Day Length and Reproduction:
One area of our research focuses on how the perception of exogenous cues leads to an internal representation of seasonal time, and how this sense of time is communicated to the hypothalamic-pituitary-gonadal axis. Changes in day length (also called 'photoperiod') are the most noise-free time-of-year cues in nature; harbingers of climatic change, they are the principal input cue for the phasing of seasonal rhythms. Changes in day length are perceived by the brain, then translated into an endocrine signal in the form of nocturnal melatonin secretion. Decreasing day lengths in late-summer yield longer nightly patterns of melatonin, and this melatonin acts in the brain and the periphery to inhibit reproductive physiology and behavior and thereby terminate the breeding season.
The empirical goals of our work in this area are: to construct formal models of how day length signals are processed by the brain, and to identify the mechanisms (neural, hormonal, molecular) which collectively participate in seasonal reproductive transitions and the internal representation of time.
This work has spanned two decades and is inspired by mentors/collaborators at U.C. Berkeley, Ohio State University, UMass Worcester and the University of Memphis.
Topics have included:
Melatonin signal processing. In nature, day lengths are always changing. Does the brain require all of the day length information it receives, or can it make due with periodic sampling of time-of-year information? In order to understand how animals integrate and make sense of a series of changing photoperiod/melatonin signals, we characterized signal processing mechanisms that permit reproductive 'interpretation' of a series of heterogeneous melatonin stimuli. [PubMed]
Photoperiodic memory. Short winter days are sufficient to inhibit reproduction, but in nature, seasonal adaptations must be in place months before winter day lengths ever occur. Intermediate-duration photoperiods trigger these adaptations. Discrimination of intermediate photoperiods at different times of year requires a form of memory for prior day lengths. This ‘photoperiodic memory’ permits decoding direction of photoperiod change. This memory trace endures only a few months, ensuring that only recent information provides a context for the evaluation of prevailing photoperiods. [PubMed]
Social and energetic control of reproduction. This work examines how the brain integrates photic and non-photic cues in the control of reproduction. The influence of non-photic cues on reproduction is more prominent during intermediate photoperiods (early spring or late summer), when day length per se is an unreliable predictor of year-to-year fluctuations in food availability. Intermediate day lengths unmask robust reproductive responsiveness to cues such as the presence/absence of conspecifics or small changes in food availability. Non-photic cues and intermediate day lengths also affect hypothalamic kisspeptin and RFamide related peptide (GnIH) gene expression. [PubMed]
Seasonal clocks and interval timing (photorefractoriness). Whereas the termination of reproductive behavior in autumn is a direct response to changes in day length, the timing of vernal reproduction is governed by an altogether different mechanism: an endogenous seasonal clock. This seasonal interval timer renders the brain unresponsive, or ‘refractory’, to winter photoperiods after an interval of ~5 months, triggering the transition back to reproductive competence. My work has investigated the seasonal interval timer at formal, evolutionary, and physiological levels of analysis. [PubMed]
Click on the topics/links above to access these papers via PubMed.