Like the vertebrate heart, the pharynx is a neuromuscular pump thatis active throughout the life of the worm. In order to examine theelectrical events that occur during a pharyngeal pump, theelectropharyngeogram (EPG) - a fast, easy method for measuring electricalsignals generated by the pharyngeal muscles of an intact worm (Neuron12:483-495)- was developed. This method measures the currents that flow asa result of voltage changes during a muscle action potential, and sorepresents the temporal derivative of the action potential. We have usedintracellular recording to measure the absolute voltage levels across themuscle membranes during the action potential. This method requiresdissecting the pharynx out of the worm into saline (Joseph Dent WBG 13(1):44) and then, using a large diameter suction pipette to hold the terminalbulb, inserting a glass electrode into the muscle. The electrode filledwith 500mM potassium acetate has a resistance around 250Mohms. The trans-membrane voltage is measured with an Axoclamp 1D amplifier in bridge mode.A series of three action potentials from a wild-type worm is shown infigure 1a. These records closely resemble recordings form the pharyngealmuscle of Ascaris (Del Castillo and Morales (1967). J. Gen. Physiol. 50:603-629) and are somewhat similar to vertebrate heart action potentials.A small depolarizing event- possibly an excitatory postsynaptic potential-precedes these pumps, but is not always seen. The rising phase of theaction potential is fast and is followed by a longer plateau phase thatends with a rapid repolarization that overshoots the resting potential.
eat-6 encodes a Na,K-ATPase gene and the EPG from
eat-6 worms has asmaller excitation spike and a very small relaxation spike (WBG 13(2):52).Recordings from
eat-6 mutant worms (figure 1b) show a more positiveresting potential, smaller depolarizations, and slower repolarizationswith less overshoot. Further work using ionic substitution, pharmacology,and of course mutants will hopefully tell us about the variety of channelsthat must be working to cause these events.