Brain aromatization of testosterone in the male Syrian hamster: effects of androgen and photoperiod

Estrogen formed by aromatization of testosterone (T) is involved in the activation of sexual behavior and control of the neuroendocrine system in the male Syrian hamster. Our study examined whether daylength influences formation of estrogen in the preoptic area (POA) and other neuroendocrine areas (anterior hypothalamus, AHT, and medial amygdala, MA) which are targets for T in behaviorally active males. Estrogen formation in individual brain samples was assayed in vitro using the stereospecific production of 3H2O from (1 beta-3H) T as a measurement of aromatase activity. Serum levels of PRL, LH, FSH and T were compared with brain aromatase activity. Groups of intact, castrated and T-treated (chronic silastic T implants) male hamsters, previously selected on behavioral criteria as being sexually active, were maintained on long (16:8LD) or short (8:16LD) daylength for 16 weeks. Two further groups of males either intact or castrated and T-treated were shifted after 7 weeks from the long photoperiod to 12:12LD. POA, AHT and MA areas of sexually active males contained active aromatase systems which converted 3H-T to estrogens. Enzyme activity differed between the areas (POA, MA greater than AHT). Aromatase activity was low in medial septum and cerebral samples. Castration, which reduced serum T to undetectable levels and elevated LH and FSH, had no effect on preoptic aromatase activity. Although estrogen formation in POA did not differ between 8:16LD and 16:8LD males, castration reduced aromatase activity in AHT of both short- and long-day groups. Aromatase activity in AHT was also significantly reduced in photo-inhibited 12:12LD intact males. There was no analogous decrease in 5 alpha-reductase or 17 beta-hydroxysteroid dehydrogenase (HSD) activity, indicating a specific effect on the aromatase. The effect of photoperiod on aromatase activity was not reversed by T treatment. Therefore, photoinhibition acts in part through the effects of reduced T levels on the anterior hypothalamus, but it also acts independently of circulating T. Our results suggest that both androgen and photoperiod may regulate the AHT aromatase system and that this occurs by different mechanisms. The more active aromatase system in POA is insensitive to both castration and photoperiod. Behavioral deficits in short-day males are not due to changes in the preoptic aromatase system, but may be related to changes in aromatase activity within AHT. We conclude that there is a difference in the regulation of two locally active aromatase systems within the preoptic-anterior hypothalamic complex of the male hamster.