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Sex Differences in Antinociceptive Effects Induced by Gravity Stress in Rats

A number of studies have demonstrated that sex 
differentially affects responses to stress and pain. In this study, sex-related 
differences in pain responding were investigated in a gravity-induced analgesia 
model, where the effects of stressful high-gravity loading (1.5G or 2.0G for 10 
min) on nociceptive behavior in male and female rats were investigated. 
In each rat, eight sites (nose, both forepaws, upper and lower back, both hind paws 
and tail) were selected to apply noxious stimuli using a von Frey-type 
needle stimulator. The threshold values of the withdrawal responses were 
measured. In order to confirm the involvement of endogenous opioids in 
gravity-induced antinociceptive effects, naloxone-HCl (an 
opioid antagonist) was used. Effective analgesic effects could be induced by 
strong (2.0G) gravity loading, and clear sex differences were observed. 
Gravity-induced analgesic effects were more effective in males than in females, 
indicating that males are more sensitive to stress than females judging from 
nociceptive modulation. Naloxone-HCl produced a more pronounced suppression 
of nociceptive behavior in male rats, suggesting that gravity loading may 
activate endogenous opioids more readily in males than in females.