3.7 Neuropathic Pain: MOPs in primary sensory neurons contribute
to the antihyperalgesic effect of systemic opioid agonists
We sought to determine whether peripheral MOPs in primary sensory
neurons mediate the analgesic effect of opioids under neuropathic pain
conditions. In WT mice, s.c. injection of 5 mg∙kg-1DALDA reversed the SNIt-induced hyperalgesia, as
evidenced by a significant decrease in PWF to high force mechanical
stimuli from 62 ± 5.93% to 23 ± 5.17% 60 min after administration. The
same dose of DALDA produced no significant analgesic effect in theOprm1 cKO group (Figure 8A) . Previous investigations
have provided evidence for a peripheral component of the antinociceptive
effect of systemic morphine in rodents with peripheral mononeuropathy
and spinal nerve ligation-induced neuropathy
(Kayser, Lee & Guilbaud, 1995;
Pertovaara & Wei, 2001). In our study,
s.c. injection of 5 mg∙kg-1 morphine decreased PWF
from a SNIt-induced peak of 61 ± 2.77% to 18 ± 2.91%
30 min after administration (Figure 8B) . Morphine’s analgesic
effect in WT mice peaked at 30 min and lasted for approximately 90 min.
The same dose of morphine in Oprm1 cKO mice induced an attenuated
analgesic effect. Thirty minutes after drug injection, the Oprm1cKO group exhibited a decrease in ipsilateral PWF from 61 ± 3.79% to 35
± 5.43%. Similar to the time course in WT mice, morphine’s peak
response in Oprm1 cKO mice occurred after 30 min. However, the
overall effect of morphine in Oprm1 cKO mice lasted for only
approximately 60 min. Notably, morphine did not induce an analgesic
response in the contralateral hind paw of Oprm1 cKO mice.