Role of orexin receptor subtypes in the inhibitory effects of orexin-A on potassium chloride-induced increases in intracellular calcium ion levels in neurons derived from dorsal root ganglion of carrageenan-treated rats

Abstract

We analysed the roles of orexin receptors in the effects of orexin-A on KCl-induced increases in intracellular calcium ion levels ([Ca²⁺]_i) in C-fiber-like small neurons of rats with inflammation induced by intraplantar injection of carrageenan into the hind paw. Controls were treated with saline. Paw withdrawal and threshold forces in response to tactile stimuli were determined using von Frey filaments. [Ca²⁺]_i in C-fiber-like neurons derived from dorsal root ganglia was visualised using a calcium fluorescence probe. Changes in neuronal [Ca²⁺]_i were assessed as relative fluorescence intensity (F/F₀). One day after carrageenan injection, the paw withdrawal response to tactile stimuli and the paw withdrawal threshold were increased and reduced, respectively. KCl loading of neurons from either carrageenan-treated or control rats increased F/F₀ to about 2.0. KCl-induced increases in F/F₀ of carrageenan-treated, but not control, rats were inhibited by orexin-A. The OX₁ and OX₂ receptor antagonist MK-4305, but not the OX₁ receptor antagonist SB334867, counteracted the effects of orexin-A on the KCl-induced increase in F/F₀. These results suggest that OX₂, but not OX₁ receptors mediate the inhibitory effect of orexin-A on KCl-induced increases in [Ca²⁺]_i in C-fiber-like neurons of rats with inflammation.