Expression of 5α- and 5β-reductase in spinal cord and muscle of birds with different courtship repertoires

Background: Through the actions of one or more isoforms of the enzyme 5 alpha-reductase in many male reproductive tissues, circulating testosterone (T) undergoes metabolic conversion into 5 alpha-dihydrotestosterone (DHT), which binds to and activates androgen receptors (AR) with greater potency than T. In birds, T is also subject to local inactivation into 5 beta-DHT by the enzyme 5 beta-reductase. Male golden-collared manakins perform an androgen-dependent and physically elaborate courtship display, and these birds express androgen receptors in skeletal muscles and spinal cord at levels far greater than those expressed in species with more limited courtship routines, including male zebra finches. To determine if local T metabolism facilitates or impedes activation of male manakin courtship, we examined expression of two isoforms of 5 alpha-reductase, as well as 5 beta-reductase, in forelimb muscles and spinal cords of males and females of the two aforementioned species. Results: We found that all enzymes were expressed in all tissues, with patterns that partially predict a functional role for 5 alpha-reductase in these birds, especially in both muscle and spinal cord of male manakins. Moreover, we found that 5 beta-reductase was markedly different between species, with far lower levels in golden-collared manakins, compared to zebra finches. Thus, modification to neuromuscular deactivation of T may also play a functional role in adaptive behavioral modulation. Conclusions: Given that such a role for 5 alpha-reductase in androgen-sensitive mammalian skeletal muscle is in dispute, our data suggest that, in birds, local metabolism may play a key role in providing active androgenic substrates to peripheral neuromuscular systems. Similarly, we provide the first evidence that 5 beta-reductase is expressed broadly through an organism and may be an important factor that regulates androgenic modulation of neuromuscular functioning.