The effect of food restriction on the regulation of gonadotropin-releasing hormone in male house finches (Haemorhous mexicanus)

Seasonal activation of the vertebrate hypothalamic-pituitary-gonadal (HPG) axis and gonadal development is initiated by gonadotropin-releasing hormone-I (GnRH) release from the hypothalamus. In photoperiodic species, the consistent annual change in photoperiod is the primary environmental signal affecting GnRH cell activity, including changes in the synthesis and secretion of this neuropeptide. Non-photoperiodic environmental cues such as energy availability also influence HPG axis activity, but the mechanisms mediating this influence, in particular on the GnRH system, are unclear. Understanding how the neuroendocrine system integrates environmental information is critical in determining the plasticity and adaptability of physiological responses to changing environments. The primary objective of this study was to investigate GnRH-mediated changes in HPG axis activity and gonadal development in response to energy availability in a wild bird. We hypothesized that negative energy balance inhibits HPG axis activity by affecting GnRH secretion. Moderate food restriction for several weeks in male house finches, Haemorhous mexicanus, decreased body condition and inhibited photoinduced testicular growth compared to birds fed ad libitum. Food restriction did not affect plasma luteinizing hormone (LH; a correlate of GnRH release) or plasma testosterone, but it enhanced the plasma LH response to an injection of the glutamatergic agonist, N-methyl-D-aspartate (NMDA). Thus, food restriction may decrease photoinduced HPG axis activation by acting centrally, in particular by attenuating the release of accumulated GnRH stores.