P2.121A Thursday, Jan. 5 Melatonin and seasonal variation in GnRH: Lessons for interpreting changes in immunoreactive cell number LUTTERSCHMIDT, D.I.*; MAINE, A.R.; WILCZYNSKI, W.; Portland State University, Oregon; Portland State University, Oregon; Georgia State University, Atlanta email@example.com
Seasonal rhythms in reproductive physiology and behavior are regulated by changes in the activity of the hypothalamus-pituitary-gonad axis. The neuroendocrine factors mediating these changes, however, are poorly understood. We investigated whether the pineal hormone melatonin alters seasonal changes in gonadotropin-releasing hormone (GnRH) and sex steroids in green treefrogs (Hyla cinerea). Male and female frogs collected during the winter (nonreproductive) or summer (reproductive) were implanted subcutaneously with either a melatonin-filled or blank silastic capsule for 4 weeks. Melatonin did not modulate GnRH-immunoreactive (GnRH-ir) cell number or sex steroid hormones in either sex, suggesting that the reproductive axis of green treefrogs is not directly sensitive to melatonin signaling. As expected, male testosterone and female estradiol concentrations were higher in reproductive frogs collected during the summer breeding season. Seasonal differences in GnRH immunoreactivity were also observed, although the pattern was not as expected. Reproductively active frogs had significantly fewer GnRH-ir cells than nonreproductive frogs (F = 8.76; P = 0.006). Our data suggest that increased release of GnRH during the summer breeding season leads to both increased sex steroid hormone concentrations and a decrease in GnRH-ir cell number. Several studies have provided data consistent with this alternative interpretation of increased immunoreactive cell number, although current examples represent neuropeptides with complex roles in seasonal and social behaviors (e.g., arginine vasotocin). Our results provide support for this alternative hypothesis with a relatively simple example of GnRH in a seasonal breeder.