Henri Korn

Calcium influx through nicotinic receptor in rat central neurons: Its relevance to cellular regulation

The Ca2+ permeability of a nicotinic acetylcholine receptor (nAChR) in the rat CNS was determined using both current and fluorescence measurements on medial habenula neurons. The elementary slope conductance of the nAChR channel was 11 pS in pure external Ca2+ (100 mM) and 42 pS in standard solution. Ca2+ influx through nAChRs resulted in the rise of cytosolic Ca2+ concentration ([Ca2+]i) to the micromolar range. This increase was maximal under voltage conditions (below -50 mV) in which Ca2+ influx through voltage-activated channels was minimal. Ca2+ influx through nAChRs directly activated a Ca(2+)-dependent Cl- conductance. In addition, it caused a decrease in the GABAA response that outlasted the rise in [Ca2+]i. These results underscore the physiological significance of Ca2+ influx through nAChR channel in the CNS.

Quantal analysis and synaptic efficacy in the CNS.

Quantal analysis of synaptic transmission at connections between neurons in the CNS has provided insights concerning the structural constraints on transmitter release and postsynaptic responsiveness. However, it has proven difficult in many cases to resolve the size and variability of a single quantum or to distinguish clear peaks in amplitude histograms of evoked responses, due in part to the superposition of background instrumental and biological noise. These limitations raise questions about recent attempts to use direct or indirect methods of quantal analysis in order to distinguish between pre- and postsynaptic loci of the modifications underlying long-term potentiation, particularly since the interpretations are model-dependent and the statistical treatments and experimental techniques employed incorporate simplifying assumptions not yet proven.

Is there chaos in the brain? I. Concepts of nonlinear dynamics and methods of investigation

In the light of results obtained during the last two decades in a number of laboratories, it appears that some of the tools of nonlinear dynamics, first developed and improved for the physical sciences and engineering, are well-suited for studies of biological phenomena. In particular it has become clear that the different regimes of activities undergone by nerve cells, neural assemblies and behavioural patterns, the linkage between them, and their modifications over time, cannot be fully understood in the context of even integrative physiology, without using these new techniques. This report, which is the first of two related papers, is aimed at introducing the non expert to the fundamental aspects of nonlinear dynamics, the most spectacular aspect of which is chaos theory. After a general history and definition of chaos the principles of analysis of time series in phase space and the general properties of chaotic trajectories will be described as will be the classical measures which allow a process to be classified as chaotic in ideal systems and models. We will then proceed to show how these methods need to be adapted for handling experimental time series; the dangers and pitfalls faced when dealing with non stationary and often noisy data will be stressed, and specific criteria for suspecting determinism in neuronal cells and/or assemblies will be described. We will finally address two fundamental questions, namely i) whether and how can one distinguish, deterministic patterns from stochastic ones, and, ii) what is the advantage of chaos over randomness: we will explain why and how the former can be controlled whereas, notoriously, the latter cannot be tamed. In the second paper of the series, results obtained at the level of single cells and their membrane conductances in real neuronal networks and in the study of higher brain functions, will be critically reviewed. It will be shown that the tools of nonlinear dynamics can be irreplaceable for revealing hidden mechanisms subserving, for example, neuronal synchronization and periodic oscillations. The benefits for the brain of adopting chaotic regimes with their wide range of potential behaviours and their aptitude to quickly react to changing conditions will also be considered.

Characteristics of miniature inhibitory postsynaptic currents in CA1 pyramidal neurones of rat hippocampus.

1. Recordings were made in vitro from chloride‐loaded CA1 rat hippocampal pyramidal neurones in the presence of tetrodotoxin (TTX) to examine miniature inhibitory postsynaptic currents (IPSCs). 2. Most spontaneous synaptic events recorded before TTX was applied, and all events that were resolved in the presence of TTX, were blocked by the GABAA receptor antagonist bicuculline. 3. At 25 degrees C, averaged miniature IPSCs had time to peak of about 3 ms and in most cases decayed with a single time constant close to 25 ms. 4. With a driving force for chloride ions between 70 and 80 mV, the mean miniature IPSC amplitude was 19.6‐27.9 pA, yielding a conductance of 258‐326 pS. The mean amplitude of unitary IPSCs recorded before TTX was applied was in the range of 31‐73 pA. 5. When intervals between miniature IPSCs were compared with an exponential distribution, there was an excess of events at intervals shorter than 5 ms. Some individual events appeared to represent the nearly simultaneous release of two inhibitory quanta. 6. Miniature IPSC amplitude distributions were better fitted with the sum of two Gaussians than with one Gaussian. The variance in amplitude of a single quantal event exceeded that of the baseline noise. 7. Comparison of the conductance changes corresponding to the first Gaussian distribution with single GABA channel data suggests that one inhibitory quantum opens twelve to twenty chloride channels and that GABA molecules bind once to a postsynaptic receptor.

Calcium mobilization elicited by two types of nicotinic acetylcholine receptors in mouse substantia nigra pars compacta

Nicotinic acetylcholine receptors (nAChRs) are expressed in the midbrain ascending dopaminergic system, a target of many addictive drugs. Here we assessed the intracellular Ca2+ level by imaging fura‐2‐loaded cells in substantia nigra pars compacta in mouse brain slices, and we examined the influence on this level of prolonged exposures to nicotine using mice lacking the nAChR β2‐subunit. In control cells, superfusion with nicotine (10–100 μm) caused a long‐lasting rise of intracellular Ca2+ level which depended on extracellular Ca2+. This nicotinic response was almost completely absent in β2–/– mutant mice, leaving a small residual response to a high concentration (100 μm) of nicotine which was inhibited by the α7‐subunit‐selective antagonist, methyllycaconitine. Conversely, the α7‐subunit‐selective agonist choline (10 mm) caused a methyllycaconitine‐sensitive increase in intracellular Ca2+ level both in wild‐type and β2–/– mutant mice. Nicotine‐elicited Ca2+ mobilization was reduced by the Na+ channel blocker tetrodotoxin (TTX) and by T‐type Ca2+ channel blocking agents, whereas the choline‐elicited Ca2+ increase was insensitive to TTX. Neither nicotine nor choline produced Ca2+ increase following inhibition of the release of Ca2+ from intracellular stores by dantrolene. These results demonstrate that in nigral dopaminergic neurons, nicotine can elicit Ca2+ mobilization via activation of two distinct nAChR subtypes: that of β2‐subunit‐containing nAChR followed by activation of Na+ channel and T‐type Ca2+ channels, and/or activation of α7‐subunit‐containing nAChR. The Ca2+ influx due to nAChR activation is subsequently amplified by the recruitment of intracellular Ca2+ stores. This Ca2+ mobilization may possibly contribute to the long‐term effects of nicotine on the dopaminergic system.

Automatic detection of spontaneous synaptic responses in central neurons

A fully automatized software package for detection and measurements of randomly occurring synaptic transients embedded in background noise is described. It is based on waveform recognition protocols, allows analysis of long data segments, and provides quantitative information about event amplitudes and kinetics. Simulated postsynaptic recordings have been used to assess its performance over a wide range of conditions mimicking those seen in physiological experiments.

A new method to estimate the Kolmogorov entropy from recurrence plots: its application to neuronal signals

Recurrence plots are a useful graphic tool for studying dynamical systems. A new variable is proposed to quantify their deterministic structure. It allows an easy computation of the K2 entropy since the distance between points needs to be calculated only once instead of every time for each new embedding dimension, and it bypasses some of the limitations inherent to the classical Grassberger and Procaccia method. It is valuable for biological data, as illustrated here by tests with various dynamical models and its application to experimental signals recorded from neurons.

Role of medullary networks and postsynaptic membrane properties in regulating Mauthner cell responsiveness to sensory excitation.

A benefit of studying well-defined networks at a cellular level is that it might be possible both to place these details in the context of the specific function of the network and to extract general principles applicable to more complex systems. The Mauthner cell system in teleosts is one such vertebrate network where a single impulse can trigger a vital escape reaction, the C start, in response to auditory or visual stimuli. We review here experiments concerned with the organization, at the cellular level, of the afferent circuits impinging on the Mauthner cell and with certain intrinsic membrane properties of the Mauthner cell that contribute to shaping the threshold and expression of the C start. One concept that emerges is related to the interaction between excitatory and inhibitory drives to the Mauthner cell. It seems that every major afferent drive to this neuron also excites a feedforward inhibitory network which, in turn, exerts a major role in establishing and regulating the threshold of the escape response. This design feature is complemented by the Mauthner cells membrane properties which contribute to the behavioral threshold but exhibit nonlinearities, as excitation begins to overcome inhibition. Finally, we have compared in detail the frequency-dependent characteristics of inhibition and excitation, as revealed by studies of individual identified synaptic connections. This comparison emphasizes the notion that although inhibition is maximized for weak transient stimuli, it becomes depressed at auditory stimulus frequencies that facilitate excitatory transmission and evoke the escape response.

Activation of human α1 and α2 homomeric glycine receptors by taurine and GABA

1 Two ligand binding α subunits, α1 and α2, of the human (H) glycine receptor (GlyR) are involved at inhibitory synapses in the adult and neonatal spinal cord, respectively. The ability of homomeric αH1 and αH2 GlyRs to be activated by glycine, taurine and GABA was studied in Xenopus oocytes or in the human embryonic kidney HEK‐293 cell line. 2 In outside‐out patches from HEK cells, glycine, taurine and GABA activated both GlyRs with the same main unitary conductance, i.e. 85 ± 3 pS (n= 6) for αH1, and 95 ± 5 pS (n = 4) for αH2. 3 The sensitivity of both αH1 and αH2 GlyRs to glycine was highly variable. In Xenopus oocytes the EC50 for glycine (EC50gly) was between 25 and 280 μm for αH1 (n= 44) and between 46 and 541 μm for αH2 (n= 52). For both receptors, the highest EC50gly values were found on cells with low maximal glycine responses. 4 The actions of taurine and GABA were dependent on the EC50gly: (i) their EC50 values were linearly correlated to EC50gly, with EC50tau≈ 10 EC50gly and EC50GABA≈ 500‐800 EC50gly; (ii) they could act either as full or weak agonists depending on the EC50gly. 5 The Hill coefficient (nH) of glycine remained stable regardless of the EC50gly whereas nH for taurine decreased with increasing EC50tau. 6 The degree of desensitization, evaluated by fast application of saturating concentrations of agonist on outside‐out patches from Xenopus oocytes, was similar for glycine and taurine on both GlyRs and did not exceed 50 %. 7 Our data concerning the variations of EC50gly and the subsequent behaviour of taurine and GABA could be qualitatively described by the simple del Castillo‐Katz scheme, assuming that the agonist gating constant varies whereas the binding constants are stable. However, the stability of the Hill coefficient for glycine was not explained by this model, suggesting that other mechanisms are involved in the modulation of EC50.

Interleukin-1β induces hyperpolarization and modulates synaptic inhibition in preoptic and anterior hypothalamic neurons

Most of the inflammatory effects of the cytokine interleukin 1beta (IL-1beta) are mediated by induction of cyclooxygenase (COX)2 and the subsequent synthesis and release of prostaglandin E2. This transcription-dependent process takes 45-60 min, but IL-1beta, a well-characterized endogenous pyrogen also exerts faster neuronal actions in the preoptic area/anterior hypothalamus. Here, we have studied the fast (1-3 min) signaling by IL-1beta using whole-cell patch clamp recordings in preoptic area/anterior hypothalamus neurons. Exposure to IL-1beta (0.1-1 nM) hyperpolarized a subset ( approximately 20%) of preoptic area/anterior hypothalamus neurons, decreased their input resistance and reduced their firing rate. These effects were associated with an increased frequency of bicuculline-sensitive spontaneous inhibitory postsynaptic currents and putative miniature inhibitory postsynaptic currents, strongly suggesting a presynaptic mechanism of action. These effects require the type 1 interleukin 1 receptor (IL-1R1), and the adapter protein myeloid differentiation primary response protein (MyD88), since they were not observed in cultures obtained from IL-1R1 (-/-) or from MyD88 (-/-) mice. Ceramide, a second messenger of the IL-1R1-dependent fast signaling cascade, is produced by IL-1R1-MyD88-mediated activation of the neutral sphingomyelinase. C2-ceramide, its cell penetrating analog, also increased the frequency of miniature inhibitory postsynaptic currents in a subset of cells. Both IL-1beta and ceramide reduced the delayed rectifier and the A-type K(+) currents in preoptic area/anterior hypothalamus neurons. The latter effect may account in part for the increased spontaneous inhibitory postsynaptic current frequency as suggested by experiments with the A-type K(+) channel blockers 4-aminopyridine. Taken together our data suggest that IL-1beta inhibits the activity of preoptic area/anterior hypothalamus neurons by increasing the presynaptic release of GABA.