Modulation of Forebrain Electroencephalographic Activity and Behavioral State by the Locus Ceruleus-Noradrenergic System: Involvement of the Medial Septal Area

Abstract

This chapter summarizes a series of studies in the halothane-anesthetized rat to directly assess the causal relationship between locus coeruleus (LC) neuronal discharge activity and forebrain electroencephalographic activity (EEG) state. The chapter demonstrates substantial tonic excitatory actions of the LC noradrenergic system on electroencephalographic activity. The results indicate that the LC-noradrenergic system is a potent modulator of forebrain EEG state via actions of β receptors in the halothane-anesthetized rat. Under these experimental conditions, noradrenergic efferents, presumably arising from LC, modulate forebrain EEG state via actions at MS β receptors. However, to obtain a clearer understanding of the behavioral functions of the LC-noradrenergic system additional studies have been conducted. Based on these observations and those of others, it is posited that one function of LC is to facilitate the induction of a behavioral state appropriate for acquisition of sensory information. This is proposed to be dependent on changes in the tonic activity of LC neurons associated with changes in behavioral state and, at least in part, involves actions of norepinephrine (NE) at β receptors located within the MS. Superimposed on these actions may be facilitatory actions on state-dependent cognitive processes that may, in part, involve phasic fluctuations in LC neuronal activity and possibly actions of NE within cortical and thalamic terminal fields. Combined, these actions would serve to facilitate appropriate behavioral responding to sensory information.