Abstract
The pentapeptide proctolin modulates the activity of the rhythmic pattern generators in the crustacean stomatogastric nervous system. We report that proctolin depolarizes these cells maximally at membrane potentials close to the threshold for action potential generation. In voltage clamp, proctolin evokes an inward current, carried at least partially by Na super(+), that shows strong outward rectification. Removal of extracellular Ca super(2+) markedly increases the amplitude of the proctolin-evoked current and linearizes its current-voltage curve.