Testosterone, corticosterone, and photoperiod interact to regulate plasma levels of binding globulin and free steroid hormone in Dark-eyed Juncos, Junco hyemalis

The pharmacology and regulation of corticosteroid binding globulins (CBG) i n Dark-eyed Juncos,Junco hyemalis, was investigated. The equilibrium dissoc iation constant for [H-3]corticosterone (CORT) binding to plasma was < 5 nM . This binding site had a similar high affinity for progesterone, approxima tely fivefold lower affinity for androgens, and negligible affinity for est radiol. The following data suggested that plasma CBG levels are regulated b y both testosterone and day length: (1) CBG binding capacity in free-living adult males was greater in early than in late breeding season and greater in males than in females and (2) CBG levels were higher in testosterone-tre ated, castrated males than in castrated males receiving no testosterone and still higher in testosterone-treated males exposed to long days than in si milar males exposed to short days. Birds apparently lack a sex steroid-spec ific binding globulin, but it was estimated that more than 90% of testoster one in junco plasma should bind to CBG. An increase in plasma CORT, such as occurs during a stress response, was judged to acutely increase free testo sterone levels as much as fivefold. Corticosterone and testosterone may thu s interact in a complex manner in species that lack sex hormone binding pro teins. <(c)> 2001 Academic Press.