The role of NK₁ and NK₂ neurokinin receptors in the acute and sustained nociceptive activation of dorsal horn neurons

Abstract

The tachykinins substance P (SP) and neurokinin A (NKA) can be released from fine somatosensory afferents into the spinal cord by noxious cutaneous stimuli. This study assessed the role of their respective NKj and NK2 receptors and some putative intracellular mediators in both acute and sustained nociceptive inputs to dorsal horn neurons:(a) In anaesthetised rats, extracellular recordings were made from laminae III-V multireceptive neurons. The ionophoretic administration of NK₁ antagonists L-668,169, GR 82334 and [D-Pro⁴,D-Trp⁷,⁹,¹⁰Phe¹¹]substance P-(4-ll) failed to influence neuronal responses to noxious pinch or heat, but often enhanced responses to innocuous brush, whilst the NK₂ antagonist L-659,874 inhibited responses to noxious heat, but not pinch or brush. Selective NK₁ and NK₂ receptor agonists, [N-acetyl-Arg⁶,Sar⁹,Met(O₂)¹¹]SP₆₋₁₁ and GR 64349 respectively, both excited dorsal horn neurons. The contribution of NK₁ and NK₂ receptors to sustained neuronal activity induced by peripheral application of the Cfibre selective algogen mustard oil was then investigated. The mustard oilinduced activity was inhibited by the selective NK₂ receptor antagonist L-659,874 but not by the selective NK₁ antagonists L-668,169 or GR 82334. The role of NK₁ and NK₂ receptors was further examined in the central neuronal sensitisation of both innocuous and noxious inputs, induced by mustard oil. Prior to mustard oil application, the NK₁ antagonists RP 67580 and GR 82334 selectively enhanced neuronal responses to innocuous brush, whilst NK₂ antagonists SR 48968 and L-659,874 selectively inhibited responses to noxious heat, however, following repeated mustard oil application, the facilitated neuronal responses to brush and heat were blocked by both selective NK₁ and NK₂ antagonists.(b) Evidence for a role of protein kinase C (PKC) in mediating sustained nociceptive responses of rat dorsal horn neurons was provided by the blockade of mustard oil-, but not brush-evoked neuronal activation by the PKC inhibitors GF 109203X and chelerythrine and by SR 48968-sensitive subcellular translocation of [³H]phorbol 12,13-dibutyrate binding sites ipsilateral to mustard oil stimulation.(c) In Situ hybridisation histochemistry (ISHH) demonstrated that expression of c-fos mRNA, induced in the superficial dorsal horn by peripheral application of mustard oil was inhibited by systemic administration of both RP 67580 and SR 48968.(d) The effects of intrathecally-applied NK\ and NK2 antagonists were assessed on thermally-evoked tail-flick and paw-flick behavioural responses. GR 82334 and L-659,874 had no effect alone, but in combination inhibited paw-flick. After inflammation induced by intraplantar injection of carrageenan, each was effective individually.These results provide evidence that spinal NK₂ receptors are involved in mediating both acute and sustained nociceptive transmission, probably acting through phosphoinositide hydrolysis and stimulation of protein kinase C (PKC). However, evidence for NK₁ receptor involvement was only obtained in sustained or inflammatory models of nociceptive transmission in the spinal dorsal horn.