Detection and photoaffinity labeling of the Ca2+-activated K+ channel-associated apamin receptor in cultured astrocytes from rat brain

SK Channels Modulation Accelerates Equilibrium Recovery in Unilateral Vestibular Neurectomized Rats

We have previously reported in a feline model of acute peripheral vestibulopathy (APV) that the sudden, unilateral, and irreversible loss of vestibular inputs induces selective overexpression of small conductance calcium-activated potassium (SK) channels in the brain stem vestibular nuclei. Pharmacological blockade of these ion channels by the selective antagonist apamin significantly alleviated the evoked vestibular syndrome and accelerated vestibular compensation. In this follow-up study, we aimed at testing, using a behavioral approach, whether the antivertigo (AV) effect resulting from the antagonization of SK channels was species-dependent or whether it could be reproduced in a rodent APV model, whether other SK channel antagonists reproduced similar functional effects on the vestibular syndrome expression, and whether administration of SK agonist could also alter the vestibular syndrome. We also compared the AV effects of apamin and acetyl-DL-leucine, a reference AV compound used in human clinic. We demonstrate that the AV effect of apamin is also found in a rodent model of APV. Other SK antagonists also produce a trend of AV effect when administrated during the acute phase of the vertigo syndrome. Conversely, the vertigo syndrome is worsened upon administration of SK channel agonist. It is noteworthy that the AV effect of apamin is superior to that of acetyl-DL-leucine. Taken together, these data reinforce SK channels as a pharmacological target for modulating the manifestation of the vertigo syndrome during APV.

Electrophysiological and structural aspects in the frontal cortex after the bee (Apis mellifera) venom experimental treatment

The aim of this study is to evaluate the bioelectrical and structural-functional changes in frontal cortex after the bee venom (BV) experimental treatments simulating both an acute envenomation and a subchronic BV therapy. Wistar rats were subcutaneously injected once with three different BV doses: 700 μg/kg (T(1) group), 2100 μg/kg (T(3) group), and 62 mg/kg (sublethal dose-in T(SL) group), and repeated for 30 days with the lowest dose (700 μg/kg-in T(S) group). BV effects were assessed by electrophysiological, histological, histochemical, and ultrastructural methods. Single BV doses produced discharges of negative and biphasic sharp waves, and epileptiform spike-wave complexes. The increasing frequency of these elements suggested a dose-dependent neuronal hyperexcitation or irritation. As compared to the lower doses, the sublethal dose was responsible for a pronounced toxic effect, confirmed by ultrastructural data in both neurons and glial cells that underwent extensive, irreversible changes, triggering the cellular death. Subchronic BV treatment in T(S) group resulted in a slower frequency and increased amplitude of cortical activity suggesting neuronal loss. However, neurons were still stimulated by the last BV dose. Structural-functional data showed a reduced cellular density in frontal cortex of animals in this group, while the remaining neurons displayed both specific (stimulation of neuronal activity) and unspecific modifications (moderate alterations to necrotic phenomena). Molecular mechanisms involved in BV interactions with the nervous tissue are also discussed. We consider all these data very important for clinicians who manage patients with multiple bee stings, or who intend to set an appropriate BV therapy.

Differential alterations of ion channel binding sites in temporal and occipital regions of the cerebral cortex in Alzheimer's disease

Three ion channel binding sites were examined by means of quantitative ligand binding autoradiography in temporal and occipital cortex from 9 patients with neuropathologically confirmed Alzheimer's disease (AD) and 7 matched control subjects. The following ligands were used: 125I-apamin to label a population of Ca(2+)-sensitive K+ channels; [3H]PN200-110 to label L-type voltage-sensitive Ca2+ channels and [3H]glibenclamide to label ATP-sensitive K+ channels. Ion channel binding sites were compared to: choline acetyltransferase (ChAT) activity and plaque densities measured in the same tissue. In the temporal cortex in AD 125I-apamin binding was increased compared to controls (e.g. superficial layers: control = 0.71 +/- 0.07; AD = 1.02 +/- 0.07, mean +/- S.E.M. pmol/g tissue). In contrast, in adjacent sections [3H]glibenclamide binding was reduced in AD compared to controls (e.g. superficial layers: control = 25.3 +/- 1.7; AD = 17.9 +/- 1.4 pmol/g tissue). [3H]PN200-110 binding in temporal cortex was not altered in AD compared to controls. In the occipital cortex 125I-apamin binding was increased in AD while both [3H]glibenclamide and [3H]PN-200-110 binding sites in this cortical area were not different from controls. Plaque density (per mm2) was higher in temporal (e.g. layers I-III, 43 +/- 6) than in occipital cortex (layers I-III, 27 +/- 4) in the AD patients while ChAT activity was reduced by 40% in temporal cortex and by 50% in occipital cortex compared to controls. The results suggest that the three ion channel binding sites are located on structural elements in the brain which are differentially affected by the pathophysiology of AD.

Selective reduction of [125I]apamin binding sites in Alzheimer hippocampus: a quantitative autoradiographic study

[125I]Apamin binding sites were examined using quantitative autoradiography in the hippocampus of 9 patients with Alzheimer's disease and 8 age-matched controls. Within the hippocampal formation from control subjects, [125I]apamin binding sites were highly concentrated in the subiculum and CA1. In Alzheimer's disease there was a marked and discrete loss of [125I]apamin binding sites in the subiculum (control = 1.10 +/- 0.10 pmol/g; Alzheimer = 0.71 +/- 0.09 pmol/g) and CA1 (control = 1.41 +/- 0.09 pmol/g; Alzheimer = 0.85 +/- 0.11 pmol/g; values are mean +/- S.E.M.). This reduction of [125I]apamin binding sites in the subiculum correlated with cell density but not neuritic plaque density. These results indicate that an anatomically discrete loss of Ca(2+)-dependent K+ channels within the hippocampal formation occurs in Alzheimer's disease.

Use of toxins to study potassium channels

Potassium channels comprise groups of diverse proteins which can be distinguished according to each member's biophysical properties. Some types of K+ channels are blocked with high affinity by specific peptidyl toxins. Three toxins, charybdotoxin, iberiotoxin, and noxiustoxin, which display a high degree of homology in their primary amino acid sequences, have been purified to homogeneity from scorpion venom. While charybdotoxin and noxiustoxin are known to inhibit more than one class of channel (i.e., several Ca(2+)-activated and voltage-dependent K+ channels), iberiotoxin appears to be a selective blocker of the high-conductance, Ca(2+)-activated K+ channel that is present in muscle and neuroendocrine tissue. A distinct class of small-conductance Ca(2+)-activated K+ channel is blocked by two other toxins, apamin and leiurotoxin-1, that share no sequence homology with each other. A family of homologous toxins, the dendrotoxins, have been purified from venom of various related species of snakes. These toxins inhibit several inactivating voltage-dependent K+ channels. Although molecular biology approaches have been employed to identify and characterize several species of voltage-gated K+ channels, toxins directed against a particular channel can still be useful in defining the physiological role of that channel in a particular tissue. In addition, for those K+ channels which are not yet successfully probed by molecular biology techniques, toxins can be used as biochemical tools with which to purify the target protein of interest.

Polypeptide components of the apamin receptor associated with a calcium activated potassium channel

Photoaffinity labeling of rat brain membranes with [125I]ANPAA-apamin incorporated radioactivity into polypeptides of 86 and 59 kDa and occasionally a more weakly labeled component of 45 kDa. These polypeptides were immunoprecipitated with anti-apamin antibodies and treated with glycosidases. Neither the 86 nor the 59 kDa polypeptide appeared to be N-glycosylated. Partial proteolytic mapping of affinity labeled polypeptides with chymotrypsin or V8 protease generated an identical pattern. These results suggest that the 59 and 45 kDa components are not additional subunits of an oligomeric protein but result from cleavage of the 86 kDa polypeptide.

Electrophysiological properties of reactive glial cells in the kainate-lesioned hippocampal slice

Kainate-lesioned hippocampal slices provide an excellent system for examining the electrophysiological properties of non-cultured glial cells without interfering signals from surrounding neurons. Intracellular recordings in the intensely gliotic CA3 region indicated that the resting membrane potential and input resistance of these reactive glial cells were similar to those reported for non-reactive astrocytes. Dye-coupling typical of astrocytes was also demonstrated amongst these cells, and was considerably reduced by cytoplasmic acidification. Occasionally these cells demonstrated spontaneous, rhythmic oscillations of membrane potential associated with large changes in whole cell input resistance. The action potentials reported in cultured astrocytes were not observed in reactive glial cells, even under conditions that maximize the observation of Ca2(+)-dependent responses. This suggests that reactive glial cells in this preparation have either no voltage-activated Ca2+ channels or a very low density of such channels. These cells also lack a bicarbonate conductance, but they do appear to have an apamin-sensitive conductance, possibly a Ca2(+)-activated gK.

Potassium channel toxins

Many venom toxins interfere with ion channel function. Toxins, as specific, high affinity ligands, have played an important part in purifying and characterizing many ion channel proteins. Our knowledge of potassium ion channel structure is meager because until recently, no specific potassium channel toxins were known, or identified as such. This review summarizes the sudden explosion of research on potassium channel toxins that has occurred in recent years. Toxins are discussed in terms of their structure, physiological and pharmacological properties, and the characterization of toxin binding sites on different subtypes of potassium ion channels.

Volatile anesthetics inhibit the ion flux through Ca2+-activated K+ channels of rat glioma C6 cells

Ca2+-activated K+ channels in rat glioma C6 cells were investigated using monolayers of these cells in petri dishes. The ion flux through the channels was studied with 86Rb+ after addition of a Ca2+-ionophore to the incubation medium. Both the influx and efflux of 86Rb+ through these Ca2+-activated K+ channels were inhibited by the general anesthetic halothane (at clinical concentrations). Other volatile anesthetics such as isoflurane, enflurane and methoxyflurane also inhibited the Ca2+-activated K+ channels at clinical concentrations. Inhibition of these channels by general anesthetics could have profound effects on signal transmission in the brain.

Presence of a charybdotoxin sensitive Ca2+-activated K+ channel in rat glioma C6 cells

A study was made of the 86Rb+ influx and efflux through Ca2+-activated K+ channels of intact rat glioma C6 cells after addition of a Ca2+ ionophore to the incubation medium. Half-maximal activation of the channels was obtained at a cytoplasmic Ca2+ concentration of approximately 400 nM. The 86Rb+ ion flux through the Ca2+-activated K+ channels was insensitive to apamin, but was inhibited by low concentrations of charybdotoxin (IC50 = 1.6 nM). This is the first evidence for the presence of charybdotoxin-sensitive Ca2+-activated K+ channels in glial cells.

An emerging pharmacology of peptide toxins targeted against potassium channels

Voltage-dependent ion channels are a difficult class of proteins to approach biochemically. Many such channels are present at low density in relevant tissues and exist as multiple subtypes that can be distinguished electrophysiologically. In particular, K channels appear to be a diverse family of proteins characterized by many different conductance properties, gating behaviors and regulatory phenomena. Fortunately, specific peptide toxins for K channels are present in the venoms of insects, scorpions, snakes and possibly other species. The available sequences of these peptides define several different families of toxins. Electrophysiological and radioligand binding studies suggest that these toxins can be used to distinguish subclasses of K channels that share similar toxin binding sites. The growing databank of sequence homologies for both toxins and channels is, in essence, a codebook for identifying common elements of structure and function. The continuing development of toxins as biochemical probes should help to uncover the molecular basis and physiological significance of K-channel diversity.

Photoaffinity labeling of the K+-channel-associated apamin-binding molecule in smooth muscle, liver and heart membranes

High-affinity binding sites for mono[125I]iodoapamin were detected in membranes (Kd = 59 pM, Bmax = 24 fmol/mg protein) and cultured cells (Kd = 69 pM, Bmax = 2.8 fmol/mg protein) from rat heart and in membranes from guinea-pig ileum (Kd = 67 pM, Bmax 42 fmol/mg protein) and liver (Kd = 15 pM, Bmax = 43 fmol/mg protein). Binding was stimulated by K+ ions (K0.5 = 0.3-0.5 mM). Covalent labeling with arylazide [125I]iodoapamin derivatives showed that smooth muscle, liver and heart binding molecules are associated with a 85-87-kDa polypeptide. A second strongly labeled 57-kDa component was identified in liver membranes only.