Copulatory stimuli in rats induce heat abbreviation through effects on genitalia but not through effects on central nervous mechanisms supporting the steroid hormone-induced sexual responsiveness.

Male copulatory stimuli in various animal species abbreviate the length of the period of their female conspecifics' heat. Such effect can be explained in some species as the result of copulation-induced alterations in ovarian hormone secretion (e.g. reflex ovulators). Other species require a different explanation: specifically, interactions in the hypothalamus between effects of steroid hormones and inhibitory neural afferents from the genital area have been hypothesized to play a role in some laboratory animals such as guinea pigs, hamsters and rats. However, the evidence in support of heat abbreviation through copulatory stimuli in rats is at best equivocal. There is obvious need for its unequivocal demonstration prior to any further analysis of the potential mechanism of action. The present study intended to find a model in support of the concept of 'centrally induced' heat abbreviation in rats. Virgin rats during 'natural heat' were examined with or without allowing them the opportunity to pace male copulatory activities. Ovariectomized, steroid hormone-treated rats were studied with minor or ample experience with sexual activities in prior tests and, also, with or without the opportunity to 'pace' male sexual acts. None of the experimental models revealed unequivocal support for detrimental effect of large numbers of intromittive copulations on 'central mechanisms' regulating heat duration. It rather seemed that frequent intromittive copulations and ejaculates affected the 'peripheral' genital tract (vulva, vagina, cervix) making further copulation of a highly aversive quality so that sexual encounters were avoided or prevented. This effect was noticed with great inter-individual variability. The conclusion is drawn that rats do not show the counterpart of heat abbreviation reportedly occurring rapidly and reliably in guinea pigs after a limited amount of vaginal/cervical stimulation through copulation or insertions of a glass-rod.