CONTRIBUTION OF NK(1) AND NK(2) RECEPTOR ACTIVATION TO HIGH-THRESHOLD AFFERENT FIBER EVOKED VENTRAL ROOT RESPONSES IN THE RAT SPINAL-CORD IN-VITRO

The contribution of neurokinin and NMDA receptor activation to the generation of the prolonged high threshold evoked ventral root potential (VRP) and its temporal summation has been assessed in the neonatal rat hemisected spinal cord maintained in vitro. High intensity single shock stimulation of the dorsal roots evoked a prolonged VRP (9.81 +/- 0.9 s, n = 11). A low frequency (1-10 Hz) repetitive stimulation (20 s duration) of high threshold afferent fibres evoked a summated VRP. This summated VRP reflected the temporal summation of EPSP's in spinal cord neurones which underlies the phenomenon of 'Windup'. The integrated area and duration of the high threshold evoked VRP were significantly reduced following superfusion of the spinal cord with the NK2 receptor antagonist MEN,10376 (100 nM). In the presence of D-AP5 (20 muM) the area of the C-fibre evoked VRP was also significantly reduced. The VRP duration was unaffected. Superfusion with either CP-96,345 (500 nM) or RP,67580 (100 nM), both non-peptide NK1 antagonist, did not have any significant effect upon the area or duration of the prolonged VRP following high threshold stimulation. The simultaneous application of D-AP5 (20 muM) with either MEN,10376 (100 nM) or CP-96,345 (500 nM) together produced a reduction in the area of the evoked VRP which was comparable to the value obtained by addition of their individual effects. The amplitude of the summated VRP was significantly reduced following application of D-AP5 (20 muM). No significant effect upon the amplitude was observed following separate application of either MEN,10376 (100 nM), CP-96,345 (500 nM) or RP,67580 (100 nM). The effect upon the amplitude of the summated VRP, of the simultaneous application of D-AP5 and MEN,10376, was not significantly different from the effect of D-AP5 alone. In contrast, the effect of the simultaneous application of D-AP5 and CP-96,345 was significantly different from the effect of D-AP5 alone. These data suggest that the long duration synaptic events evoked following brief activation of high threshold dorsal root afferent fibres mainly involves the activation of the NK2 tachykinin and NMDA receptor subtypes. More sustained nociceptive input however is associated with the additional activation of the NK1 receptor subtype. This contribution however is only revealed when in co-operation with the action of the NMDA receptor antagonist D-AP5.