Seasonal variation in brain GnRH in free-living breeding and photorefractory house finches (Carpodacus mexicanus)

Absolute photorefractoriness in captive birds of several species correlates with reduced hypothalamic content of gonadotropin-releasing hormone (GnRH) relative to photosensitive or photostimulated birds. We used immunocytochemistry to examine GnRH immunostaining in free-living breeding and photorefractory house finches (Carpodacus mexicanus). Photorefractory birds with regressed gonads in early autumn had lower GnRH immunoreactivity (fewer, smaller, less intensely stained cell bodies and fewer immunopositive axon fibers) than did breeding birds with enlarged gonads in spring. These results confirm that absolute refractoriness in house finches is associated with dramatic changes in the hypothalamic GnRH system, as in other species with this type of refractoriness, and show that these changes occur in free- living birds.