In vitro characterization of neurons in the ventral part of the nucleus tractus solitarius. I. Identification of neuronal types and repetitive firing properties.

1. An in vitro brain stem slice preparation from adult guinea pigs was used to determine the properties of neurons located in the ventral part of the nucleus tractus solitarius (NTS), an area associated with the dorsal respiratory group. Based upon their morphology and their repetitive firing properties, three classes of ventral NTS neurons, termed types I, II, and III, were observed. 2. Type I neurons were multipolar with pyramidal-shaped cell bodies. These neurons responded to prolonged depolarizations from a resting level of -50 mV with a discrete, high-frequency burst of spikes, which rapidly adapted to a low steady-state level. When depolarized from levels more negative than -65 mV, the initial burst was diminished. 3. Type II neurons were multipolar with fusiform-shaped cell bodies. Type II neurons responded to depolarizations from -50 mV with an initial high spike frequency, which gradually adapted to a steady-state level. When depolarized from levels more negative than -60 mV, these neurons displayed a delay between the onset of the stimulus and the first spike. This delay has been termed "delayed excitation." The expression of delayed excitation was modulated by both the size and duration of hyperpolarizing prepulses that preceded depolarization. 4. Type III neurons were multipolar with spherical shaped-cell bodies. In response to depolarizations from -50 mV, these neurons displayed high-frequency firing with little adaptation. The repetitive firing properties of type III neurons were not modulated by hyperpolarization. 5. Bulbospinal neurons in the ventral NTS were identified using retrograde transport of rhodamine-labeled latex beads injected into the region of the phrenic motor nucleus at spinal cord levels C4 through C6. Only type I and type II neurons were labeled in the ventral NTS (0.2-1.0 mm rostral to the obex). Both contralateral and ipsilateral projections were observed. Contralaterally, type I and II neurons were evenly distributed. Ipsilaterally, however, type II neurons accounted for two-thirds of the labeled neurons. 6. Type I and II neurons had similar input resistances and time constants: 97.0 +/- 17.6 M omega and 14.4 +/- 2.2 ms (n = 5) for type I and 107.0 +/- 11.2 M omega and 13.7 +/- 1.6 ms for type II (n = 5).(ABSTRACT TRUNCATED AT 400 WORDS)