OX2 receptors mediate the inhibitory effects of orexin-A on potassium chloride-induced increases in intracellular calcium ion levels in neurons derived from rat dorsal root ganglion in a chronic pain model.

AIMS: Orexin-A is known to induce anti-nociceptive effects in animal models of chronic pain. We have found that orexin-A inhibits KCl loading-induced increases in the intracellular calcium ion levels ([Ca2+ ]i ) in C-fiber-like neurons of rats showing inflammatory nociceptive behavior. Here, we examined the effects of orexin-A on the depolarization of C-fiber-like neurons derived from a rat model for another type of chronic pain, namely neuropathic pain. Thus, we analyzed the effects of orexin-A on KCl-induced increases in [Ca2+ ]i in C-fiber-like neurons of rats with sciatic nerve ligation.

METHODS: Paw withdrawal and threshold force in response to tactile stimuli were evaluated using von Frey filaments. Sham-operated rats served as controls. [Ca2+ ]i in neurons were visualized by calcium fluorescent probe. Changes in [Ca2+ ]i were assessed using relative fluorescence intensity.

RESULTS: Seven days after sciatic nerve ligation, paw withdrawal and threshold force for tactile stimuli were increased and reduced, respectively. KCl loading to neurons from either sciatic nerve-ligated or control rats increased relative fluorescence intensity. The KCl-induced increase in relative fluorescence intensity in sciatic nerve-ligated, but not that of control, rats was inhibited by orexin-A. The OX1 and OX2 receptor antagonist MK-4305 and OX2 receptor antagonist EMPA, but not the OX1 receptor antagonist SB 334867, each counteracted orexin-A-induced inhibition of KCl-provoked increases in relative fluorescence intensity.

CONCLUSION: The present findings constitute neuropharmacological evidence that OX2 but not OX1 receptors mediate the inhibitory effects of orexin-A on KCl-induced increases in [Ca2+ ]i in C-fiber-like neurons of rats showing hyperalgesia provoked by sciatic nerve ligation.