A morning surge in plasma luteinizing hormone coincides with elevated Fos expression in gonadotropin-releasing hormone-immunoreactive neurons in the diurnal rodent, Arvicanthis niloticus

Arvicanthis niloticus is a diurnal murid rodent from sub-Saharan Africa. He re we report on processes associated with mating in this species in an atte mpt to elucidate how the neural mechanisms governing temporal organization differ in nocturnal and diurnal species. First, we systematically mapped th e distribution of GnRH neurons in adult females. Second, we tested the hypo thesis that Arvicanthis differ from nocturnal murid rodents with respect to the timing of the LH surge and the associated increase in Fos expression i n GnRH-immunoreactive (IR) neurons. We examined these events around a postp artum estrus. When parturition occurred between zeitgeber time (ZT) 2 and 1 7 (lights on at ZT 0 and off at ZT 12; there are 24 ZT units a day, each eq uivalent to 1 standard hour), we collected blood and perfused females at ZT 17, 20, 23, or 2. A sharp peak in plasma LH occurred at ZT 20, and a 10-fo ld increase in the percentage of GnRH-IR neurons that expressed Fos-IR occu rred between ZT 17 and 20. By contrast, this rise occurs in nocturnal roden ts during the last few hours of the light period. This is the first indicat ion of a difference between nocturnal and diurnal animals with respect to n eural mechanisms associated with a precisely timed event of known significa nce.