The effect of intracerebro ventricular (ICV) injection of histamine and intraperitonal injection of histidine on the acute trigeminal pain in rats

The present study explored the interaction between histaminergic system. Orofacial pain was induced by subcutaneous (sc) injection of formalin (50 ?l, 1%) in the upper lip region and the time spent face rubbing was recorded in 3 min blocks for 45 min. Histamine and morphine suppressed both phases of pain. Naloxone alone non-significantly increased pain intensity and inhibited the antinociceptive effects of morphine and histamine. The results of the present study indicate that at the level of the hippocampus, histamine through its H1 and H2 receptors, mediates orofacial region pain.