Modification by nickel of instrumental thermoregulatory behavior in rats

The effects of NiCl2 on the colonic temperature and thermoregulatory behavior (TRB) of rats were examined. TRB was evaluated in an instrumental (operant) setting in which rats were required to press a level to obtain convectional heat (SEEK) or to avoid heat (ESCAPE). Orthogonal polynomial regression was used to describe the response patterns in both the SEEK and ESCAPE situations. Two milligrams per kilogram of Ni (ip) caused rapid, transient hypothermia at an ambient temperature of 21 degrees C. When given access to heat reinforcement, rats responded for heat at a lower rate immediately after 2 or 5 mg/kg of Ni (up to 5-15 min) than after saline. Subsequently, response rates rose 30 min or more after Ni injection. A converse pattern was found with the heat escape situation. These observations, confirmed by two contrasting procedures, indicate that the changes were thermoregulatory in nature and cannot be explained by nonspecific suppressive or excitatory effects of Ni. They further suggest that Ni-induced hypothermia results from an altered body temperature set point. The subsequent reversal in behavior probably arises from a direct action of Ni on autonomic effector mechanisms. The origin and biological significance of these findings require further investigation. Physical requirements and response topography are discussed as critical variables in the interpretation of experiments requiring similar responses under different ambient temperatures.