Inhibition of the CNA trigger process for arousal from hibernation

Ground squirrels (Citellus lateralis) produced three distinct types of thermogenic response during hibernation. These responses were evoked spontaneously as well as after stimulation produced by brief handling, or after microinjection of acetylcholine into the midbrain reticular formation. Type I responses were characterized by small magnitude and a slow (mean rate, 0.03 degrees C/min), variable rising phase. Type II responses were characterized by a smooth, rapid rising phase with a mean rate of increase of 0.11 degrees C/min and by an abrupt reversal of the rising phase within a restricted ceiling temperature band with a mean value of 9.4 degrees C. The third type of response, full arousal, was characterized by a return of body temperature to euthermic (nonhibernating) levels and by an early rising phase that was indistinguishable from the rising phase of type II responses. This indicates that the rising phase of type II responses and the duplicate portion of full arousals are produced by a common neuronal mechanism that functions as the trigger for arousal from hibernation, and that this mechanism can be spontaneously inhibited when increasing internal temperature reaches a hibernation ceiling level.