Central NO is involved in the antinociceptive action of intracisternal antidepressants in freely moving rats

The present study was performed to examine the central effects of antidepressants on nociceptive jaw opening reflex after intracisternal injection. We also investigated the mechanisms of central antinociceptive action of intracisternal antidepressants. We recorded the jaw opening reflex in freely moving rats and chose to administer antidepressants intracisternally in order to eliminate the effects of anesthetic agents on the pain assessment and evaluate the importance of the spinal site of action of antidepressants. After intracisternal injection of 15 μg imipramine, digastric electromyogram (dEMG) was decreased to 76 ± 6% of the control. Intracisternal administration of 30 μg desipramine, nortriptyline or imipramine suppressed dEMG remarkably to 48 ± 2, 27 ± 8, or 25 ± 5% of the control, respectively. The suppression of dEMG was maintained for 50 min. L- N(G)-Nitroarginine methyl ester (NAME) blocked the suppression of dEMG from 32 ± 2 to 81 ± 5% of the control. These results indicate that antidepressants produce antinociception through central mechanisms in the orofacial area. The central NO pathway seems to be involved in the antinociception of intracisternal antidepressants at supraspinal sites.