Facilitatory transmitters and cAMP can modulate accommodation as well as transmitter release in Aplysia sensory neurons: Evidence for parallel processing in a single cell

Presynaptic facilitation of transmission from sensory to motor neurons contributes significantly to behavioral sensitization of defensive withdrawal reflexes in Aplysia. Presynaptic facilitation is associated with a decrease in the serotonin-sensitive K⁺ conductance. This decrease broadens the presynaptic action potential. In addition, the procedures that cause facilitation - stimulation of the connective (the pathway from the tail and head), application of modulatory transmitters, or injection of cAMP - also increase the excitability of the sensory neurons as tested with intracellular depolarizing pulses injected into the cell body. The increased excitability is reflected in a decreased threshold for generating action potentials and a reduction in accomodation to prolonged constant current stimuli. By influencing the excitability of the peripheral processes of the sensory neurons, stimulation of the connectives or serotonin also produces a small enhancement of the response of the sensory neurons to a tactile stimulus applied to the siphon. The excitability changes appear to result, at least in part, from the same cellular mechnaisms that lead to broadening of the action potential, a cAMP-mediated closure of K⁺ channels. Therefore, these findings indicate that the same class of mechanisms can, in principle, have a dual action and provide further evidence of parallel processing in the modulation of transmitter release from a single neuron.