Effect of day length on plasma FSH and LH in castrated and intact white-crowned sparrows

Abstract

Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) were measured in the plasma of male white-crowned sparrows ( Zonotrichia leucophrys gambelii) using heterologous radioimmunoassay systems. Levels were determined in intact birds during a photo-periodically induced (20L:4D) gonadal cycle and compared with those in castrates on the same photoregime. In the intact birds testicular weight increased rapidly between Days 10 and 55 while plasma LH reached a maximum level by Day 6 and remained high until Day 50. Between Days 50 and 75 there was a decline in circulating LH to basal levels, coincident with the onset of photorefractoriness and spontaneous gonadal regression. Control birds on short days (8L:16D) showed no changes in plasma levels of LH. In contrast, levels of FSH increased gradually on 20L:4D to reach a maximum by Day 36 when the testes and almost attained maximum size. Thereafter, they steadily declined to basal levels by Day 75. Control birds on 8L:16D had plasma FSH levels below the detectable limits of the assay. During photostimulation (20L:4D) of castrated sparrows, plasma levels of FSH and LH reached levels up to tenfold those in intact birds. The temporal patterns in both gonadotropins were similar with maxima at Day 16, and declining to basal levels by Day 75. Short-day castrates sampled over the same time period showed a slight increase in plasma LH levels, whereas plasma FSH remained undectable.