Inward rectification and low threshold calcium conductance in rat cerebellar Purkinje cells. An in vitro study

GLP-1 enhances hyperpolarization-activated currents of mouse cerebellar Purkinje cell in vitro

Glucagon-like peptide-1 (GLP-1) is mainly secreted by preglucagonergic neurons in the nucleus tractus solitarius, which plays critical roles in regulation of neuronal activity in the central nervous system through its receptor. In the cerebellar cortex, GLP-1 receptor is abundantly expressed in the molecular layer, Purkinje cell (PC) layer and granular layer, indicating that GLP-1 may modulate the cerebellar neuronal activity. In this study, we investigated the mechanism by which GLP1 modulates mouse cerebellar PC activity in vitro. After blockade of glutamatergic and GABAergic synaptic transmission in PCs, GLP1 increased the spike firing rate accompanied by depolarization of membrane potential and significantly depressed the after-hyperpolarizing potential and outward rectifying current of spike firing discharges via GLP1 receptors. In the presence of TTX and Ba2+, GLP1 significantly enhanced the hyperpolarized membrane potential-evoked instant current, steady current, tail current (I-tail) and hyperpolarization-activated (IH) current. Application of a selective IH channel antagonist, ZD7288, blocked IH and abolished the effect of GLP1 on PC membrane currents. The GLP1 induced enhancement of membrane currents was also abolished by a selective GLP1 receptor antagonist, exendin-9-39, as well as by protein kinase A (PKA) inhibitors, KT5720 and H89. In addition, immunofluorescence detected GLP1 receptor in the mouse cerebellar cortex, mostly in PCs. These results indicated that GLP1 receptor activation enhanced IH channel activity via PKA signaling, resulting in increased excitability of mouse cerebellar PCs in vitro. The present findings indicate that GLP1 plays a critical role in modulating cerebellar function by regulating the spike firing activity of mouse cerebellar PCs.

Lobule-Related Action Potential Shape- and History-Dependent Current Integration in Purkinje Cells of Adult and Developing Mice

Purkinje cells (PCs) are the principal cells of the cerebellar cortex and form a central element in the modular organization of the cerebellum. Differentiation of PCs based on gene expression profiles revealed two subpopulations with distinct connectivity, action potential firing and learning-induced activity changes. However, which basal cell physiological features underlie the differences between these subpopulations and to what extent they integrate input differentially remains largely unclear. Here, we investigate the cellular electrophysiological properties of PC subpopulation in adult and juvenile mice. We found that multiple fundamental cell physiological properties, including membrane resistance and various aspects of the action potential shape, differ between PCs from anterior and nodular lobules. Moreover, the two PC subpopulations also differed in the integration of negative and positive current steps as well as in size of the hyperpolarization-activated current. A comparative analysis in juvenile mice confirmed that most of these lobule-specific differences are already present at pre-weaning ages. Finally, we found that current integration in PCs is input history-dependent for both positive and negative currents, but this is not a distinctive feature between anterior and nodular PCs. Our results support the concept of a fundamental differentiation of PCs subpopulations in terms of cell physiological properties and current integration, yet reveals that history-dependent input processing is consistent across PC subtypes.

The hyperpolarization-activated, cyclic nucleotide-gated channel resides on myocytes in mouse bladders and contributes to adrenergic-induced detrusor relaxation

Control of urinary continence is predicated on sensory signaling about bladder volume. Bladder sensory nerve activity is dependent on tension, implicating autonomic control over detrusor myocyte activity during bladder filling. Hyperpolarization-activated cyclic nucleotide-gated (HCN) ion channels are known contributors to bladder control, but their mechanism of action is not well understood. The lack of a definitive identification of cell type(s) expressing HCN in the bladder presents a significant knowledge gap. We recently reported a complete transcriptomic atlas of the C57BL/6 mouse bladder showing the dominant HCN paralog in mouse bladder, Hcn1, is limited to a subpopulation of detrusor smooth myocytes (DSMs). Here, we report details of these findings, along with results of patch-clamp experiments, immunohistochemistry, and functional myobath/tension experiments in bladder strips. With the use of a transgenic mouse expressing fluorescence-tagged α-smooth muscle actin, our data confirmed location and function of DSM HCN channels. Despite previous associations of HCN with postulated bladder interstitial cells, neither evidence of specific interstitial cell types nor an association of nonmyocytes with HCN was discovered. We confirm that HCN activation participates in reducing sustained (tonic) detrusor tension via cAMP, with no effect on intermittent (phasic) detrusor activity. In contrast, blockade of HCN increases phasic activity induced by a protein kinase A (PKA) blocker or a large-conductance Ca²⁺-activated K⁺ (BK) channel opener. Our findings, therefore, suggest a central role for detrusor myocyte HCN in regulating and constraining detrusor myocyte activity during bladder filling.

Afferents of the mouse linear nucleus

The linear nucleus (Li) was identified in 1978 from its projections to the cerebellum. However, there is no systematic study of its connections with other areas of the central nervous system possibly due to the challenge of injecting retrograde tracers into this nucleus. The present study examines its afferents from some nuclei involved in motor and cardiovascular control with anterograde tracer injections. BDA injections into the central amygdaloid nucleus result in labeled fibers to the ipsilateral Li. Bilateral projections with an ipsilateral dominance were observed after injections in a) jointly the paralemniscal nucleus, the noradrenergic group 7/ Köllike -Fuse nucleus/subcoeruleus nucleus, b) the gigantocellular reticular nucleus, c) and the solitary nucleus/the parvicellular/intermediate reticular nucleus. Retrogradely labeled neurons were observed in Li after BDA injections into all these nuclei except the central amygdaloid and the paralemniscal nuclei. Our results suggest that Li is involved in a variety of physiological functions apart from motor and balance control it may exert via its cerebellar projections.

Mechanisms of Spontaneous Climbing Fiber Discharge-Evoked Pauses and Output Modulation of Cerebellar Purkinje Cell in Mice

Climbing fiber (CF) afferents modulate the frequency and patterns of cerebellar Purkinje cell (PC) simple spike (SS) activity, but its mechanism is unclear. In the present study, we investigated the mechanisms of spontaneous CF discharge-evoked pauses and the output modulation of cerebellar PCs in urethane-anesthetized mice using in vivo whole-cell recording techniques and pharmacological methods. Under voltage-clamp recording conditions, spontaneous CF discharge evoked strong inward currents followed by small conductance calcium-activated potassium (SK) channels that mediated outward currents. The application of a GABA_A receptor antagonist did not significantly alter the spontaneous SS firing rate, although an AMPA receptor blocker abolished complex spike (CS) activity and induced significantly increased SS firing rates and a decreased coefficient of variation (CV) SS value. Either removal of extracellular calcium or chelated intracellular calcium induced a decrease in amplitude of CS-evoked after-hyperpolarization (AHP) potential accompanied by an increase in SS firing rate. In addition, blocking SK channels activity with a selective antagonist, dequalinium decreased the amplitude of AHP and increased SS firing rate. Moreover, we found repeated CF stimulation at 1 Hz induced a significant decrease in the spontaneous firing rate of SS, and accompanied with an increase in CV of SS in cerebellar slices, which was also abolished by dequalinium. These results indicated that the spontaneous CF discharge contributed to decreasing SS firing rate via activation of SK channels in the cerebellar PCs in vivo in mice.

Electrophysiological and Immunohistochemical Evidence for an Increase in GABAergic Inputs and HCN Channels in Purkinje Cells that Survive Developmental Ethanol Exposure

Ethanol exposures during the early postnatal period of the rat result in significant death of Purkinje cells (PCs). The magnitude, time-course, and lobular specificity of PC death have been well characterized in several studies. Additionally, significant reduction of climbing fiber inputs to the surviving PCs has been characterized. This study investigates whether further alterations to the cerebellar cortical circuits might occur as a result of developmental ethanol exposures. We first examined the firing pattern of PCs in acute slice preparations on postnatal days 13-15. While the basic firing frequency was not significantly altered, PCs from rat pups treated with ethanol on postnatal days 4-6 showed a significantly increased number of inhibitory postsynaptic potentials (IPSCs) and a larger Ih current. We conducted immunofluorescent studies to identify the probable cause of the increased IPSCs. We found a significant 21 % increase in the number of basket cells per PC and a near doubling of the volume of co-localized basket cell axonal membrane with PC. In addition, we identified a significant (~147 %) increase in HCN1 channel volume co-localized to PC volume. Therefore, the cerebellar cortex that survives targeted postnatal ethanol exposure is dramatically altered in development subsequent to PC death. The cerebellar cortical circuit that results is one that operates under a significant degree of increased resting inhibition. The alterations in the development of cerebellar circuitry following ethanol exposure, and the significant loss of PCs, could result in modifications of the structure and function of other brain regions that receive cerebellar inputs.

HCN1 channels in cerebellar Purkinje cells promote late stages of learning and constrain synaptic inhibition

Neural computations rely on ion channels that modify neuronal responses to synaptic inputs. While single cell recordings suggest diverse and neurone type-specific computational functions for HCN1 channels, their behavioural roles in any single neurone type are not clear. Using a battery of behavioural assays, including analysis of motor learning in vestibulo-ocular reflex and rotarod tests, we find that deletion of HCN1 channels from cerebellar Purkinje cells selectively impairs late stages of motor learning. Because deletion of HCN1 modifies only a subset of behaviours involving Purkinje cells, we asked whether the channel also has functional specificity at a cellular level. We find that HCN1 channels in cerebellar Purkinje cells reduce the duration of inhibitory synaptic responses but, in the absence of membrane hyperpolarization, do not affect responses to excitatory inputs. Our results indicate that manipulation of subthreshold computation in a single neurone type causes specific modifications to behaviour.

Contributions of T-type voltage-gated calcium channels to postsynaptic calcium signaling within Purkinje neurons

Low threshold voltage-gated T-type calcium channels have long been implicated in the electrical excitability and calcium signaling of cerebellar Purkinje neurons although the molecular composition, localization, and modulation of T-type channels within Purkinje cells have only recently been addressed. The specific functional roles that T-type channels play in local synaptic integration within Purkinje spines are also currently being unraveled. Overall, Purkinje neurons represent a powerful model system to explore the potential roles of postsynaptic T-type channels throughout the nervous system. In this review, we present an overview of T-type calcium channel biophysical, pharmacological, and physiological characteristics that provides a foundation for understanding T-type channels within Purkinje neurons. We also describe the biophysical properties of T-type channels in context of other voltage-gated calcium channel currents found within Purkinje cells. The data thus far suggest that one specific T-type isoform, Ca_v3.1, is highly expressed within Purkinje spines and both physically and functionally couples to mGluR1 and other effectors within putative signaling microdomains. Finally, we discuss how the selective potentiation of Ca_v3.1 channels via activation of mGluR1 by parallel fiber inputs affects local synaptic integration and how this interaction may relate to the overall excitability of Purkinje neuron dendrites.

Expression and properties of hyperpolarization-activated current in rat dorsal root ganglion neurons with known sensory function

The hyperpolarization-activated current (I(h)) has been implicated in nociception/pain, but its expression levels in nociceptors remained unknown. We recorded I(h) magnitude and properties by voltage clamp from dorsal root ganglion (DRG) neurons in vivo, after classifying them as nociceptive or low-threshold-mechanoreceptors (LTMs) and as having C-, Aδ- or Aα/β-conduction velocities (CVs). For both nociceptors andLTMs, I(h) amplitude and I(h) density (at -100 mV) were significantly positively correlated with CV.Median I(h) magnitudes and I(h) density in neuronal subgroupswere respectively:muscle spindle afferents(MSAs):-4.6 nA,-33 pA pF(-1); cutaneous Aα/β LTMs: -2.2 nA, -20 pA pF(-1); Aβ-nociceptors: -2.6 nA, -21 pA pF(-1); both Aδ-LTMs and nociceptors: -1.3 nA, ∼-14 pA pF(-1); C-LTMs: -0.4 nA, -7.6 pA pF(-1); and C-nociceptors: -0.26 nA, -5 pApF(-1). I(h) activation slow time constants (slow τ values) were strongly correlated with fast τ values; both were shortest in MSAs. Most neurons had τ values consistent with HCN1-related I(h); others had τ values closer to HCN1+HCN2 channels, or HCN2 in the presence of cAMP. In contrast, median half-activation voltages (V(0.5)) of -80 to -86 mV for neuronal subgroups suggest contributions of HCN2 to I(h). τ values were unrelated to CV but were inversely correlated with I(h) and I(h) density for all non-MSA LTMs, and for Aδ-nociceptors. From activation curves ∼2-7% of I(h)would be activated at normal membrane potentials. The high I(h) may be important for excitability of A-nociceptors (responsible for sharp/pricking-type pain) and Aα/β-LTMs (tactile sensations and proprioception). Underlying HCN expression in these subgroups therefore needs to be determined. Altered high I(h) may be important for excitability of A-nociceptors (responsible for sharp/pricking-type pain) and Aα/β-LTMs (tactile sensations and proprioception). Underlying HCN expression in these subgroups therefore needs to be determined. Altered Ih expression and/or properties (e.g. in chronic/pathological pain states) may influence both nociceptor and LTM excitability.expression and/or properties (e.g. in chronic/pathological pain states) may influence both nociceptor and LTM excitability.

Lobule-specific membrane excitability of cerebellar Purkinje cells

  Cerebellar Purkinje cells (PCs) are the sole output of the cerebellar cortex and function as key to a variety of learning-related behaviours by integrating multimodal afferent inputs. Intrinsic membrane excitability of neurons determines the input-output relationship, and therefore governs the functions of neural circuits. Cerebellar vermis consists of ten lobules (lobules I-X), and each lobule receives different sensory information. However, lobule-specific differences of electrophysiological properties of PC are incompletely understood. To address this question, we performed a systematic comparison of membrane properties of PCs from different lobules (lobules III-V vs. X). Two types of firing patterns (tonic firing and complex bursting) were identified in response to depolarizing current injections in lobule III-V PCs, whereas four distinct firing patterns (tonic firing, complex bursting, initial bursting and gap firing) were observed in lobule X. A-type K(+) current and early inactivation of fast Na(+) conductance with activation of 4-aminopyridine-sensitive conductances were shown to be responsible for the formation of gap firing and initial bursting patterns, respectively, which were observed only in lobule X. In response to current injection, PCs in lobule X spiked with wider dynamic range. These differences in firing pattern and membrane properties probably contribute to signal processing of afferent inputs in lobule-specific fashion, and particularly diversity of discharge patterns in lobule X, as a part of the vestibulocerebellum, might be involved in strict coordination of a precise temporal response to a wide range of head movements.

Hyperpolarization-activated currents in gonadotropin-releasing hormone (GnRH) neurons contribute to intrinsic excitability and are regulated by gonadal steroid feedback

Pulsatile release of gonadotropin-releasing hormone (GnRH) is required for fertility and is regulated by steroid feedback. Hyperpolarization-activated currents (I(h)) play a critical role in many rhythmic neurons. We examined the contribution of I(h) to the membrane and firing properties of GnRH neurons and the modulation of this current by steroid milieu. Whole-cell voltage- and current-clamp recordings were made of GFP-identified GnRH neurons in brain slices from male mice that were gonad-intact, castrated, or castrated and treated with estradiol implants. APV, CNQX, and bicuculline were included to block fast synaptic transmission. GnRH neurons (47%) expressed a hyperpolarization-activated current with pharmacological and biophysical characteristics of I(h). The I(h)-specific blocker ZD7288 reduced hyperpolarization-induced sag and rebound potential, decreased GnRH neuron excitability and action potential firing, and hyperpolarized membrane potential in some cells. ZD7288 also altered the pattern of burst firing and reduced the slope of recovery from the after-hyperpolarization potential. Activation of I(h) by hyperpolarization increased spike frequency, whereas inactivation of I(h) by depolarization reduced spike frequency. Castration increased I(h) compared with that in gonad-intact males. This effect was reversed by in vivo estradiol replacement. Together, these data indicate I(h) provides an excitatory drive in GnRH neurons that contributes to action potential burst firing and that estradiol regulates I(h) in these cells. As estradiol is the primary central negative feedback hormone on GnRH neuron firing in males, this provides insight into the mechanisms by which steroid hormones potentially alter the intrinsic properties of GnRH neurons to change their activity.

Interneurons of the cerebellar cortex toggle Purkinje cells between up and down states

We demonstrate that single interneurons can toggle the output neurons of the cerebellar cortex (the Purkinje cells) between their two states. The firing of Purkinje cells has previously been shown to alternate between an "up" state in which the cell fires spontaneous action potentials and a silent "down" state. We show here that small hyperpolarizing currents in Purkinje cells can bidirectionally toggle Purkinje cells between down and up states and that blockade of the hyperpolarization-activated cation channels (H channels) with the specific antagonist ZD7288 (10 microM) blocks the transitions from down to up states. Likewise, hyperpolarizing inhibitory postsnyaptic potentials (IPSPs) produced by small bursts of action potentials (10 action potentials at 50 Hz) in molecular-layer interneurons induce these bidirectional transitions in Purkinje cells. Furthermore, single interneurons in paired interneuron --> Purkinje cell recordings, produce bidirectional switches between the two states of Purkinje cells. The ability of molecular-layer interneurons to toggle Purkinje cells occurs when Purkinje cells are recorded under whole-cell patch-clamp conditions as well as when action potentials are recorded in an extracellular loose cell-attached configuration. The mode switch demonstrated here indicates that a single presynaptic interneuron can have opposite effects on the output of a given Purkinje cell, which introduces a unique type of synaptic interaction that may play an important role in cerebellar signaling.

Dendritic Kv3.3 potassium channels in cerebellar purkinje cells regulate generation and spatial dynamics of dendritic Ca2+ spikes

Purkinje cell dendrites are excitable structures with intrinsic and synaptic conductances contributing to the generation and propagation of electrical activity. Voltage-gated potassium channel subunit Kv3.3 is expressed in the distal dendrites of Purkinje cells. However, the functional relevance of this dendritic distribution is not understood. Moreover, mutations in Kv3.3 cause movement disorders in mice and cerebellar atrophy and ataxia in humans, emphasizing the importance of understanding the role of these channels. In this study, we explore functional implications of this dendritic channel expression and compare Purkinje cell dendritic excitability in wild-type and Kv3.3 knockout mice. We demonstrate enhanced excitability of Purkinje cell dendrites in Kv3.3 knockout mice, despite normal resting membrane properties. Combined data from local application pharmacology, voltage clamp analysis of ionic currents, and assessment of dendritic Ca(2+) spike threshold in Purkinje cells suggest a role for Kv3.3 channels in opposing Ca(2+) spike initiation. To study the physiological relevance of altered dendritic excitability, we measured [Ca(2+)](i) changes throughout the dendritic tree in response to climbing fiber activation. Ca(2+) signals were specifically enhanced in distal dendrites of Kv3.3 knockout Purkinje cells, suggesting a role for dendritic Kv3.3 channels in regulating propagation of electrical activity and Ca(2+) influx in distal dendrites. These findings characterize unique roles of Kv3.3 channels in dendrites, with implications for synaptic integration, plasticity, and human disease.

Hyperpolarization-activated cation channels: from genes to function

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels comprise a small subfamily of proteins within the superfamily of pore-loop cation channels. In mammals, the HCN channel family comprises four members (HCN1-4) that are expressed in heart and nervous system. The current produced by HCN channels has been known as I(h) (or I(f) or I(q)). I(h) has also been designated as pacemaker current, because it plays a key role in controlling rhythmic activity of cardiac pacemaker cells and spontaneously firing neurons. Extensive studies over the last decade have provided convincing evidence that I(h) is also involved in a number of basic physiological processes that are not directly associated with rhythmicity. Examples for these non-pacemaking functions of I(h) are the determination of the resting membrane potential, dendritic integration, synaptic transmission, and learning. In this review we summarize recent insights into the structure, function, and cellular regulation of HCN channels. We also discuss in detail the different aspects of HCN channel physiology in the heart and nervous system. To this end, evidence on the role of individual HCN channel types arising from the analysis of HCN knockout mouse models is discussed. Finally, we provide an overview of the impact of HCN channels on the pathogenesis of several diseases and discuss recent attempts to establish HCN channels as drug targets.

Enhanced Ih depresses rat entopeduncular nucleus neuronal activity from high-frequency stimulation or raised Ke+

High-frequency stimulation (HFS) is used to treat a variety of neurological diseases, yet its underlying therapeutic action is not fully elucidated. Previously, we reported that HFS-induced elevation in [K(+)](e) or bath perfusion of raised K(e)(+) depressed rat entopeduncular nucleus (EP) neuronal activity via an enhancement of an ionic conductance leading to marked depolarization. Herein, we show that the hyperpolarization-activated (I(h)) channel mediates the HFS- or K(+)-induced depression of EP neuronal activity. The perfusion of an I(h) channel inhibitor, 50 microM ZD7288 or 2 mM CsCl, increased input resistance by 23.5 +/- 7% (ZD7288) or 35 +/- 10% (CsCl), hyperpolarized cells by 3.4 +/- 1.7 mV (ZD7288) or 2.3 +/- 0.9 mV (CsCl), and decreased spontaneous action potential (AP) frequency by 51.5 +/- 12.5% (ZD7288) or 80 +/- 13.5% (CsCl). The I(h) sag was absent with either treatment, suggesting a block of I(h) channel activity. Inhibition of the I(h) channel prior to HFS or 6 mM K(+) perfusion not only prevented the previously observed decrease in AP frequency, but increased neuronal activity. Under voltage-clamp conditions, I(h) currents were enhanced in the presence of 6 mM K(+). Calcium is also involved in the depression of EP neuronal activity, since its removal during raised K(e)(+) application prevented this attenuation and blocked the I(h) sag. We conclude that the enhancement of I(h) channel activity initiates the HFS- and K(+)-induced depression of EP neuronal activity. This mechanism could underlie the inhibitory effects of HFS used in deep brain stimulation in output basal ganglia nuclei.

The leaner P/Q-type calcium channel mutation renders cerebellar Purkinje neurons hyper-excitable and eliminates Ca2+-Na+ spike bursts

The leaner mouse mutation of the Cacna1a gene leads to a reduction in P-type Ca2+ current, the dominant Ca2+ current in Purkinje cells (PCs). Here, we compare the electro-responsiveness and structure of PCs from age-matched leaner and wild-type (WT) mice in pharmacological isolation from synaptic inputs in cerebellar slices. We report that compared with WT, leaner PCs exhibit lower current threshold for Na+ spike firing, larger subthreshold membrane depolarization, rapid adaptation followed by complete block of Na+ spikes upon strong depolarization, and fail to generate Ca2+-Na+ spike bursts. The Na+ spike waveforms in leaner PCs have slower kinetics, reduced spike amplitude and afterhyperpolarization. We show that a deficit in the P-type Ca2+ current caused by the leaner mutation accounts for most but not all of the changes in mutant PC electro-responsiveness. The selective P-type Ca2+ channel blocker, omega-agatoxin-IVA, eliminated differences in subthreshold membrane depolarization, adaptation of Na+ spikes upon strong current-pulse stimuli, Na+ spike waveforms and Ca2+-Na+ burst activity. In contrast, a lower current threshold for eliciting repetitive Na+ spikes in leaner PCs was still observed after blockade of the P-type Ca2+ current, suggesting secondary effects of the mutation that render PCs hyper-excitable. Higher input resistance, reduced whole-cell capacitance and smaller dendritic size accompanied the enhanced excitability in leaner PCs, indicative of developmental retardation in these cells caused by P/Q-type Ca2+ channel malfunction. Our data indicate that a deficit in P-type Ca2+ current leads to complex functional and structural changes in PCs, impairing their intrinsic and integrative properties.

Local and global effects of I(h) distribution in dendrites of mammalian neurons

The hyperpolarization-activated cation current I(h) exhibits a steep gradient of channel density in dendrites of pyramidal neurons, which is associated with location independence of temporal summation of EPSPs at the soma. In striking contrast, here we show by using dendritic patch-clamp recordings that in cerebellar Purkinje cells, the principal neurons of the cerebellar cortex, I(h) exhibits a uniform dendritic density, while location independence of EPSP summation is observed. Using compartmental modeling in realistic and simplified dendritic geometries, we demonstrate that the dendritic distribution of I(h) only weakly affects the degree of temporal summation at the soma, while having an impact at the dendritic input location. We further analyze the effect of I(h) on temporal summation using cable theory and derive bounds for temporal summation for any spatial distribution of I(h). We show that the total number of I(h) channels, not their distribution, governs the degree of temporal summation of EPSPs. Our findings explain the effect of I(h) on EPSP shape and temporal summation, and suggest that neurons are provided with two independent degrees of freedom for different functions: the total amount of I(h) (controlling the degree of temporal summation of dendritic inputs at the soma) and the dendritic spatial distribution of I(h) (regulating local dendritic processing).

The mGlu1 antagonist CPCCOEt enhances the climbing fibre response in Purkinje neurones independently of glutamate receptors

CPCCOEt (7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester) is frequently used to test for the involvement of mGlu1 receptors. Using whole-cell voltage recording from Purkinje cells in slices of rat cerebellum we find that CPCCOEt, at concentrations used to block mGlu1 receptors, causes an enhancement of the climbing fibre response. Application of alternative antagonists with activity at mGlu1 neither mimicked nor occluded the effects of CPCCOEt. Receptor antagonists demonstrated that this non-mGlu1 action of CPCCOEt was not mediated by other mGlu receptors or GABA(B) receptors. Voltage-clamped climbing fibre EPSCs are unaffected by CPCCOEt whilst application of a glutamate transport blocker did not occlude the CPCCOEt effect. This suggests that a postsynaptic voltage-dependent component of the complex climbing fibre response is the target. We have found no evidence for the involvement of the hyperpolarisation-activated current, I(h), and calcium-activated conductances. Voltage-gated sodium, calcium and potassium channels are possible targets with inhibition of a potassium channel the most likely. Awareness of this non-mGlu-mediated effect of CPCCOEt is likely to be important for the correct interpretation of its actions.

Dendritic amplification of inhibitory postsynaptic potentials in a model Purkinje cell

In neurons with large dendritic arbors, the postsynaptic potentials interact in a complex manner with active and passive membrane properties, causing not easily predictable transformations during the propagation from synapse to soma. Previous theoretical and experimental studies in both cerebellar Purkinje cells and neocortical pyramidal neurons have shown that voltage-dependent ion channels change the amplitude and time-course of postsynaptic potentials. We investigated the mechanisms involved in the propagation of inhibitory postsynaptic potentials (IPSPs) along active dendrites in a model of the Purkinje cell. The amplitude and time-course of IPSPs recorded at the soma were dependent on the synaptic distance from the soma, as predicted by passive cable theory. We show that the effect of distance on the amplitude and width of the IPSP was significantly reduced by the dendritic ion channels, whereas the rise time was not affected. Somatic IPSPs evoked by the activation of the most distal synapses were up to six times amplified owing to the presence of voltage-gated channels and the IPSP width became independent of the covered distance. A transient deactivation of the Ca(2+) channels and the Ca(2+)-dependent K(+) channels, triggered by the hyperpolarization following activation of the inhibitory synapse, was found to be responsible for these dynamics. Nevertheless, the position of activated synapses had a marked effect on the Purkinje cell firing pattern, making stellate cells and basket cells most suitable for controlling the firing rate and spike timing, respectively, of their target Purkinje cells.

Specific T-type calcium channel isoforms are associated with distinct burst phenotypes in deep cerebellar nuclear neurons

T-type calcium channels are thought to transform neuronal output to a burst mode by generating low voltage-activated (LVA) calcium currents and rebound burst discharge. In this study we assess the expression pattern of the three different T-type channel isoforms (Ca(v)3.1, Ca(v)3.2, and Ca(v)3.3) in cerebellar neurons and focus on their potential role in generating LVA spikes and rebound discharge in deep cerebellar nuclear (DCN) neurons. We detected expression of one or more Ca(v)3 channel isoforms in a wide range of cerebellar neurons and selective expression of different isoforms in DCN cells. We further identify two classes of large-diameter DCN neurons that exhibit either a strong or weak capability for rebound discharge, despite the ability to generate LVA spikes when calcium currents are pharmacologically isolated. By correlating the Ca(v)3 channel expression pattern with the electrophysiological profile of identified DCN cells, we show that Ca(v)3.1 channels are expressed in isolation in DCN-burst cells, whereas Ca(v)3.3 is expressed in DCN-weak burst cells. Ca(v)3.1-expressing DCN cells correspond to excitatory or GABAergic neurons, whereas Ca(v)3.3-expressing cells are non-GABAergic. The Ca(v)3 class of LVA calcium channels is thus expressed in specific combinations in a wide range of cerebellar neurons but contributes to rebound burst discharge in only a select number of cell classes.

Slow oscillatory activity of rat globus pallidus neurons in vitro

The neurons in the external segment of the pallidum in the primate develop a characteristic firing pattern consisting of alternately occurring long, 2-20 s, strongly active phases and long completely silent phases when the subthalamo-pallidal excitatory inputs are blocked. The induction of the activity might be a factor in the development of dyskinesias after the loss of subthalamic output. In this study, we used globus pallidus (GPe) slice preparations obtained from juvenile rats to examined the conditions that support the alternatively occurring long depolarized and hyperpolarized phases which we refer to as the slow oscillation (SO). SO was not induced by the blockade of glutamatergic inputs but was induced by treatments that depolarized dendrites and, at the same time, hyperpolarized the somata with current injections. The treatments included elevation of extracellular K(+), application of K-current blockers and the lowering of extracellular Ca(2+). Application of TTX or intracellular BAPTA injection blocked the SO, while the SO could be maintained in hyperpolarization-activated inward current blockers, organic Ca-current blockers and up to 200 microm CdCl(2). These results suggest that Na currents play a major role in the generation of SO in vitro. It can be speculated that Na currents are involved in the development of active phases observed in the GPe after blockade of the glutamatergic inputs in vivo and that the unique property of GPe neurons in maintaining strong activity after the elimination of the glutamatergic driving force contributes to the development of motor disorders such as dyskinesia.

Physiological and morphological development of the rat cerebellar Purkinje cell

Cerebellar Purkinje cells integrate multimodal afferent inputs and, as the only projection neurones of the cerebellar cortex, are key to the coordination of a variety of motor- and learning-related behaviours. In the neonatal rat the cerebellum is undeveloped, but over the first few postnatal weeks both the structure of the cerebellum and cerebellar-dependent behaviours mature rapidly. Maturation of Purkinje cell physiology is expected to contribute significantly to the development of cerebellar output. However, the ontogeny of the electrophysiological properties of the Purkinje cell and its relationship to maturation of cell morphology is incompletely understood. To address this problem we performed a detailed in vitro electrophysiological analysis of the spontaneous and intracellularly evoked intrinsic properties of Purkinje cells obtained from postnatal rats (P0 to P90) using whole-cell patch clamp recordings. Cells were filled with neurobiotin to enable subsequent morphological comparisons. Three stages of physiological and structural development were identified. During the early postnatal period (P0 to approximately P9) Purkinje cells were characterized by an immature pattern of Na(+)-spike discharge, and possessed only short multipolar dendrites. This was followed by a period of rapid maturation (from approximately P12 to approximately P18), consisting of changes in Na(+)-spike discharge, emergence of repetitive bursts of Na(+) spikes terminated by Ca(2+) spikes (Ca(2+)-Na(+) bursts), generation of the trimodal pattern, and a significant expansion of the dendritic tree. During the final stage (> P18 to P90) there were minor refinements of cell output and a plateau in dendritic area. Our results reveal a rapid transition of the Purkinje cell from morphological and physiological immaturity to adult characteristics over a short developmental window, with a close correspondence between changes in cell output and dendritic growth. The development of Purkinje cell intrinsic electrophysiological properties further matches the time course of other measures of cerebellar structural and functional maturation.

GABAergic activation of an inwardly rectifying K+ current in mouse cerebellar Purkinje cells

Cerebellar Purkinje cells integrate motor information conveyed by excitatory synaptic inputs from parallel and climbing fibres. Purkinje cells abundantly express B-type G-protein-coupled gamma-aminobutyric acid receptors (GABABR) that are assumed to mediate major responses, including postsynaptic modulation of the synaptic inputs. However, the identity and function of effectors operated by GABABR are not fully elucidated. Here we characterized an inwardly rectifying current activated by baclofen (Ibacl), a GABABR agonist, in cultured mouse Purkinje cells using a ruptured-patch whole-cell technique. Ibacl is operated by GABABR via Gi/o-proteins, as it is not inducible in pertussis-toxin-pretreated cells. Ibacl is carried by K+ because its reversal potential shifts with the equilibrium potential of K+. Ibacl is blocked by 10(-3) M Ba2+ or Cs+, and 10(-8) M tertiapin-Q. Upon the onset and offset of a hyperpolarizing step, Ibacl is activated and deactivated, respectively, with double-exponential time courses (time constants, <1 ms and 30-80 ms). Based on similarities in the above properties, G-protein-coupled inwardly rectifying K+ (GIRK) channels are thought to be responsible for Ibacl. Perforated-patch recordings from cultured Purkinje cells demonstrate that Ibacl hyperpolarizes the resting potential and the peak level achieved by glutamate-evoked potentials initiated in the dendrites. Moreover, cell-attached recordings from Purkinje cells in cerebellar slices demonstrate that Ibacl impedes spontaneous firing. Therefore, Ibacl may reduce the postsynaptic and intrinsic excitability of Purkinje cells under physiological conditions. These findings give a new insight into the role of GABABR signalling in cerebellar information processing.

Control of neuronal discharge timing by afferent fiber number and the temporal pattern of afferent impulses

We employ computer simulations to explore the effect of different temporal patterns of afferent impulses on the evoked discharge of a model cerebellar Purkinje cell. We show that the frequency and temporal correlation of impulses across afferent fibers determines which of four regimes of discharge activity is evoked. In the uncorrelated, here Poissonian, case, (i) cell discharge is determined by the total stimulation rate and temporal patterns of discharge are the same for different combinations of afferent fiber number and mean impulse rate per fiber giving the same total stimulation. Alternatively, if temporal correlations are present in the stimulus, (ii) for stimulation frequencies of 4 to at least 64 Hz there is a narrow range of afferent fiber number for which every stimulus pulse (composed of a single impulse on each afferent fiber) evokes a single action potential. In this case cell discharge is frequency locked to the stimulus with a concomitant reduction in discharge variability. (iii) For lower fiber numbers and thus discharge frequencies lower than the locking frequency, the variability of cell discharge is typically independent of afferent impulse timing, whereas, (iv) at higher fiber numbers and thus higher discharge frequencies, the reverse is true. We conclude that in case (iii) the cell acts as an integrator and discharge is determined by the stimulation rate, whereas in case (iv) the cell acts as a coincidence detector and the timing of discharge is determined by the temporal pattern of afferent stimulation. We discuss our results in terms of their significance for neuronal activity at the network level and suggest that the reported effects of varying stimulus timing and afferent convergence can be expected to obtain also with other principal cell types within the central nervous system.

Dendritic control of spontaneous bursting in cerebellar Purkinje cells

We investigated the mechanisms that contribute to spontaneous regular bursting in adult Purkinje neurons in acutely prepared cerebellar slices. Bursts consisted of 3-20 spikes and showed a stereotypic waveform. Each burst developed with an increase in firing rate and was terminated by a more rapid increase in firing rate and a decrease in spike height. Whole-cell current-clamp recordings showed that each burst ended with a rapid depolarization followed by a hyperpolarization. Dual dendritic and somatic extracellular recordings revealed that each burst was terminated by a dendritic calcium spike. The contributions of T- and P/Q-type calcium current, large (BK) and small (SK) conductance calcium-activated potassium currents, and hyperpolarization-activated (I(H)) current to bursting were investigated with specific channel blockers. None of the currents, except for P/Q, were required to sustain spontaneous bursting or the stereotypic burst waveform. T-type calcium, BK, and SK channels contributed to interspike and interburst intervals. The effect of T-type calcium channel block was more pronounced after BK channel block and vice versa, indicating that these two currents interact to regulate burst firing. Block of I(H) current had no effect on bursting. Partial block of P/Q-type calcium channels concurrently eliminated dendritic calcium spikes and caused a switch from regular bursting to tonic firing or irregular bursting. Dendritic calcium spikes persisted in the presence of tetrodotoxin, indicating that their initiation did not require somatic sodium spikes. Our results demonstrate an important role for dendritic conductances in burst firing in intact Purkinje neurons.

The hyperpolarization-activated HCN1 channel is important for motor learning and neuronal integration by cerebellar Purkinje cells

In contrast to our increasingly detailed understanding of how synaptic plasticity provides a cellular substrate for learning and memory, it is less clear how a neuron's voltage-gated ion channels interact with plastic changes in synaptic strength to influence behavior. We find, using generalized and regional knockout mice, that deletion of the HCN1 channel causes profound motor learning and memory deficits in swimming and rotarod tasks. In cerebellar Purkinje cells, which are a key component of the cerebellar circuit for learning of correctly timed movements, HCN1 mediates an inward current that stabilizes the integrative properties of Purkinje cells and ensures that their input-output function is independent of the previous history of their activity. We suggest that this nonsynaptic integrative function of HCN1 is required for accurate decoding of input patterns and thereby enables synaptic plasticity to appropriately influence the performance of motor activity.

Dendritic low-threshold Ca2+ channels in rat cerebellar Purkinje cells: possible physiological implications

Low-voltage activated (LVA) Ca2+ currents have been characterized in a large variety of neurons including cerebellar Purkinje cells (PCs). This review summarizes and discusses the biophysical, pharmacological properties, as well as the molecular identity of LVA Ca2+ channels described in PCs in various experimental conditions. Putative functional roles for LVA Ca2+ currents include generation of low-threshold Ca2+ spikes (LTS) that underlie burst firing, promotion of intrinsic oscillatory behaviour, Ca2+ entry close to the resting membrane potential and synaptic potentiation. Based on our recent findings on cerebellar rat PCs in slice cultures, this review presents the major evidence demonstrating that LVA Ca2+ channels produce a dendritic initiated LTS with a regulated propagation to the soma. This new role for LVA Ca2+ channels is particularly important in determining firing patterns in PCs.

Functional roles of an ERG current isolated in cerebellar Purkinje neurons

Transcripts encoding ERG potassium channels are expressed by most neurons of the CNS. By patch-clamp whole cell recording from Purkinje neurons in slices of young (5-9 days old) mouse cerebellum we have been able to isolate a tail current [IK(ERG)] with the same characteristics as previously described for ERG channels. In zero external Ca2+ and high K+ (40 mM) the V1/2 of activation was -50.7 mV, the V1/2 of inactivation was -70.6 mV, and the deactivation rate was double exponential and voltage dependent. IK(ERG) was 93.0% blocked by WAY-123,398 (1 microM) and 78.2% by haloperidol (2 microM). The role of IK(ERG) on evoked firing was studied in adult mice, where WAY-123,398 application decreased the first spike latency, increased the firing frequency, and suppressed the frequency adaptation. However, the shape of individual action potentials was not affected. Stimulation of presynaptic climbing fibers evoked the Purkinje neuron "complex spike," composed of an initial spike and several spikelets. IK(ERG) block caused an increase of the number of spikelets of the "complex spike." These data show, for the first time, an IK(ERG) in a neuron of the CNS, the cerebellar Purkinje neuron, and indicate that such a current is involved in the control of membrane excitability, firing frequency adaptation, and in determining the effects of the climbing fiber synapse.

Distinct contributions of small and large conductance Ca2+-activated K+ channels to rat Purkinje neuron function

The cerebellum is important for many aspects of behaviour, from posture maintenance and goal-oriented reaching movements to timing tasks and certain forms of learning. In every case, information flowing through the cerebellum passes through Purkinje neurons, which receive input from the two primary cerebellar afferents and generate continuous streams of action potentials that constitute the sole output from the cerebellar cortex to the deep nuclei. The tonic firing behaviour observed in Purkinje neurons in vivo is maintained in brain slices even when synaptic inputs are blocked, suggesting that Purkinje neuron activity relies to a significant extent on intrinsic conductances. Previous research has suggested that the interplay between Ca2+ currents and Ca2+-activated K+ channels (KCa channels) is important for Purkinje cell activity, but how many different KCa channel types are present and what each channel type contributes to cell behaviour remains unclear. In order to better understand the ionic mechanisms that control the behaviour of these neurons, we investigated the effects of different Ca2+ channel and KCa channel antagonists on Purkinje neurons in acute slices of rat cerebellum. Our data show that Ca2+ entering through P-type voltage-gated Ca2+ channels activates both small-conductance (SK) and large-conductance (BK) KCa channels. SK channels play a role in setting the intrinsic firing frequency, while BK channels regulate action potential shape and may contribute to the unique climbing fibre response.

Active contribution of dendrites to the tonic and trimodal patterns of activity in cerebellar Purkinje neurons

The cerebellum is responsible for coordination of movement and maintenance of balance. Cerebellar architecture is based on repeats of an anatomically well defined circuit. At the center of these functional circuits are Purkinje neurons, which form the sole output of the cerebellar cortex. It is proposed that coordination of movement is achieved by encoding timing signals in the rate of firing and pattern of activity of Purkinje cells. An understanding of cerebellar timing requires an appreciation of the intrinsic firing behavior of Purkinje cells and the extent to which their activity is regulated within the functional circuits. We have examined the spontaneous firing of Purkinje neurons in isolation from the rest of the cerebellar circuitry by blocking fast synaptic transmission in acutely prepared cerebellar slices. We find that, intrinsically, mature Purkinje cells show a complex pattern of activity in which they continuously cycle among tonically firing, bursting, and silent modes. This trimodal pattern of activity emerges as the cerebellum matures anatomically and functionally. Concurrent with the transformation of the immature tonically firing cells to those with the trimodal pattern of activity, the dendrites assume a prominent role in regulating the excitability of Purkinje cells. Thus, alterations in the rate and pattern of activity of Purkinje neurons are not solely the result of synaptic input but also arise as a consequence of the intrinsic properties of the cells.

Active contribution of dendrites to the tonic and trimodal patterns of activity in cerebellar Purkinje neurons

The cerebellum is responsible for coordination of movement and maintenance of balance. Cerebellar architecture is based on repeats of an anatomically well defined circuit. At the center of these functional circuits are Purkinje neurons, which form the sole output of the cerebellar cortex. It is proposed that coordination of movement is achieved by encoding timing signals in the rate of firing and pattern of activity of Purkinje cells. An understanding of cerebellar timing requires an appreciation of the intrinsic firing behavior of Purkinje cells and the extent to which their activity is regulated within the functional circuits. We have examined the spontaneous firing of Purkinje neurons in isolation from the rest of the cerebellar circuitry by blocking fast synaptic transmission in acutely prepared cerebellar slices. We find that, intrinsically, mature Purkinje cells show a complex pattern of activity in which they continuously cycle among tonically firing, bursting, and silent modes. This trimodal pattern of activity emerges as the cerebellum matures anatomically and functionally. Concurrent with the transformation of the immature tonically firing cells to those with the trimodal pattern of activity, the dendrites assume a prominent role in regulating the excitability of Purkinje cells. Thus, alterations in the rate and pattern of activity of Purkinje neurons are not solely the result of synaptic input but also arise as a consequence of the intrinsic properties of the cells.

Properties of unitary granule cell-->Purkinje cell synapses in adult rat cerebellar slices

The cerebellar cortex contains huge numbers of synapses between granule cells and Purkinje cells. These synapses are thought to be a major storage site for information required to execute coordinated movements. To obtain a quantitative description of this connection, we recorded unitary synaptic responses between granule cell and Purkinje cell pairs in adult rat cerebellar slices. Our results are consistent with parallel fiber-->Purkinje cell synapses having high release probabilities and modest paired pulse facilitation. However, a wide range of response amplitudes was observed. Indeed, we detected many fewer parallel fiber connections (7% of the granule cells that were screened) than expected (54%), leading us to suggest that up to 85% of parallel fiber-->Purkinje cell synapses do not generate detectable electrical responses. We also investigated the possible role of granule cell ascending axons by recording granule cells near the Purkinje cell. A high proportion (up to 50%) of local granule cells generated detectable synaptic responses. However, most of these connections were indistinguishable from parallel fiber connections, suggesting that powerful ascending axon connections are rare. The existence of many very weak synapses would provide a mechanism for Purkinje cells to extract information selectively from the mass provided by parallel fibers.

Properties of unitary granule cell-->Purkinje cell synapses in adult rat cerebellar slices

The cerebellar cortex contains huge numbers of synapses between granule cells and Purkinje cells. These synapses are thought to be a major storage site for information required to execute coordinated movements. To obtain a quantitative description of this connection, we recorded unitary synaptic responses between granule cell and Purkinje cell pairs in adult rat cerebellar slices. Our results are consistent with parallel fiber-->Purkinje cell synapses having high release probabilities and modest paired pulse facilitation. However, a wide range of response amplitudes was observed. Indeed, we detected many fewer parallel fiber connections (7% of the granule cells that were screened) than expected (54%), leading us to suggest that up to 85% of parallel fiber-->Purkinje cell synapses do not generate detectable electrical responses. We also investigated the possible role of granule cell ascending axons by recording granule cells near the Purkinje cell. A high proportion (up to 50%) of local granule cells generated detectable synaptic responses. However, most of these connections were indistinguishable from parallel fiber connections, suggesting that powerful ascending axon connections are rare. The existence of many very weak synapses would provide a mechanism for Purkinje cells to extract information selectively from the mass provided by parallel fibers.

Responses to Metabotropic Glutamate Receptor Activation in Cerebellar Purkinje Cells: Induction of an Inward Current

The responses to activation of metabotropic glutamate receptors (mGluRs) of Purkinje cells in rat cerebellar slice cultures were investigated using intracellular recordings in single-electrode voltage-clamp mode combined with microfluorometric measurements of cytosolic free calcium using fura-2. Purkinje cells were perfused with saline containing 0.5 microM tetrodotoxin and 10 microM bicuculline and voltage-clamped at -60 mV. Bath-applied trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid (t-ACPD, 50 - 100 microM), a selective agonist of mGluRs, induced a transient inward current that was followed by an outward current. The response induced by t-ACPD was not affected by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, up to 40 microM). In contrast, inward currents caused by (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA, 1 - 2 microM) were completely abolished, while inward currents caused by quisqualate (0.25 microM) were only partially depressed by CNQX (5 - 40 microM). The inward current induced by t-ACPD was unaffected by external Ba2+ (1 mM), tetraethylammonium (10 mM) and Cs+ (1 mM), and was associated with an increase in apparent input conductance of the cell membrane. The extrapolated reversal potential of inward currents induced by t-ACPD was +18 mV while Cl- currents induced by muscimol reversed at -66 mV. Inward currents induced by t-ACPD, but not those induced by AMPA, were associated with a rise in cytosolic Ca2+ concentration and suppressed by intracellular injection of a calcium chelator. Replacement of external Na+ by choline or Li+ depressed the inward current and resulted in a slower decay of the Ca2+ signal.

Control of the propagation of dendritic low-threshold Ca(2+) spikes in Purkinje cells from rat cerebellar slice cultures

To investigate the ionic mechanisms controlling the dendrosomatic propagation of low-threshold Ca(2+) spikes (LTS) in Purkinje cells (PCs), somatically evoked discharges of action potentials (APs) were recorded under current-clamp conditions. The whole-cell configuration of the patch-clamp method was used in PCs from rat cerebellar slice cultures. Full blockade of the P/Q-type Ca(2+) current revealed slow but transient depolarizations associated with bursts of fast Na(+) APs. These can occur as a single isolated event at the onset of current injection, or repetitively (i.e. a slow complex burst). The initial transient depolarization was identified as an LTS Blockade of P/Q-type Ca(2+) channels increased the likelihood of recording Ca(2+) spikes at the soma by promoting dendrosomatic propagation. Slow rhythmic depolarizations shared several properties with the LTS (kinetics, activation/inactivation, calcium dependency and dendritic origin), suggesting that they correspond to repetitively activated dendritic LTS, which reach the soma when P/Q channels are blocked. Somatic LTS and slow complex burst activity were also induced by K(+) channel blockers such as TEA (2.5 x 10(-4) M) charybdotoxin (CTX, 10(-5) M), rIberiotoxin (10(-7) M), and 4-aminopyridine (4-AP, 10(-3) M), but not by apamin (10(-4) M). In the presence of 4-AP, slow complex burst activity occurred even at hyperpolarized potentials (-80 mV). In conclusion, we suggest that the propagation of dendritic LTS is controlled directly by 4-AP-sensitive K(+) channels, and indirectly modulated by activation of calcium-activated K(+) (BK) channels via P/Q-mediated Ca(2+) entry. The slow complex burst resembles strikingly the complex spike elicited by climbing fibre stimulation, and we therefore propose, as a hypothesis, that dendrosomatic propagation of the LTS could underlie the complex spike.

Membrane potential bistability is controlled by the hyperpolarization-activated current I(H) in rat cerebellar Purkinje neurons in vitro

We investigated the role of the hyperpolarization-activated mixed cation current, I(H), in the control of spontaneous action potential firing of rat cerebellar Purkinje neurons in brain slices. Extracellular recordings revealed that the continual action potential firing of Purkinje neurons was disrupted by the pharmacological blockade of I(H). Blockade of I(H) revealed spontaneous transitions between periods of tonic action potential firing and quiescence, without effects on the frequency or variance of action potential generation. Whole-cell recordings revealed that blockade of I(H) unmasked a form of membrane potential bistability, where transitions between tonic firing and quiescent states (separated by approximately 20 mV) were evoked by excitatory and inhibitory postsynaptic potentials, or by the delivery of brief (20 ms) somatic or dendritic positive and negative current pulses. The stable upper state of tonic action potential firing was maintained by the recruitment of axo-somatic voltage-activated sodium, but not calcium, channels. Negative modulation of I(H) by serotonin unmasked bistability, indicating that bistability of Purkinje neurons is likely to occur under physiological conditions. These data indicate that I(H) acts as a 'safety net', maintaining the membrane potential of Purkinje neurons within the range necessary for the generation of tonic action potential firing. Following the downregulation of I(H), synaptic inhibition can generate long periods (seconds) of quiescence, the duration of which can be controlled by climbing fibre activation and by the underlying 'tone' of parallel fibre activity.

The composite neuron: a realistic one-compartment Purkinje cell model suitable for large-scale neuronal network simulations

We present a simple method for the realistic description of neurons that is well suited to the development of large-scale neuronal network models where the interactions within and between neural circuits are the object of study rather than the details of dendritic signal propagation in individual cells. Referred to as the composite approach, it combines in a one-compartment model elements of both the leaky integrator cell and the conductance-based formalism of Hodgkin and Huxley (1952). Composite models treat the cell membrane as an equivalent circuit that contains ligand-gated synaptic, voltage-gated, and voltage- and concentration-dependent conductances. The time dependences of these various conductances are assumed to correlate with their spatial locations in the real cell. Thus, when viewed from the soma, ligand-gated synaptic and other dendritically located conductances can be modeled as either single alpha or double exponential functions of time, whereas, with the exception of discharge-related conductances, somatic and proximal dendritic conductances can be well approximated by simple current-voltage relationships. As an example of the composite approach to neuronal modeling we describe a composite model of a cerebellar Purkinje neuron.

Compartmental models of rat cerebellar Purkinje cells based on simultaneous somatic and dendritic patch-clamp recordings

1. Simultaneous dendritic and somatic patch-clamp recordings were made from Purkinje cells in cerebellar slices from 12- to 21-day-old rats. Voltage responses to current impulses injected via either the dendritic or the somatic pipette were obtained in the presence of the selective I(h) blocker ZD 7288 and blockers of spontaneous synaptic input. Neurons were filled with biocytin for subsequent morphological reconstruction. 2. Four neurons were reconstructed and converted into detailed compartmental models. The specific membrane capacitance (C(m)), specific membrane resistance (R(m)) and intracellular resistivity (R(i)) were optimized by direct fitting of the model responses to the electrophysiological data from the same cell. Mean values were: C(m), 0.77 +/- 0.17 microF cm(-2) (mean +/- S.D.; range, 0.64-1.00 microF cm(-2)), R(m), 122 +/- 18 kOmega cm(2) (98-141 kOmega cm(2)) and R(i), 115 +/- 20 Omega cm (93-142 Omega cm). 3. The steady-state electrotonic architecture of these cells was compact under the experimental conditions used. However, somatic voltage-clamp recordings of parallel fibre and climbing fibre synaptic currents were substantially filtered and attenuated. 4. The detailed models were compared with a two-compartment model of Purkinje cells. The range of synaptic current kinetics that can be faithfully recorded using somatic voltage clamp is predicted fairly well by the two-compartment model, even though some of its underlying assumptions are violated. 5. A model of I(h) was constructed based on voltage-clamp data, and inserted into the passive compartmental models. Somatic EPSP amplitude was substantially attenuated compared to the amplitude of dendritic EPSPs at their site of generation. However, synaptic efficacy of the same quantal synaptic conductance, as measured by the somatic EPSP amplitude, was only weakly dependent on synaptic location on spiny branchlets. 6. The passive electrotonic structure of Purkinje cells is unusual in that the steady-state architecture is very compact, while voltage transients such as synaptic potentials and action potentials are heavily filtered.

Modulation of a presynaptic hyperpolarization-activated cationic current (I(h)) at an excitatory synaptic terminal in the rat auditory brainstem

1. A hyperpolarization-activated non-specific cation current, I(h), was examined in bushy cell bodies and their giant presynaptic terminals (calyx of Held). Whole-cell patch clamp recordings were made using an in vitro brain slice preparation of the cochlear nucleus and the superior olivary complex. The aim was to characterise I(h) in identified cell bodies and synaptic terminals, to examine modulation by presynaptic cAMP and to test for modulatory effects of I(h) activation on synaptic transmission. 2. Presynaptic I(h) was activated by hyperpolarizing voltage-steps, with half-activation (V(1/2)) at -94 mV. Activation time constants were voltage dependent, showing an e-fold acceleration for hyperpolarizations of -32 mV (time constant of 78 ms at -130 mV). The reversal potential of I(h) was -29 mV. It was blocked by external perfusion of 1 mM CsCl but was unaffected by BaCl(2). 3. Application of internal cAMP shifted the activation curve to more positive potentials, giving a V(1/2) of -74 mV; hence around half of the current was activated at resting membrane potentials. This shift in half-activation was mimicked by external perfusion of a membrane-permeant analogue, 8-bromo-cAMP. 4. The bushy cell body I(h) showed similar properties to those of the synaptic terminal; V(1/2) was -94 mV and the reversal potential was -33 mV. Somatic I(h) was blocked by CsCl (1 mM) and was partially sensitive to BaCl(2). Somatic I(h) current density increased with postnatal age from 5 to 16 days old, suggesting that I(h) is functionally relevant during maturation of the auditory pathway. 5. The function of I(h) in regulating presynaptic excitability is subtle. I(h) had little influence on EPSC amplitude at the calyx of Held, but may be associated with propagation of the action potential at branch points. Presynaptic I(h) shares properties with both HCN1 and HCN2 recombinant channel subunits, in that it gates relatively rapidly and is modulated by internal cAMP.

Calcium dynamics and electrophysiological properties of cerebellar Purkinje cells in SCA1 transgenic mice

Cerebellar Purkinje cells (PCs) from spinocerebellar ataxia type 1 (SCA1) transgenic mice develop dendritic and somatic atrophy with age. Inositol 1,4,5-trisphosphate receptor type 1 and the sarco/endoplasmic reticulum Ca(2+) ATPase pump, which regulate [Ca(2+)](i), are expressed at lower levels in these cells compared with the levels in cells from wild-type (WT) mice. To examine PCs in SCA1 mice, we used whole-cell patch clamp recording combined with fluorometric [Ca(2+)](i) and [Na(+)](i) measurements in cerebellar slices. PCs in SCA1 mice had Na(+) spikes, Ca(2+) spikes, climbing fiber (CF) electrical responses, parallel fiber (PF) electrical responses, and metabotropic glutamate receptor (mGluR)-mediated, PF-evoked Ca(2+) release from intracellular stores that were qualitatively similar to those recorded from WT mice. Under our experimental conditions, it was easier to evoke the mGluR-mediated secondary [Ca(2+)](i) increase in SCA1 PCs. The membrane resistance of SCA1 PCs was 3.3 times higher than that of WT cells, which correlated with the 1.7 times smaller cell body size. Most SCA1 PCs (but not WT) had a delayed onset (about 50--200 ms) to Na(+) spike firing induced by current injection. This delay was increased by hyperpolarizing prepulses and was eliminated by 4-aminopyridine, which suggests that this delay was due to enhancement of the A-like K(+) conductance in the SCA1 PCs. In response to CF stimulation, most PCs in mutant and WT mice had rapid, widespread [Ca(2+)](i) changes that recovered in <200 ms. Some SCA1 PCs showed a slow, localized, secondary Ca(2+) transient following the initial CF Ca(2+) transient, which may reflect release of Ca(2+) from intracellular stores. Thus, with these exceptions, the basic physiological properties of mutant PCs are similar to those of WT neurons, even with dramatic alteration of their morphology and downregulation of Ca(2+) handling molecules.

Cloning and localization of the hyperpolarization-activated cyclic nucleotide-gated channel family in rat brain

Rhythmic firing in brain and heart is mediated by pacemaker channels that are activated by hyperpolarization and regulated directly by cyclic nucleotides. Recent work has identified a new gene family that encodes such channels, which are termed hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels. In this study, we report the molecular cloning and localization by in situ hybridization of HCN1-4 in adult rat brain. The rat HCN1-4 clones show high homology to the deduced amino acid sequence of the mouse channels (>97% identity). The mRNA expression of the four channels in adult brain was evaluated in a systematic manner from the olfactory bulb to lower brain stem nuclei. Each mRNA demonstrated a unique pattern of distribution. HCN1 expression is highly enriched in cerebral cortex, hippocampus, cerebellum, and facial motor nucleus; HCN2 is highly abundant in mamillary bodies, pontine nucleus, ventral cochlear nucleus, and nucleus of the trapezoid body; HCN3 expression is most pronounced in supraoptic nucleus of hypothalamus; and HCN4 expression is most abundant in medial habenula and anterior and principal relay nuclei of the thalamus. These variations in regional specificity of HCN channels could generate important differences in neuronal pacemaker activity across brain systems.

Properties and role of I(h) in the pacing of subthreshold oscillations in entorhinal cortex layer II neurons

Various subsets of brain neurons express a hyperpolarization-activated inward current (I(h)) that has been shown to be instrumental in pacing oscillatory activity at both a single-cell and a network level. A characteristic feature of the stellate cells (SCs) of entorhinal cortex (EC) layer II, those neurons giving rise to the main component of the perforant path input to the hippocampal formation, is their ability to generate persistent, Na(+)-dependent rhythmic subthreshold membrane potential oscillations, which are thought to be instrumental in implementing theta rhythmicity in the entorhinal-hippocampal network. The SCs also display a robust time-dependent inward rectification in the hyperpolarizing direction that may contribute to the generation of these oscillations. We performed whole cell recordings of SCs in in vitro slices to investigate the specific biophysical and pharmacological properties of the current underlying this inward rectification and to clarify its potential role in the genesis of the subthreshold oscillations. In voltage-clamp conditions, hyperpolarizing voltage steps evoked a slow, noninactivating inward current, which also deactivated slowly on depolarization. This current was identified as I(h) because it was resistant to extracellular Ba(2+), sensitive to Cs(+), completely and selectively abolished by ZD7288, and carried by both Na(+) and K(+) ions. I(h) in the SCs had an activation threshold and reversal potential at approximately -45 and -20 mV, respectively. Its half-activation voltage was -77 mV. Importantly, bath perfusion with ZD7288, but not Ba(2+), gradually and completely abolished the subthreshold oscillations, thus directly implicating I(h) in their generation. Using experimentally derived biophysical parameters for I(h) and the low-threshold persistent Na(+) current (I(NaP)) present in the SCs, a simplified model of these neurons was constructed and their subthreshold electroresponsiveness simulated. This indicated that the interplay between I(NaP) and I(h) can sustain persistent subthreshold oscillations in SCs. I(NaP) and I(h) operate in a "push-pull" fashion where the delay in the activation/deactivation of I(h) gives rise to the oscillatory process.

Atm-deficient mice Purkinje cells show age-dependent defects in calcium spike bursts and calcium currents

Ataxia telangiectasia in humans results from homozygous loss-of-function mutations in ATM. Neurological deterioration is the major cause of death in ataxia telangiectasia patients: in the cerebellum, mainly Purkinje cells are affected. We have generated Atm-deficient mice which display neurological abnormalities by several tests of motor function consistent with an abnormality of cerebellar function, but without histological evidence of neuronal degeneration. Here we performed a more detailed morphological analysis and an electrophysiological study on Purkinje cells from Atm-deficient mice of different ages. We found no histological or immunohistochemical abnormalities. Electrophysiology revealed no abnormalities in resting membrane potential, input resistance or anomalous rectification. In contrast, there was a significant decrease in the duration of calcium and sodium firing. The calcium deficit became significant between six to eight and 12-20 weeks of age, and appeared to be progressive. By voltage-clamp recording, we found that the firing deficits were due to a significant decrease in calcium currents, while inactivating potassium currents seem unaffected. In other mutant mice, calcium current deficits have been shown to be related to cell death.Our experiments suggest that the electrophysiological defects displayed by Atm-deficient mice are early predegenerative lesions and may be a precursor of Purkinje cell degeneration displayed by ataxia telangiectasia patients.

Developmental changes in eye-blink conditioning and neuronal activity in the cerebellar interpositus nucleus

Neuronal activity was recorded in the cerebellar interpositus nucleus in infant rats during classical conditioning of the eye-blink response. The percentage and amplitude of eye-blink conditioned responses increased as a function of postnatal age. Learning-specific neuronal activity in the cerebellum emerged ontogenetically in parallel with the eye-blink conditioned response. There were also age-specific changes in neuronal activity after the onset of the conditioned and unconditioned stimuli. The results indicate that the development of the eye-blink conditioned response may depend on the development of stimulus-evoked neuronal responses and learning-specific plasticity in the cerebellum. Functional immaturity in the afferent neural pathways may limit the induction of neural plasticity in the cerebellum and thereby limit the development of the eye-blink conditioned response.

Developmental changes in eye-blink conditioning and neuronal activity in the cerebellar interpositus nucleus

Neuronal activity was recorded in the cerebellar interpositus nucleus in infant rats during classical conditioning of the eye-blink response. The percentage and amplitude of eye-blink conditioned responses increased as a function of postnatal age. Learning-specific neuronal activity in the cerebellum emerged ontogenetically in parallel with the eye-blink conditioned response. There were also age-specific changes in neuronal activity after the onset of the conditioned and unconditioned stimuli. The results indicate that the development of the eye-blink conditioned response may depend on the development of stimulus-evoked neuronal responses and learning-specific plasticity in the cerebellum. Functional immaturity in the afferent neural pathways may limit the induction of neural plasticity in the cerebellum and thereby limit the development of the eye-blink conditioned response.

Chronic ethanol treatment alters AMPA-induced calcium signals in developing Purkinje neurons

Cerebellar Purkinje neurons developing in culture were treated chronically with 30 mM (140 mg%; 3-11 days in vitro) ethanol to study the actions of prolonged ethanol exposure on responses to exogenous application of AMPA, a selective agonist at the AMPA subtype of ionotropic glutamate receptors. There was no consistent difference between control and chronic ethanol-treated neurons in resting membrane potential, input resistance, or the amplitude or duration of the membrane responses to AMPA (1 or 5 microM applied by brief microperfusion) as measured using the nystatin patch method of whole cell recording. In additional studies, the Ca2+ signal to AMPA was examined using the Ca2+ sensitive dye fura-2. The mean peak Ca2+ signal elicited by 5 microM AMPA was enhanced in the dendritic region (but not the somatic region) of chronic ethanol-treated Purkinje neurons compared to control neurons. In contrast, there was no difference between control and chronic ethanol-treated neurons in the peak amplitude of the Ca2+ signal to 1 microM AMPA, whereas the recovery of the Ca2+ signals was more rapid in both somatic and dendritic regions of ethanol-treated neurons. Resting Ca2+ levels in the somatic and dendritic regions were similar between control and ethanol-treated neurons. These data show that the membrane and Ca2+ responses to AMPA in Purkinje neurons are differentially affected by prolonged ethanol exposure during development. Moreover, chronic ethanol exposure produces a selective enhancement of AMPA-evoked dendritic Ca2+ signals under conditions reflecting intense activation (i.e., 5 microM AMPA), whereas both somatic and dendritic Ca2+ signals are attenuated with smaller levels of activation (i.e., 1 microM AMPA). Because Ca2+ is an important regulator of numerous intracellular functions, chronic ethanol exposure during development could produce widespread changes in the development and function of the cerebellum.

The HCN gene family: molecular basis of the hyperpolarization-activated pacemaker channels

The molecular basis of the hyperpolarization-activated cation channels that underlie the anomalous rectifying current variously termed Ih, Iq, or I(f) is discussed. On the basis of the expression patterns and biophysical properties of the newly cloned HCN ion channels, an initial attempt at defining the identity and subunit composition of channels underlying native Ih is undertaken. By comparing the sequences of HCN channels to other members of the K channel superfamily, we discuss how channel opening may be coupled to membrane hyperpolarization and to direct binding of cyclic nucleotide. Finally, we consider some of the questions in cardiovascular physiology and neurobiology that can be addressed as a result of the demonstration that Ih is encoded by the HCN gene family.

Ionic currents underlying spontaneous action potentials in isolated cerebellar Purkinje neurons

Acutely dissociated cell bodies of mouse Purkinje neurons spontaneously fired action potentials at approximately 50 Hz (25 degrees C). To directly measure the ionic currents underlying spontaneous activity, we voltage-clamped the cells using prerecorded spontaneous action potentials (spike trains) as voltage commands and used ionic substitution and selective blockers to isolate individual currents. The largest current flowing during the interspike interval was tetrodotoxin-sensitive sodium current (approximately -50 pA between -65 and -60 mV). Although the neurons had large voltage-dependent calcium currents, the net current blocked by cobalt substitution for calcium was outward at all times during spike trains. Thus, the electrical effect of calcium current is apparently dominated by rapidly activated calcium-dependent potassium currents. Under current clamp, all cells continued firing spontaneously (though approximately 30% more slowly) after block of T-type calcium current by mibefradil, and most cells continued to fire after block of all calcium current by cobalt substitution. Although the neurons possessed hyperpolarization-activated cation current (Ih), little current flowed during spike trains, and block by 1 mM cesium had no effect on firing frequency. The outward potassium currents underlying the repolarization of the spikes were completely blocked by 1 mM TEA. These currents deactivated quickly (<1 msec) after each spike. We conclude that the spontaneous firing of Purkinje neuron cell bodies depends mainly on tetrodotoxin-sensitive sodium current flowing between spikes. The high firing rate is promoted by large potassium currents that repolarize the cell rapidly and deactivate quickly, thus preventing strong hyperpolarization and restoring a high input resistance for subsequent depolarization.

Ionic currents underlying spontaneous action potentials in isolated cerebellar Purkinje neurons

Acutely dissociated cell bodies of mouse Purkinje neurons spontaneously fired action potentials at approximately 50 Hz (25 degrees C). To directly measure the ionic currents underlying spontaneous activity, we voltage-clamped the cells using prerecorded spontaneous action potentials (spike trains) as voltage commands and used ionic substitution and selective blockers to isolate individual currents. The largest current flowing during the interspike interval was tetrodotoxin-sensitive sodium current (approximately -50 pA between -65 and -60 mV). Although the neurons had large voltage-dependent calcium currents, the net current blocked by cobalt substitution for calcium was outward at all times during spike trains. Thus, the electrical effect of calcium current is apparently dominated by rapidly activated calcium-dependent potassium currents. Under current clamp, all cells continued firing spontaneously (though approximately 30% more slowly) after block of T-type calcium current by mibefradil, and most cells continued to fire after block of all calcium current by cobalt substitution. Although the neurons possessed hyperpolarization-activated cation current (Ih), little current flowed during spike trains, and block by 1 mM cesium had no effect on firing frequency. The outward potassium currents underlying the repolarization of the spikes were completely blocked by 1 mM TEA. These currents deactivated quickly (<1 msec) after each spike. We conclude that the spontaneous firing of Purkinje neuron cell bodies depends mainly on tetrodotoxin-sensitive sodium current flowing between spikes. The high firing rate is promoted by large potassium currents that repolarize the cell rapidly and deactivate quickly, thus preventing strong hyperpolarization and restoring a high input resistance for subsequent depolarization.