Neurohypophysial Opiate Receptors: Are They On Pituicytes?

Abstract

The administration of low doses of morphine and β-endorphin into the cerebral ventricles results in a fall in plasma vasopressin concentration, and the location of the opiate receptors responsible for this effect, is presumed to be on pre-terminal sites on the neurosecretory fibres themselves. This chapter describes and evaluates the hypothesis that enkephalinergic fibres in the neural lobe acts on the pituicytes rather than directly on neurosecretory fibres, by measuring the opiate receptor population of the neural lobe in control rats and in rats in which the neurohypophysial neurosecretory fibres have degenerated following pituitary stalk transection two weeks previously. The chapter shows that two weeks after the transection of the pituitary stalk, there is shrinkage of the neural lobe and an increase in the concentration of opiate receptor sites. Although the shrinkage of the neural lobe means that there has been an overall diminution of the opiate receptor population found in the intact gland, a very significant preservation of receptors and an increased concentration of opiate receptor sites have been shown, at a time when there is reported to be complete loss of neurosecretory axons. However, alterations in ionic environment, GABA uptake, or uptake and removal of vasopressin and oxytocin by the pituicytes, might inhibit or facilitate neurohypophysial hormone release.