Coexpression of vesicular glutamate transporter-3 and γ-aminobutyric acidergic markers in rat rostral medullary raphe and intermediolateral cell column

Markers of serotonergic, gamma-aminobutyric acid (GABA)-ergic (glutamic acid decarboxylase, 67 kDa isoform; GAD-67), and glutamatergic transmission (vesicular glutamate transporter 3; VGLUT3) have been detected in presumed sympathetic premotor neurons of the medullary raphe, a region that controls sympathetic tone to brown fat, skin blood vessels, and heart. In this study, the degree of coexpression of these markers was examined in raphe neurons by simultaneous histological detection of tryptophan hydroxylase (TrpOH) immunoreactivity with GAD-67 mRNA and VGLUT3 mRNA. Over half (52%) of the VGLUT3 mRNA-positive neurons expressed one or both of the other markers. The proportion of VGLUT3 neurons containing at least one of the other two markers was even higher (89%) for VGLUT3 spinally projecting neurons. VGLUT3 neurons containing markers for both serotonin and GABA were especially numerous (50-72%, depending on rostrocaudal level) within the marginal layer of raphe pallidus and the parapyramidal region. The dual GABAergic and glutamatergic nature of some bulbospinal raphe neurons was suggested by the presence of nerve terminals immunoreactive (ir) for both VGLUT3 and GABA in the intermediolateral cell column (IML) as detected by electron microscopy. VGLUT3-ir terminals formed approximately equal numbers of symmetric and asymmetric synapses onto presumed preganglionic neurons (nitric oxide synthase-ir profiles) or GABA-ir dendrites in IML, and terminals immunoreactive for both VGLUT3 and GABA always formed symmetric synapses. These data suggest that medullary raphe VGLUT3 neurons could inhibit sympathetic outflow and that their spinal targets include both preganglionic neurons and GABAergic interneurons.