Modulation of Spontaneous and Sensory-Evoked Discharge of Locus Coeruleus Neurons by Behavioral State: Functional Implications

Abstract

Although many hypotheses have been generated concerning the role of the norepinephrine (NE)-containing locus coeruleus (LC) system in behavior, there is as yet no single theory to unify the vast array of observations relevant to this nucleus. In order to more fully elucidate the overall role of the LC in brain and behavioral processes, my collaborators and I have sought a more complete understanding of the cellular physiologic properties of this system (see Aston-Jones et al., 1984). There are at least 3 essential questions in achieving such a cellular physiologic understanding: (1) where do LC neurons project, (2) what effect does NE released from LC terminals have on target neurons, and (3) when are LC neurons active (and presumably releasing NE) during ongoing behavior. For the first of these questions, I assume that the reader is familiar with the vast efferent trajectories of the LC system (for review, see Foote et al., 1983). In sum, for the present purpose one may consider the LC projection system to be extremely global, innervating nearly all parts of the neuraxis with few exceptions. Regarding the second question, the effects of NE on LC target neuronal activity are described for one sample target system by Segal in the present volume. These and other studies have found that while NE is inhibitory overall, it may also enhance the selectivity of target cell discharge, so that in the presence of this neurotransmitter neurons in many brain regions respond with increased preference to their most strongly determined inputs.