[3H]Dihydroergocryptine Binding in Anterior Pituitary and Prolactin Secretion: Further Evidence of Brain Regulation of Adenohypophyseal Receptors*

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Abstract
Secretion of PRL by the pituitary gland is under tonic inhibitory control by tuberoinfundibular dopaminergic neurons. The aim of the present work was to assess the influence of the hypothalamus on the binding of [3H]dihydroergocryptine, a dopamine agonist, in the adenohypophysis and its role in the hypoprolactinemic action of exogenous dopamine. Serum PRL levels were elevated in ovariectomized adult rats with electrolytic lesions of the median eminence compared to levels in sham-operated animals; [3H]dihydroergocryptine binding to anterior pituitary was elevated from a control value of 161.2 ± 7.7 to 229.0 ± 10.0 fmol/mg protein in rats studied 7–21 days after placement of the lesion (P < 0.05). Scatchard analysis of [3H]dihydroergocryptine binding in lesioned and sham-operated animals indicated an increase of 62% in the number of binding sites without a change in binding affinity. In a second group of rats, with or without median eminence destruction, the effects of different doses of ip dopamine on serum PRL were determined. The hypoprolactinemic effect of each dose of the amine was more evident in median eminencelesioned rats than in sham-operated controls. These results suggest that when hypothalamic influences on the anterior pituitary are removed, there is increased binding of dopamine by the gland and increased hypoprolactinemic effects of the exogenous amine. This indicates that receptors in the pituitary are under hypothalamic control, and this could be an important regulatory mechanism in neuroendocrine events.