No abstract

ATP has been proposed to mediate synaptic transmission in the spinal cord dorsal horn, particularly in the pathway carrying nociceptive information. Using transverse spinal cord slices from postnatal rats, we show that EPSCs mediated by P 2X receptors, and presumably activated by synaptically released ATP, are evoked in a subpopulation of spinal cord lamina II neurons, a region known to receive strong input from nociceptive primary afferents. The P 2X receptors on acutely dissociated dorsal horn neurons are nondesensitizing, insensitive to ␣␤ methylene ATP, and show strong but variable sensitivity to the antagonists suramin and pyridoxal-phosphate-6-azophenyl-2Ј,4Ј-disulfonic acid (PPADS). These characteristics are consistent with a heterogeneous population of P 2X receptors, the composition of which includes P 2X2 , P 2X4 , and P 2X6 receptor subtypes. Our results suggest that ATP-activated P 2X receptors in lamina II of the rat spinal cord may play a role in transmitting or modulating nociceptive information. Holton and Holton (1954) first proposed ATP as a possible neurotransmitter in the dorsal horn over 40 years ago. Since then, its role as a fast neurotransmitter in the peripheral nervous system has been demonstrated (Burnstock et al., 1972;Evans et al., 1992;Silinsky and Gerzanich, 1993;Galligan and Bertrand, 1994). In the C NS only one study, performed on neurons in the medial habenula, has demonstrated that ATP can act as a fast neurotransmitter (Edwards et al., 1992). Within the spinal cord dorsal horn, despite strong evidence implicating ATP as a putative neurotransmitter (Jahr and Jessell, 1983;Fyffe and Perl, 1984;Salter and Henry, 1985;Salter and Hicks, 1994), its role in this regard has not been demonstrated (Li and Perl, 1995).Several ATP receptor subunits have been cloned from different tissues (Brake et al., 1994;Valera et al., 1994;Chen et al., 1995;Lewis et al., 1995;Buell et al., 1996;Collo et al., 1996;Vulchanova et al., 1996). Of those cloned, the ATP P 2X2 , P 2X4 , and P 2X6 receptor subunit RNAs have been shown to be expressed in the spinal cord dorsal horn, particularly in the superficial laminae of the dorsal horn (Brake et al., 1994;Buell et al., 1996;Vulchanova et al., 1996), lending f urther support to the idea that ATP receptors may participate in synaptic function there. Recently, P 2X7 mRNA also was detected in both brain and spinal cord (and elsewhere), but its exact distribution was not described . When P 2X2 , P 2X4 , and P 2X6 receptor subunits are expressed in heterologous cellular systems in homomeric form, they all mediate a nondesensitizing response to ATP and are insensitive to the agonist ␣␤ methylene ATP (Brake et al., 1994;Collo et al., 1996;Séguéla et al., 1996), making them pharmacologically distinguishable from the other P 2X subunits. A further pharmacological distinction among subunits is that, whereas responses mediated by the P 2X2 subunit are sensitive to the antagonists suramin and pyridoxal-phosphate-6-azophenyl-2Ј,4Ј-disulfonic acid (PPADS), P 2X4 and P 2X...

NMDA receptors have the potential to produce complex activity-dependent regulation of transmitter release when localized presynaptically. In the somatosensory system, NMDA receptors have been immunocytochemically detected on presynaptic terminals of primary afferents, and these have been proposed to drive release of substance P from central terminals of a subset of nociceptors in the spinal cord dorsal horn. Here we report that functional NMDA receptors are indeed present at or near the central terminals of primary afferent fibers. Furthermore, we show that activation of these presynaptic receptors results in an inhibition of glutamate release from the terminals. Some of these NMDA receptors may be expressed in the preterminal axon and regulate the extent to which action potentials invade the extensive central arborizations of primary sensory neurons.

To study the postnatal development of nociceptive synaptic inputs in the superficial dorsal horn of the neonatal rat spinal cord, we examined the effect of capsaicin and menthol on glutamatergic mEPSCs in postnatal day (P) 0–1, P5–6 and P9–11 slices of spinal cord. Capsaicin (100 nm to 2 μm) increased the mEPSC frequency in a concentration‐dependent manner at all ages tested, with a significant enhancement of the effect between P5 and P10. This effect was sensitive to vanilloid receptor (VR) antagonists. The elevation in mEPSC frequency occurred at concentrations of capsaicin (100 nm) that did not alter the distribution of mEPSC amplitudes and was abolished by a dorsal rhizotomy, demonstrating that capsaicin acts via presynaptic VR1 receptors localized on primary afferents. Menthol significantly increased the mEPSC frequency with a similar developmental pattern to capsaicin without consistently affecting mEPSC amplitude. The increase in mEPSC frequency following capsaicin did not depend on transmembrane calcium influx since it persisted in zero [Ca2+]o. The facilitation of spontaneous glutamate release by capsaicin was sufficient to evoke action potentials in neonatal dorsal horn neurons but was accompanied by a block of EPSCs evoked by electrical stimulation of the dorsal root. These results indicate that VR1‐expressing nociceptive primary afferents form functional synaptic connections in the superficial dorsal horn from birth and that activation of the VR1 receptor increases spontaneous glutamate release via an undetermined mechanism. In addition, the data suggest that immature primary afferents express functional menthol receptors that are capable of modulating transmitter release. These results have important functional implications for infant pain processing.