Vesicular Glutamate Transporter 3-expressing Nonserotonergic Projection Neurons Constitute a Subregion in the Rat Midbrain Raphe Nuclei

We previously reported that about 80% of vesicular glutamate transporter 3 (VGLUT3)-positive cells displayed immunoreactivity for serotonin, but the others were negative in the rat midbrain raphe nuclei, such as the dorsal (DR) and median raphe nuclei (MnR). In the present study, to investigate the precise distribution of VGLUT3-expressing nonserotonergic neurons in the DR and MnR, we performed double fluorescence in situ hybridization for VGLUT3 and tryptophan hydroxylase 2 (TPH2). According to the distribution of VGLUT3 and TPH2 mRNA signals, we divided the DR into six subregions. In the MnR and the rostral (DRr), ventral (DRV), and caudal (DRc) parts of the DR, VGLUT3 and TPH2 mRNA signals were frequently colocalized (about 80%). In the lateral wings (DRL) and core region of the dorsal part of the DR (DRDC), TPH2-producing neurons were predominantly distributed, and about 94% of TPH2-producing neurons were negative for VGLUT3 mRNA. Notably, in the shell region of the dorsal part of the DR (DRDSh), VGLUT3 mRNA signals were abundantly detected, and about 75% of VGLUT3-expressing neurons were negative for TPH2 mRNA. We then examined the projection of VGLUT3-expressing nonserotonergic neurons in the DRDSh by anterograde and retrograde labeling after chemical depletion of serotonergic neurons. The projection was observed in various brain regions such as the ventral tegmental area, substantia nigra pars compacta, hypothalamic nuclei, and preoptic area. These results suggest that VGLUT3-expressing nonserotonergic neurons in the midbrain raphe nuclei are preferentially distributed in the DRDSh and modulate many brain regions with the neurotransmitter glutamate via ascending axons.