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Mechanisms regulating the reliability of synaptic transmission Mackenzie, Paul James

Abstract

Calcium imaging, current clamp recording, electron microscopy and confocal microscopy were used to explore the reliability of synaptic transmission in cultured cortical neurons. The coupling between action potentials and rises in calcium at distal release sites was first evaluated. Local domains of enhanced calcium influx occurred at axonal varicosities that appeared to be functional sites of transmitter release. Single somatic action potentials reliably resulted in calcium transients at all presumed release sites, including those on distal collaterals. Variability in the amplitude of presynaptic calcium transients at individual boutons was estimated to be less than 20%. The coupling of somatic action potentials to calcium influx at distal release sites is a reliable process, although non-linearity in the relationship between calcium influx and transmitter release may amplify the effects of small fluctuations in calcium influx. Next we compared action potential conduction reliability in axons and dendrites. Extracellular medium with low sodium or dilute tetrodotoxin decreased action potential amplitude recorded in the soma without influencing axonal conduction. In contrast, the calcium transient measured along dendrites was markedly reduced in both low sodium and in dilute tetrodotoxin. These data provide further evidence that strongly excitable axons are functionally compartmentalized from weakly excitable dendrites. Modulation of sodium currents or membrane potential by neurotransmitters or repetitive firing is more likely to influence neuronal firing prior to action potential generation than the propagation of signals to axonal terminals. Finally, we tested the hypothesis that functional differences between synapses are associated with ultrastructure. Using imaging techniques, we measured dendritic miniature synaptic calcium transients attributed to the spontaneous release of single transmitter quanta. Identified neurons were then processed for serial transmission electron microscopy. At sites of NMDA receptor-mediated quantal calcium transients, we measured spine volume, synaptic contact area, and vesicle number. The number of vesicles at a synapse did not correlate with spontaneous activity. However, synapse size correlated positively with the amplitude of the NMDA component of the quantal postsynaptic response, suggesting that the number of postsynaptic receptors may place a limit on the postsynaptic response. Therefore, modulation of quantal amplitude may involve processes that alter synapse size.

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