Electrotonic coupling between rat sympathetic preganglionic neurones in vitro.

1. Using the whole-cell recording technique in rat spinal cord slices we have shown that 26% of sympathetic preganglionic neurones (SPNs) show spontaneous membrane potential oscillations. These oscillations consist of trains of biphasic waves, which we have termed spikelets because of their similarity to truncated action potentials. 2. The spikelets were inhibited by TTX and anaesthetics such as alpha-chloralose but not by the intracellular application of lidocaine N-ethyl bromide (QX-314). 3. By stimulating the ventral roots we have demonstrated the presence of short-latency depolarizations (SLDs) in oscillating neurones. These SLDs have a similar waveform to the spontaneous spikelets, and also show the ability to override the frequency of occurrence of the spontaneous spikelets. These observations suggest that the spikelets result from electrotonic coupling between the oscillating SPNs. 4. SLDs were also observed in a population of non-oscillating, electrotonically coupled, quiescent SPNs. It was possible to induce oscillations in these neurones by the injection of depolarizing current (in the presence of QX-314), suggesting that these neurones are also gap-junction coupled. 5. Simultaneous whole-cell recordings were obtained from twenty-three pairs of SPNs. Two pairs displayed both spontaneous, synchronized oscillations and action potentials. Electrotonic coupling was confirmed by the detection of membrane polarization in both neurones in response to current injected into one neurone. In a further two pairs of quiescent SPNs, injection of depolarizing current pulses into one neurone induced action potential discharge in that neurone and a depolarization and oscillations in the other neurone. 6. The ability of groups of electrotonically coupled SPNs to generate spontaneous discharges within the spinal cord provides a novel mechanism for the integration and synchronization of information within the sympathetic nervous system.