Opioid modulation of magnocellular neurosecretory cell activity

Magnocellular neurosecretory cells of the hypothalamic supraoptic and parav entricular nuclei secrete the hormones. oxytocin and vasopressin, into the systemic circulation from the posterior pituitary gland. Oxytocin is import ant for parturition and is essential for lactation. Vasopressin regulates b ody fluid homeostasis. The secretion of these hormones is altered in respon se to peripheral stimuli that are conveyed via projections from other parts of the brain. Endogenous opioid peptide systems interact with the magnocel lular neurosecretory system at several levels to restrain the basal secreti on of these hormones as well as their secretory responses to various physio logical stimuli. The inhibition of basal secretion can occur at the level o f the neurosecretory terminals where endogenous opioids inhibit the release of oxytocin, and at the cell bodies of magnocellular cells to modulate the activity pattern of vasopressin cells. The responses of the magnocellular neurosecretory system to physiological stimuli are also regulated by these mechanisms but in addition probably also by pre-synaptic inhibition of affe rent inputs to magnocellular cells as well as direct effects on the cell bo dies of efferent input cells to modulate their activity. Here, Lye review t he mechanisms and functional consequences of opioid interactions with oxyto cin and vasopressin cells. (C) 2000 Elsevier Science Ireland Ltd. AU rights reserved.