Establishment of partner preference in male rats: Effect of prenatal letrozole and sexual experience

Cohabitation with receptive females under D2-type agonism in adulthood restores partner preference and brain dimorphism in the SDN-POA following neonatal gonadectomy in male rats

Perinatal testosterone, or its metabolite estradiol, organize the brain toward a male phenotype. Male rodents with insufficient testosterone during this period fail to display sexual behavior and partner preference for receptive females in adulthood. However, cohabitation with non-reproductive conspecifics under the influence of a D2 agonist facilitates the expression of conditioned partner preference via Pavlovian learning in gonadally intact male rats. In the present experiment, three groups of neonatal PD1 males (N = 12/group) were either gonadectomized (GDX), sham-GDX, or left intact and evaluated for social preferences and sexual behaviors as adults. We then examined whether the effects of GDX could be reversed by conditioning the males via cohabitation with receptive females under the effects of the D2 agonist quinpirole (QNP) or saline, along with the size of some brain regions, such as the sexually dimorphic nucleus of the preoptic area (SDN-POA), suprachiasmatic nucleus (SCN), posterior dorsal medial amygdala (MeApd) and ventromedial hypothalamus (VMH). Results indicated that neonatal GDX resulted in the elimination of male-typical sexual behavior, an increase in same-sex social preference, and a reduction of the area of the SDN-POA. However, GDX-QNP males that underwent exposure to receptive females in adulthood increased their social preference for females and recovered the size in the SDN-POA. Although neonatal GDX impairs sexual behavior and disrupts partner preference and brain dimorphism in adult male rats, Pavlovian conditioning under enhanced D2 agonism ameliorates the effects on social preference and restores brain dimorphism in the SDN-POA without testosterone.

Multiparity favors same-sex partner preference in male rats

Same-sex partner preference is present in many mammals, including rodents. Several possible causal factors have been proposed for the establishment of this preference. The Fraternal Birth Order effect refers to the observation that older brothers increase the probability of homosexuality in men, but no experiment has analyzed this possibility. In this study, partner preference (tested in a three compartments box) and female and male sexual behavior (studied in a cylindrical arena) were evaluated in young male rats (3 months) born to multiparous mothers that had 4-6 previous gestations and around 12 months of age. Control groups were young male rats born to primiparous young (4 months) or aged (12 months) mothers. In the partner preference test, the males born to multiparous dams spent less time interacting with the receptive female and more time interacting with the sexually active male, and a 39% exhibited same-sex partner preference. This high percentage seems related to multiparity of their mothers and not to maternal age, because the males born to primiparous aged females (12 months) showed a similar low proportion of same-sex partner preference than the males born to young (4 months) primiparous females (4%). In the sexual behavior tests, no male born of a multiparous dam and with same-sex preference ejaculated and 54% displayed proceptivity and lordosis. Present results suggest that the fraternal birth order effect may occur also in rats.

Distribution of estrogen receptors alpha and beta in the brain of male rats with same-sex preference

Two distinct estrogen receptors (ERs) exist, ERα and ERβ. Both receptors participate in the sexual differentiation of the rat brain and likely participate in the regulation of adult sexual orientation (i.e. partner preference). This last idea was investigated herein by examining males treated with the aromatase inhibitor, letrozole, administered prenatally (0.56 μg/kg G10-22). This treatment usually provokes same-sex preference in 1-2 males per litter. Vehicle-treated males (with female preference) and females in spontaneous proestrus (with male preference) were included as controls. ERα and ERβ expression was analyzed by immunohistochemistry in brain areas known to control masculine sexual behavior and partner preference, like the medial preoptic area (MPOA), bed nucleus of the stria terminalis (BNST), medial amygdala (MeA) and ventromedial hypothalamic nucleus (VMH), as well as other brain regions suspected to participate in these processes. In addition, serum levels of estradiol were determined in all male groups. Letrozole-treated male rats that preferred sexually experienced males (LPM) showed over-expressed ERα in the hippocampal cornu Ammonis (CA 1, 3, 4) and dentate gyrus. The LPM group showed up-regulated ERβ expression in the CA2 and reticular thalamic nucleus. The levels of estradiol did not differ between the groups. The higher expression of ERs in these males was different than their expression in females, with male sex-preference. This suggests that males with same-sex preference showed a unique brain, this sui generis steroid receptor expression probably participates in the biological underpinnings of sexual preference.

Androgen receptors immunoreactivity in the rat brain of males with same-sex preference

Androgen receptors (AR) are crucial in the control of male sexual behavior and sex preference. AR are particularly concentrated in areas related with the neuroendocrine control of sex preference including the medial amygdala (MeA), the ventromedial nucleus of the hypothalamus (VMH), the bed nucleus of the stria terminalis (BNST), the medial preoptic area (MPOA), the nucleus accumbens (Acb), the suprachiasmatic (SCh) and supraoptic (SO) nuclei, but also seem to be important for the control of reproductive processes in the hippocampus (CA1-CA4 and dentate gyrus, DG). In the present study we analyzed the density of AR in these brain areas of adult male rats with sexual preference (established in a three-compartment box). Same-sex preference was produced in male rats by the prenatal administration of the aromatase inhibitor, letrozole (0.56 μg/kg/ml s.c. G10-22) that usually produces 1-2 animals per litter with same sex preference, while the others retain a female sex preference. We also included a group of proestrus females that had a clear preference for a sexually active male. AR were analyzed by immunocytochemistry using PG21 as primary antibody. We also measured total plasma testosterone concentrations by radioimmunoassay. In males with same sex preference there was a specific AR overexpression in CA3 and CA4 that suggests a feminized pattern because females in proestrus trend to show a higher density of AR in these hippocampal areas. Sex differences in AR density were found in the anterior cingulate cortex (ACg) and frontoparietal cortex (FrPa). Serum levels of testosterone did not differ between groups. Data are discussed based on the role of AR in the hippocampus.

Retracted: Sexual orientation, neuropsychiatric disorders and the neurotransmitters involved

This article has been retracted: please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy). This article has been retracted at the request of the Editor in Chief of Neuroscience and Biobehavioral Reviews after concerns were raised with respect to the phrasing of comparisons drawn between humans and animal models. These comparisons were deemed unsupportable, and thus in the best interests of publication standards the Editor has concluded it is necessary to retract the paper. The authors disagree with the reason for the retraction.

Prenatal exposure to valproic acid and treatment with intranasal oxytocin have sex-specific effects on behavior in Long Evans rats

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by deficits in social behaviors and communication. In rodents and humans, prenatal exposure to antiepileptic valproic acid is associated with an increased risk for autistic-like characteristics. One potential treatment is oxytocin, a prosocial neuropeptide that can be delivered intranasally. However, the sex-specific effects of valproic acid exposure and intranasal oxytocin treatment on behavior have not been fully explored. Pregnant Long Evans rats were administered valproic acid (500 mg/kg) or saline midday on gestational day 12, and after weaning, male and female pups were assigned to control (saline-saline), valproic acid-saline, or valproic acid-oxytocin groups. Oxytocin (0.8 IU/kg) or saline was delivered intranasally 30-60 min before tests for anxiety-like behaviors (elevated plus maze), social interactions (sociability) and sociosexual behaviors (partner preference, 50 kHz vocalizations and scent marking). Prenatal exposure to valproic acid resulted in sex-specific differences in behavior. When compared to controls, valproic acid males showed enhanced anxiety-like behaviors in adolescence and fewer scent marks in adulthood, while valproic acid females showed reduced sexual (partner) preference as adults. Intranasal oxytocin was anxiolytic for valproic acid males, but moderately anxiogenic for valproic acid females, and in both sexes it surprisingly impaired social interactions in the sociability test. Furthermore, intranasal oxytocin failed to improve sociosexual deficits in valproic acid rats. These findings highlight the importance of conducting preclinical studies in both sexes, and suggest that oxytocin may be an effective treatment in animal models with heightened anxiety-like behaviors.

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.

Sexual partner preference in animals and humans

Sex differences in brain and behavior of animals including humans result from an interaction between biological and environmental influences. This is also true for the differences between men and women concerning sexual orientation. Sexual differentiation is mediated by three groups of biological mechanisms: early actions of sex steroids, more direct actions of sex-specific genes not mediated by gonadal sex steroids and epigenetic mechanisms. Differential interactions with parents and conspecifics have additionally long-term influences on behavior. This presentation reviews available evidence indicating that these different mechanisms play a significant role in the control of sexual partner preference in animals and humans, in other words the homosexual versus heterosexual orientation. Clinical and epidemiological studies of phenotypically selected populations indicate that early actions of hormones and genetic factors clearly contribute to the determination of sexual orientation. The maternal embryonic environment also modifies the incidence of male homosexuality via immunological mechanisms. The relative contribution of each of these mechanisms remains however to be determined.

Lack of interaction between prenatal stress and prenatal letrozole to induce same-sex preference in male rats

Same-sex partner preference between males has been observed in all species in which this behavior has been studied. Disruption of brain estradiol synthesis during development has been proposed as one of the biological causes underlying this behavior in some mammals. In support of this possibility, perinatal administration of aromatase inhibitors (such as letrozole) to male rat pups, induces around half of them to have same-sex preference and female sexual behavior in adulthood. Another putative factor that modifies sex preference is prenatal stress. Several stress protocols, applied to the pregnant dam, cause some of the adult male progeny to have an increased male preference, a decreased preference for the female, and lordosis behavior. Interestingly, these effects of stress might be mediated by its inhibitory action on brain aromatase. The aim of the present study was to analyze a possible interaction between these two factors in male rats. Pregnant dams were exposed to one of the four treatments across gestation days 10-22 (G10-G22): 1) vehicle-treated non-stressed controls; 2) letrozole (0.56 µg/kg); 3) 30 min immobilization stress); 4) both letrozole and stress combined. The male offspring were tested in adulthood for partner preference in a three-chambered arena, where we also recorded the masculine and feminine sexual behaviors. One week later males were tested for masculine and feminine sexual behavior in cylindrical arenas where they interacted for 30 min with a receptive female and thereafter with a sexually active male for another 30 min. Letrozole, stress and their combination resulted in same-sex preference in 40, 31 and 50% of males, respectively, compared to 5% in the control group. In the sexual behavior tests, prenatal stress reduced the percentage of males displaying intromissions and ejaculation (impaired masculinization), while letrozole mainly increased lordosis (impaired defeminization). The males prenatally submitted to stress and treated with letrozole presented these behavioral features but did not differ from both treatments given independently. The results indicate that the changes induced by stress or the aromatase inhibition produced by letrozole only accounts for a shift in partner preference in around half of the males and that there was no interaction between these two factors.