Different types of blockade of crustacean acetylcholine-induced currents

What We Have Gained from Ibogaine: α3β4 Nicotinic Acetylcholine Receptor Inhibitors as Treatments for Substance Use Disorders

For decades, ibogaine─the main psychoactive alkaloid found in Tabernanthe iboga─has been investigated as a possible treatment for substance use disorders (SUDs) due to its purported ability to interrupt the addictive properties of multiple drugs of abuse. Of the numerous pharmacological actions of ibogaine and its derivatives, the inhibition of α3β4 nicotinic acetylcholine receptors (nAChRs), represents a probable mechanism of action for their apparent anti-addictive activity. In this Perspective, we examine several classes of compounds that have been discovered and developed to target α3β4 nAChRs. Specifically, by focusing on compounds that have proven efficacious in pre-clinical models of drug abuse and have been evaluated clinically, we highlight the promising potential of the α3β4 nAChRs as viable targets to treat a wide array of SUDs. Additionally, we discuss the challenges faced by the existing classes of α3β4 nAChR ligands that must be overcome to develop them into therapeutic treatments.

Potential therapeutic uses of mecamylamine and its stereoisomers

Mecamylamine (3-methylaminoisocamphane hydrochloride) is a nicotinic parasympathetic ganglionic blocker, originally utilized as a therapeutic agent to treat hypertension. Mecamylamine administration produces several deleterious side effects at therapeutically relevant doses. As such, mecamylamine's use as an antihypertensive agent was phased out, except in severe hypertension. Mecamylamine easily traverses the blood-brain barrier to reach the central nervous system (CNS), where it acts as a nicotinic acetylcholine receptor (nAChR) antagonist, inhibiting all known nAChR subtypes. Since nAChRs play a major role in numerous physiological and pathological processes, it is not surprising that mecamylamine has been evaluated for its potential therapeutic effects in a wide variety of CNS disorders, including addiction. Importantly, mecamylamine produces its therapeutic effects on the CNS at doses 3-fold lower than those used to treat hypertension, which diminishes the probability of peripheral side effects. This review focuses on the pharmacological properties of mecamylamine, the differential effects of its stereoisomers, S(+)- and R(-)-mecamylamine, and the potential for effectiveness in treating CNS disorders, including nicotine and alcohol addiction, mood disorders, cognitive impairment and attention deficit hyperactivity disorder.

Mechanisms of the inhibition of endplate acetylcholine receptors by antiseptic chlorhexidine (experiments and models)

Mechanisms of the inhibition of evoked multiquantal endplate currents (EPC) by chlorhexidine (CHX) were studied in electrophysiological experiments and by mathematical modeling to discriminate between possible channel, receptor, and non-receptor effects of this common antiseptic drug. Experiments were carried out on the isolated neuromuscular preparation of the cut m. sartorius of the frog Rana ridibunda. The nerve-stimulation-evoked endplate currents were measured by standard double microelectrode technique. For the mathematical simulation, a method based on the solution of a system of ordinary differential equations was used. CHX in milimolar concentrations suppressed the amplitude and shortened the evoked EPC. Recovery of the EPC amplitude was very slow, and EPC shortening persisted during 30-40 min washout of the drug. There is no indication that CHX competes for acetylcholine or carbachol binding site(s). A comparison of the experimental data with mathematical simulation made it possible to construct a reliable kinetic scheme, which describes the action of CHX. CHX induces a combined slow blockade of the open ionic channel and long-lasting allosteric inhibition of the nicotinic acetylcholine receptor. The very slow washout of the drug in terms of EPC amplitude and virtually no recovery of the shortened EPC time course might substantiate certain caution to avoid unintentional high-dose application during its antibacterial application.

Modulation of neuronal nicotinic receptor function by the neuropeptides CGRP and substance P on autonomic nerve cells

1. One classical example of how neuropeptides can affect the function of ligand-gated receptors is the modulation of neuronal nicotinic receptors (nAChRs) by substance P. The present review updates current understanding of this action by substance P and compares it with other neuropeptides more recently found to modulate nAChRs in the autonomic nervous system. 2. Calcitonin gene-related peptide (CGRP) and its N-terminal fragments have been shown to exert complex inhibitory as well facilitatory actions on nAChRs. Fragments such as CGRP(1-4), CGRP(1-5) and CGRP(1-6) rapidly and reversibly enhance agonist sensitivity of nAChRs without directly activating those receptors. Longer fragments or the full-length peptide potently inhibit responses mediated by nAChRs via an apparently competitive-type antagonism. This phenomenon differs from the substance P-induced block, which is agonist use-dependent and preferential towards large nicotinic responses. 3. It is argued that the full-length peptides CGRP and substance P might play distinct roles in the activity-dependent modulation of cholinergic neurotransmission, by inhibiting background noise in the case of CGRP or by reducing excessive excitation in the case of substance P. Hence, multiple neuropeptide mechanisms may represent a wide array of fine-tuning processes to regulate nicotinic synaptic transmission. 4. The availability of novel CGRP derivatives with a strong enhancing action on nAChRs may offer new leads for the drug design targeted for potentiation of nAChRs in the autonomic nervous system as well as in the brain, a subject of interest to counteract the deficit of the nAChR function associated with neurodegenerative diseases like Alzheimer's and Parkinson's diseases.

Rapid relief of block by mecamylamine of neuronal nicotinic acetylcholine receptors of rat chromaffin cells in vitro: an electrophysiological and modeling study

The mechanism responsible for the blocking action of mecamylamine on neuronal nicotinic acetylcholine receptors (nAChRs) was studied on rat isolated chromaffin cells recorded under whole-cell patch clamp. Mecamylamine strongly depressed (IC(50) = 0.34 microM) inward currents elicited by short pulses of nicotine, an effect slowly reversible on wash. The mecamylamine block was voltage-dependent and promptly relieved by a protocol combining membrane depolarization with a nicotine pulse. Either depolarization or nicotine pulses were insufficient per se to elicit block relief. Block relief was transient; response depression returned in a use-dependent manner. Exposure to mecamylamine failed to block nAChRs if they were not activated by nicotine or if they were activated at positive membrane potentials. These data suggest that mecamylamine could not interact with receptors either at rest or at depolarized level. Other nicotinic antagonists like dihydro-beta-erythroidine or tubocurarine did not share this action of mecamylamine although proadifen partly mimicked it. Mecamylamine is suggested to penetrate and block open nAChRs that would subsequently close and trap this antagonist. Computer modeling indicated that the mechanism of mecamylamine blocking action could be described by assuming that 1) mecamylamine-blocked receptors possessed a much slower, voltage-dependent isomerization rate, 2) the rate constant for mecamylamine unbinding was large and poorly voltage dependent. Hence, channel reopening plus depolarization allowed mecamylamine escape and block relief. In the presence of mecamylamine, therefore, nAChRs acquire the new property of operating as coincidence detectors for concomitant changes in membrane potential and receptor occupancy.

Two related forms of long-term habituation in the crab Chasmagnathus are differentially affected by scopolamine

An opaque screen moving overhead elicits an escape response in the crab Chasmagnathus, which, after a few presentations, habituates for a long period (long-term habituation, LTH). Previous results distinguished two types of LTH: the (context-signal)-LTH yielded by spaced training, determined by an association between context and habituating stimulus, and cycloheximide sensitive: and the (signal)-LTH produced by massed training, context independent, and cycloheximide insensitive. Present experiments were aimed at studying the possible involvement of cholinergic mechanisms in one or both types of LTH, using the muscarinic antagonist, scopolamine (SCP). Results indicate that LTH acquired by spaced training (30 trials separated by 85 s) is blocked in a dose-dependent manner by posttraining SCP. Amnesia is shown with 100 ng SCP/g injected immediately before or after spaced training but not delayed 1-h posttraining. No effect of SCP on LTH acquired by massed training (300 trials separated by 4 s) is detected. Pretraining SCP induces a decrease in the response level at the initial trials of either a spaced or a massed training. It is concluded that the storage of (context-signal)-LTH may be selectively regulated by a muscarinic-cholinergic mechanism. However, the possibility that other cholinergic receptors would be involved in the consolidation of the (signal)-LTH is discussed.

Open channel block and alteration of N-methyl-D-aspartic acid receptor gating by an analog of phencyclidine

We investigated inhibition of the N-methyl-D-aspartic acid (NMDA) receptor-channel complex by N-ethyl-1,4,9, 9alpha-tetrahydro-4alphaR-cis-4alphaH-fluoren-++ +4alpha-amine (NEFA), a structural analog of phencyclidine (PCP). Using the whole-cell recording technique, we demonstrated that NEFA inhibits NMDA responses with an IC50 of 0.51 microM at -66 mV. We determined that NEFA binds to the open channel, and subsequently the channel can close and trap the blocker. Once the channel has closed, NEFA is unable to dissociate until the channel reopens. Single-channel recordings revealed that NEFA reduces the mean open time of single NMDA-activated channels in a concentration-dependent manner with a forward blocking rate (k+) of 39.9 microM-1 s-1. A computational model of antagonism by NEFA was developed and constrained using kinetic measurements of single-channel data. By multiple criteria, only models in which blocker binding in the channel causes a change in receptor operation adequately fit or predicted whole-cell data. By comparing model predictions and experimental measurements of NEFA action at a high NMDA concentration, we determined that NEFA affects receptor operation through an influence on channel gating. We conclude that inhibition of NMDA receptors by PCP-like blockers involves a modification of channel gating as well as block of current flow through the open channel.

An analysis of philanthotoxin block for recombinant rat GluR6(Q) glutamate receptor channels

1. The action of philanthotoxin 343 (PhTX) on rat homomeric GluR6(Q) recombinant glutamate receptor channels was analysed using concentration-jump techniques and outside-out patches from HEK 293 cells. Both onset and recovery from block by external PhTX were dependent on the presence of agonist, indicating that channels must open for PhTX to bind and that channel closure can trap PhTX. 2. Block by external PhTX developed with double-exponential kinetics. The rate of onset of the fast component of block showed an exponential increase per 27 mV hyperpolarization over the range -40 to -100 mV. The rate of onset of the slow component of block showed a non-linear concentration dependence indicating a rate-limiting step in the blocking mechanism. 3. The extent of block by 1 microM external PhTX was maximal at -40 mV and did not increase with further hyperpolarization; the rate of recovery from block by external PhTX increased 6-fold on hyperpolarization from -40 to -100 mV suggesting that PhTX permeates at negative membrane potentials. 4. Apparent Kd values for block by external PhTX estimated from dose-inhibition experiments decreased 300-fold on hyperpolarization from +40 mV (Kd, 19.6 microM) to -40 mV (Kd, 69 nM); there was little further increase in affinity with hyperpolarization to -80 mV (Kd, 56 nM), consistent with permeation of PhTX at negative membrane potentials. 5. Block by internal PhTX showed complex kinetics and voltage dependence. Analysis with voltage ramps from -120 to +120 mV indicated a Kd at 0 mV of 20 microM, decreasing e-fold per 16 mV depolarization. However, at +90 mV the extent of block by 1 and 10 microM internal PhTX (73 % and 95 %, respectively) reached a maximum and did not increase with further depolarization. 6. Voltage-jump analysis of block by 100 microM internal PhTX revealed partial trapping. With 100 ms jumps from -100 to -40 mV, onset and recovery from block were complete within 5 ms. With jumps of longer duration the extent of block increased, with a time constant of 8.1 s, reaching 84 % at 30 s. On repolarization to -100 mV, recovery from block showed fast and slow components. 7. The amplitude of the slow component of block by internal PhTX showed a biphasic voltage dependence, first increasing then decreasing with progressive depolarization. Maximum block was obtained at 0 mV. 8. Our results suggest that PhTX acts as an open channel blocker; however, provided that the toxin remains bound to the channel, an allosteric mechanism destabilizes the open state, inducing channel closing and trapping PhTX. Strong depolarization for internal PhTX, or strong hyperpolarization for external PhTX, forces the toxin to permeate before it triggers entry into closed blocked states.

Nicotinic antagonists (piperidines and quinuclidines) reduce the susceptibility of early sea urchin embryos to agents evoking calcium shock

1. Some nicotinic antagonists (piperidine and quinuclidine derivatives and bis-quaternary compounds) protect early embryos of the sea urchin Lytechinus pictus against a calcium shock evoked by ionomycin or a mixture of phorbol myristate acetate and nicotine. 2. Maximal protective potency was found for drugs that did not penetrate the plasma membrane. 3. Early sea urchin embryos have nicotinic acetylcholine receptors (nAChR) or nAChR-like structures localized on the cell surface that, apparently, take part in the control of Ca2+ influx.

The channel-blocking action of methonium compounds on rat submandibular ganglion cells. 1983

The effects of drugs of the polymethylene bis-trimethylammonium (methonium) series on the characteristics of the synaptic currents (e.s.cs) recorded from voltage-clamped rat submandibular ganglion cells have been studied. The drugs studied were from C4 to C10 (decamethonium).

All of the drugs except C4 shortened the initial decay phase of the e.s.c.; C9 and C10 produced an additional slowly decaying component. These effects were interpreted in terms of an open channel block mechanism, the calculated rate constants for association with the open channel at − 80 mV being fairly similar (5.9 × 10⁶ to 18.1 × 10⁶M⁻¹ s⁻¹) for all of the compounds except C4, which had no effect on the e.s.c. decay.

All of the compounds produced use-dependent block when tested with short trains of stimuli at 10 Hz, or with trains of ionophoretic pulses of acetylcholine, consistent with their channel blocking property. Tubocurarine had a similar effect, but not trimetaphan or mecamylamine.

Recovery from use-dependent block with short chain methonium compounds, up to C8, was very slow in the absence of agonist, being incomplete even after several minutes. With C9 or C10 or tubocurarine, recovery from use-dependent block was complete within a few seconds. With C6 recovery in the absence of agonist was unaffected by membrane potential, but could be accelerated by applying acetylcholine with the cell depolarized to − 40 mV. This persistent block was ascribed to the ability of the blocking molecule to become trapped by closure of the channel. With C9 and C10 it is assumed that their presence inhibits channel closure, so they can escape without the help of agonist.

When use-dependent block is avoided by leaving the ganglion unstimulated during equilibration with the blocking drug, the first e.s.c. elicited shows no appreciable reduction of amplitude, though with C6, C7 or C8 subsequent responses elicited at 0.1 Hz become progressively more blocked.

Even at 1 Mm, C6 does not prevent acetylcholine from opening ionic channels.

It is concluded that all of the effects on e.s.c. amplitude can be interpreted in terms of channel block, there being no evidence of any receptor blocking action.

Intraneuronal accumulation and persistence of radiolabel in rat brain following in vivo administration of [3H]-chlorisondamine

1. Chlorisondamine (CHL), a bisquaternary amine, produces a remarkably long-lasting blockade of central responses to nicotine. The mechanism underlying this blockade is not known. The main aim of this study was to test for possible accumulation of [3H]-CHL in rat brain during the period of chronic blockade. 2. Rats received CHL, either systemically (10 mg kg-1) or centrally (10 micrograms i.c.v.). Seven days later, striatal synaptosomes prepared from these animals were tested for nicotine-induced [3H]-dopamine release. This experiment showed that i.c.v. administration of CHL was as effective as systemic administration in producing ex vivo blockade of central nicotinic receptors. 3. Rats received bilateral i.c.v. infusions of [3H]-CHL (10 micrograms) and radioactivity was subsequently quantified in dissected cerebral cortex, striatum, hippocampus, midbrain and cerebellum. Radiolabel was detected at all three survival times (1, 7, and 21 days). Regional heterogeneity was apparent at 7 and 21 days survival. Radiolabel was almost exclusively confined to the insoluble subcellular fraction in all areas sampled. 4. The anatomical distribution of radiolabel was also visualized in brain sections. Rats received bilateral i.c.v. infusions of [3H]-CHL (10 micrograms) and were killed at 1, 7, 21 or 84 days. Immediately before they were killed, all rats were tested behaviourally, and central nicotinic blockade was demonstrated at 1, 7 and 21 days; partial recovery was observed at 84 days. Particularly at longer survival times, tritium was found to be heavily concentrated in the substantia nigra pars compacta, ventral tegmental area, dorsal raphé nucleus, and the granular layer of the cerebellum. 5. The possibility of retrograde axonal transport of radiolabel was then examined. Rats received a unilateral intrastriatal infusion of [3H]-CHL (0.34 or 0.034 micrograms) one week before they were killed. Autoradiographic labelling was largely confined to the site of infusion and to the ipsilateral substantia nigra pars compacta and dorsal raphé nucleus. 6. Thus, after i.c.v. administration, CHL (and/or centrally-formed derivatives) is initially widely distributed within the brain and is then selectively retained within a few brain areas. A persistent accumulation occurs within putative dopaminergic and 5-hydroxytryptaminergic neurones, at least partly through uptake by terminals and/or axons followed by retrograde transport. This persistent and anatomically-selective intraneuronal accumulation possibly underlies the long-term central nicotinic blockade associated with chlorisondamine.

Blockade of nicotinic receptor-mediated release of dopamine from striatal synaptosomes by chlorisondamine administered in vivo

1. The chronic nicotinic blockade produced following in vivo administration of chlorisondamine was investigated in vitro. Nicotine-induced [3H]-dopamine release from striatal synaptosomes was used as a measure of central nicotinic receptor function. 2. In synaptosomal preparations from rats pretreated with a single administration of chlorisondamine (10 mg kg-1, s.c.), 1, 7, 21, 42, 63 or 84 days before they were killed, responses to (-)-nicotine (10(-6) M) were blocked. 3. In vivo administration of chlorisondamine (10 mg kg-1, s.c.), 7 days before rats were killed, produced a nicotinic blockade in vitro that was insurmountable even with a high concentration of (-)-nicotine (10(-4) M). 4. Both in vitro and in vivo administration of chlorisondamine blocked nicotinic responses to acetylcholine (10(-4) M). In contrast, neither in vitro nor in vivo administration of chlorisondamine reduced [3H]-dopamine release induced by high K+ (20 x 10(-3) M) or (+)-amphetamine (10(-6) M). 5. Nicotinic blockade resulting from in vitro administration of chlorisondamine (10(-5) M) recovered partially after 60 min wash-out, and completely by 90 min. In contrast, no recovery was seen in synaptosomes prepared from rats pretreated with chlorisondamine (10 mg kg-1, s.c.) in vivo. 6. Thus, in vivo treatment with chlorisondamine results in a quasi-irreversible, insurmountable block of CNS nicotinic receptors. The persistence of this block ex vivo indicates that physical trapping by the blood brain barrier is not solely responsible for the persistent blockade seen in vivo. The resistance of this blockade to prolonged in vitro wash-out suggests that the underlying mechanism differs from that associated with in vitro administration.

Blockade of nicotinic receptor-mediated release of dopamine from striatal synaptosomes by chlorisondamine and other nicotinic antagonists administered in vitro

1. Central nicotinic receptor function examined in vitro, by measuring nicotine-induced [3H]-dopamine release from rat striatal synaptosomes. 2. The agonists (-)-nicotine, acetylcholine, 1,1-dimethyl-4-phenylpiperazinium (DMPP) and cytisine (10(-7)-10(-4) M) all increased [3H]-dopamine release in a concentration-dependent manner. Cytisine did not produce a full agonist response, compared to the other agonists. 3. The actions of nicotine, acetylcholine and cytisine were largely dependent on external Ca2+. In contrast, DMPP (10(-5) and 10(-4) M) evoked a marked release of [3H]-dopamine even in the absence of Ca2+. Nevertheless, in the presence of external Ca2+, responses to DMPP were completely blocked by the nicotinic antagonists chlorisondamine and mecamylamine (5 x 10(-5) M); in the absence of external Ca2+, blockade was only partial. 4. Chlorisondamine, mecamylamine and dihydro-beta-erythroidine (10(-8)-10(-4) M) produced a concentration-dependent block of responses to nicotine (10(-6) M). Approximate IC50 values were 1.6, 0.3 and 0.2 x 10(-6), respectively. Chlorisondamine and mecamylamine blocked responses to nicotine (10(-7)-10(-4) M) insurmountably, whereas dihydro-beta-erythroidine behaved in a surmountable fashion. 5. The occurrence of use-dependent block was tested by briefly pre-exposing the synaptosomes to nicotine during superfusion with antagonist, and determining the response to a subsequent nicotine application. Consistent with a possible channel blocking action, brief pre-exposure to agonist increased the antagonist potency of chlorisondamine (approximately 25 fold). No significant use-dependent block was detected with dihydro-beta-erythroidine.

Activation of nicotinic acetylcholine receptors on cultured Drosophila and other insect neurones

1. Using whole-cell and single channel recordings, we have examined the properties of acetylcholine (ACh)-activated currents in neurones from larval and pupal Drosophila melanogaster (fruit fly), larval and embryonic Musca domestica (house fly), and nymphal Schistocerca gregaria (locust). 2. In all preparations, single channel recordings revealed two major classes of ACh-activated channels, with average conductances of approximately 32 and 59 pS. 3. At ACh concentrations from 1 to 10 microM, channel activity in Drosophila larval neurones occurs in bursts with an average of 1-2 openings. Open times and burst durations are described by one or two exponentials. Burst durations for the 32 pS channel (approximately 3 ms, slow component) were longer than those for the 59 pS channel (approximately 1.0 ms). The mean open interval duration for the 32 pS channel (slow component) was also longer than that of the 59 pS channel. 4. At high ACh (20-200 microM) concentrations, bursts of the smaller conductance channel occur in clusters separated by long-lived periods without channel activity. Considerable kinetic heterogeneity was observed among clusters. 5. The whole-cell dose-response curve suggests that activation of current by ACh increases up to at least 100 microM and that multiple ligand binding steps are involved. 6. Drosophila and Musca larval neuronal ACh-activated channels show some unique features in their cholinergic pharmacological properties: (a) they are only weakly activated by the potent neuromuscular nicotinic agonist suberyldicholine, (b) hexamethonium and decamethonium are weak, but approximately equi-effective blockers, and (c) alpha- and kappa-bungarotoxin (BTX) both blocked reversibly, though alpha-BTX appears to be the more potent inhibitor.

A patch clamp study of the nicotinic acetylcholine receptor of bovine adrenomedullary chromaffin cells in culture

1. Acetylcholine-induced currents recorded from bovine adrenal medullary chromaffin cells maintained in culture were studied during pressure or ionophoretic applications of ACh, using the 'whole-cell' and 'outside-out' configurations of the patch clamp technique. In standard salines, ACh evoked whole-cell currents of -38 pA to -1 nA at -60 mV, which had a reversal potential (EACh) of -7.1 +/- 0.6 mV. The ACh current-voltage relationship was characteristically linear at negative holding potentials and biphasic at positive holding potentials, displaying a region of almost zero slope conductance between 0 and +40 mV followed by a region of positive slope conductance at more positive potentials. 2. Relative permeation to cations was examined. Substitution of external Na+ by sucrose resulted in a -42 mV shift of EACh for a 10-fold reduction in [Na+]o. Using isotonic substitutions, the permeability ratios (relative to Na+) for monovalent cations were determined to be 1.32 +/- 0.02 for Cs+ (n = 11), 1.03 +/- 0.02 for Li+ (n = 8) and 0.18 +/- 0.02 for Tris+ (n = 7). Elevated external Ca2+ salines were found to shift EACh to more positive potentials, especially in the presence of low external Na+. 3. The nicotinic agonists nicotine, tetramethylammonium and lobeline evoked inward currents in bovine chromaffin cells. In contrast, decamethonium and the muscarinic agonist, methacholine, had no effect. 4. The nicotinic antagonists mecamylamine, trimetaphan, (+)-tubocurarine and hexamethonium caused dose-dependent reductions in the amplitude of ACh-evoked inward currents. The estimated IC50's were 0.25, 0.33, 0.63 and 2.2 microM respectively, for cells voltage clamped at -60 mV. High concentrations (> 2 microM) of the muscarinic antagonist, atropine, also produced a dose-dependent reduction in the amplitude of ACh-induced currents. 5. Inhibition by trimetaphan was voltage independent. With the other drugs the antagonism was voltage sensitive, increasing with membrane hyperpolarization. The voltage sensitivity was most marked for hexamethonium. Neither hexamethonium nor mecamylamine were found to depress ACh-evoked outward currents at concentrations which severely depressed inward currents. In addition to its antagonist actions, (+)-tubocurarine activated unitary currents in these cells and on isolated membrane patches. 6. The results indicate that nicotinic ion channels of bovine chromaffin cells have a similar ionic selectivity to monovalent cations, but that Ca2+ ions permeate the channels to a greater degree than at the motor endplate. The ACh current-voltage relationship resembles that described for other types of 'neuronal' nicotinic receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacology of nicotine

In recent years progress in basic neuropsychopharmacology and clinical addiction research have allowed the conclusion that tobacco smoking essentially represents an addiction to nicotine. Parallel to this work, experimental research in biochemistry, physiology and pharmacology has provided detailed descriptions of the structure and function of the nicotinic receptor, the biologic mediator of the many actions of nicotine. This article reviews current knowledge of nicotinic mechanisms in the peripheral and central nervous systems as well as some implications for the notion of smoking as an addiction to nicotine. In particular this review will focus on the effects of nicotine on brain dopamine and noradrenaline systems since these neuronal systems appear to be crucially involved in the rewarding and stimulant effects of addictive drugs.

Anomalous voltage dependence of channel blockade at a crustacean glutamate-mediated synapse

1. The voltage dependence and concentration dependence of blockade of glutamate-activated currents by the diquaternary amine, chlorisondamine, were examined in a marine crustacean muscle. 2. Chlorisondamine results in the splitting of focally recorded synaptic current decays into two exponential components. The fast component becomes faster with increases in drug concentration and with hyperpolarization. The slow decay rate is unchanged or faster with hyperpolarization and the relative amplitude of the slow component is increased with hyperpolarization. 3. The alteration of synaptic current decay rates by chlorisondamine over the range of 5 to 100 microM and -80 to -140 mV is quantitatively consistent with a simple channel blockade model with a zero-voltage blocking rate of 6 x 10(5) M-1 s-1 at 12 degrees C with a voltage dependence of about 40 mV per e-fold change. The unblocking rate is about 5 s-1 at 0 mV and increases with hyperpolarization with a voltage dependence of about 30 mV per e-fold change. 4. The dose dependence and voltage dependence of blockade of ionophoretically activated glutamate currents by chlorisondamine are qualitatively consistent with the kinetic estimates. 5. The anomalous voltage dependence of the unblocking process is considered in terms of the possibility that the relief from blockade by chlorisondamine occurs by transit of chlorisondamine through the ion channel opened by glutamate.

Mecamylamine is a selective non-competitive antagonist of N-methyl-D-aspartate- and aspartate-induced currents in horizontal cells dissociated from the catfish retina

The nicotinic acetylcholine channel blockers mecamylamine (MECA) and pempidine (PEMP) blocked voltage-clamped isolated catfish retina cone horizontal responses to aspartate (Asp) and N-methyl-D-aspartate (NMDA) but had little effect on currents induced by kainate and quisqualate. Concentration response curves for NMDA and Asp in the presence of MECA suggested that MECA was a non-competitive inhibitor of NMDA and Asp responses. Further, the MECA and PEMP block of NMDA and Asp-induced currents was highly voltage-sensitive. The non-competitive and voltage-sensitive block of NMDA and Asp-induced currents by MECA suggest that MECA (and PEMP) block the NMDA receptor ion channel.

Trapping of an open-channel blocker at the frog neuromuscular acetylcholine channel

At the ganglionic nicotinic acetylcholine channel (Gurney, A. M., and H. P. Rang, 1984, Br. J. Pharmacol., 82:623-642) and on some cholinergic neuromuscular synapses of Crustacea (Lingle, C., 1983a, J. Physiol. (Lond.), 339:395-417; Lingle, C., 1983b, J. Physiol. (Lond.), 339:419-437), some agents that block cholinergic currents by an open-channel block mechanism appear to become trapped within the channel when it subsequently closes. It is unknown whether trapping of some open-channel blockers might also occur at the neuromuscular nicotinic acetylcholine channel. Here we show that the long-lived cholinergic blocking action of chlorisondamine, a ganglionic nicotinic blocker, can in part be most simply explained by an open-channel block mechanism followed by a subsequent trapping of the blocking molecule within the closed ion channel. Unique structural characteristics of the chlorisondamine molecule place several provocative constraints on the mechanism by which trapping may be occurring.

A comparison between mechanisms of action of different nicotinic blocking agents on rat submandibular ganglia

The blocking properties of tubocurarine, decamethonium, hexamethonium and trimetaphan on nicotinic agonists applied by repetitive ionophoretic pulses were examined in rat submandibular ganglion cells using a two-microelectrode voltage-clamp technique at 30 degrees C. Hexamethonium, a proposed slowly dissociating, open-channel blocker at concentrations of 2-20 microM did not produce a 'use-dependent' run-down of responses, but its antagonism was clearly dependent on membrane potential. The voltage-dependent reduction of agonist response by hexamethonium was not dependent on the nature of agonist used. Similar results were obtained with acetylcholine (ACh) and carbamylcholine (CCh) ionophoresis. (+)-Tubocurarine (5 microM) and decamethonium (10 microM) produced 'use-dependent' run-down of agonist responses which became more pronounced at higher frequency and as the cell was hyperpolarized, consistent with open-channel blockade. In contrast, trimetaphan (2.5 microM), a receptor antagonist did not cause 'use-dependent' run-down of responses. Hence, the antagonism produced by hexamethonium, unlike tubocurarine and decamethonium, could not be accounted for in terms of open-channel blockade but requires an alternative mechanism, the nature of which is discussed.

Acetylcholine responses of identified neurons in Helix pomatia--II. Pharmacological properties of acetylcholine responses

A pharmacological separation of depolarizing and hyperpolarizing mechanisms involved in the generation of acetylcholine (ACh) depolarizations was attempted in the identified neurons B1 and B3 of the buccal ganglia of Helix pomatia. The selectivity of the drugs employed was assayed in non-identified buccal neurons in which ACh increased a hyperpolarizing Cl- conductance. Voltage clamp techniques were used. Under control conditions the depolarizing ACh currents increased non-linearly with more negative membrane potentials. The hyperpolarizing ACh currents showed a linear potential dependence. The buffer substance Tris (5 mmol/l) depressed the depolarizing ACh currents. The effect was accentuated with more negative membrane potentials. Tris failed to affect hyperpolarizing ACh responses. HEPES (5 mmol/l) did not change depolarizing or hyperpolarizing ACh responses. d-Tubocurarine (0.02-0.2 mmol/l), hexamethonium (0.5-5.0 mmol/l) and atropine (0.1 mmol/l) blocked the depolarizing and hyperpolarizing ACh responses. Arecoline (0.1 mmol/l) had neither an agonistic nor an antagonistic effect on the identified and on the non-identified neurons. It displayed an anticholinesterase activity. Anthracene-9-carbonic acid (0.5 mmol/l) depressed selectively the hyperpolarizing ACh responses. In the neurons B1 and B3 no pharmacologically separable hyperpolarizing ACh responses were detected to be superimposed on the ACh depolarizations.

The channel-blocking action of methonium compounds on rat submandibular ganglion cells

The effects of drugs of the polymethylene bis-trimethylammonium (methonium) series on the characteristics of the synaptic currents (e.s.cs) recorded from voltage-clamped rat submandibular ganglion cells have been studied. The drugs studied were from C4 to C10 (decamethonium). All of the drugs except C4 shortened the initial decay phase of the e.s.c.; C9 and C10 produced an additional slowly decaying component. These effects were interpreted in terms of an open channel block mechanism, the calculated rate constants for association with the open channel at -80 mV being fairly similar (5.9 X 10(6) to 18.1 X 10(6)M-1S-1) for all of the compounds except C4, which had no effect on the e.s.c. decay. All of the compounds produced use-dependent block when tested with short trains of stimuli at 10 Hz, or with trains of ionophoretic pulses of acetylcholine, consistent with their channel blocking property. Tubocurarine had a similar effect, but not trimetaphan or mecamylamine. Recovery from use-dependent block with short chain methonium compounds, up to C8, was very slow in the absence of agonist, being incomplete even after several minutes. With C9 or C10 or tubocurarine, recovery from use-dependent block was complete within a few seconds. With C6 recovery in the absence of agonist was unaffected by membrane potential, but could be accelerated by applying acetylcholine with the cell depolarized to -40 mV. This persistent block was ascribed to the ability of the blocking molecule to become trapped by closure of the channel. With C9 and C10 it is assumed that their presence inhibits channel closure, so they can escape without the help of agonist. When use-dependent block is avoided by leaving the ganglion unstimulated during equilibration with the blocking drug, the first e.s.c. elicited shows no appreciable reduction of amplitude, though with C6, C7 or C8 subsequent responses elicited at 0.1 Hz become progressively more blocked. Even at 1 mM, C6 does not prevent acetylcholine from opening ionic channels. It is concluded that all of the effects on e.s.c. amplitude can be interpreted in terms of channel block, there being no evidence of any receptor blocking action.

Blockade of cholinergic channels by chlorisondamine on a crustacean muscle

Details of the blocking action of chlorisondamine, a ganglionic nicotinic blocker, on the excitatory cholinergic currents of the spiny lobster gastric mill 1 (g.m.1) muscle are described. The steady-state block of cholinergic ionophoretic currents produced by chlorisondamine is strongly voltage-dependent. During a hyperpolarizing voltage step, a sequence of ionophoretic agonist pulses in the presence of chlorisondamine shows a large interpulse interaction manifested as a gradual diminution in response amplitude. The extent of diminution is dependent on the number of the pulse in a series and not on the duration of the interval between pulses. The slowly developing blockade is entirely dependent on agonist application. If agonist application is suspended for various time intervals following the development of a given blocked level in chlorisondamine, no recovery from the block is observed whether the rest interval is at the step potential or at more depolarized potentials. Recovery from a given blocked level can be observed if, during a depolarizing voltage step (to -60 mV) away from the potential at which the block was established (-140 mV), agonist is applied before return to the initial potential (-140 mV). Chlorisondamine produces a dose-dependent reduction in excitatory junctional current (e.j.c.) decay rate that is linear with chlorisondamine concentration and markedly dependent on voltage (approximately equal to 35 mV/e-fold change). Reduction in the amplitude of e.j.c.s occurred at concentrations of chlorisondamine that produced no detectable effect on e.j.c. decay. Alterations in e.j.c. amplitude showed time- and use-dependent aspects similar to those observed for ionophoretic currents. These results are discussed primarily in terms of a sequential model in which, following the binding of chlorisondamine to the opened ion channel, the channel can undergo a transition to a stable-blocked state that requires reactivation by agonist to become unblocked. This stable-blocked state is considered a closed-blocked channel.