Modulation of inhibitory post-synaptic currents (IPSCs) in mouse cerebellar Purkinje and basket cells by snake and scorpion toxin K+ channel blockers

An activator of voltage-gated K+ channels Kv1.1 as a therapeutic candidate for episodic ataxia type 1

Loss-of-function mutations in the KCNA1(Kv1.1) gene cause episodic ataxia type 1 (EA1), a neurological disease characterized by cerebellar dysfunction, ataxic attacks, persistent myokymia with painful cramps in skeletal muscles, and epilepsy. Precision medicine for EA1 treatment is currently unfeasible, as no drug that can enhance the activity of Kv1.1-containing channels and offset the functional defects caused by KCNA1 mutations has been clinically approved. Here, we uncovered that niflumic acid (NFA), a currently prescribed analgesic and anti-inflammatory drug with an excellent safety profile in the clinic, potentiates the activity of Kv1.1 channels. NFA increased Kv1.1 current amplitudes by enhancing the channel open probability, causing a hyperpolarizing shift in the voltage dependence of both channel opening and gating charge movement, slowing the OFF-gating current decay. NFA exerted similar actions on both homomeric Kv1.2 and heteromeric Kv1.1/Kv1.2 channels, which are formed in most brain structures. We show that through its potentiating action, NFA mitigated the EA1 mutation-induced functional defects in Kv1.1 and restored cerebellar synaptic transmission, Purkinje cell availability, and precision of firing. In addition, NFA ameliorated the motor performance of a knock-in mouse model of EA1 and restored the neuromuscular transmission and climbing ability in Shaker (Kv1.1) mutant Drosophila melanogaster flies (Sh5). By virtue of its multiple actions, NFA has strong potential as an efficacious single-molecule-based therapeutic agent for EA1 and serves as a valuable model for drug discovery.

Targeting inhibitory cerebellar circuitry to alleviate behavioral deficits in a mouse model for studying idiopathic autism

Autism spectrum disorder (ASD) encompasses wide-ranging neuropsychiatric symptoms with unclear etiology. Although the cerebellum is a key region implicated in ASD, it remains elusive how the cerebellar circuitry is altered and whether the cerebellum can serve as a therapeutic target to rectify the phenotype of idiopathic ASD with polygenic abnormalities. Using a syndromic ASD model, e.g., Black and Tan BRachyury T+Itpr3tf/J (BTBR) mice, we revealed that increased excitability of presynaptic interneurons (INs) and decreased intrinsic excitability of postsynaptic Purkinje neurons (PNs) resulted in low PN firing rates in the cerebellum. Knowing that downregulation of Kv1.2 potassium channel in the IN nerve terminals likely augmented their excitability and GABA release, we applied a positive Kv1.2 modulator to mitigate the presynaptic over-inhibition and social impairment of BTBR mice. Selective restoration of the PN activity by a new chemogenetic approach alleviated core ASD-like behaviors of the BTBR strain. These findings highlight complex mechanisms converging onto the cerebellar dysfunction in the phenotypic model and provide effective strategies for potential therapies of ASD.

Identification of a molecular locus for normalizing dysregulated GABA release from interneurons in the Fragile X brain

Principal neurons encode information by varying their firing rate and patterns precisely fine-tuned through GABAergic interneurons. Dysregulation of inhibition can lead to neuropsychiatric disorders, yet little is known about the molecular basis underlying inhibitory control. Here, we find that excessive GABA release from basket cells (BCs) attenuates the firing frequency of Purkinje neurons (PNs) in the cerebellum of Fragile X Mental Retardation 1 (Fmr1) knockout (KO) mice, a model of Fragile X Syndrome (FXS) with abrogated expression of the Fragile X Mental Retardation Protein (FMRP). This over-inhibition originates from increased excitability and Ca²⁺ transients in the presynaptic terminals, where Kv1.2 potassium channels are downregulated. By paired patch-clamp recordings, we further demonstrate that acutely introducing an N-terminal fragment of FMRP into BCs normalizes GABA release in the Fmr1-KO synapses. Conversely, direct injection of an inhibitory FMRP antibody into BCs, or membrane depolarization of BCs, enhances GABA release in the wild type synapses, leading to abnormal inhibitory transmission comparable to the Fmr1-KO neurons. We discover that the N-terminus of FMRP directly binds to a phosphorylated serine motif on the C-terminus of Kv1.2; and that loss of this interaction in BCs exaggerates GABA release, compromising the firing activity of PNs and thus the output from the cerebellar circuitry. An allosteric Kv1.2 agonist, docosahexaenoic acid, rectifies the dysregulated inhibition in vitro as well as acoustic startle reflex and social interaction in vivo of the Fmr1-KO mice. Our results unravel a novel molecular locus for targeted intervention of FXS and perhaps autism.

Cerebellar learning modulates surface expression of a voltage-gated ion channel in cerebellar cortex

Numerous experiments using ex vivo electrophysiology suggest that mammalian learning and memory involves regulation of voltage-gated ion channels in terms of changes in function. Yet, little is known about learning-related regulation of voltage-gated ion channels in terms of changes in expression. In two experiments, we examined changes in cell surface expression of the voltage-gated potassium channel alpha-subunit Kv1.2 in a discrete region of cerebellar cortex after eyeblink conditioning (EBC), a well-studied form of cerebellar-dependent learning. Kv1.2 in cerebellar cortex is expressed almost entirely in basket cells, primarily in the axon terminal pinceaux (PCX) region, and Purkinje cells, primarily in dendrites. Cell surface expression of Kv1.2 was measured using both multiphoton microscopy, which allowed measurement confined to the PCX region, and biotinylation/western blot, which measured total cell surface expression. In the first experiment, rats underwent three sessions of EBC, explicitly unpaired stimulus exposure, or context-only exposure and the results revealed a decrease in Kv1.2 cell surface expression in the unpaired group as measured with microscopy but no change as measured with western blot. In the second experiment, the same three training groups underwent only one half of a session of training, and the results revealed an increase in Kv1.2 cell surface expression in the unpaired group as measured with western blot but no change as measured with microscopy. In addition, rats in the EBC group that did not express conditioned responses (CRs) exhibited the same increase in Kv1.2 cell surface expression as the unpaired group. The overall pattern of results suggests that cell surface expression of Kv1.2 is changed with exposure to EBC stimuli in the absence, or prior to the emergence, of CRs.

Alteration of Neuronal Excitability and Short-Term Synaptic Plasticity in the Prefrontal Cortex of a Mouse Model of Mental Illness

Using a genetic mouse model that faithfully recapitulates a DISC1 genetic alteration strongly associated with schizophrenia and other psychiatric disorders, we examined the impact of this mutation within the prefrontal cortex. Although cortical layering, cytoarchitecture, and proteome were found to be largely unaffected, electrophysiological examination of the mPFC revealed both neuronal hyperexcitability and alterations in short-term synaptic plasticity consistent with enhanced neurotransmitter release. Increased excitability of layer II/III pyramidal neurons was accompanied by consistent reductions in voltage-activated potassium currents near the action potential threshold as well as by enhanced recruitment of inputs arising from superficial layers to layer V. We further observed reductions in both the paired-pulse ratios and the enhanced short-term depression of layer V synapses arising from superficial layers consistent with enhanced neurotransmitter release at these synapses. Recordings from layer II/III pyramidal neurons revealed action potential widening that could account for enhanced neurotransmitter release. Significantly, we found that reduced functional expression of the voltage-dependent potassium channel subunit K_v1.1 substantially contributes to both the excitability and short-term plasticity alterations that we observed. The underlying dysregulation of K_v1.1 expression was attributable to cAMP elevations in the PFC secondary to reduced phosphodiesterase 4 activity present in Disc1 deficiency and was rescued by pharmacological blockade of adenylate cyclase. Our results demonstrate a potentially devastating impact of Disc1 deficiency on neural circuit function, partly due to K_v1.1 dysregulation that leads to a dual dysfunction consisting of enhanced neuronal excitability and altered short-term synaptic plasticity.SIGNIFICANCE STATEMENT Schizophrenia is a profoundly disabling psychiatric illness with a devastating impact not only upon the afflicted but also upon their families and the broader society. Although the underlying causes of schizophrenia remain poorly understood, a growing body of studies has identified and strongly implicated various specific risk genes in schizophrenia pathogenesis. Here, using a genetic mouse model, we explored the impact of one of the most highly penetrant schizophrenia risk genes, DISC1, upon the medial prefrontal cortex, the region believed to be most prominently dysfunctional in schizophrenia. We found substantial derangements in both neuronal excitability and short-term synaptic plasticity-parameters that critically govern neural circuit information processing-suggesting that similar changes may critically, and more broadly, underlie the neural computational dysfunction prototypical of schizophrenia.

Action potential broadening in a presynaptic channelopathy

Brain development and interictal function are unaffected in many paroxysmal neurological channelopathies, possibly explained by homoeostatic plasticity of synaptic transmission. Episodic ataxia type 1 is caused by missense mutations of the potassium channel Kv1.1, which is abundantly expressed in the terminals of cerebellar basket cells. Presynaptic action potentials of small inhibitory terminals have not been characterized, and it is not known whether developmental plasticity compensates for the effects of Kv1.1 dysfunction. Here we use visually targeted patch-clamp recordings from basket cell terminals of mice harbouring an ataxia-associated mutation and their wild-type littermates. Presynaptic spikes are followed by a pronounced afterdepolarization, and are broadened by pharmacological blockade of Kv1.1 or by a dominant ataxia-associated mutation. Somatic recordings fail to detect such changes. Spike broadening leads to increased Ca²⁺ influx and GABA release, and decreased spontaneous Purkinje cell firing. We find no evidence for developmental compensation for inherited Kv1.1 dysfunction.

Episodic ataxia type 1 is caused by mutations in the potassium channel Kv1.1, which is found in cerebellar basket cells. Here, the authors use electrophysiology techniques to characterize these mutant channels, and observe that the changes result in decreased spontaneous Purkinje cell firing with no evidence for developmental compensation.

Intracerebellar infusion of the protein kinase M zeta (PKMζ) inhibitor zeta-inhibitory peptide (ZIP) disrupts eyeblink classical conditioning

Protein kinase M zeta (PKM-ζ), a constitutively active N-terminal truncated form of PKC-ζ, has long been implicated in a cellular correlate of learning, long-term potentiation (LTP). Inhibition of PKM-ζ with zeta-inhibitory peptide (ZIP) has been shown in many brain structures to disrupt maintenance of AMPA receptors, irreversibly disrupting numerous forms of learning and memory that have been maintained for weeks. Delay eyeblink conditioning (EBC) is an established model for the assessment of cerebellar learning; here, we show that PKC-ζ and PKM-ζ are highly expressed in the cerebellar cortex, with highest expression found in Purkinje cell (PC) nuclei. Despite being highly expressed in the cerebellar cortex, no studies have examined how regulation of cerebellar PKM-ζ may affect cerebellar-dependent learning and memory. Given its disruption of learning in other brain structures, we hypothesized that ZIP would also disrupt delay EBC. We have shown that infusion of ZIP into the lobulus simplex of the rat cerebellar cortex can indeed significantly disrupt delay EBC. (PsycINFO Database Record

Selective Loss of Presynaptic Potassium Channel Clusters at the Cerebellar Basket Cell Terminal Pinceau in Adam11 Mutants Reveals Their Role in Ephaptic Control of Purkinje Cell Firing

A specialized axonal ending, the basket cell "pinceau," encapsulates the Purkinje cell axon initial segment (AIS), exerting final inhibitory control over the integrated outflow of the cerebellar cortex. This nonconventional axo-axonic contact extends beyond the perisomatic chemical GABAergic synaptic boutons to the distal AIS, lacks both sodium channels and local exocytotic machinery, and yet contains a dense cluster of voltage-gated potassium channels whose functional contribution is unknown. Here, we show that ADAM11, a transmembrane noncatalytic disintegrin, is the first reported Kv1-interacting protein essential for localizing Kv1.1 and Kv1.2 subunit complexes to the distal terminal. Selective absence of these channels at the pinceau due to mutation of ADAM11 spares spontaneous GABA release from basket cells at the perisomatic synapse yet eliminates ultrarapid ephaptic inhibitory synchronization of Purkinje cell firing. Our findings identify a critical role for presynaptic K(+) channels at the pinceau in ephaptic control over the speed and stability of spike rate coding at the Purkinje cell AIS in mice.

SIGNIFICANCE STATEMENT

This study identifies ADAM11 as the first essential molecule for the proper localization of potassium ion channels at presynaptic nerve terminals, where they modulate excitability and the release of neural transmitters. Genetic truncation of the transmembrane disintegrin and metalloproteinase protein ADAM11 resulted in the absence of Kv1 channels that are normally densely clustered at the terminals of basket cell axons in the cerebellar cortex. These specialized terminals are responsible for the release of the neurotransmitter GABA onto Purkinje cells and also display electrical signaling. In the ADAM11 mutant, GABAergic release was not altered, but the ultrarapid electrical signal was absent, demonstrating that the dense presynaptic cluster of Kv1 ion channels at these terminals mediate electrical transmission. Therefore, ADAM11 plays a critical role at this central synapse.

Cerebellar secretin modulates eyeblink classical conditioning

We have previously shown that intracerebellar infusion of the neuropeptide secretin enhances the acquisition phase of eyeblink conditioning (EBC). Here, we sought to test whether endogenous secretin also regulates EBC and to test whether the effect of exogenous and endogenous secretin is specific to acquisition. In Experiment 1, rats received intracerebellar infusions of the secretin receptor antagonist 5-27 secretin or vehicle into the lobulus simplex of cerebellar cortex immediately prior to sessions 1-3 of acquisition. Antagonist-infused rats showed a reduction in the percentage of eyeblink CRs compared with vehicle-infused rats. In Experiment 2, rats received intracerebellar infusions of secretin or vehicle immediately prior to sessions 1-2 of extinction. Secretin did not significantly affect extinction performance. In Experiment 3, rats received intracerebellar infusions of 5-27 secretin or vehicle immediately prior to sessions 1-2 of extinction. The secretin antagonist did not significantly affect extinction performance. Together, our current and previous results indicate that both exogenous and endogenous cerebellar secretin modulate acquisition, but not extinction, of EBC. We have previously shown that (1) secretin reduces surface expression of the voltage-gated potassium channel α-subunit Kv1.2 in cerebellar cortex and (2) intracerebellar infusions of a Kv1.2 blocker enhance EBC acquisition, much like secretin. Kv1.2 is almost exclusively expressed in cerebellar cortex at basket cell-Purkinje cell pinceaus and Purkinje cell dendrites; we propose that EBC-induced secretin release from PCs modulates EBC acquisition by reducing surface expression of Kv1.2 at one or both of these sites.

The knockout of secretin in cerebellar Purkinje cells impairs mouse motor coordination and motor learning

Secretin (SCT) was first considered to be a gut hormone regulating gastrointestinal functions when discovered. Recently, however, central actions of SCT have drawn intense research interest and are supported by the broad distribution of SCT in specific neuronal populations and by in vivo physiological studies regarding its role in water homeostasis and food intake. The direct action of SCT on a central neuron was first discovered in cerebellar Purkinje cells in which SCT from cerebellar Purkinje cells was found to potentiate GABAergic inhibitory transmission from presynaptic basket cells. Because Purkinje neurons have a major role in motor coordination and learning functions, we hypothesize a behavioral modulatory function for SCT. In this study, we successfully generated a mouse model in which the SCT gene was deleted specifically in Purkinje cells. This mouse line was tested together with SCT knockout and SCT receptor knockout mice in a full battery of behavioral tasks. We found that the knockout of SCT in Purkinje neurons did not affect general motor ability or the anxiety level in open field tests. However, knockout mice did exhibit impairments in neuromuscular strength, motor coordination, and motor learning abilities, as shown by wire hanging, vertical climbing, and rotarod tests. In addition, SCT knockout in Purkinje cells possibly led to the delayed development of motor neurons, as supported by the later occurrence of key neural reflexes. In summary, our data suggest a role in motor coordination and motor learning for SCT expressed in cerebellar Purkinje cells.

The central mechanisms of secretin in regulating multiple behaviors

Secretin (SCT) was firstly discovered as a gut peptide hormone in stimulating pancreatic secretion, while its novel neuropeptide role has drawn substantial research interests in recent years. SCT and its receptor (SCTR) are widely expressed in different brain regions, where they exert multiple cellular functions including neurotransmission, gene expression regulation, neurogenesis, and neural protection. As all these neural functions ultimately can affect behaviors, it is hypothesized that SCT controls multiple behavioral paradigms. Current findings support this hypothesis as SCT-SCTR axis participates in modulating social interaction, spatial learning, water and food intake, motor coordination, and motor learning behaviors. This mini-review focuses on various aspects of SCT and SCTR in hippocampus, hypothalamus, and cerebellum including distribution profiles, cellular functions, and behavioral phenotypes to elucidate the link between cellular mechanisms and behavioral control.

Cellular mechanisms and behavioral consequences of Kv1.2 regulation in the rat cerebellum

The potassium channel Kv1.2 α-subunit is expressed in cerebellar Purkinje cell (PC) dendrites where its pharmacological inhibition increases excitability (Khavandgar et al., 2005). Kv1.2 is also expressed in cerebellar basket cell (BC) axon terminals (Sheng et al., 1994), where its blockade increases BC inhibition of PCs (Southan and Robertson, 1998a). Secretin receptors are also expressed both in PC dendrites and BC axon terminals (for review, see (Yuan et al., 2011). The effect of secretin on PC excitability is not yet known, but, like Kv1.2 inhibitors, secretin potently increases inhibitory input to PCs (Yung et al., 2001). This suggests secretin may act in part by suppressing Kv1.2. Receptor-mediated endocytosis is a mechanism of Kv1.2 suppression (Nesti et al., 2004). This process can be regulated by protein kinase A (PKA) (Connors et al., 2008). Since secretin receptors activate PKA (Wessels-Reiker et al., 1993), we tested the hypothesis that secretin regulates Kv1.2 trafficking in the cerebellum. Using cell-surface protein biotinylation of rat cerebellar slices, we found secretin decreased cell-surface Kv1.2 levels by modulating Kv1.2 endocytic trafficking. This effect was mimicked by activating adenylate cyclase (AC) with forskolin, and was blocked by pharmacological inhibitors of AC or PKA. Imaging studies identified the BC axon terminal and PC dendrites as loci of AC-dependent Kv1.2 trafficking. The physiological significance of secretin-regulated Kv1.2 endocytosis is supported by our finding that infusion into the cerebellar cortex of either the Kv1.2 inhibitor tityustoxin-Kα, or of the Kv1.2 regulator secretin, significantly enhances acquisition of eyeblink conditioning in rats.

Increased Kv1 channel expression may contribute to decreased sIPSC frequency following chronic inhibition of NR2B-containing NMDAR

Numerous studies have documented the effects of chronic N-methyl-D-aspartate receptor (NMDAR) blockade on excitatory circuits, but the effects on inhibitory circuitry are not well studied. NR2A- and NR2B-containing NMDARs play differential roles in physiological processes, but the consequences of chronic NR2A- or NR2B-containing NMDAR inhibition on glutamatergic and GABAergic neurotransmission are unknown. We investigated altered GABAergic neurotransmission in dentate granule cells and interneurons following chronic treatment with the NR2B-selective antagonist, Ro25,6981, the NR2A-prefering antagonist, NVP-AAM077, or the non-subunit-selective NMDAR antagonist, D-APV, in organotypic hippocampal slice cultures. Electrophysiological recordings revealed large reductions in spontaneous inhibitory postsynaptic current (sIPSC) frequency in both granule cells and interneurons following chronic Ro25,6981 treatment, which was associated with minimally altered sIPSC amplitude, miniature inhibitory postsynaptic current (mIPSC) frequency, and mIPSC amplitude, suggesting diminished action potential-dependent GABA release. Chronic NVP-AAM077 or D-APV treatment had little effect on these measures. Reduced sIPSC frequency did not arise from downregulated GABA(A)R, altered excitatory or inhibitory drive to interneurons, altered interneuron membrane properties, increased failure rate, decreased action potential-dependent release probability, or mGluR/GABA(B) receptor modulation of GABA release. However, chronic Ro25,6981-mediated reductions in sIPSC frequency were occluded by the K+ channel blockers, dendrotoxin, margatoxin, and agitoxin, but not dendrotoxin-K or XE991. Immunohistochemistry also showed increased Kv1.2, Kv1.3, and Kv1.6 in the dentate molecular layer following chronic Ro25,6981 treatment. Our findings suggest that increased Kv1 channel expression/function contributed to diminished action potential-dependent GABA release following chronic NR2B-containing NMDAR inhibition and that these Kv1 channels may be heteromeric complexes containing Kv1.2, Kv1.3, and Kv1.6.

Spatial variation in membrane excitability modulated by 4-AP-sensitive K+ channels in the axons of the crayfish neuromuscular junction

Current-clamp recordings were made from the primary (1°) and secondary (2°) branching points (BPs) of axons at the crayfish neuromuscular junction. Action potential (AP) firing initiated by current injected at the 2° BP showed strong adaptation or high-frequency firing at threshold current, whereas AP firing frequency at the 1° BP exhibited a gradual rise with increasing current amplitude. The voltage threshold for AP (V(TH)) was higher at the 2° BP than the 1° BP. 4-Aminopyridine (4-AP) at 200 μM increased AP amplitude and duration at both BPs but reduced threshold current at the 2° BP more than at the 1° BP. This blocker lowered V(TH) at both BPs, but the difference between the BPs remained. Firing patterns evoked at the 2° BP became similar to those evoked at the 1° BP in 4-AP. Thus 4-AP-sensitive channels may be more concentrated in the distal axon and control AP initiation and firing patterns there. Orthodromic APs between the two BPs were also compared. There was no difference in AP amplitude between the two BPs, but AP half-width recorded at the 2° BP was longer than that at the 1° BP. AP duration at both BPs increased gradually, by ∼17%, during a 100-Hz, 500-ms train (in-train rise). Normalized AP half-widths revealed a smaller fractional in-train rise at the 2° BP. Thus, although distal APs were broader, AP duration there was under more stringent control than that of the proximal axon. 4-AP increased AP amplitude and duration of the entire orthodromic train and reduced the magnitude of the in-train rise in AP half-width at both BPs. However, this blocker did not uncover a clear difference between the two BPs. Thus 4-AP-sensitive channels concentrated in distal axon may be essential in preventing unintended firing and modulating AP waveform without interfering with orthodromic AP propagation.

Ethanol increases GABAergic transmission and excitability in cerebellar molecular layer interneurons from GAD67-GFP knock-in mice

AIMS

This study assessed the acute effect of ethanol on GABAergic transmission at molecular layer interneurons (MLIs; i.e. basket and stellate cells) in the cerebellar cortex. The actions of ethanol on spontaneous firing of these pacemaker neurons were also measured.

METHODS

Transgenic mice (glutamic acid-decarboxylase 67-green fluorescent protein knock-in mice) that express green fluorescence protein in GABAergic interneurons were used to aid in the identification of MLIs. Parasagittal cerebellar slices were prepared and whole-cell patch-clamp electrophysiological techniques were used to measure GABA(A) receptor-mediated spontaneous and miniature inhibitory postsynaptic currents (sIPSCs and mIPSCs). Loose-seal cell-attached recordings were used to measure spontaneous action potential firing.

RESULTS

Stellate cells received spontaneous GABAergic input in the form of a mixture of action potential-dependent events (sIPSCs) and quantal events (mIPSCs); ethanol increased sIPSC frequency to a greater extent than mIPSC frequency. Ethanol increased spontaneous action potential firing of MLIs, which could explain the increase in sIPSC frequency in stellate cells. Basket cells received GABAergic input in the form of quantal events only. Ethanol significantly increased the frequency of these events, which may be mediated by a different type of interneuron (perhaps, the Lugaro cell) or Purkinje cell collaterals.

CONCLUSIONS

Ethanol exposure differentially increases GABA release at stellate cell vs. basket cell-to-Purkinje cell synapses. This effect may contribute to the abnormalities in cerebellar function associated with alcohol intoxication.

Kv1.3 channels regulate synaptic transmission in the nucleus of solitary tract

The voltage-gated K(+) channel Kv1.3 has been reported to regulate transmitter release in select central and peripheral neurons. In this study, we evaluated its role at the synapse between visceral sensory afferents and secondary neurons in the nucleus of the solitary tract (NTS). We identified mRNA and protein for Kv1.3 in rat nodose ganglia using RT-PCR and Western blot analysis. In immunohistochemical experiments, anti-Kv1.3 immunoreactivity was very strong in internal organelles in the soma of nodose neurons with a weaker distribution near the plasma membrane. Anti-Kv1.3 was also identified in the axonal branches that project centrally, including their presynaptic terminals in the medial and commissural NTS. In current-clamp experiments, margatoxin (MgTx), a high-affinity blocker of Kv1.3, produced an increase in action potential duration in C-type but not A- or Ah-type neurons. To evaluate the role of Kv1.3 at the presynaptic terminal, we examined the effect of MgTx on tract evoked monosynaptic excitatory postsynaptic currents (EPSCs) in brain slices of the NTS. MgTx increased the amplitude of evoked EPSCs in a subset of neurons, with the major increase occurring during the first stimuli in a 20-Hz train. These data, together with the results from somal recordings, support the hypothesis that Kv1.3 regulates the duration of the action potential in the presynaptic terminal of C fibers, limiting transmitter release to the postsynaptic cell.

Contribution of Kv1.2 voltage-gated potassium channel to D2 autoreceptor regulation of axonal dopamine overflow

Impairments in axonal dopamine release are associated with neurological disorders such as schizophrenia and attention deficit hyperactivity disorder and pathophysiological conditions promoting drug abuse and obesity. The D2 dopamine autoreceptor (D2-AR) exerts tight regulatory control of axonal dopamine (DA) release through a mechanism suggested to involve K(+) channels. To evaluate the contribution of Kv1 voltage-gated potassium channels of the Shaker gene family to the regulation of axonal DA release by the D2-AR, the present study employed expression analyses, real time measurements of striatal DA overflow, K(+) current measurements and immunoprecipitation assays. Kv1.1, -1.2, -1.3, and -1.6 mRNA and protein were detected in midbrain DA neurons purified by fluorescence-activated cell sorting and in primary DA neuron cultures. In addition, Kv1.1, -1.2, and -1.6 were localized to DA axonal processes in the dorsal striatum. By means of fast scan cyclic voltammetry in striatal slice preparations, we found that the inhibition of stimulation-evoked DA overflow by a D2 agonist was attenuated by Kv1.1, -1.2, and -1.6 toxin blockers. A particular role for the Kv1.2 subunit in the process whereby axonal D2-AR inhibits DA overflow was established with the use of a selective Kv1.2 blocker and Kv1.2 knock-out mice. Moreover, we demonstrate the ability of D2-AR activation to increase Kv1.2 currents in co-transfected cells and its reliance on Gβγ subunit signaling along with the physical coupling of D2-AR and Kv1.2-containing channels in striatal tissue. These findings underline the contribution of Kv1.2 in the regulation of nigrostriatal DA release by the D2-AR and thereby offer a novel mechanism by which DA release is regulated.

GABA release from cerebellar stellate cells is developmentally regulated by presynaptic GABA(B) receptors in a target-cell-specific manner

Transmitter release from boutons along a common axon is often regulated depending on the postsynaptic target. Here, GABA release from cerebellar stellate cells onto Purkinje cells and other stellate cells was examined in acute cerebellar slices of 2- and 4-week-old mice. Consistent with previous findings on action potential-dependent GABA release, we found a developmental decrease in inhibitory inputs onto Purkinje cells but not onto stellate cells when recording miniature inhibitory postsynaptic currents (mIPSCs). Although amplitudes of mIPSCs were developmentally reduced in both cell types, mIPSC frequencies were decreased in Purkinje cells but were increased in stellate cells. Similarly, modulation of GABA release by presynaptic GABA(B) receptors changed during development in Purkinje cells but not in stellate cells, as demonstrated by the baclofen-mediated depression of mIPSC frequency and evoked IPSC (eIPSC) amplitudes. The selectively diminished baclofen effect in 4-week-old Purkinje cells correlated with a selective downregulation of presynaptic GABA(B) receptors at stellate cell-to-Purkinje cell synapses observed by immunoelectron microscopy analysis. Thus, expression of GABA(B) receptors in stellate cell axons and presynaptic modulation of GABA release appear to change during development in a target-cell-specific manner.

GABA(A) receptor expression and inhibitory post-synaptic currents in cerebellar Purkinje cells in dystrophin-deficient mdx mice

1. Duchenne muscular dystrophy (DMD) is the second most common fatal genetic disease and arises as a consequence of an absence or disruption of the protein dystrophin. In addition to wasting of the skeletal musculature, boys with DMD have a significant degree of cognitive impairment. 2. We show here that there is no difference between littermate control and mdx mice (a murine model of DMD) in the overall expression of the GABA(A) receptor a1-subunit, supporting the suggestion that it is the clustering at the synapse that is affected and not the expression of the GABA(A) receptor protein. 3. We report a significant reduction in both the frequency and amplitude of spontaneous inhibitory post-synaptic currents in cerebellar Purkinje cells of mdx mice compared with littermate controls, consistent with the reported reduction in the number and size of GABA(A) receptor clusters immunoreactive for a1- and a2-subunits at the post-synaptic densities. 4. These results may explain some of the behavioural problems and cognitive impairment reported in DMD.

Seizures and reduced life span in mice lacking the potassium channel subunit Kv1.2, but hypoexcitability and enlarged Kv1 currents in auditory neurons

Genes Kcna1 and Kcna2 code for the voltage-dependent potassium channel subunits Kv1.1 and Kv1.2, which are coexpressed in large axons and commonly present within the same tetramers. Both contribute to the low-voltage-activated potassium current I Kv1, which powerfully limits excitability and facilitates temporally precise transmission of information, e.g., in auditory neurons of the medial nucleus of the trapezoid body (MNTB). Kcna1-null mice lacking Kv1.1 exhibited seizure susceptibility and hyperexcitability in axons and MNTB neurons, which also had reduced I Kv1. To explore whether a lack of Kv1.2 would cause a similar phenotype, we created and characterized Kcna2-null mice (-/-). The -/- mice exhibited increased seizure susceptibility compared with their +/+ and +/- littermates, as early as P14. The mRNA for Kv1.1 and Kv1.2 increased strongly in +/+ brain stems between P7 and P14, suggesting the increasing importance of these subunits for limiting excitability. Surprisingly, MNTB neurons in brain stem slices from -/- and +/- mice were hypoexcitable despite their Kcna2 deficit, and voltage-clamped -/- MNTB neurons had enlarged I Kv1. This contrasts strikingly with the Kcna1-null MNTB phenotype. Toxin block experiments on MNTB neurons suggested Kv1.2 was present in every +/+ Kv1 channel, about 60% of +/- Kv1 channels, and no -/- Kv1 channels. Kv1 channels lacking Kv1.2 activated at abnormally negative potentials, which may explain why MNTB neurons with larger proportions of such channels had larger I Kv1. If channel voltage dependence is determined by how many Kv1.2 subunits each contains, neurons might be able to fine-tune their excitability by adjusting the Kv1.1:Kv1.2 balance rather than altering Kv1 channel density.

Mechanism of progesterone neuroprotection of rat cerebellar Purkinje cells following oxygen-glucose deprivation

The survival of rat Purkinje cell (PCs) cerebellar cultures was used to test the hypothesis that progesterone is protective against oxygen-glucose deprivation through potentiation of GABA(A) receptor activity. Electrophysiological recordings confirm that PCs develop robust excitatory and inhibitory synapses in culture. Exposure of cultured PCs to increasing concentrations of progesterone during oxygen-glucose deprivation revealed a concentration-dependent protection by progesterone, with significant protection observed at physiological concentrations, as low as 10 nm. The concurrent application of the GABA(A) receptor antagonist picrotoxin (100 microm) completely abolished the neuroprotection afforded by progesterone, indicating that progesterone is neuroprotective through activation of GABA(A) receptors. Progesterone potentiates GABA(A) receptor activity indirectly through its metabolites, such as allopregnanolone (ALLO). Therefore, ALLO was applied to PC cultures and was observed to produce significant protection at all concentrations tested, from 10 to 1000 nm. Finally, the inhibition of progesterone metabolism with finasteride abolished the protection afforded by progesterone without having any effect on the neuroprotection caused by ALLO. These data indicate that progesterone protects cerebellar PCs at physiological concentrations through a GABA-active metabolite.

Involvement of Kv channel subtypes on GABA release in mechanically dissociated neurons from the rat substantia nigra

The seven members of Shaker-related K(+) channel family, which are known to regulate membrane excitability and transmitter release, have been identified in the CNS. It is known that the substantia nigra pars reticulata (SNr) receives GABAergic inputs mainly from the striatum and sends GABAergic outputs to the thalamus. An immunohistochemical study shows that the Kv1 family, particularly Kv1.4, is expressed in the SNr, while it is reported that Kv1.2 mRNA is detected in the striatal neurons. Therefore, which Kv channels may be involved in the release of GABA in the SNr remains still controversial. To address this issue, we tested the effects of various K(+) channel blockers on the synaptic transmission using mechanically dissociated SNr neurons known as "synaptic bouton preparation", that retained functional presynaptic nerve terminals which enable us to examine miniature inhibitory postsynaptic currents (mIPSCs) by conventional whole-cell patch clamp recordings. Based on the sensitivities of mIPSCs to the Kv channel blockers, we concluded that Kv channels, in particular Kv1.2 subunit play the most significant role in the release of GABA at the presynaptic terminals projecting to the SNr neurons.

Kv1.1 deletion augments the afferent hypoxic chemosensory pathway and respiration

Mutations in the potassium channel gene Kv1.1 are associated with human episodic ataxia type 1 (EA-1) syndrome characterized by movement disorders and epilepsy. Ataxic episodes in EA-1 patients are often associated with exercise or emotional stress, which suggests a prominent role for the autonomic nervous system. Many of these alterations are reproduced in the Kv1.1-null mouse. Kv1.1 also regulates excitability of sensory neurons essential in cardiovascular and respiratory reflexes. We examined the neural control of the respiratory system of littermate wild-type (control) and Kv1.1-null mice during low O2 (hypoxia). Immunohistochemical studies demonstrated Kv1.1 in the afferent limb of the carotid body chemoreflex (the major regulator in the response to hypoxia), consisting of the carotid body, petrosal ganglion, and nucleus of the solitary tract (NTS). Respiration was examined by plethysmography. Null mice exhibited a greater increase in respiration during hypoxia compared with controls. In vitro carotid body sensory discharge during hypoxia was greater in null than control mice. In the caudal NTS, evoked EPSCs in brainstem slices were similar between control and null mice. However, the frequency of spontaneous and miniature EPSCs was greater in null mice. Null mice also exhibited more asynchronous release after a stimulus train. These results demonstrate the important role of Kv1.1 in afferent chemosensory activity and suggest that mutations in the human Kv1.1 gene have functional consequences during stress responses that involve respiratory reflexes.

Kv1 channels selectively prevent dendritic hyperexcitability in rat Purkinje cells

Purkinje cells, the sole output of the cerebellar cortex, encode the timing signals required for motor coordination in their firing rate and activity pattern. Dendrites of Purkinje cells express a high density of P/Q-type voltage-gated calcium channels and fire dendritic calcium spikes. Here we show that dendritic subthreshold Kv1.2 subunit-containing Kv1 potassium channels prevent generation of random spontaneous calcium spikes. With Kv1 channels blocked, dendritic calcium spikes drive bursts of somatic sodium spikes and prevent the cell from faithfully encoding motor timing signals. The selective dendritic function of Kv1 channels in Purkinje cells allows them to effectively suppress dendritic hyperexcitability without hindering the generation of somatic action potentials. Further, we show that Kv1 channels also contribute to dendritic integration of parallel fibre synaptic input. Kv1 channels are often targeted to soma and axon and the data presented support a major dendritic function for these channels.

Kv1 K+ channels control Purkinje cell output to facilitate postsynaptic rebound discharge in deep cerebellar neurons

Purkinje cells (PCs) generate the sole output of the cerebellar cortex and govern the timing of action potential discharge from neurons of the deep cerebellar nuclei (DCN). Here, we examine how voltage-gated Kv1 K+ channels shape intrinsically generated and synaptically controlled behaviors of PCs and address how the timing of DCN neuron output is modulated by manipulating PC Kv1 channels. Kv1 channels were studied in cerebellar slices at physiological temperatures with Kv1-specific toxins. Outside-out voltage-clamp recordings indicated that Kv1 channels are present in both somatic and dendritic membranes and are activated by Na+ spike-clamp commands. Whole-cell current-clamp recordings revealed that Kv1 K+ channels maintain low frequencies of Na+ spike and Ca-Na burst output, regulate the duration of plateau potentials, and set the threshold for Ca2+ spike discharge. Kv1 channels shaped the characteristics of climbing fiber (CF) responses evoked by extracellular stimulation or intracellular simulated EPSCs. In the presence of Kv1 toxins, CFs discharged spontaneously at approximately 1 Hz. Finally, "Kv1-intact" and "Kv1-deficient" PC tonic and burst outputs were converted to stimulus protocols and used as patterns to stimulate PC axons and synaptically activate DCN neurons. We found that the Kv1-intact patterns facilitated short-latency and high-frequency DCN neuron rebound discharges, whereas DCN neuron output timing was markedly disrupted by the Kv1-deficient stimulus protocols. Our results suggest that Kv1 K+ channels are critical for regulating the excitability of PCs and CFs and optimize the timing of PC outputs to generate appropriate discharge patterns in postsynaptic DCN neurons.

Involvement of kv1 potassium channels in spreading acidification and depression in the cerebellar cortex

Spreading acidification and depression (SAD) is a form of propagated activity in the cerebellar cortex characterized by acidification and a transient depression in excitability. This study investigated the role of Kv1 potassium channels in SAD using neutral red, flavoprotein autofluorescence, and voltage-sensitive dye optical imaging in the mouse cerebellar cortex, in vivo. The probability of evoking SAD was greatly increased by blocking Kv1.1 as well as Kv1.2 potassium channels by their specific blockers dendrotoxin K (DTX-K) and tityustoxin (TsTX), respectively. DTX-K not only greatly lowered the threshold for evoking SAD but also resulted in multiple cycles of spread and spontaneous SAD. The occurrence of spontaneous SAD originating from spontaneous parallel fiber-like beams of activity suggests that blocking Kv1 channels increased parallel fiber excitability. This was confirmed by the generation of parallel fiber-like beams with the microinjection of glutamate into the upper molecular layer in the presence of DTX-K. The dramatic effects of DTX-K suggest a possible connection between SAD and episodic ataxia type 1 (EA1), a Kv1.1 potassium channelopathy. The threshold for evoking SAD was significantly lowered in the Kv1.1 heterozygous knockout mouse compared with wild-type littermates. Carbamazepine and acetazolamide, both effective in the treatment of EA1, significantly decreased the likelihood of evoking SAD. Blocking GABAergic neurotransmission did not alter the effectiveness of DTX-K. The cyclin D2 null mouse, which lacks cerebellar stellate cells, also exhibited SAD. Therefore blocking Kv1 potassium channels establishes the conditions needed to generate SAD. Furthermore, the results are consistent with the hypothesis that SAD may underlie the transient attacks of ataxia characterizing EA1.

Potassium channel subunit Kv3.2 and the water channel aquaporin-4 are selectively localized to cerebellar pinceau

The pinceau is a cerebellar structure formed by descending GABA-ergic basket cell axonal terminals converging on the initial axonal segment of Purkinje cell. Although basket cells exert a powerful inhibitory influence on the output of the cerebellar cortex, the function and mode of action of the pinceau are not understood because the majority of basket cell axons fail to make identifiable synaptic contacts with the Purkinje cell axon. Several proteins were previously reported to cluster specifically in this area, including a number of voltage-activated potassium channel subunits. In this study, we used immunohistochemistry, electron microscopy, and electron tomography to examine the ultrastructural localization of a novel voltage-gated potassium channel subunit, Kv3.2, in the pinceau. We found strong, selective localization of Kv3.2 to basket cell axons. Additionally, because potassium buffering is often conducted through water channels, we studied the extent of a brain-specific water channel, aquaporin-4 (AQP4), using confocal and electron microscopy. As expected, we found AQP4 was heavily localized to astrocytic processes of the pinceau. The abundance of potassium channels and AQP4 in this area suggests rapid ionic dynamics in the pinceau, and the unusual, highly specialized morphology of this region implies that the structural features may combine with the molecular composition to regulate the microenvironment of the initial segment of the Purkinje cell axon.

Mechanism of GABAB receptor-mediated inhibition of spontaneous GABA release onto cerebellar Purkinje cells

gamma-Aminobutyric acid (GABA)(B) receptor-mediated modulation of spontaneous GABA release onto Purkinje cells was investigated in cerebellar slices from 3- to 5-week-old mice. The GABA(B) receptor agonists baclofen and CGP 44533 each reduced the frequency of miniature inhibitory postsynaptic currents (mIPSCs), with no significant effect on mIPSC amplitude; together, consistent with a presynaptic site of action. The GABA(B) receptor antagonist CGP 55845 blocked baclofen-induced inhibition. The sulphydryl alkylating agent N-ethylmaleimide occluded baclofen effects, implicating G(i/o) subunits in mediating a GABA(B) G protein-coupled receptor pathway. Baclofen-induced inhibition persisted in the presence of Ba(2+), a blocker of K(+) channels, and Cd(2+), a blocker of Ca(2+) channel-mediated GABA release. Application of nominally Ca(2+)-free extracellular solutions reduced mIPSC frequency and amplitude; however, baclofen produced a significant inhibition in mIPSC frequency, further suggesting that this pathway was independent of Ca(2+) influx. Spontaneous GABA release was increased by the adenylate cyclase activator, forskolin, and the phorbol ester, phorbol 12,13-dibutyrate. However, baclofen-induced inhibition was not significantly changed in either condition. Baclofen action was also not affected by the adenylate cyclase inhibitor SQ 22536 or the protein kinase C inhibitor chelerythrine chloride. Baclofen still reduced mIPSC frequency in the presence of the polyvalent cation ruthenium red, which acts as a secretagogue here; however, baclofen-induced inhibition was reduced significantly. Furthermore, baclofen produced no clear inhibition during high-frequency mIPSCs bursts induced by the potent secretagogue alpha-Latrotoxin. Together, these results suggest that GABA(B) inhibition occurs downstream of Ca(2+) influx and may be mediated, in part, by an inhibition of the vesicular release mechanism.

Presynaptic K+ channels: electrifying regulators of synaptic terminal excitability

Potassium channels are crucial regulators of neuronal excitability, setting resting membrane potentials and firing thresholds, repolarizing action potentials and limiting excitability. Although most of our understanding of K+ channels is based on somatic recordings, there is good evidence that these channels are present in synaptic terminals. In recent years the improved access to presynaptic compartments afforded by direct recording techniques has indicated diverse roles for native K+ channels, from suppression of aberrant firing to action potential repolarization and activity-dependent modulation of synaptic activity. This article reviews the growing evidence for multiple roles and discrete localization of distinct K+ channels at presynaptic terminals.

Altered inhibitory input to Purkinje cells of dystrophin-deficient mice

We investigated evoked EPSPs and spontaneous IPSPs in cerebellar slices from dystrophin-deficient mdx mice. In the presence of the GABA(A) antagonist bicuculline the increase in EPSP amplitude was less in mdx Purkinje cells compared to control, and the amplitude of miniature IPSCs in mdx cells was also significantly less than in controls. This reduced inhibitory input is most likely due to the reported reduction in the size of GABA(A) channel clusters.

Dendrotoxin poisoning in a neurobiochemist

BACKGROUND

Dendrotoxin is a highly potent blocker of KV1.1, KV1.2, and KV1.6 potassium channels that is derived from the venom of the green mamba (Dendroaspis angusticeps). It is commonly used to inhibit the function of whole nerve preparations in vitro. Despite the widespread use and potency of this compound, neurotoxicity in humans has not been described from refined toxin. We report a case of dendrotoxin toxicity from dermal exposure.

CASE REPORT

A healthy 40-year-old female neurobiochemist presented with complaints of progressive numbness of the left malar region and lateral orbit that progressed to include the medial orbit and tongue. One hour prior to presentation she used her bare hands to remove residual petroleum jelly from a dish that had previously contained 500 nanoliters of 500 nanomolar dendrotoxin. She recalled rubbing her left eye prior to the onset of symptoms. Before touching the dish, she had washed it with running water and then 70% ethanol while using latex gloves. Physical examination was remarkable only for weakness to superior gaze and some mild tongue fasciculations. Within 12 hours of exposure, she was completely asymptomatic.

CONCLUSION

Dendrotoxin is a highly potent neurotoxin that can cause localized impairment of nerve function after mucous membrane exposure.

Hyperexcitability and reduced low threshold potassium currents in auditory neurons of mice lacking the channel subunit Kv1.1

A low voltage-activated potassium current, IKL, is found in auditory neuron types that have low excitability and precisely preserve the temporal pattern of activity present in their presynaptic inputs. The gene Kcna1 codes for Kv1.1 potassium channel subunits, which combine in expression systems to produce channel tetramers with properties similar to those of IKL, including sensitivity to dendrotoxin (DTX). Kv1.1 is strongly expressed in neurons with IKL, including auditory neurons of the medial nucleus of the trapezoid body (MNTB). We therefore decided to investigate how the absence of Kv1.1 affected channel properties and function in MNTB neurons from mice lacking Kcna1. We used the whole cell version of the patch clamp technique to record from MNTB neurons in brainstem slices from Kcna1-null (-/-) mice and their wild-type (+/+) and heterozygous (+/-) littermates. There was an IKL in voltage-clamped -/- MNTB neurons, but it was about half the amplitude of the IKL in +/+ neurons, with otherwise similar properties. Consistent with this, -/- MNTB neurons were more excitable than their +/+ counterparts; they fired more than twice as many action potentials (APs) during current steps, and the threshold current amplitude required to generate an AP was roughly halved. +/- MNTB neurons had excitability and IKL amplitudes identical to the +/+ neurons. The IKL remaining in -/- neurons was blocked by DTX, suggesting the underlying channels contained subunits Kv1.2 and/or Kv1.6 (also DTX-sensitive). DTX increased excitability further in the already hyperexcitable -/- MNTB neurons, suggesting that -/- IKL limited excitability despite its reduced amplitude in the absence of Kv1.1 subunits.

Spreading acidification and depression in the cerebellar cortex

Optical imaging of activity-dependent pH changes using neutral red has revealed a novel form of propagated activity in the cerebellar cortex: spreading acidification and depression (SAD). Evoked by surface stimulation, SAD is characterized by a propagation geometry that reflects the parasagittal architecture of the cerebellum, high speed of propagation across several folia, and a transient depression of the molecular layer circuitry. The properties of SAD differentiate it from other forms of propagating activity in the nervous system including spreading depression and Ca++ waves. Involving several factors, SAD is hypothesized to be a regenerative process that requires a functioning parallel fibers-Purkinje cell circuit, glutamatergic neurotransmission, and is initiated by increased neuronal excitability. Three possible neuronal and glia substrates in the cerebellar cortex could account for the propagation geometry of SAD. Recently, the authors demonstrated that blocking voltage-gated Kv1.1 potassium channels plays a major role in the generation of SAD. This observation has lead to the hypothesis that the episodic and transient disruption in cerebellar function that characterizes episodic ataxia type 1, a Kv1.1 channelopathy, is due to SAD occurring in the cerebellar cortex.

Presynaptic internal Ca2+ stores contribute to inhibitory neurotransmitter release onto mouse cerebellar Purkinje cells

1. Miniature inhibitory postsynaptic currents (mIPSCs) were recorded in mouse Purkinje cells in the presence of 1 micro M tetrodotoxin (TTX). Under these conditions, which eliminated Ca(2+) influx through voltage-dependent Ca(2+) channels (VDCCs), the contribution of Ca(2+) stores to spontaneous GABA release was examined. 2. The plant alkaloid ryanodine acts as an inhibitor of endoplasmic reticulum ryanodine-sensitive Ca(2+) release channels (ryanodine receptors) at low micromolar concentrations. Ryanodine effects were confined to a subpopulation of cells tested. At 10 micro M ryanodine, 4/12 cells showed a significant increase in mean mIPSC frequency of +19.6+/-4.0% (n=4). 3. The sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA) pump inhibitor cyclopiazonic acid (CPA) produced a more robust effect. In 8/10 cells, 25 micro M CPA caused a significant increase in mean mIPSC frequency; the mean increase being +26.0+/-3.0% (n=8). Similar results were seen with thapsigargin (1-2 micro M), another SERCA pump inhibitor. 4. Ruthenium red (RuR) has been proposed to either act directly on the release machinery or block Ca(2+) pumps on internal stores. At 10 micro M RuR, all cells showed a rapid, large increase in mean mIPSC frequency of +90.4+/-16.4% (n=9). This increase was greater than that seen by agents known to modulate Ca(2+) stores and was more consistent with a direct action. At this concentration, RuR also occluded the effects of CPA. 5. For all reagents, there were no obvious effects on mean mIPSC amplitude. However, the effects on mIPSC frequency were consistent with a presynaptic action and indicate that Ca(2+) stores may contribute to spontaneous GABA release onto mouse Purkinje cells.

Dendrotoxin sensitive potassium channels modulate GABA but not glutamate release in the rat entorhinal cortex in vitro

We have previously shown that the anticonvulsant drug, phenytoin, increases the frequency and amplitude of spontaneous inhibitory postsynaptic currents at GABA synapses on principal neurones in the rat entorhinal cortex. This effect is similar to that seen at other GABA synapses following blockade of voltage-gated potassium channels (Kv1.1, 1.2 and 1.6) with alpha-dendrotoxin. In the present study we examined whether dendrotoxins can alter GABA release at synapses in the entorhinal cortex. We recorded spontaneous inhibitory postsynaptic currents using whole cell voltage clamp techniques in slices of rat entorhinal cortex in vitro. alpha-Dendrotoxin evoked an increase in frequency and amplitude of spontaneous inhibitory postsynaptic currents, an effect that was blocked by prior perfusion with tetrodotoxin. The effect of the toxin did not occlude the increase in spontaneous inhibitory postsynaptic currents seen with phenytoin. Indeed, the effect of the two drugs together was, at least, additive on GABA release. Perfusion with the specific Kv1.1 blocker, dendrotoxin-K had no effect on GABA release. In addition, alpha-dendrotoxin had no effect on frequency or amplitude of spontaneous excitatory postsynaptic currents at glutamate synapses on entorhinal cortex neurones. We conclude that K-channels containing the Kv1.2 and/or 1.6 subunits modulate the release of GABA, but not glutamate in the entorhinal cortex. The modulation of GABA release by phenytoin is unlikely to be due to an effect on these channels.

The Cav2.1/alpha1A (P/Q-type) voltage-dependent calcium channel mediates inhibitory neurotransmission onto mouse cerebellar Purkinje cells

The effects of voltage-dependent calcium channel (VDCC) antagonists on spontaneous inhibitory postsynaptic currents (sIPSCs) in mouse Purkinje cells were examined using in vitro cerebellar slices. The inorganic ion Cd2+ reduced sIPSC amplitude and frequency. No additional block was seen with the Na+ channel antagonist tetrodotoxin (TTX) suggesting that all action potential-evoked inhibitory GABA release was mediated by high-voltage-activated VDCCs. No evidence was found for involvement of Cav1/alpha1C and alpha1D (L-type), Cav2.2/alpha1B (N-type) or Cav2.3/alpha1E (R-type) high-voltage-activated VDCCs or low-voltage-activated Cav3/alpha1G, alpha1H and alpha1I (T-type) VDCCs in mediating presynaptic GABA release. Blockade of sIPSCs by 200 nM omega-agatoxin IVA implicated the Cav2.1/alpha1A (P/Q-type) subtype of high-voltage-activated VDCCs in mediating inhibitory transmission. Inhibition by omega-agatoxin IVA was similar to that seen with Cd2+ and TTX. Selective antibodies directed against the Cav2.1 subunit revealed staining in regions closely opposed to Purkinje cell somata. Cav2.1 staining was colocalized with staining for antibodies against glutamic acid decarboxylase and corresponded well with the pericellular network formed by GABAergic basket cell interneurons. Antibody labelling of Cav2.3 revealed a region-specific expression. In the cerebellar cortex anterior lobe, Cav2.3 staining was predominantly somatodendritic; whilst in the posterior lobe, perisomatic staining was seen primarily. However, electrophysiological data was not consistent with a role for the Cav2.3 subunit in mediating presynaptic GABA release. No consistent staining was seen for other Cav (alpha1) subunits. Electrophysiological and immunostaining data support a predominant role for Cav2.1 subunits in mediating action potential-evoked inhibitory GABA release onto mouse Purkinje cells.

The inherited episodic ataxias: how well do we understand the disease mechanisms?

The past few years have seen the elucidation of several neurological diseases caused by inherited mutations of ion channels. In contrast to many other types of genetic disorders, the "channelopathies" can be studied with high precision by applying electrophysiological methods. This review evaluates the success of this approach in explaining the mechanisms of two forms of episodic ataxia that are known to be caused by mutations of ion channels: episodic ataxia type 1 (EA1, caused by K+ channel mutations) and episodic ataxia type 2 (EA2, caused by Ca2+ channel mutations). Although both of these disorders are rare, they raise many important questions about the roles of identified channels in brain function. Indeed, a resolution of the mechanisms by which both diseases occur will represent a major milestone in understanding diseases of the CNS, in addition to opening the way to novel possible treatments.

Twenty years of dendrotoxins

Dendrotoxins are small proteins that were isolated 20 years ago from mamba (Dendroaspis) snake venoms (Harvey, A.L., Karlsson, E., 1980. Dendrotoxin from the venom of the green mamba, Dendroaspis angusticeps: a neurotoxin that enhances acetylcholine release at neuromuscular junctions. Naunyn-Schmiedebergs Arch. Pharmacol. 312, 1-6.). Subsequently, a family of related proteins was found in mamba venoms and shown to be homologous to Kunitz-type serine protease inhibitors, such as aprotinin. The dendrotoxins contain 57-60 amino acid residues cross-linked by three disulphide bridges. The dendrotoxins have little or no anti-protease activity, but they were demonstrated to block particular subtypes of voltage-dependent potassium channels in neurons. Studies with cloned K(+) channels indicate that alpha-dendrotoxin from green mamba Dendroaspis angusticeps blocks Kv1.1, Kv1.2 and Kv1.6 channels in the nanomolar range, whereas toxin K from the black mamba Dendroaspis polylepis preferentially blocks Kv1.1 channels. Structural analogues of dendrotoxins have helped to define the molecular recognition properties of different types of K(+) channels, and radiolabelled dendrotoxins have also been useful in helping to discover toxins from other sources that bind to K(+) channels. Because dendrotoxins are useful markers of subtypes of K(+) channels in vivo, dendrotoxins have become widely used as probes for studying the function of K(+) channels in physiology and pathophysiology.

Electrophysiological characterization of voltage-gated K(+) currents in cerebellar basket and purkinje cells: Kv1 and Kv3 channel subfamilies are present in basket cell nerve terminals

To understand the processes underlying fast synaptic transmission in the mammalian CNS, we must have detailed knowledge of the identity, location, and physiology of the ion channels in the neuronal membrane. From labeling studies we can get clues regarding the distribution of ion channels, but electrophysiological methods are required to determine the importance of each ion channel in CNS transmission. Dendrotoxin-sensitive potassium channel subunits are highly concentrated in cerebellar basket cell nerve terminals, and we have previously shown that they are responsible for a significant fraction of the voltage-gated potassium current in this region. Here, we further investigate the characteristics and pharmacology of the voltage-dependent potassium currents in these inhibitory nerve terminals and compare these observations with those obtained from somatic recordings in basket and Purkinje cell soma regions. We find that alpha-DTX blocks basket cell nerve terminal currents and not somatic currents, and the IC(50) for alpha-DTX in basket cell terminals is 3.2 nM. There are at least two distinct types of potassium currents in the nerve terminal, a DTX-sensitive low-threshold component, and a second component that activates at much more positive voltages. Pharmacological experiments also reveal that nerve terminal potassium currents are also markedly reduced by 4-AP and TEA, with both high-sensitivity (micromolar) and low-sensitivity (millimolar) components present. We suggest that basket cell nerve terminals have potassium channels from both the Kv1 and Kv3 subfamilies, whereas somatic currents in basket cell and Purkinje cell bodies are more homogeneous.

Electrophysiological characterization of voltage-gated K(+) currents in cerebellar basket and purkinje cells: Kv1 and Kv3 channel subfamilies are present in basket cell nerve terminals

To understand the processes underlying fast synaptic transmission in the mammalian CNS, we must have detailed knowledge of the identity, location, and physiology of the ion channels in the neuronal membrane. From labeling studies we can get clues regarding the distribution of ion channels, but electrophysiological methods are required to determine the importance of each ion channel in CNS transmission. Dendrotoxin-sensitive potassium channel subunits are highly concentrated in cerebellar basket cell nerve terminals, and we have previously shown that they are responsible for a significant fraction of the voltage-gated potassium current in this region. Here, we further investigate the characteristics and pharmacology of the voltage-dependent potassium currents in these inhibitory nerve terminals and compare these observations with those obtained from somatic recordings in basket and Purkinje cell soma regions. We find that alpha-DTX blocks basket cell nerve terminal currents and not somatic currents, and the IC(50) for alpha-DTX in basket cell terminals is 3.2 nM. There are at least two distinct types of potassium currents in the nerve terminal, a DTX-sensitive low-threshold component, and a second component that activates at much more positive voltages. Pharmacological experiments also reveal that nerve terminal potassium currents are also markedly reduced by 4-AP and TEA, with both high-sensitivity (micromolar) and low-sensitivity (millimolar) components present. We suggest that basket cell nerve terminals have potassium channels from both the Kv1 and Kv3 subfamilies, whereas somatic currents in basket cell and Purkinje cell bodies are more homogeneous.

Modulation by K+ channels of action potential-evoked intracellular Ca2+ concentration rises in rat cerebellar basket cell axons

1. Action potential-evoked [Ca2+]i rises in basket cell axons of rat cerebellar slices were studied using two-photon laser scanning microscopy and whole-cell recording, to identify the K+ channels controlling the shape of the axonal action potential. 2. Whole-cell recordings of Purkinje cell IPSCs were used to screen K+ channel subtypes which could contribute to axonal repolarization. alpha-Dendrotoxin, 4-aminopyridine, charybdotoxin and tetraethylammonium chloride increased IPSC rate and/or amplitude, whereas iberiotoxin and apamin failed to affect the IPSCs. 3. The effects of those K+ channel blockers that enhanced transmitter release on the [Ca2+]i rises elicited in basket cell axons by action potentials fell into three groups: 4-aminopyridine strongly increased action potential-evoked [Ca2+]i; tetraethylammonium and charybdotoxin were ineffective alone but augmented the effects of 4-aminopyridine; alpha-dendrotoxin had no effect. 4. We conclude that cerebellar basket cells contain at least three pharmacologically distinct K+ channels, which regulate transmitter release through different mechanisms. 4-Aminopyridine-sensitive, alpha-dendrotoxin-insensitive K+ channels are mainly responsible for repolarization in basket cell presynaptic terminals. K+ channels blocked by charybdotoxin and tetraethylammonium have a minor role in repolarization. alpha-Dendrotoxin-sensitive channels are not involved in shaping the axonal action potential waveform. The two last types of channels must therefore exert control of synaptic activity through a pathway unrelated to axonal action potential broadening.