METABOTROPIC GLUTAMATE-RECEPTOR ACTIVATION CONTRIBUTES TO NOCICEPTIVEREFLEX ACTIVITY IN THE RAT SPINAL-CORD IN-VITRO

The contribution of metabotropic glutamate receptor activation to the spinal segmental reflex response evoked at high-intensity electrical s timulation, suggesting a role in nociception, has been examined in an in vitro preparation of neonatal rat spinal cord. Segmental reflex res ponses were recorded as a ventral root depolarization evoked following drug perfusion to the spinal cord or by electrical activation of high -threshold nociceptive afferent fibres. Superfusion of the selective m etabotropic glutamate receptor agonist, (1S,3R)-1-aminocyclopentane-1, 3-dicarboxylic acid [(IS,3R)-ACPD], to the spinal cord produced a dose -dependent, reversible ventral root depolarization (EC(50) = 58 +/- 7 mu M; n = 4), which was antagonized by the selective metabotropic glut amate receptor antagonist, (+)-alpha-methyl-4-carboxyphenylglycine (MC PG; IC50 = 243 +/- 61 mu M; n = 4). MCPG, over the same concentration range (10 mu M-5.0 mM) did not affect N-methyl-D-aspartate-induced ven tral root depolarizations. In contrast, the specific N-methyl-D-aspart ate receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (D-AP5) reduced N-methyl-D-aspartate-evoked ventral root depolarization but d id not affect the depolarization evoked by (1S,3R)-ACPD, thus indicati ng the specificity of the antagonists for these aggregate responses. M CPG significantly reduced the prolonged phase of the single shock C-fi bre-evoked ventral root depolarization (IC50 = 2.9 +/- 0.2 mM; n = 3-5 ). Low frequency high intensity stimulation of the dorsal root evoked a wind-up response, the amplitude of which was attenuated by both D-AP 5 and MCPG in a dose-dependent manner. The ventral root depolarization evoked by capsaicin application (1.0 mu M, 30 s) was blocked by both MCPG (IC50 = 809 +/- 35 mu M; n = 4) and D-AP5 (IC50 = 143 +/- 43 mu M ; n = 4). These data suggest that both D-AP5 and MCPG reduced C-fibre- induced ventral root responses. In addition to N-methyl-D-aspartate re ceptor, metabotropic glutamate receptor activation appears to be invol ved in the generation of the segmental spinal reflex evoked by high-in tensity stimulation in the neonatal rat spinal cord in vitro. Copyrigh t (C) 1996 IBRO.