Single-unit response of noradrenergic neurons in the locus coeruleus of freely moving cats. I. Acutely presented stressful and nonstressful stimuli.

The present experiment was designed to explore the stress-relatedness of activity in noradrenergic neurons of the locus coeruleus (LC) of behaving cats. A stressor was defined as a stimulus that elicited a significant sympathoadrenal activation as measured by plasma norepinephrine level and heart rate. According to this definition, exposure to 15 min of 100 dB white noise or 15 min of restraint was stressful in cats. In contrast, exposure to inaccessible rats for 15 min was behaviorally activating but nonstressful. The single-unit activity of noradrenergic neurons in the LC of behaving cats was examined under these conditions. The stressful stimuli elicited a significant increase in LC neuronal activity for the entire 15 min stressor duration, whereas the behaviorally activating but nonstressful stimulus elicited no significant change in the activity of these neurons. These results provide evidence that behavioral activation per se is not sufficient to evoke a tonic activation of these neurons. Rather, these data support the hypothesis that the LC is involved in the CNS response to stress and provide additional evidence that the activity of LC noradrenergic neurons increases in association with sympathoadrenal activation.