Quantal potential fields around individual active zones of amphibian motor-nerve terminals

Sub-cellular Electrical Heterogeneity Revealed by Loose Patch Recording Reflects Differential Localization of Sarcolemmal Ion Channels in Intact Rat Hearts

The cardiac action potential (AP) is commonly recoded as an integral signal from isolated myocytes or ensembles of myocytes (with intracellular microelectrodes and extracellular macroelectrodes, respectively). These signals, however, do not provide a direct measure of activity of ion channels and transporters located in two major compartments of a cardiac myocyte: surface sarcolemma and the T-tubule system, which differentially contribute to impulse propagation and excitation-contraction (EC) coupling. In the present study we investigated electrical properties of myocytes within perfused intact rat heart employing loose patch recording with narrow-tip (2 μm diameter) extracellular electrodes. Using this approach, we demonstrated two distinct types of electric signals with distinct waveforms (single peak and multi-peak AP; AP1 and AP2, respectively) during intrinsic pacemaker activity. These two types of waveforms depend on the position of the electrode tip on the myocyte surface. Such heterogeneity of electrical signals was lost when electrodes of larger pipette diameter were used (5 or 10 μm), which indicates that the electric signal was assessed from a region of <5 μm. Importantly, both pharmacological and mathematical simulation based on transverse (T)-tubular distribution suggested that while the AP1 and the initial peak of AP2 are predominantly attributable to the fast, inward Na⁺ current in myocyte's surface sarcolemma, the late components of AP2 are likely representative of currents associated with L-type Ca²⁺ channel and Na⁺/Ca²⁺ exchanger (NCX) currents which are predominantly located in T-tubules. Thus, loose patch recording with narrow-tip pipette provides a valuable tool for studying cardiac electric activity on the subcellular level in the intact heart.

Seasonal factors influence quantal transmitter release and calcium dependence at amphibian neuromuscular junctions

Amphibian neuromuscular junctions (NMJs) are composed of hundreds of neurotransmitter release sites that exhibit nonuniform transmitter release probabilities and demonstrated seasonal modulation. We examined whether recruitment of release sites is variable when the extracellular calcium concentration ([Ca²⁺]_o) is increased in the wet and dry seasons. The amount of transmitter released from the entire nerve terminal increases by approximately the fourth power as [Ca²⁺]_o is increased. Toad (Bufo marinus) NMJs were visualized using 3,3'-diethyloxardicarbocyanine iodide [DiOC₂(5)] fluorescence, and focal loose patch extracellular recordings were used to record the end-plate currents (EPCs) from small groups of release sites. Quantal content (m̄_e ), average probability of quantal release (p_e ), and the number of active release sites (n_e ) were determined for different [Ca²⁺]_o Our results indicated that the recruitment of quantal release sites with increasing [Ca²⁺]_o differs spatially (between different groups of release sites) and also temporally (in different seasons). These differences were reflected by the nonuniform alterations in p_e and n_e Most release site groups demonstrated an increase in both p_e and n_e when [Ca²⁺]_o increased. In ~30% of release site groups examined, p_e decreased while n_e increased only during the active period (wet season). Although the dry season induced parallel right shift in the quantal release versus extracellular calcium concentration when compared with the wet season, the dependence of quantal content on [Ca²⁺]_o was not changed. These results demonstrate the flexibility, reserve, and adaptive capacity of neuromuscular junctions in maintaining appropriate levels of neurotransmission.

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.

Electric fields of synaptic currents could influence diffusion of charged neurotransmitter molecules

Rapid activation of synaptic receptor-channels evokes an ion current that flows through the narrow synaptic cleft; this exerts a significant voltage drop and therefore strong electric field (10(4) V/m range) directed towards the current sinks in the cleft. To what extent this field affects fast diffusion of charged neurotransmitter molecules is not known. We draw a theoretical framework for this complex electrodiffusion phenomenon and establish the basic relationships between the synaptic current and the time course of neurotransmitter in the cleft. The analyses predict that excitatory currents could significantly accelerate the dispersion of negatively charged molecules from the cleft while attracting the positively charged molecules towards the current sinks. This previously unrecognized mechanism should affect the kinetics of synaptic receptor currents, thus contributing to fast synaptic signaling in the brain.

Quantal current fields around individual boutons in sympathetic ganglia

The release of a quantum of neurotransmitter from an active zone of a bouton is accompanied by the flow of extracellular current that creates a potential field about the site of transmitter action beneath the bouton. It is shown theoretically that the density of the field at the peak of the quantal current gives rise to an extracellular potential that declines to values of less than 5 microV at 1.3 microm distance in the circumferential direction around the neuron and equally rapidly in the radial direction away from the neuron. A loose-patch electrode placed over a bouton distorts the quantal field about the bouton and calculations show that under current-clamp conditions, potentials of over 40 microV can be recorded with an electrode of tip diameter 2 microm, provided the separation between the tip and the neuron's surface is about 0.1 microm. Quantal release recorded from visualized boutons on rat monopolar pelvic ganglion cells with loose-patch electrodes is in agreement with the properties of the quantal potential field given in the theoretical analysis.

Formation and function of synapses with respect to Schwann cells at the end of motor nerve terminal branches on mature amphibian (Bufo marinus) muscle

A study has been made of the formation and regression of synapses with respect to Schwann cells at the ends of motor nerve terminal branches in mature toad (Bufo marinus) muscle. Synapse formation and regression, as inferred from the appearance and loss of N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl) pyridinium dibromide (FM1-43)-stained vesicle clusters, occurred at the ends of terminal branches over a 16 hr period. Multiple microelectrodes placed in an array about FM1-43 blobs at the ends of terminal branches detected the electrical signs of neurotransmitter being released onto receptors. Injection of a calcium indicator (Oregon Green 488 BAPTA-1) into the motor nerve with subsequent imaging of the calcium transients, in response to stimulation, often showed a reduced calcium influx in the ends of terminal branches. Injection of a fluorescent dye into motor nerves revealed the full extent of their terminal branches and growing processes. Injection of the terminal Schwann cells (TSCs) often revealed pseudopodial TSC processes up to 10-microm-long. Imaging of these TSC processes over minutes or hours showed that they were highly labile and capable of extending several micrometers in a few minutes. Injection of motor nerve terminals with a different dye to that injected into their TSCs revealed that terminal processes sometimes followed the TSC processes over a few hours. It is suggested that the ends of motor nerve terminals in vivo are in a constant state of remodeling through the formation and regression of processes, that TSC processes guide the remodeling, and that it can occur over a relatively short period of time.

Formation and function of synapses with respect to Schwann cells at the end of motor nerve terminal branches on mature amphibian (Bufo marinus) muscle

A study has been made of the formation and regression of synapses with respect to Schwann cells at the ends of motor nerve terminal branches in mature toad (Bufo marinus) muscle. Synapse formation and regression, as inferred from the appearance and loss of N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl) pyridinium dibromide (FM1-43)-stained vesicle clusters, occurred at the ends of terminal branches over a 16 hr period. Multiple microelectrodes placed in an array about FM1-43 blobs at the ends of terminal branches detected the electrical signs of neurotransmitter being released onto receptors. Injection of a calcium indicator (Oregon Green 488 BAPTA-1) into the motor nerve with subsequent imaging of the calcium transients, in response to stimulation, often showed a reduced calcium influx in the ends of terminal branches. Injection of a fluorescent dye into motor nerves revealed the full extent of their terminal branches and growing processes. Injection of the terminal Schwann cells (TSCs) often revealed pseudopodial TSC processes up to 10-microm-long. Imaging of these TSC processes over minutes or hours showed that they were highly labile and capable of extending several micrometers in a few minutes. Injection of motor nerve terminals with a different dye to that injected into their TSCs revealed that terminal processes sometimes followed the TSC processes over a few hours. It is suggested that the ends of motor nerve terminals in vivo are in a constant state of remodeling through the formation and regression of processes, that TSC processes guide the remodeling, and that it can occur over a relatively short period of time.

Quantal and non-quantal current and potential fields around individual sympathetic varicosities on release of ATP

The electrical phenomena that occur at sympathetic varicosities due to the release of ATP include spontaneous and evoked excitatory junction potentials (SEJPs and EJPs; recorded with an intracellular electrode) as well as fast and slow excitatory junctional currents (EJCs; recorded with a loose-patch electrode placed over varicosities). The electrical analysis of these transients is hampered by lack of a detailed theory describing how current and potential fields are generated upon the release of a quantum of ATP. Here, we supply such a theory and develop a computational model for the electrical properties of a smooth muscle syncytium placed within a volume conductor, using a distributed representation for the individual muscle cells. The amplitudes and temporal characteristics of both SEJPs and fast EJCs are predicted by the theory, but those of the slow EJCs are not. It is shown that these slow components cannot arise as a consequence of propagation of fast quantal components from their site of origin in the muscle syncytium to the point of recording. The possibility that slow components arise by a mechanism of transmitter secretion that is different from quantal release is examined. Experiments that involve inserting peptide fragments of soluble N-ethylmaleimide-sensitive fusion attachment protein (alpha-SNAP) into varicosities, a procedure that is known to block quantal release, left the slow component of release unaffected. This work provides an internally consistent description of quantal potential and current fields about the varicosities of sympathetic nerve terminals and provides evidence for a non-quantal form of transmitter release.

NANC transmission at a varicosity: the individuality of single synapses

Nerve terminals consist of several hundred varicosities or synapses, each with a single active zone. The smooth muscle membrane apposing varicosities within about 50 nm is occupied by a 1-microm diameter cluster of P2X(1) receptors together with a mixture of other P2X subtypes; the rest of the membrane possesses small (0.4 microm diameter) clusters of P2X(1) to P2X(6) subunits. The small P2X clusters appear to form large clusters during development. This is supported by the observation that chimeras of P2X(1) subunits and green fluorescent protein (P2X(1)-GFP), when packaged into adenoviruses used to infect excitable cells, initially form a diffuse distribution of small clusters of P2X(1)-GFP in the membrane; these can be later observed in real time to form large clusters. Recording the electrical signs of ATP release from single adjacent varicosities, or using antibodies to label the extent of exocytosis from them, shows that they release with quite different probabilities. There are large quantitative differences in the extent of P2X autoreceptors on the membranes of individual varicosities. These will contribute to the differences in the probability of secretion from individual varicosities. The present analysis of NANC transmission at single varicosities indicates that individual synapses possess different probabilities for the secretion of transmitter as well as different complements of autoreceptors and mixtures of postjunctional receptor subunits.