There are in contrast many

There are in contrast many studies demonstrating that sex steroids modulate gssg processing of stimuli related to reproduction and this correlates with the observation that steroid receptors and aromatase are expressed in many brain nuclei that are part of the olfactory, visual and auditory pathways. Nearly 50 years ago, Pfaff and Pfaffmann already discovered that sexually relevant olfactory inputs only reach the preoptic area of male rats if they are exposed to elevated concentrations of testosterone (Pfaff and Pfaffmann, 1969). Castration does not modify the detection of these signals by the olfactory receptors as shown by the fact that they elicit neural activity in the olfactory bulbs, but the surgery completely blocks the transfer of this information to the preoptic region, and this effect is reversed by a treatment with exogenous testosterone. Importantly, this effect of testosterone is specific to olfactory signals controlling sexual behavior (pheromones): other olfactory signals never reach the preoptic area independently of the endocrine condition of the subjects. This concept (central rather than peripheral controls) is supported by multiple studies based namely on the detection of brain activity via the demonstration of increased immediate early gene expression (fos or egr-1) that have subsequently confirmed the role of sex steroids in the selective processing of sexually relevant olfactory stimuli (Baum and Bakker, 2013; Petrulis, 2013; Wood and Newman, 1995). Other sensory modalities are similarly affected by sex steroids. This is not extensively documented for the visual system (Little, 2013) and in this specific case effects of estrogen seem to be mostly impacting the eye (Gupta et al., 2005; Hutchinson et al., 2014). The work of Richmond Thompson and collaborators in goldfishes (Carassius auratus) similarly demonstrates rapid actions of estrogens on the processing of visual stimuli and effects are mediated at least in part in the eye although this work does not exclude so far additional effects in the central nervous system (Lord et al., 2009; Mangiamele et al., 2017; Yue et al., 2018). In contrast, there is an extensive literature demonstrating effects of sex steroids on the perception and processing of somatosensory inputs, in particular pain stimuli. This topic is however complex and the modulatory effect of sex steroids on nociception seems to vary as a function of the sex of the subjects, the type of painful stimuli and other specific aspects of the experimental protocols. This topic has been reviewed multiple times recently (see for recent reviews: (Amandusson and Blomqvist, 2013; Gintzler and Liu, 2012; Traub and Ji, 2013)) and is beyond the scope of the present paper. The modulation of audition by steroids has also received a substantial attention in a variety of animal models including humans (Caras, 2013) and there is in particular a very substantial literature of the effect of estrogens on the processing of auditory stimuli, which is the focus of this review.
Estrogens and audition There is multidimensional correlative evidence suggesting that estrogens modulate auditory function in humans. Women demonstrate a better perceptual sensitivity for high frequency sounds compared to males, their auditory function fluctuates in parallel with circulating estrogen concentrations during the menstrual cycle, and auditory function is decreased during pregnancy and after menopause but can be restored by a treatment with exogenous estrogens (reviewed in (Caras, 2013)). The causal value of these correlations has not been tested in humans for obvious ethical reasons but there is a variety of animal studies in fishes, frogs, birds and mammals demonstrating in a causal way the impact of estrogens on the processing of auditory stimuli (Caras, 2013; Maney and Pinaud, 2011; Miranda and Liu, 2009; Sisneros, 2009; Wilczynski et al., 1993; Zornik and Kelley, 2011). We briefly review here the available information related to songbirds that are the focus of this review and were used in our functional Magnetic Resonance Imaging (fMRI) studies.