Regulation of prolactin secretion in a nonphotoperiodic breeder, the zebra finch

In avian species, high levels of prolactin (PRL) released from the anterior pituitary are associated with reproductive behaviors such as incubation and rearing of young. We studied reproduction-associated PRL levels in breeding and nonbreeding zebra finches. The general hypothesis of this dissertation was: The general pattern of PRL release in zebra finches is similar to other avian species, but differences in regulation of release can be expected due to the unique breeding environment in which these animals live.;The research was divided into four parts: (1) Determining the pattern of PRL release and secretion in response to vasoactive intestinal peptide (VIP); (2) Determining PRL and growth hormone (GH) presence and distribution in the anterior pituitary; (3) Determining differences in PRL molecular weight isoforms in the anterior pituitary during different phases of reproduction; and (4) Determining the effects of dopaminergic agents and anti-VIP antibodies in regulating PRL release.;Plasma PRL levels were low in nonbreeding zebra finches and high in breeding zebra finches. Males and females did not differ. Exogenous VIP raised PRL levels in nonbreeding zebra finches to a level comparable to breeding finches. The VIP-induced PRL rise occurred rapidly and had a prolonged effect. Zebra finches who were in the parental stage of reproduction showed only a slight increase in plasma PRL following VIP injection.;PRL-immunoreactive (IR) cells were concentrated in the cephalic lobe and along the ventral margin of the anterior pituitary (AP) and extended into the caudal lobe in birds of all reproductive conditions. GH-IR cells were concentrated in the caudal lobe of the AP and also extended into the cephalic lobe. Zebra finches with previous reproductive experience had significantly more PRL-IR cells in the AP than inexperienced birds, regardless of breeding status at the time sampled. Breeding zebra finches tended to have more PRL-IR cells than nonbreeders. There were no differences in the number of PRL-IR or GH-IR cells in the APs of birds of different ages or sex, and neither PRL-IR nor GH-IR cell counts correlated with plasma PRL levels. Further, the anterior pituitary of breeding zebra finches expressed two molecular weight isoforms of PRL, compared with only one found in nonbreeding birds.;Given alone, BROMO and DOMP produced little change in plasma PRL of nonbreeding birds. Nonbreeders receiving VIP + DOMP had higher post-injection PRL levels than other treatment groups, with females responding more robustly than males. Among breeding zebra finches, only females receiving DOMP alone had significantly higher post-injection plasma PRL levels than controls. All other treatments given to breeding zebra finches resulted in plasma PRL levels that were intermediate between controls and females that had been given DOMP. Antibodies against VIP at the dose used were ineffective at changing plasma PRL levels in breeding birds.;These are the first detailed studies on factors regulating plasma PRL release in zebra finches. Based on these findings, we hypothesize that zebra finches are more responsive to agents that stimulate PRL secretion rather than to inhibitory agents, and this may be part of the suite of characteristics that allow for their opportunistic breeding strategy. Additionally, we have conducted one of the few studies examining the effects of previous reproductive experience separate from age and have found that experience may induce long-lasting anatomical changes in the pituitary that prime it for future reproductive events