PRODUCTION OF ENDOGENOUS NITRIC-OXIDE AND ACTIVATION OF SOLUBLE GUANYLATE-CYCLASE ARE REQUIRED FOR N-METHYL-D-ASPARTATE-PRODUCED FACILITATION OF THE NOCICEPTIVE TAIL-FLICK REFLEX

The intrathecal (i.t.) administration of either N-methyl-D-aspartate (NMDA, 10 pmol to 10 pmol) or L-arginine (1 pmol to 10 nmol), but not D-arginine (1 pmol to 10 nmol), produced a rapid, transient, dose-dependent facilitation (maximal response of 30.9 +/- 6.0% and 33.7 +/- 1.5%, respectively) of the nociceptive tail-flick reflex (ED50 = 47.8 +/- 15.4 fmol and 11.4 +/- 2.7 pmol, respectively). Maximal NMDA-produced facilitation of the tail-flick reflex (1 pmol i.t.) was completely abolished by prior treatment (10 min prior) with either N(omega)-nitro-L-arginine methyl ester (L-NAME, 10 nmol i.t.), methylene blue (10 nmol i.t.) or DL-5-aminophosphonovaleric acid (AP5, 100 pmol i.t.). NMDA-produced facilitation was completely recovered 40 min after L-NAME, 50 min after methylene blue and 30 min after AP5. L-NAME, methylene blue or AP5 did not significantly alter baseline tail-flick latency. These results suggest that NMDA-produced facilitation of a thermal nociceptive reflex is mediated through activation of an NMDA receptor that results in an increase in endogenous nitric oxide and activation of soluble guanylate cyclase in lumbar spinal cord.