MODULATION OF SWIMMING BEHAVIOR IN THE MEDICINAL LEECH .2. IONIC CONDUCTANCES UNDERLYING SEROTONERGIC MODULATION OF SWIM-GATING CELL-204

In the previous paper we showed that serotonin had several effects on the electrical properties of swim-gating neurons (cells 204) of the leech. These included membrane potential depolarization, induction of a sag voltage response, and enhancement of rebound responses. Here we investigate the ionic basis of these changes by comparing responses of cell 204 to injected current pulses in experimental salines containing modified concentrations of Na+, K+, Ca2+, or Cl-. Our data indicate that serotonin modulates multiple conductances in cell 204. However, most effects of serotonin can be explained by enhancement of two Na+-dependent conductances, a Cs+-sensitive cation conductance (g(h)) and a persistent Na+ conductance (g(NaS)). Both conductances contribute to the resting potential depolarization and increased amplitude of postinhibitory rebound responses induced by serotonin. In addition, enhanced gh underlies a sag potential elicited by hyperpolarizing current pulses in serotonin-treated cells. Hyperpolarizing rebound responses following depolarizing current pulses are composed of Na+-dependent and Na+-independent components, both of which are enhanced by serotonin. Activation of an electrogenic Na+/K+ pump may underlie the prolonged Na+-dependent component. The Na+-independent component decays within 1 s and may be produced by a voltage- or Ca2+-activated K+ conductance.