Subthreshold oscillation of the membrane potential in magnocellular neurones of the rat supraoptic nucleus

Green Tea Catechins, (-)-Catechin Gallate, and (-)-Gallocatechin Gallate are Potent Inhibitors of ABA-Induced Stomatal Closure

Stomatal movement is indispensable for plant growth and survival in response to environmental stimuli. Cytosolic Ca²⁺ elevation plays a crucial role in ABA-induced stomatal closure during drought stress; however, to what extent the Ca²⁺ movement across the plasma membrane from the apoplast to the cytosol contributes to this process still needs clarification. Here the authors identify (-)-catechin gallate (CG) and (-)-gallocatechin gallate (GCG), components of green tea, as inhibitors of voltage-dependent K⁺ channels which regulate K⁺ fluxes in Arabidopsis thaliana guard cells. In Arabidopsis guard cells CG/GCG prevent ABA-induced: i) membrane depolarization; ii) activation of Ca²⁺ permeable cation (I_Ca ) channels; and iii) cytosolic Ca²⁺ transients. In whole Arabidopsis plants co-treatment with CG/GCG and ABA suppressed ABA-induced stomatal closure and surface temperature increase. Similar to ABA, CG/GCG inhibited stomatal closure is elicited by the elicitor peptide, flg22 but has no impact on dark-induced stomatal closure or light- and fusicoccin-induced stomatal opening, suggesting that the inhibitory effect of CG/GCG is associated with Ca²⁺ -related signaling pathways. This study further supports the crucial role of I_Ca channels of the plasma membrane in ABA-induced stomatal closure. Moreover, CG and GCG represent a new tool for the study of abiotic or biotic stress-induced signal transduction pathways.

The voltage and spiking responses of subthreshold resonant neurons to structured and fluctuating inputs: persistence and loss of resonance and variability

We systematically investigate the response of neurons to oscillatory currents and synaptic-like inputs and we extend our investigation to non-structured synaptic-like spiking inputs with more realistic distributions of presynaptic spike times. We use two types of chirp-like inputs consisting of (i) a sequence of cycles with discretely increasing frequencies over time, and (ii) a sequence having the same cycles arranged in an arbitrary order. We develop and use a number of frequency-dependent voltage response metrics to capture the different aspects of the voltage response, including the standard impedance (Z) and the peak-to-trough amplitude envelope ([Formula: see text]) profiles. We show that Z-resonant cells (cells that exhibit subthreshold resonance in response to sinusoidal inputs) also show [Formula: see text]-resonance in response to sinusoidal inputs, but generally do not (or do it very mildly) in response to square-wave and synaptic-like inputs. In the latter cases the resonant response using Z is not predictive of the preferred frequencies at which the neurons spike when the input amplitude is increased above subthreshold levels. We also show that responses to conductance-based synaptic-like inputs are attenuated as compared to the response to current-based synaptic-like inputs, thus providing an explanation to previous experimental results. These response patterns were strongly dependent on the intrinsic properties of the participating neurons, in particular whether the unperturbed Z-resonant cells had a stable node or a focus. In addition, we show that variability emerges in response to chirp-like inputs with arbitrarily ordered patterns where all signals (trials) in a given protocol have the same frequency content and the only source of uncertainty is the subset of all possible permutations of cycles chosen for a given protocol. This variability is the result of the multiple different ways in which the autonomous transient dynamics is activated across cycles in each signal (different cycle orderings) and across trials. We extend our results to include high-rate Poisson distributed current- and conductance-based synaptic inputs and compare them with similar results using additive Gaussian white noise. We show that the responses to both Poisson-distributed synaptic inputs are attenuated with respect to the responses to Gaussian white noise. For cells that exhibit oscillatory responses to Gaussian white noise (band-pass filters), the response to conductance-based synaptic inputs are low-pass filters, while the response to current-based synaptic inputs may remain band-pass filters, consistent with experimental findings. Our results shed light on the mechanisms of communication of oscillatory activity among neurons in a network via subthreshold oscillations and resonance and the generation of network resonance.

Three-dimensional transistor arrays for intra- and inter-cellular recording

Electrical impulse generation and its conduction within cells or cellular networks are the cornerstone of electrophysiology. However, the advancement of the field is limited by sensing accuracy and the scalability of current recording technologies. Here we describe a scalable platform that enables accurate recording of transmembrane potentials in electrogenic cells. The platform employs a three-dimensional high-performance field-effect transistor array for minimally invasive cellular interfacing that produces faithful recordings, as validated by the gold standard patch clamp. Leveraging the high spatial and temporal resolutions of the field-effect transistors, we measured the intracellular signal conduction velocity of a cardiomyocyte to be 0.182 m s^-1, which is about five times the intercellular velocity. We also demonstrate intracellular recordings in cardiac muscle tissue constructs and reveal the signal conduction paths. This platform could provide new capabilities in probing the electrical behaviours of single cells and cellular networks, which carries broad implications for understanding cellular physiology, pathology and cell-cell interactions.

Distinct response properties of rat prepositus hypoglossi nucleus neurons classified on the basis of firing patterns

Neurons in the prepositus hypoglossi nucleus (PHN), which is involved in controlling horizontal gaze, show distinct firing patterns in response to depolarizing current pulses. Although the firing patterns are commonly used to classify neuron types, whether the classified PHN neurons show differences in voltage response properties when stimulated with various types of current inputs remains unclear. In this study, we investigated the response properties of PHN neurons to various current stimuli using whole-cell recordings in rat brainstem slices. In response to pulse currents, neurons that exhibited oscillatory firing (OSC type) showed greater gain than other types, and neurons with a low firing rate (LFR type) showed strong overshooting firing responses to ramp currents. In response to triangular ramp currents, the late-spiking type and the LFR type showed a marked hysteretic frequency-current relationship. In response to sinusoidal currents, the gain was larger in the OSC type than in the other types, although the gain and phase of all types of neurons were similarly modulated by an increase in the input frequency. These findings suggest that distinct neuron types show distinct response properties, depending on the type of stimulus. These neuron types may represent the functionally different populations in the PHN.

The shaping of intrinsic membrane potential oscillations: positive/negative feedback, ionic resonance/amplification, nonlinearities and time scales

The generation of intrinsic subthreshold (membrane potential) oscillations (STOs) in neuronal models requires the interaction between two processes: a relatively fast positive feedback that favors changes in voltage and a slower negative feedback that opposes these changes. These are provided by the so-called resonant and amplifying gating variables associated to the participating ionic currents. We investigate both the biophysical and dynamic mechanisms of generation of STOs and how their attributes (frequency and amplitude) depend on the model parameters for biophysical (conductance-based) models having qualitatively different types of resonant currents (activating and inactivating) and an amplifying current. Combinations of the same types of ionic currents (same models) in different parameter regimes give rise to different types of nonlinearities in the voltage equation: quasi-linear, parabolic-like and cubic-like. On the other hand, combinations of different types of ionic currents (different models) may give rise to the same type of nonlinearities. We examine how the attributes of the resulting STOs depend on the combined effect of these resonant and amplifying ionic processes, operating at different effective time scales, and the various types of nonlinearities. We find that, while some STO properties and attribute dependencies on the model parameters are determined by the specific combinations of ionic currents (biophysical properties), and are different for models with different such combinations, others are determined by the type of nonlinearities and are common for models with different types of ionic currents. Our results highlight the richness of STO behavior in single cells as the result of the various ways in which resonant and amplifying currents interact and affect the generation and termination of STOs as control parameters change. We make predictions that can be tested experimentally and are expected to contribute to the understanding of how rhythmic activity in neuronal networks emerge from the interplay of the intrinsic properties of the participating neurons and the network connectivity.

Multi-timescale systems and fast-slow analysis

Mathematical models of biological systems often have components that vary on different timescales. This multi-timescale character can lead to problems when doing computer simulations, which can require a great deal of computer time so that the components that change on the fastest time scale can be resolved. Mathematical analysis of these multi-timescale systems can be greatly simplified by partitioning them into subsystems that evolve on different time scales. The subsystems are then analyzed semi-independently, using a technique called fast-slow analysis. In this review we describe the fast-slow analysis technique and apply it to relaxation oscillations, neuronal bursting oscillations, canard oscillations, and mixed-mode oscillations. Although these examples all involve neural systems, the technique can and has been applied to other biological, chemical, and physical systems. It is a powerful analysis method that will become even more useful in the future as new experimental techniques push forward the complexity of biological models.

Electrophysiological and morphological properties of neurons in the prepositus hypoglossi nucleus that express both ChAT and VGAT in a double-transgenic rat model

Although it has been proposed that neurons that contain both acetylcholine (ACh) and γ-aminobutyric acid (GABA) are present in the prepositus hypoglossi nucleus (PHN), these neurons have not been characterized because of the difficulty in identifying them. In the present study, PHN neurons that express both choline acetyltransferase and the vesicular GABA transporter (VGAT) were identified using double-transgenic rats, in which the cholinergic and inhibitory neurons express the fluorescent proteins tdTomato and Venus, respectively. To characterize the neurons that express both tdTomato and Venus (D+ neurons), the afterhyperpolarization (AHP) profiles and firing patterns of these neurons were investigated via whole-cell recordings of brainstem slice preparations. Regarding the three AHP profiles and four firing patterns that the D+ neurons exhibited, an AHP with an afterdepolarization and a firing pattern that exhibited a delay in the generation of the first spike were the preferential properties of these neurons. In the three morphological types classified, the multipolar type that exhibited radiating dendrites was predominant among the D+ neurons. Immunocytochemical analysis revealed that the VGAT-immunopositive axonal boutons that expressed tdTomato were primarily located in the dorsal cap of inferior olive (IO) and the PHN. Although the PHN receives cholinergic inputs from the pedunculopontine tegmental nucleus and laterodorsal tegmental nucleus, D+ neurons were absent from these brain areas. Together, these results suggest that PHN neurons that co-express ACh and GABA exhibit specific electrophysiological and morphological properties, and innervate the dorsal cap of the IO and the PHN.

Variation in sodium current amplitude between vasopressin and oxytocin hypothalamic supraoptic neurons

Biophysical characteristics of tetrodotoxin-sensitive sodium (Na(+)) currents were studied in vasopressin (VP) and oxytocin (OT) supraoptic neurons acutely isolated from rat hypothalamus. Na(+) current density (pA/pF) was significantly greater in VP neurons than in OT neurons. No significant difference between VP and OT neurons was detected regarding the voltage dependence of activation and steady-state inactivation, or rate of recovery from inactivation of Na(+) currents. In both VP and OT neurons, the macroscopic inactivation of the Na(+) currents was best fitted with a double-exponential expression suggesting two rates of inactivation. Also in both types, the time course of recovery from inactivation proceeded with fast and slow time constants averaging around 8 and 350 ms, respectively, suggesting the presence of multiple pathways of recovery from inactivation. The slower time constant of recovery of inactivation may be involved in the decrease in action potential (AP) amplitude that occurs after the first spike during burst firing in both neuronal types. The larger amplitude of Na(+) currents in VP vs. OT neurons may explain the previous observations that VP neurons exhibit a lower AP threshold and greater AP amplitude than OT neurons, and may serve to differently tune the firing properties and responses to neuromodulators of the respective neuronal types.

Membrane resonance enables stable and robust gamma oscillations

Neuronal mechanisms underlying beta/gamma oscillations (20–80 Hz) are not completely understood. Here, we show that in vivo beta/gamma oscillations in the cat visual cortex sometimes exhibit remarkably stable frequency even when inputs fluctuate dramatically. Enhanced frequency stability is associated with stronger oscillations measured in individual units and larger power in the local field potential. Simulations of neuronal circuitry demonstrate that membrane properties of inhibitory interneurons strongly determine the characteristics of emergent oscillations. Exploration of networks containing either integrator or resonator inhibitory interneurons revealed that: (i) Resonance, as opposed to integration, promotes robust oscillations with large power and stable frequency via a mechanism called RING (Resonance INduced Gamma); resonance favors synchronization by reducing phase delays between interneurons and imposes bounds on oscillation cycle duration; (ii) Stability of frequency and robustness of the oscillation also depend on the relative timing of excitatory and inhibitory volleys within the oscillation cycle; (iii) RING can reproduce characteristics of both Pyramidal INterneuron Gamma (PING) and INterneuron Gamma (ING), transcending such classifications; (iv) In RING, robust gamma oscillations are promoted by slow but are impaired by fast inputs. Results suggest that interneuronal membrane resonance can be an important ingredient for generation of robust gamma oscillations having stable frequency.

Mechanisms contributing to the dopamine induction of crawl-like bursting in leech motoneurons

Dopamine (DA) activates fictive crawling behavior in the medicinal leech. To identify the cellular mechanisms underlying this activation at the level of crawl-specific motoneuronal bursting, we targeted potential cAMP-dependent events that are often activated through DA(1)-like receptor signaling pathways. We found that isolated ganglia produced crawl-like motoneuron bursting after bath application of phosphodiesterase inhibitors (PDIs) that upregulated cAMP. This bursting persisted in salines in which calcium ions were replaced with equimolar cobalt or nickel, but was blocked by riluzole, an inhibitor of a persistent sodium current. PDI-induced bursting contained a number of patterned elements that were statistically similar to those observed during DA-induced fictive crawling, except that one motoneuron (CV) exhibited bursting during the contraction rather than the elongation phase of crawling. Although DA and the PDIs produced similar bursting profiles, intracellular recordings from motoneurons revealed differences in altered membrane properties. For example, DA lowered motoneuron excitability whereas the PDIs increased resting discharge rates. We suggest that PDIs (and DA) activate a sodium-influx-dependent timing mechanism capable of setting the crawl rhythm and that multiple DA receptor subtypes are involved in shaping and modulating the phase relationships and membrane properties of cell-specific members of the crawl network to generate crawling.

Neural precursors (NPCs) from adult L967Q mice display early commitment to "in vitro" neuronal differentiation and hyperexcitability

The pathogenic factors leading to selective degeneration of motoneurons in ALS are not yet understood. However, altered functionality of voltage-dependent Na(+) channels may play a role since cortical hyperexcitability was described in ALS patients and riluzole, the only drug approved to treat ALS, seems to decrease glutamate release via blockade or inactivation of voltage-dependent Na(+) channels. The wobbler mouse, a murine model of motoneuron degeneration, shares some of the clinical features of human ALS. At early stages of the wobbler disease, increased cortical hyperexcitability was observed. Moreover, riluzole reduced motoneuron loss and muscular atrophy in treated wobbler mice. Here, we focussed our attention on specific electrophysiological properties, like voltage-activated Na(+) currents and underlying regenerative electrical activity, as read-outs of the neuronal maturation process of neural stem/progenitor cells (NPCs) isolated from the subventricular zone (SVZ) of adult early symptomatic wobbler mice. In self-renewal conditions, the rate of wobbler NPC proliferation "in vitro" was 30% lower than that of healthy mice. Conversely, the number of wobbler NPCs displaying early neuronal commitment and action potentials was significantly higher. Upon switching from proliferative to differentiative conditions, NPCs underwent significant changes in the key properties of voltage gated Na(+) currents. The most notable finding, in cells with neuronal morphology, was an increase in Na(+) current density that strictly correlated with an increased probability to generate action potentials. This feature was remarkably more pronounced in neurons differentiated from wobbler NPCs that upon sustained stimulation, displayed short trains of pathological facilitation. In agreement with this result, an increase in the number of c-Fos positive cells, a surrogate marker of neuronal network activation, was observed in the mesial cortex of the wobbler mice "in situ". Thus these findings, all together, suggest that a state of early neuronal hyperexcitability may be a major contributor of motoneuron vulnerability.

Mixed mode oscillations in mouse spinal motoneurons arise from a low excitability state

We explain the mechanism that elicits the mixed mode oscillations (MMOs) and the subprimary firing range that we recently discovered in mouse spinal motoneurons. In this firing regime, high-frequency subthreshold oscillations appear a few millivolts below the spike voltage threshold and precede the firing of a full blown spike. By combining intracellular recordings in vivo (including dynamic clamp experiments) in mouse spinal motoneurons and modeling, we show that the subthreshold oscillations are due to the spike currents and that MMOs appear each time the membrane is in a low excitability state. Slow kinetic processes largely contribute to this low excitability. The clockwise hysteresis in the I-F relationship, frequently observed in mouse motoneurons, is mainly due to a substantial slow inactivation of the sodium current. As a consequence, less sodium current is available for spiking. This explains why a large subprimary range with numerous oscillations is present in motoneurons displaying a clockwise hysteresis. In motoneurons whose I-F curve exhibits a counterclockwise hysteresis, it is likely that the slow inactivation operates on a shorter time scale and is substantially reduced by the de-inactivating effect of the afterhyperpolarization (AHP) current, thus resulting in a more excitable state. This accounts for the short subprimary firing range with only a few MMOs seen in these motoneurons. Our study reveals a new role for the AHP current that sets the membrane excitability level by counteracting the slow inactivation of the sodium current and allows or precludes the appearance of MMOs.

Neurophysiological and computational principles of cortical rhythms in cognition

Synchronous rhythms represent a core mechanism for sculpting temporal coordination of neural activity in the brain-wide network. This review focuses on oscillations in the cerebral cortex that occur during cognition, in alert behaving conditions. Over the last two decades, experimental and modeling work has made great strides in elucidating the detailed cellular and circuit basis of these rhythms, particularly gamma and theta rhythms. The underlying physiological mechanisms are diverse (ranging from resonance and pacemaker properties of single cells to multiple scenarios for population synchronization and wave propagation), but also exhibit unifying principles. A major conceptual advance was the realization that synaptic inhibition plays a fundamental role in rhythmogenesis, either in an interneuronal network or in a reciprocal excitatory-inhibitory loop. Computational functions of synchronous oscillations in cognition are still a matter of debate among systems neuroscientists, in part because the notion of regular oscillation seems to contradict the common observation that spiking discharges of individual neurons in the cortex are highly stochastic and far from being clocklike. However, recent findings have led to a framework that goes beyond the conventional theory of coupled oscillators and reconciles the apparent dichotomy between irregular single neuron activity and field potential oscillations. From this perspective, a plethora of studies will be reviewed on the involvement of long-distance neuronal coherence in cognitive functions such as multisensory integration, working memory, and selective attention. Finally, implications of abnormal neural synchronization are discussed as they relate to mental disorders like schizophrenia and autism.

Activation of mGluR5 induces spike afterdepolarization and enhanced excitability in medium spiny neurons of the nucleus accumbens by modulating persistent Na+ currents

The involvement of metabotropic glutamate receptors type 5 (mGluR5) in drug-induced behaviours is well-established but limited information is available on their functional roles in addiction-relevant brain areas like the nucleus accumbens (NAc). This study demonstrates that pharmacological and synaptic activation of mGluR5 increases the spike discharge of medium spiny neurons (MSNs) in the NAc. This effect was associated with the appearance of a slow afterdepolarization (ADP) which, in voltage-clamp experiments, was recorded as a slowly inactivating inward current. Pharmacological studies showed that ADP was elicited by mGluR5 stimulation via G-protein-dependent activation of phospholipase C and elevation of intracellular Ca(2+) levels. Both ADP and spike aftercurrents were significantly inhibited by the Na(+) channel-blocker, tetrodotoxin (TTX). Moreover, the selective blockade of persistent Na(+) currents (I(NaP)), achieved by NAc slice pre-incubation with 20 nm TTX or 10 \#956;m riluzole, significantly reduced the ADP amplitude, indicating that this type of Na(+) current is responsible for the mGluR5-dependent ADP. mGluR5 activation also produced significant increases in I(NaP), and the pharmacological blockade of this current prevented the mGluR5-induced enhancement of spike discharge. Collectively, these data suggest that mGluR5 activation upregulates I(NaP) in MSNs of the NAc, thereby inducing an ADP that results in enhanced MSN excitability. Activation of mGluR5 will significantly alter spike firing in MSNs in vivo, and this effect could be an important mechanism by which these receptors mediate certain aspects of drug-induced behaviours.

Electrical properties of neurons in the intact rat major pelvic ganglion

The aim of this investigation was to characterize the electrical properties of neurons in the rat major pelvic ganglia (MPG) using intracellular recording techniques. MPG were dissected from male rats euthanized by isoflurane and thoracotomy. Neurons were classified as "phasic" or "tonic" according to their rate of accommodation during a 500-ms depolarizing current pulse. Phasic cells were further subdivided into rapidly or slowly adapting. The firing pattern of tonic cells was divided into regular high frequency, low frequency or irregular firing. In tonic cells, onset spikes showed TTX-resistant discharges; whereas sustained spikes were TTX sensitive. Changing the current pulse amplitude or the stimulation interval could alter the firing pattern in both types of neurons. Subthreshold membrane potential oscillations (SMPOs) were primarily observed when neurons were depolarized. SMPOs were Na(+) dependent and TTX sensitive. The majority of tonic and phasic neurons generated rebound spikes, most of which were partially Na(+) dependent. A small percentage (<6%) of neurons exhibited spontaneous activity. Taken together these findings are consistent with the concept that neurons in the MPG exhibit heterogeneous electrical properties.

Intrinsic membrane properties and morphological characteristics of interneurons in the rat supratrigeminal region

The membrane properties and morphological features of interneurons in the supratrigeminal area (SupV) were studied in rat brain slices using whole-cell patch clamp recording techniques. We classified three morphological types of neurons as fusiform, pyramidal, and multipolar and four physiological types of neurons according to their discharge pattern in response to a 1-sec depolarizing current pulse from -80 mV. Single-spike neurons responded with a single spike, phasic neurons showed an initial burst of spikes and were silent during the remainder of the stimulus, delayed-firing (DF) neurons exhibited a slow depolarization and delay to initial spike onset, and tonic (T) neurons showed maintained a discharge throughout the stimulus pulse. In a subpopulation of neurons (10%), membrane depolarization to around -44 mV produced a rhythmic burst discharge (RB) that was associated with voltage-dependent subthreshold membrane oscillations. Both these phenomena were blocked by the sodium channel blocker riluzole at a concentration that did not affect the fast transient spike. Low doses of 4-AP, which blocks low-threshold K+ currents, transformed bursting into low-frequency tonic discharge. In contrast, bursting occurred with exposure to cadium, a calcium-channel blocker. This suggests that persistent sodium currents and low-threshold K+ currents have a role in intrinsic burst generation. Importantly, RB cells were most often associated with multipolar neurons that exhibited either a DF or a T discharge. Thus, the SupV contains a variety of physiological cell types with unique morphologies and discharge characteristics. Intrinsic bursting neurons form a unique group in this region. .

Serotonergic modulation of persistent sodium currents and membrane excitability via cyclic AMP-protein kinase A cascade in mesencephalic V neurons

In rat mesencephalic trigeminal (Mes V) neurons, persistent sodium currents in conjunction with low-threshold potassium currents are critical for generation of subthreshold membrane oscillations and onset of burst behavior. Here we demonstrate that the cAMP/protein kinase A (PKA) signaling pathway modulates persistent sodium currents. In particular, we show that elevation of cAMP suppresses a low-threshold I(NaP) via a PKA intracellular pathway. Bath application of forskolin (20 microM), a stimulant for the production of cAMP, reduced the peak I(NaP). 1,9-Dideoxy-forskolin (20 microM), an inactive form of forskolin, produced minimal effects on I(NaP), and the membrane-permeable cAMP analogue 8-bromo-cAMP (500 microM) mimicked the effect of forskolin. Additionally, preapplication of H89 (2 microM), a specific PKA inhibitor, suppressed the effect of forskolin, suggesting the involvement of the cAMP/PKA intracellular signaling pathway in this modulation. 5-HT receptor stimulation (20 microM) also mimicked the modulation of I(NaP) by forskolin via the cAMP/PKA-dependent signaling pathway. Current clamp analysis demonstrated that voltage-dependent membrane resonance in response to a ZAP input current at depolarized holding potentials (approximately -50 mV) was specifically suppressed by forskolin or 5-HT. Moreover, drug application enhanced frequency adaptation in response to a 1-sec current pulse. These results indicate that modulation of persistent sodium currents by a cAMP/PKA pathway can significantly alter the membrane excitability and discharge characteristics of Mes V neurons.

Participation of sodium currents in burst generation and control of membrane excitability in mesencephalic trigeminal neurons

Subthreshold sodium currents are important in sculpting neuronal discharge and have been implicated in production and/or maintenance of subthreshold membrane oscillations and burst generation in mesencephalic trigeminal neurons (Mes V). Moreover, recent data suggest that, in some CNS neurons, resurgent sodium currents contribute to production of high-frequency burst discharge. In the present study, we sought to determine more directly the participation of these currents during Mes V electrogenesis using the action potential-clamp method. In postnatal day 8-14 rats, the whole-cell patch-clamp method was used to record sodium currents by subtraction in response to application of TTX in voltage-clamp mode using the action potential waveform as the command protocol. We found that TTX-sensitive sodium current is the main inward current flowing during the interspike interval, compared with the h-current (Ih) and calcium currents. Furthermore, in addition to the transient sodium current that flows during the upstroke of action potential, we show that resurgent sodium current flows at the peak of afterhyperpolarization and persistent sodium current flows in the middle of the interspike interval to drive high-frequency firing. Additionally, transient, resurgent, and persistent sodium current components showed voltage- and time-dependent slow inactivation, suggesting that slow inactivation of these currents can contribute to burst termination. The data suggest an important role for these components of the sodium current in Mes V neuron electrogenesis.

Endogenous activation of supraoptic nucleus kappa-opioid receptors terminates spontaneous phasic bursts in rat magnocellular neurosecretory cells

Phasic activity in magnocellular neurosecretory vasopressin cells is characterized by alternating periods of activity (bursts) and silence. During phasic bursts, action potentials (spikes) are superimposed on plateau potentials that are generated by summation of depolarizing after-potentials (DAPs). Burst termination is believed to result from autocrine feedback inhibition of plateau potentials by the kappa-opioid peptide, dynorphin, which is copackaged in vasopressin neurosecretory vesicles and exocytosed from vasopressin cell dendrites during phasic bursts. Here we tested this hypothesis, using intracellular recording in vitro to show that kappa-opioid receptor antagonist administration enhanced plateau potential amplitude to increase postspike excitability during spontaneous phasic activity. The antagonist also increased postburst DAP amplitude in vitro, indicating that endogenous dynorphin probably reduces plateau potential amplitude by inhibiting the DAP mechanism. However, the kappa-opioid receptor antagonist did not affect the slow depolarization that follows burst termination, suggesting that recovery from endogenous kappa-opioid inhibition does not contribute to the slow depolarization. We also show, by extracellular single-unit recording, that that there is a strong random element in the timing of burst initiation and termination in vivo. Administration of a kappa-opioid receptor antagonist eliminated the random element of burst termination but did not alter the timing of burst initiation. We conclude that dendritic dynorphin release terminates phasic bursts by reducing the amplitude of plateau potentials to reduce the probability of spike firing as bursts progress. By contrast, dendritic dynorphin release does not greatly influence the membrane potential between bursts and evidently does not influence the timing of burst initiation.

Fast Pseudo-Periodic Oscillation in the Rat Brain Voltage-gated Sodium Channel α Subunit

In the work reported here, we have investigated the changes in the activation and fast inactivation properties of the rat brain voltage-gated sodium channel (rNa(v) 1.2a) alpha subunit, expressed heterologously in the Chinese Hamster Ovary (CHO) cells, by short depolarizing prepulses (10-1000 ms). The time constant of recovery from fast inactivation (tau(fast)) and steady-state parameters for activation and inactivation varied in a pseudo-oscillatory fashion with the duration and amplitude of a sustained prepulse. A consistent oscillation was observed in most of the steady-state and non-inactivating current parameters with a time period close to 225 ms, although a faster oscillation of time period 125 ms was observed in the tau(fast). The studies on the non-inactivating current and steady-state activation indicate that the phase of oscillation varies from cell to cell. Co-expression of the beta1 subunit with the alpha subunit channel suppressed the oscillation in the charge movement per single channel and free energy of steady-state inactivation, although the oscillation in the half steady-state inactivation potential remained unaltered. Incidentally, the frequencies of oscillation in the sodium channel parameters (4-8 Hz) correspond to the theta component of network oscillation. This fast pseudo-oscillatory mechanism, together with the slow pseudo-oscillatory mechanism found in these channels earlier, may contribute to the oscillations in the firing properties observed in various neuronal subtypes and many pathological conditions.

Persistent Sodium Currents in Mesencephalic V Neurons Participate in Burst Generation and Control of Membrane Excitability

The functional and biophysical properties of a persistent sodium current ( INaP) previously proposed to participate in the generation of subthreshold oscillations and burst discharge in mesencephalic trigeminal sensory neurons (Mes V) were investigated in brain stem slices (rats, p7–p12) using whole cell patch-clamp methods. INaPactivated around −76 mV and peaked at −48 mV, with V1/2of −58.7 mV. Ramp voltage-clamp protocols showed that INaPundergoes time- as well as voltage-dependent inactivation and recovery from inactivation in the range of several seconds (τonset= 2.04 s, τrecov= 2.21 s). Riluzole (≤5 μM) substantially reduced INaP, membrane resonance, postinhibitory rebound (PIR), and subthreshold oscillations, and completely blocked bursting, but produced modest effects on the fast transient Na+current ( INaT). Before complete cessation, burst cycle duration was increased substantially, while modest and inconsistent changes in burst duration were observed. The properties of the INaTwere obtained and revealed that the amplitude and voltage dependence of the resulting “window current” were not consistent with those of the observed INaPrecorded in the same neurons. This suggests an additional mechanism for the origin of INaP. A neuronal model was constructed using Hodgkin-Huxley parameters obtained experimentally for Na+and K+currents that simulated the experimentally observed membrane resonance, subthreshold oscillations, bursting, and PIR. Alterations in the model gNaPparameters indicate that INaPis critical for control of subthreshold and suprathreshold Mes V neuron membrane excitability and burst generation.

Light-evoked oscillatory discharges in retinal ganglion cells are generated by rhythmic synaptic inputs

In the visual system, optimal light stimulation sometimes generates gamma-range (ca. 20 approximately 80 Hz) synchronous oscillatory spike discharges. This phenomenon is assumed to be related to perceptual integration. Applying a planar multi-electrode array to the isolated frog retina, Ishikane et al. demonstrated that dimming detectors, off-sustained type ganglion cells, generate synchronous oscillatory spike discharges in response to diffuse dimming illumination. In the present study, applying the whole cell current-clamp technique to the isolated frog retina, we examined how light-evoked oscillatory spike discharges were generated in dimming detectors. Light-evoked oscillatory ( approximately 30 Hz) spike discharges were triggered by rhythmic ( approximately 30 Hz) fluctuations superimposed on a depolarizing plateau potential. When a suprathreshold steady depolarizing current was injected into a dimming detector, only a few spikes were evoked at the stimulus onset. However, repetitive spikes were triggered by a gamma-range sinusoidal current superimposed on the steady depolarizing current. Thus the light-evoked rhythmic fluctuations are likely to be generated presynaptically. The light-evoked rhythmic fluctuations were suppressed not by intracellular application of N-(2,6-dimethyl-phenylcarbamoylmethyl)triethylammonium bromide (QX-314), a Na(+) channel blocker, to the whole cell clamped dimming detector but by bath-application of tetrodotoxin to the retina. The light-evoked rhythmic fluctuations were suppressed by a GABA(A) receptor antagonist but potentiated by a GABA(C) receptor antagonist, whereas these fluctuations were little affected by a glycine receptor antagonist. Because amacrine cells are spiking neurons and because GABA is one of the main transmitters released from amacrine cells, amacrine cells may participate in generating rhythmically fluctuated synaptic input to dimming detectors.

Phasic bursts in rat magnocellular neurosecretory cells are not intrinsically regenerative in vivo

Vasopressinergic hypothalamic magnocellular neurosecretory cells fire in phasic bursts. Burst initiation involves summation of postsynaptic potentials to generate action potentials. Action potentials are each followed by a nonsynaptic depolarizing after-potential that summates temporally to generate a plateau potential and so sustain activity throughout the burst. It is unknown whether this plateau potential exceeds spike threshold in vivo to cause intrinsic regenerative firing or simply approaches threshold to increase the probability that excitatory postsynaptic potentials will trigger further action potentials. Here we show that pharmacological blockade of ionotropic glutamatergic transmission by microdialysis application of kynurenic acid into the supraoptic nucleus of anaesthetized rats prevents spontaneous bursts and bursts (after-discharge) evoked by short trains of antidromically stimulated action potentials in magnocellular neurosecretory cells. Even during prolonged depolarization induced by 1 m NaCl infusion, kynurenic acid microdialysis application still blocked after-discharge. The ability of kynurenic acid to block after-discharge during osmotic stimulation was not caused by an unmasking of inhibitory postsynaptic potentials as kynurenic acid was equally effective in the presence of the ionotropic gamma-aminobutyric acid receptor antagonist, bicuculline, nor did it result from inhibition of plateau potential amplitude as this was unaffected by kynurenic acid and bicuculline in vitro, as was after-discharge evoked in vitro. We conclude that phasic bursts are nonregenerative in vivo but rather require continued excitatory synaptic input activity superimposed upon a subthreshold plateau potential to sustain burst activity.

Induction of pseudo-periodic oscillation in voltage-gated sodium channel properties is dependent on the duration of prolonged depolarization

The neuronal voltage-gated sodium channels play a vital role in the action potential waveform shaping and propagation. Here, we report the effects of prolonged depolarization (1-160 s) on the detailed kinetics of activation, fast inactivation and recovery from slow inactivation in the rNa(v)1.2a voltage-gated sodium channel alpha-subunit expressed in Chinese hamster ovary (CHO) cells. Wavelet analysis revealed that the duration and amplitude of a prolonged sustained depolarization altered all the steady state and kinetic parameters of the channel in a pseudo-oscillatory fashion with time-variable period and amplitude, often superimposed on a linear trend. The half steady state activation potential showed a reversible depolarizing shift of 5-10 mV with duration of prolonged depolarization, while half steady state inactivation potential showed a hyperpolarizing shift of 43-55 mV. The time periods for most of the parameters relating to activation and fast and slow inactivation, lie close to 28-30 s, suggesting coupling of these kinetic processes through an oscillatory mechanism. Co-expression of the beta1-subunit affected the time periods of oscillation (close to 22 s for alpha + beta1) in steady state activation parameters. Application of a pulse protocol that mimicked paroxysmal depolarizing shift (PDS), a kind of depolarization seen in epileptic discharges, instead of a sustained depolarization, also caused oscillatory behaviour in the rNav1.2a alpha-subunit. This inherent pseudo-oscillatory mechanism may regulate excitability of the neurons, account for the epileptic discharges and subthreshold membrane potential oscillation and offer a molecular memory mechanism intrinsic to the neurons, independent of synaptic plasticity.

Intrinsic spontaneous activity and subthreshold oscillations in neurones of the rat dorsal column nuclei in culture

The basis of rhythmic activity observed at the dorsal column nuclei (DCN) is still open to debate. This study has investigated the electrophysiological properties of isolated DCN neurones deprived of any synaptic influence, using the perforated-patch technique. About half of the DCN neurones (64/130) were spontaneously active. More than half of the spontaneous neurones (36/64) showed a low threshold membrane oscillation (LTO) with a mean frequency of 11.4 Hz (range: 4.3-22.1 Hz, n = 20; I = 0). Cells showing LTOs also invariably showed a rhythmic 1.2 Hz clustering activity (groups of 2-5 action potentials separated by silent LTO periods). Also, more than one-third of the silent neurones presented clustering activity, always accompanied by LTOs, when slightly depolarised. The frequency of LTOs was voltage dependent and could be abolished by TTX (0.5 microM) and riluzole (30 microM), suggesting the participation of a sodium current. LTOs were also abolished by TEA (15 mM), which transformed clustering into tonic activity. In voltage clamp, most DCN neurones (85%) showed a TTX-/riluzole-sensitive persistent sodium current (INa,p), which activated at about -60 mV and had a half-maximum activation at -49.8 mV. An M-like, non-inactivating outward current was present in 95% of DCN neurones, and TEA (15 mM) inhibited this current by 73.7 %. The non-inactivating outward current was also inhibited by barium (1 mM) and linopirdine (10 microM), which suggests its M-like nature; both drugs failed to block the LTOs, but induced a reduction in their frequency by 56 and 20%, respectively. These results demonstrate for the first time that DCN neurones have a complex and intrinsically driven clustering discharge pattern, accompanied by subthreshold membrane oscillations. Subthreshold oscillations rely on the interplay of a persistent sodium current and a non-inactivating TEA-sensitive outward current.

Voltage-dependent membrane potential oscillations of rat striatal fast-spiking interneurons

We used whole-cell recordings to investigate subthreshold membrane potential oscillations and their relationship with intermittent firing in striatal fast-spiking interneurons. During current injections (100-500 pA, 1 s), these cells displayed a highly variable pattern of spike bursts (comprising 1-30 action potentials) interspersed with membrane potential oscillations. The oscillation threshold was -42 +/- 10 mV, and coincided with that for action potentials. The oscillation frequency was voltage dependent and ranged between 20 and 100 Hz. Oscillations were unaffected by the calcium channel blockers cadmium and nickel and by blockers of ionotropic glutamate and GABA receptors. Conversely, the sodium channel blocker tetrodotoxin fully abolished the oscillations and the spike bursts. The first spike of a burst appeared to be triggered by an oscillation, since the timing and rate of rise of the membrane potential in the subthreshold voltage region was similar for the two events. Conversely, the second spike (and the subsequent ones) displayed much faster depolarisations in the subthreshold voltage range, indicating that they were generated by a different mechanism. Consistent with these notions, a small pulse of intracellular current delivered during the oscillation was effective in triggering a burst of action potentials that largely outlasted the pulse. We conclude that fast-spiking interneuron oscillations are generated by an intrinsic membrane mechanism that does not require fast synaptic transmission, and which depends on sodium conductance but not calcium conductance, and that such oscillations are responsible for triggering the intermittent spike bursts that are typical of these neurons.

Pacemaker neurons for the theta rhythm and their synchronization in the septohippocampal reciprocal loop

Hippocampal theta (4-10 Hz) oscillation represents a well-known brain rhythm implicated in spatial cognition and memory processes. Its cellular mechanisms remain a matter of debate, and previous computational work has focused mostly on mechanisms intrinsic to the hippocampus. On the other hand, experimental data indicate that GABAergic cells in the medial septum play a pacemaker role for the theta rhythm. We have used biophysical modeling to address two major questions raised by the septal pacemaker hypothesis: what is the ion channel mechanism for the single-cell pacemaker behavior and how do these cells become synchronized? Our model predicts that theta oscillations of septal GABAergic cells depend critically on a low-threshold, slowly inactivating potassium current. Network simulations show that theta oscillations are not coherent in an isolated population of pacemaker cells. Robust synchronization emerges with the addition of a second GABAergic cell population. Such a reciprocally inhibitory circuit can be realized by the hippocampo-septal feedback loop.

Selective potentiation of N-type calcium channels by angiotensin II in rat subfornical organ neurones

1. Here we have characterized Ca(2+) currents in rat subfornical organ neurones, and their modulation by angiotensin II. Currents were of the high voltage-activated (HNA) subtype, as the threshold for activation was near -30 mV (mid-point potential (V(50)) of activation -14 mV). Using Ba(2+) as the charge carrier, little inactivation was observed, and it occurred only at depolarized potentials (V(50) of inactivation -12 mV). More inactivation was observed using Ca(2+) as the charge carrier, indicating that Ca(2+)-dependent inactivation plays a role in regulating Ca(2+) channel function in subfornical organ (SFO) neurones. 2. The net Ba(2+) current could be blocked by Cd(2+) (EC(50) 1.6 microM), confirming that currents are of the HVA variety. By using selective antagonists, we identified the presence of both L- and N-type channels; 20 microM nifedipine blocked 22 +/- 1 % of the current, while omega-conotoxin GVIA blocked 65 +/- 7 %, indicating that these currents make up the net current through Ca(2+) channels. 3. Angiotensin II potentiated the inward current throughout the range of activation. Using depolarizing voltage ramps, 1 nM angiotensin potentiated the peak current by 14 +/- 5 %. We then used selective blockade of the HVA component currents; 20 microM nifedipine failed to prevent the potentiation by angiotensin II (12 +/- 5 %), while blocking N-type channels with omega-conotoxin GVIA blocked the facilitation by ANG (2.3 +/- 2 %). Losartan (1 microM) prevented the actions of ANG on the inward current (1.6 +/- 1 %), indicating that the selective effects of ANG on N-type channels in SFO neurones are mediated by AT(1) receptors.