Nonneural Androgen Receptors Affect Sexual Differentiation of Brain and Behavior

Abstract

Testosterone, acting via estrogenic and androgenic pathways, is the major endocrine mechanism promoting sexual differentiation of the mammalian nervous system and behavior, but we have an incomplete knowledge of which cells and tissues mediate these effects. To distinguish between neural and nonneural actions of androgens in sexual differentiation of brain and behavior, we generated a loxP-based transgenic mouse, which overexpresses androgen receptors (ARs) when activated by Cre. We used this transgene to overexpress AR globally in all tissues using a cytomegalovirus (CMV)-Cre driver (CMV-AR), and we used a Nestin-Cre driver to overexpress AR only in neural tissue (Nes-AR). We then examined whether neural or global AR overexpression can affect socio-sexual behaviors using a resident-intruder paradigm. We found that both neural and global AR overexpression resulted in decreased aggressive behaviors and increased thrusting during mounting of intruders, consistent with a neural site of action. Global, but not neural, AR overexpression in males led to an increase in same-sex anogenital investigation. Together, these results suggest novel roles for nonneural AR in sexual differentiation of mice, and indicate that excess AR can lead to a paradoxical reduction of male-typical behavior.

Non‐neural androgen receptors affect sexual differentiation of brain and behaviour

“…Validation of the genetic manipulation was carried out by measuring AR immunoreactivity in neural and non‐neural tissues, as well as by examining the effect of transgene expression on a known phenotypic endpoint of androgen signalling. We find that AR‐immunoreactive nuclei are more numerous in the brain but not in the skeletal muscle of Nestin‐AR mice relative to WT mice, whereas AR‐immunoreactive nuclei are more numerous in both the brain and skeletal muscle of CMV‐AR mice relative to WT mice . Furthermore, masculinisation of the glans clitoris was observed in female CMV‐AR mice but not Nestin‐AR mice, indicating that the transgene AR are functional …”

“…This explanation appears to be unlikely because many features of mutant males were masculine (Table ). Furthermore, we saw no decrease in testosterone production in either Nestin‐AR or CMV‐AR males . To account for these results, we must therefore invoke tissue and/or system‐specific mechanisms that alter the dose relationship between AR and masculine phenotype.…”

“…The loss of function approach to studying gene function is undeniably powerful, although it does leave several questions unresolved. Gene overexpression provides a complementary approach that allows us to examine the upper limits of gene dose and this can yield insights into the regulatory systems in place for the phenotypes in question . In the case of AR, we can consider whether a higher androgen dose results in a greater masculine phenotype.…”

“…The base transgene in these combinations is CMV‐STOP‐AR. This transgene has a strong universal promoter (CMV [cytomegalovirus]) that is prevented from driving expression of an AR coding sequence by an intervening stop sequence flanked by loxp sites . These CMV‐STOP‐AR mice are then bred with driver mice that either express Cre globally using a CMV‐Cre transgene (yielding CMV‐AR mice) or specifically in the nervous system using a Nestin‐Cre transgene (yielding Nestin‐AR mice).…”

“…However, in CMV‐AR mice, which globally overexpress AR, we saw no evidence of loss of masculine phenotype in the genitals or urogenital system and masculine sexual behaviours were increased, indicating that the apparent loss of AR function was not global. These observations lead us to consider that a simple cellular mechanism where overexpression results in loss of AR function is unlikely to account for the phenotype of CMV‐AR mice . A satisfying explanation would therefore require that a cellular mechanism would be differentiated according to affected system, so that some brain structures, for example, would be affected but not others.…”