Sex, surges and circadian rhythms: The timing of reproductive events in a diurnal roden

Rhythms in the timing of reproductive events are reversed in diurnal and nocturnal rodents, but little is known about the neural mechanisms underlying these differences. I examined these issues by comparing the diurnal murid rodent Arvicanthis niloticus (grass rat) to nocturnal Rattus norvegicus (lab rat). In the first set of experiments, I examined the hypothesis that differences in the timing of estrous events in diurnal and nocturnal species are due to differences in rhythms in responsiveness to steroid hormones. I found that steroids were able to induce a rise in activity of neurons containing gonadotropin releasing hormone (GnRH) at only one time of day, which was 12 hours apart in grass rats and lab rats. These temporal patterns persisted in both species when they were housed in constant darkness for five days suggesting that an endogenous circadian clock drives the rhythms in responsiveness to hormones. Secondly, I determined whether cells within the suprachiasmatic nucleus (SCN), the site of the principle mammalian circadian clock, project to neuroendocrine and steroid sensitive cells in grass rats, and whether these pathways differed from those of lab rats. Anterograde tract-tracing in grass rats revealed that both GnRH and estrogen receptor (ER) containing cells appear to receive input from the SCN, as has been seen in nocturnal rodents. I then found that arginine vasopressin and vasoactive intestinal polypeptide, two neuropeptides made in the SCN, were contained in axon terminals that contacted GnRH neurons, as is also the case in lab rats and hamsters. In the third set of experiments, I examined the hypothesis that inverted rhythms in the timing of estrus-related behaviors in diurnal and nocturnal rodents are due to differences in rhythms in sensitivity to steroid hormones. Pairs of males and hormone-primed females were tested for mating at four different times of day. I found that grass rats had a daily rhythm in sexual behavior that was 12 hours out of phase relative to that seen in nocturnal rodents. Specifically, both the lordosis quotient and rate of copulation were relatively low at zeitgeber time (ZT) 17 and then rose to a peak at ZT 23. I also observed a bimodal rhythm in male mounting behavior that peaked at ZT 11 and 23. Taken together, these results indicate that steroid-primed grass rats and lab rats are similar with respect to the temporal relationship among estrous-related events, but that the timing of these events relative to the light:dark cycle is dramatically different. These differences appear to be due to rhythms in responsiveness to steroid hormones whereas the structure of the neural pathways communicating temporal information from the SCN to cells within the reproductive axis appear to be the same. The mechanisms underlying the differences in rhythms of sensitivity to steroids might involve temporal patterns of signals emitted by the SCN and/or patterns of sensitivity of neural targets to these signals.