Receptors and neurohormones in human pituitary adenomas

In order to go further into the pathogenesis of human pituitary adenomas, we studied receptors for neurohormones (thyroliberin, TRH; dopamine, DA; somatostatin, SRIH), for estradiol and epidermal growth factor (EGF) thought to influence hormone secretion and/or cell growth. The following results were obtained: (1) the receptors listed above, with the exception of EGF receptors in the adenomas, are present in normal pituitary tissue and in prolactin (PRL)- and growth hormone (GH)-secreting adenomas; (2) they are functional and their affinities are not different in normal or tumoral tissues; (3) their density is variable and depends on the type of secreting adenoma (GH or PRL), the size of the tumor and the plasma level of the hormone which is secreted, and (4) in nonsecreting adenomas, only TRH receptors are found with characteristics identical to those observed in secreting adenomas. We also showed that TRH is contained in normal and tumoral pituitary tissues. TRH and SRIH are released in vitro from adenomatous cells in large amounts, suggesting their possible synthesis by the pituitary. In both cases a local regulation is observed. TRH release is stimulated in the presence of DA while SRIH is inhibited in the presence of TRH. This neuropeptide release may be implicated in the pituitary hormone regulation through a paracrine or an autocrine mechanism. Thus, the neurohormone receptors found in pituitary adenomas should be dependent on a more complex regulation than it has been envisaged till now.