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ACTIVATION OF MU AND KAPPA RECEPTORS ON INTESTINAL DORSAL ROOT GANGLIA (DRG) NERVES INHIBIT NOCICEPTIVE AND NEUROGENIC INFLAMMATORY SIGNALING
I Spreadbury, S VannerQueen's University, Kingston, Ontario
AIMS: Recent studies suggest that the release of endogenous opioids from immune cells during chronic inflammation has an anti-nociceptive action, but the mechanisms by which this may occur in the GI tract have not been clarified. We proposed that opioid agonists inhibit nociceptive currents and neurotransmitter release by acting on intestinal DRG neurons.
METHODS: To study TRPV1 currents in these neurons, we employed patch clamp techniques where nociceptive colonic DRG neurons were identified using Fast Blue retrograde labeling and by their small size. Isolated DRG neurons were recorded using the amphotericin perforated
patch technique at a holding potential of −70 mV. The neurons had been incubated at 37°C in 10 μM forskolin for 15 minutes prior to patch
clamping. In a separate series of experiments we examined whether opioid agonists inhibit neurotransmitter release from capsaicin-sensitive nerves in the guinea pig ileum. Using in vitro guinea pig ileal submucosal preparations, submucosal arterioles were pre-constricted with PGF2alpha 300 nM, and vasodilator responses recorded using videomicroscopy following stimulation of capsaicin-sensitive perivascular nerves (20 Hz, 2 s).
RESULTS: In patch clamp recordings of DRG nerves, at 1-2 minute intervals 5 s applications of 400 nM capsaicin were made via a rapid super-
fusion system, producing inward currents of reproducible amplitude (mean = 2.39 +/- 0.93 nA, n = 5). Subsequent superfusion of 10 μM DAMGO
between capsaicin applications reduced the TRPV1 current amplitudes by 55.6% (+/- 7.8%, n = 5; p = 0.0021) within 5 minutes. In the guinea
pig submucosal preparation, compared to control vasodilator responses a 3 min application of the kappa agonist U50488H (30 μM) inhibited vasodilator responses by over 50% (p < 0.01). U50488H had a dose-dependent effect (EC50 = 7.6 μM) and was blocked by 1 μM naloxone. To
demonstrate that this was a pre-synaptic action, dilations evoked by exogenous application of the putative neurotransmitter substance P (100 nM) were studied in the presence and absence of U50488H. No difference was observed in the substance P evoked dilations between groups, suggesting U50488H had no effect on the post-synaptic membrane.
CONCLUSIONS: In summary we have shown that the μ agonist DAMGO inhibits TRPV1 currents, which are known to be involved in nociception, and kappa agonists inhibit neuropeptide release from the sensory nerve terminals in the gut, which can play a role in neurogenic inflammation. Together, these demonstrate that endogenous opioids could reduce nociceptive signalling in two ways; a direct effect by inhibiting ion channels on neurons, and an indirect action by suppression of neurogenic inflammation, thereby decreasing cytokine signalling to the neurons.