We have used a direct, non-immunochemical and highly accurate method to quantify the effects of testosterone and oestrogen on mitotic and apoptotic activity in the young, male rat anterior pituitary in vivo. Surgical gonadectomy resulted in a 3-fold increase in mitotic activity by the fourth post-operative day, which returned gradually to levels seen in intact animals over the subsequent 3–4 weeks. Both a single dose of Sustanon, a mixture of long-acting testosterone esters in arachis oil, and

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... he same dose divided over 7 days (starting 6 days after gonadectomy), initially suppressed mitotic activity to levels seen in intact animals, but was associated after 48–96 h with a wave of increased mitotic activity. The latter was blocked by co-administration of Sustanon with the non-steroidal aromatase inhibitor letrozole and was not seen when the non-aromatisable androgen dihydrotestosterone was substituted for Sustanon. Oestrogen alone in gonadectomised and intact rats produced a marked increase in mitosis as expected. With the exception of a transient increase in response to a single high-dose injection of Sustanon in gonadectomised animals, apoptotic activity was unaffected by all of the above. This study suggests that pituitary mitotic activity is tonically inhibited by gonadal hormone production (at least in the short term) in adult male rats. The study also suggests that supraphysiological testosterone treatment – while unable to reduce anterior pituitary mitotic activity in untreated, intact animals –suppresses the early increase in mitotic activity induced by gonadectomy. Oestrogen, either exogenous or generated locally by aromatisation, stimulates anterior pituitary mitotic activity in a time-dependent manner.