Regulation of locus coeruleus neurons and splanchnic, sympathetic nerves by cardiovascular afferents.

The brain norepinephrine (NE) neurons in the nucleus locus coeruleus (LC) have been claimed to be involved both in the regulation of behavioral functions, e.g. vigilance and arousal reactions, and in cardiovascular control. Recent studies from this laboratory have also shown that cardiovascular, vagal afferents can participate in the regulation of the LC neurons in the rat. Utilizing electrophysiological techniques, we have now studied the effects of activation of blood volume receptors or arterial baroreceptors on the firing rate of single cells in the LC and, parallelly, on splanchnic, sympathetic discharge in the chloral hydrate anesthetized rat. Blood volume load (0.5-5 ml heparinized blood, intravenously administered) induced a reduction in both LC neuronal firing rate and splanchnic nerve activity (SNA), effects which were readily and completely reversed by withdrawal of the corresponding amount of blood. In comparison, the central LC neurons were more sensitive to blood volume expansion than the peripheral splanchnic nerves. The effects of blood volume load on LC and SNA remained unaffected after deafferention of arterial baroreceptors. Blood pressure elevation, induced by slow intravenous infusion of NE or angiotensin (AII) (total dose 2 micrograms/kg), caused an immediate reduction in both the firing rate of most of the LC cells tested as well as in SNA. While the effect on SNA was abolished by deafferentation of arterial baroreceptors, the effect on central LC activity remained largely unaffected. Consequently, these data strengthen the concept that brain NE neurons in the LC are subject to control by peripheral blood volume receptors, analogously to peripheral sympathetic nerves. Arterial baroreceptors may still participate in the control of central noradrenergic nerve activity, but in contrast to their function for SNA they are not critical for the inhibition of LC neurons by blood pressure elevation. Rather, these two cardiovascular afferent systems may participate in the physiological regulation of the LC activity in a complimentary and convergent fashion.