A quantitative description of stimulation-induced changes in transmitter release at the frog neuromuscular junction.

Endplate potentials were recorded from frog sartorius neuromuscular junctions under conditions of greatly reduced quantal contents to develop a quantitative description of stimulation-induced changes in transmitter release. Four general models relating potentiation, augmentation, and the first and second components of facilitation to transmitter release were developed. These models were then tested by incorporating equations for the kinetic properties of the four components of increased transmitter release and examining the ability of the resulting sets of equations to predict stimulation-induced changes in transmitter release. Three of the models were essentially consistent with the observation that augmentation had a multiplicative type relationship to facilitation. These models could also predict the effect of frequency and duration of stimulation on endplate potential (EPP) amplitude during and after prolonged (40 s) trains including the response to step changes in stimulation rate. These models extend by about two orders of magnitude the duration of stimulationinduced changes in transmitter release that can be accounted for, and show that the combined kinetic properties of potentiation, augmentation, and the two components of facilitation are generally sufficient to account for these changes

Determining the neurotransmitter concentration profile at active synapses

Establishing the temporal and concentration profiles of neurotransmitters during synaptic release is an essential step towards understanding the basic properties of inter-neuronal communication in the central nervous system. A variety of ingenious attempts has been made to gain insights into this process, but the general inaccessibility of central synapses, intrinsic limitations of the techniques used, and natural variety of different synaptic environments have hindered a comprehensive description of this fundamental phenomenon. Here, we describe a number of experimental and theoretical findings that has been instrumental for advancing our knowledge of various features of neurotransmitter release, as well as newly developed tools that could overcome some limits of traditional pharmacological approaches and bring new impetus to the description of the complex mechanisms of synaptic transmission

Data-Driven Modelling of the Inositol Trisphosphate Receptor (IPR) and its Role in Calcium-Induced Calcium Release (CICR)

We review the current state of the art of data-driven modelling of the inositol trisphosphate receptor (IPR). After explaining that the IPR plays a crucial role as a central regulator in calcium dynamics, several sources of relevant experimental data are introduced. Single ion channels are best studied by recording single-channel currents under different ligand concentrations via the patch-clamp technique. The particular relevance of modal gating, the spontaneous switching between different levels of channel activity that occur even at constant ligand concentrations, is highlighted. In order to investigate the interactions of IPRs, calcium release from small clusters of channels, so-called calcium puffs, can be used. We then present the mathematical framework common to all models based on single-channel data, aggregated continuous-time Markov models, and give a short review of statistical approaches for parameterising these models with experimental data. The process of building a Markov model that integrates various sources of experimental data is illustrated using two recent examples, the model by Ullah et al. and the “Park–Drive” model by Siekmann et al. (Biophys. J. 2012), the only models that account for all sources of data currently available. Finally, it is demonstrated that the essential features of the Park–Drive model in different models of calcium dynamics are preserved after reducing it to a two-state model that only accounts for the switching between the inactive “park” and the active “drive” modes. This highlights the fact that modal gating is the most important mechanism of ligand regulation in the IPR. It also emphasises that data-driven models of ion channels do not necessarily have to lead to detailed models but can be constructed so that relevant data is selected to represent ion channels at the appropriate level of complexity for a given application

The effect of repetitive stimulation on facilitation of transmitter release at the frog neuromuscular junction

1. End-plate potentials (e.p.p.s) were recorded from frog neuromuscular junctions blocked with high Mg and/or low Ca to characterize the processes underlying increased transmitter release during repetitive stimulation. 2. There was a progressive increase in the amplitude of successive e.p.p.s during repetitive stimulation. Increasing the frequency or duration of stimulation increased this facilitation of e.p.p. amplitudes. Facilitation is defined as the fractional increase in amplitude of a test e.p.p. over a control. 3. By assuming that each impulse in a train contributes an identical increment of facilitation that sums linearly with the facilitation contributed by the previous impulses, estimates of the facilitation contributed by a single impulse, f(t), were made from the incremental increase in e.p.p. amplitudes during repetitive stimulation. The average value of f(t) contributed by the first impulse in the train during stimulation at 20/sec is given by f(t) = 0·8 e(-t/50) + 0·12 e (-t/300) + 0·025 e(-t/3000),where t is in msec. The first two terms in this equation were independent of the stimulation rate used to determine f(t) while the coefficient of the third term was a function of the stimulation rate, decreasing 2 to 3 times when the stimulation rate was decreased from 20/sec to 1/sec. 4. This linear facilitation model predicted growth of e.p.p. amplitudes during the first several hundred msec of repetitive stimulation. Thereafter, e.p.p. amplitudes were typically facilitated more than predicted by the linear model. 5. Several new methods are presented which can be used to obtain estimates of the magnitude and time course of facilitation contributed by specific impulses during repetitive stimulation. 6. It is found that the value of short-term f(t) in the tested range of 25-300 msec progressively increases during repetitive stimulation while its time course of decay remains unchanged. After 9 sec of stimulation at 20/sec, the short-term f(t) increased to 1·4 times control. 7. The increase in short-term f(t) was independent of whether it was determined from a step increase or decrease in total facilitation, excluding the possibility that the observed increase in short-term f(t) resulted from a change in the rate of decay of facilitation. 8. It is suggested with supporting data from the following paper (Magleby, 1973) that each impulse contributes two types of facilitation that are responsible for the growth of e.p.p.s during repetitive stimulation: a short-term facilitation with linear summation properties described by the first two terms in the expression in paragraph 3 and a long-term cumulative facilitation approximated by the third term. The long-term facilitation is expressed as an increase in both the short-term facilitation and in the base level of transmitter release. The relative contribution of these two expressions of the long-term facilitation to the third term is a function of the stimulation rate and is given by the ratio of facilitation to the base level of transmitter release

The effect of tetanic and post-tetanic potentiation on facilitation of transmitter release at the frog neuromuscular junction

1. End-plate potentials (e.p.p.s) were recorded from frog neuromuscular junctions blocked with high Mg and/or low Ca. 2. Estimates of f(t), the facilitation contributed by each impulse, were obtained during and following repetitive stimulation from the incremental change in e.p.p. amplitudes following step changes in the stimulation rate during the conditioning and testing trains. 3. Estimates of f(t) increased during the conditioning stimulation and returned to control in the post-tetanic period. This increase in f(t) was proportional to the magnitude of tetanic or post-tetanic potentiation (PTP) present. 4. These results are described by: [Formula: see text] where h(t)/h(c) is the e.p.p. amplitude at time t expressed in terms of the control, P(t) is potentiation, F(t) is facilitation and 1 is the base level of transmitter release. Thus, potentiation has a multiplicative (gain) effect on facilitation and the base level of transmitter release. 5. PTP was present immediately following the conditioning train. However, if depression occurred during the conditioning train, PTP developed after a delay. 6. It is suggested that facilitation and potentiation represent increases in two independent factors which act jointly to increase the probability of transmitter release

Testing for microscopic reversibility in the gating of maxi K+ channels using two-dimensional dwell-time distributions.

An assumption usually made when developing kinetic models for the gating of ion channels is that the transitions among the various states involved in the gating obey microscopic reversibility. If this assumption is incorrect, then the models and estimated rate constants made with the assumption would be in error. This paper examines whether the gating of a large conductance Ca-activated K+ channel in skeletal muscle is consistent with microscopic reversibility. If microscopic reversibility is obeyed, then the number of forward and backward transitions per unit time for each individual reaction step will, on average, be identical and, consequently, the gating must show time reversibility. To look for time reversibility, two-dimensional dwell-time distributions of the durations of open and closed intervals were obtained from single-channel current records analyzed in the forward and in the backward directions. Two-dimensional dwell-time distributions of pairs of open intervals and of pairs of closed intervals were also analyzed to extend the resolution of the method to special circumstances in which intervals from different closed (or open) states might have similar durations. No significant differences were observed between the forward and backward analysis of the two-dimensional dwell-time distributions, suggesting time reversibility. Thus, we find no evidence to indicate that the gating of the maxi K+ channel violates microscopic reversibility