Effects of environmental temperature on the development of a noradrenergic thermoregulatory mechanism in the rat.

Rats reared at 30 degrees C do not exhibit the same thermoregulatory competence during cold exposure as do rats reared at 20 degrees C. They are even more clearly distinguished by the absence of an hypothermic response to intrahypothalamic noradrenaline (IH-NA). In one series of experiments, different groups of rats all received 120 days 30 degrees C-exposure and 20 days 20 degrees C-exposure. The 20 degrees C-exposure occurred at different ages in different treatment groups. At 140 days of age, bilateral IH-NA injections (each 10 micrograms/1 microliter CSF) were administered in conscious rats and the body temperature response observed. An hypothermic response to IH-NA was observed in groups whose exposure to 20 degrees C terminated between 20 and 80 days of age. In a second series of experiments, duration of rearing at 20 degrees C varied but always started at 40 days of age. Responses to IH-NA in 140-day-old adults indicated that the exposure required to induce 50% of the hypothermic response of control (20 degrees C-reared) rats was approximately 17 days. These data suggest that there is an hypothalamic noradrenergic mechanism implicated in the control of body temperature whose development is affected by environmental temperature in a duration-dependent manner. The period during which this effect may be exerted extends into adulthood.