BEHAVIORAL-RESPONSES TO UTERINE OR VAGINAL DISTENSION IN THE RAT

Afferent fibers in the rat hypogastric and pelvic nerves supply the ut erus and vagina, respectively, the former being responsive mainly to i ntense uterine stimuli, the latter to gentle and intense vaginal stimu li (Berkley et al. 1993c). If such responses result in sensory experie nces, 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 th e rat's uterine horn or vagina were then distended to various volumes and the metestrous rat's detection behaviors and operant escape respon se probabilities measured. Approximately 26% of the 23 rats tested fai led to respond to uterine stimulation, even when it produced ischemia. For the rest, detection and escape responses occurred only to ischemi c stimuli and never to all of them, even at the highest volumes. In co ntrast, all 25 rats tested responded readily to vaginal distension, of ten to all of them at high volumes. Detection behaviors occurred at di stension magnitudes lower than those that evoked escape responses. The se results support the hypothesis that sensory consequences of uterine and vaginal stimulation differ. Because effective uterine stimuli wer e larger than any that would occur in a normal physiological state in non-pregnant/parturient rats, normally occurring uterine states in suc h rats are probably insensible. In addition, while the behavioral resp onses did indeed reflect differences in hypogastric and pelvic nerve r esponse properties, the results also indicated that activity produced in those fibers, even by abnormal stimuli, does not inevitably result in behavior.