Gonadotropin-releasing hormone (GnRH) effects on the lactotroph function have been widely studied, but they probably result from paracrine interactions. No visual data about GnRH receptor in the pituitary are available. In order to identify the GnRH target cells in the pituitary of adult rats, the cellular distribution of rat GnRH receptor mRNA was investigated by electron microscopy, using in situ hybridization on ultrathin pituitary frozen sections. In situ hybridization was performed using a digoxigenin-labeled ol igonucleotide probe revealed by an indirect immunogold reaction. Gonadotropin-releasing hormone receptor mRNA was found in the cytoplasmic matrix, apposed to the endoplasmic reticulum and the nucleus of the gonadotrophs, which were identified by their ultrastructural characteristics, and by the presence of luteinizing hormone (LH) immunoreactivity. It was also found in the lactotrophs, which were revealed by the immunocytological detection of prolactin. No GnRH receptor mRNA was detected in corticotrophs, somatotrophs, thyrotrophs or hepatocytes. This result, without excluding paracrine effects, clearly showed that in addition to the gonadotrophs, the lactototrophs are likely to be direct target cells for the hypothalamic GnRH.
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