Meeting Abstract

29.2  Saturday, Jan. 4 13:45  Sex and seasonal differences in the effects of melatonin on brain arginine vasotocin: Relationship to melatonin receptor type 1a immunoreactivity in green treefrogs (Hyla cinerea) HOWARD, CM*; LUTTERSCHMIDT, DI; Portland State University chow@pdx.edu

Melatonin (MEL) modulates arginine vasotocin (AVT) immunoreactive (ir) cell number in the brain of green treefrogs (Hyla cinerea) during the summer breeding season, and this modulation is sexually dimorphic. In this study, we asked if MEL’s influence on AVT varies seasonally. We show that treatment of nonreproductive frogs with MEL-filled silastic implants for 4 wks during the winter does not alter AVT-ir cell number in any brain region (i.e., nucleus accumbens, amygdala, preoptic area, hypothalamus, or suprachiasmatic nucleus). These results suggest that MEL’s influence on AVT is associated with sex and seasonal variation in melatonin receptor expression. We tested this hypothesis by using immunohistochemistry to characterize the distribution of MEL type 1a receptor (MEL1a-R) in the brain of reproductive and nonreproductive male and female frogs. We quantified MEL1a-R-ir cell number in regions known to contain AVT cell populations. When boundaries between regions were unclear, they were combined for statistical analysis. During the breeding season, frogs had significantly more MEL1a-R-ir cells than nonreproductive frogs in all brain regions, including the combined nucleus accumbens, diagonal band of Broca and septum (p =0.031), striatum (p = 0.020), amygdala (p < 0.001), pre-optic area and suprachiasmatic nucleus (p < 0.001), and caudal hypothalamus (p < 0.001). In the accumbens region, where MEL’s effect on AVT is known to be sexually dimorphic, males had significantly more MEL1a-R-ir cells than females during the summer breeding season (p = 0.014). These results suggest that variation in MEL1a-R expression mediates both sex and seasonal differences in the sensitivity of AVT neurons to MEL signaling.