Identification of periaqueductal gray and dorsal raphe nucleus neurons projecting to both the trigeminal sensory complex and forebrain structures: a fluorescent retrograde double-labeling study in the rat.

The midbrain periaqueductal gray (PAG) including the dorsal raphe nucleus (DR) has been known to contain serotoninergic neurons projecting to many brain regions. Employing fluorescent retrograde double labeling combined with immunofluorescence histochemistry for serotonin (5-HT), we examined in the rat whether or not single PAG/DR neurons with 5-HT send their axons to both the trigeminal sensory complex and forebrain regions. Stereotaxic injections of Diamidino Yellow (DY) and Fast Blue (FB) were performed unilaterally; DY was injected into the caudal spinal trigeminal nucleus or principal sensory trigeminal nucleus, and FB into the ventrolateral orbital cortex, nucleus accumbens or amygdala. A small percentage of PAG/DR neurons were doubly labeled with DY and FB, and the majority of them showed 5-HT-like immunoreactivity (5-HT-LI). Most of these 5-HT-LI PAG/DR neurons that were indicated to send their axons simultaneously to both the trigeminal sensory complex and forebrain regions were distributed in the ventrolateral PAG subdivision and ventral aspects of the medial PAG subdivision at the middle and caudal PAG levels, bilaterally with a predominant distribution on the side ipsilateral to the injections. This indicates a possible role of these PAG/DR neurons in the limbic or affective-motivational aspect of the pain-related neural system.