The anterior pituitary is active mitotically and apoptotically under basal conditions and in response to a variety of physiological and pathophysiological stimuli. Hypothyroidism in man is associated with a modest but very occasionally dramatic increase in overall pituitary size. The mechanisms underlying this reversible phenomenon remain obscure. In the present study we have examined young adult rat anterior pituitary following surgical thyroidectomy and subsequent thyroid hormone treatment and withdrawal using an extremely accurate system for quantifying directly identified mitotic and apoptotic events. Despite the expected increase in the number and/or proportion of immunohistochemically identifiable thyrotrophs three weeks after thyroidectomy, mitotic and apoptotic activity remained unchanged, as did pituitary wet weight, in comparison with sham-operated and intact controls. In contrast, mitotic but not apoptotic activity was enhanced by treatment of thyroidectomized animals with thyroid hormones (triiodothyronine (T 3 ) and thyroxine (T 4 ) 1·8 µg and 3·6 µg/100 g body weight per day respectively), and once again declined to levels seen in intact animals within 72 h of subsequent thyroid hormone withdrawal. Thyroid hormone-induced enhancement of mitotic activity was also seen in intact rats treated with similar doses of thyroid hormones for 7 days and in thyroidectomized rats treated for a similar period with very low dose thyroid hormone replacement at a level that had no effect on raised hypothalamic TRH-or pituitary TSH -transcript prevalence (0·018 µg T 3 plus 0·036 µg T 4 /100 g body weight per day). Thus changes in mitotic and apoptotic activity are unlikely to be the principle mechanism for the apparent increase in thyrotrophs up to 4 weeks after thyroidectomy. In contrast, the data indicate that thyroid hormones have a permissive effect on anterior pituitary mitotic activity in thyroidectomized male rats. Thyroid hormone-induced enhancement of mitotic activity in intact rats further suggests that in euthyroid rats, ambient thyroid hormone levels are a limiting factor for anterior pituitary mitotic activity. In summary, this time course study of young, male rats has shown for the first time that thyroidectomy, thyroid hormone replacement and subsequent withdrawal has no significant effect on anterior pituitary apoptotic activity. Secondly, it has shown that the anterior pituitary mitotic response to thyroidectomy is blocked by complete thyroid hormone deprivation, but can be restored by very low level thyroid hormone replacement, and thirdly that in intact animals thyroid hormone levels significantly limit anterior pituitary mitotic activity.

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 hypothalamohypophyseal 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 adrenalectomyinduced 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.

Objectives: Glucocorticoid withdrawal in man is associated with transient but sometimes prolonged impairment of hypothalamo -pituitary -adrenal axis secretory responsiveness. This has led to continued concern in the clinical arena. The acute anterior pituitary response to glucocorticoids in the rat includes apoptosis-mediated deletion of a cell population. Whilst continued cell turnover following glucocorticoid withdrawal and the potential for differentiation of uncommitted precursor cells and transdifferentiation of other secretory cell types predicts that, given sufficient time, complete anterior pituitary trophic recovery is likely, this hypothesis has not previously been tested. Design and methods: We have quantified pituitary mitotic and apoptotic rate, as well as corticotroph number and pro-opiomelanocortin transcript prevalence, together with hypothalamic corticotrophin-releasing hormone and vasopressin transcript prevalence 5 weeks after three short dexamethasone treatments each separated by a week. Bilateral adrenalectomy was then carried out as a maximal secretory and trophic stimulus, and the response to a fourth dexamethasone treatment assessed 1 week later. Results: Anterior pituitary mitotic index was significantly higher in rats previously exposed to dexamethasone compared with age-matched controls exposed to dexamethasone for the first time. No differences were found in the subsequent apoptotic response to a fourth dexamethasone treatment or in the levels of paraventricular corticotrophin-releasing hormone or vasopressin transcripts or pituitary pro-opiomelanocortin transcripts. Conclusions: These data indicate that full recovery of pituitary mitotic activity takes longer than the recovery of secretory parameters, and suggest that, for several weeks after glucocorticoid exposure, the ability of the pituitary to meet fresh demands placed on it may be suboptimal.