Effects of physiological manipulations on locus coeruleus neuronal activity in freely moving cats. II. Cardiovascular challenge.

Several cardiovascular manipulations were examined for their effects on single-unit activity of locus coeruleus noradrenergic (LC-NE) neurons in unanesthetized, unrestrained cats: hydralazine (1 mg/kg, i.v.) was administered to present a tonic hypotensive stimulus, and to activate preferentially the neural component of the sympathoadrenal system; hemorrhage was used to decrease blood volume and to activate both the neural and hormonal components of the sympathoadrenal system; intravenous infusion of isotonic saline was used to increase blood volume. LC-NE neurons were activated by hydralazine, in parallel with the sympathetic response (indicated by elevated heart rate and plasma NE). LC-NE unit activity was decreased following a volume load. However, contrary to previous findings in anesthetized animals, hemorrhage had no effect on LC-NE unit activity, but did activate both components of the sympathetic response. It is concluded that: (1) cardiovascular stimuli can influence the activity of LC-NE neurons, though they show less sensitivity to such stimuli than do primary regulatory mechanisms; (2) the response of LC-NE neurons to physiological stimuli can occur independent of changes in behavioral state; (3) these neurons do not appear to play a specific role in cardiovascular regulation, but may respond to physiological challenges in general; (4) finally, in agreement with previous studies, our data show that LC-NE neurons are generally co-activated with the sympathetic nervous system, but also that the two can be dissociated (e.g. hemorrhage).