Mammotroph and gonadotroph volume percentage in the rat anterior pituitary after lesions of the medial basal hypothalamus.

A lesion of the medial basal hypothalamus (MBH) results in enhanced secretion of prolactin and reduced release of the other hormones from the anterior pituitary. We measured the volume percentage of the prolactin-secreting mammotrophs, stained immunocytochemically, under several experimental conditions to assess the morphological correlated of the high secretion of prolactin. In addition, gonadotrophs were stained both tinctorially with the periodic acid-Schiff reagent and immunocytochemically with antibodies to luteinizing hormone. Anterior pituitaries were studied from female rats which were: (1) cycling, (2) ovariectomized for 2 weeks, (3) ovariectomized for 2 weeks with a lesion of the MBH for 1, 14 and 21 days. The volume percentage of mammotrophs and gonadotrophs in the lateral, central and middle fields of the anterior pituitary was determined and this measurement was expressed relative to endocrine cellular area so that vascular space and necrotic regions would not be included in the assessment. Ovariectomy significantly decreased mammotroph and increased gonadotroph volume percentage when compared to the intact animals. MBH lesions had the opposite effect. At 14 and 21 days after a lesion, the volume percentage of mammotrophs was significantly greater than in the intact rat, while the volume percentage of gonadotrophs was identical to that observed in the intact rat. These data suggest that the elimination of the prolactin-inhibitory hormone(s) of the MBH result in hypertrophy and/or hyperplasia of mammotrophs. This effect occurs in the absence of ovarian and may be of importance in the induction of prolactin-secreting pituitary adenomas. The data also support the hypothesis that destruction of gonadotropin-releasing hormone neuron terminals countermands the hypertrophy of gonadotrophs induced by ovariectomy.