Effects of age and reproductive experience on the distribution of prolactin and growth hormone secreting cells in the anterior pituitary of a passerine

Prior parental experience attenuates hormonal stress responses and alters hippocampal glucocorticoid receptors in biparental rock doves

In the face of challenges, animals must balance investments in reproductive effort versus their own survival. Physiologically, this trade-off may be mediated by glucocorticoid release by the hypothalamic-pituitary-adrenal axis and prolactin release from the pituitary to maintain parental care. The degree to which animals react to and recover from stressors likely affects maintenance of parental behavior and, ultimately, fitness. However, less is known about how gaining parental experience may alter hormonal stress responses and their underlying neuroendocrine mechanisms. To address this gap, we measured the corticosterone (CORT) and prolactin (PRL) stress response in individuals of both sexes of the biparental rock dove (Columba livia) that had never raised chicks versus birds that had fledged at least one chick. We measured both CORT and PRL at baseline and after an acute stressor (30 min restraint). We also measured negative feedback ability by administering dexamethasone, a synthetic glucocorticoid that suppresses CORT release, and measured CORT and PRL after 60 min. All hormones were measured when birds were not actively nesting to assess whether effects of parental experience extend beyond the breeding bout. Experienced birds had lower stress-induced and negative-feedback CORT, and higher stress-induced PRL than inexperienced birds. In a separate experiment, we measured glucocorticoid receptor subtype expression in the hippocampus, a key site of negative feedback regulation. Experienced birds showed higher glucocorticoid receptor expression than inexperienced controls, which may mediate their ability to attenuate CORT release. Together, these results shed light on potential mechanisms by which gaining experience may improve parental performance and fitness.

Effects of Parental Experience and Age On Expression of Prolactin, Vasoactive Intestinal Peptide and Their Receptors in a Biparental Bird (Columba Livia)

As animals gain parental experience, they often show more rapid and efficient parental care responses that likely improve offspring survival and fitness. Changes in circulating hormones that underlie reproductive behaviors, including prolactin, have been found to correlate with parental experience in birds and mammals. Altered responsiveness to prolactin in key behavioral centers of the brain may also underlie the effects of experience on parental behaviors. Further, experience may also affect responsiveness to prolactin stimulatory hormones, such as hypothalamic vasoactive intestinal peptide (VIP). While experience has been shown to upregulate neural prolactin receptors and responsiveness in rodents, its effects on prolactin receptor gene expression remain unstudied in birds. To address this, we examined gene expression of pituitary prolactin, hypothalamic prolactin receptors in the preoptic area, hypothalamic VIP, and pituitary VIP receptors in both sexes of the biparental rock dove (Columba livia) when birds were not actively nesting. As age and parental experience are often confounded (i.e.,experienced parents tend to be older than their inexperienced counterparts), we measured gene expression in birds of varying combinations of age (0.6-3 years) and prior reproductive experience (0-12 chicks raised). We found that increasing experience with chicks correlated with lower PRLR expression in the preoptic area, and age correlated with lower VIP expression in birds of both sexes. Pituitary PRL and VIPR expression was not associated with parental experience or age. These results suggest there may be persistent effects of experience and age on neural responsiveness to, and regulation of, prolactin in birds.

Relationship between prolactin, reproductive experience, and parental care in a biparental songbird, the zebra finch (Taeniopygia guttata)

Hormonal systems have long been thought to play an important role in stimulating the onset of parental behavior, a critical component of reproductive success in a variety of taxa. Elevations in the peptide hormone prolactin (PRL) have been repeatedly positively correlated with the onset and maintenance of parental care across vertebrate species. A causal role for PRL in parental care has been established in several mammalian species, but less evidence for a causal role of PRL and parental care exists in birds. The zebra finch, a socially monogamous, biparental songbird, is an exceptionally useful animal model to study parental care and other close social relationships. Both sexes share parental care equally, exhibit the same parental behaviors, and show a marked improvement in breeding success with experience. We hypothesize that PRL is critically involved in the expression of zebra finch parental care and predict that circulating PRL levels will increase with breeding experience. To begin testing this, we measured plasma PRL concentrations in 14 male-female zebra finch pairs (N=28) across two breeding cycles, using a repeated measures design. PRL was measured in the birds' first, reproductively inexperienced, breeding cycle beginning at courtship and extending through chick fledging. PRL was measured again during the birds' second, reproductively experienced, breeding cycle, beginning with egg laying until chick fledging. We found that plasma PRL is significantly elevated from non-breeding concentrations during late incubation and early post-hatch care and that this elevation is greater in the reproductively experienced cycle compared to the inexperienced cycle. Findings of this study will be used to inform hypotheses and predictions for future experimental manipulations of PRL during parental care.