K+ channel sub-types in rat brain: characteristic locations revealed using beta-bungarotoxin, alpha- and delta-dendrotoxins

Gambierol inhibition of voltage-gated potassium channels augments spontaneous Ca2+ oscillations in cerebrocortical neurons

Gambierol is a marine polycyclic ether toxin produced by the marine dinoflagellate Gambierdiscus toxicus and is a member of the ciguatoxin toxin family. Gambierol has been demonstrated to be either a low-efficacy partial agonist/antagonist of voltage-gated sodium channels or a potent blocker of voltage-gated potassium channels (Kvs). Here we examined the influence of gambierol on intact cerebrocortical neurons. We found that gambierol produced both a concentration-dependent augmentation of spontaneous Ca(2+) oscillations, and an inhibition of Kv channel function with similar potencies. In addition, an array of selective as well as universal Kv channel inhibitors mimicked gambierol in augmenting spontaneous Ca(2+) oscillations in cerebrocortical neurons. These data are consistent with a gambierol blockade of Kv channels underlying the observed increase in spontaneous Ca(2+) oscillation frequency. We also found that gambierol produced a robust stimulation of phosphorylation of extracellular signal-regulated kinases 1/2 (ERK1/2). Gambierol-stimulated ERK1/2 activation was dependent on both inotropic [N-methyl-d-aspartate (NMDA)] and type I metabotropic glutamate receptors (mGluRs) inasmuch as MK-801 [NMDA receptor inhibitor; (5S,10R)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate], S-(4)-CGP [S-(4)-carboxyphenylglycine], and MTEP [type I mGluR inhibitors; 3-((2-methyl-4-thiazolyl)ethynyl) pyridine] attenuated the response. In addition, 2-aminoethoxydiphenylborane, an inositol 1,4,5-trisphosphate receptor inhibitor, and U73122 (1-[6-[[(17b)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione), a phospholipase C inhibitor, both suppressed gambierol-induced ERK1/2 activation, further confirming the role of type I mGluR-mediated signaling in the observed ERK1/2 activation. Finally, we found that gambierol produced a concentration-dependent stimulation of neurite outgrowth that was mimicked by 4-aminopyridine, a universal potassium channel inhibitor. Considered together, these data demonstrate that gambierol alters both Ca(2+) signaling and neurite outgrowth in cerebrocortical neurons as a consequence of blockade of Kv channels.

Dopamine enhances the excitability of somatosensory thalamocortical neurons

The thalamus conveys sensory information from peripheral and subcortical regions to the neocortex in a dynamic manner that can be influenced by several neuromodulators. Alterations in dopamine (DA) receptor function in thalami of Schizophrenic patients have recently been reported. In addition, schizophrenia is associated with sensory gating abnormalities and sleep-wake disturbances, thus we examined the role of DA on neuronal excitability in somatosensory thalamus. The ventrobasal (VB) thalamus receives dopaminergic innervation and expresses DA receptors; however, the action of DA on VB neurons is unknown. In the present study, we performed whole cell current- and voltage-clamp recordings in rat brain slices to investigate the role of DA on excitability of VB neurons. We found that DA increased action potential discharge and elicited membrane depolarization via activation of different receptor subtypes. Activation of D2-like receptors (D(2R)) leads to enhanced action potential discharge, whereas the membrane depolarization was mediated by D1-like receptors (D(1R)). The D(2R-mediated) increase in spike discharge was mimicked and occluded by α-dendrotoxin (α-DTX), indicating the involvement of a slowly inactivating K(+) channels. The D1R-mediated membrane depolarization was occluded by barium, suggesting the involvement of a G protein-coupled K(+) channel or an inwardly rectifying K(+) channel. Our results indicate that DA produces dual modulatory effects acting on subtypes of DA receptors in thalamocortical relay neurons, and likely plays a significant role in the modulation of sensory information.

Effects of various K+ channel blockers on spontaneous glycine release at rat spinal neurons

Molecular biology approaches have identified more than 70 different K+ channel genes that assemble to form diverse functional classes of K+ channels. Although functional K+ channels are present within presynaptic nerve endings, direct studies of their precise identity and function have been generally limited to large, specialized presynaptic terminals such as basket cell terminals and Calyx of Held. In the present study, therefore, we investigated the functional K+ channel subtypes on the small glycinergic nerve endings (< 1 microm diameter) projecting to spinal sacral dorsal commissural nucleus (SDCN) neurons. In the presence of TTX, whole-cell patch recording of mIPSCs was made from mechanically dispersed SDCN neurons in which functional nerve endings remain attached. Glycinergic responses were isolated by blocking glutamatergic and GABAergic inputs with CNQX, AP5 and bicuculline. The K+ channel blockers, 4-AP, TEA, delta-dendrotoxin, margatoxin, iberiotoxin, charybdotoxin and apamin, significantly increased 'spontaneous' mIPSC frequency without affecting mIPSC amplitude. The results suggest the existence of the following K+ channel subtypes on glycinergic nerve endings that are involved in regulating 'spontaneous' glycine release (mIPSCs): the Shaker-related K+ channels Kv1.1, Kv1.2, Kv1.3, Kv1.6 and Kv1.7 and the intracellular Ca2+ -sensitive K+ channels BKCa, IKCa and SKCa. Ca2+ channel blockers by themselves, including L-type (nifedipine), P/Q-type (omega-agatoxin IVA, AgTX) and N-type (omega-conotoxin GVIA, CgTX), did not alter the 'spontaneous' mIPSC frequency or amplitude, but inhibited the increase of the mIPSC frequency evoked by 4-AP, indicating the participation of L-, P/Q- and N-type Ca2+ channels regulating 'spontaneous' glycine release from the nerve terminals.

Presynaptic enzymatic neurotoxins

Botulinum neurotoxins produced by anaerobic bacteria of the genus Clostridium are the most toxic proteins known, with mouse LD50 values in the 1-5 ng/kg range, and are solely responsible for the pathophysiology of botulism. These metalloproteinases enter peripheral cholinergic nerve terminals and cleave proteins of the neuroexocytosis apparatus, causing a persistent, but reversible, inhibition of neurotransmitter release. They are used in the therapy of many human syndromes caused by hyperactive nerve terminals. Snake presynaptic PLA2 neurotoxins block nerve terminals by binding to the nerve membrane and catalyzing phospholipid hydrolysis with production of lysophospholipids and fatty acids. These compounds change the membrane conformation, causing enhanced fusion of synaptic vesicle via hemifusion intermediate with release of neurotransmitter and, at the same time, inhibition of vesicle fission and recycling. It is possible to envisage clinical applications of the lysophospholipid/fatty acid mixture to inhibit hyperactive superficial nerve terminals.

Immunohistochemical localization of the voltage-gated potassium channel subunit Kv1.4 in the central nervous system of the adult rat

A large set of voltage-gated potassium channels is involved in regulating essential aspects of neuronal function in the central nervous system, thus contributing to the ability of neurons to respond to a given input. In the present study, we used immunocytochemical methods to elucidate the regional, cellular and subcellular distribution of the voltage-gated potassium channel subunit Kv1.4, a member of the Shaker subfamily, in the brain. At the light microscopic level, the Kv1.4 subunit showed a unique distribution pattern, being localized in specific neuronal populations of the rat brain. The neuronal regions expressing the highest levels of Kv1.4 protein included the cerebral cortex, the hippocampus, the posterolateral and posteromedial ventral thalamic nuclei, the dorsolateral and medial geniculate nuclei, the substantia nigra and the dorsal cochlear nucleus. The Kv1.4 subunit was also present in other neuronal populations, with different levels of Kv1.4 immunoreactivity. In all immunolabeled regions, the Kv1.4 subunit was mostly diffusely distributed and, to a lesser extent, it stained cell bodies and proximal dendrites. Furthermore, Kv1.4 immunoreactivity was also detected in nerve terminals and axonal terminal fields. At the electron microscopic level, Kv1.4 was located postsynaptically in dendritic spines and shafts at extrasynaptic sites, as well as presynaptically in axon and active zone of axon terminals, in the neocortex and hippocampus. The findings indicate that Kv1.4 channels are widely distributed in the rat brain and suggest that activation of this channel would have different modulatory effects on neuronal excitability.

Gamma-dendrotoxin blocks large conductance Ca2+-activated K+ channels in neuroblastoma cells

In N1E 115 neuroblastoma cells, gamma-dendrotoxin (DTX, 200 nM) blocked the outward K(+) current by 31.1 +/- 3.5% (n = 4) with approximately 500 nM Ca(2+) in the pipet solution, but had no effect on the outward K(+) current when internal Ca(2+) was reduced. Using a ramp protocol, iberiotoxin (IbTX, 100 nM) inhibited a component of the whole cell current, but in the presence of 200 nM gamma-DTX, no further inhibition by IbTX was observed. Two types of single channels were seen using outside-out patches when the pipette free Ca(2+) concentration was approximately 500 nM; a 63 pS and a 187 pS channel. The 63 pS channel was TEA-, IbTX- and gamma-DTX-insensitive, while the 187 pS channel was blocked by 1 mM TEA, 100 nM IbTX or 200 nM gamma-DTX. Both channels were activated by external application of ionomycin, when the pipet calcium concentration was reduced. gamma-DTX (200 nM) reduced the probability of openings of the 187 pS channel, with an IC(50) of 8.5 nM. In GH(3) cells gamma-DTX (200 nM) also blocked an IbTX-sensitive component of whole-cell K(+) currents. These results suggest that gamma-DTX blocks a large conductance Ca(2+) activated K(+) current in N1E 115 cells. This is the first indication that any of the dendrotoxins, which have classically been known to block voltage-gated (Kv) channels, can also block Ca(2+) activated K(+) channels.

APETx1, a new toxin from the sea anemone Anthopleura elegantissima, blocks voltage-gated human ether-a-go-go-related gene potassium channels

A new peptide, APETx1, which specifically inhibits human ether-a-go-go-related gene (HERG) channels, was purified from venom of the sea anemone Anthopleura elegantissima. APETx1 is a 42-amino acid peptide cross-linked by three disulfide bridges and shares 54% homology with BDS-I, another sea anemone K+ channel inhibitor. Although they differ in their specific targets, circular dichroism spectra and molecular modeling indicate that APETx1 and BDS-I have a common molecular scaffold and belong to the same structural family of K+ channel blocking peptides. APETx1 inhibits HERG currents in a heterologous system with an IC50 value of 34 nM by modifying the voltage dependence of the channel gating. Central injections in mice failed to induce any neurotoxic symptoms. APETx1, which has no sequence homologies with scorpion toxins acting on HERG, defines a new structural group of HERG gating modifiers isolated from a sea anemone.

Reduction of P/Q-type calcium channels in the postmortem cerebellum of paraneoplastic cerebellar degeneration with Lambert-Eaton myasthenic syndrome

The aim of this study was to clarify whether autoimmunity against P/Q-type voltage-gated calcium channels (VGCCs) in the cerebellum was associated with the pathogenesis of paraneoplastic cerebellar degeneration (PCD) with Lambert-Eaton myasthenic syndrome (LEMS). We used human autopsy cerebellar tissues from three PCD-LEMS patients and six other disease patients including one with LEMS as the controls. We compared cerebellar P/Q-type VGCC in these patients and controls for the amount and ratio of autoantibody-channel complex using an 125I-omega-conotoxin MVIIC-binding assay with Scatchard analysis, and their distribution using autoradiography. The quantity of cerebellar P/Q-type VGCC measured by Scatchard analysis were reduced in PCD-LEMS patients (63.0 +/- 7.0 fmol/mg, n = 3), compared with the controls (297.8 +/- 38.9 fmol/mg, n = 6). The ratio of autoantibody-VGCC complexes to total P/Q-type VGCCs measured by immunoprecipitation assay were increased in PCD-LEMS patients. We analysed cerebellar specimens by autoradiography using (125)I-omega-conotoxin MVIIC, which specifically binds to P/Q-type VGCCs. In PCD-LEMS cerebellum, the toxin binding sites of P/Q-type VGCCs were markedly reduced compared with controls, especially in the molecular layer, which is the richest area of P/Q-type VGCCs in the normal cerebellum. This suggests that P/Q-type VGCCs of the cerebellar molecular layer is the immunological target in developing PCD-LEMS.

Novel tarantula toxins for subtypes of voltage-dependent potassium channels in the Kv2 and Kv4 subfamilies

Three novel peptides with the ability to inhibit voltage-dependent potassium channels in the shab (Kv2) and shal (Kv4) subfamilies were identified from the venom of the African tarantulas Stromatopelma calceata (ScTx1) and Heteroscodra maculata (HmTx1, HmTx2). The three toxins are 34- to 38-amino acid peptides that belong to the structural family of inhibitor cystine knot spider peptides reticulated by three disulfide bridges. Electrophysiological recordings in COS cells show that these toxins act as gating modifier of voltage-dependent K+ channels. ScTx1 is the first high-affinity inhibitor of the Kv2.2 channel subtype (IC50, 21.4 nM) to be described. ScTx1 also inhibits the Kv2.1 channels, with an IC50 of 12.7 nM, and Kv2.1/Kv9.3 heteromultimers that have been proposed to be involved in O2 sensing in pulmonary artery myocytes. In addition, it is the most effective inhibitor of Kv4.2 channels described thus far, with an IC50 of 1.2 nM. HmTx toxins share sequence similarities with both the potassium channel blocker toxins (HmTx1) and the calcium channel blocker toxin omega-GsTx SIA (HmTx2). They inhibit potassium current associated with Kv2 subtypes in the 100 to 300 nM concentration range. HmTx2 seems to be a specific inhibitor of Kv2 channels, whereas HmTx1 also inhibits Kv4 channels, including Kv4.1, with the same potency. HmTx1 is the first described peptide effector of the Kv4.1 subtype. Those novel toxins are new tools for the investigation of the physiological role of the different potassium channel subunits in cellular physiology.

Characteristics of brain Kv1 channels tailored to mimic native counterparts by tandem linkage of alpha subunits: implications for K+ channelopathies

Most neuronal Kv1 channels contain Kv1.1, Kv1.2 alpha, and Kvbeta2.1 subunits, yet the influences of their stoichiometries on properties of the (alpha)(4)(beta)(4) variants remain undefined. cDNAs were engineered to contain 0, 1, 2, or 4 copies of Kv1.1 with the requisite number of Kv1.2 and co-expressed in mammalian cells with Kvbeta2.1 to achieve "native-like" hetero-oligomers. The monomeric (Kv1.1 or 1.2), dimeric (Kv1.1-1.2 or 1.2-1.2), and tetrameric (Kv1.1-(1.2)(3)) constructs produced proteins of M(r) approximately 62,000, 120,000, and 240,000, which assembled into (alpha)(4)(beta)(4) complexes. Each alpha cRNA yielded a distinct K(+) current in oocytes, with voltage dependence of activation being shifted negatively as the Kv1.1 content in tetramers was increased. Channels containing 1, 2, or 4 copies of Kv1.1 were blocked by dendrotoxin k (DTX)(k) with similarly high potencies, whereas Kv(1.2)(4) proved nonsusceptible. Accordingly, Kv1.2/beta2.1 expressed in baby hamster kidney cells failed to bind DTX(k); in contrast, oligomers containing only one Kv1.1 subunit in a tetramer exhibited high affinity, with additional copies causing modest increases. Thus, one Kv1.1 subunit largely confers high affinity for DTX(k), whereas channel electrophysiological properties are tailored by the content of Kv1.1 relative to Kv1.2. This notable advance could explain the diversity of symptoms of human episodic ataxia I, which is often accompanied by myokymia, due to mutated Kv1.1 being assembled in different combinations with wild-type and Kv1.2.

Regionally selective alterations in local cerebral glucose utilization evoked by charybdotoxin, a blocker of central voltage-activated K+-channels

The quantitative [14C]-2-deoxyglucose autoradiographic technique was employed to investigate the effect of charybdotoxin, a blocker of certain voltage-activated K+ channels, on functional activity, as reflected by changes in local rates of cerebral glucose utilization in rat brain. Intracerebroventricular administration of charybdotoxin, at doses below those producing seizure activity, produced a heterogeneous effect on glucose utilization throughout the brain. Out of the 75 brain regions investigated, 24 displayed alterations in glucose utilization. The majority of these changes were observed with the intermediate dose of charybdotoxin administered (12.5 pmol), with the lower (6.25 pmol) and higher (25 pmol) doses of charybdotoxin producing a much more restricted pattern of change in glucose utilization. In brain regions which displayed alterations in glucose at all doses of charybdotoxin administered, no dose dependency in terms of the magnitude of change was observed. The 21 brain regions which displayed altered functional activity after administration of 12.5 pmol charybdotoxin were predominantly limited to the hippocampus, limbic and motor structures. In particular, glucose utilization was altered within three pathways implicated within learning and memory processes, the septohippocampal pathway, Schaffer collaterals within the hippocampus and the Papez circuit. The nigrostriatal pathway also displayed altered local cerebral glucose utilization. These data indicate that charybdotoxin produces alterations in functional activity within selected pathways in the brain. Furthermore the results raise the possibility that manipulation of particular subtypes of Kv1 channels in the hippocampus and related structures may be a means of altering cognitive processes without causing global changes in neural activity throughout the brain.

Beta-bungarotoxin is a potent inducer of apoptosis in cultured rat neurons by receptor-mediated internalization

The neurotoxic phospholipase A(2), beta-bungarotoxin (beta-BuTx), is a component of the snake venom from the Taiwanese banded krait Bungarus multicinctus. beta-BuTx affects presynaptic nerve terminal function of the neuromuscular junction and induces widespread neuronal cell death throughout the mammalian and avian CNS. To analyse the initial events of beta-BuTx-mediated cell death, the toxin was applied to cultured rat hippocampal neurons where it induced neuronal cell death in a concentration-dependent manner (EC(50) approximately equal to 5 x 10(-13) M) within 24 h. Fluorescence labelled beta-BuTx was completely incorporated by neurons within < 10 min. Binding and uptake of beta-BuTx, as well as induction of cell death, were efficiently antagonized by preincubation with dendrotoxin I, a blocker of voltage-gated potassium channels devoid of phospholipase activity. Binding of beta-BuTx was selective for neurofilament-positive cells. As evident from intense annexin-V and TUNEL stainings, application of beta-BuTx induced apoptotic cell death exclusively in neurons, leaving astrocytes unaffected. No evidence was obtained for any contribution of either caspases or calpains to beta-BuTx-induced apoptosis, consistent with the inability of the inhibitors Z-Asp-DCB and calpeptin, respectively, to protect neurons from beta-BuTx-induced cell death. These observations indicate that induction of cell death by beta-BuTx comprises several successive phases: (i) binding to neuronal potassium channels is the initial event, followed by (ii) internalization and (iii) induction of apoptotic cell death via a caspase-independent pathway.

Protective effect of a new hypothalamic peptide against cobra venom and trauma-induced neuronal injury

A study of separate and combined actions of cobra venom (CV) and a new hypothalamic proline-rich polypeptide (PRP) isolated from magnocellular cells (NPV and NSO) on intoxication- and trauma-induced neuronal injury (during 3-4 weeks after hemisection with and without PRP treatment) was carried out. The registration of background and evoked impulse activity flow, changes in spinal cord (SC) inter- and motoneurons, responding to flexor, extensor, and mixed nerve stimulation in both acute and chronic experimental neurodegeneration was performed. The facilitating effect of PRP on the abovementioned neurons was revealed. High doses of CV that evoked the neurodegenerative changes demonstrated an inhibitory effect. In this case PRP treatment both before and after intoxication restored electrical neuronal activity to baseline level and higher. These results are evidence of protective action of PRP. The low doses of CV induced a facilitating effect. The combination of CV and PRP displayed an additive facilitating effect; in a number of cases the repeated administration of CV led to decrease of significant PRP effect till baseline level (for example, the inhibition after primary response prior to secondary late discharge). Greater liability of the secondary early and late long-time discharges of poststimulus responses, differently expressed in various neuron types of SC to chemical influences is of interest. PRP-induced inhibition of the paroxysmal activity related with CV action is also very interesting. Morpho-functional experiments with SC injury demonstrated the abolition of difference in the background and evoked SC neuronal activity below the section and on intact symmetric side after daily PRP administration for 3 weeks. PRP hindered the scar formation and activated neuroglia proliferation; it promoted white matter element growth, hampered the degeneration of cellular elements, and protected against tissue stress. Our results favor the combined use of PRP and CV in clinical practice for the treatment of neurodegeneration of toxic and traumatic origin, as well as specific neurodegenerative diseases such as Alzheimer's.

A new class of scorpion toxin binding sites related to an A-type K+ channel: pharmacological characterization and localization in rat brain

A new scorpion toxin (3751.8 Da) was isolated from the Buthus martensi venom, sequenced and chemically synthesized (sBmTX3). The A-type current of striatum neurons in culture completely disappeared when 1 microM sBmTX3 was applied (Kd=54 nM), whereas the sustained K+ current was unaffected. 125I-sBmTX3 specifically bound to rat brain synaptosomes (maximum binding=14 fmol x mg(-1) of protein, Kd=0.21 nM). A panel of toxins yet described as specific ligands for K+ channels were unable to compete with 125I-sBmTX3. A high density of 125I-sBmTX3 binding sites was found in the striatum, hippocampus, superior colliculus, and cerebellum in the adult rat brain.

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.

Inhibition of hKv2.1, a major human neuronal voltage-gated K+ channel, by meclofenamic acid

Using the standard patch clamp whole cell recording method, we assessed the pharmacological activity of four fenamate nonsteroidal anti-inflammatory drugs, meclofenamic acid, flufenamic acid, mefenamic acid and niflumic acid, on hKv2.1, a major human neuronal voltage-gated potassium channel stably expressed heterologously in Chinese hamster ovary cells. Meclofenamic acid inhibited hKv2.1 in a concentration-dependent manner whereas the other three fenamates had weaker or no effect on these channels at a concentration of 100 microM. The estimated IC50 of meclofenamic acid was 56.0 microM for hKv2.1 compared an IC50 of 155.9 microM for another human neuronal K channel (hKv1.1). Meclofenamic acid reached its maximum inhibition within 5 min of bath application and its effect was readily reversed upon wash. Kinetic analysis revealed that this drug did not alter the channel activation or deactivation time courses. Moreover, the effect of meclofenamic acid on hKv2.1 channels was not voltage-dependent. Indomethacin, another inhibitor of the cyclooxygenase that catalyses the synthesis of prostaglandin from arachidonic acid, had no effect on either hKv2.1 or hKv1.1. These results indicate that meclofenamic acid inhibits hKv2.1 more potently than hKv1.1 and it is likely that this compound acts directly on the channel proteins.

Identification of residues in dendrotoxin K responsible for its discrimination between neuronal K+ channels containing Kv1.1 and 1.2 alpha subunits

Dendrotoxin (DTX) homologues are powerful blockers of K+ channels that contain certain subfamily Kv1 (1.1-1.6) alpha- and beta-subunits, in (alpha)4(beta)4 stoichiometry. DTXk inhibits potently Kv1.1-containing channels only, whereas alphaDTX is less discriminating, but exhibits highest affinity for Kv1.2. Herein, the nature of interactions of DTXk with native K+ channels composed of Kv1.1 and 1.2 (plus other) subunits were examined, using 15 site-directed mutants in which amino acids were altered in the 310-helix, beta-turn, alpha-helix and random-coil regions. The mutants' antagonism of high-affinity [125I]DTXk binding to Kv1. 1-possessing channels in rat brain membranes and blockade of the Kv1. 1 current expressed in oocytes were quantified. Also, the levels of inhibition of [125I]alphaDTX binding to brain membranes by the DTXk mutants were used to measure their high- and low-affinity interactions, respectively, with neuronal Kv1.2-containing channels that possess Kv1.1 as a major or minor constituent. Displacement of toxin binding to either of these subtypes was not altered by single substitution with alanine of three basic residues in the random-coil region, or R52 or R53 in the alpha-helix; accordingly, representative mutants (K17A, R53A) blocked the Kv1.1 current with the same potency as the natural toxin. In contrast, competition of the binding of the radiolabelled alphaDTX or DTXk was dramatically reduced by alanine substitution of K26 or W25 in the beta-turn whereas changing nearby residues caused negligible alterations. Consistently, W25A and K26A exhibited diminished functional blockade of the Kv1.1 homo-oligomer. The 310-helical N-terminal region of DTXk was found to be responsible for recognition of Kv1.1 channels because mutation of K3A led to approximately 1246-fold reduction in the inhibitory potency for [125I]DTXk binding and a large decrease in its ability to block the Kv1.1 current; the effect of this substitution on the affinity of DTXk for Kv1.2-possessing oligomers was much less dramatic (approximately 16-fold).

alpha subunit compositions of Kv1.1-containing K+ channel subtypes fractionated from rat brain using dendrotoxins

K+ channels from the Kv1 subfamily contain four alpha-subunits and the combinations (from Kv1.1-1.6) determine susceptibility to dendrotoxin (DTX) homologues. The subunit composition of certain subtypes in rat brain was investigated using DTXk which only interacts with Kv1.1-containing channels and alphaDTX (and its closely related homologue DTXi) that binds preferentially to Kv1. 2-possessing homo- or hetero-oligomers. Covalent attachment of [125I]DTXk bound to channels in synaptic membranes unveiled subunits of Mr = 78 000 and 96 000. Immunoprecipitation of these solubilized and dissociated cross-linked proteins with IgG specific for each of the alpha-subunits identified Kv1.1, 1.2 and 1.4; this led to assemblies of Kv1.1/1.2 and 1.1/1.4 being established. Kv1. 2-enriched channels, purified from rat brain by chromatography on immobilized DTXi, contained Kv1.1, 1.2 and 1.6 confirming one of the above-noted pairs and indicating an additional Kv1.1-containing oligomer (Kv1.1/1.2/1.6); the notable lack of Kv1.4 excludes a Kv1. 1/1.2/1.4 combination. On the other hand, channels with Kv1.1 as a constituent, isolated using DTXk, possessed Kv1.4 in addition to those found in the DTXi-purified oligomers; this provides convergent support for the occurrence of the three combinations established above but adds a possible fourth (Kv1.1/1.4/1.6), though this was not confirmed. Moreover, sequential purification on DTXi and DTXk resins yielded channels containing only Kv1.1/1.2 but with an apparent predominance of Kv1.1, reaffirming the latter multimer.

Vesicle exocytosis stimulated by alpha-latrotoxin is mediated by latrophilin and requires both external and stored Ca2+

alpha-Latrotoxin (LTX) stimulates massive neurotransmitter release by two mechanisms: Ca2+-dependent and -independent. Our studies on norepinephrine secretion from nerve terminals now reveal the different molecular basis of these two actions. The Ca2+-dependent LTX-evoked vesicle exocytosis (abolished by botulinum neurotoxins) is 10-fold more sensitive to external Ca2+ than secretion triggered by depolarization or A23187; it does not, however, depend on the cation entry into terminals but requires intracellular Ca2+ and is blocked by drugs depleting Ca2+ stores and by inhibitors of phospholipase C (PLC). These data, together with binding studies, prove that latrophilin, which is linked to G proteins and inositol polyphosphate production, is the major functional LTX receptor. The Ca2+-independent LTX-stimulated release is not inhibited by botulinum neurotoxins or drugs interfering with Ca2+ metabolism and occurs via pores in the presynaptic membrane, large enough to allow efflux of neurotransmitters and other small molecules from the cytoplasm. Our results unite previously contradictory data about the toxin's effects and suggest that LTX-stimulated exocytosis depends upon the co-operative action of external and intracellular Ca2+ involving G proteins and PLC, whereas the Ca2+-independent release is largely non-vesicular.

Sea anemone peptides with a specific blocking activity against the fast inactivating potassium channel Kv3.4

Sea anemone venom is known to contain toxins that are active on voltage-sensitive Na+ channels, as well as on delayed rectifier K+ channels belonging to the Kv1 family. This report describes the properties of a new set of peptides from Anemonia sulcata that act as blockers of a specific member of the Kv3 potassium channel family. These toxins, blood depressing substance (BDS)-I and BDS-II, are 43 amino acids long and differ at only two positions. They share no sequence homologies with other K+ channel toxins from sea anemones, such as AsKS, AsKC, ShK, or BgK. In COS-transfected cells, the Kv3.4 current was inhibited in a reversible manner by BDS-I, with an IC50 value of 47 nM. This inhibition is specific because BDS-I failed to block other K+ channels in the Kv1, Kv2, Kv3, and Kv4 subfamilies. Inward rectifier K+ channels are also insensitive to BDS-I. BDS-I and BDS-II share the same binding site on brain synaptic membranes, with K0.5 values of 12 and 19 nM, respectively. We observed that BDS-I and BDS-II have some sequence homologies with other sea anemone Na+ channels toxins, such as AsI, AsII, and AxI. However, they had a weak effect on tetrodotoxin-sensitive Na+ channels in neuroblastoma cells and no effect on Na+ channels in cardiac and skeletal muscle cells. BDS-I and BDS-II are the first specific blockers identified so far for the rapidly inactivating Kv3.4 channel.

A phospholipase A2-related snake venom (from Crotalus durissus terrificus) stimulates neuroendocrine and immune functions: determination of different sites of action

Immune neuroendocrine interactions are vital for the individual's survival in certain physiopathological conditions, such as sepsis and tissular injury. It is known that several animal venoms, such as those from different snakes, are potent neurotoxic compounds and that their main component is a specific phospholipase A type 2 (PLA2). It has been described recently that the venom from Crotalus durissus terrificus [snake venom (SV), in the present study] possesses some cytotoxic effect in different in vitro and in vivo animal models. In the present study, we investigated whether SV and its main component, PLA2 (obtained from the same source), are able to stimulate both immune and neuroendocrine functions in mice, thus characterizing this type of neurotoxic shock. For this purpose, several in vivo and in vitro designs were used to further determine the sites of action of SV-PLA2 on the hypothalamo-pituitary-adrenal (HPA) axis function and on the release of the pathognomonic cytokine, tumor necrosis factor alpha (TNF alpha), of different types of inflammatory stress. Our results indicate that SV (25 microg/animal) and PLA2 (5 microg/animal), from the same origin, stimulate the HPA and immune axes when administered (i.p.) to adult mice; both preparations were able to enhance plasma glucose, ACTH, corticosterone (B), and TNF alpha plasma levels in a time-related fashion. SV was found to activate CRH- and arginine vasopressin-ergic functions in vivo and, in vitro, SV and PLA2 induced a concentration-related (0.05-10 microg/ml) effect on the release of both neuropeptides. SV also was effective in changing anterior pituitary ACTH and adrenal B contents, also in a time-dependent fashion. Direct effects of SV and PLA2 on anterior pituitary ACTH secretion also were found to function in a concentration-related fashion (0.001-1 microg/ml), and the direct corticotropin-releasing activity of PLA2 was additive to those of CRH and arginine vasopressin; the corticotropin-releasing activity of both SV and PLA2 were partially reversed by the specific PLA2 inhibitor, manoalide. On the other hand, neither preparation was able to directly modify spontaneous and ACTH-stimulated adrenal B output. The stimulatory effect of SV and PLA2 on in vivo TNF alpha release was confirmed by in vitro experiments on peripheral mononuclear cells; in fact, both PLA2 (0.001-1 microg/ml) and SV (0.1-10 microg/ml), as well as concavalin A (1-100 microg/ml), were able to stimulate TNF alpha output in the incubation medium. Our results clearly indicate that PLA2-dependent mechanisms are responsible for several symptoms of inflammatory stress induced during neurotoxemia. In fact, we found that this particular PLA2-related SV is able to stimulate both HPA axis and immune functions during the acute phase response of the inflammatory processes.

Site-directed mutagenesis of dendrotoxin K reveals amino acids critical for its interaction with neuronal K+ channels

Dendrotoxin K (DTXK) is a 57-residue protein from mamba venom that blocks certain non-inactivating, voltage-activated K+ currents in neurones. In order to pinpoint the residues responsible for its specificity, structure-activity relations of DTX(K) were investigated by mutagenesis. A previously cloned gene encoding this toxin [Smith et al. (1993) Biochemistry 32, 5692-5697] was used to make single mutations; after expression in Escherichia coli as fusion proteins and enzymatic cleavage of the conjugates isolated from the periplasmic space, nine toxins were purified. Structural analysis of the native DTXK and representative mutants by circular dichroism showed that no significant differences were detectable in their folded structures. The biological activity of the mutants, which contained alterations of positively charged and other amino acids, was determined from their abilities to compete for the binding of 125I-labeled DTX(K) to K+ channels in synaptic plasma membranes from rat cerebral cortex. Mutants with residues substituted in the alpha-helix near the C-terminus (R52A or R53A) yielded binding parameters similar to those of wild-type and native DTX(K). In the case of the beta-turn (residues 24-28), however, altering single amino acids reduced binding to the high-affinity site of K+ channels, with the rank order of decreases being K26A >> W25A > K24A = K28A. Also, substitutions made in the 3(10)-helix (residues 3-7), a region located close to the beta-turn, produced equivalent effects (K3A > K6A). Thus, it is deduced that residues in the distorted beta-turn and neighboring 3(10)-helix of DTX(K) are critical for its interaction with neuronal K+ channels.

Effects of excitatory amino acid antagonists on dendrotoxin-induced increases in neurotransmitter release and epileptiform bursting in rat hippocampus in vitro

Alpha-dendrotoxin (alpha-DTx), a snake venom toxin which blocks several types of fast-activating voltage-dependent potassium channels, induces limbic seizures and neuronal damage when injected into the brain. The mechanisms underlying these convulsant and neuropathological actions are not fully understood. We have studied the effects of alpha-DTx on neurotransmitter release and electrical activity in rat hippocampal brain slices and the role of excitatory amino acid receptors in mediating these actions of the toxin. alpha-DTx increased the basal release of acetylcholine, glutamate, aspartate, and GABA in a concentration-dependent manner and induced epileptiform bursting in the CA1 and CA3 regions of the slice. The increase in neurotransmitter release was evident during the first 4 min after toxin addition, whereas the bursting appeared after a concentration-dependent delay (20-40 min with 250 nM toxin). The N-methyl-D-aspartate (NMDA) receptor antagonists AP5 and MK-801 had no effect on the frequency or amplitude of dendrotoxin-induced epileptiform bursts, but the non-NMDA antagonists CNQX and DNQX abolished bursting in both CA1 and CA3 within 4-6 min. In contrast, the toxin-induced increases in neurotransmitter release were not blocked by DNQX. This study has demonstrated that, following exposure to alpha-DTx, there is a rapid increase in the release of neurotransmitters which precedes the onset of epileptiform bursting in the hippocampus. Since DNQX abolished the bursting but had no effect on the increase in neurotransmitter release, these results suggest that DNQX blocks alpha-DTx-induced epileptiform activity by antagonism of postsynaptic non-NMDA receptors.

IA-type K+ channel blockers promote survival of cortical neurons in culture: involvement of L-type Ca2+ channels

Dendrotoxin (DTX), a well characterized IA-type potassium channel blocker, directly added to the culture medium had no effect on survival of cultured cortical neurons at 6 or 14 days in vitro. On the contrary, neurons exposed to DTX remained in better condition than untreated ones. In an attempt to demonstrate the mechanisms by which DTX may affect neuronal survival we studied its effect in co-cultures of cortical neurons and astrocytes submitted to successive medium changes. After the second change of medium, at 9 days in vitro, the neuronal number in controls decreased by 43%, while in cultures receiving astrocyte-conditioned medium the cell loss was significantly reduced (15%, p < 0.01) with respect to control conditions. When DTX was added to the culture medium neuronal loss was also significantly prevented (25% for 1 microM DTX, p < 0.01) with respect to control conditions. 4-Aminopyridine (4-AP) and 21 mM K+ also preserved neurons. The L-type calcium channel antagonist nifedipine (5 microM) abolished the protective effect of DTX and 4-AP. These results show that K+ channel blockade induces protection against damage produced by repetitive medium change and that this effect is mediated by L-type Ca2+ channels.

Recent studies on dendrotoxins and potassium ion channels

1. Dendrotoxins are small proteins isolated from mamba (Dendroaspis) snake venoms. They block some subtypes of voltage-dependent potassium channels in neurons. 2. Dendrotoxins contain 57-60 amino acid residues crosslinked by three disulfide bridges. They are homologous to Kunitz-type serine protease inhibitors, such as aprotinin, although they have little or no antiprotease activity. 3. Dendrotoxins act mainly on neuronal K+ channels. Studies with cloned K+ channels indicate that alpha-dendrotoxin from green mamba Dendroaspis angusticeps blocks Kv1.1 and Kv1.2 channels in the nanomolar range. In native cells, dendrotoxin appears preferentially to block inactivating forms of K+ current. 4. Dendrotoxins can induce repetitive firing in neurons and facilitate transmitter release. On direct injection to the CNS, dendrotoxins can induce epileptiform activity. 5. Radiolabeled dendrotoxins are useful markers of subtypes of K+ channels in vivo, and structural analogs help to define the molecular recognition properties of different types of K+ channels.

Molecular properties of voltage-gated K+ channels

Subfamilies of voltage-activated K+ channels (Kv1-4) contribute to controlling neuron excitability and the underlying functional parameters. Genes encoding the multiple alpha subunits from each of these protein groups have been cloned, expressed and the resultant distinct K+ currents characterized. The predicted amino acid sequences showed that each alpha subunit contains six putative membrane-spanning alpha-helical segments (S1-6), with one (S4) being deemed responsible for the channels' voltage sensing. Additionally, there is an H5 region, of incompletely defined structure, that traverses the membrane and forms the ion pore; residues therein responsible for K+ selectively have been identified. Susceptibility of certain K+ currents produced by the Shaker-related subfamily (Kv1) to inhibition by alpha-dendrotoxin has allowed purification of authentic K+ channels from mammalian brain. These are large (M(r) approximately 400 kD), octomeric sialoglycoproteins composed of alpha and beta subunits in a stoichiometry of (alpha)4(beta)4, with subtypes being created by combinations of subunit isoforms. Subsequent cloning of the genes for beta 1, beta 2 and beta 3 subunits revealed novel sequences for these hydrophilic proteins that are postulated to be associated with the alpha subunits on the inner side of the membrane. Coexpression of beta 1 and Kv1.4 subunits demonstrated that this auxiliary beta protein accelerates the inactivation of the K+ current, a striking effect mediate by an N-terminal moiety. Models are presented that indicate the functional domains pinpointed in the channel proteins.

N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors mediate seizures and CA1 hippocampal damage induced by dendrotoxin-K in rats

The epileptogenic and neurodegenerative effects of dendrotoxin K, from Dendroaspis polylepis, a specific blocker of a non-inactivating, voltage-sensitive K+ channel, were studied after focal injection into one dorsal hippocampus in rats. Administration of 35 pmol dendrotoxin K elicited motor seizures and bilateral electrocortical discharges after a latent period (5.3 +/- 2.1 min), in all of the treated animals (n = 6). At 24 h, histological examination of brain (n = 5) coronal sections (10 microns; n = 6 per brain) detected bilateral damage to the hippocampal formation which extended 300 microns rostral and caudal to the injection tract. Quantitation of the damage revealed significant bilateral neuronal cell loss in the CA1 and CA4 pyramidal cell layer relative to the corresponding brain regions of rats (n = 3) injected with bovine serum albumin (105 pmol), which per se was ineffective in all respects. Dendrotoxin K (35 pmol) also caused a significant loss of CA3 pyramidal neurons and dentate gyrus granule cells ipsilateral to the site of toxin injection. In one out of six rats, a lower dose (3.5 pmol) of dendrotoxin K produced convulsive behaviour and electrocortical seizures but after a longer latency and these were accompanied by significant neuronal loss in the CA1, CA3 and CA4 pyramidal cell layer ipsilateral to the injected side. The lowest dose (0.35 pmol; n = 6 rats) of dendrotoxin K used failed to induce seizures and did not cause hippocampal damage (n = 6 rats). Systemic (i.p.) treatment with dizocilpine maleate (3 mg/kg) or LY 274614 (5 mg/kg i.p.), two N-methyl-D-aspartate receptor antagonists (given 15 min beforehand), prevented dendrotoxin K (35 pmol)-induced motor seizures and electrocortical epileptogenic discharges in 100% of the animals (n = 6 per group) treated. Similarly, these antagonists minimized the damage typically produced in the rat hippocampus, with no significant neuronal loss being observed. By contrast, NBQX (30 mg/kg, i.p. given 15 min previously), a non-N-methyl-D-aspartate antagonist, failed to prevent seizures normally evoked by dendrotoxin K (35 pmol; n = 6 rats); also, this treatment was unable to abolish CA1 pyramidal cell loss but minimized the loss in hippocampal sectors distant to the site of dendrotoxin K injection. However, complete protection against motor and electrocortical seizures and hippocampal damage was afforded by GYKI 52466 (10 mg/kg i.p.; n = 6 rats), a more effective non-N-methyl-D-aspartate receptor antagonist. These findings differ from the reported lack of protection by N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor antagonists to rats receiving intra-hippocampal injection of alpha-dendrotoxin; this difference may stem from the ability of alpha-dendrotoxin to block predominantly a slowly inactivating K+ current whereas dendrotoxin K inhibits a non inactivating variant. In conclusion, the present data on dendrotoxin K, together with the previously described pattern of neurotoxicity for alpha-dendrotoxin, show that these homologues act via different mechanisms and, thus, can be used effectively as complementary tools to study seizures and neuronal cell death.

Ultrastructural localization of a voltage-gated K+ channel alpha subunit (KV 1.2) in the rat cerebellum

A highly specific monoclonal antibody and pre-embedding immunocytochemistry were employed to examine the distribution of the K+ channel, alpha subunit K(V)1.2 in the rat cerebellum. At the light microscopic level, the heaviest immunoreactivity was seen in the basket cell pinceau at the base of Purkinje cells, with lighter staining of basket and Golgi cell bodies and a punctate pattern in the granule cell and molecular layers. Electron microscopy was performed to identify the ultrastructural location of K(V)1.2 alpha subunit in these labelled structures. This revealed that the labelling of the pinceau was confined to the preterminal axonal plexus, the area immediately around the Purkinje axon initial segment being relatively devoid of staining. Basket cell parent axons were not immunostained, but gave rise to heavily stained fine processes. Immunoreactivity was also seen in myelinated axons in the granule cell layer and in the medial cerebellar nucleus, the staining being most concentrated at the juxtaparanodal regions of the axons. An unusual pattern of staining was seen in some mossy fibre terminals, with staining restricted to fine protuberances of mossy fibre glomeruli. Structures contacted by these protuberances included adjoining glial processes. Immunostaining was absent from Purkinje cell bodies, dendrites, their axon initial segments and their terminals in the medial cerebellar nucleus. In this study, the alpha subunit K(V)1.2 was localized to a number of different cell types in the cerebellum. Each neuronal type displays a distinct subcellular distribution of the subunit.

Kalicludines and kaliseptine. Two different classes of sea anemone toxins for voltage sensitive K+ channels

New peptides have been isolated from the sea anemone Anemonia sulcata which inhibit competitively the binding of 125I-dendrotoxin I (a classical ligand for K+ channel) to rat brain membranes and behave as blockers of voltage-sensitive K+ channels. Sea anemone kalicludines are 58-59-amino acid peptides cross-linked with three disulfide bridges. They are structurally homologous both to dendrotoxins which are snake venom toxins and to the basic pancreatic trypsin inhibitor (Kunitz inhibitor) and have the unique property of expressing both the function of dendrotoxins in blocking voltage-sensitive K+ channels and the function of the Kunitz inhibitor in inhibiting trypsin. Kaliseptine is another structural class of peptide comprising 36 amino acids with no sequence homology with kalicludines or with dendrotoxins. In spite of this structural difference, it binds to the same receptor site as dendrotoxin and kalicludines and is as efficient as a K+ channel inhibitor as the most potent kalicludine.

Blockade by dendrotoxin homologues of voltage-dependent K+ currents in cultured sensory neurones from neonatal rats

1. Homologues of dendrotoxin (Dtx) were isolated from the crude venom of Green and Black Mamba snakes and examined for K+ channel blocking activity in neonatal rat dorsal root ganglion cells (DRGs) by whole-cell patch clamp recording. 2. Outward potassium current activated by depolarization was composed of two major components: a slowly inactivating current (SIC, tau decay approximately 50 ms, 200 ms and 2s), and a non-inactivating current (NIC, tau decay > 2 min). Tail current analysis revealed two time constants of deactivation of total outward current, 3-12 ms and 50-150 ms (at -80 mV) which corresponded to SIC and NIC, respectively. 3. All the homologues (alpha-, beta-, gamma- and delta-Dtx and toxins I and K) blocked outward current activated by depolarization in a dose-dependent manner. The most potent in blocking total outward current was delta-Dtx (EC50 of 0.5 +/- 0.2 nM), although there were no statistically significant differences in potency between any of the homologues. 4. Qualitative differences in the nature of the block were noted between homologues. In particular, the block by delta-Dtx was time-dependent, whereas that by alpha-Dtx was not. 5. alpha-Dtx was a much better blocker of SIC (EC50 = 1.0 +/- 0.4 nM) than was delta-Dtx (EC50 = 17.6 +/- 5.8 nM). Furthermore, delta-Dtx was selective for NIC (EC50 +/- 0.24 +/- 0.03 nM) over SIC and reduced the slow component of tail currents (NIC), preferentially. On the other hand, a-Dtx did not significantly distinguish between SIC and NIC although tail current analysis showed that a-Dtxpreferentially reduced the fast component of tail currents (SIC).6. The results confirm, using direct electrophysiological methods, that homologues of dendrotoxins from Mamba snake venom block K+ channels in rat sensory neurones. Furthermore, a-Dtx and 6-Dtx distinguish between sub-types of K+ channels in these cells and may thus be useful pharmacological tools in other neuronal K+ channel studies.

Antibodies specific for distinct Kv subunits unveil a heterooligomeric basis for subtypes of alpha-dendrotoxin-sensitive K+ channels in bovine brain

The authentic subunit compositions of neuronal K+ channels purified from bovine brain were analyzed using a monoclonal antibody (mAb 5), reactive exclusively with the Kv1.2 subunit of the latter and polyclonal antibodies specific for fusion proteins containing C-terminal regions of four mammalian Kv proteins. Western blotting of the K+ channels isolated from several brain regions, employing the selective blocker alpha-dendrotoxin (alpha-DTX), revealed the presence in each of four different Kvs. Variable amounts of Kv1.1 and 1.4 subunits were observed in the K+ channels purified from cerebellum, corpus striatum, hippocampus, cerebral cortex, and brain stem; on the other hand, contents of Kv1.6 and 1.2 subunits appeared uniform throughout. Each Kv-specific antibody precipitated a different proportion (anti-Kv1.2 > 1.1 >> 1.6 > 1.4) of the channels detectable with radioiodinated alpha-DTX in every brain region, consistent with a widespread distribution of these oligomeric subtypes. Such heterooligomeric combinations were further documented by the lack of additivity upon their precipitation with a mixture of antibodies to Kv1.1 and Kv1.2; moreover, cross-blotting of the multimers precipitated by mAb 5 showed that they contain all four Kv proteins. Collectively, these findings demonstrate that subtypes of alpha-DTX-susceptible K+ channels are prevalent throughout mammalian brain which are composed of different Kv proteins assembled in complexes, shown previously to also contain auxiliary beta-subunits [Parcej, D. N., Scott, V. E. S., & Dolly, J.O. (1992) Biochemistry 31, 11084-11088].

Prominent location of a K+ channel containing the alpha subunit Kv 1.2 in the basket cell nerve terminals of rat cerebellum

A panel of monoclonal antibodies specific for a family of voltage-dependent, fast-activating K+ channels, raised against alpha-dendrotoxin acceptors purified from bovine brain, were used to probe the distribution of these important proteins in rat cerebellum. All the antibodies reacted with their antigens in the folial white matter, the granular cell layer and the basket cell nerve termini within the Purkinje cell layer. However, a very intense staining pattern was exhibited by only one monoclonal that reacts exclusively with Kv 1.2 alpha subunit, the predominant isoform present in alpha-dendrotoxin sensitive K+ channels. Double-labelling procedures with neuronal and glial markers were used to verify this discrete antibody staining of the basket cell terminals that synapse with the base of Purkinje cell bodies in a readily recognizable and characteristic fashion. This is the first direct demonstration, using a monoclonal antibody, of a presynaptic location for a voltage-activated K+ channel; its discrete distribution in the basket cell pinceau suggests that it could control release of the inhibitory transmitter GABA and, thereby, influence excitability of Purkinje cells in the cerebellum.

Structural features important for the biological activity of the potassium channel blocking dendrotoxins

1. Dendrotoxins from mamba snake venoms are small proteins that block neuronal K+ channels. In order to investigate structural features associated with their biological activity, partially folded versions of dendrotoxins I and K from black mamba (Dendroaspis polylepis) were prepared by selectively reducing one or more of their three S-S bonds. 2. The modified toxins were tested for ability to compete with 125I-labelled native toxin I to high affinity binding sites on rat brain synaptosomal membranes and for the ability to increase acetylcholine release in a neuromuscular preparation. 3. Binding affinity increased progressively as the toxins folded to the native conformation and the most biologically active of the modified species were those in which only the disulphide bond between residues 14 and 38 was not formed. These intermediates had native-like conformations as determined by circular dichroism but still had about 5-10 times lower affinity than native toxins. 4. Addition of negatively charged groups to block the free sulthydryls at positions 14 and 38 caused a further, marked loss of activity. 5. The results are consistent with the existence of two important regions in the dendrotoxin molecules. The region containing two of the disulphide bonds (around Cys5-Cys55 and Cys30-Cys51) and much of the secondary structure is essential for the binding affinity of the toxins, while the region around Cys14 and Cys38, equivalent to part of the antiprotease site of the homologous protease inhibitor from bovine pancreas (BPTI), plays an important role in the potency of dendrotoxins.

Inhibition of phosphorylation of synapsin I and other synaptosomal proteins by beta-bungarotoxin, a phospholipase A2 neurotoxin

Some snake venom neurotoxins, such as beta-bungarotoxin (beta-BuTX), which possess relatively low phospholipase A2 (PLA2) activity, act presynaptically to alter acetylcholine (ACh) release both in the periphery and in the CNS. In investigating the mechanism of this action, we found that beta-BuTX (5 and 15 nM) inhibited phosphorylation, in both resting and depolarized synaptosomes, of a wide range of proteins, including synapsin I. Naja naja atra PLA2, which has higher PLA2 activity, also inhibited phosphorylation but was less potent than beta-BuTX. At 1 nM, beta-BuTX and N. n. atra PLA2 inhibited phosphorylation of synapsin I only in depolarized synaptosomes. Synaptosomal ATP levels were not affected by 5 or 15 nM beta-BuTX or by 5 nM N. n. atra PLA2. Limited proteolysis, using Staphylococcus aureus V-8 protease, indicated that beta-BuTX inhibited phosphorylation of synapsin I in both the head and the tail regions. The inhibition of phosphorylation was not antagonized by nordihydroguaiaretic acid or indomethacin, suggesting that arachidonic acid derivatives do not mediate this inhibition. Furthermore, inhibition of phosphorylation by beta-BuTX and N. n. atra PLA2 was not altered in the presence of the phosphatase inhibitor okadaic acid, suggesting that stimulation of phosphatase activity is not responsible for this inhibition. Inhibition of protein phosphorylation by PLA2 neurotoxins and enzymes may be associated with an inhibition of ACh release.

Production of seizures and brain damage in rats by alpha-dendrotoxin, a selective K+ channel blocker

alpha-Dendrotoxin (Dtx), a snake polypeptide, increases neuronal excitability by blocking certain fast-activating, voltage-dependent K+ channels. Thus, the behavioural, electrocortical (ECoG) and neuropathological effects of Dtx, injected into rat brain areas, were studied. A unilateral injection of 35 pmol of Dtx into the CA1 hippocampal area or the dendate gyrus (DG; upper blade) immediately produced motor and ECoG seizures, followed at 24 h by multi-focal brain damage and significant neuronal loss. Whilst brain damage was seen bilaterally, significant neuronal loss occurred only in regions (CA1, CA3, CA4 and DG) ipsilateral to the site of injection. A lower dose (3.5 pmol) of toxin elicited motor and ECoG seizures but failed to produce brain damage. Seizures were observed 50 min after injecting Dtx (35 pmol) into the amygdala, though significant neuronal loss was not evident. 4-Aminopyridine (100 nmol), given into the CA1 area elicited a similar motor and ECoG pattern to that of Dtx except no brain damage could be seen at 24 h. Systemic pretreatment with antagonists of N-methyl-D-aspartate receptors (MK-801 or CGP 37849) did not protect against the effects typically evoked by injecting Dtx into the CA1 area.

Subcellular segregation of two A-type K+ channel proteins in rat central neurons

In the mammalian nervous system, K+ channels regulate diverse aspects of neuronal function and are encoded by a large set of K+ channel genes. The roles of different K+ channel proteins could be dictated by their localization to specific subcellular domains. We report that two K+ channel polypeptides, Kv1.4 and Kv4.2, which form transient (A-type) K+ channels when expressed in Xenopus oocytes, are segregated in rat central neurons. Kv1.4 protein is targeted to axons and possibly terminals, while Kv4.2 is concentrated in dendrites and somata. This differential distribution implies distinct roles for these channel proteins in vivo. Their localizations suggest that Kv1.4 and Kv4.2 may regulate synaptic transmission via presynaptic, or postsynaptic mechanisms, respectively.

Differential expression of K+ channel mRNAs in the rat brain and down-regulation in the hippocampus following seizures

K+ channels are major determinants of membrane excitability. Differences in neuronal excitability within the nervous system may arise from differential expression of K+ channel genes, regulated spatially in a cell type-specific manner, or temporally in response to neuronal activity. We have compared the distribution of mRNAs of three K+ channel genes, Kv1.1, Kv1.2, and Kv4.2 in rat brain, and examined activity-dependent changes following treatment with the convulsant drug pentylenetetrazole. Both regional and cell type-specific differences of K+ channel gene expression were found. In addition, seizure activity caused a reduction of Kv1.2 and Kv4.2 mRNAs in the dentate granule cells of the hippocampus, raising the possibility that K+ channel gene regulation may play a role in long-term neuronal plasticity.

Protection against dendrotoxin-induced clonic seizures in mice by anticonvulsant drugs

Various anticonvulsant drugs were evaluated for their ability to protect against clonic seizures induced in mice by intraventricular injection of the K+ channel blocking peptide dendrotoxin (DTX). Phenytoin, the phenytoin-like anticonvulsant carbamazepine and the broad spectrum drug valproate were effective in this model, whereas the GABA-enhancers diazepam and tiagabine, the NMDA antagonists (+/-)-CPP and (+)-MK-801, the AMPA antagonist NBQX, the antiabsence drug ethosuximide and the Ca2+ channel antagonist nimodipine were inactive. In contrast to the lack of activity of other NMDA antagonists, phencyclidine and ADCI [(+/-)-aminocarbonyl-10,11-dihydro-5H-dibenzo [a,d]cyclohepten-5,10-imine] were potent antagonists of DTX-induced seizures.

A novel K+ channel with unique localizations in mammalian brain: molecular cloning and characterization

Using a cDNA library prepared from circumvallate papillae of rat tongue, we have identified, cloned, and sequenced a novel K+ channel, designated cdrk. The cdrk channel appears to be a member of the Shab subfamily, most closely resembling drk1. Electrophysiologic analysis of expressed cdrk channels reveals delayed rectifier properties similar to those of drk1 channels. Localizations of cdrk mRNA in rat brain and peripheral tissues, assessed by in situ hybridization and Northern blot analysis, differ from any other reported K+ channels. In the brain cdrk mRNA is most concentrated in granule cells of the olfactory bulb and cerebellum. In peripheral tissues, mRNAs for cdrk and drk1 are reciprocally localized, indicating that the K+ channel properties contributed by mammalian Shab homologs may be important in a variety of excitable tissues.

Comparison of the effects of four dendrotoxin peptides, 4-aminopyridine and tetraethylammonium on the electrically evoked [3H]noradrenaline release from rat hippocampus

We have examined the effects of four dendrotoxin (DaTX) peptides, alpha-, beta-, gamma- and delta-DaTX, separated from the venom of the green mamba (Dendroaspis angusticeps), on field stimulation-evoked [3H]noradrenaline (NA) release from rat hippocampus and compared their effects with those of two other inhibitors of K+ channels, 4-aminopyridine (4-AP) and tetraethylammonium (TEA). 4-AP (10-300 microM) and TEA (0.1-5 mM) facilitated the evoked [3H]NA release in a concentration-dependent manner. The evoked [3H]NA release was reduced to about half by alpha 2-adrenoceptor stimulation (UK 14,304; 100 nM) and this reduction was antagonized by 4-AP (10-100 microM), whereas TEA even at 5 mM was a poor inhibitor of alpha 2-effects. alpha-DaTX (10-200 nM) mimicked 4-AP in increasing the electrically evoked [3H]NA release and diminishing the inhibitory effects of UK 14,304 in a concentration-dependent manner. delta-DaTX did not itself alter the electrically evoked [3H]NA release, but at 200 nM, it reduced the effects of alpha 2-receptor stimulation. beta- and gamma-DaTX (up to 200 nM) had no significant effects. 4-AP, 3,4-diaminopyridine (3,4-DAP), TEA and the four dendrotoxins displaced the binding of [3H]p-aminoclonidine ([3H]PAC) from alpha 2-receptors. The IC50 values were 6.6 x 10(-4), 1.42 x 10(-3), 5.6 x 10(-2) for 4-AP, 3,4-DAP and TEA, respectively, and 3.19 x 10(-6) M for alpha-DaTX. Thus, their potency as inhibitors of alpha 2-receptors is apparently too low to account alone for the antagonism by K+ channel inhibitors of alpha 2-effects on NA release.(ABSTRACT TRUNCATED AT 250 WORDS)