Projections from dorsomedial part of the subnucleus oralis to the mesencephalic trigeminal neurons innervating the masseter muscle--a PHA-L and HRP double labeling study in the rat.

When PHA-L was injected to dorsomedial part of the subnucleus oralis of the spinal trigeminal nucleus (Vodm), anterogradely labeled varicosities and boutons were observed in the mesencephalic trigeminal nucleus (Vme). The PHA-L labeling was seen mainly in the Vine ipsilateral to the injection side. If the injection sites were placed more medially containing both Vodm and parvocellular reticular nucleus (PCRt), more PHA-L labeling in contralateral Vme was observed. It appeared that the Vodm neurons projected predominantly to the ipsilateral Vme, while neurons in the PCRt adjacent to the Vodm projected bilaterally to the Vme. It was also seen that many PHA-L labeled varicosities and boutons surrounded or contacted on the Vme neurons counterstained with cresyl violet. These contacts were seen frequently in the ipsilateral Vme, occasionally in the contralateral nucleus. Some Vme neurons ipsilateral to the injection side were retrogradely labeled in a solidly-filled appearance following injection of PHA-L into the Vodm. When PHA-L was applied to the Vodm and HRP was injected to the masseter nerve on the same side, the relationship between PHA-L labeled terminals and HRP labeled Vme neurons was studied. The contacts between PHA-L labeled boutons and HRP labeled somata or processes of the Vme neurons were observed occasionally. PHA-L labeled terminals sometimes contacted on none-HRP-labeled somata counterstained with neutral red. Besides the PHA-L labeling in the Vme, the labeling was also seen in the medial parabrachial nucleus ipsilaterally, in the motor trigeminal nucleus bilaterally with ipsilateral predominance. In addition, an outstanding labeling band along the medial border of the principal sensory trigeminal nucleus (Vp) was observed following injection of PHA-L into the Vodm. The labeled terminals were concentrated in dorsomedial part of the Vp (Vpdm) and in juxtatrigeminal nucleus (JX), meanwhile many labeled fibres were distributed between the Vpdm and JX in the intertrigeminal region.