect produced by the peptide. However, the respective slope of each rise in body temperature in both NT and CSF groups during exposure to all three warm temperatures was nevertheless identical. Overall, the results show that in the animal given NT centrally the core temperature tends to follow the ambient temperature, either cold or warm. Thus, it is concluded that NT possesses a clear-cut poikilothermic action on the central neutral systems underlying the control of body temperature. Therefore, it remains uncertain whether NT in fact could play any role in the mechanism within the diencephalon which mediates normal thermoregulatory function.