26RFa is an endogenous ligand for the QRFP receptor. We previously found that intracerebroventricular injection of 26RFa produces an analgesic effect in a rat formalin test. In the present study, we directly tested the hypothesis that the analgesic effects of 26RFa in the formalin test are mediated in well-recognized regions of the descending inhibitory pain pathways, such as the rostral ventromedial medulla (RVM), locus coeruleus (LC), and periaqueductal grey (PAG) in rats. Injection cannulae were stereotaxically placed in the RVM, LC, or PAG through a burr hole. 26RFa (15 μg) or saline was delivered in a total volume of 0.5 μL. In a formalin test, 50 μL of 5% formalin was injected subcutaneously into the hind paw. In an antagonist study, idazoxan, an α-2 antagonist, or naloxone, an opioid receptor antagonist, was administered. Microinjection of 26RFa into the RVM had no effect compared with that in saline-injected rats. Microinjection of 26RFa into the LC contralateral, but not ipsilateral, to the formalin injection site significantly decreased the number of flinching behaviors compared with that of saline-injected rats. This effect was antagonized by intrathecal injection of idazoxan. Microinjection of 26RFa into the contralateral, but not ipsilateral, PAG produced an analgesic effect, and this effect was partly antagonized by intraperitoneal naloxone. These data suggest that 26RFa microinjected into the contralateral LC induced noradrenaline release in the spinal cord and produced an analgesic effect. In the contralateral PAG, 26RFa activated the opioid system, and some analgesic effects were mediated by opioid system activation.
Keywords: 26RFa; Descending inhibitory system; Formalin test.
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