Organization of synaptic transmission in the mammalian solitary complex, studied in vitro.

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Abstract
1. Synaptic transmission and neuronal morphology were studied in the nucleus tractus solitarius and in the dorsal vagal motor nucleus (solitary complex), in coronal brain-stem slices of rat or cat, superfused in vitro. 2. Electrical stimulation of afferent fibres of the solitary tract evoked two different types of post-synaptic response recorded intracellularly in different solitary complex neurones. Labelling with horseradish peroxidase showed that these two sorts of orthodromically evoked responses were correlated with different post-synaptic neuronal morphologies. 3. The majority of recorded neurones (n = 93) showed a prolonged reduction in excitability following the intial solitary-tract-evoked excitatory post-synaptic potential (e.p.s.p.). A smaller number of neurones (n = 53) showed a prolonged increase in excitability following solitary tract stimulation. In no case did the solitary tract stimulation induce a burst of action potentials at high frequency. 4. the time-to-peak and the half-width of the initial solitary-tract-evoked e.p.s.p. were shorter in neurones with prolonged increased excitability than in those with prolonged reduced excitability. In neurones with prolonged reduced excitabilty, this e.p.s.p. was followed by a hyperpolarization lasting 60-100 ms. The latency of this inhibitory post-synaptic potential (i.p.s.p.) was 3-5 ms longer than that of the initial e.p.s.p. and its reversal potential was 10 mV more negative than the reversal potential of the response measured following application of .gamma.-aminobutyric acid or glycine. In neurones with prolonged increased excitability, at a membrane potential of -40 to -50 mV, the initial solitary tract e.p.s.p. was followed by a prolonged depolarization lasting 100-400 ms. 5. Background synaptic activity was high in neurones with prolonged increased excitability, consisting of unitary e.p.s.p.s with an amplitude of more than 0.8 mV. This activity was increased, for a period of 300-800 ms following solitary tract stimulation. Spontaneous excitatory potentials of more than 0.5 mV were not seen in neurones with prolonged reduced excitability. In these neurones, after intracellular injection of choride ions, reversed unitary i.p.s.p.s formed a background activity which was increased following stimulation of the solitary tract. 6. Neurones with prolonged reduced excitability were found in the medial, ventral and ventrolateral part of the nucleus tractus solitarius and in the dorsal vagal motor nucleus where they were identified by their antidromic response to stimulation ventral and lateral to the tractus solitarius. In contrast, neurones with prolonged increased excitability were encountered frequently in the dorsomedial and commissural regions of the nucleus tractus solitarius. 7. Neurones with prolonged reduced excitability were of medium size with a dendritic arborization extending 200-300 .mu.m from the soma. Neurones with prolonged increased excitability were small with a dendritic arborization restricted to a region not nore than 100 .mu.m for the cell body. 8. The existence of two different types of neurones in the solitary complex is discussed. It was found that the solitary tract nucleus in vitro contains the neuronal circuitry necessary for the generation of complex activity lasting several hundred milliseconds. It is proposed that neurones with prolonged reduced excitability integrate several different synaptic inputs. Neurones with prolonged increased excitability may be local interneurones which form the network responsible for the maintenance of long-lasting activity.