Social regulation of cortisol receptor gene expression

Abstract

In many social species, individuals influence the reproductive capacity of conspecifics. In a well-studied African cichlid fish species, Astatotilapia burtoni, males are either dominant (D) and reproductively competent or non-dominant (ND) and reproductively suppressed as evidenced by reduced gonadotropin releasing hormone (GnRH1) release, regressed gonads, lower levels of androgens and elevated levels of cortisol. Here, we asked whether androgen and cortisol levels might regulate this reproductive suppression. Astatotilapia burtoni has four glucocorticoid receptors (GR1a, GR1b, GR2 and MR), encoded by three genes, and two androgen receptors (ARα and ARβ), encoded by two genes. We previously showed that ARα and ARβ are expressed in GnRH1 neurons in the preoptic area (POA), which regulates reproduction, and that the mRNA levels of these receptors are regulated by social status. Here, we show that GR1, GR2 and MR mRNAs are also expressed in GnRH1 neurons in the POA, revealing potential mechanisms for both androgens and cortisol to influence reproductive capacity. We measured AR, MR and GR mRNA expression levels in a microdissected region of the POA containing GnRH1 neurons, comparing D and ND males. Using quantitative PCR (qPCR), we found D males had higher mRNA levels of ARα, MR, total GR1a and GR2 in the POA compared with ND males. In contrast, ND males had significantly higher levels of GR1b mRNA, a receptor subtype with a reduced transcriptional response to cortisol. Through this novel regulation of receptor type, neurons in the POA of an ND male will be less affected by the higher levels of cortisol typical of low status, suggesting GR receptor type change as a potential adaptive mechanism to mediate high cortisol levels during social suppression. KEY WORDS: Cichlid, Glucocorticoid receptors, Social stress, Androgen receptors INTRODUCTIONSocial status hierarchies are a ubiquitous organizing principle of social systems in many animal species from ants (Wilson, 2000) to primates (Cheney and Seyfarth, 1990), including humans (Chiao et al., 2009). Such hierarchies typically regulate access to resources including territories, food and/or mates, granting individuals of high status access to these resources and requiring those of low status to find alternative solutions to survive. In many species, the reproductive capacity of low status individuals is reduced and mating is not possible because of behavioral and physiological changes (e.g. Willisch et al., 2012). Typically, aggressive encounters establish and maintain social rank, which, in turn, produces significant differences in reproductive capacity. In such social systems, levels of stress and reproductive hormones directly reflect the social rank of individuals and may play a causal role in behavioral and physiological changes. RESEARCH ARTICLEThe relationship between social dominance and reproductive physiology has been well studied in an African cichlid fish, Astatotilapia burtoni (Günther 1894), in which status regulates both reproductive access and...

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