Evidence for a Persistent Na⁺ Conductance In Neurons of the Gastric Mill Rhythm Generator of Spiny Lobsters

Evidence for a persistent Na⁺ conductance was obtained in identified motor neurons of the gastric mill network in the stomatogastric ganglion of the spiny lobster Panulirus interruptus. The cells studied were the lateral gastric and lateral posterior gastric motor neurons, which in vivo control chewing movements of the lateral teeth of the gastric mill. We examined basic cellular properties in the quiescent network of the isolated stomatogastric ganglion. In current-clamp recordings, we found two types of evidence for a persistent Na⁺ conductance. First, tetrodotoxin-sensitive inward rectification occurred during depolarization from rest to spike threshold. Second, 5 mmol l⁻¹ tetraethylammonium (a K⁺ channel blocker) induced plateau potentials that persisted in the presence of Mn²⁺ or a low [Ca²⁺]_o but were blocked by a low [Na⁺]_o or 100 nmol l⁻¹ tetrodotoxin. The plateau potentials could drive trains of fast spikes in the motor axon and strong transmitter release at central output synapses within the ganglion.

This conductance probably corresponds to the persistent Na⁺ current, I_NaP, described in cultured stomatogastric neurons and in neurons from several other preparations. During normal neuronal activity, it may contribute to the prolonged plateau depolarizations and long spike trains typical of motor neuronal activity during gastric rhythm generation. Persistent inward currents of this type are likely to be important in neurons that must fire prolonged bursts in cycle after cycle of rhythmical activity.