Photoreceptive oscillators within neurons of the premammillary nucleus (PMM) and seasonal reproduction in temperate zone birds

The pathway for light transmission regulating the reproductive neuroendocrine system in temperate zone birds remains elusive. Based on the evidence provided from our studies with female turkeys, it is suggested that the circadian clock regulating reproductive seasonality is located in putatively photosensitive dopamine–melatonin (DA–MEL) neurons residing in the premammillary nucleus (PMM) of the caudal hypothalamus. Melanopsin is expressed by these neurons; a known photopigment which mediates light information pertaining to the entrainment of the clock. Exposure to a gonad stimulatory photoperiod enhances the activity of the DAergic system within DA–MEL neurons. DAergic activity encoding the light information is transmitted to the pars tuberalis, where thyroid-stimulating hormone, beta (TSHβ) cells reside, and induces the release of TSH. TSH stimulates tanycytes lining the base of the third ventricle and activates type 2 deiodinase in the ependymal which enhances triiodothyronine (T3) synthesis. T3 facilitates the release of gonadotropin-releasing hormone-I which stimulates luteinizing hormone/follicle stimulating hormone release and gonad recrudescence. These data taken together with the findings that clock genes are rhythmically expressed in the PMM where DA–MEL neurons are localized imply that endogenous oscillators containing photoreceptors within DA–MEL neurons are important in regulating the DA and MEL rhythms that drive the circadian cycle controlling seasonal reproduction.

► Putatively photosensitive dopamine–melatonin (DA–MEL) neurons express melanopsin. ► Clock genes are rhythmically expressed in DA–MEL neurons. ► DAergic terminal from DA–MEL neurons intermingle with TSHβ cells in pars tuberalis. ► DA–MEL neurons may encode light information and regulate reproductive seasonality.