Estrous and sex variations in vocalization thresholds to hindpaw and tail pressure stimulation in the rat

This study examined sex and estrous differences in vocalization thresholds of rats to hindpaw and tail pressure stimulation tested daily throughout at least 3 weeks. When all the measures were pooled, compared to males, female rats had higher thresholds for tail pressure (499 +/- 6 g, n = 188 measures vs. 466 +/- 2 g, n = 144 measures, respectively), but equal thresholds for hindpaw pressure (321 +/- 6 g, n = 188 measures vs. 319 +/- 2 g, n = 144 measures, respectively). Thresholds of female rats in proestrus and estrus were lower than those of rats in metestrus and diestrus for both tail and hindpaw stimulation, whereas those of males did not vary systematically. Thresholds at the two stimulation sites covaried in females but not in males. These results add to the growing list of important interacting factors that underly behavioral sensitivity to noxious somatic stimulation.