VGF in the Medial Preoptic Nucleus Increases Sexual Activity Following Sexual Arousal Induction in Male Rats

Mating experiences with the same partner enhanced mating activities of naïve male medaka fish

Mating experience shapes male mating behavior across species, from insects, fish, and birds, to rodents. Here, we investigated the effect of multiple mating experiences on male mating behavior in "naïve" (defined as sexually inexperienced) male medaka fish. The latency to mate with the same female partner significantly decreased after the second encounter, whereas when the partner was changed, the latency to mate was not decreased. These findings suggest that mating experiences enhanced the mating activity of naïve males for the familiar female, but not for an unfamiliar female. In contrast, the mating experiences of "experienced" (defined as those having mated > 7 times) males with the same partner did not influence their latency to mate. Furthermore, we identified 10 highly and differentially expressed genes in the brains of the naïve males after the mating experience and revealed 3 genes that are required for a functional cascade of the thyroid hormone system. Together, these findings suggest that the mating experience of naïve male medaka fish influences their mating behaviors, with neural changes triggered by thyroid hormone activation in the brain.

Sexual Experience Induces the Expression of Gastrin-Releasing Peptide and Oxytocin Receptors in the Spinal Ejaculation Generator in Rats

Male sexual function in mammals is controlled by the brain neural circuits and the spinal cord centers located in the lamina X of the lumbar spinal cord (L3–L4). Recently, we reported that hypothalamic oxytocin neurons project to the lumbar spinal cord to activate the neurons located in the dorsal lamina X of the lumbar spinal cord (dXL) via oxytocin receptors, thereby facilitating male sexual activity. Sexual experiences can influence male sexual activity in rats. However, how this experience affects the brain–spinal cord neural circuits underlying male sexual activity remains unknown. Focusing on dXL neurons that are innervated by hypothalamic oxytocinergic neurons controlling male sexual function, we examined whether sexual experience affects such neural circuits. We found that >50% of dXL neurons were activated in the first ejaculation group and ~30% in the control and intromission groups in sexually naïve males. In contrast, in sexually experienced males, ~50% of dXL neurons were activated in both the intromission and ejaculation groups, compared to ~30% in the control group. Furthermore, sexual experience induced expressions of gastrin-releasing peptide and oxytocin receptors in the lumbar spinal cord. This is the first demonstration of the effects of sexual experience on molecular expressions in the neural circuits controlling male sexual activity in the spinal cord.

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.

Neuronal activation of the sexually dimorphic nucleus of the preoptic area in female and male rats during copulation and its sex differences

The medial preoptic area, which plays an essential role in the control of sexual behavior in rats, contains a sexually dimorphic nucleus that consists of neurons expressing calbindin-D28 K (Calb) that is referred to as the CALB-SDN. The CALB-SDN is larger and contains more Calb neurons in males than in females. The physiological functions of the CALB-SDN are not fully understood; however, CALB-SDN neurons are activated during sexual behavior in males, suggesting that the male CALB-SDN is involved in regulation of sexual behavior. However, no information exists about the physiological functions of the female CALB-SDN. In the present study, we performed an immunohistochemical analysis of c-Fos, a neuronal activity marker, in the CALB-SDN of female and male rats that had copulated with conspecifics of the opposite sex to determine whether neurons of the female CALB-SDN are activated during copulation and whether the neuronal activity of the CALB-SDN differs between sexes. The numbers of c-Fos-immunoreactive cells with or without Calb-immunoreactivity (c-Fos⁺/Calb⁺ and c-Fos⁺/Calb^- cells) were greater in the CALB-SDN of rats that had copulated than in rats that had not copulated in each sex. Although the number of Calb⁺ cells in the CALB-SDN was smaller in females than in males, the increase in the number of c-Fos⁺/Calb⁺ cells in the female CALB-SDN with copulation was comparable to that in the male CALB-SDN with copulation. The increase in the number of c-Fos⁺/Calb^- cells in the CALB-SDN with copulation was more prominent in males than in females. These results suggest that CALB-SDN neurons are activated during copulation in both sexes. The patterns of neuronal activation in the CALB-SDN during copulation may differ between sexes.

Cellular Composition of the Preoptic Area Regulating Sleep, Parental, and Sexual Behavior

The preoptic area (POA) has long been recognized as a sleep center, first proposed by von Economo. The POA, especially the medial POA (MPOA), is also involved in the regulation of various innate functions such as sexual and parental behaviors. Consistent with its many roles, the MPOA is composed of subregions that are identified by different gene and protein expressions. This review addresses the current understanding of the molecular and cellular architecture of POA neurons in relation to sleep and reproductive behavior. Optogenetic and pharmacogenetic studies have revealed a diverse group of neurons within the POA that exhibit different neural activity patterns depending on vigilance states and whose activity can enhance or suppress wake, non-rapid eye movement (NREM) sleep, or rapid eye movement (REM) sleep. These sleep-regulating neurons are not restricted to the ventrolateral POA (VLPO) region but are widespread in the lateral MPOA and LPOA as well. Neurons expressing galanin also express gonadal steroid receptors and regulate motivational aspects of reproductive behaviors. Moxd1, a novel marker of sexually dimorphic nuclei (SDN), visualizes the SDN of the POA (SDN-POA). The role of the POA in sleep and other innate behaviors has been addressed separately; more integrated observation will be necessary to obtain physiologically relevant insight that penetrates the different dimensions of animal behavior.

Sexually Dimorphic Formation of the Preoptic Area and the Bed Nucleus of the Stria Terminalis by Neuroestrogens

Testicular androgens during the perinatal period play an important role in the sexual differentiation of the brain of rodents. Testicular androgens transported into the brain act via androgen receptors or are the substrate of aromatase, which synthesizes neuroestrogens that act via estrogen receptors. The latter that occurs in the perinatal period significantly contributes to the sexual differentiation of the brain. The preoptic area (POA) and the bed nucleus of the stria terminalis (BNST) are sexually dimorphic brain regions that are involved in the regulation of sex-specific social behaviors and the reproductive neuroendocrine system. Here, we discuss how neuroestrogens of testicular origin act in the perinatal period to organize the sexually dimorphic structures of the POA and BNST. Accumulating data from rodent studies suggest that neuroestrogens induce the sex differences in glial and immune cells, which play an important role in the sexually dimorphic formation of the dendritic synapse patterning in the POA, and induce the sex differences in the cell number of specific neuronal cell groups in the POA and BNST, which may be established by controlling the number of cells dying by apoptosis or the phenotypic organization of living cells. Testicular androgens in the peripubertal period also contribute to the sexual differentiation of the POA and BNST, and thus their aromatization to estrogens may be unnecessary. Additionally, we discuss the notion that testicular androgens that do not aromatize to estrogens can also induce significant effects on the sexually dimorphic formation of the POA and BNST.

Blunt olfaction in sexually sluggish male rats

Among the intact male rats, a subpopulation has been found to show little or no sexual behavior, even after experiencing several mating sessions. This study investigated whether sexually sluggish (SS) males show behavioral differences from normal copulatory (NC) males, other than those concerning sexual behavior. The olfactory preference of males was measured through the time spent displaying nose-poking behavior directed at sexually active males and estrous females for odor exploration in a three-chamber apparatus. Both the NC and SS males showed a significant preference for the odor of estrous females compared with that of male odors. However, SS males spent significantly less time nose-poking estrous females than NC males. The food-finding test was performed after overnight fasting. Our findings showed that all the NC males found the buried pellet within 5 min, whereas over 60% of the SS males failed to find it. The males were also tested for their ability to find a buried bag containing soiled bedding from estrous female cages. The bag was found by 80% of NC males, but only by 20% of SS males. Our results suggest that SS and NC male rats differ not only in sexual behavior but also in other functions such as olfaction.

Region-specific effects of copulation on dendritic spine morphology and gene expression related to spinogenesis in the medial preoptic nucleus of male rats

The medial preoptic nucleus (MPN) plays an essential role in the control of male sexual behavior. In rats, the central part of the MPN (MPNc) contains a sexually dimorphic nucleus exhibiting male-biased morphological sex differences. Although it has been suggested that the MPNc of male rats functions to induce sexual arousal, the mechanisms by which male rats are sexually aroused to successfully achieve copulation are poorly understood. We recently showed that increased neuronal activity in the MPNc of male rats during copulation is higher at their first copulation compared with later copulations, indicating that a plastic change in excitatory synaptic transmission occurs with copulatory experience. In this study, we tested the hypothesis that changes to dendritic spines at structural and molecular levels occur following copulatory experience. First, we examined the effects of at least two copulations on the morphology of dendrites and spines in the MPNc and in the lateral and medial parts of the MPN (MPNlm) of male rats. In the MPNc, the total number of dendrites and their branches, and the surface area of dendrites were not significantly affected by copulation. However, the copulatory experience, specifically experience of ejaculation, significantly reduced the density of mushroom spines but not of filopodia, thin or stubby spines in the MPNc. In the MPNlm, the copulatory experience, specifically experience of ejaculation, significantly increased the surface area of dendrites, although there was no significant effect of copulation on spine density. Next, we measured the mRNA levels of genes encoding actin-binding proteins related to spinogenesis after male rats had copulated for their first and second times. Copulatory stimuli, especially stimuli from ejaculation, significantly reduced the mRNA levels of drebrin A and spinophilin in the MPNc but not in the MPNlm. These results indicate that copulatory experiences, especially experience of ejaculation, reduce spine density in the MPNc of male rats, which may result, in part, from downregulation of genes encoding actin-binding proteins.