Electrophysiology: a method to investigate the functional properties of ligand-gated channels

Automated Higher-Throughput Compound Screening on Ion Channel Targets Based on theXenopus laevisOocyte Expression System

As numerous diseases have been shown to be related to dysfunction of ion channels and neurotransmitter receptors and to affect regulatory pathways, ion channels have attracted increasing attention as a target class for drug discovery. The concomitant demand of the pharmaceutical industry for adequate electrophysiological methods to investigate drug effects on specific ion channels in secondary and safety screening has resulted in the development of electrophysiological instrumentation that allows automated monitoring of ion channel function with a higher throughput. Here we tested a fully automated screening system based on the Xenopus laevis oocyte expression system. We addressed the questions of data quality and reproducibility obtained by automated oocyte injection and two-electrode voltage-clamp (TEVC) recording using the Roboocyte (Multi Channel Systems GmbH, Reutlingen, Germany) technology compared to conventional oocyte recording. A gamma-aminobutyric acid (GABA)A-receptor subtype (alpha(1)beta(2)) was chosen as an example for a ligand-gated ion channel, and the slowly activating potassium current I(Ks) as a voltage-activated ion channel. Oocytes were injected with cDNA or cRNA via the Roboocyte injection stage. Ion channel currents were successfully recorded after 2-7 days in about 40% of the oocytes injected with GABA(A) receptor cDNA, and after 2-4 days in about 60% of the oocytes injected with KCNE1 cRNA. EC(50) values for the GABA(A) receptor and IC(50) values for blockers of I(Ks) were comparable to values obtained with conventional TEVC recording techniques. In conclusion, our results show that the Roboocyte is a valuable automated tool for oocyte injection and TEVC recording that can be used in drug screening and target validation to enhance the number of compounds and oocytes tested per day.

A vertical flow chamber for Xenopus oocyte electrophysiology and automated drug screening

Xenopus laevis oocytes are used extensively in the study of ion channel coupled receptors. Efficient use of oocytes for ion channel characterization requires a system that is inherently stable, reproducible, minimizes drug volumes, and maximizes oocyte longevity. We have constructed a vertical flow oocyte recording chamber to address the aforesaid issues, where the oocyte is placed in a funnel-shaped chamber and perfused from the bottom of the funnel. The vertical rather than horizontal flow of perfusate results in an unusually stable environment for oocyte recording. Two-electrode voltage clamp recordings from a single oocyte are acquired easily and routinely over several hours while maintaining stable baseline currents and reproducible response profiles. Chamber characteristics were tested using a serotonin ligand-gated ion channel receptor (5-HT3R). Data obtained from this system corresponds well with published data. To further test the stability and reliability of this perfusion chamber, we constructed an automated oocyte perfusion system utilizing a commonly available HPLC autosampler. We were able to obtain dose-response curves for various 5-HT3AR ligands using the automated perfusion system with minimal user intervention. Such a system can easily satisfy need for automated oocyte electrophysiology in academic settings, especially small to medium sized laboratories.

Membrane potential fluorescence: a rapid and highly sensitive assay for nicotinic receptor channel function

Seven cell lines expressing native and transfected nicotinic receptor subtypes were evaluated functionally by using fluorescent assays based on membrane potential and calcium dynamics with "no-wash" dye systems. Both assays provided the same rank orders of potency for (+/-)-epibatidine, 2S-(-)-nicotine, 7R,9S-(-)-cytisine, and 1,1-dimethyl-4-phenylpiperazinium in a cell line expressing rat alpha 3 beta 4 receptors. Nicotinic antagonists mecamylamine and dihydro-beta-erythroidine inhibited responses in both assays. Both agonist and antagonist activity were assessed within the same experiment. Agonists seemed more potent in the membrane potential assay than in the calcium assay, whereas the converse was true for antagonists. The membrane potential assay afforded robust responses in K-177 cells expressing human alpha 4 beta 2 receptors, in IMR-32 and SH-SY5Y cells expressing human ganglionic receptors, and in TE-671 cells expressing human neuromuscular receptors. These lines gave weak to modest calcium responses. Moreover, membrane potential responses were obtained in cell lines expressing rat alpha 4 beta 2 and alpha 4 beta 4 receptors, which were devoid of calcium responses. Thus, membrane potential serves as a sensitive measure of nicotinic activity, and the resulting depolarization may be as important as calcium in cell signaling.

Synthesis and electrophysiological studies of a novel epibatidine analogue

The new epibatidine analogue exo-2-(2-pyridyl)-7-azabicyclo[2.2.1]heptane (2PABH) was synthesised. Separation of enantiomers was performed on chiral HPLC chromatography in polar-organic phase mode at 0 degree C. Enantiomeric purity was greater than 99.8%ee for the (-)- and 90.5%ee for the (+)-enantiomer respectively. Optical rotation was determined to be [alpha]23D = +/- 13 degrees. Electrophysiological studies of 2PABH were carried out on alpha 4 beta 2, alpha 3 beta 4 and alpha 7 nAChR subtypes cloned from rat and reconstituted in Xenopus oocytes. Both enantiomers could not significantly activate the heteromeric subtypes. The homomeric alpha 7 nAChR displays a high sensitivity only towards (-)-2PABH. The EC50 for (-)-2PABH and ACh were determined (32.5 +/- 9.5 microM, 137.3 +/- 16.5 microM). (-)-2PABH was shown to be a partial agonist (80% of ACh). Thus the efficacy of 2PABH differs markedly from that of epibatidine. The intramolecular N-N-distance and the spatial pyridine nitrogen orientation play a central role in nAChR recognition.

Allosteric modulation of neuronal nicotinic acetylcholine receptors

The structure-function relationship of the neuronal nicotinic acetylcholine receptor is examined in the light of the allosteric concepts. Effects of site-directed mutagenesis as well as those caused by allosteric effector of the physiological and pharmacological receptor properties are discussed.

Open-channel blockers at the human alpha4beta2 neuronal nicotinic acetylcholine receptor

To extend our knowledge of the pharmacological profile of human alpha4beta2 neuronal nicotinic receptors, we investigated the action of hexamethonium on the major brain human nicotinic acetylcholine receptor (nAChR) stably expressed in human embryonic kidney 293 cells. This compound displays all of the characteristics of an open-channel blocker at the human alpha4beta2 nAChR: a voltage-dependent inhibition (more pronounced at hyperpolarized potentials), absence of competition, and use dependence. Moreover, we observed that classic N-methyl-D-aspartate open-channel blockers amantadine, 3,5-dimethyl-1-adamantanamine (memantine), and dizocilpine [(+)-MK-801] and the calcium channel antagonist 8-(diethylamino)octyl-3,4,5-trimethoxybenzoate are powerful inhibitors of the human alpha4beta2 nAChR. Dose-inhibition curves yield, at -100 mV, IC50 values in the micromolar range for all of compounds and Hill coefficients below unity. Whole-cell current-voltage relationships display a strong rectification profile at hyperpolarized potentials, and current blockades are fitted adequately by a mathematical model that describes the mechanism of an ion channel block. We conclude that these molecules are powerful human alpha4beta2 open-channel blockers ranking in the following order of potency: amantadine > memantine = hexamethonium > 8-(diethylamino)octyl-3,4,5-trimethoxybenzoate approximately (+)-MK-801.

Ivermectin: a positive allosteric effector of the alpha7 neuronal nicotinic acetylcholine receptor

We report that preapplication of ivermectin, in the micromolar range, strongly enhances the subsequent acetylcholine-evoked current of the neuronal chick or human alpha7 nicotinic acetylcholine receptors reconstituted in Xenopus laevis oocytes and K-28 cells. This potentiation does not result from nonspecific Cl- currents. The concomitant increase in apparent affinity and cooperativity of the dose-response curve suggest that ivermectin acts as a positive allosteric effector. This interpretation is supported by the observation of an increase in efficiency of a partial agonist associated with the potentiation and by the differential effect of ivermectin on mutants within the M2 channel domain. Ivermectin effects reveal a novel allosteric site for pharmacological agents on neuronal alpha7 nicotinic acetylcholine receptors.

Modulation of cholinergic nucleus basalis neurons by acetylcholine and N-methyl-D-aspartate

Known to exert an important modulatory influence on the cerebral cortex, the cholinergic neurons of the basal forebrain are modulated in turn by neurotransmitters which may include acetylcholine released from processes of brainstem or forebrain neurons. In the present study, we examined the effect of carbachol, a non-specific cholinergic agonist, either alone or in the presence of N-methyl-D-aspartate upon electrophysiologically identified cholinergic basalis neurons in guinea-pig basal forebrain slices. Carbachol produced a direct postsynaptic hyperpolarization, accompanied by a decrease in membrane resistance. Muscarine could mimic this hyperpolarizing effect, whereas nicotine produced a direct postsynaptic membrane depolarization. The interaction of carbachol with N-methyl-D-aspartate was subsequently tested since, in a prior study, N-methyl-D-aspartate was shown to induce rhythmic bursting in cholinergic cells when they were hyperpolarized by continuous injection of outward current. Applied simultaneously with N-methyl-D-aspartate in the absence of current injection, carbachol was also found to promote rhythmic bursting in half of the cells tested. Since the bursts under these conditions were markedly longer in duration than those observed in the presence of N-methyl-D-aspartate alone, it was hypothesized that carbachol might have another action, in addition to the membrane hyperpolarization. Using dissociated cells, it was found that brief applications of carbachol could indeed diminish the slow afterhyperpolarizations that follow single spikes, short bursts or long trains of action potentials in cholinergic basalis neurons. These results indicate that, through its dual ability to hyperpolarize cholinergic neurons and to reduce their afterhyperpolarizations, acetylcholine can promote the occurrence of rhythmic bursting in the presence of N-methyl-D-aspartate. Accordingly, whether derived from brainstem or local sources, acetylcholine may facilitate rhythmic discharge in cholinergic basalis neurons which could in turn impose a rhythmic modulation upon cortical activity during particular states across the sleep-waking cycle.

Distinct contributions of high- and low-voltage-activated calcium currents to afterhyperpolarizations in cholinergic nucleus basalis neurons of the guinea pig

The contributions made by low- (LVA) and high-voltage-activated (HVA) calcium currents to afterhyperpolarizations (AHPs) of nucleus basalis (NB) cholinergic neurons were investigated in dissociated cells. Neurons with somata >25 microM were studied because 80% of them stained positively for choline acetyltransferase and had electrophysiological characteristics identical to those of cholinergic NB neurons previously recorded in basal forebrain slices. Calcium currents of cholinergic NB neurons first were dissected pharmacologically into an amiloride-sensitive LVA and at least five subtypes of HVA currents. Approximately 17% of the total HVA current was sensitive to nifedipine (3 microM), 35% to omega-conotoxin-GVIA (200-400 nM), 10% to omega-Agatoxin-IVA (100 nM), and 20% to omega-Agatoxin-IVA (300-500 nM), suggesting the presence of L-, N-, P-, and Q-type channels, respectively. A remaining current (R-type) resistant to these antagonists was blocked by cadmium (100-200 microM). We then assessed pharmacologically the role that LVA and HVA currents had in activating the apamin-insensitive AHP elicited by a long train of action potentials (sAHP) and the AHP evoked either by a short burst of action potentials or by a single action potential (mAHP) that is known to be apamin-sensitive. During sAHPs, approximately 60% of the hyperpolarization was activated by calcium flowing through N-type channels and approximately 20% through P-type channels, whereas T-, L-, and Q-type channels were not involved significantly. In contrast, during mAHPs, N- and T-type channels played key roles (approximately 60 and 30%, respectively), whereas L-, P-, and Q-type channels were not implicated significantly. It is concluded that in cholinergic NB neurons various subtypes of calcium channels can differentially activate the apamin-sensitive mAHP and the apamin-insensitive sAHP.

Facilitation of N-type calcium current is dependent on the frequency of action potential-like depolarizations in dissociated cholinergic basal forebrain neurons of the guinea pig

Voltage-dependent inhibition of high voltage-activated (HVA) calcium currents by G-proteins can be transiently relieved (facilitated) by strong depolarizing prepulses. However, with respect to the physiological significance of facilitation, it remains to be established if it can be induced by action potentials (AP) in central neurons. With the use of whole-cell recordings of dissociated cholinergic basal forebrain neurons of the guinea pig, it is shown that the GTPgammaS-inhibited HVA currents that occur through N-ethylmaleimide (NEM)-sensitive Gi-Go subtypes of G-proteins can be facilitated. Furthermore, although different types of HVA channels are present in these neurons, facilitation occurred mostly through disinhibition of the N-type current. On the basis of data indicating that the recovery from facilitation was relatively slow, we tested if more physiological stimuli that crudely mimicked APs (2 msec long depolarizations to 40 mV from a holding of -50 mV) potentially could induce facilitation of HVA currents inhibited by GTPgammaS and cholinergic agonists. Indeed, evidence is provided that the extent of facilitation is dependent on both the number and frequency of AP-like depolarizations. These results suggest that firing rates and patterns of discharge of neurons could influence their responsiveness to transmitters acting on N-type HVA calcium channels.

Human alpha4beta2 neuronal nicotinic acetylcholine receptor in HEK 293 cells: A patch-clamp study

The cloning and expression of genes encoding for the human neuronal nicotinic acetylcholine receptors (nAChRs) has opened new possibilities for investigating their physiological and pharmacological properties. Cells (HEK 293) stably transfected with two of the major brain subunits, alpha4 and beta2, were characterized electrophysiologically using the patch-clamp technique. Fast application of the natural ligand ACh can evoke currents up to 3500 pA, with an apparent affinity (EC50) of 3 microM and a Hill coefficient of 1.2. The rank order of potency of four nAChR ligands to activate human alpha4beta2 receptors is (-)-nicotine > ACh > (-)-cytisine > ABT-418. At saturating concentrations, the efficacy of these ligands is ABT-418 >> (-)-nicotine > ACh >> (-)-cytisine > GTS-21 (previously named DMXB). Coapplication of 1 microM ACh with known nAChR inhibitors such as dihydro-beta-erythroidine and methyllycaconitine reversibly reduces the current evoked by the agonist with respective IC50 values of 80 nM and 1.5 microM. The current-voltage relationship of human alpha4beta2 displays a strong rectification at positive potentials. Experiments of ionic substitutions suggest that human alpha4beta2 nAChRs are permeable to sodium and potassium ions. In the "outside-out" configuration, ACh evokes unitary currents (main conductance 46 pS) characterized by a very fast rundown. Potentiation of the ACh-evoked currents is observed when the extracellular calcium concentration is increased from 0.2 to 2 mM. In contrast, however, a reduction of the evoked currents is observed when calcium concentration is elevated above 2 mM.