Nitric oxide-induced increase of excitatory amino acid levels in the trigeminal nucleus caudalis of the rat with tactile hypersensitivity evoked by the loose-ligation of the inferior alveolar nerves

To investigate whether or not N-methyl-D-aspartate (NMDA)/nitric oxide (NO) pathway in the trigeminal system is involved in the development and/or maintenance of such pathological pain states as the hyperalgesia and allodynia observed after dental surgery, we examined the alteration patterns of excitatory amino acid (EAA) level in the superficial layer of subnucleus caudalis of the brain-stem trigeminal sensory nuclear complex (SpVc-I,II) by in vivo microdialysis. A very high EAA release response was observed immediately after the start of the perfusion in ligated animals compared with sham-operated rats. The EAA level evoked by application of the 40-V tooth pulp-stimulation or 1% capsaicin cream was significantly higher in the ligated animals than those in the sham-operated animals. This increase of EAA level induced by capsaicin cream was inhibited by adding carboxy-PTIO (100 microM) to the perfusate. The applications of SNAP (2 mM) into the perfusate enhanced the level of EAAs in ligated animals and sham-operated animals. However, SNAP-evoked EAA levels in ligated animals were not significantly different compared with those of sham-operated animals. These results suggest that alterations in the stimulus-evoked raised EAA levels that occur in the site of the first synaptic relay of the dental pain pathway and which are expressed via endogenous NO, and which play an important role in development and/or maintenance of pathological pain states following dental peripheral nerve injury.