Neuropeptides and reproductive flexibility in songbirds: A mini review

Synchronization of physiological and behavioral activities associated with avian reproduction requires corresponding changes in the activity of the hypothalamus-pituitary-gonadal axis. This involves complex brain peptidergic pathways, which show spatial and temporal differences in their expression and distribution during the annual reproductive cycle. The well-studied pathways include gonadotropin-releasing and inhibiting hormones (GnRH, GnIH), neuropeptide Y (NPY), cocaine- and amphetamine-regulated transcript (CART), vasoactive intestinal peptide (VIP) and other peptides like arginine vasotocin (VT), oxytocin (mesotocin), and spexin. Together, these peptides form a neurochemical framework for the integration of both internal and external (environmental) cues; this results in a neuroendocrine response. Conceivably, therefore, the neurochemical framework within which brain peptides possibly interact and perform reproductive regulatory roles might show species differences. Here, we aim to review briefly the roles of these neuropeptides in reproduction in both opportunistically and seasonally breeding birds. Much of the discussion will be based on our own research on the opportunistic breeding zebra finch and the seasonally breeding redheaded bunting, Indian weaverbird, and spotted munia. The summer breeding redheaded bunting and weaverbird are typical photosensitive long-day species, but they show qualitative differences in response to stimulatory photoperiods during the post-reproductive period of their annual cycle. Buntings exhibit absolute photorefractoriness, while weaverbirds exhibit relative photorefractoriness. The autumn breeding spotted munia, on the other hand, is an atypical photosensitive species. It responds to both short and long photoperiods and presumably lacks photorefractoriness.