Role of A delta afferent fibers in modulation of primary afferent input to the adult rat spinal cord.

ABSTRACT

To address the question of whether fine myelinated and unmyelinated primary afferent fibers contribute to the mechanism of presynaptic inhibition in the spinal cord, we studied dorsal root-evoked dorsal root potentials (DR-DRPs) using a newly developed longitudinal spinal cord slice preparations in the adult rat. Single stimuli applied to the L6 dorsal root elicited a DR-DRP in the L5 dorsal root which had an amplitude of 50-150 microV and had a half decay time of 20-66 ms. The DR-DRP was depressed by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10-20 microM), while DL-2-amino-5-phosphonovaleric acid (APV, 50-100 microM) had no significant effect. DR-DRP was markedly depressed by bicuculline or picrotoxin. The evoked DR-DRP was unchanged in rats treated with capsaicin which eliminated the majority of unmyelinated C afferent fibers. Taken together with the higher voltages (> or = 1.9 V) required to elicit DR-DRP, this observation strongly suggests that the A delta afferent fibers are primarily responsible for producing and receiving 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 described for larger myelinated fibers in vivo. This pathway may contribute importantly to synaptic modulation of somatosensory information, including nociception at the superficial dorsal horn through an interneuronal connection which are mediated by the non-NMDA and GABAA receptors.