Lack of response in the release of oxytocin and vasopressin from isolated neurohypophyses to dopamine, met-enkephalin and leu-enkephalin

Abstract

We investigated the effects of dopamine, met-enkephalin and leu-enkephalin on basal and ouabain-stimulated release of oxytocin and vasopressin from isolated neurointermediate lobes. The present study revealed that neurohypophyseal hormone release was not affected by dopamine, neither from lobes of untreated rats nor from those of rats with dopamine-deficiency (pretreated with alpha-methyl-p-tyrosine-methylester). Likewise, metoclopramide, a dopamine antagonist, was unable to alter the neurohypophyseal hormone release. Our results also indicate that the opioid peptides met-enkephalin and leu-enkephalin do not influence spontaneous or ouabain-stimulated oxytocin and vasopressin release, which is in accordance with our findings that naloxone under our experimental conditions is also ineffective.

Corticotrophin and vasopressin responses to metoclopramide in patients with hypothalamic amenorrhoea

“…Metoclopramide stimulation of AVP release is likely to occur at the hypothalamic-pituitary level. The finding that metoclopramide did not alter AVP release from isolated rat neurohypophyseal tissue (Pitzel & Konig, 1984) indicates that the effect is on the neurosecretory nerve terminal in the hypothalamic nuclei. Therefore, the plasma AVP level may reflect hypothalamic neuronal activity and AVP levels in the hypothalamus.…”

Neurochemical Regulation of Oxytocin Secretion in Lactation*

“…In addition, enkephalin mRNA was not detectable in magnocellular neurons unless neurohypophysial hormone secretion was stimulated by hypertonic saline (119), suggesting that this peptide may be co-expressed only under particular conditions. Also arguing against involvement of an enkephalin-like peptide are the findings that neither met-nor leu-enkephalin, nor several of their analog agonists affect electrically evoked OT release in vitro, under conditions where other opioid agonists are effective (317,321,(330)(331)(332). There is also evidence that /3-endorphin, released from intermediate lobe, is not involved in the opioid inhibition of OT release (315).…”

Effect of 6-hydroxydopamine injection into the arcuate hypothalamic nucleus on the osmotic release of vasopressin in conscious rats

“…It is possible that the dopaminergic neurons may act to inhibit AVP secretion, because electrical stimulation of the rostral arcuate nucleus caused a decrease in the firing activity of the neurohypophysial neurosecretory axons abolished by application of a dopamine antago¬ nist (9), and because dopamine diminished AVP release from the isolated rat neurohypophysis in vitro (10). However, such an effect of dopamine on in vitro AVP release could not be confirmed by other authors (11,12). Furthermore, when injected into the cerebral ventricle in rats, dopamine facilitated AVP secretion and its antagonists blocked osmotically stimulated AVP release (13)(14)(15).…”