A scriptable DSP-based system for dynamic conductance injection

Kv4.2 channel activity controls intrinsic firing dynamics of arcuate kisspeptin neurons

Key points

Neurons in the hypothalamus of the brain which secrete the peptide kisspeptin are important regulators of reproduction, and normal reproductive development.

Electrical activity, in the form of action potentials, or spikes, leads to secretion of peptides and neurotransmitters, influencing the activity of downstream neurons; in kisspeptin neurons, this activity is highly irregular, but the mechanism of this is not known.

In this study, we show that irregularity depends on the presence of a particular type of potassium ion channel in the membrane, which opens transiently in response to electrical excitation.

The results contribute to understanding how kisspeptin neurons generate and time their membrane potential spikes, and how reliable this process is.

Improved understanding of the activity of kisspeptin neurons, and how it shapes their secretion of peptides, is expected to lead to better treatment for reproductive dysfunction and disorders of reproductive development.

Abstract

Kisspeptin neurons in the hypothalamus are critically involved in reproductive function, via their effect on GnRH neuron activity and consequent gonadotropin release. Kisspeptin neurons show an intrinsic irregularity of firing, but the mechanism of this remains unclear. To address this, we carried out targeted whole‐cell patch‐clamp recordings of kisspeptin neurons in the arcuate nucleus (Kiss1^Arc), in brain slices isolated from adult male Kiss‐Cre:tdTomato mice. Cells fired irregularly in response to constant current stimuli, with a wide range of spike time variability, and prominent subthreshold voltage fluctuations. In voltage clamp, both a persistent sodium (NaP) current and a fast transient (A‐type) potassium current were apparent, activating at potentials just below the threshold for spiking. These currents have also previously been described in irregular‐spiking cortical interneurons, in which the A‐type current, mediated by Kv4 channels, interacts with NaP current to generate complex dynamics of the membrane potential, and irregular firing. In Kiss1^Arc neurons, A‐type current was blocked by phrixotoxin, a specific blocker of Kv4.2/4.3 channels, and consistent expression of Kv4.2 transcripts was detected by single‐cell RT‐PCR. In addition, firing irregularity was correlated to the density of A‐type current in the membrane. Using conductance injection, we demonstrated that adding Kv4‐like potassium conductance (g_Kv4) to a cell produces a striking increase in firing irregularity, and excitability is reduced, while subtracting g_Kv4 has the opposite effects. Thus, we propose that Kv4 interacting dynamically with NaP is a key determinant of the irregular firing behaviour of Kiss1^Arc neurons, shaping their physiological function in gonadotropin release.

Neurons in the hypothalamus of the brain which secrete the peptide kisspeptin are important regulators of reproduction, and normal reproductive development.

Electrical activity, in the form of action potentials, or spikes, leads to secretion of peptides and neurotransmitters, influencing the activity of downstream neurons; in kisspeptin neurons, this activity is highly irregular, but the mechanism of this is not known.

In this study, we show that irregularity depends on the presence of a particular type of potassium ion channel in the membrane, which opens transiently in response to electrical excitation.

The results contribute to understanding how kisspeptin neurons generate and time their membrane potential spikes, and how reliable this process is.

Improved understanding of the activity of kisspeptin neurons, and how it shapes their secretion of peptides, is expected to lead to better treatment for reproductive dysfunction and disorders of reproductive development.

Facilitation of mossy fibre-driven spiking in the cerebellar nuclei by the synchrony of inhibition

KEY POINTS

Large premotor neurons of the cerebellar nuclei (CbN cells) integrate synaptic inhibition from Purkinje neurons and synaptic excitation from mossy fibres to generate cerebellar output. We find that mossy fibre inputs to CbN cells generate unitary AMPA receptor EPSCs of ∼1 nS that decay in ∼1 ms and mildly voltage-dependent NMDA receptor EPSCs of ∼0.6 nS that decay in ∼7 ms. A few hundred mossy fibres active at a few tens of spikes s-1 must converge on CbN cells to generate physiological CbN spike rates (∼60 spikes s-1 ) during convergent inhibition from spontaneously active Purkinje cells. Dynamic clamp studies in cerebellar slices from weanling mice demonstrate that synaptic excitation from mossy fibres becomes more effective at increasing the rate of CbN cell spiking when the coherence (synchrony) of convergent inhibition is increased.

ABSTRACT

Large projection neurons of the cerebellar nuclei (CbN cells), whose activity generates movement, are inhibited by Purkinje cells and excited by mossy fibres. The high convergence, firing rates and strength of Purkinje inputs predict powerful suppression of CbN cell spiking, raising the question of what activity patterns favour excitation over inhibition. Recording from CbN cells at near-physiological temperatures in cerebellar slices from weanling mice, we measured the amplitude, kinetics, voltage dependence and short-term plasticity of mossy fibre-mediated EPSCs. Unitary EPSCs were small and brief (AMPA receptor, ∼1 nS, ∼1 ms; NMDA receptor, ∼0.6 nS, ∼7 ms) and depressed moderately. Using these experimentally measured parameters, we applied combinations of excitation and inhibition to CbN cells with dynamic clamp. Because Purkinje cells can fire coincident simple spikes during cerebellar behaviours, we varied the proportion (0-20 of 40) and precision (0-4 ms jitter) of synchrony of inhibitory inputs, along with the rates (0-100 spikes s-1 ) and number (0-800) of excitatory inputs. Even with inhibition constant, when inhibitory synchrony was higher, excitation increased CbN cell firing rates more effectively. Partial inhibitory synchrony also dictated CbN cell spike timing, even with physiological rates of excitation. These effects were present with ≥10 inhibitory inputs active within 2-4 ms of each other. Conversely, spiking was most effectively suppressed when inhibition was maximally asynchronous. Thus, the rate and relative timing of Purkinje-mediated inhibition set the rate and timing of cerebellar output. The results suggest that increased coherence of Purkinje cell activity can facilitate mossy fibre-driven spiking by CbN cells, in turn driving movements.

Stochastic and deterministic dynamics of intrinsically irregular firing in cortical inhibitory interneurons

Most cortical neurons fire regularly when excited by a constant stimulus. In contrast, irregular-spiking (IS) interneurons are remarkable for the intrinsic variability of their spike timing, which can synchronize amongst IS cells via specific gap junctions. Here, we have studied the biophysical mechanisms of this irregular spiking in mice, and how IS cells fire in the context of synchronous network oscillations. Using patch-clamp recordings, artificial dynamic conductance injection, pharmacological analysis and computational modeling, we show that spike time irregularity is generated by a nonlinear dynamical interaction of voltage-dependent sodium and fast-inactivating potassium channels just below spike threshold, amplifying channel noise. This active irregularity may help IS cells synchronize with each other at gamma range frequencies, while resisting synchronization to lower input frequencies.

DOI:http://dx.doi.org/10.7554/eLife.16475.001

Neurons send information to other neurons in the brain by generating fast electrical pulses called action potentials (or spikes). When stimulated by input signals of a constant size, neurons generally respond with regular patterns of spiking leading to rhythmical brain activity. However, neurons known as irregular spiking interneurons are unique: the relationship between the input they receive and whether or not they produce a spike appears to be random. The molecular mechanism behind this phenomenon is not clear.

Mendonça et al. set out to investigate whether irregular spiking is truly random, or whether there is some degree of predictability. The experiments used genetically modified mice in which irregular spiking interneurons were specifically labeled with a fluorescent protein, which made them easier to find to record their electrical activity. Sophisticated statistical analyses showed that these neurons are not firing at random. Instead, there is a pattern to the timings of the spikes they produce.

It was previously known that electrical spikes in neurons are generated by sodium ions and potassium ions moving across the membrane that surrounds each cell. Proteins called ion channels provide routes for these ions to pass through the membrane. Mendonça et al. show that compared to other neurons, irregular spiking interneurons have larger numbers of a specific type of potassium ion channel. Mimicking the effect of increasing the number of these potassium ion channels in the interneurons made the firing pattern of these neurons more irregular, while decreasing the number of these channels made the firing patterns more regular and predictable.

A computer model of an irregular spiking interneuron showed that the activity of these potassium ion channels and a type of sodium ion channel plays a key role in producing irregular electrical spiking. Further analysis showed that irregular spiking interneurons can synchronize their activity with fast, but not slow, rhythms in brain activity.

The findings of Mendonça et al. suggest that irregular spiking interneurons can disrupt slow regular electrical activity in the brain. Rhythms in brain activity vary depending on whether we are awake or asleep, and are altered in diseases such as epilepsy and schizophrenia. Now that we have a better understanding of how irregular spiking interneurons work, it should be possible to find out how they coordinate their activity with each other, and what effect they have on animal behavior.

DOI:http://dx.doi.org/10.7554/eLife.16475.002

Non-linear developmental trajectory of electrical phenotype in rat substantia nigra pars compacta dopaminergic neurons

Neurons have complex electrophysiological properties, however, it is often difficult to determine which properties are the most relevant to neuronal function. By combining current-clamp measurements of electrophysiological properties with multi-variate analysis (hierarchical clustering, principal component analysis), we were able to characterize the postnatal development of substantia nigra dopaminergic neurons' electrical phenotype in an unbiased manner, such that subtle changes in phenotype could be analyzed. We show that the intrinsic electrical phenotype of these neurons follows a non-linear trajectory reaching maturity by postnatal day 14, with two developmental transitions occurring between postnatal days 3-5 and 9-11. This approach also predicted which parameters play a critical role in phenotypic variation, enabling us to determine (using pharmacology, dynamic-clamp) that changes in the leak, sodium and calcium-activated potassium currents are central to these two developmental transitions. This analysis enables an unbiased definition of neuronal type/phenotype that is applicable to a range of research questions.

Single electrode dynamic clamp with StdpC

Dynamic clamp is a powerful approach for electrophysiological investigations allowing researchers to introduce artificial electrical components into target neurons to simulate ionic conductances, chemical or electrotonic inputs or connections to other cells. Due to the rapidly changing and potentially large current injections during dynamic clamp, problematic voltage artifacts appear on the electrode used to inject dynamic clamp currents into a target neuron. Dynamic clamp experiments, therefore, typically use two separate electrodes in the same cell, one for recording membrane potential and one for injecting currents. The requirement for two independent electrodes has been a limiting factor for the use of dynamic clamp in applications where dual recordings of this kind are difficult or impossible to achieve. The recent development of an active electrode compensation (AEC) method has overcome some of these prior limitations, permitting artifact-free dynamic clamp experimentation with a single electrode. Here we describe an AEC method for the free dynamic clamp software StdpC. The AEC component of StdpC is the first such system implemented for the use of non-expert users and comes with a set of semi-automated configuration and calibration procedures that facilitate its use. We briefly introduce the AEC method and its implementation in StdpC and then validate it with an electronic model cell and in two different biological preparations.

Ca²⁺/cAMP-sensitive covariation of I(A) and I(H) voltage dependences tunes rebound firing in dopaminergic neurons

The level of expression of ion channels has been demonstrated to vary over a threefold to fourfold range from neuron to neuron, although the expression of distinct channels may be strongly correlated in the same neurons. We demonstrate that variability and covariation also apply to the biophysical properties of ion channels. We show that, in rat substantia nigra pars compacta dopaminergic neurons, the voltage dependences of the A-type (I(A)) and H-type (I(H)) currents exhibit a high degree of cell-to-cell variability, although they are strongly correlated in these cells. Our data also demonstrate that this cell-to-cell covariability of voltage dependences is sensitive to cytosolic cAMP and calcium levels. Finally, using dynamic clamp, we demonstrate that covarying I(A) and I(H) voltage dependences increases the dynamic range of rebound firing while covarying their amplitudes has a homeostatic effect on rebound firing. We propose that the covariation of voltage dependences of ion channels represents a flexible and energy-efficient way of tuning firing in neurons.

Inhibition of post-synaptic Kv7/KCNQ/M channels facilitates long-term potentiation in the hippocampus

Activation of muscarinic acetylcholine receptors (mAChR) facilitates the induction of synaptic plasticity and enhances cognitive function. In the hippocampus, M₁ mAChR on CA1 pyramidal cells inhibit both small conductance Ca²⁺-activated KCa2 potassium channels and voltage-activated Kv7 potassium channels. Inhibition of KCa2 channels facilitates long-term potentiation (LTP) by enhancing Ca²⁺calcium influx through postsynaptic NMDA receptors (NMDAR). Inhibition of Kv7 channels is also reported to facilitate LTP but the mechanism of action is unclear. Here, we show that inhibition of Kv7 channels with XE-991 facilitated LTP induced by theta burst pairing at Schaffer collateral commissural synapses in rat hippocampal slices. Similarly, negating Kv7 channel conductance using dynamic clamp methodologies also facilitated LTP. Negation of Kv7 channels by XE-991 or dynamic clamp did not enhance synaptic NMDAR activation in response to theta burst synaptic stimulation. Instead, Kv7 channel inhibition increased the amplitude and duration of the after-depolarisation following a burst of action potentials. Furthermore, the effects of XE-991 were reversed by re-introducing a Kv7-like conductance with dynamic clamp. These data reveal that Kv7 channel inhibition promotes NMDAR opening during LTP induction by enhancing depolarisation during and after bursts of postsynaptic action potentials. Thus, during the induction of LTP M₁ mAChRs enhance NMDAR opening by two distinct mechanisms namely inhibition of KCa2 and Kv7 channels.

The mechanism of ethanol action on midbrain dopaminergic neuron firing: a dynamic-clamp study of the role of I(h) and GABAergic synaptic integration

Hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels are expressed in dopaminergic (DA) neurons of the ventral tegmental area (VTA) as well as in DA and GABAergic neurons of the substantia nigra (SN). The excitation of DA neurons induced by ethanol has been proposed to result from its enhancing HCN channel current, I(h). Using perforated patch-clamp recordings in rat midbrain slices, we isolated I(h) in these neurons by voltage clamp. We showed that ethanol reversibly increased the amplitude and accelerated the activation kinetics of I(h) and caused a depolarizing shift in its voltage dependence. Using dynamic-clamp conductance injection, we injected artificial I(h) and fluctuating GABAergic synaptic conductance inputs into neurons following block of intrinsic I(h). This demonstrated directly a major role of I(h) in promoting rebound spiking following phasic inhibition, which was enhanced as the kinetics and amplitude of I(h) were changed in the manner induced by ethanol. Similar effects of ethanol were observed on I(h) and firing rate in non-DA, putatively GABAergic interneurons, indicating that in addition to its direct effects on firing, ethanol will produce large changes in the inhibition and disinhibition (via GABAergic interneurons) converging on DA neurons. Thus the overall effects of ethanol on firing of DA cells of the VTA and SN in vivo, and hence on phasic dopamine release in the striatum, appear to be determined substantially by its action on I(h) in both DA cells and GABAergic interneurons.

Dynamic clamp with StdpC software

Dynamic clamp is a powerful method that allows the introduction of artificial electrical components into target cells to simulate ionic conductances and synaptic inputs. This method is based on a fast cycle of measuring the membrane potential of a cell, calculating the current of a desired simulated component using an appropriate model and injecting this current into the cell. Here we present a dynamic clamp protocol using free, fully integrated, open-source software (StdpC, for spike timing-dependent plasticity clamp). Use of this protocol does not require specialist hardware, costly commercial software, experience in real-time operating systems or a strong programming background. The software enables the configuration and operation of a wide range of complex and fully automated dynamic clamp experiments through an intuitive and powerful interface with a minimal initial lead time of a few hours. After initial configuration, experimental results can be generated within minutes of establishing cell recording.

Synchronization of firing in cortical fast-spiking interneurons at gamma frequencies: a phase-resetting analysis

Fast-spiking (FS) cells in the neocortex are interconnected both by inhibitory chemical synapses and by electrical synapses, or gap-junctions. Synchronized firing of FS neurons is important in the generation of gamma oscillations, at frequencies between 30 and 80 Hz. To understand how these synaptic interactions control synchronization, artificial synaptic conductances were injected in FS cells, and the synaptic phase-resetting function (SPRF), describing how the compound synaptic input perturbs the phase of gamma-frequency spiking as a function of the phase at which it is applied, was measured. GABAergic and gap junctional conductances made distinct contributions to the SPRF, which had a surprisingly simple piecewise linear form, with a sharp midcycle break between phase delay and advance. Analysis of the SPRF showed how the intrinsic biophysical properties of FS neurons and their interconnections allow entrainment of firing over a wide gamma frequency band, whose upper and lower frequency limits are controlled by electrical synapses and GABAergic inhibition respectively.

Oscillations of the electrical field in the brain at 30–80 Hz (gamma oscillations) reflect coordinated firing of neurons during cognitive, sensory, and motor activity, and are thought to be a key phenomenon in the organization of neural processing in the cortex. Synchronous firing of a particular type of neuron, the inhibitory fast-spiking (FS) cell, imposes the gamma rhythm on other cells in the network. FS cells are highly interconnected by both gap junctions and chemical inhibition. In this study, we probed FS cells with a synthetic conductance stimulus which mimics the electrical effect of these complex connections in a controlled way, and directly measured how the timing of their firing should be affected by nearby FS neighbours. We were able to fit a mathematically simple but accurate model to these measurements, the “synaptic phase-resetting function”, which predicts how FS neurons synchronize at different frequencies, noise levels, and synaptic connection strengths. This model gives us deeper insight into how the FS cells synchronize so effectively at gamma oscillations, and will be a building-block in large-scale simulations of the FS cell network aimed at understanding the onset and stability of patterns of gamma oscillation in the cortex.

Electrical Coupling and Passive Membrane Properties of AII Amacrine Cells

AII amacrine cells in the mammalian retina are connected via electrical synapses to on-cone bipolar cells and to other AII amacrine cells. To understand synaptic integration in these interneurons, we need information about the junctional conductance ( gj), the membrane resistance ( rm), the membrane capacitance ( Cm), and the cytoplasmic resistivity ( Ri). Due to the extensive electrical coupling, it is difficult to obtain estimates of rm, as well as the relative contribution of the junctional and nonjunctional conductances to the total input resistance of an AII amacrine cell. Here we used dual voltage-clamp recording of pairs of electrically coupled AII amacrine cells in an in vitro slice preparation from rat retina and applied meclofenamic acid (MFA) to block the electrical coupling and isolate single AII amacrines electrically. In the control condition, the input resistance ( Rin) was ∼620 MΩ and the apparent rm was ∼760 MΩ. After block of electrical coupling, determined by estimating gj in the dual recordings, Rin and rm were ∼4,400 MΩ, suggesting that the nongap junctional conductance of an AII amacrine cell is ∼16% of the total input conductance. Control experiments with nucleated patches from AII amacrine cells suggested that MFA had no effect on the nongap junctional membrane of these cells. From morphological reconstructions of AII amacrine cells filled with biocytin, we obtained a surface area of ∼900 μm2 which, with a standard value for Cm of 0.01 pF/μm2, corresponds to an average capacitance of ∼9 pF and a specific membrane resistance of ∼41 kΩ cm2. Together with information concerning synaptic connectivity, these data will be important for developing realistic compartmental models of the network of AII amacrine cells.

Causes of transient instabilities in the dynamic clamp

The dynamic clamp is a widely used method for integrating mathematical models with electrophysiological experiments. This method involves measuring the membrane voltage of a cell, using it to solve computational models of ion channel dynamics in real-time, and injecting the calculated current(s) back into the cell. Limitations of this technique include those associated with single electrode current clamping and the sampling effects caused by the dynamic clamp. In this study, we show that the combination of these limitations causes transient instabilities under certain conditions. Through physical experiments and simulations, we show that dynamic clamp instability is directly related to the sampling delay and the maximum simulated conductance being injected. It is exaggerated by insufficient electrode series resistance and capacitance compensation. Increasing the sampling rate of the dynamic clamp system increases dynamic clamp stability; however, this improvement, is constrained by how well the electrode series resistance and capacitance are compensated. At present, dynamic clamp sampling rates are justified solely on the temporal dynamics of the models being simulated; here we show that faster rates increase the stable range of operation for the dynamic clamp system. In addition, we show that commonly accepted levels of resistance compensation nevertheless significantly compromise the stability of a dynamic clamp system.

Electrical Synapses Between AII Amacrine Cells: Dynamic Range and Functional Consequences of Variation in Junctional Conductance

AII amacrine cells form a network of electrically coupled interneurons in the mammalian retina and tracer coupling studies suggest that the junctional conductance ( Gj) can be modulated. However, the dynamic range of Gjand the functional consequences of varying Gjover the dynamic range are unknown. Here we use whole cell recordings from pairs of coupled AII amacrine cells in rat retinal slices to provide direct evidence for physiological modulation of Gj, appearing as a time-dependent increase from about 500 pS to a maximum of about 3,000 pS after 30–90 min of recording. The increase occurred in recordings with low- but not high-resistance pipettes, suggesting that it was related to intracellular washout and perturbation of a modulatory system. Computer simulations of a network of electrically coupled cells verified that our recordings were able to detect and quantify changes in Gjover a large range. Dynamic-clamp electrophysiology, with insertion of electrical synapses between AII amacrine cells, allowed us to finely and reversibly control Gjwithin the same range observed for physiologically coupled cells and to examine the quantitative relationship between Gjand steady-state coupling coefficient, synchronization of subthreshold membrane potential fluctuations, synchronization and transmission of action potentials, and low-pass filter characteristics. The range of Gjvalues over which signal transmission was modulated depended strongly on the specific functional parameter examined, with the largest range observed for action potential transmission and synchronization, suggesting that the full range of Gjvalues observed during spontaneous run-up of coupling could represent a physiologically relevant dynamic range.