Serotonin and cyclic adenosine 3':5'-monophosphate modulate the potassium current in tail sensory neurons in the pleural ganglion of Aplysia.

Tail sensory neurons in the pleural ganglion that mediate the afferent portion of the tail withdrawal reflex in Aplysia californica undergo heterosynaptic facilitation of transmitter release during sensitization. As in the siphon sensory neurons, the transmitter serotonin produces facilitation and also elicits a slow, decreased conductance excitatory postsynaptic potential (EPSP) in these neurons. Using voltage clamp and biochemical analyses, we have found that the slow EPSP in the pleural sensory neurons is due to a decrease in a potassium conductance identical to the S potassium current characterized in siphon sensory neurons. Like the S current, the current modulated by serotonin in the pleural sensory neurons is a non-inactivating potassium current, and it contributes to both the resting and action potentials. The current reverses in 120 mM external K+ at -20 mV, close to the predicted Nernst equilibrium potential. Intracellular cesium blocks the serotonin response, but the current is not blocked by equimolar substitution of barium for calcium, nor by 50 mM tetraethylammonium chloride. The effect of serotonin is cAMP dependent, since serotonin elevates cAMP and both cAMP injection and forskolin mimic the serotonin response. These results indicate that the mechanism associated with sensitization of the siphon-gill withdrawal reflex, a slow decreased potassium conductance, is also a component of the neuronal circuitry underlying modulation of another reflex, the tail withdrawal reflex. Therefore, two distinct populations of neurons subserving similar behavioral functions have related biophysical and biochemical properties.