Unilateral
ligation of the tendon of anterior superficial part of rat masseter muscle (TASM) leads to long-lasting
allodynia. Sex differences in peripheral
mu-opioid receptor (MOR)-mediated
analgesia under persistent myogenic
pain are not well understood. In this study, we examined (1) whether locally applied MOR agonists attenuate persistent
pain following TASM
ligation in a sex dependent manner, (2) whether there are sex differences of MOR expression changes in rat trigeminal ganglia (TG). The effects of MOR agonist, D-Ala2, N-Me-Phe4, Gly5-ol]-
Enkephalin acetate salt (
DAMGO), were assessed 14 days after TASM
ligation in male, female and orchidectomized (GDX) male rats. MOR
mRNA and
protein levels in TG 14 days following tendon
ligation were also determined. The mechanical thresholds of the injured side were significantly decreased in both male and female rats, from 3 days to 28 days after TASM
ligation.
A10 μg
DAMGO significantly attenuated
allodynia in male rats. A 10-fold higher dose of
DAMGO was required in female and GDX male rats to produce the level of anti-
allodynia achieved in male rats. The level of MOR
mRNA in TG from male rats was significantly greater 14 days after TASM
ligation compared with the
sham-operated male rats, but not from female and GDX male rats. After TASM
ligation, males had significantly more MOR immunoreactivity in TG compared to
sham-operated males. The MOR levels increased to 181.8% of the
sham level in male rats receiving
tendon injury. But there was no significant change in female rats receiving
tendon injury compared to the
sham female rats. Taken together, our data suggest that there were sex differences in the effects of peripheral MOR agonists between male and female rats under TASM
ligation developing long-lasting
pain condition, which is partly mediated by sex differences in the changes of MOR expressions and
testosterone is an important factor in the regulation of MOR.