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

Rats reared at 30°C do not exhibit the same thermoregulatory competence during cold exposure as do rats reared at 20° 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° C-exposure and 20 days 20° C-exposure. The 20° C-exposure occurred at different ages in different treatment groups. At 140 days of age, bilateral IH-NA injections (each 10 g/1 l 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° C terminated between 20 and 80 days of age. In a second series of experiments, duration of rearing at 20° 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° C-reared) rats was 17days. 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.