Nociceptive threshold can be changed by gravity stimulation in rats

Abstract

It has been reported that stress exposure could suppress pain in various animal species. However, very few studies have been performed on the effect of gravity stimulation on nociceptors. The aim of the present study is to reveal the effect of gravity stimulation on nociceptive withdrawal responses in rats. A von Frey type apparatus was used to evoke withdrawal reflexes at eight different body sites. Reflex thresholds were measured before and after gravity stimulation, which was applied with a centrifugal-type apparatus at 116 r.p.m. (2g), for 10min. Naloxone-HCl (0.1 mg kg-1 i.p.) was used before centrifugal stimulation to investigate whether the analgesic effect was mediated via endogenous opioids. The mean threshold values significantly increased in all body sites after 2.0g exposure. Moreover, the analgesic effect was greater in upper body sites. The increase in threshold remained long after gravity stimulation, suggesting that humoral factors are involved in the analgesic mechanism. Gravity-induced analgesia was antagonized by naloxone, indicating that an opioid system is included in its mediation. Copyright © 2004 John Wiley & Sons, Ltd.