We have previously identified a series of age-dependent, temporally constrained and closely interdependent mitotic and apoptotic events in the male rat anterior pituitary that occur in response to timed single and repeated hypothalamo-pituitary-adrenal axis stimuli. One of the most dramatic of these is the short burst of apoptosis that occurs 24-48 h after exposure to dexamethasone. If bilateral adrenalectomy precedes exposure to dexamethasone by 1-2 weeks, mitotic activity is transiently increased and the subsequent apoptotic response to dexamethasone greatly enhanced. This study was designed to determine whether adrenalectomy-induced augmentation of the apoptotically sensitive pituitary cell population is mediated via glucocorticoid withdrawal at the level of the pituitary, or whether increased exposure to hypothalamo-hypophyseal trophic hormones of paraventricular origin is responsible. We used stereotaxic surgery to isolate both paraventricular nuclei without disturbing either median eminence input from the arcuate and supraoptic nuclei, or the hypothalamo-hypophyseal-portal blood flow that carries a significant proportion of the pituitary systemic supply. When bilateral adrenalectomy and paraventricular nucleus disconnection were combined, the adrenalectomy-induced increase in anterior pituitary pro-opiomelanocortin (POMC) transcript prevalence was abolished, confirming the loss of paraventricular corticotrophin-releasing hormone (CRH) input. However, the amplitude and pattern of the adrenalectomy-induced anterior pituitary mitotic response and enhancement of the apoptotic response to dexamethasone 1 week later remained completely intact. These data demonstrate that anterior pituitary trophic responses following bilateral adrenalectomy are more likely to be mediated through direct glucocorticoid withdrawal at the level of the pituitary rather than via changes in hypothalamo-hypophyseal releasing factor exposure. This finding highlights the presence of distinct control systems for pituitary hormone gene expression and pituitary mitotic and apoptotic responses.

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