The timing of the appearance of miniature end-plate potentials

A method of estimating the rate of miniature end-plate potential occurrences based on parametric time series modeling

This paper presents a parametric method of estimating the rate of miniature end-plate potential (MEPP) occurrences. We consider the case where the rate of MEPP occurrences is raised by the constant deporalization of presynaptic terminals by using high-concentration potassium solutions. Under such conditions, since MEPP occurrences cannot be identified by eye due to waveform superposition, it is necessary to estimate the rate with the aid of statistical techniques instead of counting the occurrences by eye. In this paper it is assumed according to the literatures that the MEPP data are modeled as a stationary Poisson impulse process filtered by the linear system the impulse response function of which is the sum of two exponentials. Then, the discretized MEPP data are shown to be a second-order autoregressive (AR(2)) process, driven by the sum of 2 first-order moving average (MA(1)) processes (the residual time series). An explicit formula for estimating the rate can be derived by combining the second- and third-order moments of the residual time series. The validity of the proposed estimation method is verified through Monte Carlo simulations in which the rate is varied ranging from 100 to 10,000 s-1. Likewise, the proposed method is applied to estimation of the rate of actual MEPP data, which were observed at the frog's neuromuscular junction under high-concentration potassium solutions.

Sensitization and multiplicative noise in the descending contralateral movement detector (DCMD) of the locust

Spike discharges from the descending contralateral movement detector (DCMD) were recorded extracellularly from the ventral nerve cord of the locust in complete darkness, in response to dim flashes of constant-intensity light, and in response to pairs of identical flashes presented different intervals apart. Three phenomena were discovered: novel long-term sensitization which changes the DCMD's sensitivity to light, a multiplicative cascade process driven by shot events, and the suppression of the spike discharge shortly after a dim flash. The DCMD's spike discharge is stochastic. It can be considered as a two-stage cascade process producing intrinsic multiplicative noise. An effective photon, or thermal isomerization in complete darkness, produces an unseen shot event which in turn initiates a random number of DCMD spikes in a cluster. A short initiates a variable number of spikes when it directs the rate of a Poisson process. The results of statistical analyses are consistent with this model when the amplitudes of shot events are variable. The transmission efficiency is low because at least 2.4-9.6 quantum bumps are required to produce one extra DCMD spike. The DCMD has a constant mean discharge rate of 0.25-1.5 spikes/s in complete darkness. Clustering about particular points in time (shots) leads to a lack of independence between interspike intervals, and the overdispersion of interspike interval and number distributions compared with those from a simple Poisson process. The mean cluster size is 1.3-1.6 spikes in darkness. Similar clustering was found in response to flashes of light. A dim flash changes the DCMD's sensitivity to light, even at threshold when no spike discharge results. Sensitization occurs because the average number of shot events produced by isoquantal flashes depends on the history of visual stimulation. This contributes to the nonlinear response-intensity function. The evolution of sensitization is roughly constant in different DCMD cells, lasting approximately 3 s after a flash. Sensitization was observed in response to light only, presumably because the intensity of dark-light is too low. It is proposed that sensitization is associated with a set of processes or molecular state in the presynaptic region of a chemical synapse.

Perpetual inhibitory activity in mammalian brain slices generated by spontaneous GABA release

Miniature spontaneous inhibitory postsynaptic currents (sIPSCs) mediated by GABAA receptors were recorded using whole-cell patch clamp recordings in rat brain slices maintained in vitro at 34 +/- 1 degree C. We have found that firing of action potentials by principal neurons or by GABAergic interneurons is not necessary to the generation of sIPSCs since they persist in the presence of 1-5 microM tetrodotoxin (TTX). The average frequency of the discrete sIPSCs exhibits a large cell-to-cell variability and is between 5-15 Hz. The amplitudes of the sIPSCs depend on the difference between the membrane potential and the equilibrium potential for Cl- (ECl). Generally, 70-80 mV away from ECl, sIPSCs have a mean amplitude of 30-80 pA (i.e. peak conductance of 400-1000 pS) with an average decay time constant of 5.8 ms. Accordingly, unitary single sIPSCs arise from the simultaneous activation of no more than 20 GABAA receptor/channels. The perpetual barrage of spontaneous GABAergic activity is very likely to be a critical factor in the regulation of neuronal excitability and the mechanism of action of several neuroactive compounds.

Characteristics of slow-miniature endplate currents show a subunit composition

The normal neuromuscular junction shows two classes of spontaneous miniature endplate potentials. These classes are based on a discontinuity in the profile of miniature endplate potential amplitude distributions. The amplitude of one class of miniature endplate potentials from a bell-shaped amplitude distribution and the remaining miniature endplate potentials compose a population which forms a left-hand skew distribution with a mode 1/7 to 1/10 that of the bell-miniature endplate potentials [Kriebel M. E. and Gross C. E. (1974) J. gen. Physiol, 64, 85-103]. Some skew-miniature endplate potentials have a slow time-to-peak and show breaks on the rising phase. Most treatments that alter the miniature endplate potential frequency change the ratio of skew-miniature endplate potentials/bell-miniature endplate potentials [Kriebel M. E. et al. (1976) J. Physiol. 262, 553-581]. The time characteristics of miniature endplate currents were readily altered in the isolated frog and mouse neuromuscular junctions with several agents known to increase the percentage of slow-miniature endplate potentials (heat, botulinum toxin, 4-aminoquinoline and increases in bath osmolarity). The slow-miniature endplate potential amplitudes were a continuum of amplitudes from skew- to giant miniature endplate potentials. The rising phases of miniature endplate potentials were a continuum from smooth to many with breaks and offsets. In a series of sequentially recorded slow-miniature endplate currents, many had congruent rising phases of constant slope regardless of amplitude or of time-to-peak. The rising phases of congruent slow-miniature endplate currents which showed a change in slope deviated at similar amplitudes. The least value of the slope of a slow-miniature endplate current was that of the sub-miniature endplate current; and, miniature endplate currents with overall lower slope values showed a wave pattern and/or irregular breaks which suggests summation of sequentially delayed sub-miniature endplate currents. Plots of the amplitude vs time-to-peak of miniature endplate currents from identified junctions demonstrated that the normal percentage of slow-miniature endplate currents was greatly increased with the treatments used here and that the time-to-peak of giant miniature endplate currents usually was longer than that of normally occurring bell-miniature endplate currents. Giant miniature endplate currents with short time-to-peak values are probably from two miniature endplate currents occurring, by chance, almost simultaneously. During and/or after treatments, miniature endplate currents formed clusters of similar size miniature endplate currents, not randomly distributed in time, which graded from distinct miniature endplate currents to giant miniature endplate currents.(ABSTRACT TRUNCATED AT 400 WORDS)

Verapamil, neuromuscular transmission and the nicotinic receptor

The effect of verapamil on neuromuscular transmission was studied in the frog by analysing ionophoretic endplate current (iEPC) trains and the growth and decay phases of miniature endplate currents (mepcs). In addition, single channel data on the interaction of verapamil with the nicotinic acetylcholine receptor were obtained from cultured embryonic chick skeletal muscle cells. Verapamil caused both open and closed channel blockade in the iEPC trains. Mepc amplitude was decreased at low micromolar concentrations, and at higher concentrations there was also accelerated mepc decay indicating open channel blockade. The latter effect could not be explained by a sequential channel occlusion mechanism. Analysis of the mepc rising phase showed that low micromolar concentrations of the drug decreased the pool of receptors which could be activated. Single channel data confirmed the specific interaction of verapamil with the nicotinic receptor, showing closed channel blockade at low concentrations, and at higher levels the shortening of open channel lifetime. It is suggested that both forms of blockade may involve desensitization processes.

Timing of seizure recurrence in adult epileptic patients: a statistical analysis

Seizure diaries were maintained prospectively in 24 epileptic patients (19 with partial complex, three with partial simple, and three with primary generalized seizures) who were selected consecutively, had stable seizure patterns, were reliable historians, and were known to be compliant with medications. Diaries were maintained for an average of 237 days (range, 61-365), and an average of 18 seizures were recorded per patient (range, 5-76). Seizure patterns were analyzed by using the methods appropriate for a time series of events (point process). Two patients had a decreasing trend in seizure frequency. For 12 patients, seizure occurrence was indistinguishable from that of a Poisson process. The remaining 10 patients had an exponential distribution of seizure intervals, but did not fit other criteria for a Poisson process; 3 of these showed evidence for seizure clustering; none showed evidence for a seizure cycle. It is concluded that the pattern of seizure occurrence in most epileptic people is random, but in approximately 50%, it is not occurring according to a Poisson process. These observations indicate that seizure cycling and/or clustering are not common in epileptic patients, but do not exclude the possibility that seizures have been precipitated by some randomly occurring event, such as sleep deprivation or increased stress.