QUANTAL SECRETION FROM VISUALIZED BOUTONS ON RAT PELVIC GANGLION NEURONS

Synaptic transmission from single preganglionic hypogastric nerves inn ervating monopolar pelvic ganglion neurones has been studied with intr acellular electrodes to record transmission from all the boutons and w ith extracellular electrodes placed over boutons visualized with DiOC( 2) (5) in order to record transmission from selected boutons. Intracel lular electrodes revealed spontaneous excitatory postsynaptic potentia ls (EPSPs) with amplitude histograms that show increasing numbers of l arge EPSPs as the external calcium ([Ca2+](o)) was increased from 0.15 to 1.0 mM. These histograms were in general well fitted by a Poisson mixture of gamma distributions. Extracellular electrodes placed over v isualized boutons revealed evoked excitatory postsynaptic potentials ( extracellular EPSPs) with amplitude histograms that were best describe d by single gamma distributions in most cases in low [Ca2+](o) (less t han 0.5 mM). The standard deviation of these gammas was not much large r than that of the electrical noise. In a minority of extracellular re cordings the amplitude histogram of evoked extracellular EPSPs was bes t described by a gamma distribution in which the standard deviation wa s much greater than that of the noise. Confocal microscopy of boutons orthogradely labelled with dextran-rhodamine showed that about 30% of these formed closely apposing pairs on the surface of the neurones. Th ese observations are discussed in terms of the hypothesis that multiqu antal release at boutons occurs as a consequence of the coupled secret ion from closely apposed boutons.