Unpredictable sound stress evokes PMA, hind paw mechanical allodynia, and grimacing pain behavior
We first determine the development of nociception post stress in the 1, 3, 7, 10, and 14 days and address whether there would be a sexually dimorphic effect. We observed in this stress model, that on day 1 after stress induction there was an enhancement in the plasma corticosterone level, but not after 7 days of stress exposure (data not shown), as described before for this model using rats (Khasar et al., 2008).
Male and female mice showed hind paw mechanical allodynia in the 3-, 7-, and 10-days post stress induction. Mice returned to baseline withdrawal thresholds in 14 days post stress induction (Fig. 2 A and B). Similarly, when verifying PMA, in both sexes, the allodynic effect of stress started on the first-day post-stress and was maintained until the 10th day (Fig 2. D and E). To measure spontaneous non evoked pain in these animals, we assessed grimace. We observed that both males and females ST, compared to NST mice had grimacing pain behavior on the first and 7th-day post stress (Fig 2. G and H).
Thus, we verified that the nociceptive peak was on the 7th-day post-stress for PMA, hind paw mechanical allodynia, and grimacing pain behavior. Interestingly, on the nociceptive peak, it was observed a sexual dimorphism for hind paw mechanical allodynia, PMA, and grimacing pain behavior. These data showed that among the ST animals, females had higher nociception compared to males in the 7th-day post-stress induction (Fig. 2 C, F, I). However, male and female mice had similar development of nociception at 1-, 3-, 10-, and 14-days post-stress induction (data not shown).