Morphology and projections of neurobiotin-labeled nucleus tractus solitarii neurons recorded in vitro.

The suitability of the anterograde tracer neurobiotin to provide information about the morphology and projections of extracellularly or intracellularly recorded medial nucleus tractus solitarii (nTS) neurons was evaluated in horizontally oriented rat dorsal medulla in vitro slices. After responsiveness to angiotensin (Ang) II, substance P (SP), and L-glutamate was evaluated, neurons were labeled by electrophoresis of neurobiotin at the recording site. Extracellular application (2 microA for 2 min) produced discrete injection sites (40-70 microns) with a small group of labeled neurons. Ejections into the solitary tract documented that the tracer was not taken up by axons traversing the injection site. Neuronal perikarya, primary and secondary dendrites, and axons exhibited a dense Golgi-like appearance, with well-defined dendritic spines and axonal varicosities. Dendritic or axonal processes could be followed for more than 1 mm from the cell soma in a 50 microns thick section, documenting the horizontal architecture of the medial nTS. Intracellular electrophoresis filled the soma, primary and secondary dendrites, and axons of neurons characterized for responsiveness to peptides, L-glutamate and solitary tract stimulation. The location within the nTS and axonal projections of neurons responsive to Ang II and SP appeared to differ from those of cells responsive to Ang II and L-glutamate. Thus, either extracellular or intracellular application of neurobiotin in the in vitro slice can reveal differences in axonal or dendritic targets of neuronal subgroups responsive to different neurotransmitters or peptides and provide evidence for the likely autonomic significance of the neurons.