Figure 3.

Proposed model by which melatonin influences RFRP-3 and the reproductive axis in long-day-breeding rodents. Melatonin, coding for day length, acts on melatonin receptors in the pars tuberalis to influence TSH production (133–135). The long duration melatonin signal in short days suppresses TSH production, whereas short duration melatonin during long days is associated with robust TSH release. In long days, TSH stimulates tanycytes to upregulate Dio2 and, consequently, increases T3 (129–132, 142). Long days are associated with high RFRP-3 expression, with neuron numbers markedly reduced in short-day animals. Although T3 increases expression of RFRP-3 in short-day animals to resemble that of long-day rodents (142), whether RFRP-3 cells are direct targets of T3 remains to be determined.