Formalin-induced release of excitatory amino acids in the skin of the rat hindpaw

Application of glutamate to skin evokes pain-related behaviors [S.M. Carlton, G.L. Hargett, R.E. Coggeshall, Localization and activation of glutamate receptors in unmyelinated axons of rat glabrous skin, Neurosci. Lett., 197 (1995) 25–28; D.L. Jackson, C.B. Graff, J.D. Richardson, K.M. Hargreaves, Glutamate participates in the peripheral modulation of thermal hyperalgesia in rats, Eur. J. Pharmacol., 284 (1995) 321–325.] and peripherally-administered glutamate antagonists can prevent the nociception produced by inflammation [E.M. Davidson, R.E. Coggeshall, S.M. Carlton, Peripheral NMDA and non-NMDA glutamate receptors contribute to nociceptive behaviors in the rat formalin test, NeuroReport, 8 (1997) 941–946; Jackson et al., 1995.] In this study, the concentrations of glutamate and aspartate in the plantar of the rat hindpaws were measured before and after the subcutaneous administration of formalin. Increases in glutamate and aspartate concentrations were observed on the ipsilateral side, but not on the contralateral side, to the injection. This shows that nociception and inflammation caused by formalin injection induces the release of peripheral glutamate and aspartate, which would contribute to nociception and inflammatory pain.