ROLE OF A-DELTA AFFERENT-FIBERS IN MODULATION OF PRIMARY AFFERENT INPUT TO THE ADULT-RAT SPINAL-CORD

To address the question of whether fine myelinated and unmyelinated pr imary afferent fibers contribute to the mechanism of presynaptic inhib ition in the spinal cord, we studied dorsal root-evoked dorsal root po tentials (DR-DRPs) using a newly developed longitudinal spinal cord sl ice preparations in the adult rat. single stimuli applied to the L6 do rsal root elicited a DR-DRP in the L5 dorsal root which had an amplitu de of 50-150 mu V and had a half decay time of 20-66 ms. The DR-DRP wa s depressed by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10-20 mu M) , while DL-2-amino-5-phosphonovaleric acid (APV, 50-100 mu M) had no s ignificant effect. DR-DRP was markedly depressed by bicuculline or pic rotoxin. The evoked DR-DRP was unchanged in rats treated with capsaici n which eliminated the majority of unmyelinated C afferent fibers. Tak en together with the higher voltages (greater than or equal to 1.9 V) required to elicit DR-DRP, this observation strongly suggests that the A delta afferent fibers are primarily responsible for producing and r eceiving the DR-DRP. The present study shows that the DR-DRP mediated by the A delta fibers in the slice preparation is analogous to those d escribed for larger myelinated fibers in vivo. This pathway may contri bute importantly to synaptic modulation of somatosensory information, including nociception at the superficial dorsal horn through an intern euronal connection which are mediated by the non-NMDA and GABA(A) rece ptors.