CHRISTENSEN, D.E. *; VLECK, C.M.; Iowa State Univ., Ames: Lactotroph Distribution and Response to VIP Stimulation Across the Breeding Cycle in the Zebra Finch

In arid regions of Australia, Zebra Finches are opportunistic breeders whose breeding cycles are coupled to nonseasonal rainfall rather than photoperiod, requiring them to be reproductively competent on short notice. We measured plasma prolactin (PRL) levels by RIA in breeding and nonbreeding Zebra Finches both before and after exogenously administered vasoactive intestinal peptide (VIP), the avian prolactin-releasing hormone. In incubating birds, basal PRL levels were high and did not change after VIP injection. In nonbreeding birds, basal PRL levels were low but increased to levels comparable to those found in breeding birds following a single VIP challenge. This pattern differs from photosensitive species in which only during a breeding cycle do birds transiently, but significantly, elevate their level of PRL in response to exogenous VIP. In photosensitive birds, the seasonal increase in PRL is associated with an increased number of lactotrophs that densely populate the cranial lobe of the anterior pituitary and then extend caudally during breeding. When these birds become photorefractory, their lactotroph distribution once again recedes to the cranial lobe of the anterior pituitary. Our results show no difference in lactotroph distribution in the anterior pituitary between breeding and nonbreeding Zebra Finches. However, an increase in lactotroph number is seen in those birds that have previously raised a clutch to fledge, and this increase persists even when these birds are subsequently separated from their mates. This suggests that even when not actively breeding, Zebra Finches maintain competent pituitary lactotrophs that can secrete PRL at maximal rates as part of the suite of characters enabling these birds to take advantage of opportunistic breeding conditions.