Limnaea stagnalis neurones have been used to study the functioning of membrane receptor-channel complexes. The experiments were performed using a fixed membrane potential (E) and the intracellular perfusion technique. The cells employed responded to acetylcholine (ACh) by changing only their Cl- conductance. 2. ACh-induced currents, their fluctuations and relaxations resulting from a jump of E were studied. 3. The following facts have been established based on analysis of ACh currents, their fluctuations and relaxations: (1) the characteristic time of the exponential decay of the autocorrelation function, tau N, is in the range of 15-20 ms; (2) the characteristic relaxation time, tau R, equals 50-60 ms (ACh concentration = 0.25 microM, desensitization is not observed); (3) E does not exert any functionally significant effect upon tau N or tau R which could have governed the non-linearity of the membrane voltage-current characteristic; (4) variation of ACh concentration from 0.25 to 1 microM has a significant effect on tau R but not on tau N; (5) lowering of the ACh solution temperature from 22 to 8.5 degrees C results in a 20% increase of the ACh current, a 3- to 4-fold decrease of the single-channel conductance (gamma), a 20% increase in tau N and a 3- to 4-fold increase in tau R. 4. The suberylcholine (SCh)-induced membrane current has approximately the same value as the ACh-induced current at equal concentrations of ACh and SCh (0.25 microM); the tau N and gamma values were also quite close, but tau R was 2.3 times lower for SCh than for ACh. 5. An essentially two-stage scheme of functioning of membrane receptor-channel complexes is proposed. The scheme has two distinguishable and measurable stages and involves five closed states and one open state; it offers an explanation for our experimental data as well as the results of other workers.
