SEPARATION OF NEURON TYPES IN THE GUSTATORY ZONE OF THE NUCLEUS-TRACTUS-SOLITARII ON THE BASIS OF INTRINSIC FIRING PROPERTIES

1. Whole-cell current-clamp recordings were made from neurons in the rostral nucleus tractus solitarii (NTS) in an in vitro brain slice preparation in rats. On the basis of previous investigations, these neurons are believed to be involved with processing of gustatory as well as somatosensory information. 2. Rostral NTS neurons had a mean resting membrane potential of -47 mV. The mean input resistance was 336 M-OMEGA, and by fitting a double exponential function the membrane time constant had fast (2.3 ms) and slow (20.6 ms) components. 3. Neurons were separated into four different groups on the basis of their responses to a current injection pulse paradigm consisting of membrane hyperpolarization of different magnitudes and durations immediately followed by a long (1,200 ms) depolarizing pulse. The regular repetitive discharge pattern of the first group of neurons (Group I neurons) was changed into an irregular spike train by membrane hyperpolarization. Hyperpolarization of Group II neurons either delayed the occurrence of the first action potential or increased the length of the first interspike interval in the action-potential train produced by membrane depolarization. The length of the delay was related both to the magnitude and duration of the hyperpolarizing prepulse. Hyperpolarization had the least effect on the discharge pattern of Group III neurons. The discharge pattern of Group IV neurons consisted of a short burst of action potentials that was often shortened by prior hyperpolarization of the neuron. 4. Differences exist in other intrinsic properties of the four neuron groups. Group I and III neurons were capable of initiating the highest frequency of action potentials to a 100-pA 1,200-ms depolarizing pulse. In response to a short depolarizing pulse, Group II neurons had the longest latency to the first spike and responded with the fewest action potentials. Group IV neurons tended to have higher input resistance and membrane time constants than the other neuron groups. A subset of neurons in each neuron group showed membrane afterhyperpolarizations (AHP) after depolarization-induced action-potential trains (postburst AHP). Postburst AHP amplitudes ranged from 1.0 to 12.9 mV and were of greatest magnitude in Group II neurons. Postburst AHP durations ranged from 75 to 3,538 ms and were of longest duration in neurons belonging to Group III. Group II neurons, which had the largest postburst AHP magnitude. had the shortest postburst AHP duration. 5. These results demonstrate that neurons in the rostral NTS can be separated on the basis of their intrinsic membrane properties. These intrinsic properties probably reflect the different roles these neurons play in processing afferent gustatory and somatosensory information in the NTS. Regulation of neuronal responses based on hyperpolarization and depolarization of the membrane can Presumably transform the afferent message during neural processing by the NTS.