Control of release of vasopressin by neuroendocrine reflexes

The neurones in the supraoptic and paraventricular nuclei (SON and PVN) which secrete vasopressin are separate from those which secrete oxytocin and are distributed in different parts of the nuclei. They may be distinguished electrophysiologically by a characteristic phasic pattern of firing. A selective afferent neural input to these neurones would provide a mechanism for the release of vasopressin independently of oxytocin in response to appropriate physiological stimuli. Release of vasopressin is controlled by changes in blood volume or pressure ('volume control') and in plasma osmolality ('osmotic control'). Stimuli involved in volume control such as haemorrhage, hypotension and carotid occlusion cause vasopressin to be released into the circulation with little or no detectable oxytocin. An osmotic stimulus releases vasopressin alone in some species but not apparently in the rat in which both hormones are released. Volume control is mediated reflexly by peripheral receptors in the cardiovascular system. Activation of baro- and stretch receptors results in inhibition, and activation of chemoreceptors in stimulation, of release. Afferent impulses from these receptors are conveyed in the vagi and carotid sinus nerves to the NTS on the dorsal surface of the brain stem. All afferent impulses to the NTS are excitatory. It follows that the afferents from chemoreceptors must stimulate an excitatory, and those from baro- and stretch receptors an inhibitory, projection from the NTS to the vasopressin-secreting cells in the SON and PVN. Two alternative models are presented of the neural pathways and transmitters involved. The model of Fig. 2 shows an excitatory relay through a cholinoceptive area on the ventral surface of the brain stem which has been termed the 'nicotine-sensitive area' because topical application of nicotine to this area in the cat released vasopressin without oxytocin. An inhibitory relay is shown through the A1 group of noradrenergic neurones on the ventral surface which selectively innervate the vasopressin-secreting neurones in the SON. This model implies an inhibitory role for noradrenaline acting on beta- or alpha 2-receptors. However the most recent investigations suggest an excitatory, rather than inhibitory, function of the A1 noradrenergic neurones involving alpha 1-receptors. This is the basis of the model in Fig. 3. The A1 neurones project either directly to the SON and PVN or indirectly through the lateral preoptic nucleus which lies in close proximity to the SON. The nicotine-sensitive area may be coincident with the A1 group of noradrenergic neurones.(ABSTRACT TRUNCATED AT 400 WORDS)