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  • Title: Effects of morphine on the distribution of Fos protein in the trigeminal subnucleus caudalis neurons during experimental tooth movement of the rat molar.
    Author: Aihara Y, Maeda T, Hanada K, Wakisaka S.
    Journal: Brain Res; 1999 Feb 20; 819(1-2):48-57. PubMed ID: 10082860.
    Abstract:
    The present study was undertaken to disclose temporal changes in the distribution of Fos-like immunoreactive (-IR) neurons in the trigeminal subnucleus caudalis (SpVc), one of the important relay nuclei for processing the nociceptive information from the oro-facial regions, following induction of experimental tooth movement in rat upper molars. Furthermore, the effect of morphine and naloxone on the levels of Fos-IR neurons in the SpVc was examined. The experimental tooth movement was induced by insertion of an elastic rubber between the first and second upper molars. In normal animals, Fos-IR neurons were rarely observed in the SpVc. Immediately after insertion of the elastic band, the distribution of Fos-IR neurons was comparable to that observed in normal animals. The number of Fos-IR neurons increased significantly from 1 to 4 h following the induction of experimental tooth movement, reaching a maximum at 2 h, and then decreasing gradually. Most of the neurons were localized in the dorsomedial portion of the superficial layers of the ipsilateral SpVc near the obex, but a few were observed at the ventral portion of the SpVc. The neurons at the superficial layers and ventral portion of the contralateral SpVc also showed Fos-like immunoreactivity, but their numbers were significantly smaller than those on the ipsilateral side. Pretreatment with morphine (3 and 10 mg/kg, i.p.) significantly reduced the induction of Fos-IR neurons at the superficial layers of the ipsilateral SpVc in a dose-dependent manner, and its effect was antagonized by the subsequent treatment of naloxone (2 mg/kg, i.p.). Naloxone pretreatment enhanced the expression of Fos-IR neurons on the ipsilateral SpVc. The present results of a reduction of Fos-IR neurons by morphine pretreatment suggest that the induction of Fos-IR neurons may be due to the noxious stimulation caused by induction of experimental tooth movement.
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