Isolation and characterization of slow, depolarizing responses of cardiac ganglion neurons in the crab, Portunus sanguinolentus

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Abstract
1. Tetrodotoxin-resistant, active responses to depolarization of the large cardiac ganglion cells were studied in semi-isolated preparations from the crab, Portunus sanguinolentus. Impulse activity was monitored with extracellular electrodes, simultaneous recordings from two or three large cells were made with intracellular electrodes, and current was passed via a bridge or second intracellular electrode. Preparations were continuously perfused with saline containing 3 x 10(-7) M tetrodotoxin (TTX). 2. About 20 min after introduction of TTX, small-cell impulses and resultant EPSPs in large cells cease, while rhythmic, spontaneous bursting of large cells continues. A pacemaker depolarization between bursts and slow depolarizations underlying the impulse bursts are prominent at this time. Shortly after, spontaneous burst rate slows, and at ca. 25 min, the ganglion becomes electrically quiescent. 3. In the quiescent, TTX-perfused ganglion, injection of depolarizing current into any one of the large cells results in active responses. At current strengths of sufficient intensity and duration (e.g., 20 nA, 20 ms; 5 nA, 500 ms) to depolarize a large cell by ca. 10 mV from resting potential (-53 mV, avg), the graded responses become regenerative and of constant form, provided the stimulation rate is less thna 0.15/s. Such responses have been termed "driver potentials." At more rapid rates, thresholds are increased and responses reduced. 4. Driver potentials of anterior large cells reach peak amplitudes of ca. 20 mV (to -32 mV), have maximum rates of rise of 0.45 V/s and of fall of 0.2 V/s, and a duration of ca. 250 ms. They are followed by hyperpolarizing afterpotentials, a rapidly decaying one (1 s) to -58 mV, followed by a slowly decaying one (7.5 s), -55 mV. Responses of posterior large cells are smaller (16 mV) and slower; the site of active response may be at a distance from the soma. 5. The ability of elicit near-synchronous responses and the identity of amplitude and form of responses among anterior cells and of posterior cells, regardless of which cell receives depolarizing current, indicates that all cells undergo active responses and are stimulated by electrotonic spread of depolarization. 6. The responses involve a conductance increase since memses during a driver potential are much reduced. 7. Depolarization by steady current increases the absolute threshold, decreases the maximum depolarization of the peak, and slows rates of rise and fall. Hyperpolarization increases rates of rise and fall; the absolute value reached by the peak depolarization is unchanged. Hyperpolarization reduces the amplitude of the rapid after-potential relative to the displaced resting potential. 8. Hyperpolarizing current pulses imposed during the rise and peak of driver-potential responses are followed by redevelopment of a complete response. Sufficiently strong hyperpolarization can terminate a response. The current strength needed to terminate a response decreases the later during the response the pulse is given...