NK3 receptor blockade prevents hyperalgesia and the associated spinal cord substance P release in monoarthritic rats

Previous studies in vitro have shown that NK3 receptors exist on primary afferent terminals in rat spinal cord and mediate potentiation of the depolarisation-evoked substance P (SP) release. In the present study we have investigated the role of the NK3 receptor-mediated SP release system in a model of inflammatory pain. Monoarthritis was induced in rats by unilateral injection of complete Freund's adjuvant (CFA); withdrawal latencies to a thermal stimulus were subsequently measured at various times following CFA. The CFA-treated paw displayed hyperalgesia as early as 4 h after CFA injection and hyperalgesia was maintained until day 4 but had disappeared by day 21. The thermal hyperalgesia was associated with an increase in basal SP release from spinal cord synaptosomes. The possible involvement of endogenous neurokinin B acting at NK3 receptors was tested by using SB 223412-A [(S)-(-)-N-(alpha-ethylbenzyl)-3-hydroxy-2-phenylquinoline-4-carbo xamide hydrochloride], a novel, potent (Ki=30 nM) and selective (Ki>10,000 nM for NK1 and NK2 receptors), non-peptidic NK3 receptor antagonist. In vitro SB 223412-A antagonised the potentiation of SP release produced by senktide in spinal cord synaptosomes. Administered systemically to monoarthritic rats (50 mg/kg, p.o., b.i.d., for 4 days), the NK3 receptor antagonist SB 223412-A significantly reduced thermal hyperalgesia and normalised the basal release of SP from spinal cord synaptosomes. The data suggest that neurokinin B acting at NK3 receptors that mediate SP release within the spinal cord play a role in inflammation. These NK3 receptors may represent, therefore, appropriate targets in the therapy of inflammatory pain.