Intracellular study of nucleus parabrachialis and nucleus tractus solitarii interconnections

Responses of the nucleus parabrachialis (PBN) neurons to electrical stimulation of the nucleus tractus solitarii (NTS) were investigated by intracellular recording technique in anesthetized rats. Excitatory postsynaptic potentials (EPSPs) were evoked by ipsilateral NTS stimulation in 38 PBN neurons. They were considered monosynaptic because their latencies did not change with either variations in stimulus intensity or high-frequency repetitive stimulation. The latencies of EPSPs ranged from 1.2 to 6.9 ms. PBN neurons were also antidromically activated by NTS stimulation, giving a mean axonal conduction velocity of 4.6 m/s. Some of these neurons also responded with monosynaptic EPSPs to NTS stimulation. Direct stimulation of these neurons by depolarizing current pulses elicited repetitive firing with frequencies up to 350 Hz. The morphological analysis of 5 PBN neurons labeled with horseradish peroxidase (HRP) indicates that the soma were fusiform in shape, and the size varied from 163 to 783 microns 2. All neurons had 3-5 spiny primary dendrites which extended in a predominantly mediolateral direction. Axons arose from a proximal dendritic trunk, close to the soma. The results indicated that PBN is reciprocally connected with the NTS which elicits an excitatory effect on PBN neurons.