Pars tuberalis

Ecology

Biology

Neuroendocrinology

Kisspeptin

photoperiodism

Endocrinology

Hypothalamus

Gonadotropin-releasing hormone

Preoptic area

Triiodothyronine

Deiodinase

Seasonal breeder

Luteinizing hormone

Pituitary gland

Internal medicine

DIO2

Iodothyronine deiodinase

Melatonin

Hormone

Medicine

Journal of Neuroendocrinology

Synchronization of mammalian breeding activity to the annual change of photoperiod and environmental conditions is of the utmost importance for individual survival and species perpetuation. Subsequent to the early 1960s, when the central role of melatonin in this adaptive process was demonstrated, our comprehension of the mechanisms through which light regulates gonadal activity has increased considerably. The current model for the photoperiodic neuroendocrine system points to pivotal roles for the melatonin-sensitive pars tuberalis (PT) and its seasonally-regulated production of thyroid-stimulating hormone (TSH), as well as for TSH-sensitive hypothalamic tanycytes, radial glia-like cells located in the basal part of the third ventricle. Tanycytes respond to TSH through increased expression of thyroid hormone (TH) deiodinase 2 (Dio2), which leads to heightened production of intrahypothalamic triiodothyronine (T3) during longer days of spring and summer. There is strong evidence that this local, long-day driven, increase in T3 links melatonin input at the PT to gonadotropin-releasing hormone (GnRH) output, to align breeding with the seasons. The mechanism(s) through which T3 impinges upon GnRH remain(s) unclear. However, two distinct neuronal populations of the medio-basal hypothalamus, which express the (Arg)(Phe)-amide peptides kisspeptin and RFamide-related peptide-3, appear to be well-positioned to relay this seasonal T3 message towards GnRH neurons. Here, we summarize our current understanding of the cellular, molecular and neuroendocrine players, which keep track of photoperiod and ultimately govern GnRH output and seasonal breeding.

<scp>GnRH</scp>and the photoperiodic control of seasonal reproduction: Delegating the task to kisspeptin and<scp>RFRP</scp>‐3

GnRHand the photoperiodic control of seasonal reproduction: Delegating the task to kisspeptin andRFRP‐3