Sexual hearing: the influence of sex hormones on acoustic communication in frogs

To modulate and be modulated: estrogenic influences on auditory processing of communication signals within a socio-neuro-endocrine framework

Gonadal hormones modulate behavioral responses to sexual stimuli, and communication signals can also modulate circulating hormone levels. In several species, these combined effects appear to underlie a two-way interaction between circulating gonadal hormones and behavioral responses to socially salient stimuli. Recent work in songbirds has shown that manipulating local estradiol levels in the auditory forebrain produces physiological changes that affect discrimination of conspecific vocalizations and can affect behavior. These studies provide new evidence that estrogens can directly alter auditory processing and indirectly alter the behavioral response to a stimulus. These studies show that: 1) Local estradiol action within an auditory area is necessary for socially relevant sounds to induce normal physiological responses in the brains of both sexes; 2) These physiological effects occur much more quickly than predicted by the classical time-frame for genomic effects; 3) Estradiol action within the auditory forebrain enables behavioral discrimination among socially relevant sounds in males; and 4) Estradiol is produced locally in the male brain during exposure to particular social interactions. The accumulating evidence suggests a socio-neuro-endocrinology framework in which estradiol is essential to auditory processing, is increased by a socially relevant stimulus, acts rapidly to shape perception of subsequent stimuli experienced during social interactions, and modulates behavioral responses to these stimuli. Brain estrogens are likely to function similarly in both songbird sexes because aromatase and estrogen receptors are present in both male and female forebrain. Estrogenic modulation of perception in songbirds and perhaps other animals could fine-tune male advertising signals and female ability to discriminate them, facilitating mate selection by modulating behaviors.

Steroid receptor expression in the fish inner ear varies with sex, social status, and reproductive state

Background

Gonadal and stress-related steroid hormones are known to influence auditory function across vertebrates but the cellular and molecular mechanisms responsible for steroid-mediated auditory plasticity at the level of the inner ear remain unknown. The presence of steroid receptors in the ear suggests a direct pathway for hormones to act on the peripheral auditory system, but little is known about which receptors are expressed in the ear or whether their expression levels change with internal physiological state or external social cues. We used qRT-PCR to measure mRNA expression levels of multiple steroid receptor subtypes (estrogen receptors: ERα, ERβa, ERβb; androgen receptors: ARα, ARβ; corticosteroid receptors: GR2, GR1a/b, MR) and aromatase in the main hearing organ of the inner ear (saccule) in the highly social African cichlid fish Astatotilapia burtoni, and tested whether these receptor levels were correlated with circulating steroid concentrations.

Results

We show that multiple steroid receptor subtypes are expressed within the main hearing organ of a single vertebrate species, and that expression levels differ between the sexes. We also show that steroid receptor subtype-specific changes in mRNA expression are associated with reproductive phase in females and social status in males. Sex-steroid receptor mRNA levels were negatively correlated with circulating estradiol and androgens in both males and females, suggesting possible ligand down-regulation of receptors in the inner ear. In contrast, saccular changes in corticosteroid receptor mRNA levels were not related to serum cortisol levels. Circulating steroid levels and receptor subtype mRNA levels were not as tightly correlated in males as compared to females, suggesting different regulatory mechanisms between sexes.

Conclusions

This is the most comprehensive study of sex-, social-, and reproductive-related steroid receptor mRNA expression in the peripheral auditory system of any single vertebrate. Our data suggest that changes in steroid receptor mRNA expression in the inner ear could be a regulatory mechanism for physiological state-dependent auditory plasticity across vertebrates.

Topography of estradiol-modulated genomic responses in the songbird auditory forebrain

Sex steroids facilitate dramatic changes in behavioral responses to sociosexual signals and are increasingly implicated in the sensory processing of those signals. Our previous work demonstrated that in female white-throated sparrows, which are seasonal breeders, genomic responses in the auditory forebrain are selective for conspecific song over frequency-matched tones only when plasma estradiol (E2) reaches breeding levels. Here, we sought to map this E2-dependent selectivity in the best-studied area of the auditory forebrain, the caudomedial nidopallium (NCM). Nonbreeding females with low endogenous levels of E2 were treated with E2 or a placebo and exposed to conspecific song, tones, or no sound playback. Immunoreactive protein product of the immediate early gene zenk (egr-1) was then quantified within seven distinct subregions, or domains, of NCM. We report three main findings: (1) regardless of hormone treatment, the zenk response is significantly higher in dorsal than in ventral NCM, and higher in medial than in lateral NCM; (2) E2-dependent selectivity of the response is limited to the rostral and medial domains of NCM; in the more caudal domains, song induces more zenk expression than tones regardless of hormone treatment; (3) even when no sound stimuli were presented, E2 treatment significantly increased zenk expression in the rostral, but not the caudal, domains of NCM. Together, the latter two findings suggest that E2-dependent plasticity in NCM is concentrated in rostral NCM, which is hodologically and neurochemically distinct from caudal NCM. Activity in rostral NCM may therefore be seasonally regulated in this species.