Editorial: Reproductive Neuroendocrinology and Social Behavior

Reproduction consists of various physiological events including fertilization, development of sexual characteristics, social behavior, maturation, and aging. Reproductive functions are ultimately regulated by the hypothalamus-pituitary-gonadal (HPG) axis. Gonadotropin-releasing hormone (GnRH) is a pivotal hypothalamic neuropeptide that regulates vertebrate reproduction (Schally et al., 1972). In tetrapods, GnRH neurons are located in the preoptic-hypothalamic region and project their fibers to the median eminence to regulate gonadotropin secretion from the anterior pituitary gland, which stimulates sex steroid secretion and gametogenesis in the gonads. It was also shown that central administration of GnRH can stimulate female sexual behavior in rats (Moss and McCann, 1973; Pfaff, 1973). GnRH release is regulated by other neuropeptides, neurotransmitters, and steroid hormones. Watanabe et al. summarize the role of gamma-amino butyric acid (GABA) in the regulation of GnRH neuronal activity and discuss functional consequences of GABAergic inputs to GnRH neurons in physiological aspects of reproduction. Recently, two neuropeptides containing the C-terminal Arg-Phe-NH 2 (RFamide peptides), kisspeptin, and gonadotropin-inhibitory hormone (GnIH), emerged as critical accelerator, and suppressor, respectively, of vertebrate reproduction. Parhar et al. highlight classical and recent findings regarding the role of GnRH, kisspeptin, and GnIH in the regulation of social behaviors in fish, birds, and mammals, and discuss their importance in future biological and biomedical researches (Perspectives). As social behaviors such as courtship, mating, and aggression are strongly associated with sex steroids (Adkins-Regan, 2005), hypothalamic neuropeptides can regulate social behaviors by regulating the HPG axis. It was originally thought that males display male-typical behaviors because they are exposed to androgen and females display female-typical behaviors because they are exposed to estrogen or progestogen. However, it was later discovered that central actions of androgen in males for the expression of certain male-typical behaviors require its aromatization into neuroestrogen (aromatization hypothesis; Yahr, 1979). Ubuka and Tsutsui summarize investigations on how aromatase expression and activity are regulated in the brain and discuss how neuroestrogen regulates socio-sexual behavior of males. Change in androgen concentration in response to social challenges has been hypothesized as one of the regulatory mechanisms of behavior in response to the perceived social environment (Challenge hypothesis, Wingfield et al., 1990). Oliveira and Oliveira review studies on the mechanism and function of androgen response to social challenges and discuss the modulatory mechanism of social decision-making by peripheral hormones. Sakuma summarizes detailed mechanisms of estrogen-sensitive preoptic area (POA) neurons regulating sexual behavior of female rats. According to