Arginine Vasotocin and Neuropeptide Y Vary with Seasonal Life-History Transitions in Garter Snakes.

Transitions between life-history stages are often accompanied by dramatic behavioral switches that result from a shift in motivation to pursue one resource over another. While the neuroendocrine mechanisms that regulate such behavioral transitions are poorly understood, arginine vasotocin (AVT) and neuropeptide Y (NPY) are excellent candidates because they modulate reproductive and feeding behavior, respectively. We asked if seasonal changes in AVT and NPY are concomitant with the seasonal migration to and from the feeding grounds in red-sided garter snakes (Thamnophis sirtalis parietalis). Male and female snakes were collected in different migratory states during both the spring and fall. The total number of AVT- and NPY-immunoreactive (ir) cells was then quantified in each brain region of interest. To correct for potential variation in region volume related to sexually dimorphic body size in this species, we first determined that snout-vent length is an accurate proxy for regional brain volume. We then corrected each individual's ir cell number by its SVL to directly compare seasonal changes in AVT and NPY between males and females. Within the supraoptic nucleus, both males and females had more AVT-ir cells during the fall compared with spring. As predicted, males had significantly more AVT-ir cells during the spring mating season in the hypothalamus (HYP) and bed nucleus of the stria terminalis, brain regions important in regulating reproductive behavior. Females also had significantly more AVT-ir cells in the HYP during the spring, as well as a significantly higher number of hypothalamic AVT cells than males. During the fall, males had significantly more NPY-ir cells in the cortex and posterior HYP compared with spring, possibly reflecting increased feeding behavior during summer foraging. Females did not exhibit significant main effects of season on NPY-ir cell number in any region. Neither AVT- nor NPY-ir cell number varied significantly with migratory status, but we did observe significant changes related to seasonal transitions in reproductive state. Our results indicate that changes in brain AVT and NPY are associated with seasonal transitions in reproductive and foraging behaviors, and may be involved in mediating sex differences in the timing of life-history events.