The neural integration of environmental information by seasonally breeding birds

SYNOPSIS. Songbirds show dramatic seasonal changes in their propensity to exhibit an endocrine response to environmental stimuli that regulate the timing of reproduction. There is a clear cycle in their ability to respond to changes in photoperiod but less is known about seasonal variation in their sensitivity to supplementary factors that augment the photo-induction of gonadal growth and fine tune the final onset of breeding. Such factors include behavioral interactions with a mate and the availability of a nest site. In birds, seasonal variations in the neural processing of photoperiod are mediated by substantial modifications in the availability of hypothalamic Gonadotropin-Releasing Hormone (GnRH). There is also evidence that the activity of certain monoamines, such as norepinephrine and serotonin, changes in discrete parts of the hypothalamus as a function of the bird's photoperiodic condition. However, it is not known whether the response to supplementary factors involves similar changes. In this paper, I review what is known about how the avian central nervous system might integrate initial predictive information, such as photoperiod, and supplementary cues such as behavioral interactions. Photoperiodic information is perceived by an encephalic photoreceptor while supplementary stimuli are perceived by the visual and auditory systems. Both types of information seem to converge on the GnRH system of neurons in the preoptic area and the septum that regulate pituitary function. This suggests that future studies should concentrate on the connectivity of these GnRH positive cells to other parts of the central nervous system and on the regulation of GnRH activitiy in these cells by endogenous and exogenous factors.