Opioid-noradrenergic interactions in the control of oxytocin cells.

The nucleus tractus solitarii (NTS) projects directly to the oxytocin neurones of the supraoptic nucleus (SON), and relays afferent stimuli arising from the birth canal during parturition. About 80% of these projecting neurones are noradrenergic, and these same neurones are activated following systemic administration of cholecystokinin (CCK), which also results in an increased electrical and secretory activity in oxytocin neurones. Oxytocin release in response to CCK is abolished following selective neurotoxic destruction of these noradrenergic neurones. Oxytocin release following CCK (and that during parturition) is potently inhibited by morphine, which blocks the local noradrenaline release in the supraoptic nucleus. This acute opiate action involves presynaptic inhibition of the noradrenergic terminals, and occurs without marked suppression of the activity of noradrenergic cells in the NTS. During chronic exposure to morphine the oxytocin system becomes tolerant to, and dependent upon morphine. In the course of tolerance, oxytocin cell activation in response to CCK recovers from initial inhibition. However, the pathway that mediates this response does not appear to become dependent: the oxytocin cell response to CCK is unchanged by opiate withdrawal induced by naloxone, despite a large increase in the background electrical activity of oxytocin cells provoked by withdrawal. Nevertheless, expression of withdrawal excitation by oxytocin neurones is curiously contingent upon the activity of the noradrenergic input in that prior lesioning of this input has no effect upon the subsequent withdrawal excitation of oxytocin cells. Yet under urethane anaesthesia, acute pharmacological blockade of the noradrenergic input suppresses withdrawal. We discuss how these paradoxical observations might be reconciled, and note that the difference may be related to differing levels of tonic activity in the noradrenergic input. It is possible that dependence relies upon the input when it is there, but not when it is not.