Changes in pituitary responsiveness to luteinizing hormone-releasing hormone during an annual cycle in the domestic duck, Anas platyrhynchos L.

On four occasions during an annual cycle, 5--7 male domestic ducks were injected with two different doses (5 and 20 micrograms) of synthetic luteinizing hormone-releasing hormone (LHRH) to study the possible changes in responsiveness of the pituitary. The luteinizing hormone (LH) and the follicle-stimulating hormone (FSH) were measured in the plasma samples collected after these injections. The induced release of LH changes from one period of the year to another, being minimum in March at the height of the reproductive season. The LHRH injection also induces the release of some FSH but only in limited amounts. The changes in pituitary responsiveness to LHRH are negatively correlated to changes in the circulating LH level (it is high when the plasma LH is low and vice versa). This suggests that the hypothalamic synthesis and release of LHRH must also change during the year