Transient reversal of olfactory preference following castration in male rats: Implication for estrogen receptor involvement

Reducing local synthesis of estrogen in the tubular striatum promotes attraction to same-sex odors in female mice

Brain-derived 17β-estradiol (E2) confers rapid effects on neural activity. The tubular striatum (TuS, also called the olfactory tubercle) is both capable of local E2 synthesis due to its abundant expression of aromatase and is a critical locus for odor-guided motivated behavior and odor hedonics. TuS neurons also contain mRNA for estrogen receptors α, β, and the G protein-coupled estrogen receptor. We demonstrate here that mRNA for estrogen receptors appears to be expressed upon TuS dopamine 1 receptor-expressing neurons, suggesting that E2 may play a neuromodulatory role in circuits which are important for motivated behavior. Therefore, we reasoned that E2 in the TuS may influence attraction to urinary odors which are highly attractive. Using whole-body plethysmography, we examined odor-evoked high-frequency sniffing as a measure of odor attaction. Bilateral infusion of the aromatase inhibitor letrozole into the TuS of gonadectomized female adult mice induced a resistance to habituation over successive trials in their investigatory sniffing for female mouse urinary odors, indicative of an enhanced attraction. All males displayed resistance to habituation for female urinary odors, indicative of enhanced attraction that is independent from E2 manipulation. Letrozole's effects were not due to group differences in basal respiration, nor changes in the ability to detect or discriminate between odors (both monomolecular odorants and urinary odors). Therefore, de novo E2 synthesis in the TuS impacts females' but not males' attraction to female urinary odors, suggesting a sex-specific influence of E2 in odor hedonics.

Neural and Hormonal Basis of Opposite-Sex Preference by Chemosensory Signals

In mammalian reproduction, sexually active males seek female conspecifics, while estrous females try to approach males. This sex-specific response tendency is called sexual preference. In small rodents, sexual preference cues are mainly chemosensory signals, including pheromones. In this article, we review the physiological mechanisms involved in sexual preference for opposite-sex chemosensory signals in well-studied laboratory rodents, mice, rats, and hamsters of both sexes, especially an overview of peripheral sensory receptors, and hormonal and central regulation. In the hormonal regulation section, we discuss potential rodent brain bisexuality, as it includes neural substrates controlling both masculine and feminine sexual preferences, i.e., masculine preference for female odors and the opposite. In the central regulation section, we show the substantial circuit regulating sexual preference and also the influence of sexual experience that innate attractants activate in the brain reward system to establish the learned attractant. Finally, we review the regulation of sexual preference by neuropeptides, oxytocin, vasopressin, and kisspeptin. Through this review, we clarified the contradictions and deficiencies in our current knowledge on the neuroendocrine regulation of sexual preference and sought to present problems requiring further study.

Why does castrated male odor attract sexually active male rats?-Attractivity induced by hypothalamus-pituitary-gonad axis block

Sexually experienced male rats show an olfactory preference for estrous female odor compared to male odor. Notably, they also prefer castrated male over gonadally intact male odor. This study examined the role of elevated circulating gonadotropins and gonadotropin-releasing hormone (GnRH) induced by disinhibition of the hypothalamus-pituitary-gonad axis in the castration-induced attractiveness of male rats. Experiment I compared preference of sexually experienced males between odors of castrated males injected with a GnRH antagonist (Cast+Ant), castrated males injected with saline (Cast), and gonadally intact males (Sham). These olfactory preference tests revealed that probe males significantly preferred Cast over Sham and Cast+Ant odors but had no preference between Sham and Cast+Ant odors. Experiment II used hypophysectomy (HPx) to remove the gonadotropin source in place of pharmacologically antagonizing GnRH. Similarly, the probe males preferred Cast (high both GnRH and gonadotropins) over HPx (high GnRH but no gonadotropin) odors, but also preferred HPx than Sham (low both GnRH and gonadotropins) odors. We then examined the effects of exogenous gonadotropins, human chorionic gonadotropin (hCG) and equine chorionic gonadotropin (eCG), to reveal which gonadotropins critically affect on the attractiveness of HPx males. The olfactory preference tests revealed that injecting eCG, but not hCG, significantly increased the probe males` preference compared to HPx male odor, suggesting that the attractiveness is augmented via follicle stimulating hormone (FSH) receptor rather than luteinizing hormone receptor. These results indicate that sexually active male rats show olfactory preference for castrated male rats due to elevated GnRH and FSH blood levels produced by a lack of androgens.

Social and neuromolecular phenotypes are programmed by prenatal exposures to endocrine-disrupting chemicals

Exposures to endocrine-disrupting chemicals (EDCs) affect the development of hormone-sensitive neural circuits, the proper organization of which are necessary for the manifestation of appropriate adult social and sexual behaviors. We examined whether prenatal exposure to polychlorinated biphenyls (PCBs), a family of ubiquitous industrial contaminants detectable in virtually all humans and wildlife, caused changes in sexually-dimorphic social interactions and communications, and profiled the underlying neuromolecular phenotype. Rats were treated with a PCB commercial mixture, Aroclor 1221 (A1221), estradiol benzoate (EB) as a positive control for estrogenic effects of A1221, or the vehicle (4% DMSO), on embryonic day (E) 16 and 18. In adult F1 offspring, we first conducted tests of ultrasonic vocalization (USV) calls in a sociosexual context as a measure of motivated communications. Numbers of certain USV call types were significantly increased by prenatal treatment with A1221 in males, and decreased by EB in females. In a test of sociosexual preference for a hormone-vs. a non-hormone-primed opposite sex conspecific, male (but not female) nose-touching with opposite-sex rats was significantly diminished by EDCs. Gene expression profiling was conducted in two brain regions that are part of the social decision-making network in the brain: the medial preoptic nucleus (MPN) and the ventromedial nucleus (VMN). In both regions, many more genes were affected by A1221 or EB in females than males. In female MPN, A1221 changed expression of steroid hormone receptor and neuropeptide genes (e.g., Ar, Esr1, Esr2, and Kiss1). In male MPN, only Per2 was affected by A1221. The VMN had a number of genes affected by EB compared to vehicle (females: Kiss1, Kiss1r, Pgr; males: Crh) but not A1221. These differences between EB and A1221 indicate that the mechanism of action of A1221 goes beyond estrogenic pathways. These data show sex-specific effects of prenatal PCBs on adult behaviors and the neuromolecular phenotype.

Aging and estradiol effects on gene expression in the medial preoptic area, bed nucleus of the stria terminalis, and posterodorsal medial amygdala of male rats

Studies on the role of hormones in male reproductive aging have traditionally focused on testosterone, but estradiol (E2) also plays important roles in the control of masculine physiology and behavior. Our goal was to examine the effects of E2 on the expression of genes selected for E2-sensitivity, involvement in behavioral neuroendocrine functions, and impairments with aging. Mature adult (MAT, 5 mo) and aged (AG, 18 mo) Sprague-Dawley male rats were castrated, implanted with either vehicle or E2 subcutaneous capsules, and euthanized one month later. Bilateral punches were taken from the bed nucleus of the stria terminalis (BnST), posterodorsal medial amygdala (MePD) and the preoptic area (POA). RNA was extracted, and expression of 48 genes analyzed by qPCR using Taqman low-density arrays. Results showed that effects of age and E2 were age- and region-specific. In the POA, 5 genes were increased with E2 compared to vehicle, and there were no age effects. By contrast the BnST showed primarily age-related changes, with 6 genes decreasing with age. The MePD had 5 genes that were higher in aged than mature males, and 17 genes with significant interactions between age and E2. Gene families identified in the MePD included nuclear hormone receptors, neurotransmitters and neuropeptides and their receptors. Ten serum hormones were assayed in these same males, with results revealing both age- and E2-effects, in several cases quite profound. These results support the idea that the male brain continues to be highly sensitive to estradiol even with aging, but the nature of the response can be substantially different in mature and aging animals.