Dopaminergic mechanisms regulating prolactin secretion in patients with prolactin-secreting pituitary adenoma. Long-term studies after selective transsphenoidal surgery

Twenty-seven female patients with prolactin-secreting pituitary microadenoma, were studied at different intervals following selective transsphenoidal removal of the tumor. Postoperatively, all patients had normal prolactin (PRL) levels and regular menstrual cycles were restored. Sixteen of 27 patients showed positive responses to TRH and metoclopramide (MCP) within 1 mo after surgery. On the contrary, 9 patients showed extremely low PRL levels and exhibited negative responses to provocative stimuli immediately after surgery. The long-term evaluation of these patients demonstrated that normal neuroendocrine relationships were restored after several months since positive PRL responses to TRH and MCP could be elicited in such patients. The remaining 2 patients who showed basal PRL levels in the upper range of normal, exhibited negative responses to TRH and MCP. These patients had progressively increasing basal PRL levels, with negative responses to provocative stimuli. In addition, 8 patients with normal responses to TRH and MCP exhibited 10–20 mo after surgery a normal decrease in PRL levels following administration of carbidopa plus L-Dopa. Negative responses to carbidopa plus L-Dopa were instead obtained in 6 postoperative patients with elevated PRL levels and negative responses to TRH and MCP. These results suggest that: 1) Hyperprolactinemia induced by “autonomous” pituitary adenomas increases hypothalamic dopamine (DA) secretion, which in turn inhibits PRL secretion by nonadenomatous lactotropes. 2) Total selective removal of the microadenoma acutely decreases PRL concentration, but a functional inhibition of the normal lactotrope can persist for a period of few months following surgery in a certain number of patients. 3) Prolonged reduction of PRL concentration is accompanied to a normal DA tone with reestablishment of normal neuroendocrine relationships.