OX(2) 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 ([Ca(2+)](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 [Ca(2+)](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. [Ca(2+)](i) in neurons were visualized by calcium fluorescent probe. Changes in [Ca(2+)](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 OX(1) and OX(2) receptor antagonist MK‐4305 and OX(2) receptor antagonist EMPA, but not the OX(1) 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 OX(2) but not OX(1) receptors mediate the inhibitory effects of orexin‐A on KCl‐induced increases in [Ca(2+)](i) in C‐fiber‐like neurons of rats showing hyperalgesia provoked by sciatic nerve ligation.