Ionic currents in cultured rat suprachiasmatic neurons

L-Type calcium channel blockade reduces network activity in human epileptic hypothalamic hamartoma tissue

Purpose

Human hypothalamic hamartomas (HHs) are associated with gelastic seizures, intrinsically epileptogenic, and notoriously refractory to medical therapy. We previously reported that the L-type calcium channel antagonist nifedipine blocks spontaneous firing and γ-aminobutyric acid (GABA)_A–induced depolarization of single cells in HH tissue slices. In this study, we examined whether blocking L-type calcium channels attenuates emergent activity of HH neuronal networks.

Methods

A high-density multielectrode array was used to record extracellular signals from surgically resected HH tissue slices. High-frequency oscillations (HFOs, ripples and fast ripples), field potentials, and multiunit activity (MUA) were studied (1) under normal and provoked [4-aminopyridine (4-AP)] conditions; and (2) following nifedipine treatment.

Key Findings

Spontaneous activity occurred during normal artificial cerebrospinal fluid (aCSF) conditions. Nifedipine reduced the total number and duration of HFOs, abolished the association of HFOs with field potentials, and increased the inter-HFO burst intervals. Notably, the number of active regions was decreased by 45 ± 9% (mean ± SEM) after nifedipine treatment. When considering electrodes that detected activity, nifedipine increased MUA in 58% of electrodes and reduced the number of field potentials in 67% of electrodes. Provocation with 4-AP increased the number of events and, as the number of electrodes that detected activity increased 248 ± 62%, promoted tissue-wide propagation of activity. During provocation with 4-AP, nifedipine effectively reduced HFOs, the association of HFOs with field potentials, field potentials, MUA, and the number of active regions, and limited propagation.

Significance

This is the first study to report (1) the presence of HFOs in human subcortical epileptic brain tissue in vitro; (2) the modulation of “pathologic” high-frequency oscillations (i.e., fast ripples) in human epileptic tissue by L-type calcium channel blockers; and (3) the modulation of network physiology and synchrony of emergent activity in human epileptic tissue following blockade of L-type calcium channels. Attenuation of activity in HH tissue during normal and provoked conditions supports a potential therapeutic usefulness of L-type calcium channel blockers in epileptic patients with HH.

On the role of calcium and potassium currents in circadian modulation of firing rate in rat suprachiasmatic nucleus neurons: multielectrode dish analysis

The master circadian clock of mammals in the suprachiasmatic nucleus (SCN) of the hypothalamus entrains to a 24-h daily light-dark cycle and regulates circadian rhythms. The SCN is composed of multiple neurons with cell autonomous clocks exhibiting robust firing rhythms with a high firing rate during the subjective day. The membrane target(s) of the cellular clock responsible for circadian modulation of the firing rate in SCN neurons still remain unclear. Previously, L-type Ca(2+) currents and fast delayed rectifier (FDR) K(+) currents have been suggested to contribute directly to circadian modulation of electrical activity. Using long-term continuous recording of activity from dispersed rat SCN neurons in multielectrode dish and ionic channel blockers, we tested these hypotheses. Neither an L-type Ca(2+) current blocker (20 microM of nifedipine for 2 days) nor an FDR current blocker (500 microM of 4-aminopyridine (4-AP) for 4 days) suppressed the circadian modulation of firing rate. A specific blocker of Na(+) persistent current (5 microM of riluzole for 1 day followed by 10 microM during the next day) reversibly suppressed firing activity in a dose-dependent manner. These data indicate that neither nifedipine-sensitive Ca(2+) current(s) nor 4-AP-sensitive K(+) current(s) are key membrane targets for circadian modulation of electrical firing rate in SCN neurons.

Modeling the electrophysiology of suprachiasmatic nucleus neurons

Neurons in the SCN act as the central circadian (approximately 24-h) pacemaker in mammals. Using measurements of the ionic currents in SCN neurons, the authors fit a Hodgkin-Huxley-type model that accurately reproduces slow (approximately 28 Hz) neural firing as well as the contributions of ionic currents during an action potential. When inputs of other SCN neurons are considered, the model accurately predicts the fractal nature of firing rates and the appearance of random bursting. In agreement with experimental data, the molecular clock within these neurons modulates the firing rate through small changes in the concentration of internal calcium, calcium channels, or potassium channels. Predictions are made on how signals from other neurons can start, stop, speed up, or slow down firing. Only a slow sodium inactivation variable and voltage do not reach equilibrium during the interval between action potentials, and based on this finding, a reduced model is formulated.

Electrophysiology of the suprachiasmatic circadian clock

In mammals, an internal timekeeping mechanism located in the suprachiasmatic nuclei (SCN) orchestrates a diverse array of neuroendocrine and physiological parameters to anticipate the cyclical environmental fluctuations that occur every solar day. Electrophysiological recording techniques have proved invaluable in shaping our understanding of how this endogenous clock becomes synchronized to salient environmental cues and appropriately coordinates the timing of a multitude of physiological rhythms in other areas of the brain and body. In this review we discuss the pioneering studies that have shaped our understanding of how this biological pacemaker functions, from input to output. Further, we highlight insights from new studies indicating that, more than just reflecting its oscillatory output, electrical activity within individual clock cells is a vital part of SCN clockwork itself.

Persistent calcium current in rat suprachiasmatic nucleus neurons

The suprachiasmatic nuclei contain the primary circadian clock, and suprachiasmatic nuclei neurons exhibit a diurnal modulation of spontaneous firing rate. The present study examined the voltage-gated persistent Ca(2+) current, in acutely isolated rat suprachiasmatic nuclei neurons using a ramp-type voltage-clamp protocol. Slow triangular voltage-clamp commands from a holding potential of -85 mV to +5 mV elicited inward current (100-400 pA) that was completely blocked by Cd(2+). This current showed little or no hysteresis, and was identified as persistent Ca(2+) current. The threshold for persistent Ca(2+) current ranged between -60 and -45 mV, and it was maximal at about -8 mV. Nifedipine at 10-20 microM blocked 80-100%. To assess the role of persistent Ca(2+) current in the generation of spontaneous action potentials in both acutely isolated and intact suprachiasmatic nuclei neurons, the effect of Cd(2+) and nifedipine on firing rate was studied using on-cell recording. Application of Cd(2+) exerted a weak excitatory effect and nifedipine had no significant effect on the spontaneous firing rate of isolated suprachiasmatic nuclei neurons. In all intact suprachiasmatic nuclei neurons in slice preparations (n=15), Cd(2+) slowly inhibited spontaneous firing; in high-frequency firing cells (four of 15), a transient increase of firing rate preceded inhibition. No significant effect of nifedipine on firing rate of intact suprachiasmatic nuclei neurons was found. Therefore, persistent Ca(2+) current itself (as carrier of charge) does not appear to contribute significantly to spontaneous firing of suprachiasmatic nuclei neurons. A slowly developing inhibitory effect of Cd(2+) on spontaneous firing of intact suprachiasmatic nuclei neurons in slice preparations may be due to penetration of Cd(2+) through Ca(2+) channels, and its subsequent effect on intracellular mechanisms, while the transient increase of firing rate in high-frequency firing neurons is probably due to inhibition of Ca(2+)-activated K(+) current.

Daily rhythmicity of large-conductance Ca2+ -activated K+ currents in suprachiasmatic nucleus neurons

Neurons within the suprachiasmatic nucleus (SCN) comprise the master circadian pacemaker in mammals. These neurons exhibit circadian rhythms in spontaneous action potential frequency and in the transcription of core circadian clock genes, including Period1 (Per1). Targeted electrophysiological recordings from SCN neurons marked with a green fluorescent protein (GFP) reporter of Per1 gene transcription have previously indicated that K(+) currents are critically involved in the expression of neurophysiological rhythmicity. The present study examined the role of large conductance, Ca(2+)-activated K(+) channels (BK) in the daily rhythmicity of mouse SCN neurons. BK-mediated currents were examined in Per1::GFP neurons under voltage clamp using iberiotoxin, a specific BK channel blocker. BK current was a greater proportion of whole-cell outward currents during the night than during the day. Analysis of iberiotoxin difference currents also demonstrated that BK current amplitude and density were greater during the night and that the day/night difference in steady state amplitude was not due to altered inactivation. Single cell RT-PCR demonstrated the presence of the BK channel transcript, KCNMA1, in Per1-expressing neurons. In situ hybridization analysis further showed that KCNMA1 mRNA was rhythmically expressed in the SCN under light:dark (LD) conditions, peaking during the middle of the night phase. Acute inhibition of BK currents blunted the circadian rhythm SCN neuron spike frequency. These results establish that BK channel function is elevated at night, thus altering SCN neuron activity.

Mechanism of spontaneous firing in dorsomedial suprachiasmatic nucleus neurons

We studied acutely dissociated neurons from the dorsomedial (shell) region of the rat suprachiasmatic nucleus (SCN) with the aim of determining the ionic conductances that underlie spontaneous firing. Most isolated neurons were spontaneously active, firing rhythmically at an average frequency of 8 +/- 4 Hz. After application of TTX, oscillatory activity generally continued, but more slowly and at more depolarized voltages; these oscillations were usually blocked by 2 microm nimodipine. To quantify the ionic currents underlying normal spontaneous activity, we voltage clamped cells using a segment of the spontaneous activity of each cell as voltage command and then used ionic substitution and selective blockers to isolate individual currents. TTX-sensitive sodium current flowed throughout the interspike interval, averaging -3 pA at -60 mV and -11 pA at -55 mV. Calcium current during the interspike interval was, on average, fourfold smaller. Except immediately before spikes, calcium current was outweighed by calcium-activated potassium current, and in current clamp, nimodipine usually depolarized cells and slowed firing only slightly (average, approximately 8%). Thus, calcium current plays only a minor role in pacemaking of dissociated SCN neurons, although it can drive oscillatory activity with TTX present. During normal pacemaking, the early phase of spontaneous depolarization (-85 to -60 mV) is attributable mainly to background conductance; cells have relatively depolarized resting potentials (with firing stopped by TTX and nimodipine) of -55 to -50 mV, although input resistance is high (9.5 +/- 4.1 GOmega). During the later phase of pacemaking (positive to -60 mV), TTX-sensitive sodium current is dominant.

Persistent subthreshold voltage-dependent cation single channels in suprachiasmatic nucleus neurons

The hypothalamic suprachiasmatic nucleus (SCN) contains the primary circadian pacemaker in mammals, and transmits circadian signals by diurnal modulation of neuronal firing frequency. The ionic mechanisms underlying the circadian regulation of firing frequency are unknown, but may involve changes in membrane potential and voltage-gated ion channels. Here we describe novel tetrodotoxin- and nifedipine-resistant subthreshold, voltage-dependent cation (SVC) channels that are active at resting potential of SCN neurons and increase their open probability (P(o)) with membrane depolarization. The increased P(o) reflects changes in the kinetics of the slow component of the channel closed-time, but not the channel open-time or fast closed-time. This study provides a background for investigation of the possible role of SVC channels in regulation of circadian oscillations of membrane excitability in SCN neurons.

Role of Neuronal Membrane Events in Circadian Rhythm Generation

Circadian clock systems are composed of an input or "entrainment" pathway by which synchronization to the external environment occurs, a pacemaker responsible for generating rhythmicity, and an output or "expression" pathway through which rhythmic signals act to modulate physiology and behavior. The circadian pacemaker contains molecular feedback loops of rhythmically expressed genes and their protein products, which, through interactions, generate a circa 24-h cycle of transcription and translation of clock and clock-controlled genes. Neuronal membrane events appear to play major roles in entrainment of circadian rhythms in mollusks and mammals. In mammals, the suprachiasmatic nuclei of the hypothalamus receive photic information via the retinohypothalamic tract. Retinal signals, mediated by glutamate, induce calcium release and activate a number of intracellular cascades involved in photic gating and phase shifting. Membrane events are also involved in rhythm expression. Calcium and potassium currents influence the electrical output of pacemaker neurons by altering shape and intervals of impulse prepotentials, afterhyperpolarization periods, and interspike intervals, as well as altering membrane potentials and thereby shaping the spontaneous rhythmic spiking patterns. Unlike the involvement of membrane events in circadian entrainment and expression, it is less clear whether electrical activity, postsynaptic events, and transmembrane ion fluxes also are essential elements in rhythm generation. Studies, however, suggest that neuronal membrane activity may indeed play a crucial role in circadian rhythm generation.

Membranes, Ions, and Clocks: Testing the Njus–Sulzman–Hastings Model of the Circadian Oscillator

Current circadian clock models based on interlocking autoregulatory transcriptional?translational negative feedback loops have arisen out of an explosion of molecular genetic data obtained over the last decade (for review, see Stanewsky, 2003; Young and Kay, 2001). An earlier model of circadian oscillation was based on feedback interactions between membrane ion transport systems and ion concentration gradients (Njus et al., 1974, 1976). This membrane model was posited as a more plausible alternative at the time to the even earlier "chronon" model, which was based on autoregulatory genetic feedback loops (Ehret and Trucco, 1967). The membrane model has been tested in a number of experimental systems by pharmacologically manipulating either ionic gradients across the plasma membrane or ion transport systems, but with inconsistent results. In the meantime, the scope and explanatory power of the genetic models overshadowed inquiries into the role of membrane ion fluxes in clock function. However, several recently developed techniques described in this article have provided a new glimpse into the essential role that membrane ion fluxes play in the mechanism of the core circadian oscillator and indicate that a complete understanding of the clock must include both genetic and membrane-based feedback loops.

Rhythmic regulation of membrane potential and potassium current persists in SCN neurons in the absence of environmental input

Neurons of the mammalian suprachiasmatic nucleus (SCN) generate self-sustained rhythms of action potential frequency having a period of approximately 24 h. It is generally believed that cell autonomous circadian oscillation of a network of biological clock genes drives the circadian rhythm in neuronal firing rate through as yet unspecified effects on the neuronal membrane. While it is clear that cyclic gene expression continues in constant darkness, previous studies have not examined which specific membrane properties of SCN neurons continue to oscillate in constant conditions. Here, we demonstrate that SCN neurons exhibit robust rhythms in resting membrane potential and input resistance in constant darkness. Furthermore, application of the K+ channel blocker tetraethylammonium revealed a rhythm in K+ current amplitude that persists in constant darkness and underlies the rhythm in membrane potential.

Mechanism of Irregular Firing of Suprachiasmatic Nucleus Neurons in Rat Hypothalamic Slices

The mechanisms of irregular firing of spontaneous action potentials in neurons from the rat suprachiasmatic nucleus (SCN) were studied in hypothalamic slices using cell-attached and whole cell recording. The firing pattern of spontaneous action potentials could be divided into regular and irregular, based on the interspike interval (ISI) histogram and the membrane potential trajectory between action potentials. Similar to previous studies, regular neurons had a firing rate about >3.5 Hz and irregular neurons typically fired about <3.5 Hz. The ISI of irregular-firing neurons was a linear function of the sum of inhibitory postsynaptic potentials (IPSPs) between action potentials. Bicuculline (10–30 μM) suppressed IPSPs and converted an irregular pattern to a more regular firing. Bicuculline also depolarized SCN neurons and induced bursting-like activity in some SCN neurons. Gabazine (20 μM), however, suppressed IPSPs without depolarization, and also converted irregular activity to regular firing. Thus GABAA receptor–mediated IPSPs appear responsible for irregular firing of SCN neurons in hypothalamic slices.

Circadian Dynamics of Cytosolic and Nuclear Ca2+ in Single Suprachiasmatic Nucleus Neurons

Intracellular free Ca(2+) regulates diverse cellular processes, including membrane potential, neurotransmitter release, and gene expression. To examine the cellular mechanisms underlying the generation of circadian rhythms, nucleus-targeted and untargeted cDNAs encoding a Ca(2+)-sensitive fluorescent protein (cameleon) were transfected into organotypic cultures of mouse suprachiasmatic nucleus (SCN), the primary circadian pacemaker. Circadian rhythms in cytosolic but not nuclear Ca(2+) concentration were observed in SCN neurons. The cytosolic Ca(2+) rhythm period matched the circadian multiple-unit-activity (MUA)-rhythm period monitored using a multiple-electrode array, with a mean advance in phase of 4 hr. Tetrodotoxin blocked MUA, but not Ca(2+) rhythms, while ryanodine damped both Ca(2+) and MUA rhythms. These results demonstrate cytosolic Ca(2+) rhythms regulated by the release of Ca(2+) from ryanodine-sensitive stores in SCN neurons.

Role of membrane conductances and protein synthesis in subjective day phase advances of the hamster circadian clock by neuropeptide Y

Neurons of the mammalian circadian pacemaker in the hypothalamic suprachiasmatic nuclei exhibit a rhythm in firing rate that can be reset by neuropeptide Y. We recorded the effects of neuropeptide Y on Na+ and K+ conductances of hamster suprachiasmatic nuclei neurons using whole-cell, perforated-patch and cell-attached patch-clamp recordings, both in dissociated and brain slice preparations. While neuropeptide Y had no effect on voltage-gated Na+ currents, neuropeptide Y activated a leak K+ current. Neuropeptide Y phase advances in the suprachiasmatic nuclei brain slice preparation were blocked by a number of K+ channel blockers (tetraethylammonium chloride, dendrotoxin-I, glybenclamide). However, a K+ ionophore, valinomycin, did not shift the rhythm. The inhibition by tetraethylammonium chloride did not persist in the presence of glutamatergic receptor blockers. We have previously shown that glutamate can oppose neuropeptide Y phase-shifting actions, suggesting that K+ channel inhibition acts by inducing glutamate release. Protein synthesis inhibitors had no effect on clock phase when applied during the subjective day, and had no influence on neuropeptide Y-induced phase shifts. On the other hand, glutamate's ability to inhibit neuropeptide Y shifts was abolished by protein synthesis inhibition. Thus, while neuropeptide Y phase shifts do not require protein synthesis, glutamate blocks neuropeptide Y shifts via increased gene expression during the subjective day, at a time when it does not reset the clock. These results indicate that neuropeptide Y phase shifts via a mechanism that does not involve changes in membrane conductance or protein synthesis.

The SCN is comprised of a population of coupled oscillators

The circadian clock in the suprachiasmatic nucleus (SCN) of the mammalian hypothalamus exhibits two necessary properties: (1) a mechanism for the generation of autonomous circadian rhythms in individual pacemaker cells, and (2) a means to synchronize the autonomous pacemaker cells. A variety of potential components of the endogenous pacemaker, including ion channels, second messengers, transcriptional factors, and the protein targets of kinases and transcription factors are reviewed. Similarly, reverse transmitter transport, extracellular ion fluxes, small membrane-diffusible molecules, glial regulation, and neural adhesion molecules are considered as possible synchronizing factors. Provisional criteria are suggested for empirical distinction of endogenous pacemaker versus synchronizing mechanisms.

Cellular mechanisms underlying spontaneous firing in rat suprachiasmatic nucleus: involvement of a slowly inactivating component of sodium current

Neurons constituting the pacemaker of circadian rhythms, located in the suprachiasmatic nucleus, generate spontaneous firing patterns that change across the day-night cycle. Their average spontaneous firing rate is considered an important functional marker of clock activity because it is highest during daytime and low at night. In this study we investigate the ionic mechanisms underlying spontaneous firing in acutely prepared slices and dissociated neurons of the suprachiasmatic nucleus. In current-clamp mode, spontaneous action potentials were consistently preceded by depolarizing ramps. These ramps were Na+ dependent, were sensitive to tetrodotoxin (TTX), and disappeared on hyperpolarization. Ramps and associated spikes were not abolished by blockers of the H current (1 mM cesium) or calcium currents (50 microM nickel or 200 microM cadmium). In voltage-clamped neurons in slices or dissociated neurons, TTX-sensitive and Na+-dependent inward current was observed to activate well below firing threshold (-60 to -50 mV). The low-threshold component of Na+ current inactivated slowly as compared with the fast component that mediates action potentials. However, its inactivation proceeded more rapidly than has been reported for the persistent Na+ current in cortical structures. Persistent Na+ current was generally absent or small in amplitude. The voltage dependence and kinetics of the slowly inactivating component of Na+ current are consistent with the hypothesis that it is partially deinactivated during spike afterhyperpolarizations and contributes significantly to subsequent depolarizing ramps. These observations implicate the slowly inactivating component of Na+ current in ionic mechanisms governing spontaneous firing in suprachiasmatic nucleus neurons.

Mechanisms underlying the chronotropic effect of angiotensin II on cultured neurons from rat hypothalamus and brain stem

The chronotropic effect of angiotensin II (Ang II) was studied in cultured neurons from rat hypothalamus and brain stem with the use of the patch-clamp technique. Ang II (100 nM) increased the neuronal spontaneous firing rate from 0.8 +/- 0.3 (SE) Hz in control to 1.3 +/- 0.4 Hz (n = 7, P < 0.05). The amplitude of threshold stimulation was decreased by Ang II (100 nM) from 82 +/- 4 pA to 62 +/- 5 pA (n = 4, P < 0.05). These actions of Ang II were reversed by the angiotensin type 1 (AT1) receptor antagonist losartan (1 microM). In the presence of tetrodotoxin, Ang II (100 nM) significantly increased the frequency and the amplitude of the Cd2+-sensitive subthreshold activity of the cultured neurons. Ang II also stimulated the subthreshold early afterdepolarizations (EADs) to become fully developed action potentials. Similar to the action of Ang II, the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA, 100 nM) increased the firing rate from 0.76 +/- 0.3 Hz to 2.3 +/- 0.5 Hz (n = 6, P < 0.05) and increased the neuronal subthreshold activity. After neurons were intracellularly dialyzed with PKC inhibitory peptide (PKCIP, 5 microM), PMA alone, Ang II alone, or PMA plus Ang II no longer increased the action potential firing initiated from the resting membrane potential level. However, superfusion of PMA plus Ang II or Ang II alone increased the number of EADs that reached threshold and produced action potentials even in the presence of PKCIP (5 microM, n = 4). The actions of Ang II could also be mimicked by depolarizing pulse and K+ channel blockers (tetraethylammonium chloride or 4-aminopyridine). These results indicate that Ang II by activation of AT1 receptors increases neuronal excitability and firing frequency, and that this may involve both PKC dependent and -independent mechanisms.

Membrane properties and synaptic inputs of suprachiasmatic nucleus neurons in rat brain slices

1. Whole-cell recordings were made from 390 neurons of the suprachiasmatic nucleus (SCN) in horizontal brain slices during different portions of the circadian day. The locomotor activity of the rats was measured prior to the preparation of brain slices to insure that each rat was entrained to a 12 h-12 h light-dark cycle. 2. The mean input conductance was 42% higher (1.58 nS) in neurons recorded near the subjective dawn than those (1.11 nS) recorded near the subjective dusk. The current required to hold the neurons at -60 mV also showed a circadian variation with a peak in the middle of the subjective day and a nadir in the middle of the subjective night. Analysis of the variations in the input conductance and the holding current at -60 mV suggested that at least two ion conductances are involved in the pacemaking of the circadian rhythms. 3. Voltage-clamped SCN neurons often had both outward and inward spontaneous postsynaptic currents. The outward currents were blocked by bicuculline but not by strychnine, and were identified as IPSCs mediated by GABAA receptors. The inward currents were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and were identified as EPSCs mediated by glutamate. Most spontaneous synaptic currents were miniature currents but action potential-dependent large events were seen more often in IPSCs than in EPSCs. 4. Stimulation of the optic nerve or chiasm usually evoked a monosynaptic EPSC which was mediated by both NMDA and non-NMDA receptors. In 13% of cells, optic nerve stimulation evoked an outward current or an inward current followed by an outward current; all the evoked currents were blocked by 4-aminophosphonovaleric acid (APV) and CNQX whereas the outward current only was blocked by bicuculline, suggesting involvement of an inhibitory interneuron. 5. SCN neurons sum the excitatory inputs from both optic nerves; on average each SCN cell receives innervation from at least 4.8 retinohypothalamic tract (RHT) axons. 6. Focal stimulation in the vicinity of the recorded neuron revealed that nearly all SCN neurons receive local or extranuclear GABAergic inputs operating via GABAA receptors. The EPSCs activated by such stimulation were not significantly different in amplitude and pharmacological properties from those induced by RHT stimulation. 7. One hundred and one neurons were labelled with neurobiotin during whole-cell recording. Based on the dendritic structures, four types of SCN neurons (monopolar, radial, simple bipolar and curly bipolar) were identified. The curly bipolar cells had a higher membrane conductance, holding current and hyperpolarization-activated current (Ih) amplitude than the other neuronal types. Radial neurons did not respond to optic nerve stimulation, which activated EPSCs in the other cell types.