Seasonal changes in brain GnRH immunoreactivity and song-control nuclei volumes in an opportunistically breeding songbird.

White-winged crossbills (Loxia leucoptera) are opportunistic breeders that can nest at almost any time of year if there is sufficient food. Other cardueline finches that have been shown to breed on a strictly seasonal schedule become absolutely refractory to the stimulatory effects of long-day photoperiod, dramatically down-regulate hypothalamic gonadotropin-releasing hormone (GnRH), and reduce the volume of several song-control nuclei in autumn. This study examined whether changes in photoperiod modify the GnRH and song-control systems in white-winged crossbills. Adult male and female crossbills were captured and held on a naturally changing photoperiod. Brains of male and female birds were collected in May, October, and January. GnRH content was assessed by immunocytochemistry and the volumes of Nissl-defined song-control nuclei (HVc, Area X, and the robust nucleus of the archistriatum) were reconstructed. In contrast to other cardueline finches, GnRH immunoreactivity was relatively stable across the year, exhibiting only modest seasonal variation. The song control system, on the other hand, exhibited large seasonal changes as well as sex differences. Thus, crossbills appear to maintain hypothalamic GnRH content year round, perhaps to facilitate a rapid response to favorable breeding conditions, even on short days. However, song control nuclei are dramatically affected by photoperiod. Future work should examine these systems in crossbills breeding on short days to compare photoperiod-dependent and -independent effects on neural plasticity.

Increased VIP and decreased GnRH expression in photorefractory dark-eyed juncos (Junco hyemalis).

Most temperate zone birds show dramatic seasonal cycles in responsiveness to light. In the spring the hypothalamo-pituitary-gonadal axis of photosensitive birds is stimulated by long days. In the late summer birds no longer respond to long days, their gonads regress, and they are said to be photorefractory. After several weeks of refractoriness birds regain photosensitivity. During refractoriness circulating concentrations of luteinizing hormone are low and prolactin levels are high. These fluctuations in peripheral hormones result from changes in the brain rather than in the pituitary and/or the gonads. In the present study we examined seasonal changes in expression of vasoactive-intestinal polypeptide (VIP) and gonadotropin-releasing hormone (GnRH) in the brain of dark-eyed juncos (Junco hyemalis). Birds were photosensitive and exposed to long photoperiod (20:4 LD) for 1 day, 45-60 days, or not at all, or they were photorefractory (housed in 20:4 LD). The results indicate that VIP expression was similar in all photosensitive birds. However, photorefractory birds had significantly higher numbers of VIP-positive neurons in the infundibulum compared to photosensitive birds. The number of GnRH-positive neurons in the preoptic area was significantly lower in photorefractory birds and significantly higher in long-term photostimulated birds. These results indicate that the inverse relationship between circulating prolactin (released by VIP) and luteinizing hormone (released by GnRH) during refractoriness may result from neural changes in VIP and GnRH expression, respectively.