HATTORI, Tomoko; BREUNER , Creagh W; WILCZYNSKI , Walter ; The University of Texas at Austin ; The University of Texas at Austin ; The University of Texas at Austin : Comparison of total and free steroid hormones in dominant and subordinate male green anole lizards

The presence of plasma binding globulins such as corticosterone-binding globulin (CBG) and sex hormone binding globulin (SHBG) are often ignored in assessing hormone-behavior interactions, despite the fact that plasma binding globulins play important roles in controlling levels of the free (unbound, bioactive) form of hormones. In the present study, we used radioligand binding methods to characterize the plasma binding globulins in male green anole lizards, and used the information to understand how plasma binding globulins and/or free hormone levels might relate to differences in behavior and physiology of dominant and subordinate animals. Competition studies and equilibrium saturation analysis confirmed the presence of two types of binding globulins in this species, similar to those found in tree lizards. The first, a putative SHBG, binds testosterone and estradiol (testosterone: Kd=9.4nM). The second binds both testosterone and corticosterone with relatively low affinity (testosterone: Kd=47nM, corticosterone: Kd=78nM), and is best described as an androgen-glucocorticoid binding globulin (AGBG; Jennings et al. 2000). Total (bound plus unbound) testosterone and corticosterone levels, and AGBG and SHBG capacities, were similar regardless of dominant/subordinate status. Free corticosterone levels were also similar between these groups. Free testosterone levels, however, were significantly higher in the dominant animals (p=0.041). Our findings suggest that free testosterone levels may relate to the behavioral differences between dominant and subordinate animals. This study is supported by NSF IBN 0090739.