Studying properties of neurotransmitter receptors by non-stationary noise analysis of spontaneous synaptic currents

A patient-derived mutation of epilepsy-linked LGI1 increases seizure susceptibility through regulating Kv1.1

BACKGROUND

Autosomal dominant lateral temporal epilepsy (ADLTE) is an inherited syndrome caused by mutations in the leucine-rich glioma inactivated 1 (LGI1) gene. It is known that functional LGI1 is secreted by excitatory neurons, GABAergic interneurons, and astrocytes, and regulates AMPA-type glutamate receptor-mediated synaptic transmission by binding ADAM22 and ADAM23. However, > 40 LGI1 mutations have been reported in familial ADLTE patients, more than half of which are secretion-defective. How these secretion-defective LGI1 mutations lead to epilepsy is unknown.

RESULTS

We identified a novel secretion-defective LGI1 mutation from a Chinese ADLTE family, LGI1-W183R. We specifically expressed mutant LGI1^W183R in excitatory neurons lacking natural LGI1, and found that this mutation downregulated K_v1.1 activity, led to neuronal hyperexcitability and irregular spiking, and increased epilepsy susceptibility in mice. Further analysis revealed that restoring K_v1.1 in excitatory neurons rescued the defect of spiking capacity, improved epilepsy susceptibility, and prolonged the life-span of mice.

CONCLUSIONS

These results describe a role of secretion-defective LGI1 in maintaining neuronal excitability and reveal a new mechanism in the pathology of LGI1 mutation-related epilepsy.

Inhibitory inputs to an inhibitory interneuron: Spontaneous postsynaptic currents and GABAA receptors of A17 amacrine cells in the rat retina

Amacrine cells constitute a large and heterogenous group of inhibitory interneurons in the retina. The A17 amacrine plays an important role for visual signaling in the rod pathway microcircuit of the mammalian retina. It receives excitatory input from rod bipolar cells and provides feedback inhibition to the same cells. However, from ultrastructural investigations, there is evidence for input to A17s from other types of amacrine cells, presumably inhibitory, but there is a lack of information about the identity and functional properties of the synaptic receptors and how inhibition contributes to the integrative properties of A17s. Here, we studied the biophysical and pharmacological properties of GABAergic spontaneous inhibitory postsynaptic currents (spIPSCs) and GABA_A receptors of A17 amacrines, using whole-cell and outside-out patch recordings from rat retinal slices. The spIPSCs displayed fast onsets (10-90% rise time ~740 μs) and double-exponential decays (τ_fast ~4.5 ms [43% of amplitude]; τ_slow ~22 ms). Ultrafast application of brief pulses of GABA (3 mM) to patches evoked responses with deactivation kinetics best fitted by a triple-exponential function (τ₁ ~5.3 ms [55% of amplitude]; τ₂ ~48 ms [32% amplitude]; τ₃ ~187 ms). Non-stationary noise analysis of spIPSCs and patch responses yielded single-channel conductances of ~21 and ~25 pS, respectively. Pharmacological analysis suggested that the spIPSCs are mediated by receptors with an α1βγ2 subunit composition and the somatic receptors have an α2βγ2 and/or α3βγ2 composition. These results demonstrate the presence of synaptic GABA_A receptors on A17s, which may play an important role in signal integration in these cells.

AMPA receptors at ribbon synapses in the mammalian retina: kinetic models and molecular identity

In chemical synapses, neurotransmitter molecules released from presynaptic vesicles activate populations of postsynaptic receptors that vary in functional properties depending on their subunit composition. Differential expression and localization of specific receptor subunits are thought to play fundamental roles in signal processing, but our understanding of how that expression is adapted to the signal processing in individual synapses and microcircuits is limited. At ribbon synapses, glutamate release is independent of action potentials and characterized by a high and rapidly changing rate of release. Adequately translating such presynaptic signals into postsynaptic electrical signals poses a considerable challenge for the receptor channels in these synapses. Here, we investigated the functional properties of AMPA receptors of AII amacrine cells in rat retina that receive input at spatially segregated ribbon synapses from OFF-cone and rod bipolar cells. Using patch-clamp recording from outside-out patches, we measured the concentration dependence of response amplitude and steady-state desensitization, the single-channel conductance and the maximum open probability. The GluA4 subunit seems critical for the functional properties of AMPA receptors in AII amacrines and immunocytochemical labeling suggested that GluA4 is located at synapses made by both OFF-cone bipolar cells and rod bipolar cells. Finally, we used a series of experimental observables to develop kinetic models for AII amacrine AMPA receptors and subsequently used the models to explore the behavior of the receptors and responses generated by glutamate concentration profiles mimicking those occurring in synapses. These models will facilitate future in silico modeling of synaptic signaling and processing in AII amacrine cells.

Maximum likelihood estimation of biophysical parameters of synaptic receptors from macroscopic currents

Dendritic integration and neuronal firing patterns strongly depend on biophysical properties of synaptic ligand-gated channels. However, precise estimation of biophysical parameters of these channels in their intrinsic environment is complicated and still unresolved problem. Here we describe a novel method based on a maximum likelihood approach that allows to estimate not only the unitary current of synaptic receptor channels but also their multiple conductance levels, kinetic constants, the number of receptors bound with a neurotransmitter, and the peak open probability from experimentally feasible number of postsynaptic currents. The new method also improves the accuracy of evaluation of unitary current as compared to the peak-scaled non-stationary fluctuation analysis, leading to a possibility to precisely estimate this important parameter from a few postsynaptic currents recorded in steady-state conditions. Estimation of unitary current with this method is robust even if postsynaptic currents are generated by receptors having different kinetic parameters, the case when peak-scaled non-stationary fluctuation analysis is not applicable. Thus, with the new method, routinely recorded postsynaptic currents could be used to study the properties of synaptic receptors in their native biochemical environment.

Methods for recording and measuring tonic GABAA receptor-mediated inhibition

Tonic inhibitory conductances mediated by GABA_A receptors have now been identified and characterized in many different brain regions. Most experimental studies of tonic GABAergic inhibition have been carried out using acute brain slice preparations but tonic currents have been recorded under a variety of different conditions. This diversity of recording conditions is likely to impact upon many of the factors responsible for controlling tonic inhibition and can make comparison between different studies difficult. In this review, we will firstly consider how various experimental conditions, including age of animal, recording temperature and solution composition, are likely to influence tonic GABA_A conductances. We will then consider some technical considerations related to how the tonic conductance is measured and subsequently analyzed, including how the use of current noise may provide a complementary and reliable method for quantifying changes in tonic current.

Two-photon imaging of nonlinear glutamate release dynamics at bipolar cell synapses in the mouse retina

Alpha/Y-type retinal ganglion cells encode visual information with a receptive field composed of nonlinear subunits. This nonlinear subunit structure enhances sensitivity to patterns composed of high spatial frequencies. The Y-cell's subunits are the presynaptic bipolar cells, but the mechanism for the nonlinearity remains incompletely understood. We investigated the synaptic basis of the subunit nonlinearity by combining whole-cell recording of mouse Y-type ganglion cells with two-photon fluorescence imaging of a glutamate sensor (iGluSnFR) expressed on their dendrites and throughout the inner plexiform layer. A control experiment designed to assess iGluSnFR's dynamic range showed that fluorescence responses from Y-cell dendrites increased proportionally with simultaneously recorded excitatory current. Spatial resolution was sufficient to readily resolve independent release at intermingled ON and OFF bipolar terminals. iGluSnFR responses at Y-cell dendrites showed strong surround inhibition, reflecting receptive field properties of presynaptic release sites. Responses to spatial patterns located the origin of the Y-cell nonlinearity to the bipolar cell output, after the stage of spatial integration. The underlying mechanism differed between OFF and ON pathways: OFF synapses showed transient release and strong rectification, whereas ON synapses showed relatively sustained release and weak rectification. At ON synapses, the combination of fast release onset with slower release offset explained the nonlinear response of the postsynaptic ganglion cell. Imaging throughout the inner plexiform layer, we found transient, rectified release at the central-most levels, with increasingly sustained release near the borders. By visualizing glutamate release in real time, iGluSnFR provides a powerful tool for characterizing glutamate synapses in intact neural circuits.

A novel whole-cell mechanism for long-term memory enhancement

Olfactory-discrimination learning was shown to induce a profound long-lasting enhancement in the strength of excitatory and inhibitory synapses of pyramidal neurons in the piriform cortex. Notably, such enhancement was mostly pronounced in a sub-group of neurons, entailing about a quarter of the cell population. Here we first show that the prominent enhancement in the subset of cells is due to a process in which all excitatory synapses doubled their strength and that this increase was mediated by a single process in which the AMPA channel conductance was doubled. Moreover, using a neuronal-network model, we show how such a multiplicative whole-cell synaptic strengthening in a sub-group of cells that form a memory pattern, sub-serves a profound selective enhancement of this memory. Network modeling further predicts that synaptic inhibition should be modified by complex learning in a manner that much resembles synaptic excitation. Indeed, in a subset of neurons all GABA_A-receptors mediated inhibitory synapses also doubled their strength after learning. Like synaptic excitation, Synaptic inhibition is also enhanced by two-fold increase of the single channel conductance. These findings suggest that crucial learning induces a multiplicative increase in strength of all excitatory and inhibitory synapses in a subset of cells, and that such an increase can serve as a long-term whole-cell mechanism to profoundly enhance an existing Hebbian-type memory. This mechanism does not act as synaptic plasticity mechanism that underlies memory formation but rather enhances the response of already existing memory. This mechanism is cell-specific rather than synapse-specific; it modifies the channel conductance rather than the number of channels and thus has the potential to be readily induced and un-induced by whole-cell transduction mechanisms.

Role of P2X4 receptors in synaptic strengthening in mouse CA1 hippocampal neurons

P2X4 receptors are calcium-permeable cation channels gated by extracellular ATP. They are found close to subsynaptic sites on hippocampal CA1 neurons. We compared features of synaptic strengthening between wild-type and P2X4 knockout mice (21–26 days old). Potentiation evoked by a tetanic presynaptic stimulus (100 Hz, 1 s) paired with postsynaptic depolarization was less in P2X4^−/− mice than in wild-type mice (230 vs. 50% potentiation). Paired-pulse ratios and the amplitude and frequency of spontaneous excitatory postsynaptic currents (EPSCs) were not different between wild-type and knockout mice. Prior hyperpolarization (ten 3 s pulses to −120 mV at 0.17 Hz) potentiated the amplitude of spontaneous EPSCs in wild-type mice, but not in P2X4^−/− mice; this potentiation was not affected by nifedipine, but was abolished by 10 mm 1,2-bis(o-aminophenoxy)ethane-N,N,N′,N′-tetra-acetic acid (BAPTA) in the recording pipette. The amplitude of N-methyl-d-aspartate EPSCs (in 6-cyano-7-nitroquinoxaline-2,3-dione, 10 or 30 μm, at −100 mV) facilitated during 20 min recording in magnesium-free solution. In wild-type mice, this facilitation of the N-methyl-d-aspartate EPSC was reduced by about 50% by intracellular BAPTA (10 mm), ifenprodil (3 μm) or 4-(4-fluorophenyl)-2-(4-methylsulphinylphenyl)-5-(4-pyridyl)1H-imidazole (5 μm). In P2X4^−/− mice, the facilitation was much less, and was unaffected by intracellular BAPTA, ifenprodil (3 μm) or mitogen-activated protein (MAP) kinase inhibitor 4-(4-fluorophenyl)-2-(4-methylsulphinylphenyl)-5-(4-pyridyl)1H-imidazole (5 μm). This suggests that the absence of P2X4 receptors limits the incorporation of NR2B subunits into synaptic N-methyl-d-aspartate receptors.

Profound desensitization by ambient GABA limits activation of δ-containing GABAA receptors during spillover

High-affinity extrasynaptic GABA(A) receptors (GABA(A)Rs) are a prominent feature of cerebellar granule neurons and thalamic relay neurons. In both cell types, the presence of synaptic glomeruli would be expected to promote activation of these GABA(A)Rs, contributing to phasic spillover-mediated currents and tonic inhibition. However, the precise role of different receptor subtypes in these two phenomena is unclear. To address this question, we made recordings from neurons in acute brain slices from mice, and from tsA201 cells expressing recombinant GABA(A)Rs. We found that δ subunit-containing GABA(A)Rs of both cerebellar granule neurons and thalamic relay neurons of the lateral geniculate nucleus contributed to tonic conductance caused by ambient GABA but not to spillover-mediated currents. In the presence of a low "ambient" GABA concentration, recombinant "extrasynaptic" δ subunit-containing GABA(A)Rs exhibited profound desensitization, rendering them insensitive to brief synaptic- or spillover-like GABA transients. Together, our results demonstrate that phasic spillover and tonic inhibition reflect the activation of distinct receptor populations.

Ideal observer analysis of signal quality in retinal circuits

The function of the retina is crucial, for it must encode visual signals so the brain can detect objects in the visual world. However, the biological mechanisms of the retina add noise to the visual signal and therefore reduce its quality and capacity to inform about the world. Because an organism's survival depends on its ability to unambiguously detect visual stimuli in the presence of noise, its retinal circuits must have evolved to maximize signal quality, suggesting that each retinal circuit has a specific functional role. Here we explain how an ideal observer can measure signal quality to determine the functional roles of retinal circuits. In a visual discrimination task the ideal observer can measure from a neural response the increment threshold, the number of distinguishable response levels, and the neural code, which are fundamental measures of signal quality relevant to behavior. It can compare the signal quality in stimulus and response to determine the optimal stimulus, and can measure the specific loss of signal quality by a neuron's receptive field for non-optimal stimuli. Taking into account noise correlations, the ideal observer can track the signal-to-noise ratio available from one stage to the next, allowing one to determine each stage's role in preserving signal quality. A comparison between the ideal performance of the photon flux absorbed from the stimulus and actual performance of a retinal ganglion cell shows that in daylight a ganglion cell and its presynaptic circuit loses a factor of approximately 10-fold in contrast sensitivity, suggesting specific signal-processing roles for synaptic connections and other neural circuit elements. The ideal observer is a powerful tool for characterizing signal processing in single neurons and arrays along a neural pathway.

Homosynaptic long-term synaptic potentiation of the "winner" climbing fiber synapse in developing Purkinje cells

During the developmental formation of neuronal circuits, redundant synapses are eliminated and persisting synapses strengthened. In the immature cerebellum, climbing fiber-Purkinje cell synapses undergo a pronounced synaptic rewiring, from a multiple innervation around birth to a mono-innervation in adults. An early stage of this process consists in the differentiation of initially equally strong synapses into one "large" and several "small" synaptic inputs. By performing whole-cell recordings in Purkinje cells of rat cerebellar slices, we found that the coincident activation of a Purkinje cell and one of its afferent climbing fibers induces homosynaptic long-term synaptic potentiation (LTP). This LTP requires postsynaptic Ca2+ signaling and involves an increase in the single channel conductance of the postsynaptic AMPA receptors. Interestingly, LTP occurs exclusively at large synaptic inputs. It is not observed at small inputs that are eventually eliminated. Thus, we identified a new form of LTP that is expressed uniquely and just for a restricted period of early development in the large climbing fiber inputs. Our results suggest that this LTP mediates the activity-dependent maturation of the "winner" climbing fiber.

Diminished A-type potassium current and altered firing properties in presympathetic PVN neurones in renovascular hypertensive rats

Accumulating evidence supports a contribution of the hypothalamic paraventricular nucleus (PVN) to sympathoexcitation and elevated blood pressure in renovascular hypertension. However, the underlying mechanisms resulting in altered neuronal function in hypertensive rats remain largely unknown. Here, we aimed to address whether the transient outward potassium current (I(A)) in identified rostral ventrolateral medulla (RVLM)-projecting PVN neurones is altered in hypertensive rats, and whether such changes affected single and repetitive action potential properties and associated changes in intracellular Ca(2+) levels. Patch-clamp recordings obtained from PVN-RVLM neurons showed a reduction in I(A) current magnitude and single channel conductance, and an enhanced steady-state current inactivation in hypertensive rats. Morphometric reconstructions of intracellularly labelled PVN-RVLM neurons showed a diminished dendritic surface area in hypertensive rats. Consistent with a diminished I(A) availability, action potentials in PVN-RVLM neurons in hypertensive rats were broader, decayed more slowly, and were less sensitive to the K(+) channel blocker 4-aminopyridine. Simultaneous patch clamp recordings and confocal Ca(2+) imaging demonstrated enhanced action potential-evoked intracellular Ca(2+) transients in hypertensive rats. Finally, spike broadening during repetitive firing discharge was enhanced in PVN-RVLM neurons from hypertensive rats. Altogether, our results indicate that diminished I(A) availability constitutes a contributing mechanism underlying aberrant central neuronal function in renovascular hypertension.

Studying properties of neurotransmitter receptors by non-stationary noise analysis of spontaneous postsynaptic currents and agonist-evoked responses in outside-out patches

Chemical synaptic transmission depends on neurotransmitter-gated ion channels concentrated in the postsynaptic membrane of specialized synaptic contacts. The functional characteristics of these neurotransmitter receptor channels are important for determining the properties of synaptic transmission. Whole-cell recording of postsynaptic currents (PSCs) and outside-out patch recording of transmitter-evoked currents are important tools for estimating the single-channel conductance and the number of receptors contributing to the PSC activated by a single transmitter quantum. When single-channel activity cannot be directly resolved, non-stationary noise analysis is a valuable tool for determining these parameters. Peak-scaled non-stationary noise analysis can be used to compensate for quantal variability in synaptic currents. Here, we present detailed protocols for conventional and peak-scaled non-stationary noise analysis of spontaneous PSCs and responses in outside-out patches. In addition, we include examples of computer code for individual functions used in the different stages of non-stationary noise analysis. These analysis procedures require 3-8 h.

Spontaneous IPSCs and glycine receptors with slow kinetics in wide-field amacrine cells in the mature rat retina

The functional properties of glycine receptors were analysed in different types of wide-field amacrine cells, narrowly stratifying cells considered to play a role in larger-scale integration across the retina. The patch-clamp technique was used to record spontaneous IPSCs (spIPSCs) and glycine-evoked patch responses from mature rat retinal slices (4-7 weeks postnatal). Glycinergic spIPSCs were blocked reversibly by strychnine (300 nM). Compared to previously described spIPSCs in AII amacrine cells, the spIPSCs in wide-field amacrine cells displayed a very slow decay time course (tau(fast) approximately 15 ms; tau(slow) approximately 57 ms). The kinetic properties of spIPSCs in whole-cell recordings were paralleled by even slower deactivation kinetics of responses evoked by brief pulses of glycine (3 mm) to outside-out patches from wide-field amacrine cells (tau(fast) approximately 45 ms; tau(slow) approximately 350 ms). Non-stationary noise analysis of patch responses and spIPSCs yielded similar average single-channel conductances (approximately 31 and approximately 34 pS, respectively). Similar, as well as both lower- and higher-conductance levels could be identified from directly observed single-channel gating during the decay phase of spIPSCs and patch responses. These results suggest that the slow glycinergic spIPSCs in wide-field amacrine cells involve alpha2beta heteromeric receptors. Taken together with previous work, the kinetic properties of glycine receptors in different types of amacrine cells display a considerable range that is probably a direct consequence of differential expression of receptor subunits. Unique kinetic properties are likely to differentially shape the glycinergic input to different types of amacrine cells and thereby contribute to distinct integrative properties among these cells.

Functional properties of spontaneous IPSCs and glycine receptors in rod amacrine (AII) cells in the rat retina

AII amacrine cells play a crucial role in retinal signal transmission under scotopic conditions. We have used rat retinal slices to investigate the functional properties of inhibitory glycine receptors on AII cells by recording spontaneous IPSCs (spIPSCs) in whole cells and glycine-evoked responses in outside-out patches. Glycinergic spIPSCs displayed fast kinetics with an average 10-90% rise time of approximately 500 mus, and a decay phase best fitted by a double-exponential function with tau(fast) approximately 4.8 ms (97.5% amplitude contribution) and tau(slow) approximately 33 ms. Decay kinetics were voltage dependent. Ultrafast application of brief ( approximately 2-5 ms) pulses of glycine (3 mm) to patches, evoked responses with fast deactivation kinetics best fitted with a double-exponential function with tau(fast) approximately 4.6 ms (85% amplitude contribution) and tau(slow) approximately 17 ms. Double-pulse experiments indicated recovery from desensitization after a 100-ms pulse of glycine with a double-exponential time course (tau(fast) approximately 71 ms and tau(slow) approximately 1713 ms). Non-stationary noise analysis of spIPSCs and patch responses, and directly observed channel gating yielded similar single-channel conductances ( approximately 41 to approximately 47 pS). In addition, single-channel gating occurred at approximately 83 pS. These results suggest that the fast glycinergic spIPSCs in AII cells are probably mediated by alpha1beta heteromeric receptors with a contribution from alpha1 homomeric receptors. We hypothesize that glycinergic synaptic input may target the arboreal dendrites of AII cells, and could serve to shunt excitatory input from rod bipolar cells and transiently uncouple the transcellular current through electrical synapses between AII cells and between AII cells and ON-cone bipolar cells.