Biomedical Research 2006; 17 (2):

The dorsal raphe nucleus (DR), contains 60% serotonergic neurons, which appears to be involved in the control of endogenous pain sensation. MK-801, a non-competitive antagonist of N-methyl-D-aspartate receptors, inhibits spinal nociceptive neurons, however, little information is available about the effect of its administration on the DR. To evaluate the distribution of c-Fos and serotonin expression in the DR following experimental tooth movement in the rat upper molar and/or administration of MK-801. Fifty-six Wistar strain male rats underwent experimental tooth movement according to the Waldo’s method. Of all the rats, 28 were administrated MK-801 (3 mg/kg) intraperitoneally before initiating the experimental tooth movement (n=14). These animals were sacrificed 2 and 12 h after the experimental tooth movement. The sections including DR were dissected, and subjected to immunohistochemical staining with c-Fos and serotonin antibodies. In addition, 28 anesthesia control animals with or without administration of MK-801 were used as the controls. In the DR, the numbers of single-labeled (c-Fos) and double-labeled (c-Fos/serotonin) neurons were increased at 2 h and decreased by 12 h as compared with the controls. MK-801 decreased the numbers of double-labeled neurons at 2 h to the control level. The present results suggest that although blockage of NMDA receptors decreases neuronal activity with in the DR during experimental tooth movement, serotonergic DR neurons are not involved in the MK801-induced analgesia.