Glutamate-induced excitation and sensitization of nociceptors in rat glabrous skin

Anatomical studies demonstrate the presence of glutamate receptors on unmyelinated axons in peripheral cutaneous nerves. Pharmacological studies show that intraplantar injection of glutamate or glutamate agonists in the glabrous skin results in nociceptive behaviors. The present study describes a novel in vitro skin-nerve preparation using the glabrous skin from the rat hindpaw. In the first series of experiments, recordings were obtained from 141 fibers that responded to a strong mechanical search stimulus. Based on their conduction velocity they were classified as C (27%), Aδ (28%) and Aβ (45%) fibers. The C and Aδ fibers typically exhibited sustained firing during suprathreshold mechanical stimuli whereas both rapidly (66%) and slowly (34%) adapting responses were obtained from Aβ fibers. Noxious heat excited 46% of the C fibers but only 12% of the Aδ units. In another series of experiments application of an ascending series of glutamate concentrations (10, 100, 300, and 1000 μM) to Aδ (n = 14) and C (n = 19) nociceptors resulted in a significant excitation of 43% (6/14) Aδ fibers and 68% (13/19) C fibers. At these concentrations, there was no excitation of Aβ units (n = 13). Superfusion of the receptive fields of either mechanoheat-sensitive A (AMH, n = 10) or C fibers (CMH, n = 12) for 2 min with 300 μM glutamate resulted in sensitization of 90% (9/10) AMH and 92% (11/12) CMH fibers to subsequent thermal stimulation. This was evidenced by a significant (1) decrease in thermal threshold for activation, (2) increase in discharge rate, and (3) increase in peak instantaneous frequencies during the second heat trial. Glutamate-induced sensitization to heat occurred in the absence of either a glutamate-induced excitation or an initial heat response. Exposure of Aδ or C fibers to glutamate did not result in a decrease in von Frey thresholds. These data provide a physiological basis for the nociceptive behaviors that arise following intraplantar injection of glutamate or glutamate agonists. Furthermore, demonstration of glutamate-induced excitation and heat sensitization of nociceptors indicates that local or topical administration of glutamate receptor antagonists may have therapeutic potential for the treatment of pain.