Effects of tetrodotoxin on the slowly adapting stretch receptor neurone of lobster.

1. A study has been made of the effects of tetrodotoxin on the impulse activity, resting membrane potential, input resistance, and the generator potential and its after-hyperpolarization of the slowly adapting stretch receptor neurone of the lobster.2. Tetrodotoxin was able to abolish completely within about 2 min the impulse activity in most cells, when given in a dose of 2 x 10(-8) g/ml., but in all cells, when administered in a dose of 4 x 10(-8) g/ml. After blockage by the toxin in concentrations as high as 4 x 10(-6) g/ml. for periods of up to 30 min the action potential was restored by washing the preparation in physiological solution for 1 hr.3. In a concentration of 4 x 10(-8) g/ml. tetrodotoxin produced within 1-2 min an average increase of 4.8 mV of the resting membrane potential and a simultaneous 47% reduction of the resting input resistance. These effects were reversed by washing the preparation in physiological solution for 1 hr.4. Tetrodotoxin administered in doses as high as 4 x 10(-6) g/ml. for periods of up to 30 min had no effect on the amplitude of the steady phase of the generator potential.5. In a concentration of 4 x 10(-8) g/ml. tetrodotoxin produced within 5 min a 65% reduction of the amplitude of the hyperpolarization following the generator potential. This effect was reversed by washing the preparation in physiological solution for 1 hr.6. The simultaneous increase in resting membrane potential and decrease in membrane resistance is suggested to be due to an elevated potassium permeability besides a reduced sodium conductance. The constancy in height of the generator potential in the presence of a decreased membrane resistance makes necessary the assumption of an augmented generator current. The decrease in amplitude of the hyperpolarization following the generator potential may be the result of an increase in potassium conductance and/or a reduction in acceleration of an electrogenic pump in consequence of a diminished sodium influx during the generator potential.