Observations on the function of the dopaminergic nerves innervating the pituitary gland

Abstract

Analysis of the distribution of catecholamines in the pituitary glands of six mammalian species showed that the highest concentrations occur in the combined neural and intermediate lobes. Dopamine was the major catecholamine; its concentration varied between 6ng/mg protein (guinea-pig) and 38 ng/mg protein (domestic pig). Catecholamines were hardly detectable in the anterior lobes. Possible roles for the dopaminergic innervation of the posterior and intermediate lobes of the pituitary gland have been investigated. Firstly, turnover of the dopamine present in the combined neural and intermediate lobes has been demonstrated in the rat by a fall in the dopamine concentrations after inhibiting dopamine synthesis with α-methyl-ρ-tyrosine methyl ester and by the presence of 3,4-dihydroxyphenylacetic acid in the gland. Secondly, the dopamine concentrations in the neuro-intermediate lobes of the rat hypophysis were measured under some conditions in which an increased release of pituitary hormones is known to occur. During lactation, when increased amounts of oxytocin, melanophore stimulating hormone, prolactin and other pituitary hormones are released over a prolonged period of time, a decrease in pituitary dopamine content and an increase in its turnover has been observed. Water deprivation and intake of large amounts of NaCl is known to lead to a prolonged increased release of vasopressin and oxytocin; this was found to be accompanied by a rise in pituitary dopamine content. A decrease in the pituitary dopamine content was found in rats 8 days after adrenalectomy. Acute release of large quantities of vasopressin, oxytocin or corticotrophin did not cause measurable change in pituitary dopamine contents. The results suggest that the dopaminergic innervation of the pituitary gland may modulate the release of oxytocin and vasopressin from the neural lobe under conditions in which the secretion of these hormones is increased over a prolonged period and may exert a direct inhibitory control over the secretion of polypeptides from the intermediate lobe.