Detection and transduction of daylength in birds.

Daylength is an important environmental cue used by temperate zone avian species to time the onset of seasonal reproductive activity. Photic cues are detected by extra-retinal, extra-pineal central nervous system elements, and are rapidly transduced to an efferent signal. In this paper, we describe the brain locus of putative encephalic photoreceptors in birds, and explore the pathway of information transfer from photic input to the reproductive axis. To this end, we examine how photoreceptors might communicate with the hypothalamic-pituitary axis, and how brain peptides vary seasonally. Recent studies indicate that brain photoreceptors lie in the lateral septum and in the tuberal hypothalamus, and co-express proteins characteristic of retinal photoreceptors, as well as vasoactive-intestinal polypeptide (VIP). At the light microscopic level, photoreceptor cells appear to communicate with gonadotropin-releasing hormone (GnRH) neurons, and vice versa. Expression of VIP-like immunoreactivity is highest in photorefractory animals while GnRH-like immunoreactivity is highest in photosensitive birds. Expression of these CNS peptides is correlated with changes in plasma prolactin and luteinizing hormone (LH), suggesting a mechanism mediating seasonal cyclicity.