Active and inactive central synapses in cell culture

On learning time delays between the spikes from different input neurons in a biophysical model of a pyramidal neuron

Biological systems are able to recognise temporal sequences of stimuli or compute in the temporal domain. In this paper we are exploring whether a biophysical model of a pyramidal neuron can detect and learn systematic time delays between the spikes from different input neurons. In particular, we investigate whether it is possible to reinforce pairs of synapses separated by a dendritic propagation time delay corresponding to the arrival time difference of two spikes from two different input neurons. We examine two subthreshold learning approaches where the first relies on the backpropagation of EPSPs (excitatory postsynaptic potentials) and the second on the backpropagation of a somatic action potential, whose production is supported by a learning-enabling background current. The first approach does not provide a learning signal that sufficiently differentiates between synapses at different locations, while in the second approach, somatic spikes do not provide a reliable signal distinguishing arrival time differences of the order of the dendritic propagation time. It appears that the firing of pyramidal neurons shows little sensitivity to heterosynaptic spike arrival time differences of several milliseconds. This neuron is therefore unlikely to be able to learn to detect such differences.

Single-pixel optical fluctuation analysis of calcium channel function in active zones of motor nerve terminals

We used high-resolution fluorescence imaging and single-pixel optical fluctuation analysis to estimate the opening probability of individual voltage-gated calcium (Ca(2+)) channels during an action potential and the number of such Ca(2+) channels within active zones of frog neuromuscular junctions. Analysis revealed ∼36 Ca(2+) channels within each active zone, similar to the number of docked synaptic vesicles but far less than the total number of transmembrane particles reported based on freeze-fracture analysis (∼200-250). The probability that each channel opened during an action potential was only ∼0.2. These results suggest why each active zone averages only one quantal release event during every other action potential, despite a substantial number of docked vesicles. With sparse Ca(2+) channels and low opening probability, triggering of fusion for each vesicle is primarily controlled by Ca(2+) influx through individual Ca(2+) channels. In contrast, the entire synapse is highly reliable because it contains hundreds of active zones.

Postnatal development of dendrodendritic inhibition in the Mammalian olfactory bulb

The mitral–granule cell (MC–GC) reciprocal synapse is an important source of auto- and lateral-inhibition in the olfactory bulb (OB), and this local inhibition is critical for odor discrimination. We may gain insight into the role of MC autoinhibition in olfaction by correlating the functional development of the autoinhibition with the postnatal development of olfactory function. We have studied the functional development of the MC–GC reciprocal synapse using whole-cell patch-clamp recordings from MCs and GCs in acute OB slices from 3- to 30-day-old rats. The magnitude of dendrodendritic inhibition (DDI) measured by depolarizing a single MC and recording recurrent inhibition in the same cell increased up to the fifteenth day of life (P15), but dropped between P15 and P30. The initial increase and later decrease in DDI was echoed by a similar increase and decrease in the frequency of miniature inhibitory post-synaptic currents, suggesting an accompanying modulation in the number of synapses available to participate in DDI. The late decrease in DDI could also result, in part, from a decrease in GC excitability as well as an increase in relative contribution of N-methyl d-aspartate (NMDA) receptors to γ-amino butyric acid (GABA) release from GC synapses. Changes in release probability of GABAergic synapses are unlikely to account for the late reduction in DDI, although they might contribute to the early increase during development. Our results demonstrate that the functional MC–GC circuit evolves over development in a complex manner that may include both construction and elimination of synapses.

A spiking neuron model: applications and learning

This paper presents a biologically inspired, hardware-realisable spiking neuron model, which we call the Temporal Noisy-Leaky Integrator (TNLI). The dynamic applications of the model as well as its applications in Computational Neuroscience are demonstrated and a learning algorithm based on postsynaptic delays is proposed. The TNLI incorporates temporal dynamics at the neuron level by modelling both the temporal summation of dendritic postsynaptic currents which have controlled delay and duration and the decay of the somatic potential due to its membrane leak. Moreover, the TNLI models the stochastic neurotransmitter release by real neuron synapses (with probabilistic RAMs at each input) and the firing times including the refractory period and action potential repolarisation. The temporal features of the TNLI make it suitable for use in dynamic time-dependent tasks like its application as a motion and velocity detector system presented in this paper. This is done by modelling the experimental velocity selectivity curve of the motion sensitive H1 neuron of the visual system of the fly. This application of the TNLI indicates its potential applications in artificial vision systems for robots. It is also demonstrated that Hebbian-based learning can be applied in the TNLI for postsynaptic delay training based on coincidence detection, in such a way that an arbitrary temporal pattern can be detected and recognised. The paper also demonstrates that the TNLI can be used to control the firing variability through inhibition; with 80% inhibition to concurrent excitation, firing at high rates is nearly consistent with a Poisson-type firing variability observed in cortical neurons. It is also shown with the TNLI, that the gain of the neuron (slope of its transfer function) can be controlled by the balance between inhibition and excitation, the gain being a decreasing function of the proportion of inhibitory inputs. Finally, in the case of perfect balance between inhibition and excitation, i.e. where the average input current is zero, the neuron can still fire as a result of membrane potential fluctuations. The firing rate is then determined by the average input firing rate. Overall this work illustrates how a hardware-realisable neuron model can capitalise on the unique computational capabilities of biological neurons.

Quantal GABA release in hippocampal synapses: role of local Ca2+ dynamics within the single terminals

Results of recent studies dedicated to the mechanisms of neurotransmission at a single inhibitory synaptic terminal in cultured neurones support the hypothesis that multiple quanta of neurotransmitter are released during excitation of inhibitory and excitatory central synapses. This is an important consideration as previous less direct measurements have suggested that a synapse can release no more than one quantum. Neurotransmitter release during long stimuli may occur at certain times with maximal probability, keeping the mean inter-release interval constant. This interval is not determined directly by vesicle depletion and moreover, each release event is independent of previous ones. The recent data also suggest that constant Ca(2+) influx is an important determinant of neurotransmitter release. It is speculated that the neurotransmitter release is regulated by a superposition of two processes: a continuous homogeneous process, (i.e. background Ca(2+) influx), and a periodic process that acts as a synchronizing factor of the release at definite moments.

Intrinsic dynamics in neuronal networks. I. Theory

Many networks in the mammalian nervous system remain active in the absence of stimuli. This activity falls into two main patterns: steady firing at low rates and rhythmic bursting. How are these firing patterns generated? Specifically, how do dynamic interactions between excitatory and inhibitory neurons produce these firing patterns, and how do networks switch from one firing pattern to the other? We investigated these questions theoretically by examining the intrinsic dynamics of large networks of neurons. Using both a semianalytic model based on mean firing rate dynamics and simulations with large neuronal networks, we found that the dynamics, and thus the firing patterns, are controlled largely by one parameter, the fraction of endogenously active cells. When no endogenously active cells are present, networks are either silent or fire at a high rate; as the number of endogenously active cells increases, there is a transition to bursting; and, with a further increase, there is a second transition to steady firing at a low rate. A secondary role is played by network connectivity, which determines whether activity occurs at a constant mean firing rate or oscillates around that mean. These conclusions require only conventional assumptions: excitatory input to a neuron increases its firing rate, inhibitory input decreases it, and neurons exhibit spike-frequency adaptation. These conclusions also lead to two experimentally testable predictions: 1) isolated networks that fire at low rates must contain endogenously active cells and 2) a reduction in the fraction of endogenously active cells in such networks must lead to bursting.

Possibility of multiquantal transmission at single inhibitory synapse in cultured rat hippocampal neurons

Miniature, spontaneous and evoked inhibitory postsynaptic currents were studied using the whole-cell patch-clamp technique on synaptically connected cultured hippocampal neurons, at a holding potential of -75 mV. All experiments were done in tetrodotoxin-containing solution to exclude an action potential generation. Spontaneous miniature inhibitory postsynaptic currents were observed in Ca2+-free solution. The distribution of miniature inhibitory postsynaptic currents was skewed to larger current amplitudes and could be fitted reliably by one Gaussian with the mean at 10.0 +/- 1.2 pA (n = 7). Spontaneously occurring whole-cell spontaneous inhibitory postsynaptic currents were recorded in physiological solution (Ca2+ 2 mM). The average amplitude of spontaneously occurring currents depended on membrane potential and reversed at -18 +/- 5 mV (n = 5). The amplitude distribution of spontaneous inhibitory postsynaptic currents had one peak clearly detectable with the mean of 20.0 +/- 2.0 pA (n = 6) or 10.0 +/- 2.0 pA (n = 2). Inhibitory postsynaptic stimulus-evoked currents arose in responses to gradual activation of neurotransmitter release by direct extracellular electrical stimulation of a single presynaptic bouton by short depolarizing pulses. The current-voltage relation of the averaged amplitudes of evoked inhibitory postsynaptic currents was linear and reversed at potential predicted by the Nernst equation for corresponding intra- and extracellular Cl- concentrations. The time-course of decay of miniature, spontaneous and evoked inhibitory postsynaptic currents was fitted by a sum of two exponents and their time-constants were the same in the range of standard deviation. The stimulus-evoked inhibitory postsynaptic currents fluctuated with regard to the discrete aliquot values of their peak amplitudes in all the investigated synapses from a measurable minimum of about 8 pA to 200 pA. The evoked inhibitory postsynaptic currents were assumed as superimposition of statistically independent quantal events. Fitting amplitude histograms of evoked inhibitory postsynaptic currents with several Gaussian curves resulted in peaks that were equidistant with the mean space of 20 +/- 3 pA (n = 10), which was assumed as one quantum (quantum size) to construct the Poisson's distribution. A possibility of simultaneous multiquantal release at single inhibitory synapses of rat hippocampal neurons was discussed.

Excitatory amino acids and synaptic transmission in embryonic rat brainstem motoneurons in organotypic culture

We used brainstem motoneurons recorded in organotypic slice co-cultures maintained for more than 18 days in vitro, together with multibarrel ionophoretic applications of glutamate receptor agonists and bath applications of specific blocking agents, to study the responses of rat brainstem motoneurons to glutamate receptor activation, and the contribution of these receptors to synaptic transmission. Differentiated brainstem motoneurons in vitro are depolarized by glutamate, N-methyl-d-aspartate (NMDA) and dl-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) iontophoresis, and express NMDA, AMPA and also specific kainate receptors, as evidenced by (+/-)2-amino-5-phosphonovaleric acid (APV)- and (-)1-(4-aminophenyl)-3-methyl-carbamoyl-4-methyl-7, 8-methylenedioxy-3,4-dihydro-5H-2,3-benzo-diazepine [GYKI 53784 (LY303070)]-resistant depolarizations. Electrical stimulations applied to the dorsal part of the explant trigger excitatory synaptic potentials with latencies distributed in three regularly spaced groups. Excitatory postsynaptic potentials (EPSPs) in the earliest group have a similar latency and time course and correspond to monosynaptic activation. EPSPs in later groups have more scattered latencies and time courses and correspond to polysynaptic activation. Monosynaptic EPSPs are insensitive to the specific NMDA blocker APV, and are completely and reversibly suppressed by the non-competitive AMPA receptor antagonist GYKI 53784 (LY303070). Detailed analysis of the spontaneous excitatory synaptic activity shows that APV decreases the frequency of spontaneous EPSPs without modifying their shape or amplitude. We conclude that excitatory synapses on brainstem motoneurons in vitro are mainly activated through AMPA receptors (AMPA-Rs). NMDA receptors (NMDA-Rs) are present in the membrane, but are located either at extrasynaptic sites or silent synapses, and are not directly involved in synaptic transmission on motoneurons. On the contrary, NMDA receptors contribute to synaptic transmission within the premotor interneuronal network.

Quantitative ultrastructural analysis of hippocampal excitatory synapses

From three-dimensional reconstructions of CA1 excitatory synapses in the rodent hippocampus and in culture, we have estimated statistical distributions of active zone and postsynaptic density (PSD) sizes (average area approximately 0.04 micron2), the number of active zones per bouton (usually one), the number of docked vesicles per active zone (approximately 10), and the total number of vesicles per bouton (approximately 200), and we have determined relationships between these quantities, all of which vary from synapse to synapse but are highly correlated. These measurements have been related to synaptic physiology. In particular, we propose that the distribution of active zone areas can account for the distribution of synaptic release probabilities and that each active zone constitutes a release site as identified in the standard quantal theory attributable to Katz (1969).

Presynaptically silent synapses: spontaneously active terminals without stimulus-evoked release demonstrated in cortical autapses

This study addresses the question of whether synapses that are capable of releasing transmitters spontaneously can also release them in an excitation-dependent manner. For this purpose, whole cell patch recordings were performed for a total of 48 excitatory solitary neurons in a microisland culture to observe excitatory autaptic currents elicited by spontaneous transmitter release as well as by somatic excitation. A somatic Na+-spike, induced in response to a short voltage step, evoked excitatory postsynaptic currents (EPSCs) of various amplitudes through the autapses; in some cases, no response was noticeable. To make sure that the recorded autaptic spontaneous EPSCs (sEPSCs) under a voltage clamp resulted from independent release of transmitters and were not associated with action potentials, sEPSCS in the presence and absence of tetrodotoxin (TTX) were compared in six cells. In the presence of TTX the evoked EPSCs were completely eliminated, whereas the sEPSCs were still observed and the amplitude distribution histograms were statistically not different from those recorded in the absence of TTX. A quantitative analysis of the sEPSCs (presumably miniature EPSCs) showed that the amplitude of stimulus-evoked EPSCs did not correlate with either the frequency or median amplitudes of the sEPSCs or the age of the culture. To identify whether the absence of stimulus-evoked response was caused either by conduction failure of excitation along the axons or by impairment of the release machinery that links the terminal depolarization to vesicle exocytosis, we examined whether high K+ and hypertonic solutions could facilitate the spontaneous release of transmitters. Although the hypertonic solution increased the spontaneous release in all cells tested (n = 18), the high K+ solution had a differential effect in increasing spontaneous release, i.e., the cells with larger evoked responses were more readily facilitated by the high K+ solution. Because the high K+ solution induced depolarization of presynaptic terminals, the present results indicated that the smaller evoked responses were due to the larger number of impaired or "silent" presynaptic terminals that were unable to link presynaptic depolarization to transmitter release. In summary, the present experiments provided evidence that at least some of the presynaptic terminals are silent in response to stimuli, while remaining spontaneously active at the same time. Because this phenomenon is due to the lack of sensitivity to depolarization at the terminals, these synaptic terminals seem incapable of linking terminal depolarization to transmitter release.

Biologically plausible neural computation

The function of a neuron can be described simultaneously at several levels of abstraction. For instance, a spike train represents the result of a computation done by a single neuron with its inputs, but it also represents the result of a complex function realized by the network in which the neuron is embedded. When models of large parts of the brain are considered, it may be desirable to use computational modules operating at a very abstract level. However, it is shown here that abstract neural functions depend on detailed features of the single neuron model used in the network reproducing the abstract function. Examples are given of the multiplicative function, motion detection, short-term memory and timing. All these operations rely on one or another feature of the extended Leaky Integrate-and-Fire neuron used in this paper, e.g. probabilistic synapses, post-synaptic currents modelled with alpha functions or partial reset of the somatic membrane. Consequently it is suggested that neural modelling at an abstract level does not obviate the need for a clear statement on the nature of the underlying model of biological neuron. In that sense, not many abstract functions are convincingly grounded, not even the standard formal neurons used in most artificial neural networks.

Differential modulation of chemical and electrical components of mixed synapses in the lobster stomatogastric ganglion

1. Two pairs of neurons in the pyloric network of the spiny lobster, Panulirus interruptus, communicate through mixed graded chemical and rectifying electrical synapses. The anterior burster (AB) chemically inhibits and is electrically coupled to the ventricular dilator (VD); the lateral pyloric (LP) and pyloric (PY) neurons show reciprocal chemical inhibition and electrical coupling. We examined the effects of dopamine (DA), serotonin (5HT) and octopamine (Oct) on these mixed synapses to determine the plasticity possible with opposing modes of synaptic interaction. 2. Dopamine increased net inhibition at all three pyloric mixed synapses by both reducing electrical coupling and increasing chemical inhibition. This reversed the sign of the net synaptic interaction when electrotonic coupling dominated some mixed synapses, and activated silent chemical components of other mixed synapses. 3. Serotonin weakly enhanced LP-->PY net inhibition, by reducing electrical coupling without altering chemical inhibition. Serotonin reduced AB-->VD electrical coupling, but variability in its effect on the chemical component made the net effect non-significant. 4. Octopamine enhanced LP-->PY and PY-->LP net inhibition by enhancing the chemical inhibitory component without altering electrical coupling. 5. Differential modulation of chemical and electrical components of mixed synapses markedly changes the net synaptic interactions. This contributes to the flexible outputs that modulators evoke from anatomically defined neural networks.

Quantal analysis of hippocampal long-term potentiation

Long-term potentiation (LTP) is a lasting (hours, days) increase in electrical responses after brief (seconds) high-frequency activation of monosynaptic pathways. It represents a popular model to study mechanisms of learning and memory. There is a general agreement on mechanisms of LTP induction, at least for LTP in hippocampal area CA1. However, a controversy exists about mechanisms of LTP maintenance: there is evidence for both pre- and postsynaptic locations of LTP mechanisms. Publications on statistical (quantal) analysis of fluctuations of excitatory postsynaptic potentials in hippocampal and some other structures are reviewed. The analysis suggests two independent mechanisms for LTP maintenance during the first hour. They are termed LTPm and LTPv and are expressed as changes in the mean number of transmitter quanta or quantal content (m) and changes in the effect of one quantum or quantal size (v), respectively. The increased number of transmitter quanta per presynaptic impulse (LTPm) can account for the many-fold increase in synaptic efficacy during LTP, especially when initially "silent" connections increase their release probabilities (p). The increase in the number of effective release sites is considered to be secondary to the increase in p. Appearance of new subsynaptic receptors, which can produce an apparent increase in m, is not excluded. The additional mechanism (LTPv) can account for an essential part of potentiation when the LTP magnitude is relatively small (< 60% increase over pretetanic amplitude). Experiments with paired-pulse facilitation support postsynaptic mechanisms for quantization and for LTPv. Intriguing problems for future statistical analysis of quantal synaptic mechanisms for behavioral memory and conditioning are understanding the different mechanisms for induction of LTPm and LTPv, and their contribution to the maintenance of LTP during post-tetanic periods of > 1 hour.

Nonuniform probability of glutamate release at a hippocampal synapse

A change in the probability of neurotransmitter release (Pr) is an important mechanism underlying synaptic plasticity. Although Pr is often assumed to be the same for all terminals at a single synapse, this assumption is difficult to reconcile with the nonuniform size and structure of synaptic terminals in the central nervous system. Release probability was measured at excitatory synapses on cultured hippocampal neurons by analysis of the progressive block of N-methyl-D-aspartate receptor-mediated synaptic currents by the irreversible open channel blocker MK-801. Release probability was nonuniform (range of 0.09 to 0.54) for terminals arising from a single axon, the majority of which had a low Pr. However, terminals with high Pr are more likely to be affected by the activity-dependent modulation that occurs in long-term potentiation.

Modeling of the quantal release at interneuronal synapses: analysis of permissible values of model moments

A theoretical study of effects of the different factors on fluctuation of post-synaptic potential (PSP) amplitudes was undertaken, using computation of regions of permissible values (RPV) of the ratio between the variance and the mean number of the quanta released (R1) and the ratio between the third moment and the variance (R2). The RPVs of these indexes for the binomial model were compared with regions determined for a number of models incorporating several factors. It has been shown that the involvement of temporal non-uniformity of transmitter release probability, decremental spreading of potentials along dendrites, and failure of spike propagation give the values of skewness index R2 less, compared to the binomial model. Simultaneously, a number of other factors, especially spatial non-uniformity of release probabilities in single release sites, would give amplitude histograms with high positive values of the index. The values of R1 and R2, calculated for 21 samples of sensorimotor EPSP amplitudes, were biased from RPV of these parameters constructed for the binomial model. The scattergram of R1 and R2 can be explained by the presence of two kinds of contacts which release quantum with different probabilities. The same was true for the beta-model based on the assumption that probabilities of quantal release are a sample of values of random variable that has beta-distribution. From analysis of the distribution of individual release probabilities, obtained from evaluation of beta-model parameters, is concluded that a greater part of boutons in the sensorimotor synapses release transmitter with very low probabilities, there being, however, a few boutons with probabilities close to 1.

Analysis of fluctuations of "minimal" excitatory postsynaptic potentials during long-term potentiation in guinea pig hippocampal slices

In previous studies, quantal analysis assuming a simple binomial model has shown that long-term potentiation (LTP) is accompanied by an increase in both mean quantal content (m) and quantal size (v), whereby the increase in m predominates. In the present study, "compound" binomial distributions with variable probabilities were convolved with Gaussian distributions in computer experiments to simulate amplitude histograms of intracellular excitatory postsynaptic potentials (EPSPs). A deconvolution procedure assuming equal "quantal" separation (v) between discrete components, but without assuming binomial statistics, was applied to the simulated distributions to determine v. It was found that with a small ratio of standard deviation of noise to v (Sn/v less than 0.4), a reliable estimate of v can be obtained even for small samples (N = 100). When Sn/v was larger (0.4-0.6), approximate v estimates (within +/- 10-20% of the simulated v) could be obtained by averaging estimates from about 10 small samples (N = 100). "Minimal" EPSPs were recorded in area CA1 of guinea pig hippocampal slices. 37 EPSP amplitude samples of 9 neurones were measured before and up to 55 min after 10 tetanizations of stratum radiatum. In accordance with the previous data, the increase in v accounted for only about 10% of the average post-tetanic increase in EPSP amplitude and was not correlated with the latter. However, for an EPSP subset with small LTP magnitude, the increase in v accounted for an essential part of the LTP magnitude while the increase in m did not correlate with it. The results are in agreement with previous data obtained in the context of the simple binomial model and are interpreted as indicating primarily a presynaptic mechanism of LTP maintenance. The results suggest two types of synaptic mechanism of LTP maintenance related to the increases in m and v, respectively. The latter mechanism is saturated at about 10 to 30% increase in post-tetanic amplitude above the pre-tetanic EPSP amplitude.

Statistical analysis of long-term potentiation of large excitatory postsynaptic potentials recorded in guinea pig hippocampal slices: binomial model

Excitatory postsynaptic potentials (EPSPs) were recorded in guinea pig hippocampal slices (area CA1) from 15 neurons after stimulation of stratum radiatum (str. rad.) and stratum oriens. EPSP amplitudes increased in 8 neurones (10 post-tetanic regions) recorded 15 to 45 min after tetanic stimulation of str. rad. The increase was considered to represent long-term potentiation (LTP). Quantal analysis was performed by two methods assuming binomial statistics: the histogram method using deconvolution of noise and the variance method. According to both methods, LTP was associated with an increase in mean quantal content (m) which correlated with LTP magnitude. A statistically significant increase in quantal size (v) was found only by the histogram method and the increase was not correlated with LTP magnitude. A separate analysis of EPSPs with small LTP magnitude demonstrated that with the histogram method only v was increased but not m. A smaller increase in m for the pooled data of both methods did not correlate with LTP magnitude for this EPSP subset. The increase in m for the whole EPSP set corresponds to previous results on the quantal analysis of LTP in in vivo preparations and favours a presynaptic location of major mechanisms underlying LTP maintenance. The increase in v indicates the existence of another mechanism responsible for the maintenance of a small part of LTP. This mechanism might involve either pre- or postsynaptic changes or both.

Quantal components of the synaptic potential induced in hippocampal neurons by activation of granule cells, and the effect of 2-amino-4-phosphonobutyric acid

The probabilistic nature of excitatory postsynaptic potentials (EPSPs) induced monosynaptically in CA3 neurons by impulses of granule cells was studied in thin transverse sections of the guinea pig hippocampus. More than 600 EPSPs were recorded under several conditions, their amplitudes were measured, and histograms representing the EPSP amplitude distribution were constructed. Quantal parameters were estimated by the method of maximum likelihood. Of 9 neurons examined in the control solution, one neuron showed an exceptionally large number of transmission failures. The amplitude distribution of EPSPs recorded from this neuron could be described by Pascal statistics, but not by binomial or Poisson statistics. The EPSP amplitude distribution from the other neurons could be described by either binomial, Poisson, or Pascal predictions with a minor preference for the last statistic. When an apparently homogeneous group of data was divided into two subgroups and parameters were estimated separately, inconsistent values were obtained in some neurons with no failures. 2-Amino-4-phosphonobutyric acid (APB) suppressed the EPSPs reversibly at relatively low concentrations. Theoretical curves calculated according to the Pascal statistics fit quite well to the entire amplitude distribution of EPSPs recorded under the action of APB. The suppression of EPSPs by APB was accompanied by a marked decrease in mean quantal content (m) with no significant reduction in mean quantal amplitude (q). A quantum induced an increase in membrane conductance of about 150 pS. These results suggest that the release probability of the mossy fiber terminal fluctuates temporally according to a gamma distribution, and that APB reduces the liberation of the transmitter from mossy fiber terminals, thereby suppressing transmission between mossy fibers and CA3 neurons.

Modulation of conduction at points of axonal bifurcation by applied electric fields

This study investigated how weak electric fields, on the order of 100 mV/cm, modulate action potential conduction through points of axonal bifurcation in leech touch sensory neurons. Axonal branch points in neurons are ubiquitous structures, and they are sites of low safety-factor for action potential propagation. In this study calibrated electric fields were applied around excised ganglia from the leech central nervous system. The electric fields were generated by 500 ms constant current square waves applied to the bath containing the tissue. Microelectrode penetration of the neurons was used to: 1) record transmembrane potential changes in the cell body of the neuron that resulted from the external field; 2) monitor conduction block when action potentials, evoked in the periphery, propagated into the ganglion; 3) inject current directly into the cell in an experimental analysis of the mechanism by which the externally applied field produced block. Conduction block was reliably induced by electric fields too weak to reach threshold for firing action potentials. In an experimental analysis where block was produced by the direct intracellular injection of negative current, a reversed polarity field relieved it. This indicates that when the external field induces block, it does so by membrane hyperpolarization at the branch point.

Thyrotropin-releasing hormone has profound presynaptic action on cultured spinal cord neurons

Thyrotropin-releasing hormone (TRH) receptors are widely distributed throughout the nervous system. In particular, both the dorsal and the ventral horn (VH) neurons contain a rich distribution of TRH receptors, and TRH application to these sites has profound physiological effects. Currently the mechanism of action of TRH is not known. We examined the effect of TRH on ventral horn neurons using intracellular and patch-clamp techniques. Our results indicate that TRH application profoundly increases the firing rate of VH cells by decreasing membrane conductance. More importantly, TRH causes a significant increase in frequency and amplitude of postsynaptic potentials. Under voltage-clamp condition, TRH reduces holding current and causes a significant increase in the rate of occurrence and the amplitude of excitatory postsynaptic currents (EPSCs), an effect that lasts for more than 5 minutes. This effect of TRH is not observed in cultured neurons pretreated with tetanus toxin. TRH also fails to alter the characteristics of the EPSCs when it is applied to a region of the cell that is sparsely innervated. These results provide strong evidence that presynaptic mechanisms have a significant role in the excitatory effect of TRH on the VH neurons. Because there is evidence that trophic factors are released from presynaptic terminals, by increasing synaptic activity, TRH can have a trophic influence on the spinal cord neurons. In addition, because there are a significant number of TRH containing neurons within the spinal cord, it is likely that TRH has a major role in information processing within the spinal cord.

A statistical test for demonstrating a presynaptic site of action for a modulator of synaptic amplitude

A statistical technique for demonstrating a presynaptic site of action for a modulator of synaptic amplitude was developed and tested. It requires that multiple measurements of peak synaptic amplitude be made under control and test conditions. The ratio of the coefficients of variation (CV) obtained under test and control conditions is calculated. A method was developed for determining the confidence interval for the CV ratio (CVR) statistic based on the null hypothesis that the synaptic modulation is purely postsynaptic. If the measured CVR falls outside the confidence interval, this implies that the modulator of synaptic amplitude is, at least in part, acting at a presynaptic site. The effectiveness of the technique and its limitations were investigated using Monte Carlo simulations. It was found to be sensitive and reliable under a variety of realistic recording conditions. The test was effective even in the presence of simulated presynaptic rundown of the synaptic response. Conventional deconvolution analysis was also applied to the Monte Carlo simulations and was found to be an inadequate indicator of the site of synaptic modulation when the discrete amplitude components were not well resolved. The CVR technique was applied to excitatory postsynaptic currents (epsc) recorded between pairs of cultured hippocampal neurones in control and test media containing 1 mM Ca2+ and 2 mM Ca2+, respectively. Test conditions increased the average synaptic amplitude, and the statistical analysis indicated that this modulation was produced by an action at a presynaptic site.

Computer software for development and simulation of compartmental models of neurons

A software package 'Nodus' for simulation and development of compartmental models of neurons is described. Passive or excitable membranes with voltage dependent ion conductances or synaptic conductances can be modeled. Detailed simulations of morphology and electrophysiology of neurons are possible. Neurophysiological experiments like voltage clamps and complex current injections can be simulated. Two integration methods are available: a fast hybrid method and an accurate fifth order Runge-Kutta method, with variable time steps. Nodus is implemented on Apple Macintosh microcomputers, with the standard user interface and interactive graphics. Simulations of simple test models demonstrate the accuracy and computation speed of the application.

Voltage clamping with single microelectrodes: comparison of the discontinuous mode and continuous mode using the Axoclamp 2A amplifier

The voltage clamp technique is a powerful method for studying the physiology of excitable membrane. This technique has made possible the determination of ionic responses generated by activation of either receptor-mediated or voltage-dependent processes. The development of the whole-cell, 'tight-seal' voltage clamp method has allowed the analysis and examination of membrane physiology at the single cell level. The method allows the characterization of voltage-dependent ionic conductances both at the macroscopic (whole-cell) and at the microscopic (unitary conductance or single channel) level in cells less than 10 micron in diameter, a feat difficult to achieve with 'conventional' fine-tipped micropipettes. In this paper, several methologies used for culturing neuronal and non-neuronal cells in the laboratory are described. A comparison between the two modes of voltage clamp using blunt-tipped 'patch'-microelectrodes, the switching (discontinuous) and the non-switching (continuous) modes, of the Axoclamp-2A amplifier is made. Some results on membrane currents obtained from neuronal and non-neuronal cells using the single electrode whole-cell 'tight-seal' voltage clamp is illustrated. The possible existence of two inactivating K+ currents, one dependent on Ca++ the other is not, is discussed.

Synaptic transmission between rat cerebellar granule and Purkinje cells in dissociated cell culture: effects of excitatory-amino acid transmitter antagonists

Monosynaptic excitatory connections between cerebellar granule and Purkinje cells were studied in dissociated cell cultures, and identification of the transmitter and the postsynaptic receptor at this synapse was pharmacologically investigated. The presynaptic granule cell and the postsynaptic Purkinje cell were voltage- or current-clamped simultaneously, and the excitatory postsynaptic current induced by the granule cell was examined. The neurons and monosynaptic excitatory connections were identified as in our earlier study. Several pairs of granule and Purkinje cells were stained with Lucifer yellow and propidium iodide, respectively, and their morphology was examined after electrophysiological recording. The monosynaptic excitatory postsynaptic current was suppressed by 1 mM kynurenate, an antagonist for excitatory-amino acid receptors, but was little affected by 0.2 mM DL-2-amino-5-phosphonovalerate, a selective antagonist of N-methyl-D-aspartate receptors. Glutamate and aspartate induced inward current in the Purkinje cells. These currents were suppressed by kynurenate at 1 mM. DL-2-Amino-5-phosphonovalerate at 0.2 mM suppressed the inward current induced by 100 microM aspartate but did not affect the inward current induced by 10 microM glutamate. These results are consistent with the idea that glutamate, or a glutamate-like substance, but not aspartate is the transmitter released at the synapse between granule and Purkinje cells and that non-N-methyl-D-aspartate receptor channels are functioning in the postsynaptic membrane.