Organization of excitatory and inhibitory local networks in the caudal nucleus of tractus solitarius of rats revealed in in vitro slice preparation.

Morphological and physiological properties of neurons in the caudal nucleus of tractus solitarius (NTS) of rats were studied in vitro by whole-cell recording and intracellular staining with biocytin. Synaptic responses following the solitary tract stimulation were also investigated to elucidate anatomical substrates of the underlying local circuits. Biocytin-filled NTS cells were divided into three groups according to the pattern of their axonal arborization: (1) local circuit neurons whose axon collaterals were extensively distributed within the NTS with the main axons leaving the NTS; (2) presumed interneurons whose axon collaterals seemed to be restricted within the NTS; and (3) projection neurons whose axons had few, if any, collaterals. Both local circuit neurons and presumed interneurons had small cell bodies (< 150 microns2 in somal area) and exhibited tonic regular spiking at depolarized membrane potentials. Polysynaptic excitatory background activity was increased and lasted for 300-1000 msec in these neurons following solitary tract stimulation. The projection neurons had medium to large cell bodies (> 150 microns2 in somal area). Inhibitory postsynaptic responses produced by an increased CI-conductance were recorded in these projection neurons. These findings suggest that excitatory local networks are organized by an assembly of the local circuit neurons in the caudal NTS, and that the interneurons are arranged to connect the excitatory local network with medium to large projection neurons via inhibitory synapses. Visceral afferent information is probably processed in the highly organized excitatory and inhibitory local networks within the caudal NTS and conveyed to other brain regions.