MULTIVESICULAR RELEASE AT SINGLE FUNCTIONAL SYNAPTIC SITES IN CEREBELLAR STELLATE AND BASKET CELLS

The purpose of the present work was to test the hypothesis that no mor e than one vesicle of transmitter can be liberated by an action potent ial at a single release site. Spontaneous and evoked IPSCs were record ed from interneurons in the molecular layer of cerebellar slices. Evok ed IPSCs were obtained using either extracellular stimulation or paire d recordings of presynaptic and postsynaptic neurons. Connections were identified as single-site synapses when evoked current amplitudes cou ld be grouped into one peak that was well separated from the backgroun d noise. Peak amplitudes ranged from 30 to 298 pA. Reducing the releas e probability by lowering the external Ca2+ concentration or adding Cd 2+ failed to reveal smaller quantal components, Some spontaneous IPSCs (1.4-2.4%) and IPSCs evoked at single-site synapses (2-6%) were follo wed within <5 msec by a secondary IPSC that could not be accounted for by random occurrence of background IPSCs, Nonlinear summation of clos ely timed events indicated that they involved activation of a common s et of receptors and therefore that several vesicles could be released at the same release site by one action potential. An average receptor occupancy of 0.70 was calculated after single release events. At some single-site connections, two closely spaced amplitude peaks were resol ved, presumably reflecting single and double vesicular release. Consis tent with multivesicular release, kinetics of onset, decay, and latenc y were correlated to IPSC amplitude. We conclude that the one-site, on e-vesicle hypothesis does not hold at interneuron-interneuron synapses .