Single-unit response of noradrenergic neurons in the locus coeruleus of freely moving cats. II. Adaptation to chronically presented stressful stimuli.

The preceding paper described a stress-related tonic activation of noradrenergic neurons of the locus coeruleus (LC) of behaving cats. The present study examined the degree to which adaptation to stress was reflected in LC unit activity. Adaptation was defined as a reduction in the magnitude of the physiological stress response of the animal, as reflected in plasma norepinephrine level and heart rate. Adaptation of the physiological stress response occurred after 5 repeated hourly presentations of 100 dB white noise and after 2 hr of restraint. When these stimuli ceased to elicit significant sympathoadrenal activation, they likewise ceased to elicit activation of LC single-unit activity. These results provide further support for the hypothesis that tonic elevations in LC neuronal activity are stress related and that the LC is involved in the CNS response to challenges to the organism.