Quantal components of spontaneous excitatory junction potentials at visualised varicosities.

The electrical signs of spontaneous transmitter release were recorded with an extracellular electrode from single visualized sympathetic varicosities on the mouse vas deferens. Ultrastructural examination of these varicosities with the electron microscope showed that they formed close-contacts with smooth muscle cells. Amplitude-frequency histograms of the spontaneous excitatory junction potentials (SEJPs) were constructed in order to determine the statistical nature of spontaneous transmitter release from a varicosity. SEJP histograms often possessed several peaks. Statistical tests showed that these were separate modes in the histograms indicating that the SEJPs were composed of subunits. The SEJP histograms were described by a mixture of distributions in which the components were identified as quanta and there was a Poisson release of quanta. The second mode in the SEJP histograms was sometimes twice that of the first mode but generally greater, suggesting a potentiation of the effects of one quantum released nearly simultaneously with another quantum. The components in the SEJP histograms were well fitted by either a Gaussian or a gamma distribution indicating that the quantum of transmitter could be described as a Gaussian or gamma variate.