Spike phase locking in CA1 pyramidal neurons depends on background conductance and firing rate.

Oscillatory activity in neuronal networks correlates with different behavioral states throughout the nervous system, and the frequency-response characteristics of individual neurons are believed to be critical for network oscillations. Recent in vivo studies suggest that neurons experience periods of high membrane conductance, and that action potentials are often driven by membrane potential fluctuations in the living animal. To investigate the frequency-response characteristics of CA1 pyramidal neurons in the presence of high conductance and voltage fluctuations, we performed dynamic-clamp experiments in rat hippocampal brain slices. We drove neurons with noisy stimuli that included a sinusoidal component ranging, in different trials, from 0.1 to 500 Hz. In subsequent data analysis, we determined action potential phase-locking profiles with respect to background conductance, average firing rate, and frequency of the sinusoidal component. We found that background conductance and firing rate qualitatively change the phase-locking profiles of CA1 pyramidal neurons versus frequency. In particular, higher average spiking rates promoted bandpass profiles, and the high-conductance state promoted phase-locking at frequencies well above what would be predicted from changes in the membrane time constant. Mechanistically, spike rate adaptation and frequency resonance in the spike-generating mechanism are implicated in shaping the different phase-locking profiles. Our results demonstrate that CA1 pyramidal cells can actively change their synchronization properties in response to global changes in activity associated with different behavioral states.

Membrane voltage fluctuations reduce spike frequency adaptation and preserve output gain in CA1 pyramidal neurons in a high-conductance state.

Modulating the gain of the input-output function of neurons is critical for processing of stimuli and network dynamics. Previous gain control mechanisms have suggested that voltage fluctuations play a key role in determining neuronal gain in vivo. Here we show that, under increased membrane conductance, voltage fluctuations restore Na(+) current and reduce spike frequency adaptation in rat hippocampal CA1 pyramidal neurons in vitro. As a consequence, membrane voltage fluctuations produce a leftward shift in the frequency-current relationship without a change in gain, relative to an increase in conductance alone. Furthermore, we show that these changes have important implications for the integration of inhibitory inputs. Due to the ability to restore Na(+) current, hyperpolarizing membrane voltage fluctuations mediated by GABA(A)-like inputs can increase firing rate in a high-conductance state. Finally, our data show that the effects on gain and synaptic integration are mediated by voltage fluctuations within a physiologically relevant range of frequencies (10-40 Hz).

A-type K+ currents in intralaminar thalamocortical relay neurons.

Transient A-type K+ currents (I(A)) are known to influence the firing pattern of a number of thalamic cell types, but have not been investigated in intralaminar thalamocortical (TC) relay neurons yet. We therefore combined whole-cell patch-clamp techniques, PCR analysis, and immunohistochemistry to investigate the voltage-dependent and pharmacological properties of I(A) and to determine its molecular basis in TC neurons from the centrolateral, paracentral, and centromedial thalamic nuclei. I(A) revealed half-maximal (V (h)) activation and inactivation at about -17 and -67 mV, respectively. At a concentration of 5-10 mM 4-aminopyridine (4-AP) completely blocked I(A). Furthermore, I(A) was nearly unaffected by two sea anemone toxins (blood depressing substances 1 and 2, BDS1 and BDS2; 6-8% block at a concentration of 1 µM) but strongly sensitive to the K(V)4 channel blocker Heteropoda venatoria toxin 2 (HpTx2; about 45% block at a concentration of 5 µM). PCR screening revealed the expression of K(V)4.1-4.3, with strongest expression for K(V)4.2 and weak expression for K(V)4.1. Accordingly K(V)4.1 was not detected in immunohistochemical staining. Furthermore, K(V)4.2 and K(V)4.3 revealed mainly dendritic and somatic staining, respectively. Together with current clamp recordings, these findings point to a scenario where the fast transient I(A) in intralaminar TC neurons has a depolarized threshold at potentials negative to -50 mV, is substantially generated by K(V)4.2 and K(V)4.3 channels, allows prominent burst firing at hyperpolarized potentials, prevents the generation of high-threshold potentials, generates a delayed onset of firing at more depolarized potentials, and allows fast tonic firing.

The neuroprotective impact of the leak potassium channel TASK1 on stroke development in mice.

Oxygen depletion (O(2)) and a decrease in pH are initial pathophysiological events in stroke development, but secondary mechanisms of ischemic cell death are incompletely understood. By patch-clamp recordings of brain slice preparations we show that TASK1 and TASK3 channels are inhibited by pH-reduction (42+/-2%) and O(2) deprivation (36+/-5%) leading to membrane depolarization, increased input resistance and a switch in action potential generation under ischemic conditions. In vivo TASK blockade by anandamide significantly increased infarct volumes at 24 h in mice undergoing 30 min of transient middle cerebral artery occlusion (tMCAO). Moreover, blockade of TASK channels accelerated stroke development. Supporting these findings TASK1(-/-) mice developed significantly larger infarct volumes after tMCAO accompanied by worse outcome in functional neurological tests compared to wild type mice. In conclusion, our data provide evidence for an important role of functional TASK channels in limiting tissue damage during cerebral ischemia.

Correlation of T-channel coding gene expression, IT, and the low threshold Ca2+ spike in the thalamus of a rat model of absence epilepsy.

T-type Ca(2+) current-dependent burst firing of thalamic neurons is thought to be involved in the hyper-synchronous activity observed during absence seizures. Here we investigate the correlation between the expression of T-channel coding genes (alpha1G, -H, -I), T-type Ca(2+) current, and the T-current-dependent low threshold Ca(2+) spike in three functionally distinct thalamic nuclei (lateral geniculate nucleus; centrolateral nucleus; reticular nucleus) in a rat model of absence epilepsy, the WAG/Rij rats, and a non-epileptic control strain, the ACI rats. The lateral geniculate nucleus and centrolateral nucleus were found to primarily express alpha1G and alpha1I, while the reticular thalamic nucleus expressed alpha1H and alpha1I. Expression was higher in WAG/Rij when compared to ACI. The T-type Ca(2+) current properties matched the predictions derived from the expression pattern analysis. Current density was larger in all nuclei of WAG/Rij rats when compared to ACI and correlated with LTS size and the minimum LTS generating slope, while T-type Ca(2+) current voltage dependency correlated with the LTS onset potential.

T-current related effects of antiepileptic drugs and a Ca2+ channel antagonist on thalamic relay and local circuit interneurons in a rat model of absence epilepsy.

Channel blocking, anti-oscillatory, and anti-epileptic effects of clinically used anti-absence substances (ethosuximide, valproate) and the T-type Ca2+ current (IT) blocker mibefradil were tested by analyzing membrane currents in acutely isolated local circuit interneurons and thalamocortical relay (TC) neurons, slow intrathalamic oscillations in brain slices, and spike and wave discharges (SWDs) occurring in vivo in Wistar Albino Glaxo rats from Rijswijk (WAG/Rij). Substance effects in vitro were compared between WAG/Rij and a non-epileptic control strain, the ACI rats. Ethosuximide (ETX) and valproate were found to block IT in acutely isolated thalamic neurons. Block of IT by therapeutically relevant ETX concentrations (0.25-0.75 mM) was stronger in WAG/Rij, although the maximal effect at saturating concentrations (>or=10 mM) was stronger in ACI. Ethosuximide delayed the onset of the low threshold Ca2+ spike (LTS) of neurons recorded in slice preparations. Mibefradil (>or=2 microM) completely blocked IT and the LTS, dampened evoked thalamic oscillations, and attenuated SWDs in vivo. Computational modeling demonstrated that the complete effect of ETX can be replicated by a sole reduction of IT. However, the necessary degree of IT reduction was not induced by therapeutically relevant ETX concentrations. A combined reduction of IT, the persistent sodium current, and the Ca2+ activated K+ current resulted in an LTS alteration resembling the experimental observations. In summary, these results support the hypothesis of IT reduction as part of the mechanism of action of anti-absence drugs and demonstrate the ability of a specific IT antagonist to attenuate rhythmic burst firing and SWDs.

Contribution of TWIK-related acid-sensitive K+ channel 1 (TASK1) and TASK3 channels to the control of activity modes in thalamocortical neurons.

The thalamocortical network is characterized by rhythmic burst activity during natural sleep and tonic single-spike activity during wakefulness. The change between these two activity modes is partially governed by transmitters acting on leak K+ currents in the thalamus, although the nature of the constituting ion channels is not yet known. In the present study, the contribution of members of the two-pore domain K+ channel family to the leak current was investigated using whole-cell patch-clamp techniques and molecular biological techniques. RT-PCR and in situ hybridization revealed the expression of TWIK-related acid-sensitive K+ channel 1 (TASK 1) and TASK3 channels in the rat dLGN. Voltage-clamp recordings of thalamocortical relay neurons in slice preparations demonstrated the existence of a current component sensitive to the TASK channel blocker bupivacaine, which reversed at the presumed K+ equilibrium potential, showed outward rectification, and contributed approximately 40% to the standing outward current at depolarized values of the membrane potential (-28 mV). The pharmacological profile was indicative of TASK channels, in that the current was sensitive to changes in extracellular pH, reduced by muscarine and increased by halothane, and these effects were occluded by a near-maximal action of bupivacaine. Pharmacological manipulation of this current under current-clamp conditions resulted in a shift between burst and tonic firing modes. It is concluded that TASK1 and TASK3 channels contribute to the muscarine- and halothane-sensitive conductance in thalamocortical relay neurons, thereby contributing to the change in the activity mode of thalamocortical networks observed during the sleep-wake cycle and on application of inhalational anesthetics.

Get the rhythm: modeling neuronal activity.

The simulation system NEURON is a common research tool for constructing structurally and functionally realistic models of neuronal systems. NEURON allows the development of simulations at any level of complexity, from subcellular components to single cells, cellular networks, and system-level models. Focusing on an in vitro cell model of a single, acutely isolated thalamic neuron, we used the simulation environment to address and to discuss the following questions in an undergraduate course: (i) Which parts are required to design a single compartment with passive electrical properties? (ii) Which components are necessary to model a single action potential or a train of action potentials? (iii) What can we learn from voltage-clamp and current-clamp experiments? (iv) What kind of cellular parameters are accessible from the modeling data? (v) What are the differences between single-compartment models and multi-compartment models? (vi) What are the advantages and disadvantages of artificial cell models? (vii) Can realistic modeling open up new strategies to discover the way that neurons process information?

Activity Modes in Thalamocortical Relay Neurons are Modulated by G(q)/G(11) Family G-proteins - Serotonergic and Glutamatergic Signaling.

In thalamocortical relay (TC) neurons, G-protein-coupled receptors play an important part in the control of activity modes. A conditional Gα(q) knockout on the background of a constitutive Gα(11) knockout (Gα(q)/Gα(11) (-/-)) was used to determine the contribution of Gq/G11 family G-proteins to metabotropic serotonin (5-HT) and glutamate (Glu) function in the dorsal part of the lateral geniculate nucleus (dLGN). In control mice, current clamp recordings showed that α-m-5-HT induced a depolarization of V(rest) which was sufficient to suppress burst firing. This depolarization was concentration-dependent (100 µM: +6 ± 1 mV, n = 10; 200 µM: +10 ± 1 mV, n = 7) and had a conditioning effect on the activation of other Gα(q)-mediated pathways. The depolarization was significantly reduced in Gα(q)/Gα(11) (-/-) (100 µM: 3 ± 1 mV, n = 11; 200 µM: 5 ± 1 mV, n = 6) and was apparently insufficient to suppress burst firing. Activating Gα(q)-coupled muscarinic receptors affected the magnitude of α-m-5-HT-induced effects in a reciprocal manner. Furthermore, the depolarizing effect of mGluR1 agonists was significantly reduced in Gα(q)/Gα(11) (-/-) mice. Immunohistochemical stainings revealed binding of 5-HT(2C)R- and mGluR1α-, but not of 5-HT(2A)R-specific antibodies in the dLGN of Gα(q)/Gα(11) (-/-) mice. In conclusion, these findings demonstrate that transmitters of ascending brainstem fibers and corticofugal fibers both signal via a central element in the form of Gq/G11-mediated pathways to control activity modes in the TC system.

Muscarinic ACh receptor-mediated control of thalamic activity via G(q)/G (11)-family G-proteins.

A genetic knock out was used to determine the specific contribution of G(q)/G(11)-family G-proteins to the function of thalamocortical relay (TC) neurons. Disruption of Galpha(q) function in a conditional forebrain-specific Galpha(q)/Galpha(11)-double-deficient mouse line (Galpha(q)/Galpha(11)(-/-) had no effects on the resting membrane potential (V (rest)) and the amplitude of the standing outward current (I (SO)). Stimulation of muscarinic acetylcholine (ACh) receptors (mAChR; muscarine, 50 microM) induced a decrease in I (SO) amplitude in wild-type mice (36 +/- 4%, n = 5), a constitutive Galpha(11)-deficient mouse line (Galpha(11)(-/-; 36 +/- 3%, n = 8), and Galpha(q)/Galpha(11)(-/-) (11 +/- 2%, n = 16). Current-clamp recordings revealed a muscarine-induced positive shift in V (rest) of 23 +/- 2 mV (n = 6), 18 +/- 5 mV (n = 5), and 2 +/- 1 mV (n = 9) in wild type, Galpha(11)(-/-), and Galpha(q)/Galpha(11)(-/-), respectively. This depolarization was associated with a change in TC neuron activity from burst to tonic firing in wild type and Galpha(11)(-/-), but not in Galpha(q)/Galpha(11)(-/-). The use of specific antibodies and of pharmacological agents with preferred affinity points to the contribution of m(1)AChR and m(3)AChR. In conclusion, we present two novel aspects of the physiology of the thalamocortical system by demonstrating that the depolarization of TC neurons, which is induced by the action of transmitters of ascending brainstem fibers, is governed roughly equally by both m(1)AChR and m(3)AChR and is transduced by Galpha(q) but not by Galpha(11).

Specific expression of low-voltage-activated calcium channel isoforms and splice variants in thalamic local circuit interneurons.

It has been suggested that the specific burst firing patterns of thalamic neurons reflect differential expression of low-voltage-activated (LVA) Ca(2+) channel subtypes and their splice variants. By combining electrophysiological, molecular biological, immunological, and computational modeling techniques we here show that diverging LVA Ca(2+) currents of thalamocortical relay (TC) and GABAergic interneurons of the dLGN correlate with a differential expression of LVA Ca(2+) channel splice variations and isoforms (alpha1G-a in TC; alpha1G-bc and alpha1I in interneurons). Implementation of the observed LVA Ca(2+) current differences into a TC neuron model changed the burst firing from TC-like to interneuron-like. We conclude that alternative splicing of the alpha1G isoform in dLGN TC and interneurons, and the exclusive expression of the alpha1I isoform in interneurons play a prominent role in setting the different LVA Ca(2+) current properties of TC and interneurons, which critically contribute to the diverging burst firing behavior of these neurons.

Modulation of neuronal activity by the endogenous pentapeptide QYNAD.

Inflammation and demyelination both contribute to the neurological deficits characteristic of multiple sclerosis. Neurological dysfunctions are attributable to inflammatory demyelination and, in addition, to soluble factors such as nitric oxide, cytokines and antibodies. QYNAD, an endogenous pentapeptide identified in the cerebrospinal fluid of patients with demyelinating disorders, has been proposed to promote axonal dysfunction by blocking sodium channels. The present study aimed at characterizing the properties of QYNAD in acutely isolated thalamic neurons in vitro. QYNAD, but not a scrambled peptide (NYDQA), blocked sodium channels in neurons by shifting the steady-state inactivation to more negative potentials. Blocking properties followed a dose-response curve with a maximum effect at 10 microm. A fluorescently labelled QYNAD analogue with retained biological activity specifically stained thalamic neurons, positive for type II sodium channels, thus demonstrating the specificity of QYNAD binding. Our study confirms and extends previous observations describing QYNAD as a potent sodium channel-blocking agent. These data as well as our preliminary observations in in vivo experiments in an animal model of inflammatory CNS demyelination warrant further in vivo studies in order to clarify the exact pathogenetic role of QYNAD in inflammatory neurological diseases.