Brain activation associated to olfactory conditioned same-sex partner preference in male rats

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.

Neurophysiology of male sexual arousal-Behavioral perspective

In the presented review, we analyzed the physiology of male sexual arousal and its relation to the motivational aspects of this behavior. We highlighted the distinction between these processes based on observable physiological and behavioral parameters. Thus, we proposed the experimentally applicable differentiation between sexual arousal (SA) and sexual motivation (SM). We propose to define sexual arousal as an overall autonomic nervous system response leading to penile erection, triggered selectively by specific sexual cues. These autonomic processes include both spinal and supraspinal neuronal networks, activated by sensory pathways including information from sexual partner and sexual context, as well as external and internal genital organs. To avoid misinterpretation of experimental data, we also propose to precise the term "sexual motivation" as all actions performed by the individual that increase the probability of sexual interactions or increase the probability of exposition to sexual context cues. Neuronal structures such as the amygdala, bed nucleus of stria terminalis, hypothalamus, nucleus raphe, periaqueductal gray, and nucleus paragigantocellularis play crucial roles in controlling the level of arousal and regulating peripheral responses via specific autonomic effectors. On the highest level of CNS, the activity of cortical structures involved in the regulation of the autonomic nervous system, such as the insula and anterior cingulate cortex, can visualize an elevated level of SA in both animal and human brains. From a preclinical perspective, we underlie the usefulness of the non-contact erection test (NCE) procedure in understanding factors influencing sexual arousal, including studies of sexual preference in animal models. Taken together results obtained by different methods, we wanted to focus attention on neurophysiological aspects that are distinctly related to sexual arousal and can be used as an objective parameter, leading to higher translational transparency between basic, preclinical, and clinical studies.

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.

Enhanced D2 Agonism Induces Conditioned Appetitive Sexual Responses Toward Non-reproductive Conspecifics

Brain mechanisms of sexual attraction toward reproductive partners develop from a systematic interrelationship between biology (nature) and learning (nurture). However, the causes of attraction toward non-reproductive partners are poorly understood. Here, we explored the role of Pavlovian learning under dopaminergic agonism on the development of sexual preference and brain activation for young male rats. During conditioning, adult sexually naïve males received either Saline (Saline-Paired) or the D2-receptor agonist quinpirole (QNP-Paired) and cohabited in contingency, or out of contingency (QNP-Unpaired) during 24 h with an almond-scented prepubertal juvenile male (PD25). Conditioning occurred every 4 days for three trials. Social and sexual responses were assessed four days after the last conditioning trial in a drug-free test, and males chose freely between a scented young male (PD37) and a novel receptive female. Four days later, males were exposed to the conditioned odor only and brain Fos-IR and serum testosterone were analyzed. Saline-Paired and QNP-Unpaired males displayed more non-contact erections (NCEs) and genital investigations for females, whereas QNP-Paired males expressed more NCEs and genital investigations for young males. In the QNP-Paired group, exposure to the young male-paired odor evoked more Fos-IR in limbic, hypothalamic and cortical areas, but no differences in serum testosterone were observed. Cohabitation with juvenile males during enhanced D2 agonism results in atypical appetitive sexual responses and a higher pattern of brain response for the young male-paired odor, with no changes in serum testosterone. We discuss the potential implications for the development of pedophilic disorder and perhaps other paraphilias.

Prenatal EDCs Impair Mate and Odor Preference and Activation of the VMN in Male and Female Rats

Environmental endocrine-disrupting chemicals (EDCs) disrupt hormone-dependent biological processes. We examined how prenatal exposure to EDCs act in a sex-specific manner to disrupt social and olfactory behaviors in adulthood and underlying neurobiological mechanisms. Pregnant rat dams were injected daily from embryonic day 8 to 18 with 1 mg/kg Aroclor 1221 (A1221), 1 mg/kg vinclozolin, or the vehicle (6% DMSO in sesame oil). A1221 is a mixture of polychlorinated biphenyls (weakly estrogenic) while vinclozolin is a fungicide (anti-androgenic). Adult male offspring exposed to A1221 or vinclozolin, and females exposed to A1221, had impaired mate preference behavior when given a choice between 2 opposite-sex rats that differed by hormone status. A similar pattern of impairment was observed in an odor preference test for urine-soaked filter paper from the same rat groups. A habituation/dishabituation test revealed that all rats had normal odor discrimination ability. Because of the importance of the ventrolateral portion of the ventromedial nucleus (VMNvl) in mate choice, expression of the immediate early gene product Fos was measured, along with its co-expression in estrogen receptor alpha (ERα) cells. A1221 females with impaired mate and odor preference behavior also had increased neuronal activation in the VMNvl, although not specific to ERα-expressing neurons. Interestingly, males exposed to EDCs had normal Fos expression in this region, suggesting that other neurons and/or brain regions mediate these effects. The high conservation of hormonal, olfactory, and behavioral traits necessary for reproductive success means that EDC contamination and its ability to alter these traits has widespread effects on wildlife and humans.

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.

Sexual behavior in rodents: Where do we go from here?

Hormones and Behavior was first published 50 years ago including some articles related to the hormonal regulation of sexual behavior in different species. Since then, this research field has produced outstanding discoveries that have contributed to our understanding of the control of sexual behavior. The refinement of classical techniques and the development of new experimental tools has opened the door to a new era of research that will allow us to understand different aspects of sexual behavior. It would also expand the possible extrapolation from animal models to understand human sexuality and its dysfunctions. In this review, we summarize some of the most recent findings about sexual behavior in both sexes including the refinement of classical methods of study with new approaches and questions as well as the development of new methods trying to explain mechanisms of action on motivational and consummatory elements of mating behavior. We also reviewed other aspects that modulate sexual behavior such as attractivity, olfactory signals and learning which model mate selection. Additionally, we described studies demonstrating that sexual behavior induces permanent brain modifications in neuronal circuits. Finally, we briefly describe recent contributions on animal models of human sexuality dysfunctions which, although with their own limitations, are under continuous refining.

Induction of reversible bidirectional social approach bias by olfactory conditioning in male mice

Social avoidance is a common component of neuropsychiatric disorders that confers substantial functional impairment. An unbiased approach to identify brain regions and neuronal circuits that regulate social avoidance might enable development of novel therapeutics. However, most paradigms that alter social avoidance are irreversible and accompanied by multiple behavioral confounds. Here we report a straightforward behavioral paradigm in male mice enabling the reversible induction of social avoidance or approach with temporal control. C57BL/6J mice repeatedly participated in both negative and positive social experiences. Negative social experience was induced by brief social defeat by an aggressive male CD-1 mouse, while positive social experience was induced by exposure to a female mouse, each conducted daily for five days. Each social experience valence was conducted in a specific odorant context (i.e. negative experience in odorant A, positive experience in odorant B). Odorants were equally preferred pre-conditioning. However, after conditioning, mice sniffed positive experience-paired odorants more than negative experience-paired odorants. Furthermore, positive- or negative-conditioned odorant contexts increased or decreased, respectively, the approach behavior of conditioned mice toward conspecifics. Because individual mice undergo both positive and negative conditioning, this paradigm may be useful to examine neural representations of social approach or avoidance within the same subject.