Abstract
The social environment changes circulating hormone levels and expression of social behavior in animals. Social information is perceived by sensory systems, leading to cellular and molecular changes through neural processes. Peripheral reproductive hormone levels are regulated by activity in the hypothalamic-pituitary–gonadal (HPG) axis. Until the end of the last century, the neurochemical systems that convey social information to the HPG axis were not well understood. Gonadotropin-inhibitory hormone (GnIH) was the first hypothalamic neuropeptide shown to inhibit gonadotropin release, in 2000. GnIH is now regarded as a negative upstream regulator of the HPG axis, and it is becoming increasingly evident that it responds to social cues. In addition to controlling reproductive physiology, GnIH seems to modulate the reproductive behavior of animals. Here, we review studies investigating how GnIH neurons respond to social information and describe the mechanisms through which GnIH regulates social behavior.
Highlights
Environmental factors can influence reproductive physiology and behavior in animals
gonadotropin-releasing hormone (GnRH)-I expression and GnRH-I neuronal somata devel opment occurred when cichlid fish ascended in social status. They became brightly colored, displayed territorial and reproductive behav iors, and showed enhanced reproductive capabilities (Burmeister et al, 2005; Francis et al, 1993). These findings suggest that various types of social information can alter the reproductive physiology of vertebrates through the GnRH-I system
Gonadotropin-inhibitory hormone (GnIH) neurons expressed glucocorticoid type 2b receptor mRNA. These data suggest that social defeat stress activates central GnIH biosynthesis through glucocorticoid receptors, which may lead to temporary reproductive dysfunction and changes in behavioral patterns of socially defeated fish
Summary
Environmental factors can influence reproductive physiology and behavior in animals. While photoperiod, temperature, and humidity strongly affect reproductive activity in many species, the social envi ronment plays an important role in fine-tuning the timing of breeding (Wingfield, 1983). The information converges on the hypothalamus and modulates repro ductive physiology via hypothalamic-pituitary–gonadal (HPG) axis ac tivity. This most likely involves the delivery of gonadotropin-releasing hormone (GnRH) from the hypothalamus to the pituitary gland (Ball, 1993). GnRH-I expression and GnRH-I neuronal somata devel opment occurred when cichlid fish ascended in social status They became brightly colored, displayed territorial and reproductive behav iors, and showed enhanced reproductive capabilities (Burmeister et al, 2005; Francis et al, 1993). GnIH is regarded as a negative upstream regulator of the HPG axis because it acts on the pituitary gland to inhibit gonadotropin secretion and negatively affects GnRH-I neu rons. We summarize current research on the social behavior-regulating effects of GnIH in the brain (Fig. 1)
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