Urine-induced reflex ovulation in anovulatory rats may be a vomeronasal effect

EXPOSURE to urine has been shown to influence the reproductive physiology of immature and mature female rodents. It affects the onset of puberty, as measured by vaginal opening 1,2 and by uterine growth 3,4 , and the maintenance of pregnancy in mice 5–7 , as well as the timing of oestrous cycles in both mice 8,9 and rats 10,11 . These effects seem to be elicited differentially by particular components of urine 3 . Although an ovulatory cycle eventually follows exposure to urine in all these examples 12 , reflex ovulation (that is, ovulation occurring shortly after brief exposure to a sensory stimulus) in response to male urine, has not been demonstrated previously. Also, traditionally, behavioural, as well as gonadal effects of urine were assumed to occur through the primary olfactory system 4,12 , rather than through one of the accessory olfactory systems, such as the vomeronasal system 13–16 . However, a role for the vomeronasal system was recently found for male hamster mating behaviour 17 . Furthermore, Raisman has pointed out that the vomeronasal system is brought into relationship with neural systems that are of importance in gonadotrophic release as well as sexual behaviour 18 . We demonstrate here that male urinary factors can elicit reflex ovulation in rats in the absence of coitus, mounting or removal to a novel cage. The present study is also the first to suggest that the vomeronasal system (in addition to, or instead of, the primary olfactory system) is involved in the effects of urinary chemicals on gonadal function.