Seasonal aromatase activity in the brain of the male red-sided garter snake

We investigated regional and seasonal variations in neural aromatase activity (AA), the enzyme that converts androgens into estrogens, to examine a possible indirect role of testosterone (T) in mediating spring reproductive behavior of red-sided garter snakes, a species exhibiting a dissociated reproductive pattern. Neural AA in male snakes varied significantly among brain regions. Additionally, there were significant interactions between brain region and season. In the spring, actively courting males had greater AA in the olfactory region (O) compared to the septum/anterior-hypothalamus preoptic area (S/AHPOA), nucleus sphericus (NS) and midbrain (Mb). Fall animals collected as they returned to the den prior to winter dormancy had significantly greater AA in the S/AHPOA compared to all other regions. These findings were consistent using either regional (gross) dissection or punch microdissection, which allowed us to separate the S and AHPOA. There were no significant differences in AA production between the S and AHPOA. This study provides the first documentation of seasonal and regional variations in AA in a snake brain and suggests that aromatization of androgens may play a role in regulating reproduction in red-sided garter snakes. During spring mating, elevated AA in the O may activate pathways essential for detection of courtship pheromones, while increased AA in the S and AHPOA of fall animals suggests that circulating androgens play an indirect role in programming critical neural pathways involved in reproduction. Thus, as in many other vertebrates, estrogenic metabolites of testosterone may be a critical hormonal component regulating reproductive behavior in this dissociated breeder.