Development of intrinsic electrophysiological properties in neurons from the gustatory region of rat nucleus of solitary tract.

There is no current understanding of the nature or time course of maturation of intrinsic electrophysiological properties for neurons in the gustatory region of the nucleus of the solitary tract (NST). Therefore, we used whole cell recordings in an in vitro slice preparation of the rat brainstem to characterize development of resting membrane, action potential and repetitive discharge properties of cells in gustatory NST at postnatal days 5, 10, 15, 20, and 30, and adult ages. Neurons were filled with Biocytin to verify location and characterize morphology. Membranes from younger neurons demonstrated a steeper current-voltage relation or higher input resistance, and a longer time constant than mature cells. Action potentials in younger cells had a slower rate of rise and were longer in duration. The afterhyperpolarization that typically follows the spike discharge usually had one phase in younger neurons, but was characterized by two or more phases in an increasing proportion of older cells. The repetitive discharge frequency in response to a range of depolarizing current pulses increased during development, and frequency/current plots were steeper in older compared with younger neurons. However, in all age groups there was clear accommodation of the discharge frequency. The greatest changes in resting membrane, action potential, and discharge properties were observed between P5 and P15, and mature values were generally reached by P20. At each postnatal age, neurons could be categorized in four neuron groups, based on the discharge pattern in response to a hyperpolarizing/depolarizing current protocol. Anatomical reconstructions indicated that although cells increased in overall dendritic expanse during development, neurons became less complex as illustrated by decreases in number of dendritic branch points, and in number and density of spines. The timing of major developmental differences in intrinsic electrical characteristics observed here is associated with a period of previously reported maturational changes in extracellular taste responses to number and concentration of chemical stimuli. However, further alterations in extracellular taste responses proceed after apparent maturation of intrinsic neural properties.