Publisher Summary This chapter discusses the strategy of seasonal breeding, the role of photoperiod in timing the annual reproductive cycle, the hypothalamo-pituitary mechanisms that mediate photoperiodic regulation of estrous cyclicity, and the photoperiodic pathway to luteinizing hormone (LH) pulse generator. To understand how photic input to the LH pulse generator leads to seasonal changes in gonadal activity, the sequence of endocrine events that normally leads to ovulation during the estrous cycle of the ewe must be considered. These preovulatory events occur during a 2–3 day follicular phase and include a precipitous drop in progesterone, a progressive rise in tonic LH secretion, a sustained increase in estradiol secretion, and the LH surge. The pivotal step in this sequence is the sustained increase in tonic LH secretion. A great deal of insight has been gained into the complex interplay between the neural and endocrine response systems that underlie the seasonal reproductive process in the short-day breeding ewe. Specifically, light cues activate retinal photoreceptors and are transmitted via a monosynaptic tract to the suprachiasmatic nuclei of the hypothalamus. After interacting with the circadian system, the photic information is relayed to the pineal gland that transduces the neural message into a hormonal signal in the form of a circadian rhythm of melatonin secretion. The pattern of this melatonin signal, which is interpreted as inductive or suppressive, sets the frequency of the LH pulse generator and determines the capacity of this neural oscillator to respond to the negative feedback action of estradiol. The resulting changes in the episodic pattern of gonadotropin secretion, in turn, dictate whether or not estrous cycles can occur.

Neuroendocrine basis of seasonal reproduction.

Reproduction of many temperate zone birds is under photoperiodic control. The Japanese quail is an excellent model for studying the mechanism of photoperiodic time measurement because of its distinct and marked response to changing photoperiods. Studies on this animal have suggested that the mediobasal hypothalamus (MBH) is an important centre controlling photoperiodic time measurement. Here we report that expression in the MBH of the gene encoding type 2 iodothyronine deiodinase (Dio2), which catalyses the intracellular deiodination of thyroxine (T4) prohormone to the active 3,5,3'-triiodothyronine (T3), is induced by light in Japanese quail. Intracerebroventricular administration of T3 mimics the photoperiodic response, whereas the Dio2 inhibitor iopanoic acid prevents gonadal growth. These findings demonstrate that light-induced Dio2 expression in the MBH may be involved in the photoperiodic response of gonads in Japanese quail.

Light-induced hormone conversion of T4 to T3 regulates photoperiodic response of gonads in birds.

http://naturaleater.com/science-articles/circadian-lunar-annual-rhythms.pdf

Human responses to the geophysical daily, annual and lunar cycles.

The behavior of the Japanese or domestic quail Coturnix japonica.

Animals have evolved many season-specific behavioural and physiological adaptations that allow them to both cope with and exploit the cyclic annual environment. Two classes of endogenous annual timekeeping mechanisms enable animals to track, anticipate and prepare for the seasons: a timer that measures an interval of several months and a clock that oscillates with a period of approximately a year. Here, we discuss the basic properties and biological substrates of these timekeeping mechanisms, as well as their reliance on, and encoding of environmental cues to accurately time seasonal events. While the separate classification of interval timers and circannual clocks has elucidated important differences in their underlying properties, comparative physiological investigations, especially those regarding seasonal prolactin secretions, hint at the possibility of common substrates.

/pdf/tracking-the-seasons-the-internal-calendars-of-vertebrates-49euvh4r1i.pdf

Tracking the seasons: the internal calendars of vertebrates

IN 1936 Bunning1,2 suggested that living organisms measured the length of the daily photoperiod by using an endogenous daily (circadian) rhythm which he envisaged as consisting of two half-cycles (the photophil followed by the scotophil) each of approximately 12 h duration2. Photoperiodic induction of a physiological process requiring long days occurred only when the duration of the natural photoperiod extended into the scotophil part of the endogenous cycle. A more explicit version of the Bunning hypothesis has been presented by Pittendrigh3,4, stressing the dual function of light in acting both as an entraining agent (or Zeitgeber) for the circadian oscillation and as the photoperiodic inducer. He has also emphasized that the “scotophil” may not last for a full 12 h and suggests the more restrictive term “photo-inducible phase” to refer to that portion of the daily oscillation which, if illuminated, leads to photoperiodic induction. There is now good evidence available to support Bunning's hypothesis in plants5,6, and in at least four species of finches where the photoperiodic induction of testicular growth also relies on a circadian rhythm7–16. The present studies in a Galliform species, the Japanese quail (Coturnix coturnix japonica)17–20, were undertaken not only to check whether a circadian oscillator was involved in the photo-periodic response mechanism of both sexes but also to measure the duration of the photo-inducible phase. Experiments on the latter seemed possible, for Coturnix shows an extremely high rate of gonadal development18.

P. J. Sharp

Papers

Circadian rhythmicity in photoperiodically induced gonadotrophin release and gonadal growth in the quail.