Abstract
During exocytosis of synaptic transmitters, the fusion of highly curved synaptic vesicle membranes with the relatively planar cell membrane requires the coordinated action of several proteins. The role of membrane lipids in the regulation of transmitter release is less well understood. Since it helps to control membrane fluidity, alteration of cholesterol content may alter the fusibility of membranes as well as the function of membrane proteins. We assayed the importance of cholesterol in transmitter release at crayfish neuromuscular junctions where action potentials can be measured in the preterminal axon. Methyl-beta-cyclodextrin (MbetaCD) depleted axons of cholesterol, as shown by reduced filipin labelling, and cholesterol was replenished by cholesterol-MbetaCD complex (Ch-MbetaCD). MbetaCD blocked evoked synaptic transmission. The lack of postsynaptic effects of MbetaCD on the time course and amplitude of spontaneous postsynaptic potentials or on muscle resting potential allowed us to focus on presynaptic mechanisms. Intracellular presynaptic axon recordings and focal extracellular recordings at individual boutons showed that failure of transmitter release was correlated with presynaptic hyperpolarization and failure of action potential propagation. All of these effects were reversed when cholesterol was replenished with Ch-MbetaCD. However, focal depolarization of presynaptic boutons and administration of a Ca2+ ionophore both triggered transmitter release after cholesterol depletion. Therefore, both presynaptic Ca2+ channels and Ca2+-dependent exocytosis functioned after cholesterol depletion. The frequency of spontaneous quantal transmitter release was increased by MbetaCD but recovered when cholesterol was reintroduced. The increase in spontaneous release was not through a calcium-dependent mechanism because it persisted with intense intracellular calcium chelation. In conclusion, cholesterol levels in the presynaptic membrane modulate several key properties of synaptic transmitter release.