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

Ion-Selective Carbon Nanotube Field-Effect Transistors for Monitoring Drug Effects on Nicotinic Acetylcholine Receptor Activation in Live Cells

We developed ion-selective field-effect transistor (FET) sensors with floating electrodes for the monitoring of the potassium ion release by the stimulation of nicotinic acetylcholine receptors (nAChRs) on PC12 cells. Here, ion-selective valinomycin-polyvinyl chloride (PVC) membranes were coated on the floating electrode-based carbon nanotube (CNT) FETs to build the sensors. The sensors could selectively measure potassium ions with a minimum detection limit of 1 nM. We utilized the sensor for the real-time monitoring of the potassium ion released from a live cell stimulated by nicotine. Notably, this method also allowed us to quantitatively monitor the cell responses by agonists and antagonists of nAChRs. These results suggest that our ion-selective CNT-FET sensor has potential uses in biological and medical researches such as the monitoring of ion-channel activity and the screening of drugs.

Adenovirus vector‑mediated in vivo gene transfer of nuclear factor erythroid‑2p45‑related factor 2 promotes functional recovery following spinal cord contusion

The aim of the present study was to investigate whether nuclear factor erythroid 2p45‑related factor 2 (Nrf2) overexpression by gene transfer may protect neurons/glial cells and the association between neurons/glial cells and axons in spinal cord injury (SCI). In the present study, Nrf2 recombinant adenovirus (Ad) vectors were constructed. The protein levels of Nrf2 in the nucleus and of the Nrf2‑regulated gene products heme oxygenase‑1 (HO‑1) and NAD (P)H‑quinone oxidoreductase‑1 (NQO1), were detected using western blot analysis in PC12 cells following 48 h of transfection. Furthermore, the expression of Nrf2 was localized using an immunofluorescence experiment, and the expression of Nrf2, HO‑1 and NQO1 were detected using an immunohistochemical experiment in the grey matter of spinal cord in rats. Post‑injury motor behavior was assessed via the Basso, Beattie and Bresnahan (BBB) locomotor scale method. In PC12 cells, subsequent to Ad‑Nrf2 transfection, nuclear Nrf2, HO‑1 and NQO1 levels were significantly increased compared with the control (P<0.01). There was statistically significant changes in the PC12‑Ad‑Nrf2 group [Nrf2 (1.146±0.095), HO‑1 (1.816±0.095) and NQO1 (1.421±0.138)] compared with the PC12‑control group [Nrf2 (0.717±0.055), HO‑1 (1.264±0.081) and NQO1 (0.921±0.088)] and PC12‑Ad‑green fluorescent protein group [Nrf2 (0.714±0.111), HO‑1 (1.238±0.053) and NQO1 (0.987±0.045); P<0.01]. The BBB scores of the rats indicated that they had improved functional recovery following the local injection of Ad‑Nrf2. On the third day following the operation, BBB scores in the adenovirus groups (0.167±0.408) were significantly decreased compared with the SCI group (1±0.894; P<0.05). In the injured section of the spinal cord in the rats, the number of positive cells expressing Nrf2, HO‑1 and NQO1 were raised compared with the control and SCI groups, indicating that the adenovirus vector‑mediated gene transfer of Nrf2 promotes functional recovery following spinal cord contusion in rats.

Dual Antidepressant Duloxetine Blocks Nicotinic Receptor Currents, Calcium Signals and Exocytosis in Chromaffin Cells Stimulated with Acetylcholine

The inhibition of nicotinic acetylcholine receptors (nAChRs) has been proposed as a potential strategy to develop new antidepressant drugs. This is based on the observation that antidepressants that selectively block noradrenaline (NA) or serotonin (5-HT) reuptake also inhibit nAChRs. Dual antidepressants blocking both NA and 5-HT reuptake were proposed to shorten the delay in exerting their clinical effects; whether duloxetine, a prototype of dual antidepressants, also blocks nAChRs is unknown. Here we explored this question in bovine chromaffin cells (BCCs) that express native α3, α5, and α7 nAChRs and in cell lines expressing human α7, α3β4, or α4β2 nAChRs. We have found that duloxetine fully blocked the acetylcholine (ACh)-elicited nicotinic currents in BCCs with an IC50 of 0.86 µM. Such blockade seemed to be noncompetitive, voltage dependent, and partially use dependent. The ACh-elicited membrane depolarization, the elevation of cytosolic calcium ([Ca2+]c), and catecholamine release in BCCs were also blocked by duloxetine. This blockade developed slowly, and the recovery of secretion was also slow and gradual. Duloxetine did not affect Na+ or Ca2+ channel currents neither the high-K+-elicited [Ca2+]c transients and secretion. Of interest was that in cell lines expressing human α7, α3β4, and α4β2 nAChRs, duloxetine blocked nicotinic currents with IC50 values of 0.1, 0.56, and 0.85 µM, respectively. Thus, in blocking α7 receptors, which are abundantly expressed in the brain, duloxetine exhibited approximately 10-fold to 100- fold higher potency with respect to reported IC50 values for various antidepressant drugs. This may contribute to the antidepressant effect of duloxetine.

Pharmacological Disruption of Sea Urchin Tube Foot Motility and Behavior

The understanding of the molecular basis of sea urchin behavior and sensory and motor systems lags far behind that of many other animal species. To investigate whole-animal behavior pharmacologically, we first demonstrated that immersion in drug solution is an effective drug administration route for sea urchins, whereas oral drug administration was found to be ineffective. Although intracoelomic injection was found to be effective at administering drugs, it was also found that injection itself can disrupt normal sea urchin behavior. Using the drug immersion procedure, we demonstrate that sea urchin locomotion and the sea urchin righting response are inhibited in a dose-dependent manner by the phosphodiesterase inhibitor theophylline and the transient receptor potential channel inhibitor 2-aminoethoxydiphenyl borate. The sea urchin righting response was also inhibited by the nitric oxide synthase inhibitor N(G)-nitro-l-arginine methyl ester and the Ca²⁺ channel inhibitor diltiazem, which, along with theophylline and 2-aminoethoxydiphenyl borate, would all be expected to disrupt smooth muscle function, based on studies in other animals. In addition, the removal of extracellular Ca²⁺ also inhibited the righting response, whereas an inhibitor of intracellular Ca²⁺ release, thapsigargin, did not affect the righting response, indicating that extracellular Ca²⁺ rather than intracellular Ca²⁺ stores are required for righting.

A novel method of neural differentiation of PC12 cells by using Opti-MEM as a basic induction medium

The PC12 cell line is a classical neuronal cell model due to its ability to acquire the sympathetic neurons features when deal with nerve growth factor (NGF). In the present study, the authors used a variety of different methods to induce PC12 cells, such as Opti-MEM medium containing different concentrations of fetal bovine serum (FBS) and horse serum compared with RPMI-1640 medium, and then observed the neurite length, differentiation, adhesion, cell proliferation and action potential, as well as the protein levels of axonal growth-associated protein 43 (GAP-43) and synaptic protein synapsin-1, among other differences. Compared with the conventional RPMI-1640 medium induction method, the new approach significantly improved the neurite length of induced cells (2.7 times longer), differentiation rate (30% increase), adhesion rate (21% increase) and expression of GAP-43 and synapsin-1 (three times), as well as reduced cell proliferation. The morphology of induced cells in Opti-MEM medium containing 0.5% FBS was more like that of neurons. Additionally, induced cells were also able to motivate the action potential after treatment for 6 days. Therefore, the research provided a novel, improved induction method of neural differentiation of PC12 cells using Opti-MEM medium containing 0.5% FBS, resulting in a better neuronal model cell line that can be widely used in neurobiology and neuropharmacology research.

The Nicotinic Cholinergic Pathway Contributes to Retinal Neovascularization in a Mouse Model of Retinopathy of Prematurity

Purpose

To investigate the role of nicotinic acetylcholine receptors (nAChRs) in retinal vascular development and ischemia-induced retinal neovascularization (NV).

Methods

The expression of nAChR subtypes and VEGF signaling pathway components was assessed in mice with and without oxygen-induced ischemic retinopathy by comparing expression levels at postnatal day (P) 14 and P17 in mice exposed to 75% oxygen from P7 to P12 and returned to room air versus mice pups that were exposed to ambient oxygen levels during the same period. The effect of topical or intraocular injection of mecamylamine, a nonspecific nAChR antagonist, or targeted deletion of α7- or α9-nAChRs on ischemia-induced retinal NV was determined by comparing the amount of retinal NV at P17 in these mice versus appropriate controls.

Results

The expression of nAChR subunits and components of the VEGF signaling pathways was increased in ischemic retina. Topical application or intraocular injection of mecamylamine decreased retinal NV in this model. Mecamylamine had no effect on normal retinal vascular development or on revascularization of the central retinal area of nonperfusion in mice with ischemic retinopathy. Targeted deletion of α9, but not α7, nAChR receptor subunits reduced retinal NV in mice with ischemic retinopathy.

Conclusion

These data suggest that nAChR signaling, primarily through the α9 nAChR subunit, contributes to ischemia-induced retinal NV, but not retinal vascular development. Mecamylamine or a specific α9 nAChR antagonist could be considered for treatment of retinopathy of prematurity and other ischemic retinopathies.

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 and roles of muscarinic activation in guinea-pig adrenal medullary cells

Muscarinic receptors are expressed in the adrenal medullary (AM) cells of various mammals, but their physiological roles are controversial. Therefore, the ionic mechanism for muscarinic receptor-mediated depolarization and the role of muscarinic receptors in neuronal transmission were investigated in dissociated guinea-pig AM cells and in the perfused guinea-pig adrenal gland. Bath application of muscarine induced an inward current at -60 mV. This inward current was partially suppressed by quinine with an IC(50) of 6.1 μM. The quinine-insensitive component of muscarine-induced currents changed the polarity at -78 mV and was inhibited by bupivacaine, a TWIK-related acid-sensitive K(+) (TASK) channel inhibitor. Conversely, the current-voltage relationship for the bupivacaine-insensitive component of muscarine currents showed a reversal potential of -5 mV and a negative slope below -40 mV. External application of La(3+) had a double action on muscarine currents of both enhancement and suppression. Immunoblotting and immunocytochemistry revealed expression of TASK1 channels and cononical transient receptor potential channels 1, 4, 5, and 7 in guinea-pig AM cells. Retrograde application of atropine reversibly suppressed transsynaptically evoked catecholamine secretion from the adrenal gland. The results indicate that muscarinic receptor stimulation in guinea-pig AM cells induces depolarization through inhibition of TASK channels and activation of nonselective cation channels and that muscarinic receptors are involved in neuronal transmission from the splanchnic nerve.

Monitoring extracellular K+ flux with a valinomycin-coated silicon nanowire field-effect transistor

A silicon nanowire field-effect transistor (SiNW-FET) coated with a polyvinyl chloride (PVC) membrane containing valinomycin (VAL) was employed as a biosensor (referred to as VAL-PVC/SiNW-FET) to detect the K(+)-efflux from live chromaffin cells. The detection sensitivity of K(+) with the VAL-PVC/SiNW-FET covers a broad range of concentrations from 10(-6) to 10(-2) M. The apparent association constants between VAL and Li(+), Na(+), K(+), and Cs(+) in Tris buffer solution were determined to be 67±42, 120±23, 5974±115, and 4121±140 M(-1), respectively. By culturing chromaffin cells on the VAL-PVC/SiNW-FET, the conductance was significantly increased by nicotine stimulation in a bath buffer without Na(+). The K(+) concentration at the cell surface was determined to be ~20 μM under the stimulation of 5 mM nicotine. These results demonstrate that the VAL-PVC/SiNW-FET is sensitive and selective to detect the released K(+) from cells and is suitable for applications in cellular recording investigations.

Ivermectin effects on motor coordination and contractions of isolated rat diaphragm

Ivermectin, the antiparasitic drug from the macrocyclic lactones class raises attention due to its high efficiency against nematodes and arthropods and very specific toxic and side effects that it may produce in host. Dominant clinical symptoms of adverse effects and toxicity of ivermectin in animals are tremor, ataxia, CNS depression and coma which often results in mortality. In our study increasing intravenous doses of ivermectin, (6 or more times higher than therapeutic dose: 1.25, 2.5, 3.75, 5.0, 6.25 and 7.5 mg/kg), caused dose-dependent disturbance of motor coordination in treated rats. The median effective dose (ED50) that was able to impair the rota-rod performance in rats treated 3 min before testing was 2.52 mg/kg. This effect weakens over time, while in the rats treated 60 min before the rota-rod test, ED50 of ivermectin was 4.21 mg/kg. Whereas, all tested doses of ivermectin did not cause any other clinical symptoms of toxicity. Ivermectin has no effect on the contractions of isolated diaphragm caused by the EFS, which effectively blocked mecamylamine (100 μM) and pancuronium (1 and 2 μM). Effect on motor coordination is the first detectable clinical symptom of ivermectin toxicity and apparently is a result of its central effects.

Nicotinic acetylcholine receptors of adrenal chromaffin cells

In the adrenal medulla, acetylcholine released by the sympathetic splanchnic nerves activates neuronal-type nicotinic acetylcholine receptors (nAChRs) on the membrane of chromaffin cells which liberate catecholamines into the bloodstream in preparation for the fight and flight reactions. On adrenal chromaffin cells the main class of nAChRs is a pentameric assembly of alpha3 and beta4 subunits that forms ion channels which produce membrane depolarization by increasing Na+, K+ and Ca2+ permeability. Homomeric alpha7 nicotinic receptors are expressed in a species-dependent manner and do not contribute to catecholamine secretion. Chromaffin cell nAChRs rapidly activate and desensitize with full recovery on washout. nAChR activity is subjected to various types of dynamic regulation. It is allosterically modulated by the endogenous neuropeptide substance P that stabilizes receptors in their desensitized state, thus depressing their responsiveness. The full-length peptide CGRP acts as a negative allosteric modulator by inhibiting responses without changing desensitization, whereas its N-terminal fragments act as positive allosteric modulators to transiently enhance nAChR function. nAChR expression increases when cells are chronically exposed to either selective antagonists or agonists such as nicotine, a protocol mimicking the condition of chronic heavy smokers. In this case, large upregulation of nAChRs occurs even though most of the extra nAChRs remain inside the cells, creating a mismatch between the increase in total nAChRs and increase in functional nAChRs on the cell surface. These findings highlight the plastic properties of cholinergic neurotransmission in the adrenal medulla to provide robust mechanisms for adapting catecholamine release to acute and chronic changes in sympathetic activity.

Key role of the nicotinic receptor in neurotransmitter exocytosis in human chromaffin cells

The whole-cell secretory response evoked by acetylcholine (ACh) in human chromaffin cells was examined using a new protocol based on quickly switching from the voltage-clamp to the current-clamp (CC) configuration of the patch-clamp technique. Our experiments revealed that Ca(2+) entry through the nicotinic receptor at hyperpolarized membrane potentials contributed as much to the exocytosis (100.4 +/- 27.3 fF) evoked by 200 ms pulses of ACh, as Ca(2+) flux through voltage-dependent Ca(2+) channels at depolarized membrane potentials. The nicotinic current triggered a depolarization event with a peak at +49.3 mV and a 'plateau' phase that ended at -23.9 mV, which was blocked by 10 mumol/L mecamylamine. When a long ACh stimulus (15 s) was applied, the nicotinic current at the end of the pulse reached a value of 15.45 +/- 3.6 pA, but the membrane potential depolarization still remained at the 'plateau' stage until withdrawal of the agonist. Perfusion with 200 mumol/L Cd(2+) during the 15 s ACh pulse completely abolished the plasma membrane depolarization at the end of the pulse, indicating that Ca(2+) entry through Ca(2+) channels contributed to the membrane potential depolarization provoked by prolonged ACh pulses. These findings also reflect that voltage-dependent Ca(2+) channels were recruited by the small current flowing through the desensitized nicotinic receptor to maintain the depolarization. Finally, muscarinic receptor activation triggered a delayed exocytotic process after prolonged ACh stimulation, dependent on Ca(2+) mobilization from the endoplasmic reticulum. In summary, we show here that nicotinic and muscarinic receptors contribute to the exocytosis of neurotransmitters in human chromaffin cells, and that the nicotinic receptor plays a key role in several stages of the stimulus-secretion coupling process in these cells.

Urinary catecholamine excretion in tetanus

Imperfect understanding of the pathophysiology of tetanus has limited therapeutic advances. Autonomic disturbance is a major cause of mortality and is believed to be associated with catecholamine release, predominantly norepinephrine. We measured epinephrine and norepinephrine concentrations in 24-h urine collections from tetanus and critically ill patients suffering from other severe diseases. In patients with severe tetanus, mean (SD) epinephrine was 164.18 (129.37) nmol x day(-1) compared with 45.18 (37.74) nmol x day(-1) in mild-moderate disease (p = 0.008). In the severe group, mean (SD) norepinephrine was 411.64 (208.5), and 121.00 (81.81) nmol x day(-1) in moderately ill patients (p < 0.001). Compared with critically ill control patients, median epinephrine was 331.77 in tetanus patients and 89.70 nmol x day(-1) in controls (p < 0.001). Median norepinephrine concentration was 788.02 nmol x day(-1) in tetanus and 300.05 nmol x day(-1) in control patients, p = 0.006. The study finds a novel result of increased epinephrine excretion in tetanus and confirms that catecholamine excretion in tetanus exceeds that in other critically ill patients. These results should be considered in designing more effective therapeutic strategies.

beta2-Subunit-containing nicotinic acetylcholine receptors are involved in nicotine-induced increases in conditioned reinforcement but not progressive ratio responding for food in C57BL/6 mice

RATIONALE

Nicotine administration potentiates conditioned reinforcement in rats, an effect that persists for weeks after chronic exposure. Little is known regarding the nicotinic receptor subtypes that may mediate this effect.

OBJECTIVE

The purpose of this study was to determine whether beta2-subunit-containing nicotinic acetylcholine receptors (beta2*nAChRs) are necessary for lasting effects of nicotine on conditioned and primary reinforcement in mice.

METHODS

Beta2 knockout (beta2KO) and wild-type (WT) mice received 14 days of nicotine exposure (NIC, 200 microg/ml in 2% saccharin) or saccharin alone (SAC) in their drinking water. Five days later, mice received paired presentations of a conditioned stimulus (CS) with water unconditioned stimulus (US) or explicitly unpaired presentations of the CS and US during Pavlovian discriminative approach training. Training was followed by two conditioned reinforcement tests. Mice were subsequently tested for food-reinforced responding in the absence of explicit cues followed by a progressive ratio test.

RESULTS

During conditioned reinforcement testing, only mice in the paired condition showed increased responding in the CS-reinforced aperture over inactive apertures. WT-NIC mice showed enhanced conditioned reinforcement compared to WT-SAC animals. beta2KO-SAC mice showed elevated conditioned reinforcement compared to WT-SAC subjects, but beta2KO-NIC and beta2KO-SAC mice did not differ in responding with conditioned reinforcement. Prior nicotine exposure did not alter food-reinforced responding but resulted in elevated break points for food in both genotypes.

CONCLUSION

These data show that nicotine exposure enhances conditioned reinforcement in mice and indicate that beta2*nAChRs are necessary for nicotine-dependent enhancement of incentive aspects of motivation but not motivation for primary reinforcement measured by progressive ratio responding.

Evaluation of benzyltetrahydroisoquinolines as ligands for neuronal nicotinic acetylcholine receptors

Effects of derivatives of coclaurine (C), which mimic the 'eastern' or the nonquaternary halves of the alkaloids tetrandrine or d-tubocurarine, respectively, both of which are inhibitors of nicotinic acetylcholine receptors (nACh), were examined on recombinant, human alpha7, alpha4beta2 and alpha4beta4 nACh receptors expressed in Xenopus oocytes and clonal cell lines using two-electrode voltage clamping and radioligand binding techniques. In this limited series, Cs have higher affinity and are most potent at alpha4 subunit-containing-nACh receptors and least potent at homomeric alpha7 receptors, and this trend is very marked for the N-unsubstituted C and its O,O'-bisbenzyl derivative. 7-O-Benzyl-N-methylcoclaurine (BBCM) and its 12-O-methyl derivative showed the highest affinities and potencies at all three receptor subtypes, and this suggests that lipophilicity at C7 and/or C12 increases potency. Laudanosine and armepavine (A) were noncompetitive and voltage-dependent inhibitors of alpha7, alpha4beta2 or alpha4beta4 receptors, but the bulkier C7-benzylated 7BNMC (7-O-benzyl-N-methylcoclaurine) and 7B12MNMC (7-O-benzyl-N,12-O-dimethyl coclaurine) were voltage-independent, noncompetitive inhibitors of nACh receptors. Voltage-dependence was also lost on going from A to its N-ethyl analogue. These studies suggest that C derivatives may be useful tools for studies characterising the antagonist and ion channel sites on human alpha7, alpha4beta2 or alpha4beta4 nACh receptors and for revealing structure-function relationships for nACh receptor antagonists.

Influence of lobeline on catecholamine release from the isolated perfused rat adrenal gland

It has been shown that lobeline (alpha-lobeline) is a lipophilic, nonpyridine, naturally occurring alkaloid obtained from Indian tobacco, Lobelia inflata. The present study was attempted to investigate the effect of lobeline on secretion of catecholamines (CA) evoked by ACh, high K(+), 1.1-dimethyl-4-phenyl piperazinium iodide (DMPP) and (3-(m-chloro-phenyl-carbamoyl-oxy)-2-butynyl trimethyl ammonium chloride (McN-A-343) from the isolated perfused rat adrenal gland and to establish the mechanism of its action. l-Lobeline (30-300 microM) perfused into an adrenal vein for 60 min produced dose- and time-dependent inhibition in CA secretory responses evoked by ACh (5.32 x 10(-3) M), DMPP (10(-4) M for 2 min) and McN-A-343 (10(-4) M for 2 min). However, lower dose of lobeline did not affect CA secretion by high K(+) (5.6 x 10(-2) M), higher dose of it reduced greatly CA secretion of high K(+). l-Lobeline itself did also fail to affect basal catecholamine output. Furthermore, in adrenal glands loaded with lobeline (100 microM), CA secretory response evoked by methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-pyridine-5-carboxylate (Bay-K-8644), an activator of L-type Ca(2+) channels was markedly inhibited while CA secretion by cyclopiazonic acid, an inhibitor of cytoplasmic Ca(2+)-ATPase was not affected. However, nicotine (30 microM), given into the adrenal gland for 60 min, initially rather enhanced CA secretory responses evoked by ACh (5.32 x 10(-3) M) and high K(+) (5.6 x 10(-2) M) followed by great inhibition later, while responses evoked by DMPP (10(-4) M for 2 min) and McN-A-343 (10(-4) M for 2 min) were greatly inhibited. Taken together, these results suggest that lobeline inhibits greatly CA secretion evoked by stimulation of cholinergic (both nicotinic and muscarinic) receptors. Lobeline at lower dose does not affect that by membrane depolarization, but at larger dose inhibits that. It is thought that this inhibitory effect of lobeline may be mediated by blocking the calcium influx into the rat adrenal medullary chromaffin cells without the inhibition of Ca(2+) release from the cytoplasmic calcium store, which is relevant to its nicotinic antagonistic activity. It also seems that there is a difference in the mode of action between nicotine and lobeline in rat adrenomedullary CA secretion.

Ca2+ permeability of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are expressed in muscle cells and neurons, as well as in an increasing number of other cell types. The nAChR channels are permeable to cations, including Ca(2+). Ca(2+) entry through nAChR channels has been shown to modulate several Ca(2+)-dependent cellular processes, such as neurotransmitter release, synaptic plasticity, and cell motility. The value of Ca(2+) permeability associated to a particular nAChR subtype thus represents an important indication for its physiological role. This review summarizes the quantitative data on Ca(2+) permeability obtained from several nAChR subtypes in native and heterologous systems. Different experimental approaches are compared, and the structural determinants of Ca(2+) permeability are discussed.

Differential effect of nicotinic agonists on the [3H]norepinephrine release from rat hippocampal slices

The aim of this study was to investigate the mechanisms involved in the effect of nicotinic agonists on the [3H]norepinephrine ([3H]NE) release from rat hippocampal slices. The stimulatory effect of nicotine, cytisine, epibatidine and anatoxin-A was completely blocked by the nicotinic antagonist mecamylamine (10 microM). In contrast, the effect of dimethylphenylpiperazinium (DMPP) was only partially inhibited by mecamylamine but was completely blocked by the NE uptake inhibitor desipramine (DMI, 10 microM). Finally, the effect of lobeline was not affected by mecamylamine and was only partially blocked by DMI. Our data indicate that the majority of nicotinic agonists increase the release of [3H]NE exclusively via stimulation of nicotinic acetylcholine receptors (nAChRs). DMPP, in addition to the stimulation of nAChRs, also evokes a carrier-mediated release. Lobeline has no stimulatory effect on nAChRs, induces a carrier-mediated release and has a further action of unidentified mechanism. Our results suggest that special caution is required for the interpretation of data, when DMPP or lobeline are used as nicotinic agonists.

Receptor protection studies to characterize neuronal nicotinic receptors: tubocurarine prevents alkylation of adrenal nicotinic receptors

Our laboratory has evidence that multiple nicotinic acetylcholine receptor subtypes regulate bovine adrenal catecholamine release. In the following studies, receptor protection assays were used to differentiate adrenal nicotinic receptor subpopulations. Under alkylating conditions, bromoacetylcholine (30 microM) reduced nicotinic receptor-stimulated adrenal catecholamine secretion by approximately 80%. When 100 microM tubocurarine was present during alkylation, nicotine-stimulated secretion was reduced by less than 30%. Hexamethonium (500 microM), decamethonium (500 microM), mecamylamine (50 microM), pentolinium (50 microM), adiphenine (50 microM), methyllycaconitine (1 microM) and alpha-bungarotoxin (1 microM) afforded no protection when present during alkylation. When the pharmacology of residual, tubocurarine-protected receptors was investigated, the EC50 value for nicotine's stimulatory effects on secretion significantly increased from 4.0 (2.5-6.5) microM in control cells to 9.1 (7.2-11.4) microM in tubocurarine-protected cells. In addition, the IC50 value for tubocurarine's inhibitory effects on release significantly decreased from 0.7 (0.5-0.9) microM in control cells to 0.3 (0.2-0.4) microM in tubocurarine-protected cells. These studies support the use of protection assays to characterize nicotinic receptor subpopulations.

Inhibition of nicotinic receptor-mediated catecholamine secretion by Dryobalanops aromatica in bovine adrenal chromaffin cells

Effect of the aqueous extract from a medicinal plant Dryobalanops aromatica(Dipterocarpaceae) on catecholamine secretion was investigated in bovine adrenal chromaffin cells. The aqueous extract inhibited [(3)H]norepinephrine ([(3)H]NE) secretion induced by 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP), a nicotinic acetylcholine receptor (nAChR) agonist, with a half-maximal inhibitory concentration (IC(50)) of 8.4 +/- 1.7 microgml(-1). Increases in cytosolic calcium ([Ca(2+)](i)) and sodium ([Na(+)](i)) induced by DMPP were also inhibited by the extract. However, the binding of [(3)H]nicotine to nAChRs was not affected by the addition of the extract in receptor binding competition analysis, suggesting that active components in the extract and nicotine do not share the binding site in the nAChR. On the other hand, [Ca(2+)](i)increases induced by high K(+), ionomycin, bradykinin, angiotensin II, and thapsigargin were not inhibited by the extract. The data suggest that the extract from D. aromatica specifically inhibits catecholamine secretion by blocking nAChR in a noncompetitive manner.

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.

Antagonism of nicotinic receptors of rat chromaffin cells by N,N, N-trimethyl-1-(4-trans-stilbenoxy)-2-propylammonium iodide: a patch clamp and ligand binding study

The effect of the oxystilbene derivative F3 was tested on nAChRs of whole-cell patch-clamped rat chromaffin cells in vitro and of rat adrenal gland membranes using (125)I-epibatidine. F3 (30 nM) rapidly and reversibly blocked inward currents generated by pulse applications of nicotine, shifting the dose-response curve to the right in a parallel fashion without changing the maximum response. The action of F3 was voltage insensitive and not due to altered current reversal potential. The R isomer of F3 was more potent (IC(50) = 350+/-30 nM) than its S-enantiomer (IC(50) = 1.5+/-0.3 microM). Nicotine-evoked currents were insensitive to 10 microM alpha-bungarotoxin. Equi-amplitude currents evoked by nicotine or epibatidine were similarly antagonized by R-F3 in a reversible fashion. Epibatidine-evoked currents readily produced receptor desensitization. Adrenal membranes specifically bound (125)I-epibatidine with a single population of binding sites endowed with high affinity (K(D) = 159 pM) and B(max) of 6.5+/-1.3 fmol mg(-1) of protein. (125)I-epibatidine binding was specifically displaced by cytisine (K(i) = 68 nM) or ACh (K(i) = 348 nM). F3 specifically displaced (125)I-epibatidine binding although with lower affinity (K(i) = 29.6 microM) than in electrophysiological experiments. (125)I-epibatidine binding to rat adrenal tissue was insensitive to alpha-bungarotoxin which readily antagonized (125)I-epibatidine binding to bovine adrenal tissue. The present results suggest that F3 is a relatively potent and apparently competitive antagonist of nAChRs on rat chromaffin cells. Since previous studies have indicated that F3 targets different subtypes on chick neuronal tissue, it appears that nAChRs display interspecies differences to be considered for drug development studies.

Potentiation and inhibition of neuronal alpha4beta4 nicotinic acetylcholine receptors by choline

The effects of choline on alpha4beta4 nicotinic acetylcholine receptors, expressed in Xenopus oocytes, were investigated using in the two-microelectrode voltage clamp technique. Particular attention was paid to the interaction between the effects of acetylcholine and choline. Choline was a low-affinity agonist of alpha4beta4 receptors with an efficacy of 10% as compared to acetylcholine. Responses evoked by 1 microM acetylcholine were potentiated by low concentrations of choline and inhibited by > 10mM choline, resulting in a bell-shaped concentration-effect relationship. Conversely, the effects of choline on responses evoked by 300 microM acetylcholine resulted in a monophasic inhibition curve with an IC(50) of 0.87 mM. The data were fitted by a two-site receptor occupation model, which accounts for similar effects of various cholinergic ligands on heteromeric nicotinic receptors. The results indicate that the potentiation was a competitive effect, whereas the inhibition was due to a mixture of competitive and non-competitive effects. It is concluded that choline acts as a potent, endogenous co-agonist at heteromeric alpha4beta4 nicotinic receptors.

The influence of lobeline on nucleus accumbens dopamine and locomotor responses to nicotine in nicotine-pretreated rats

In vivo brain microdialysis was used to investigate the influence of lobeline on dopamine (DA) and dihydroxyphenylacetic acid (DOPAC) overflow in the core of the nucleus accumbens of freely-moving rats pretreated with nicotine (0.4 mg x kg(-1), s.c., once per day for 5 days). Locomotion was also recorded. Lobeline, at doses of 0.7, 4.0 and 10.0 mg x kg(-1), i.p., failed to elicit any significant changes in extracellular dopamine or dihydroxyphenylacetic acid levels during the 60 min following its administration and did not stimulate locomotor. The dopamine responses to nicotine (0.4 mg x kg(-1), s.c.), were abolished (P<0.01) if the nicotine challenge was administered 10 min but not 60 min, after lobeline doses of 4.0 and 10.0 mg kg(-1), i.p., but were unaffected following lobeline at the lowest dose tested (0.7 mg x kg(-1), i.p.) at either time. The increase in locomotor activity was significantly attenuated (P<0.01), to a similar extent, when the nicotine was injected 10 min, but not 60 min, after all three doses of lobeline (0.7, 4.0 and 10.0 mg kg(-1), i.p.) when compared with the saline-treated rats. The results suggest that lobeline is a short-acting antagonist of the nicotinic AChRs which mediate the effects of nicotine on mesolimbic dopamine activity and locomotor stimulation.

Catecholamine secretion from bovine adrenal chromaffin cells induced by the dextrorotatory isomer of anatoxin-a

1. The nicotinic agonist (+)anatoxin-a was studied in acute preparations of adrenal chromaffin cells and was compared with other known stimulants in this system. 2. (+)Anatoxin-a was found to be a potent stimulant of catecholamine secretion with EC50=545.7 nM, which was 5.8 times as strong as nicotine (EC50=3,165 nM). (+)Anatoxin-a action was time dependent and saturable. 3. The pharmacological characteristics of (+)anatoxin-a were tested by using nicotinic and muscarinic antagonists (mecamylamine and atropine, respectively). Mecamylamine (1 microM) and atropine (100 microM) inhibited the secretion induced by (+)anatoxin-a (1 microM), as well as that induced by nicotine (10 microM), acetylcholine (10 microM and 100 microM) and oxotremorine-M (100 microM). 4. The calcium requirement for (+)anatoxin-a action was tested in comparison with the aforementioned stimulants. Addition of the calcium antagonist verapamil (10 microM) or the calcium chelator EGTA (3 mM) reduced all stimulants' action. 5. These results show that the (+)enantiomer of anatoxin-a is both dose and time dependent. Its action is mediated through the classical operation of the nicotinic acetylcholine receptor, by using calcium influx.

Activation and inhibition of the human alpha7 nicotinic acetylcholine receptor by agonists

To better understand the effects of weak as well as strong agonists at the human alpha7 nicotinic acetylcholine receptor (human alpha7 nAChR), the abilities of several classic nAChR agonists to both activate and inhibit (desensitize) the human alpha7 nAChR expressed in Xenopus oocytes were quantified and compared. Activation was measured during 0.2-20 s agonist application, as required to elicit a peak response. Inhibition was measured as the reduction in the agonist response to 200 microM ACh in the presence of inhibitor during a 5-20 min incubation. Acetylcholine (ACh), (-)-nicotine, (+)-nicotine, and 1,1-dimethyl-4-phenylpiperazinium (DMPP) were 62- to 130-fold more potent as inhibitors than as activators, with excellent correlation between the IC50 and EC50 values (r2 = 0.924). Agonist concentrations that elicited only 0.6-1.2% nAChR activation were sufficient to inhibit the response to ACh by 50%. Thus, even a very weak agonist could appear to be a potent and effective inhibitor through receptor desensitization. (-)-Lobeline, in contrast, acted as an antagonist at the human alpha7 nAChR, eliciting no detectable agonist-like response at concentrations up to 1 mM, but inhibiting the response to ACh with an IC50 value of 8.5 microM. (-)-Cotinine and the novel ligand ABT-089 [2-methyl-3-(2-(S)-pyrrolidinylmethoxy)pyridine] acted as weak agonists at the human alpha7 nAChR (1 and 1.5% response at 1 mM, respectively) and inhibited the response to ACh with IC50) values of 175 and 48 microM, respectively. These effects could be explained by receptor desensitization, at least in part.

An adrenal slice preparation for the study of chromaffin cells and their cholinergic innervation

Thin slices (200-300 microm) of adrenal glands were prepared from Wistar rats. Patch-clamp recordings were made from visually identified chromaffin cells using the whole-cell and amphotericin B perforated-patch techniques. Electrophysiological properties of chromaffin cells in slices were similar to those in cultured cells. Catecholamine release from single chromaffin cells or cell clusters in slices was also measured by amperometry. Immunostaining of slices with an antineurofilament antibody revealed the presence of neuronal fibers. Acetylcholine release was stimulated either by raising external [K+] or by focally applying voltage pulses. Nicotinic excitatory postsynaptic currents (EPSCs) were detected, ranging from 20 pA to several hundreds of pA. Amplitude distributions of spontaneous EPSCs revealed clear equidistant peaks, supporting a quantal model for acetylcholine release onto chromaffin cells. The adrenal slice preparation therefore appears to be an excellent model for studying both the cholinergic innervation of chromaffin cells as well as catecholamine release from these cells.

Changes in intracellular Na+ concentration evoked by nicotinic receptor activation in the guinea-pig adrenal chromaffin cells

Using the whole-cell voltage clamp technique and microfluorometry with sodium-binding benzofuran isophthalate (SBFI), a nicotine-induced inward current and increase in the intracellular Na+ concentration ([Na+]in) were examined simultaneously in guinea-pig adrenal chromaffin cells. The increase in [Na+]in expected from the time-integrated inward current was well correlated with that of [Na+]in measured with SBFI. The ratio of the expected [Na+]in to the measured [Na+]in was 0.64 at -85mV and decreased with increasing holding potentials. The decay time constant of the increased [Na+]in was not affected by ouabain. It is concluded that the Na+ entering the cell is diffusable in about 60% cell volume without fast buffering mechanisms and is eliminated by the exchange of Na+ between the pipette solution and cell interior under the patch clamp condition.

Imaging of intracellular calcium during desensitization of nicotinic acetylcholine receptors of rat chromaffin cells

1. The possible role of intracellular Ca2+ levels ([Ca2+]i) in desensitization of nicotinic acetylcholine receptors (AChRs) was investigated in rat cultured chromaffin cells by use of combined whole-cell patch clamping and confocal laser scanning microscopy with the fluorescent dye fluo-3. 2. On cells held at -70 mV, pressure-application of nicotine elicited inward currents with associated [Ca2+]i rises mainly due to influx through nicotinic AChRs. These responses were blocked by (+)-tubocurarine (10 microM) but were insensitive to alpha-bungarotoxin (1 microM) or Cd2+ (0.1 mM). 3. Pressure applications of 1 mM nicotine for 2 s (conditioning pulse) evoked inward currents which faded biexponentially to a steady state level due to receptor desensitization and were accompanied by a sustained increase in [Ca2+]i. Inward currents evoked by subsequent application of brief test pulses of nicotine were depressed but recovered with a time course reciprocal to the decay of the [Ca2+]i transient induced by the conditioning pulse. 4. Omission of intracellular Ca2+ chelators or use of high extracellular Ca2+ solution (10 mM) lengthened recovery of nicotinic AChRs from desensitization while adding BAPTA or EGTA intracellularly had the opposite effect. When the patch pipette contained fluo-3 or no chelators, after establishing whole cell conditions the rate of recovery became progressively longer presumably due to dialysis of endogenous Ca2+ buffers. None of these manipulations of external or internal Ca2+ had any effect on onset or steady state level of desensitization. 5. High spatial resolution imaging of [Ca2+]i in intact cells (in the presence of 0.1 mM Cd2+) showed that its level in the immediate submembrane area decayed at the same rate as in the rest of the cell, indicating that Ca2+ was in a strategic location to modulate (directly or indirectly) AChR desensitization. 6. The present data suggest that desensitized nicotinic AChRs are stabilized in their conformation by raised [Ca2+]i and that this phenomenon retards their recovery to full activity.

Potentiation and inhibition of neuronal nicotinic receptors by atropine: competitive and noncompetitive effects

Atropine, the classic muscarinic receptor antagonist, inhibits ion currents mediated by neuronal nicotinic acetylcholine receptors expressed in Xenopus laevis oocytes. At the holding potential of -80 mV, 1 microM atropine inhibits 1 mM acetylcholine-induced inward currents mediated by rat alpha2beta2, alpha2beta4, alpha3beta2, alpha3beta4, alpha4beta2, alpha4beta4, and alpha7 nicotinic receptors by 12-56%. Inward currents induced with a low agonist concentration are equally inhibited (alpha3beta2, alpha3beta4), less inhibited (alpha2beta4, alpha7), or potentiated (alpha4beta2, alpha4beta4) by 1 microM atropine. Effects on the more sensitive alpha4beta4 nicotinic receptors were investigated in detail by systematic variation of acetylcholine and atropine concentrations and of membrane potential. At high agonist concentration, atropine inhibits alpha4beta4 nicotinic receptor-mediated ion current in a noncompetitive, voltage-dependent way with IC50 values of 655 nM at -80 mV and of 4.5 microM at -40 mV. At low agonist concentration, 1 microM atropine potentiates alpha4beta4 nicotinic receptor-mediated ion current. This potentiating effect is surmounted by high concentrations of acetylcholine, indicating a competitive interaction of atropine with the nicotinic receptor, and potentiation is also reversed at high atropine concentrations. Steady state effects of acetylcholine and atropine are accounted for by a model for combined receptor occupation and channel block, in which atropine acts on two distinct sites. The first site is associated with noncompetitive ion channel block. The second site is associated with competitive potentiation, which appears to occur when the agonist recognition sites of the receptor are occupied by acetylcholine and atropine. The apparent affinity of atropine for the agonist recognition sites of the alpha4beta4 nicotinic acetylcholine receptor is estimated to be 29.9 microM.

Measurement of intracellular Na+ concentration by a Na+-sensitive fluorescent dye, sodium-binding benzofuran isophthalate, in porcine adrenal chromaffin cells--usage of palytoxin as a Na+ ionophore

Palytoxin was found to equilibrate sodium ions (Na+) across the cell membrane much faster than dose gramicidin, which has been frequently used to calibrate the intracellular Na+ concentration ([Na+]in), in cells treated with a Na+-sensitive fluorescent dye, sodium-binding benzofuran isophthalate (SBFI). Palytoxin was capable of equilibrating Na+ in cells treated with SBFI-acetoxymethyl ester (SBFI-AM) and in voltage-clamped cells loaded with SBFI through a patch pipette. Nicotine caused a dose-dependent increase in ([Na+]in) in porcine adrenal chromaffin cells treated with SBFI-AM and caused a simultaneous increase in [Na+]in and inward current in the voltage-clamped cells loaded with SBFI. Palytoxin has an advantage of calibrating ([Na+]in) in a shorter time than dose gramicidin because of its powerful ionophoretic activity.

Nicotine and sympathetic neurotransmission

Nicotine increases heart rate, myocardial contractility, and blood pressure. These nicotine-induced cardiovascular effects are mainly due to stimulation of sympathetic neurotransmission, as nicotine stimulates catecholamine release by an activation of nicotine acetylcholine receptors localized on peripheral postganglionic sympathetic nerve endings and the adrenal medulla. The nicotinic acetylcholine receptor is a ligand-gated cation channel with a pentameric structure and a central pore with a cation gate, which is essential for ion selectivity and permeability. Binding of nicotine to its extracellular binding site leads to a conformational change of the central pore, which results in the influx of sodium and calcium ions. The resulting depolarization of the sympathetic nerve ending stimulates calcium influx through voltage-dependent N-type calcium channels, which triggers the nicotine-evoked exocytotic catecholamine release. In the isolated perfused guinea-pig heart, cardiac energy depletion sensitizes cardiac sympathetic nerves to the norepinephrine-releasing effect of nicotine, as indicated by a leftward shift of the concentration-response curve, a potentiation of maximum transmitter release, and a delay of the tachyphylaxis of nicotine-evoked catecholamine release. This sensitization was also shown to occur in the human heart under in vitro conditions. Through the intracardiac release of norepinephrine, nicotine induces a beta-adrenoceptor-mediated increase in heart rate and contractility, and an alpha-adrenoceptor-mediated increase in coronary vasomotor tone. The resulting simultaneous increase in oxygen demand and coronary resistance has a detrimental effect on the oxygen balance of the heart, especially in patients with coronary artery disease. Sensitization of the ischemic heart to the norepinephrine-releasing effect of nicotine may be a trigger for acute cardiovascular events in humans, such as acute myocardial infarction and/or life-threatening ventricular tachyarrhythmias.

Single-channel recording in brain slices reveals heterogeneity of nicotinic receptors on individual neurons within the chick lateral spiriform nucleus

We examined the functional properties of central nicotinic acetylcholine receptors at the single-channel level using tight-seal, voltage-clamp techniques. Single-channel currents were recorded from cell-attached patches on lateral spiriform neurons in chick brain slices. These neurons are known to express functional nicotinic receptors that are insensitive to the antagonists alpha-bungarotoxin and kappa-bungarotoxin, and which exhibit a high affinity for nicotine and other nicotinic agonists. Single-channel openings were observed in 84% of patches (n = 118) when the nicotinic agonists acetylcholine (1-100 microM), carbamylcholine (3-100 microM), or nicotine (3-10 microM) were present in the patch pipette. In contrast, single-channels were markedly reduced in number or entirely absent when the nicotinic antagonist dihydro-beta-erythroidine was present along with acetylcholine (n = 7) or when no agonist was present in the pipette (n = 22). Single-channel openings displayed inward rectification at depolarized potentials, and were dependent on extracellular sodium. Between 1 and 30 microM acetylcholine, a dose-response relationship was observed between agonist concentration and single-channel open probability during the first minute following seal formation. Multiple classes of single nicotinic channels, with calculated mean slope conductances of 15, 31, 40, and approximately 70 pS, were observed in membrane patches on different neurons within the lateral spiriform nucleus, and even within single patches on individual neurons. We conclude that neurons within the lateral spiriform nucleus express functionally heterogeneous nicotinic receptors and that in some neurons different nicotinic receptor subtypes are present in close proximity to each other on the same cell surface.

Role of [Na+]i and [Ca2+]i in nicotine-induced norepinephrine release from bovine adrenal chromaffin cells

Intracellular free sodium ([Na+]i) and calcium ([Ca2+]i) concentrations were determined by sodium-binding benzofuran isophthalate (SBFI) and fura 2 microfluorimetry, respectively, in bovine adrenal chromaffin cells (BCC). Validation of SBFI microfluorimetry by in vitro and in vivo calibration revealed a reliable assessment of [Na+]i within a range of 1-30 mM in single BCC. Nicotine (0.1-10 microM) induced concentration-dependent increases of both [Na+]i (from 3.3 +/- 0.1 to 25.6 +/- 0.4 mM, n = 76, P < 0.001) and [Ca2+]i (from 64 +/- 1 to 467 +/- 16 nM, n = 87, P < 0.001), which were accompanied by an increase in [3H]norepinephrine (NE) release. Consistent with an exocytotic release mechanism, nicotine-induced increments of [Ca2+]i and [3H]NE release were reduced under calcium-free conditions and by gadolinium chloride (40 microM), whereas [Na+]i was not affected. In contrast, a parallel attenuation of nicotine-evoked changes in [Na+]i, [Ca2+]i, and [3H]NE release was observed during reduction of the extracellular sodium concentration. The nicotine-evoked responses were neutralized by the nicotinic receptor antagonist hexamethonium (100 microM) but not by blockade of voltage-dependent sodium channels (1 microM tetrodotoxin). In conclusion, the nicotine-induced exocytotic release of [3H]NE is triggered by an increase in [Ca2+]i, which is facilitated by sodium influx through the nicotinic receptor ionophore.

Cation channel activated by muscarinic agonists on porcine adrenal chromaffin cells

A large portion (70%) of the secretory response to muscarinic agonists in porcine adrenal chromaffin cells has previously been shown to be dependent on extracellular Ca2+ (Xu et al., J. Neurochem. 56: 1899-1896, 1991). Results presented here show that muscarinic agonists activate a cation-selective channel which is permeable to divalent cations. The muscarinic agonist, methacholine, was found to activate the uptake of Mn2+, which paralleled the ability of methacholine to activate 45Ca2+ uptake as shown previously. Secretion induced by methacholine was not affected by nifedipine, a compound that inhibits dihydropyridine-sensitive voltage-gated Ca2+ channels. In voltage-clamped cells, methacholine activated whole cell currents, which reversed at approximately -20 mV in standard salt solutions. However, with the standard whole cell configuration, the currents were slow to activate and were often erratic. In contrast, when the perforated-patch (nystatin) technique was used to measure whole cell currents, methacholine rapidly activated sustained inward currents. Ion-substitution experiments indicated that the inward currents were carried by Na+, Ba2+, or Ca2+ but not by Cl-. Single-channel currents activated by methacholine were observed in outside-out vesicles, which were electrically accessed using the perforated-patch technique. These channels reversed at -15 mV, had a slope conductance of 20 pS, and were 14-fold more likely to be open in the presence of methacholine. These channels are probably responsible for the extracellular Ca(2+)-dependent secretory response to muscarinic receptor stimulation in porcine adrenal chromaffin cells.

Monovalent and divalent cation permeability and block of neuronal nicotinic receptor channels in rat parasympathetic ganglia

Acetylcholine-evoked currents mediated by activation of nicotinic receptors in rat parasympathetic neurons were examined using whole-cell voltage clamp. The relative permeability of the neuronal nicotinic acetylcholine (nACh) receptor channel to monovalent and divalent inorganic and organic cations was determined from reversal potential measurements. The channel exhibited weak selectivity among the alkali metals with a selectivity sequence of Cs+ > K+ > Rb+ > Na+ > Li+, and permeability ratios relative to Na+ (Px/PNa) ranging from 1.27 to 0.75. The selectivity of the alkaline earths was also weak, with the sequence of Mg2+ > Sr2+ > Ba2+ > Ca2+, and relative permeabilities of 1.10 to 0.65. The relative Ca2+ permeability (PCa/PNa) of the neuronal nACh receptor channel is approximately fivefold higher than that of the motor endplate channel (Adams, D. J., T. M. Dwyer, and B. Hille. 1980. Journal of General Physiology. 75:493-510). The transition metal cation, Mn2+ was permeant (Px/PNa = 0.67), whereas Ni2+, Zn2+, and Cd2+ blocked ACh-evoked currents with half-maximal inhibition (IC50) occurring at approximately 500 microM, 5 microM and 1 mM, respectively. In contrast to the muscle endplate AChR channel, that at least 56 organic cations which are permeable to (Dwyer et al., 1980), the majority of organic cations tested were found to completely inhibit ACh-evoked currents in rat parasympathetic neurons. Concentration-response curves for guanidinium, ethylammonium, diethanolammonium and arginine inhibition of ACh-evoked currents yielded IC50's of approximately 2.5-6.0 mM. The organic cations, hydrazinium, methylammonium, ethanolammonium and Tris, were measureably permeant, and permeability ratios varied inversely with the molecular size of the cation. Modeling suggests that the pore has a minimum diameter of 7.6 A. Thus, there are substantial differences in ion permeation and block between the nACh receptor channels of mammalian parasympathetic neurons and amphibian skeletal muscle which represent functional consequences of differences in the primary structure of the subunits of the ACh receptor channel.

Anatoxin-a is a potent agonist of the nicotinic acetylcholine receptor of bovine adrenal chromaffin cells

(+)-Anatoxin-a is a neurotoxic alkaloid produced by the cyanobacterium Anabaena flos-aquae. In this study synthetic (+/-)-anatoxin-a was tested on isolated bovine adrenal chromaffin cells to determine its ability to evoke secretion of endogenous catecholamines through neuronal-type nicotinic receptor activation. Anatoxin-a was found to act as a potent agonist of the secretory response of chromaffin cells with an EC50 of 1-2 microM, compared with an EC50 of 4-5 microM for nicotine. The cells responded to anatoxin-a and nicotine with bell-shaped concentration-response curves consistent with desensitisation at concentrations of anatoxin-a greater than 5 microM and of nicotine greater than 20 microM. The secretion of catecholamines stimulated by anatoxin-a was completely inhibited in a non-competitive manner by the nicotinic antagonist mecamylamine with an IC50 of 0.4-0.5 microM. In the presence of depolarising concentrations of K+ (15 or 50 mM), anatoxin-a increased the secretion of catecholamines in a concentration-dependent manner up to the same maximum as that achieved by anatoxin-a alone. It is concluded that anatoxin-a acts as a potent and selective nicotinic agonist, capable of evoking secretion of endogenous catecholamines from chromaffin cells via their neuronal-type nicotinic receptor.

Inhibition of nicotinic acetylcholine receptor channels in bovine adrenal chromaffin cells by Y3-type neuropeptide Y receptors via the adenylate cyclase/protein kinase A system

The effect of neuropeptide Y [NPY(1-36)] and related peptides on the voltage-dependent currents and the nicotinic acetylcholine receptor (nAChR) currents (IACh) of bovine adrenal chromaffin cells was investigated using the whole-cell patch clamp technique. Catecholamine release from single chromaffin cells was measured by means of fast cyclic voltammetry. The potency order of these peptides in inhibiting IACh evoked by nicotine was NPY(1-36), NPY (16-36) > peptide YY(PYY) > [Leu31, Pro34]NPY. NPY(16-36) produced a similar degree of inhibition, irrespective of whether nicotine or an equipotent concentration of acetylcholine was used to evoke IACh. NPY(16-36) failed to alter voltage-dependent inward or outward currents. Intracellular cAMP, and extracellular dibutyryl-cAMP, produced a slowly developing increase in IACh. Intracellular cAMP, extracellular 8-Br-cAMP or dibutyryl-cAMP, and an inhibitor of cyclic nucleotide phosphodiesterases 3-isobutyl-1-methyl-xanthine (IBMX), decreased the inhibitory effect of NPY(16-36) on IACh. Although the intracellular application of the cAMP-dependent protein kinase A inhibitor [PKI(14-24)amide] alone did not alter IACh, it potentiated the effect of NPY(16-36) in interaction experiments. While the NPY(16-36)-induced inhibition of IACh was reversed on washout of the peptide, the slightly shorter C-terminal fragment NPY(18-36) caused a long-lasting depression of both IACh and catecholamine secretion evoked by nicotine. This depression was smaller in the presence of extracellular 8-Br-cAMP than in its absence. NPY(18-36) did not alter the secretory activity induced by a high concentration of potassium. It appears that, by activating Y3-receptors, NPY inhibits nAChR-current and the resulting secretion of catecholamines from bovine chromaffin cells. This process may involve a G protein-mediated decrease in intracellular cAMP with a subsequent decrease in the degree of phosphorylation of the nAChR-channel.

Inhibition by cyclothiazide of neuronal nicotinic responses in bovine chromaffin cells

1. The desensitizing acetylcholine (ACh) response of bovine chromaffin cells maintained in culture was examined using rapid agonist applications (of 2 s duration) which imposed nominal drug concentrations within 50 ms. This study was aimed, firstly, at identifying which of the alpha 3, alpha 4 and alpha 7 subunits known to be present in these cells is predominant in the ACh-evoked response and secondly, on the effects on these neuronal nicotinic ACh receptors (AChR) of cyclothiazide (CT), an agent acting as a modulator of a gating desensitization site on other ligand-gated channels. 2. Locally applied 100 microM ACh evoked peak currents (IACh) of -1.5 +/- 0.1 nA (n = 83) at a holding potential of -60 mV. The ACh dose-response curve yielded an estimated EC50 of 60 microM. This current was not sustained but desensitized during the application period; it displayed strong inward rectification, but desensitized equally whether the evoked current was inward or outward going. The latter observation excludes alpha 4 as a major contributor to the recorded current. Because the response was almost insensitive to a 1 microM alpha-bungarotoxin pretreatment (IACh = -1.2 +/- 0.1 nA; n = 6), and because 1, 1-dimethyl-4-phenylpiperazinium (DMPP) works as a potent agonist (peak current = -1.9 nA, n = 2 for 100 microM DMPP), the alpha 7 subunit is also a minor contributor to the response. Taken together, these observations suggest a dominant alpha 3 type of response. 3. Triple exponential fits were used to describe the characteristics of the ACh-evoked currents; one component to fit the rising phase, with 2 components to describe the decay phase. The decay times were 100 ms and 4 s for the fast and slow components respectively. The rate of the slow decay component increased systematically with recording time, approximately doubling from its initial value within 20-40 min. Furthermore there was a gradual rundown of the response, seen first as a loss of the late component of the current, measured at 2 s, with the peak current amplitude decreasing later in the recording.4. CT, when coapplied with ACh, produced a dose-dependent inhibition of the ACh-evoked peak current. The effect showed little voltage-dependency with 100 microM CT producing 46 +/- 5% (s.d.; n = 3)and 47 +/- 8% (s.d.; n = 7) inhibition at -100 and -60 mV respectively. At + 60 mV, inhibition was estimated to be 26 +/- 7% (s.d.; n = 3).5. After pre-exposure of the cells to CT by bath application, 10 and 30 microM CT produced poorly reversible 20 +/- 9% (n = 7) and 42 +/- 5% (n = 4) inhibitions of the peak current respectively. There were no discernible effects on the fitted decay constants at any CT concentration tested, although an increased inhibitory effect of CT was observed at higher concentrations (100 microM) on the amplitude of the late component measured at 2 s.6. Similar effects were observed in conditions chosen to isolate the alpha 3 type of receptor: namely when using DMPP as an agonist, or after a-bungarotoxin pretreatment.7. The 2,3-benzodiazepine, GYKI 53655, is known to antagonize the action of CT on AMPA receptors.Coapplication of 50 microM GYKI 53655 with ACh (100 microM) produced a 29 +/- 4% inhibition of the peak ACh-evoked current and 44 +/- 6% inhibition of its amplitude at 2 s (n = 4). This response was fully reversible. Brief applications of both CT (100 microM) and GYKI 53655 (50 microM) with ACh via the microperfusion system produced a fully reversible inhibition that was not significantly different from the values obtained with either CT or GYKI 53655 alone, with 37 +/- 6% inhibition of peak and 61 +/- 9%inhibition of the amplitude at 2 s (n = 3).8. The results obtained suggest that the CT effect is to impede recovery from a slow desensitization,with a more pronounced effect with longer CT applications. Globally, CT favours the 'rundown state' of the neuronal nicotinic AChR.

Anaesthetic modulation of nicotinic ion channel kinetics in bovine chromaffin cells

1. We have investigated the action of the anaesthetics methoxyflurane, methohexitone and etomidate on the nicotinic acetylcholine receptor channel of bovine adrenal chromaffin cells using the whole cell patch clamp technique. 2. Spectral analysis of macroscopic currents evoked by 25 microM carbachol revealed that each of the agents tested reduced the lifetime of the channel open state in a dose-dependent manner. The whole cell current was inhibited in a concentration-dependent fashion by each agent. 3. Channel gating parameters were calculated from single channel studies and the results used to test models explaining the modulation of nicotinic acetylcholine receptor channels by anaesthetics. 4. Each of the agents studied reduced the mean channel open time in a concentration-dependent manner. Anaesthetic concentrations reducing mean open time by 50% were: 370 microM methoxyflurane, 30 microM methohexitone or 23 microM etomidate. 5. Methohexitone and etomidate produced an increase in the number of brief closures within bursts, while no such increase was observed with methoxyflurane. Despite these inter-burst gaps, mean burst length was reduced by each of the agents tested. 6. It is concluded that a simple sequential blocking model fails to account for the action of these anaesthetics. An extended model, in which blocked channels can close, may be applicable.

ABT-200, a norepinephrine uptake inhibitor, blocks Ca2+ signals in chromaffin cells

We previously described inhibition by racemic (+/-)-(1'R*,3R*)-3-phenyl-1- [1',2',3',4'-tetrahydro-5',6'-methylenedioxy-1'- napthalenyl-methyl]-pyrrolidine methanesulfonate (ABT-200), and its two constituent enantiomers, SS,ABT-200 and RR,ABT-200, of nicotine-stimulated but not histamine-stimulated catecholamine release from bovine adrenal chromaffin cells. To test the hypothesis that this inhibition reflects a blockade of Ca2+ influx, we used fura-2 loaded chromaffin cells to investigate cytosolic Ca2+ signals. We found that SS,ABT-200 inhibited nicotine- and K(+)-stimulated Ca2+ signals, both of which depend on Ca2+ influx. However, the early phase of the histamine-stimulated Ca2+ signals, which depends on Ca2+ mobilization from intracellular stores, was unaffected. We also examined ion flux through the nicotinic receptor by measuring 86rubidium+ (86Rb+) efflux from preloaded mouse midbrain synaptosomes. We found that SS,ABT-200 partially inhibited nicotine-stimulated 86Rb+ efflux, suggesting that it blocks ion flux through the nicotinic receptor directly. These data support a model in which ABT-200 blocks nicotine-stimulated catecholamine release by inhibiting cation flux through multiple channels.

Calcium signalling in bovine adrenal chromaffin cells: additive effects of histamine and nicotine

In a previous report, we described the ability of two secretogogues, histamine and nicotine, to stimulate additive effects on catecholamine (CA) release and synapsin II phosphorylation in bovine adrenal chromaffin cells (BACC) [Firestone and Browning (1992), J. Neurochem., 58:441-447]. We hypothesized that these results were due to the combined effects on cytosolic Ca++ of the two distinct signalling pathways. We therefore examined the intracellular Ca++ signals stimulated by histamine and nicotine, alone and together. In Ca(++)-deficient medium, nicotine-stimulated signals were abolished, whereas histamine-stimulated signals were maintained, demonstrating that nicotine depended entirely on Ca++ influx for its effects. Indeed, the nicotine-stimulated signal could also be prevented using a Ca++ channel blocker, nicardipine. Further, the observation that exposure of BACC to thapsigargin reduced histamine-stimulated Ca++ signals verified that histamine mobilizes Ca++ from intracellular stores. Thus, the two secretogogues mobilize Ca++ from distinct pools. When BACC were stimulated with the two secretogogues together, the resulting Ca++ signal was greater than that from either alone. These data are consistent with a model in which two distinct sources of Ca++ can summate within the cell, producing a greater Ca++ signal and, hence, a greater effect on neurotransmitter release.

In vitro pharmacologic characterization of a cholinergic receptor on outer hair cells

Acetylcholine (ACh) is the major neurotransmitter released from the efferent fibers in the cochlea onto the outer hair cells (OHCs). The type of ACh receptor on OHCs and the events subsequent to receptor activation are unclear. Therefore we studied the effect of agonists and antagonists of the ACh receptor on isolated OHCs from the guinea pig. OHCs were recorded from in whole cell voltage and current clamp configuration. ACh induced an increase in outward K+ current (IACh) which hyperpolarized the OHCs. No desensitization to ACh application was observed. Cs+ replaced K+ in carrying the IACh. The IACh is Ca(2+)-dependent, time and voltage sensitive, and different from the IKCa induced by depolarization of the membrane potential. When tested at 100 microM, several agonists also induced outward current responses (acetylcholine > suberyldicholine > or = carbachol > DMPP) whereas nicotine, cytisine and muscarine did not. The IACh response to 10 microM ACh was blocked by low concentrations of traditional and non-traditional-nicotinic antagonists (strychnine > curare > bicuculline > alpha-bungarotoxin > thimethaphan) and by higher concentrations of muscarinic antagonists (atropine > 4-DAMP > AF-DX 116 > pirenzepine). Pharmacologically, the ACh receptor on OHCs is nicotinic.

Barium permeability of neuronal nicotinic receptor alpha 7 expressed in Xenopus oocytes

The rat alpha 7 neuronal nicotinic acetylcholine receptor was expressed and studied in Xenopus oocytes. The magnitude and reversal potential of instantaneous whole cell currents were examined in solutions containing varying concentrations of either calcium or barium, and in the presence or absence of the intracellular calcium chelator BAPTA. In external barium, application of nicotine elicits an inwardly rectifying response; in calcium the response is larger and has a linear IV relation. Pretreatment of oocytes with BAPTA-AM could not prevent activation of calcium-dependent chloride channels in external Ringer containing calcium. Using an extended GHK equation, the permeability ratio PBa/PNa of the alpha 7 receptor was determined to be about 17. Our results suggest that alpha 7 nicotinic receptors are highly permeable to divalent cations.

Mutations at two distinct sites within the channel domain M2 alter calcium permeability of neuronal alpha 7 nicotinic receptor

The relative permeability for sodium, potassium, and calcium of chicken alpha 7 neuronal nicotinic receptor was investigated by mutagenesis of the channel domain M2. Mutations in the "intermediate ring" of negatively charged residues, located at the cytoplasmic end of M2 (site 1), reduce calcium permeability without significantly modifying other functional properties (activation and desensitization) of the receptor; a similar change of ion selectivity is also noticed when mutations at site 1 are done in the context of a receptor mutant that conducts ions in a desensitized state. Moreover, mutations of two adjacent rings of leucines at the synaptic end of M2 (site 2) have multiple effects. They abolish calcium permeability, increase the apparent affinity for acetylcholine by 10- to 100-fold, augment Hill numbers (up to 4.6-5.0) of acetylcholine dose-response relationships, slow rates of ionic response onset, and lower the extent of desensitization. Mutations at these two topographically distinct sites within M2 selectively alter calcium transport without affecting the relative permeabilities for sodium and potassium.