Nonsteroidal antiinflammatory drug modulation of behavioral responses to intrathecal N-methyl-D-aspartate, but not to substance P and amino-methyl-isoxazole-propionic acid in the rat.

Antinociception by nonsteroidal antiinflammatory drugs, notably diclofenac and S(+)-ibuprofen, has traditionally been attributed to peripheral tissue cyclooxygenase inhibition. This study investigates the potential role of the nitric oxide system for the central antinociceptive effects of diclofenac, S(+)-, and R(-)-ibuprofen. Diclofenac and S(+)- but not R(-)-ibuprofen inhibited the behavioral response dose dependently, "biting, scratching, and licking (BSL)," induced by the spinal application of N-methyl-D-aspartate, but not that of amino-methylisoxazole-propionic acid or substance P. Diclofenac and S(+)-ibuprofen induced a parallel shift in the number of BSL responses and in the duration of the response in the behavioral model at their approximate median effective doses (diclofenac 1 mumol and S(+)-ibuprofen 5 mumol). Pretreatment with L-arginine, the natural substrate for the nitric oxide synthetase, antagonized diclofenac, and S(+)-ibuprofen-induced suppression of the biting, scratching, and licking response evoked by intrathecal N-methyl-D-aspartate. D-arginine did not antagonize the diclofenac- and S(+)-ibuprofen-induced antinociception. The study results indicate that analgesia after diclofenac and S(+)-ibuprofen involves a central mechanism which may add to the peripheral antinociceptive effect of these agents. The central action of diclofenac and S(+)-ibuprofen is partly mediated by an interaction with the N-methyl-D-aspartate receptor and nitric oxide-generating mechanisms.

Acetaminophen blocks spinal hyperalgesia induced by NMDA and substance P.

The hypothesis tested was that inhibition of the L-arginine-nitric oxide (NO) pathway may represent a potential central mechanism of action for acetaminophen (paracetamol). Spinal administration of N-methyl-D-aspartate (NMDA, 0.5 nmol), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA, 0.1 nmol) or substance P (SP, 0.5 nmol) to the rat provoked a specific behaviour characterized by biting, scratching and licking (BSL). This behaviour was antagonized by pretreatment with acetaminophen for NMDA and SP but not for AMPA. Further, the antinociceptive effect of acetaminophen was readily reversed by administration of the natural substrate for nitric oxide synthase (NOS), L-arginine, but not by D-arginine. This suggests that the analgesic effect of acetaminophen is related to inhibition of NO generation. Potential mechanisms for this may involve NMDA and SP. Our data suggest that a significant portion of the analgesic effect of acetaminophen, when used clinically, may be related to an interaction with the central nervous system L-arginine-NO pathway.

Localization of the central antinociceptive effects of diclofenac in the rat.

The ethacrynic acid-induced writhing response (WR) in the rat was studied after microinjections of diclofenac 0.1 ng-1 microgram/0.5 microliter (0.32 pmol to 3.2 nmol) into several brain regions involved in control of nociceptive behavior. The WR was inhibited after injections into the periaqueductal grey matter (PAG), ventromedial thalamus (VM), medial preoptic area (MPA) and the nucleus raphe magnus (NRM). Morphine 50 ng/0.5 microliter (0.16 nmol) was used as a positive control and vehicle injections were performed as negative reference. After diclofenac, there was a dose-dependent reduction of the WR with a threshold dose of approximately 1-10 ng in all brain areas studied except the nucleus reticularis paragigantocellularis interna (NRPGi). Naloxone 50 ng/0.5 microliter (0.15 nmol) administered into the same site 30 min after diclofenac injection, antagonized the diclofenac-induced inhibition of the WR almost completely in PAG and VM. Previous results demonstrate a central, naloxone-reversible component in the analgetic action of diclofenac. A qualitatively similar, centrally induced inhibition of the WR may be elicited after injections into PAG, VM and NRM. Thus, in addition to its peripheral mechanism of action, the non-steroidal anti-inflammatory drug, diclofenac, has a central mechanism of action which directly or indirectly involves a central opioid component.