The acetylcholine induced membrane current fluctuations are characteristic of the receptor-ionophore population involved

1. Clamp current recordings from voltage clamped Helix aspersa neurones show increased membrane current fluctuations during the three types of acetylcholine (ACh) response studied.2. The variance of the ACh induced membrane current fluctuations was proportional to the mean clamp current and was steady during data acquisition. The mean channel current (iel) could be calculated from these values.3. The power spectrum (PSD) of the ACh induced membrane current fluctuations for both the pure ionic responses [ACh D increased gNa+ and ACh H incrased gCl?] could be fitted by single Lorentzian curves.4. The corner frequency (fc) of the PSD differed between the D and H responses, being 15.9 Hz and 48 Hz respectively, corresponding to mean channel open times (Tm) of 10 msec for the Na+ channel and 3.3 msec for the Cl? channel.5. The PSD of the ACh induced membrane current fluctuations of the mixed ionic response [D(f Cl) increased gNa+ adn gCl?] could not be fitted by a single Lorentzian curve, but could be fitted by the arithmetic sum of two Lorentzian curves, with corner frequencies of 12.5 Hz and 50 Hz. These values for fc correspond to Tms of 15.9 msec and 3.2 msec.6. It is suggested that the double Lorentzian of the D(f Cl) response is due to activation of a heterogeneous receptor-ionophore population consisting of two classes which correspond to the D receptor/Na+ ionophore and H receptor/Cl? ionophore present on the other cell types studied