Changes in brain GnRH associated with photorefractoriness in house sparrows (Passer domesticus).

Temperate zone birds terminate reproduction when they become photorefractory. In many species, refractoriness is "absolute" in that gonadal regression occurs before day length declines, and individuals are reproductively unresponsive even to continuous light. Based on studies of a few species, this form of refractoriness appears to be associated with a reduction (compared with breeding and/or photosensitive birds) in numbers of hypothalamic cells and fibers that are immunoreactive for gonadotropin releasing hormone (GnRH). Some species display "relative" refractoriness, in that day length must decline before gonadal regression occurs, and individuals never lose the capacity to respond to very long days. A few reports suggest that GnRH levels in the brain do not change in relatively refractory birds. House sparrows (Passer domesticus) regress their gonads sooner when day length declines during summer than they do if photoperiod is not permitted to decline after the summer solstice (i.e., they appear to be "relatively" refractory). However, if held on long days they eventually regress the gonads despite no decline in photoperiod (they appear to be "absolutely" refractory). In this experiment, we tested whether gonadal regression was associated with changes in hypothalamic GnRH in adult male house sparrows. We used immunocytochemistry (primary antibody sensitive to both forms of avian GnRH) to compare the distribution and number of GnRH-immunoreactive cells and fibers among reproductively active individuals on long days, "absolutely" refractory individuals on long days, and presumably "relatively" refractory individuals induced to regress by shortened days. Similar reductions in ir-GnRH cell number and cell size occurred in both groups of refractory birds compared with birds still in breeding condition. Gonadal regression also was correlated with reduced staining intensity of cell bodies and reduced fiber staining, but these reductions were somewhat more pronounced in the "absolutely" than the "relatively" refractory birds. We discuss these results in light of other studies of avian GnRH changes with reproductive stage. Our results are consistent with the idea that house sparrows become absolutely refractory, regardless of whether exposed to a decline in photoperiod. However, the results also suggest that relative refractoriness may induce gonadal regression through a cessation of GnRH secretion, whereas absolute refractoriness involves down-regulation of peptide production as well.