Endogenous opiates regulate oxytocin but not vasopressin secretion from the neurohypophysis

Abstract

The neurohypophysis not only contains the nerve endings of the oxytocin- and vasopressin-secreting neurones of the hypothalamus, but also receives both dopamine¹- and opiate peptide^2,3-containing nerve fibres. This secondary innervation may regulate hormone secretion at the level of the nerve terminals by an action analogous to presynaptic inhibition⁴. Electrical stimulation of the isolated neurohypophysis releases oxytocin and vasopressin in amounts readily detectable by radio-immunoassay, and presumably also releases dopamine⁵ and endogenous opiates. If these neurosecretory products are indeed regulators of hormone release, then specific antagonists to dopamine and opiates should modify the electrically stimulated release of hormone. We report here that the dopamine antagonist spiperone does not affect the release of either oxytocin or vasopressin from the isolated rat neurohypophysis. In contrast, the opiate antagonist naloxone markedly enhances the electrical stimulation of oxytocin release, but has no effect on vasopressin release. Thus an endogenous opiate, released by electrical stimulation of the isolated neurohypophysis, inhibits the release of oxytocin, but this opiate control does not appear to be exerted on the vasopressin-secreting terminals. We have not found similar evidence that endogenous neuro-hypophysial dopamine regulates hormone release.