Receptors mediating tachykinin-evoked depolarisations of neurons in the neonatal rat spinal cord

We have examined the contribution of NK1, NK2 and NK3 receptors to the depolarisation of the neonatal rat spinal cord in vitro evoked by exogenously applied tachykinins. Potential changes were recorded extracellularly from a lumber ventral root. The NK1 receptor selective agonists substance P methly ester (SPOMe), septide and /Sar(9)/substance P-sulphone (/Sar(9)/P-sulphone), perfused onto the cord for 20 s, evoked ventral root potentials (VRPs) with similar EC50 values of 7.2 nM (95% confidence limits, 4.4-10.9 nM), 4.6 nM (1.8-9.3 nM) and 3.1 nM (1.6-5.1 nM), respectively. The NK3 receptor selective agonist senktibe also evoked VRPs with an EC50 of 12.0 nM (5.1-23.4 nM), whilst the NK2 receptor selective agonist /beta -Ala(8)/-neurokinin A(4-10) (/beta -Ala(8)/-NKA(4-10)) was much less potent (EC50 = 228.3 nM, 95% confidence limits 138.0-350.0 nM). The non-peptide NK1 receptor selective antagonist RP67580 inhibited responses to SPOMe, septide and /Sar(9)/-SP-sulphone to varying degrees with IC50 values against each of 16.0 nM (10.7-23.4 nM), 19.8 nM (8.9-37 M) and 58.0 nM (41-89 nM), respectively. The NK1 receptor antagonist CP-96,345 similarly inhibited responses to these agonists, although with higher IC50 estimates of 0.84 muM (0.51-1.40 uM) against SPOMe, 0.79 muM (0.50-1.17 muM) against /Sar(9)/-SP-sulphone and 0.37 muM (0.27-0.51 muM) against septide. Further analysis of the activity of RP67580 yielded a significantly higher pK(B) estimate for antagonism of responses to septide (7.67 +/- 0.04) than of responses to /Sar(9)/-SP-sulphone (7.18 +/- 0.05). In both cases Schild analysis indicated competitive antagonism. RP67580 also reversibly inhibited responses to /beta -Ala(8)/-NKA(4-10). However, in this case the Schild slope was significantly different from unity (0.44 +/- 0.05; P < 0.001), although the potency of the antagonist appeared similar to that seen with the NK1 receptor agonists (pA(2) = 7.52). There me no effect of RP67580 against responses to senktide or of the inactive isomer RP67581 against septide-evoked VRPs. The NK2 receptor antagonist MEN 10,376 at concentrations up to 1 <mu>M produced a partial but reversible inhibition of responses to a submaximal concentration of /beta -Ala(8)/-NKA(4-l0) (0.3 muM) with a maximum reduction in VRP amplitude of 25.6 +/- 7.3%. A similar inhibitory effect was seen against septide-evoked VRPs (30.2 +/- 5.6% inhibition), although there was no effect against responses to submaximal concentrations of /Sar(9)/-SP-sulphone or senktide. In contrast, the non-peptide NK2 receptor antagonist SR 48,968 (1 muM) produced a maximal 48.0 +/- 7.7%. inhibition of /beta -Ala(8)/-NKA(4-10) -evoked VRP's with no effect against responses to a submaximal concentration of septide. These data show that NK1 and NK3 receptor activation mediates depolarisation of the neonatal rat spinal cord, and suggest the presence of two NK1 receptor populations showing preference for septide and /Sar(9)/-SP-sulphone. Depolarisations mediated by /beta -Ala(8) /-NKA(4-10), previously described as a selective NK2 receptor ligand, are mediated predominantly via an action at NK1 receptors with a lesser involvement of NK2 receptors.