Estrous variations in behavioral responses to vaginal and uterine distention in the rat

Response properties of peripheral afferent fibers supplying the vagina and uterus of the rat vary with estrous stage (Robbins A, Berkley KJ, Sate Y. E strous cycle variation of afferent fibers supplying reproductive organs in the female rat. Brain Res 1992;596:353-356), suggesting that behavioral sen sitivity to vaginal and uterine stimulation might similarly vary. To test t his hypothesis, detection and escape responses of 12 rats to different volu mes of distention of the vaginal canal or uterine horn (six rats each) were compared during each of the four estrous stages, proestrus (P), estrus (E) , metestrus (M), and diestrus (D), using previously-published behavioral te chniques (Berkley KJ, Wood E, Scofield SL, Little M. Behavioral responses t o uterine or vaginal distention in the rat. Pain 1995;61:121-131). Estrous variations in vaginal or uterine tone (pressure-volume functions) were also measured in the same rats. Vaginal canal: escape response percentages incr eased significantly as vaginal distention volume increased, but neither esc ape nor detection responses varied with estrous stage. Vaginal tone, howeve r, was greater in P and E than in M and D, with the greatest tone in E and the least in D. When escape responses to different pressures were analyzed, it was found that escape response percentages to the same vaginal pressure were lower during P and E than during M and D. One outcome of these estrou s influences would be that a vaginal stimulus of a given volume (such as an erect penis) would exert higher pressures during P and E (i.e. the penis w ould be held within the vaginal canal more firmly) than during M and D, but would be less likely during P and E to provoke escape responses. This modi fication of nociceptive sensitivity has obvious reproductive advantages, be cause P and E constitute the rat's fertile period. Uterine hem: escape resp onse percentages increased significantly as uterine distention volume incre ased only during M and D, with no differences between them. Detection respo nses also occurred only during M and D. Similarly, uterine tone was signifi cantly greater in M and D than in P and E, with the greatest tone occurring during M and the least during P. Although these changes in uterine tone we re opposite to those of the vaginal canal, escape response percentages to t he same uterine pressures were, in a manner similar to vaginal pressures, l ower during P and E than during M and D. The functional significance of the se variations for the uterus is unclear, but does suggest that, under patho physiological conditions, stimulation of the uterine hem, as well as the va ginal canal, would be more likely to provoke escape behaviors in M and D th an in P and E. A similar increased sensitivity in rats during M and D to no xious stimulation of other pelvic organs has been observed by others. This situation resembles that in women, for whom many visceral pain conditions a re exacerbated during a comparable part of their ovarian cycle, i.e. perime nstrually. (C) 1999 International Association for the Study of Pain. Publis hed by Elsevier Science B.V.