Serotonergic modulation of muscle acetylcholine receptors of different subunit composition

The mechanisms of potentiation and inhibition of GABAA receptors by non-steroidal anti-inflammatory drugs, mefenamic and niflumic acids

Fenamates mefanamic and niflumic acids (MFA and NFA) induced dual potentiating and inhibitory effects on GABA currents recorded in isolated cerebellar Purkinje cells using the whole-cell patch-clamp and fast-application techniques. Regardless of the concentration, both drugs induced a pronounced prolongation of the current response. We demonstrated that the same concentration of drugs can produce both potentiating and inhibitory effects, depending on the GABA concentration, which indicates that both processes take place simultaneously and the net effect depends on the concentrations of both the agonist and fenamate. We found that the NFA-induced block is strongly voltage-dependent. The Woodhull analysis of the block suggests that NFA has two binding sites in the pore - shallow and deep. We built a homology model of the open GABA_AR based on the cryo-EM structure of the open α1 GlyR and applied Monte-Carlo energy minimization to optimize the ligand-receptor complexes. A systematic search for MFA/NFA binding sites in the GABA_AR pore revealed the existence of two sites, the location of which coincides well with predictions of the Woodhull model. In silico docking suggests that two fenamate molecules are necessary to occlude the pore. We showed that MFA, acting as a PAM, competes with an intravenous anesthetic etomidate for a common binding site. We built structural models of MFA and NFA binding at the transmembrane β(+)/α(-) intersubunit interface. We suggested a hypothesis on the molecular mechanism underlying the prolongation of the receptor lifetime in open state after MFA/NFA binding and β subunit specificity of the fenamate potentiation.

Calcium influx through muscle nAChR-channels: One route, multiple roles

Although Ca²⁺ influx through muscle nAChR-channels has been described over the past 40 years, its functions remain still poorly understood. In this review we suggest possible roles of Ca²⁺ entry at all stages of muscle development, summarizing the evidence present in literature. nAChRs are expressed in myoblasts prior to fusion, and can be activated in the absence of an ACh-releasing nerve terminal, with Ca²⁺ influx likely contributing to regulate cell fusion. Upon establishment of nerve-muscle contact, Ca²⁺ influx contributes to orchestrate the signaling required for the correct formation of the neuromuscular junction. Finally, in the mature synapse, Ca²⁺ entry through postsynaptic nAChR-channels - highly Ca²⁺ permeable, in particular in humans - acts on K⁺ and Na⁺ channels to shape endplate excitability. However, when genetic defects cause excessive channel activation, Ca²⁺ influx becomes toxic and causes endplate myopathy. Throughout the review, we highlight how Ricardo Miledi has contributed to construct this whole body of knowledge, from the initial description of Ca²⁺ permeability of endplate nAChR channels, to the rationale for the treatment of endplate excitotoxic damage under pathological conditions. This article is part of a Special Issue entitled: SI: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

Methylpiperidinium Iodides as Novel Antagonists for α7 Nicotinic Acetylcholine Receptors

The α7 nicotinic acetylcholine receptor (nAChR) is expressed in neuronal and non-neuronal cells and is involved in several physiopathological processes, and is thus an important drug target. We have designed and synthesized novel piperidine derivatives as α7 nAChR antagonists. Thus, we describe here a new series of 1-[2-(4-alkoxy-phenoxy-ethyl)]piperidines and 1-[2-(4-alkyloxy-phenoxy-ethyl)]-1-methylpiperidinium iodides (compounds 11a-11c and 12a-12c), and their actions on α7 nAChRs. The pharmacological activity of these compounds was studied in rat CA1 hippocampal interneurons by using the whole-cell voltage-clamp technique. Inhibition of the choline-induced current was less for 11a-11c than for the methylpiperidinium iodides 12a-12c and depended on the length of the aliphatic chain. Those compounds showing strong effects were studied further using molecular docking and molecular dynamics simulations. The strongest and non-voltage dependent antagonism was shown by 12a, which could establish cation–π interactions with the principal (+)-side and van der Waals interactions with the complementary (-)-side in the α7 nAChRs. Furthermore, compound 11a forms hydrogen bonds with residue Q115 of the complementary (-)-side through water molecules without forming cation–π interactions. Our findings have led to the establishment of a new family of antagonists that interact with the agonist binding cavity of the α7 nAChR, which represent a promising new class of compounds for the treatment of pathologies where these receptors need to be negatively modulated, including neuropsychiatric disorders as well as different types of cancer.

Effects of the antidepressant mirtazapine and zinc on nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) and zinc are associated with regulation of mood and related disorders. In addition, several antidepressants inhibit muscle and neuronal nAChRs and zinc potentiates inhibitory actions of them. Moreover, mirtazapine (a noradrenergic, serotonergic and histaminergic antidepressant) inhibits muscarinic AChRs and its effects on nAChRs are unknown. Therefore, we studied the modulation of muscle α1β1γd nAChRs expressed in oocytes and native α7-containing nAChRs in hippocampal interneurons by mirtazapine and/or zinc, using voltage-clamp techniques. The currents elicited by ACh in oocytes (at -60 mV) were similarly inhibited by mirtazapine in the absence and presence of 100 μM zinc (IC₅₀ ∼15 μM); however, the ACh-induced currents were stronger inhibited with 20 and 50 μM mirtazapine in the presence of zinc. Furthermore, the potentiation of ACh-induced current by zinc in the presence of 5 μM mirtazapine was 1.48 ± 0.06, and with 50 μM mirtazapine zinc potentiation did not occur. Interestingly, in stratum radiatum interneurons (at -70 mV), 20 μM mirtazapine showed less inhibition of the current elicited by choline (Ch) than at 10 μM (0.81 ± 0.02 and 0.74 ± 0.02 of the Ch-induced current, respectively). Finally, the inhibitory effects of mirtazapine depended on membrane potential: 0.81 ± 0.02 and 0.56 ± 0.05 of the control Ch-induced current at -70 and -20 mV, respectively. These results indicate that mirtazapine interacts with muscle and neuronal nAChRs, possibly into the ion channel; that zinc may increase the sensitivity of nAChRs to mirtazapine; and that mirtazapine decreases the sensitivity of nAChRs to zinc.

Dual effects of a 2-benzylquinuclidinium derivative on α7-containing nicotinic acetylcholine receptors in rat hippocampal interneurons

Nicotinic acetylcholine receptors (nAChRs) are widely distributed in the brain. Particularly α7-containing nAChRs, associated with several physiological roles and pathologies, are one of the most abundant. Here, we studied 2-(4-hexyloxybenzyl)-1-methylquinuclidin-1-ium iodide (designated as 8d), on ion currents elicited by choline, ICh, (Ch, a selective agonist for α7-containing nAChRs), recorded in interneurons from the stratum radiatum of the rat hippocampal CA1 region by using the whole-cell voltage-clamp technique. The 8d-concentration/Ch-response relationship exhibited high and low inhibitory affinities for α7-containing nAChRs, with IC50 values of 0.59 and 6.80 μM, respectively. Interestingly, 8d in a range of 3-10 μM exerted opposite effects: a short early potentiation and a long late inhibition of the ICh; and 8d alone elicited a non-decaying inward current. Furthermore, potentiation and inhibition of the ICh by 8d depended on the membrane potential, both being stronger at -20 than at -70 mV; indicating that 8d interacts with at least two sites into the ion channel/receptor complex: one for potentiating and another for inhibiting the α7-containing nAChRs. These results suggest that 8d may act as agonist, antagonist and positive modulator of α7-containing nAChRs in hippocampal interneurons.

Dopamine and serotonin modulate human GABAρ1 receptors expressed in Xenopus laevis oocytes

GABAρ1 receptors are highly expressed in bipolar neurons of the retina and to a lesser extent in several areas of the central nervous system (CNS), and dopamine and serotonin are also involved in the modulation of retinal neural transmission. Whether these biogenic amines have a direct effect on ionotropic GABA receptors was not known. Here, we report that GABAρ1 receptors, expressed in X. laevis oocytes, were negatively modulated by dopamine and serotonin and less so by octopamine and tyramine. Interestingly, these molecules did not have effects on GABA(A) receptors. 5-Carboxamido-tryptamine and apomorphine did not exert evident effects on any of the receptors. Schild plot analyses of the inhibitory actions of dopamine and serotonin on currents elicited by GABA showed slopes of 2.7 ± 0.3 and 6.1 ± 1.8, respectively, indicating a noncompetitive mechanism of inhibition. The inhibition of GABAρ1 currents was independent of the membrane potential and was insensitive to picrotoxin, a GABA receptor channel blocker and to the GABAρ-specific antagonist (1,2,5,6-tetrahydropyridine-4-yl)methyl phosphinic acid (TPMPA). Dopamine and serotonin changed the sensitivity of GABAρ1 receptors to the inhibitory actions of Zn(2+). In contrast, La(3+) potentiated the amplitude of the GABA currents generated during negative modulation by dopamine (EC(50) 146 μM) and serotonin (EC(50) 196 μM). The functional role of the direct modulation of GABAρ receptors by dopamine and serotonin remains to be elucidated; however, it may represent an important modulatory pathway in the retina, where GABAρ receptors are highly expressed and where these biogenic amines are abundant.

Muscle and neuronal nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are integral membrane proteins and prototypic members of the ligand-gated ion-channel superfamily, which has precursors in the prokaryotic world. They are formed by the assembly of five transmembrane subunits, selected from a pool of 17 homologous polypeptides (alpha1-10, beta1-4, gamma, delta, and epsilon). There are many nAChR subtypes, each consisting of a specific combination of subunits, which mediate diverse physiological functions. They are widely expressed in the central nervous system, while, in the periphery, they mediate synaptic transmission at the neuromuscular junction and ganglia. nAChRs are also found in non-neuronal/nonmuscle cells (keratinocytes, epithelia, macrophages, etc.). Extensive research has determined the specific function of several nAChR subtypes. nAChRs are now important therapeutic targets for various diseases, including myasthenia gravis, Alzheimer's and Parkinson's diseases, and schizophrenia, as well as for the cessation of smoking. However, knowledge is still incomplete, largely because of a lack of high-resolution X-ray structures for these molecules. Nevertheless, electron microscopy studies on 2D crystals of nAChR from fish electric organs and the determination of the high-resolution X-ray structure of the acetylcholine binding protein (AChBP) from snails, a homolog of the extracellular domain of the nAChR, have been major steps forward and the data obtained have important implications for the design of subtype-specific drugs. Here, we review some of the latest advances in our understanding of nAChRs and their involvement in physiology and pathology.

Antiemetics of the 5-hydroxytryptamine 3A antagonist class inhibit muscle nicotinic acetylcholine receptors

Antagonists of the serotonergic 5-hydroxytryptamine 3A receptor (5-HT(3A)R) and muscle nicotinic acetylcholine receptors (nAChR) are widely used in anesthesia practice. Both 5-HT(3A)R and nAChR are ligand-gated ion channels with known pharmacological overlap between some of their agonists and antagonists. We studied the actions of clinically used 5-HT(3A)R antagonist antiemetics and nondepolarizing muscle blockers on ionic currents elicited by the activation of mammalian 5-HT(3A)R and muscle nAChR, expressed in Xenopus laevis oocytes. Currents were recorded using a whole-cell two-electrode voltage clamp technique. Dolasetron, ondansetron, and granisetron reversibly inhibited 5-HT(3A)R function at nanomolar concentrations with 50% inhibitory concentrations (IC(50)) of 11.8, 6.4, and 0.2 nM; the rank order of inhibition correlated well with their clinical antiemetic potencies. The principal metabolite of dolasetron, hydrodolasetron, was 40 times more potent than the parent compound on 5-HT(3A)R (IC(50) = 0.29 nM). The potency of the nondepolarizing muscle blocker d-tubocurarine in blocking 5-HT(3A)R was similar to that of the antiemetics and significantly more than vecuronium and rapacuronium (IC(50) = 11.4 nM, 18.9 microM, 60.5 microM). Conversely, ondansetron, dolasetron, and granisetron also reversibly inhibited nAChR currents in a dose-dependent manner with IC(50)s of 14.2, 7.8, and 4.4 microM for the adult nAChR and 16.0, 18.6, and 13.9 microM for the embryonic nAChR. Again, hydrodolasetron showed significantly (10 times) more inhibitory potency on the adult nAChR than the parent compound dolasetron. These results indicate that drugs that target specific ligand-gated ion channels may also affect other ion channel types.

The acetylcholinesterase inhibitor BW284c51 is a potent blocker of Torpedo nicotinic AchRs incorporated into the Xenopus oocyte membrane

This work was aimed to determine if 1,5-bis(4-allyldimethylammoniumphenyl)pentan-3-one dibromide (BW284c51), the most selective acetylcholinesterase inhibitor (AchEI), affects the nicotinic acetylcholine (Ach) receptor (AchR) function. Purified Torpedo nicotinic AchRs were injected into Xenopus laevis oocytes and BW284c51 effects on Ach- and carbamylcholine (Cch)-elicited currents were assessed using the voltage-clamp technique.BW284c51 (up to 1 mM) did not evoke any change in the oocyte membrane conductance. When BW284c51 (10 pM-100 microM) and Ach were co-applied, Ach-evoked currents (I(Ach)) were reversibly inhibited in a concentration-dependent manner (Hill coefficient, 1; IC(50), 0.2-0.5 muM for 0.1-1000 microM Ach). Cch-elicited currents showed a similar inhibition by BW284c51.I(Ach) blockade by BW284c51 showed a strong voltage dependence, being only apparent at hyperpolarising potentials. BW284c51 also enhanced I(Ach) desensitisation.BW284c51 changed the Ach concentration-dependence curve of Torpedo AchR response from two-site to single-site kinetics, without noticeably affecting the EC(50) value. The BW284c51 blocking effect was highly selective for nicotinic over muscarinic receptors. BW284c51 inhibition potency was stronger than that of tacrine, and similar to that of d-tubocurarine (d-TC). Coapplication of BW284c51 with either tacrine or d-TC revealed synergistic inhibitory effects. Our results indicate that BW284c51 antagonises nicotinic AchRs in a noncompetitive way by blocking the receptor channel, and possibly by other, yet unknown, mechanisms. Therefore, besides acting as a selective AchEI, BW284c51 constitutes a powerful and reversible blocker of nicotinic AchRs that might be used as a valuable tool for understanding their function.

Marine sponges as pharmacy

Marine sponges have been considered as a gold mine during the past 50 years, with respect to the diversity of their secondary metabolites. The biological effects of new metabolites from sponges have been reported in hundreds of scientific papers, and they are reviewed here. Sponges have the potential to provide future drugs against important diseases, such as cancer, a range of viral diseases, malaria, and inflammations. Although the molecular mode of action of most metabolites is still unclear, for a substantial number of compounds the mechanisms by which they interfere with the pathogenesis of a wide range of diseases have been reported. This knowledge is one of the key factors necessary to transform bioactive compounds into medicines. Sponges produce a plethora of chemical compounds with widely varying carbon skeletons, which have been found to interfere with pathogenesis at many different points. The fact that a particular disease can be fought at different points increases the chance of developing selective drugs for specific targets.

Combined actions of zinc and fluoxetine on nicotinic acetylcholine receptors

Zinc and nicotinic acetylcholine receptors (nAChRs) seem to be associated with major depression, and some antidepressants, including fluoxetine (Prozac), antagonize nAChRs. Therefore, a study was made of the modulation of neuronal alpha4beta4 and muscle alpha1beta1gammadelta nAChRs, expressing in oocytes, by the combined action of zinc and fluoxetine. At a holding potential of -60 mV, 200 microM zinc increased by 361% the currents elicited by acetylcholine (ACh currents) for alpha4beta4 and by 182% for alpha1beta1gammadelta nAChRs. In contrast, 5 microM fluoxetine reduced the ACh currents to 31% for alpha4beta4 and to 45% for alpha1beta1gammadelta nAChRs. Additionally, fluoxetine reduced more the ACh currents in the presence of zinc: to 17% for alpha4beta4 and to 19% for alpha1beta1gammadelta nAChRs, and after washing out the fluoxetine the ACh current did not recover its zinc-potentiated value. Moreover, when ACh-activated nAChRs were exposed first to fluoxetine and then zinc was added, the potentiating effect of zinc was very small for muscle nAChRs and was nil for neuronal receptors. Thus, the inhibiting effect of fluoxetine prevails over the potentiating action of zinc. Finally, the effects of both zinc and fluoxetine were voltage independent, indicating that these substances interact outside the ion channel. As fluoxetine nullifies the effects of zinc, it appears that both substances interact in the same site. These results should help understand better the roles played by zinc, antidepressants, nAChRs and their combination in brain functions and in the treatment of depression.

Effects of clomipramine on neuronal nicotinic acetylcholine receptors

The action of the tricyclic antidepressant clomipramine on membrane currents elicited by acetylcholine was studied in Xenopus oocytes expressing neuronal alpha2beta4 nicotinic acetylcholine receptors. Clomipramine inhibited the acetylcholine responses rapidly and reversibly, with a similar IC(50) when the oocytes were preincubated with clomipramine (1.3+/-0.2 microM) or when they were exposed simultaneously with acetylcholine and clomipramine (1.5+/-0.3 microM). The EC(50) was 39.9+/-2.1 microM for acetylcholine alone and 65.7+/-3.6 microM for acetylcholine in the presence of 2 microM clomipramine. The inhibitory effect of clomipramine was weakly voltage-dependent, with an electric distance of approximately 0.14. Moreover, clomipramine increased the rate of decay of currents elicited by acetylcholine. From all of these, we conclude that clomipramine reversibly and noncompetitively regulates neuronal alpha2beta4 nicotinic acetylcholine receptors by blocking the open receptor-channel complex at a site close to the extracellular vestibule of the channel. The actions of clomipramine on neuronal nicotinic acetylcholine receptors may play an important role in the treatment of mental depression and other mood disorders.

Inhibition of neuronal nicotinic acetylcholine receptors by La(3+)

A study was made of the effects of La(3+) on neuronal alpha 2 beta 4 nicotinic acetylcholine receptors expressed in Xenopus oocytes. La(3+) by itself (up to 10 microM) did not elicit significant membrane currents. However, La(3+) reversibly inhibited the ionic currents induced by acetylcholine (IC(50)=13.5+/-4.3 microM). When La(3+) and acetylcholine were simultaneously applied onto an oocyte, the level of inhibition of the acetylcholine response was the same as when the oocyte was first preincubated with La(3+) and then exposed to acetylcholine plus La(3+). In the presence of La(3+), the EC(50) decreased from 43.8+/-6.4 to 26.5+/-5.1 microM, suggesting a small increase in the affinity of acetylcholine for the receptors through a noncompetitive mechanism. The inhibition of acetylcholine response was independent of the membrane potential. From these results we conclude that La(3+) regulates nicotinic receptors, reversibly and noncompetitively, presumably by inhibiting allosterically the receptor through interactions at an external domain of the receptor complex.

Antagonism of nicotinic acetylcholine receptors by inhibitors of monoamine uptake

A study was made of the effects of several monoamine-uptake inhibitors on membrane currents elicited by acetylcholine (ACh-currents) generated by rat neuronal alpha2beta4 and mouse muscle nicotinic acetylcholine receptors (AChRs) expressed in Xenopus laevis oocytes. For the two types of receptors the monoamine-uptake inhibitors reduced the ACh-currents albeit to different degrees. The order of inhibitory potency was norfluoxetine > clomipramine > indatraline > fluoxetine > imipramine > zimelidine > 6-nitro-quipazine > trazodone for neuronal alpha2beta4 AChRs, and norfluoxetine > fluoxetine > imipramine > clomipramine > indatraline > zimelidine > trazodone > 6-nitro-quipazine for muscle AChRs. Thus, the most potent inhibitor was norfluoxetine, whilst the weakest ones were trazodone, 6-nitro-quipazine and zimelidine. Effects of the tricyclic antidepressant imipramine were studied in more detail. Imipramine inhibited reversibly and non-competitively the ACh-current with a similar inhibiting potency for both neuronal alpha2beta4 and muscle AChRs. The half-inhibitory concentrations of imipramine were 3.65 +/- 0.30 microM for neuronal alpha2beta4 and 5.57 +/- 0.19 microM for muscle receptors. The corresponding Hill coefficients were 0.73 and 1.2 respectively. The inhibition of imipramine was slightly voltage-dependent, with electric distances of approximately 0.10 and approximately 0.12 for neuronal alpha2beta4 and muscle AChRs respectively. Moreover, imipramine accelerated the rate of decay of ACh- currents of both muscle and neuronal AChRs. The ACh-current inhibition was stronger when oocytes, expressing neuronal alpha2beta4 or muscle receptors, were preincubated with imipramine alone than when it was applied after the ACh-current had been generated, suggesting that imipramine acts also on non-activated or closed AChRs. We conclude that monoamine-uptake inhibitors reduce ACh-currents and that imipramine regulates reversibly and non- competitively neuronal alpha2beta4 and muscle AChRs through similar mechanisms, perhaps by interacting externally on a non-conducting state of the AChR and by blocking the open receptor-channel complex close to the vestibule of the channel. These studies may be important for understanding the regulation of AChRs as well as for understanding antidepressant- and side-effects of monoamine-uptake inhibitors.

Modulation of alpha2beta4 neuronal nicotinic acetylcholine receptors by zinc

A study was made of the modulation of nicotinic acetylcholine receptors by the divalent cation zinc. Rat neuronal nicotinic receptors (alpha2beta4) were expressed in Xenopus oocytes and membrane currents evoked by acetylcholine (ACh currents) were recorded using a two microelectrode voltage clamp. In non-injected oocytes, or in oocytes expressing alpha2beta4 receptors, Zn2+ by itself (1 microM-4 mM) generated only very small membrane currents. In contrast, in oocytes expressing alpha2beta4 receptors, Zn2+ greatly and reversibly increased the ACh current, without affecting considerably its time course. The ACh current potentiation by Zn2+ was weakly dependent on the membrane potential (2.33+/-0.10 times the control current at -100 mV vs 2.04+/-0.06 at -60 mV, suggesting that Zn2+ interacts with the receptor in the vestibule of the ion channel or at an external domain of the protein. The inward rectification of control and Zn2+-potentiated ACh-currents was similar. We conclude that Zn2+ positively and reversibly modulates neuronal nicotinic receptors in a practically voltage-independent manner and without affecting their rate of desensitization. These results will help to understand better the roles played by Zn2+ in brain functions.

Activation of volume-regulated Cl(-) channels by ACh and ATP in Xenopus follicles

Osmolarity-dependent ionic currents from follicle-enclosed Xenopus oocytes (follicles) were studied using electrophysiological techniques. Whole follicle currents were monitored using a two-electrode voltage clamp and single-channel activity was measured using the patch-clamp technique. In follicles held at -60 mV two chloride currents were activated in external hyposmotic solutions. One was the habitual volume-regulated current elicited by external hyposmolarity (ICl,swell), and the second was a slow and smooth current (Sin) generated by ACh or ATP application. In follicles, the permeability ratios for different anions with respect to Cl- were similar for both ICl,swell and Sin, with a sequence of: SCN- > I- > Br- >= NO3- >= Cl- > gluconate >= cyclamate > acetate > SO42-. Extracellular ATP blocked the outward component of Sin. Also, extracellular pH modulated the inactivation kinetics of Sin elicited by ACh; e.g. inactivation at +80 mV was approximately 100 % slower at pH 8.0 compared with that at pH 6.0. Lanthanides inhibited ICl, swell and Sin. La3+ completely inhibited ICl,swell with a half-maximal inhibitory concentration (IC50) of 17 +/- 1.9 microM, while Sin was blocked up to 55 % with an apparent IC50 of 36 +/- 2.6 microM. Patch-clamp recordings in follicular cells showed that hyposmotic challenge opened inward single-channel currents. The single channel conductance (4.7 +/- 0.4 pS) had a linear current-voltage relationship with a reversal membrane potential close to -20 mV. This single-channel activity was increased by application of ACh or ATP. The ICl,swell generation was not affected by pirenzepine or metoctramine, and did not affect the purinergic activation of the chloride current named Fin. Thus, ICl,swell was not generated via neurotransmitters released during cellular swelling. All together, equal discrimination for different anions, similar modulatory effects by extracellular pH, the blocking effects by ATP and La3+, and the same single-channel activity, strongly suggest that ICl,swell and Sin currents depend on the opening of the same type or a closely related class of volume-regulated chloride channels.

Nongenomic effects of steroids on the nicotinic acetylcholine receptor

A fast signaling mode of natural and synthetic steroids is exerted on some ion channels and cell-surface receptors. This activity contrasts with their classic mode of action, via intracellular receptors. Early studies from our laboratory demonstrated that spin-labeled androstanol and cholestane interact with the nicotinic acetylcholine receptor (AChR) and that lipid mobility at the lipid belt surrounding the AChR is reduced relative to that of the bulk membrane lipid. The occurrence of discrete and independent sites for phospholipids and sterols, both accessible to fatty acids, was subsequently disclosed in the native membrane. Synthetic and natural glucocorticoids were found to act as noncompetitive inhibitors of AChR function. The influence of different substituent groups in the cyclepentane perhydrophenanthrene ring on the channel-shortening potency of various steroids has also been assayed in muscle-type AChR, and we found a certain selectivity of this effect. Some organochlorine pesticides are xenoestrogens, that is, environmental agents capable of disrupting endocrine system signaling. We determined their effects on the AChR membrane using novel fluorescence techniques.

5-hydroxytryptamine interaction with the nicotinic acetylcholine receptor

The present study examines the interaction of the neurotransmitter 5-hydroxytryptamine (5-HT) with muscle-type nicotinic acetylcholine receptors. 5-HT inhibits the initial rate of [125I]alpha-bungarotoxin binding to Torpedo acetylcholine receptor membranes (IC(50)=8.5+/-0.32 mM) and [3H]5-HT can be photoincorporated into acetylcholine receptor subunits, with labeling of the alpha-subunit inhibitable by both agonists and competitive antagonists. Within the agonist-binding domain, [3H]5-HT photoincorporates into alphaTyr(190), alphaCys(192) and alphaCys(193). Functional studies using the human clonal cell line TE671/RD, show that 5-HT is a weak inhibitor (IC(50)=1.55+/-0.25 mM) of acetylcholine receptor activity. In this regard, agonist-response profiles in the absence and presence of 5-HT indicate a noncompetitive mode of inhibition. In addition, 5-HT displaces high affinity [3H]thienylcyclohexylpiperidine binding to the desensitized Torpedo acetylcholine receptor channel (IC(50)=1.61+/-0.07 mM). Collectively, these results indicate that 5-HT interacts weakly with the agonist recognition site and inhibits receptor function noncompetitively by binding to the acetylcholine receptor channel.

Modulation of nicotinic acetylcholine receptors by strychnine

Strychnine, a potent and selective antagonist at glycine receptors, was found to inhibit muscle (alpha1beta1gammadelta, alpha1beta1gamma, and alpha1beta1delta) and neuronal (alpha2beta2 and alpha2beta4) nicotinic acetylcholine receptors (AcChoRs) expressed in Xenopus oocytes. Strychnine alone (up to 500 microM) did not elicit membrane currents in oocytes expressing AcChoRs, but, when applied before, concomitantly, or during superfusion of acetylcholine (AcCho), it rapidly and reversibly inhibited the current elicited by AcCho (AcCho-current). Although in the three cases the AcCho-current was reduced to the same level, its recovery was slower when the oocytes were preincubated with strychnine. The amount of AcCho-current inhibition depended on the receptor subtype, and the order of blocking potency by strychnine was alpha1beta1gammadelta > alpha2beta4 > alpha2beta2. With the three forms of drug application, the Hill coefficient was close to one, suggesting a single site for the receptor interaction with strychnine, and this interaction appears to be noncompetitive. The inhibitory effects on muscle AcChoRs were voltage-independent, and the apparent dissociation constant for AcCho was not appreciably changed by strychnine. In contrast, the inhibitory effects on neuronal AcChoRs were voltage-dependent, with an electrical distance of approximately 0.35. We conclude that strychnine regulates reversibly and noncompetitively the embryonic type of muscle AcChoR and some forms of neuronal AcChoRs. In the former case, strychnine presumably inhibits allosterically the receptor by binding at an external domain whereas, in the latter case, it blocks the open receptor-channel complex.

Binding sites for exogenous and endogenous non-competitive inhibitors of the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) is the paradigm of the neurotransmitter-gated ion channel superfamily. The pharmacological behavior of the AChR can be described as three basic processes that progress sequentially. First, the neurotransmitter acetylcholine (ACh) binds the receptor. Next, the intrinsically coupled ion channel opens upon ACh binding with subsequent ion flux activity. Finally, the AChR becomes desensitized, a process where the ion channel becomes closed in the prolonged presence of ACh. The existing equilibrium among these physiologically relevant processes can be perturbed by the pharmacological action of different drugs. In particular, non-competitive inhibitors (NCIs) inhibit the ion flux and enhance the desensitization rate of the AChR. The action of NCIs was studied using several drugs of exogenous origin. These include compounds such as chlorpromazine (CPZ), triphenylmethylphosphonium (TPMP+), the local anesthetics QX-222 and meproadifen, trifluoromethyl-iodophenyldiazirine (TID), phencyclidine (PCP), histrionicotoxin (HTX), quinacrine, and ethidium. In order to understand the mechanism by which NCIs exert their pharmacological properties several laboratories have studied the structural characteristics of their binding sites, including their respective locations on the receptor. One of the main objectives of this review is to discuss all available experimental evidence regarding the specific localization of the binding sites for exogenous NCIs. For example, it is known that the so-called luminal NCIs bind to a series of ring-forming amino acids in the ion channel. Particularly CPZ, TPMP+, QX-222, cembranoids, and PCP bind to the serine, the threonine, and the leucine ring, whereas TID and meproadifen bind to the valine and extracellular rings, respectively. On the other hand, quinacrine and ethidium, termed non-luminal NCIs, bind to sites outside the channel lumen. Specifically, quinacrine binds to a non-annular lipid domain located approximately 7 A from the lipid-water interface and ethidium binds to the vestibule of the AChR in a site located approximately 46 A away from the membrane surface and equidistant from both ACh binding sites. The non-annular lipid domain has been suggested to be located at the intermolecular interfaces of the five AChR subunits and/or at the interstices of the four (M1-M4) transmembrane domains. One of the most important concepts in neurochemistry is that receptor proteins can be modulated by endogenous substances other than their specific agonists. Among membrane-embedded receptors, the AChR is one of the best examples of this behavior. In this regard, the AChR is non-competitively modulated by diverse molecules such as lipids (fatty acids and steroids), the neuropeptide substance P, and the neurotransmitter 5-hydroxytryptamine (5-HT). It is important to take into account that the above mentioned modulation is produced through a direct binding of these endogenous molecules to the AChR. Since this is a physiologically relevant issue, it is useful to elucidate the structural components of the binding site for each endogenous NCI. In this regard, another important aim of this work is to review all available information related to the specific localization of the binding sites for endogenous NCIs. For example, it is known that both neurotransmitters substance P and 5-HT bind to the lumen of the ion channel. Particularly, the locus for substance P is found in the deltaM2 domain, whereas the binding site for 5-HT and related compounds is putatively located on both the serine and the threonine ring. Instead, fatty acid and steroid molecules bind to non-luminal sites. More specifically, fatty acids may bind to the belt surrounding the intramembranous perimeter of the AChR, namely the annular lipid domain, and/or to the high-affinity quinacrine site which is located at a non-annular lipid domain. Additionally, steroids may bind to a site located on the extracellular hydrophi

Opposite effects of lanthanum on different types of nicotinic acetylcholine receptors

The effects of lanthanum (La3+) were studied on muscle and neuronal nicotinic acetylcholine receptors (AChRs) expressed in Xenopus oocytes. La3+ exerts a dose-dependent positive modulation on alpha1 beta1 gamma8 muscle AChRs, whereas it modulates negatively either alpha2 beta2, alpha2 beta4 or alpha3 beta4 neuronal AChRs. Moreover, La3+ appears to accelerate the desensitization of neuronal receptors. In both muscle and neuronal AChRs, the respective potentiating or inhibiting effects of La3+ on the ACh-currents are voltage-independent, suggesting that La3+ is acting at a site located in the external domain of the receptor.

Topology of ligand binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of both alpha subunits there exist the binding sites for agonists such as the neurotransmitter acetylcholine (ACh) and for competitive antagonists such as d-tubocurarine. Agonists trigger the channel opening upon binding while competitive antagonists compete for the former ones and inhibit its pharmacological action. Identification of all residues involved in recognition and binding of agonist and competitive antagonists is a primary objective in order to understand which structural components are related to the physiological function of the AChR. The picture for the localisation of the agonist/competitive antagonist binding sites is now clearer in the light of newer and better experimental evidence. These sites are mainly located on both alpha subunits in a pocket approximately 30-35 A above the surface membrane. Since both alpha subunits are sequentially identical, the observed high and low affinity for agonists on the receptor is conditioned by the interaction of the alpha subunit with the delta or the gamma chain, respectively. This relationship is opposite for curare-related drugs. This molecular interaction takes place probably at the interface formed by the different subunits. The principal component for the agonist/competitive antagonist binding sites involves several aromatic residues, in addition to the cysteine pair at 192-193, in three loops-forming binding domains (loops A-C). Other residues such as the negatively changed aspartates and glutamates (loop D), Thr or Tyr (loop E), and Trp (loop F) from non-alpha subunits were also found to form the complementary component of the agonist/competitive antagonist binding sites. Neurotoxins such as alpha-, kappa-bungarotoxin and several alpha-conotoxins seem to partially overlap with the agonist/competitive antagonist binding sites at multiple point of contacts. The alpha subunits also carry the binding site for certain acetylcholinesterase inhibitors such as eserine and for the neurotransmitter 5-hydroxytryptamine which activate the receptor without interacting with the classical agonist binding sites. The link between specific subunits by means of the binding of ACh molecules might play a pivotal role in the relative shift among receptor subunits. This conformational change would allow for the opening of the intrinsic receptor cation channel transducting the external chemical signal elicited by the agonist into membrane depolarisation. The ion flux activity can be inhibited by non-competitive inhibitors (NCIs). For this kind of drugs, a population of low-affinity binding sites has been found at the lipid-protein interface of the AChR. In addition, several high-affinity binding sites have been found to be located at different rings on the M2 transmembrane domain, namely luminal binding sites. In this regard, the serine ring is the locus for exogenous NCIs such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222, phencyclidine, and trifluoromethyliodophenyldiazirine. Trifluoromethyliodophenyldiazirine also binds to the valine ring, which is the postulated site for cembranoids. Additionally, the local anaesthetic meproadifen binding site seems to be located at the outer or extracellular ring. Interestingly, the M2 domain is also the locus for endogenous NCIs such as the neuropeptide substance P and the neurotransmitter 5-hydroxytryptamine. In contrast with this fact, experimental evidence supports the hypothesis for the existence of other NCI high-affinity binding sites located not at the channel lumen but at non-luminal binding domains. (ABSTRACT TRUNCATED)

Neuronal nicotinic threonine-for-leucine 247 alpha7 mutant receptors show different gating kinetics when activated by acetylcholine or by the noncompetitive agonist 5-hydroxytryptamine

Mutation of the highly conserved leucine residue (Leu-247) converts 5-hydroxytryptamine (5HT) from an antagonist into an agonist of neuronal homomeric alpha7 nicotinic acetylcholine receptor expressed in Xenopus oocytes. We show here that acetylcholine (AcCho) activates two classes of single channels with conductances of 44 pS and 58 pS, similar to those activated by 5HT. However, the mean open time of AcCho-gated ion channels (11 ms) is briefer than that of 5HT-gated ion channels (18 ms). Furthermore, whereas the open time of AcCho channels lengthens with hyperpolarization, that of 5HT channels is decreased. In voltage-clamped oocytes, the apparent affinity of the alpha7 mutant receptor for 5HT is not modified by the presence of dihydro-beta-erythroidine, which acts on the AcCho binding site in a competitive manner. This indicates a noncompetitive action of 5HT on nicotinic acetylcholine receptors. Considered together, our findings show that AcCho gates alpha7 mutant channels with similar conductance but with different kinetic profile than the channels gated by 5HT, suggesting that the two agonists act on different docking sites. These results will help to understand the crosstalk between cholinergic and serotonergic systems in the central nervous system.

Blockage of muscle and neuronal nicotinic acetylcholine receptors by fluoxetine (Prozac)

Fluoxetine (Prozac), a widely used antidepressant, is said to exert its medicinal effects almost exclusively by blocking the serotonin uptake systems. The present study shows that both muscle and neuronal nicotinic acetylcholine receptors are blocked, in a noncompetitive and voltage-dependent way, by fluoxetine, which also increases the rate of desensitization of the nicotinic receptors. Because these receptors are very widely distributed in the both central and peripheral nervous systems, the blocking action of fluoxetine on nicotinic receptors may play an important role in its antidepressant and other therapeutical effects. Our findings will help to understand the mode of action of fluoxetine, and they may also help to develop more specific medicinal drugs.

Threonine-for-leucine mutation within domain M2 of the neuronal alpha(7) nicotinic receptor converts 5-hydroxytryptamine from antagonist to agonist

A study was made of the effects of 5-hydroxytryptamine (5HT) on homomeric neuronal nicotinic receptors (nAcChoR) expressed in Xenopus oocytes after injection of cDNA encoding the wild-type chicken alpha(7) subunit. Acetylcholine (AcCho) elicited large currents (IAcCho) that were reduced by 5HT in a reversible and dose-dependent manner, with a half-inhibitory concentration (IC50) of 56 microM and a Hill coefficient (nH) of 1.2. The inhibition of IAcCho by 5HT was noncompetitive and voltage independent, a behavior incompatible with a channel blockade mechanism. 5HT alone did not elicit membrane currents in oocytes injected with the wild-type alpha(7) subunit cDNA. In contrast, 5HT elicited membrane currents (I5HT) in oocytes injected with cDNA encoding an alpha(7) mutant subunit with a threonine-for-leucine-247 substitution (L247T alpha(7)). I5HT was inhibited by the potent nicotinic receptor blockers alpha-bungarotoxin (100 nM) and methyllycaconitine (1 microM). Furthermore, the characteristics of I5HT, including its voltage dependence, were similar to those of IAcCho. The 5HT dose-I5HT response gave an apparent dissociation constant EC50 of 23.5 microM and a Hill coefficient nH of 1.7, which were not modified by the presence of AcCho. Similarly, the apparent affinity of L247T alpha(7) for AcCho as well as its cooperativity were not influenced by 5HT, indicating a lack of mutual interactions between 5HT and AcCho. These results show that 5HT is a potent noncompetitive antagonist of neuronal alpha(7) nAcChoR, but it becomes a noncompetitive agonist following mutation of the highly conserved leucine residue 247 located in the channel domain M2.