SUMMARY1. Aplysia neurones with an excitatory response to acetylcholine (ACh) were voltage-clamped, and the ACh-induced currents were studied using noise and relaxation techniques. The mean channel open time, T, and the amplitude of the elementary current, ie1, were determined from these experiments, and the variation of these parameters with the ionic content of the extracellular solution was analysed. The goal of this work was to test whether permeant ions may bind in a voltagedependent manner to channel sites and thereby hinder channel closing, as has been proposed before (Ascher, Marty & Neild, 1978a).2. The relation between T and the membrane potential V has a similar shape in normal sea water and after total replacement of Na ions with Li or Cs. In contrast, the shape of the r(V) relation is modified if Na is replaced by Mg, Sr, or Ba. 5. Experiments were performed in solutions containing Na and sucrose, or Mg and mannitol. In both cases r was smaller than in an isotonic Na or Mg solution.6. None of the above observations can be accounted for on the sole basis of outer surface potential changes.7. A quantitative model of the interaction between permeant ions and AChsensitive channels is proposed. The possible relevance of this model for the interpretation of r( V) curves in other systems is discussed.

No abstract

No abstract

The action of glucosamine was studied on voltage clamped neurones of Aplysia, presenting an excitatory response to acetylcholine. Noise and relaxation experiments show that glucosamine increases the mean channel open time and reduces the amplitude of the elementary current associated with the acetylcholine response. Both effects are enhanced by hyperpolarization of the cell membrane. The results are interpreted by a model assuming glucosamine binding to open channels. This binding impedes the flow of permeant ions and decreases the closing rate of the channels.