Dynamic responses of presynaptic terminal membrane pools following KCl and sucrose stimulation

Dynamin 1 controls vesicle size and endocytosis at hippocampal synapses

Following calcium-triggered vesicle exocytosis, endocytosis regenerates vesicles to maintain exocytosis and thus synaptic transmission, which underlies neuronal circuit activities. Although most molecules involved in endocytosis have been identified, it remains rather poorly understood how endocytic machinery regulates vesicle size. Vesicle size, together with the transmitter concentration inside the vesicle, determines the amount of transmitter the vesicle can release, the quantal size, that may control the strength of synaptic transmission. Here, we report that, surprisingly, knockout of the GTPase dynamin 1, the most abundant brain dynamin isoform known to catalyze fission of the membrane pit's neck (the last step of endocytosis), not only significantly slowed endocytosis but also increased the synaptic vesicle diameter by as much as ∼40-64% at cultured hippocampal synapses. Furthermore, dynamin 1 knockout increased the size of membrane pits, the precursor for endocytic vesicle formation. These results suggest an important function of dynamin other than its well-known fission function - control of vesicle size at the pit formation stage.

Porocytosis: Fusion Pore Array Secretion of Neurotransmitter

We believe that there is sufficient experimental evidence to support the premise that transmitter is secreted by the simultaneous activation of arrays of fusion pores at docked vesicles. This process is initiated by the action potential that activates calcium channels to increase the number of cytoplasmic calcium ions. Calcium ions trigger fusion pores to flicker open causing transmitter to diffuse from vesicular stores. We define the term porocytosis to identify this process and use the term synaptomere to indicate the anatomical and physiological functional unit of the synapse or junction. Our model shows that the simultaneous flicker of fusion pores in an array can generate unitary-end plate potentials (u-EPPs) and miniature end plate potentials (MEPPs) and that activation of all fusion pores produces EPPs. U-EPPs and EPPs generated with the model show mean values and coefficients of variation similar to experimental observations. The model is robust in that the number of docked vesicles can vary and these can be full to empty depending on nerve frequencies and vesicular traffic. The model shows that the overall process of excitation-secretion coupling is highly deterministic. At the neuromuscular junction, secretion from arrays of fusion pores ensures that a muscle fiber action potential is always produced over a range of frequencies because all transmitter release sites are activated. Our model shows that transmission at the synaptomere guarantees fidelity of information transfer at different frequencies. This characteristic shows a dynamic relationship of the secretory process to memory and learning.

Enhancement of the endosomal endocytic pathway increases quantal size

We combined recordings of spontaneous quantal events with electron microscopy analysis of synaptic ultrastructure to demonstrate that the size of a neurosecretory quantum increases following an activation of the endosomal endocytic pathway. We reversibly activated the endosomal endocytic pathway in Drosophila motor boutons by application of high K+ solution. This treatment produced the formation of numerous cisternae, vacuoles and enlarged vesicles. Spontaneous quantal events recorded immediately after the cessation of high K+ application were significantly enlarged, and this increase in quantal size was reversed after a 10 minute resting period. Actin depolymerization produced by latrunculin B pretreatment inhibited both the formation of endosome-like structures and the increase in quantal size. Loading the preparations with the dye FM1-43 followed by photoconversion of the dye combined with electron microscopy analysis revealed that the observed cisternae are likely to be the product of both bulk membrane retrieval and vesicle fusion.

Loading and recycling of synaptic vesicles in the Torpedo electric organ and the vertebrate neuromuscular junction

In vertebrate motor nerve terminals and in the electromotor nerve terminals of Torpedo there are two major pools of synaptic vesicles: readily releasable and reserve. The electromotor terminals differ in that the reserve vesicles are twice the diameter of the readily releasable vesicles. The vesicles contain high concentrations of ACh and ATP. Part of the ACh is brought into the vesicle by the vesicular ACh transporter, VAChT, which exchanges two protons for each ACh, but a fraction of the ACh seems to be accumulated by different, unexplored mechanisms. Most of the vesicles in the terminals do not exchange ACh or ATP with the axoplasm, although ACh and ATP are free in the vesicle interior. The VAChT is controlled by a multifaceted regulatory complex, which includes the proteoglycans that characterize the cholinergic vesicles. The drug (-)-vesamicol binds to a site on the complex and blocks ACh exchange. Only 10-20% of the vesicles are in the readily releasable pool, which therefore is turned over fairly rapidly by spontaneous quantal release. The turnover can be followed by the incorporation of false transmitters into the recycling vesicles, and by the rate of uptake of FM dyes, which have some selectivity for the two recycling pathways. The amount of ACh loaded into recycling vesicles in the readily releasable pool decreases during stimulation. The ACh content of the vesicles can be varied over eight-fold range without changing vesicle size.

Porocytosis: a new approach to synaptic function

We propose a new approach to address the question of how a single quantum of neurotransmitter is secreted from a presynaptic terminal whose clustered secretory vesicles are locally bathed in high levels of calcium ions [Proceedings of the Symposium on Bioelectrogenesis (1961) 297-309; The Physiology of Synapses (1964) Chapters 1, 4, 5, 6; How the Self Controls its Brain (1994) Chapters 1, 4, 5, 6; Science 256 (1992) 677-679]. This hypothesis, which we term 'porocytosis', posits that the post-synaptic quantal response results from transmitter secreted through an array of docked vesicle/secretory pore complexes. The transient increase in calcium ions, which results from the voltage activated calcium channels, stimulates the array of secretory pores to simultaneously flicker open to pulse transmitter. Porocytosis is consistent with the quantal nature of presynaptic secretion and transmission, and with available biochemical, morphological and physiological evidence. It explains the frequency dependency of quantal size as a function of the secretion process. It permits a signature amount of transmitter release for different frequencies allowing a given synapse to be employed in different behavioral responses. The porocytosis hypothesis permits fidelity of secretion and the seemingly apposed characteristic of synaptic plasticity. The dynamics inherent in an array insure a constant quantal size as a function of the number of units within the array. In this hypothesis, plasticity is a consequence of concurrent pre- and post-synaptic changes due to a change in array size. Changes in the number of docked vesicle-secretory pore complexes composing the array can explain facilitation, depletion, graded excitation-secretion and long term plasticity.

Transmitter quantal size in Torpedo electrocytes is determined by frequency of release

Miniature end-plate potentials (MEPPs) were focally recorded from the cytoplasmic surface of electrocytes in isolated columns of the Torpedo electric organ. Double electrode studies showed that the junctional area was restricted to 12 micron2. MEPP frequencies ranging from 1/min to 400/s were controlled with electrode advancement against the cytoplasmic surface. Stable membrane potentials and noise levels indicated constant intracellular, focal recording conditions. Focal MEPPs are only 1-3 mV and MEPP amplitudes smoothly decreased with an increase in MEPP frequency which demonstrates a process that meters quantal size at moment of release. Thus, release if not from a prepackaged store. MEPP interval analyses showed that events are weakly interactive at low frequencies and periodic at higher frequencies. The interdependency of MEPP amplitudes and intervals indicates that the mechanism of release controls both rate and quantal size. We propose that the amplitude and frequency dependencies of MEPPs at the Torpedo nerve-electrocyte junction are best described by a membrane channel (e.g., mediatophore, Israël and Dunant, Neurochem. Int. 28 (1996) 1-9) that meters transmitter from a presynaptic store.