Activity of cat locus coeruleus noradrenergic neurons during the defense reaction.

The single-unit activity of locus coeruleus noradrenergic (LC-NE) neurons was recorded in freely moving cats during naturally induced defense reactions. Defense reactions, consisting of arched back, piloerection, flattened ears and mydriasis, were elicited by exposing the cat either to a dog, or to a cat displaying aggressive behavior induced by electrical stimulation of the hypothalamus. LC-NE neurons were identified using previously established criteria, including suppression of firing during rapid eye movement (REM) sleep and in response to clonidine administration. Exposure to a dog evoked defense reactions and increased the tonic firing rate of LC-NE neurons (n = 8) from a baseline of approximately 0.9 spikes/s to approximately 2.5 spikes/s. Exposure to an aggressive cat evoked defense reactions that were qualitatively very similar to those produced by dog exposure, and elevated the tonic firing rate of LC-NE neurons (n = 8) from a baseline of approximately 1.0 spikes/s to approximately 2.5 spikes/s. In addition to these tonic elevations of activity, LC-NE neurons discharged in phasic bursts (as high as 10 spikes in a 500 ms period) in close association with specific threatening acts made by the dog or hypothalamically stimulated cat. The mere presence of a dog was sufficient to evoke tonic activation of LC-NE neurons, even in the absence of threatening advances by the dog, whereas exposure to a hypothalamically stimulated cat produced LC-NE neuronal activation only when the stimulated cat showed aggressive behavior. These results extend our previous work, which examined the response of LC-NE neurons to environmental and physiological stressors, into a more ethologically relevant domain, and suggest that LC-NE neuronal activation may play a role in the response to threatening or challenging situations.