Molecular and cellular mechanisms of opiate action: studies in the rat locus coeruleus.

We have studied the molecular and cellular mechanisms underlying the acute and chronic effects of opiate on neurons of the rat locus coeruleus (LC). Acutely, opiates inhibit LC neurons by activating K+ channels and inhibiting a novel sodium-dependent inward current. Both of these actions are mediated via pertussis toxin-sensitive G-proteins, and regulation of the sodium current occurs through inhibition of the cyclic AMP pathway. In contrast to the acute effects of opiates, chronic treatment of rats with opiates increases levels of specific G-protein subunits, adenylate cyclase, cyclic AMP-dependent protein kinase, and a number of phosphoproteins (including tyrosine hydroxylase) in this brain region. Electrophysiological data have provided direct support for the possibility that this upregulation of the cyclic AMP system contributes to opiate tolerance, dependence, and withdrawal exhibited by these noradrenergic LC neurons. As the adaptations in G-proteins and the cyclic AMP system appear to occur at least in part at the level of gene expression, current efforts are aimed at identifying the mechanisms by which opiates regulate the expression of these intracellular messenger proteins in the LC. These studies will lead to an improved understanding of the molecular and cellular basis of opiate addiction.