QUANTAL COMPONENTS OF SPONTANEOUS EXCITATORY JUNCTION POTENTIALS AT VISUALIZED VARICOSITIES

The electrical signs of spontaneous transmitter release were recorded with an extracellular electrode from single visualized sympathetic var icosities on the mouse vas deferens. Ultrastructural examination of th ese varicosities with the electron microscope showed that they formed close-contacts with smooth muscle cells. Amplitude-frequency histogram s of the spontaneous excitatory junction potentials (SEJPs) were const ructed in order to determine the statistical nature of spontaneous tra nsmitter release from a varicosity. SEJP histograms often possessed se veral peaks, Statistical tests showed that these were separate modes i n the histograms indicating that the SEJPs were composed of subunits. The SEJP histograms were described by a mixture of distributions in wh ich the components were identified as quanta and there was a Poisson r elease of quanta. The second mode in the SEJP histograms was sometimes twice that of the first mode but generally greater, suggesting a pote ntiation 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 th e quantum of transmitter could be described as a Gaussian or gamma var iate.