Two distinct phases characterize maturation of neurons in the nucleus of the tractus solitarius during early development: morphological and electrophysiological evidence.

We have used electrophysiology and light microscopy of intracellularly labeled neurons in the nucleus of the tractus solitarius (nTS) in brainstem slices of the newborn rat (P0 to P6) to examine the functional and morphological correlation of their development. Three-dimensional reconstruction of neurons injected intracellularly with biocytin, following electrophysiological recording, revealed a close correspondence between morphological immaturity (appearing as polarization of the dendritic tree) and the absence of a ramp-like voltage trajectory at the offset of hyperpolarizing current injections-IA negativity (8 of the 8 cells examined showed this correlation). These morphologically polarized IA negative neurons showed preferential dendritic sprouting in two diametrically opposite poles of the perikaryon. The orientation of the polarity differed according to the rostrocaudal location of the neuron. The appearance of a polarized dendritic tree during the first (immature) phase was transient and closely coincident with IA negativity. Following the development of adult-like electrophysiological characteristics, i.e., IA positivity, nucleus of the tractus solitarius neurons showed remarkably different morphological features (9 of 10 cells). These included a wide-spread branching of the dendritic tree in all directions, giving it a bushy appearance (cell body to dendrite ratio of 1:40). Numerous dendritic spines, growth cones on both dendrites and axons, and axon collateralization were present during both phases and indicate that nTS neurons during the two phases of early development demonstrate dynamic features of growth and maturation. The development of adult-like electrophysiological characteristics, i.e., IA positivity, progressively increased in the postnatal period. During the later part of the first postnatal week, twice as many neurons showed IA positivity in days P3 to P6 as compared with days P0 to P2. These results reveal the dynamic nature of neurons in the nTS during early development and illustrate the close link between morphology and functional characteristics in this region. We suggest that the establishment of adult-like morphology can be modified by appropriate environmental clues provided to nTS neurons during the initial (immature) phase of early postnatal development.