Kinetics Of Neurotransmitter Release Research Articles

Activity-dependent neurotransmitter release kinetics: correlation with changes in morphological distributions of small and large vesicles in central nerve terminals.

In central nerve terminals transmitter release is tightly regulated and thought to occur in a number of steps. These steps include vesicle mobilization and docking prior to neurotransmitter release. Intrasynaptic changes in vesicle distribution were determined by electron microscopical analysis and neurotransmitter release was monitored by biochemical measurements. We correlated K + -induced changes in distribution of small and large vesicles with the release of their transmitters. For small synaptic vesicles, amino acid release as well as recruitment to and docking at the active zone were activated within 1 s of depolarization. In contrast, the disappearance of large dense-cored vesicles and the release of the neuropeptide cholecystokinin were much slower, and no docking was observed. Studies with diverse Ca2 + channel blockers indicated that mobilization and neurotransmitter release from both vesicle types were regulated by multiple Ca2 + channels, although in different ways. Neurotransmitter release from small synaptic vesicles was predominantly regulated by P-type Ca2 + channels, whereas primarily Q-type Ca2 + channels regulated neurotransmitter release from large dense-cored vesicles. The different Ca2 + channnel types directly regulated mobilization of and neurotransmitter release from small synaptic vesicles whereas, by their cooperativity in raising the intracellular Ca2 + concentration above release threshold, they more indirectly regulated large dense-cored vesicle exocytosis.

Synapsins as regulators of neurotransmitter release.

One of the crucial issues in understanding neuronal transmission is to define the role(s) of the numerous proteins that are localized within presynaptic terminals and are thought to participate in the regulation of the synaptic vesicle life cycle. Synapsins are a multigene family of neuron-specific phosphoproteins and are the most abundant proteins on synaptic vesicles. Synapsins are able to interact in vitro with lipid and protein components of synaptic vesicles and with various cytoskeletal proteins, including actin. These and other studies have led to a model in which synapsins, by tethering synaptic vesicles to each other and to an actin-based cytoskeletal meshwork, maintain a reserve pool of vesicles in the vicinity of the active zone. Perturbation of synapsin function in a variety of preparations led to a selective disruption of this reserve pool and to an increase in synaptic depression, suggesting that the synapsin-dependent cluster of vesicles is required to sustain release of neurotransmitter in response to high levels of neuronal activity. In a recent study performed at the squid giant synapse, perturbation of synapsin function resulted in a selective disruption of the reserve pool of vesicles and in addition, led to an inhibition and slowing of the kinetics of neurotransmitter release, indicating a second role for synapsins downstream from vesicle docking. These data suggest that synapsins are involved in two distinct reactions which are crucial for exocytosis in presynaptic nerve terminals. This review describes our current understanding of the molecular mechanisms by which synapsins modulate synaptic transmission, while the increasingly well-documented role of the synapsins in synapse formation and stabilization lies beyond the scope of this review.

Two sites of action for synapsin domain E in regulating neurotransmitter release.

Synapsins, a family of synaptic vesicle proteins, have been shown to regulate neurotransmitter release; the mechanism(s) by which they act are not fully understood. Here we have studied the role of domain E of synapsins in neurotransmitter release at the squid giant synapse. Two squid synapsin isoforms were cloned and found to contain a carboxy (C)-terminal domain homologous to domain E of the vertebrate a-type synapsin isoforms. Presynaptic injection of a peptide fragment of domain E greatly reduced the number of synaptic vesicles in the periphery of the active zone, and increased the rate and extent of synaptic depression, suggesting that domain E is essential for synapsins to regulate a reserve pool of synaptic vesicles. Domain E peptide had no effect on the number of docked synaptic vesicles, yet reversibly inhibited and slowed the kinetics of neurotransmitter release, indicating a second role for synapsins that is more intimately associated with the release process itself. Thus, synapsin domain E is involved in at least two distinct reactions that are crucial for exocytosis in presynaptic terminals.

Regulation of neurotransmitter release kinetics by NSF.

NSF (N-ethylmaleimide-sensitive factor) is an adenosine triphosphatase (ATPase) that contributes to a protein complex essential for membrane fusion. The synaptic function of this protein was investigated by injecting, into the giant presynaptic terminal of squid, peptides that inhibit the ATPase activity of NSF stimulated by the soluble NSF attachment protein (SNAP). These peptides reduced the amount and slowed the kinetics of neurotransmitter release as a result of actions that required vesicle turnover and occurred at a step subsequent to vesicle docking. These results define NSF as an essential participant in synaptic vesicle exocytosis that regulates the kinetics of neurotransmitter release and, thereby, the integrative properties of synapses.

Neurotransmitter release kinetics

Neurotransmitter release kinetics

Estimation of in-vivo neurotransmitter release by brain microdialysis

Although microdialysis is commonly understood as a method of sampling low molecular weight compounds in the extracellular compartment of tissues, this definition appears insufficient to specifically describe brain microdialysis of neurotransmitters. In fact, transmitter overflow from the brain into dialysates is critically dependent upon the composition of the perfusing Ringer. Therefore, the dialysing Ringer not only recovers the transmitter from the extracellular brain fluid but is a main determinant of its in-vivo release. Two types of brain microdialysis are distinguished: quantitative micro-dialysis and conventional microdialysis. Quantitative microdialysis provides an estimate of neurotransmitter concentrations in the extracellular fluid in contact with the probe. However, this information might poorly reflect the kinetics of neurotransmitter release in vivo. Conventional microdialysis involves perfusion at a constant rate with a transmitter-free Ringer, resulting in the formation of a steep neurotransmitter concentration gradient extending from the Ringer into the extracellular fluid. This artificial gradient might be critical for the ability of conventional microdialysis to detect and resolve phasic changes in neurotransmitter release taking place in the implanted area. On the basis of these characteristics, conventional microdialysis of neurotransmitters can be conceptualized as a model of the in-vivo release of neurotransmitters in the brain. As such, the criteria of face-validity, construct-validity and predictive-validity should be applied to select the most appropriate experimental conditions for estimating neurotransmitter release in specific brain areas in relation to behaviour.

The 'Ca-voltage' hypothesis for neurotransmitter release.

The 'Ca-voltage' hypothesis for neurotransmitter release was reinvestigated by studying the kinetics of neurotransmitter release. These were independent of changes in intracellular or extracellular Ca2+ concentration. It is concluded that initiation and termination of release do not result from rapid entry and removal of Ca2+ although Ca2+ is essential for release. Quantal release of transmitter requires depolarization-dependent transformation of a membrane molecule from an inactive form T to a Ca2+-binding form S. The depolarization-dependent T----S transformation initiates release in the presence of Ca2+. The S----T transformation upon repolarization stops release even though the Ca2+ concentration at release sites is still high.