Kinetic properties of cholinergic desensitization in Aplysia neurons.

The kinetic properties of desensitization onset of excitatory cholinergic responses were studied in isolated, voltage-clamped Aplysia neurons. Desensitization of the acetylcholine (ACh)-induced current in response to microperfused acetylcholine occurred in two phases, and was best modelled as the sum of two exponential components plus a constant. Both exponential components were accelerated by increasing ACh dose. At the higher ACh doses the current decline was dominated by the fast exponential component, and the ratio of the plateau-peak current was reduced. Over the range of membrane potentials -50 to -110 mV, no change in the kinetics of desensitization onset was observed. The mean time constants of both exponential components were doubled by cooling from 20 degrees C to 5 degrees C. These results demonstrate that, as at the vertebrate neuromuscular junction, the onset of desensitization of this ACh response involves at least two processes which are dose- and temperature-sensitive. The lack of voltage dependence contrasts with results from vertebrate preparations, and indicates a fundamental difference between the properties of the excitatory ACh response in Aplysia neurons and the vertebrate neuromuscular junction.