An indirect projection from the nucleus of the solitary tract to the central nucleus of the amygdala via the parabrachial nucleus in the rat: a light and electron microscopic study.

The morphological basis of how visceral information from the nucleus of the solitary tract (NTS) is relayed from the parabrachial nucleus (PBN) to the central nucleus of the amygdala (Ce) was studied at the light and electron microscopic levels using the anterograde tracer, Phaseolus vulgaris leucoagglutinin (PHA-L), kainic acid degeneration, and retrograde tracing with horseradish peroxidase (HRP). After injection of PHA-L into the caudal NTS, anterogradely labeled fibers and terminals were predominantly distributed in the external lateral (el) and central lateral (cl) subnuclei of the PBN. After injection of HRP into the Ce, retrogradely labeled neurons in PBN were mainly distributed in the same areas. In double-labeling experiments, there was a clear overlap between neuronal elements labeled with HRP and PHA-L in the el and cl. At the electron microscopic level, the PHA-L-labeled axon terminals from the NTS mainly contained spherical agranular synaptic vesicles and formed asymmetric contacts with the postsynaptic dendrites or dendritic spines in PBN. After the lesioning agent kainic acid was injected into the NTS and HRP deposited in the Ce, it was found the afferent fibers from the NTS made direct synaptic contact with the lateral PBN neurons which in turn projected to Ce. Such evidence adds to our growing knowledge of regulation of visceral function in central nervous system and would be likely helpful for understanding the important roles of the NTS, PBN and Ce in the central control of cardiovascular, respiratory and gastrointestinal functions.