Numerous studies have reported seasonal variations in regional morphology in the brains of seasonally breeding vertebrates. These alterations of neuronal morphology and dendritic spine density appear to be an active process within specific brain nuclei that regulate seasonal behaviors. In many cases, this neural plasticity has been found to be in response to changes in circulating sex steroid hormone levels and occur within pathways essential for the control of reproductive behaviors. Male red-sided garter snakes (Thamnophis sirtalis parietalis) (RSGS) exhibit a dissociated reproductive pattern where mating is initiated at a time when the gonads and steroidogenesis are inactive. And, although circulating levels of sex steroid hormones are elevated at the initiation of courtship and mating, the only known cue found to initiate courtship behavior and mating, is an extended period of low temperature dormancy (LTD) followed by exposure to warm temperatures. This study was designed to examine the role of seasons, sex steroid hormones, and LTD on neuronal and dendritic spine density within the anterior hypothalamus-preoptic area (AHPOA), a region shown to be critical for the regulation of reproductive behaviors. In the male RSGS, the density of dendritic spines on neurons in the AHPOA was significantly greater in spring, actively courting animals, than summer or fall, non-courting animals. Animals maintained under conditions of LTD exhibited significantly increasing spine density as time maintained in LTD increased. Animals receiving either testosterone or estradiol had a significantly greater density of dendritic spines than control animals. This study offers evidence suggesting that the "set up" of the pathways controlling courtship behavior and mating in the male RSGS, is not due solely to an extended period of LTD, but that an extended period of LTD in conjunction with circulating sex steroid hormones are critical for the initiation of reproductive behavior.
Keywords: AHPOA; Golgi; Low temperature dormancy; Neuronal plasticity; Red-sided garter snake; Seasonal; Sex steroid hormones; dendritic spines.
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