Opposing effects of D-aspartic acid and nitric oxide on tuning of testosterone production in mallard testis during the reproductive cycle

<p>Abstract</p> <p>Background</p> <p>D-Aspartic acid (D-Asp) and nitric oxide (NO) play an important role in tuning testosterone production in the gonads of male vertebrates. In particular, D-Asp promotes either the synthesis or the release of testosterone, whereas NO inhibits it. In this study, we have investigated for the first time in birds the putative effects of D-Asp and NO on testicular testosterone production in relation to two phases of the reproductive cycle of the adult captive wild-strain mallard (Anas platyrhynchos) drake. It is a typical seasonal breeder and its cycle consists of a short reproductive period (RP) in the spring (April-May) and a non reproductive period (NRP) in the summer (July), a time when the gonads are quiescent. The presence and the localization of D-Asp and NO in the testis and the trends of D-Asp, NO and testosterone levels were assessed during the main phases of the bird's reproductive cycle. Furthermore, in vitro experiments revealed the direct effect of exogenously administered D-Asp and NO on testosterone steroidogenesis.</p> <p>Methods</p> <p>By using immunohistochemical (IHC) techniques, we studied the presence and the distributional pattern of D-Asp and NO in the testes of RP and NRP drakes. D-Asp levels were evaluated by an enzymatic method, whereas NO content, via nitrite, was assessed using biochemical measurements. Finally, immunoenzymatic techniques determined testicular testosterone levels.</p> <p>Results</p> <p>IHC analyses revealed the presence of D-Asp and NO in Leydig cells. The distributional pattern of both molecules was in some way correlated to the steroidogenic pathway, which is involved in autocrine testosterone production. Indeed, whereas NO was present only during the NRP, D-Asp was almost exclusively present during the RP. Consistently, the high testosterone testicular content occurring during RP was coupled to a high D-Asp level and a low NO content in the gonad. By contrast, in sexually inactive drakes (NRP), the low testosterone content in the gonad was coupled to a low D-Asp content and to a relatively high NO level. Consequently, to determine the exogenous effects of the two amino acids on testosterone synthesis, we carried out in vitro experiments using testis sections deriving from both the RP and NRP. When testis slices were incubated for 60 or 120 min with D-Asp, testosterone was enhanced, whereas in the presence of L-Arg, a precursor of NO, it was inhibited.</p> <p>Conclusion</p> <p>Our results provide new insights into the involvement of D-Asp and NO in testicular testosterone production in the adult captive wild-strain mallard drake. The localization of these two molecules in the Leydig cells in different periods of the reproductive cycle demonstrates that they play a potential role in regulating local testosterone production.</p

Bill color, not badge size, indicates testosterone-related information in house sparrows

The honesty of ornamental signals of quality is often argued to be enforced via costs associated with testosterone. It is still poorly understood, however, how seasonal variation of testosterone within individuals is related to the timing and extent of ornament development. Here, we studied inter- and intra-individual variability of plasma testosterone levels in a population of 150 captive male house sparrows (Passer domesticus) through the course of a full year. We further analyzed the relationship between plasma testosterone levels and two sexually dimorphic ornaments: badge size and bill coloration. Also, because of a known negative relation between molt and circulating testosterone levels, we analyzed the relationship between ornamentation and molt status during the fall. We found that testosterone levels increased towards the breeding season and decreased before the onset of annual molt. However, within individuals, relative testosterone titers demonstrated low repeatability between seasons. Plasma testosterone levels were not correlated with badge size in any season but were correlated strongly with bill coloration during all periods, except the breeding season when variation in bill color was low. Finally, we found that bill coloration strongly correlated with molt status during fall. Our results indicate that bill coloration, not badge size, is the best ornamental indicator of a “running average” of male testosterone in house sparrows and therefore the best potential indicator of qualities and/or behavioral strategies associated with testosterone