Behavioral responses to uterine or vaginal distension in the rat

Afferent fibers in the rat hypogastric and pelvic nerves supply the uterus and vagina, respectively, the former being responsive mainly to intense uterine stimuli, the latter to gentle and intense vaginal stimuli (Berkley et al. 1993c). If such responses result in sensory experiences, those produced by uterine and vaginal stimulation should differ, uterine stimuli being experienced mainly as pain and vaginal stimuli experienced in various ways, including pain. To test this hypothesis, 48 young virgin rats were first trained to make an operant response to escape a noxious tail-pinch stimulus. Latex balloons inserted into the rat's uterine horn or vagina were then distended to various volumes and the metestrous rat's detection behaviors and operant escape response probabilities measured. Approximately 26% of the 23 rats tested failed to respond to uterine stimulation, even when it produced ischemia. For the rest, detection and escape responses occurred only to ischemic stimuli and never to all of them, even at the highest volumes. In contrast, all 25 rats tested responded readily to vaginal distension, often to all of them at high volumes. Detection behaviors occurred at distension magnitudes lower than those that evoked escape responses. These results support the hypothesis that sensory consequences of uterine and vaginal stimulation differ. Because effective uterine stimuli were larger than any that would occur in a normal physiological state in non-pregnant/parturient rats, normally occurring uterine states in such rats are probably insensible. In addition, while the behavioral responses did indeed reflect differences in hypogastric and pelvic nerve response properties, the results also indicated that activity produced in those fibers, even by abnormal stimuli, does not inevitably result in behavior.