Role of local anesthetics on both cholinergic and serotonergic ionotropic receptors

0.75% ropivacaine may be a suitable drug in pregnant women undergoing urgent cesarean delivery during labor analgesia period

BACKGROUND

3% chloroprocaine (CP) has been reported as the common local anesthetic used in pregnant women undergoing urgent cesarean delivery during labor analgesia period. However, 0.75% ropivacaine is considered a promising and effective alternative. Therefore, we conducted a randomized controlled trial to compare the effectiveness and safety of 0.75% ropivacaine with 3% chloroprocaine for extended epidural anesthesia in pregnant women.

METHODS

We conducted a double-blind, randomized, controlled, single-center study from November 1, 2022, to April 30, 2023. We selected forty-five pregnant women undergoing urgent cesarean delivery during labor analgesia period and randomized them to receive either 0.75% ropivacaine or 3% chloroprocaine in a 1:1 ratio. The primary outcome was the time to loss of cold sensation at the T4 level.

RESULTS

There was a significant difference between the two groups in the time to achieve loss of cold sensation (303, 95%CI 255 to 402 S vs. 372, 95%CI 297 to 630 S, p = 0.024). There was no significant difference the degree of motor block (p = 0.185) at the Th4 level. Fewer pregnant women required additional local anesthetics in the ropivacaine group compared to the chloroprocaine group (4.5% VS. 34.8%, p = 0.011). The ropivacaine group had lower intraoperative VAS scores (p = 0.023) and higher patient satisfaction scores (p = 0.040) than the chloroprocaine group. The incidence of intraoperative complications was similar between the two groups, and no serious complications were observed.

CONCLUSIONS

Our study found that 0.75% ropivacaine was associated with less intraoperative pain treatment, higher patient satisfaction and reduced the onset time compared to 3% chloroprocaine in pregnant women undergoing urgent cesarean delivery during labor analgesia period. Therefore, 0.75% ropivacaine may be a suitable drug in pregnant women undergoing urgent cesarean delivery during labor analgesia period.

CLINICAL TRIAL NUMBER AND REGISTRY URL

The registration number: ChiCTR2200065201; http://www.chictr.org.cn , Principal investigator: MEN, Date of registration: 31/10/2022.

Effect of Local Anesthetics on Dipole Potential of Different Phase Membranes: A Fluorescence Study

The molecular mechanism behind the action of local anesthetics is not well understood. Phenylethanol (PEtOH) is an ingredient of essential oils with a rose-like odor, and it has previously been used as a local anesthetic. In this work, we explored the effect of PEtOH on dipole potential in membranes representing biologically relevant phases, employing the dual-wavelength ratiometric method utilizing the potential-sensitive probe di-8-ANEPPS. Our results show that PEtOH reduces membrane dipole potential in membranes of all biologically relevant phases (gel, liquid-ordered, and fluid) in a concentration-dependent manner. To the best of our knowledge, these results constitute one of the early reports describing reduction of membrane dipole potential induced by local anesthetics, irrespective of membrane phase.

Effect of tertiary amine local anesthetics on G protein-coupled receptor lateral diffusion and actin cytoskeletal reorganization

Although widely used clinically, the mechanism underlying the action of local anesthetics remains elusive. Direct interaction of anesthetics with membrane proteins and modulation of membrane physical properties by anesthetics are plausible mechanisms proposed, although a combination of these two mechanisms cannot be ruled out. In this context, the role of G protein-coupled receptors (GPCRs) in local anesthetic action is a relatively new area of research. We show here that representative tertiary amine local anesthetics induce a reduction in two-dimensional diffusion coefficient of the serotonin_1A receptor, an important neurotransmitter GPCR. The corresponding change in mobile fraction is varied, with tetracaine exhibiting the maximum reduction in mobile fraction, whereas the change in mobile fraction for other local anesthetics was not appreciable. These results are supported by quantitation of cellular F-actin, using a confocal microscopic approach previously developed by us, which showed that a pronounced increase in F-actin level was induced by tetracaine. These results provide a novel perspective on the action of local anesthetics in terms of GPCR lateral diffusion and actin cytoskeleton reorganization.

Mechanisms of Blockade of the Muscle-Type Nicotinic Receptor by Benzocaine, a Permanently Uncharged Local Anesthetic

Most local anesthetics (LAs) are amine compounds bearing one or several phenolic rings. Many of them are protonated at physiological pH, but benzocaine (Bzc) is permanently uncharged, which is relevant because the effects of LAs on nicotinic acetylcholine (ACh) receptors (nAChRs) depend on their presence as uncharged or protonated species. The aims of this study were to assess the effects of Bzc on nAChRs and to correlate them with its binding to putative interacting sites on this receptor. nAChRs from Torpedo electroplaques were microtransplanted to Xenopus oocytes and currents elicited by ACh (I_AChs), either alone or together with Bzc, were recorded at different potentials. Co-application of ACh with increasing concentrations of Bzc showed that Bzc reversibly blocked nAChRs. I_ACh inhibition by Bzc was voltage-independent, but the I_ACh rebound elicited when rinsing Bzc suggests an open-channel blockade. Besides, ACh and Bzc co-application enhanced nAChR desensitization. When Bzc was just pre-applied it also inhibited I_ACh, by blocking closed (resting) nAChRs. This blockade slowed down the kinetics of both the I_ACh activation and the recovery from blockade. The electrophysiological results indicate that Bzc effects on nAChRs are similar to those of 2,6-dimethylaniline, an analogue of the hydrophobic moiety of lidocaine. Furthermore, docking assays on models of the nAChR revealed that Bzc and DMA binding sites on nAChRs overlap fairly well. These results demonstrate that Bzc inhibits nAChRs by multiple mechanisms and contribute to better understanding both the modulation of nAChRs and how LAs elicit some of their clinical side effects. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

Mechanisms Underlying the Strong Inhibition of Muscle-Type Nicotinic Receptors by Tetracaine

Nicotinic acetylcholine (ACh) receptors (nAChRs) are included among the targets of a variety of local anesthetics, although the molecular mechanisms of blockade are still poorly understood. Some local anesthetics, such as lidocaine, act on nAChRs by different means through their ability to present as both charged and uncharged molecules. Thus, we explored the mechanisms of nAChR blockade by tetracaine, which at physiological pH is almost exclusively present as a positively charged local anesthetic. The nAChRs from Torpedo electroplaques were transplanted to Xenopus oocytes and the currents elicited by ACh (I_ACh s), either alone or co-applied with tetracaine, were recorded. Tetracaine reversibly blocked I_ACh , with an IC₅₀ (i.e., the concentration required to inhibit half the maximum I_ACh ) in the submicromolar range. Notably, at very low concentrations (0.1 μM), tetracaine reduced I_ACh in a voltage-dependent manner, the more negative potentials produced greater inhibition, indicating open-channel blockade. When the tetracaine concentration was increased to 0.7 μM or above, voltage-independent inhibition was also observed, indicating closed-channel blockade. The I_ACh inhibition by pre-application of just 0.7 μM tetracaine before superfusion of ACh also corroborated the notion of tetracaine blockade of resting nAChRs. Furthermore, tetracaine markedly increased nAChR desensitization, mainly at concentrations equal or higher than 0.5 μM. Interestingly, tetracaine did not modify desensitization when its binding within the channel pore was prevented by holding the membrane at positive potentials. Tetracaine-nAChR interactions were assessed by virtual docking assays, using nAChR models in the closed and open states. These assays revealed that tetracaine binds at different sites of the nAChR located at the extracellular and transmembrane domains, in both open and closed conformations. Extracellular binding sites seem to be associated with closed-channel blockade; whereas two sites within the pore, with different affinities for tetracaine, contribute to open-channel blockade and the enhancement of desensitization, respectively. These results demonstrate a concentration-dependent heterogeneity of tetracaine actions on nAChRs, and contribute to a better understanding of the complex modulation of muscle-type nAChRs by local anesthetics. Furthermore, the combination of functional and virtual assays to decipher nAChR-tetracaine interactions has allowed us to tentatively assign the main nAChR residues involved in these modulating actions.

Interaction of anesthetic molecules with α-helix and polyproline II extended helix of long-chain poly-l-lysine

The effect of halothane, enflurane, sevoflurane, and isoflurane molecules, as volatile anesthetics, on the α-helices and polyproline II extended helices (PPII) of long-chain poly-l-lysine (PLL) were studied using Fourier-transform infrared and vibrational circular dichroism spectroscopy. Uncharged and charged α-helices, as well as charged extended PPII helices, were subjected to anesthetic actions in solvents with different pD values or methanol to water ratios. A crucial factor responsible for hindering the anesthetic-PLL interactions is shown to be the ionization of amino groups of the PLL side chains. The α-helix to β-sheet transition was triggered only for the uncharged α-helical structures of PLL by the nonpolar anesthetics under study.

At therapeutic concentration bupivacaine causes neuromuscular blockade and enhances rocuronium-induced blockade

BACKGROUND

Partially paralyzed patients may be placed in the risk of pharyngeal dysfunction. Bupivacaine acts as acetylcholine receptor ion channel blocker and may synergistically interact with rocuronium to augment NM blockade. Thus, this study aims to elucidate whether or not, at a therapeutic concentration, bupivacaine by itself may cause NM blockade and reduce an effective concentration of rocuronium.

METHODS

Twenty-two left phrenic nerve-hemidiaphragms (Male SD rats, 150-250 g) were hung in Krebs solution. Three consecutive ST, 0.1 Hz and one TT, 50 Hz for 1.9 s were obtained before drug application and at each new drug concentration. A concentration of bupivacaine in Krebs solution (n = 5) was cumulatively increased by way of 0.01, 0.1, 1, (1, 2, 3, 4, 5, 6, 7) × 10 µM. In a Krebs solution, pre-treated with bupivacaine 0 (n = 5), 0.1 (n = 5), 1.0 (n = 5), 10 (n = 2) µM, and then concentrations of rocuronium were cumulatively increased by way of 1, 3, 5, 7, 9, 12, 14, 16, 18, 20 µM. EC for each experiment were determined by a probit. The EC(50)'s of rocuronium were compared using a Student's t-test with Bonferroni's correction. Differences were considered significant when P < 0.05.

RESULTS

The potency of bupivacaine for normalized TF was 11.4 (± 1.1) µM. Below 30 µM of bupivacaine, the single twitch potentiation sustained despite the development of tetanic fade and partial inhibition of PTT. Bupivacaine significantly facilitated the NM blockade induced by rocuronium.

CONCLUSIONS

Clinicians should be aware that bupivacaine by itself at its therapeutic concentration inhibit NM conduction and enhances rocuronium-induced muscle relaxation.

5-HT(3) receptors: role in disease and target of drugs

Serotonin type 3 (5-HT(3)) receptors are pentameric ion channels belonging to the superfamily of Cys-loop receptors. Receptor activation either leads to fast excitatory responses or modulation of neurotransmitter release depending on their neuronal localisation. 5-HT(3) receptors are known to be expressed in the central nervous system in regions involved in the vomiting reflex, processing of pain, the reward system, cognition and anxiety control. In the periphery they are present on a variety of neurons and immune cells. 5-HT(3) receptors are known to be involved in emesis, pain disorders, drug addiction, psychiatric and GI disorders. Progress in molecular genetics gives direction to personalised medical strategies for treating complex diseases such as psychiatric and functional GI disorders and unravelling individual drug responses in pharmacogenetic approaches. Here we discuss the molecular basis of 5-HT(3) receptor diversity at the DNA and protein level, of which our knowledge has greatly extended in the last decade. We also evaluate their role in health and disease and describe specific case-control studies addressing the involvement of polymorphisms of 5-HT3 subunit genes in complex disorders and responses to drugs. Furthermore, we focus on the actual state of the pharmacological knowledge concerning not only classical 5-HT(3) antagonists--the setrons--but also compounds of various substance classes targeting 5-HT(3) receptors such as anaesthetics, opioids, cannabinoids, steroids, antidepressants and antipsychotics as well as natural compounds derived from plants. This shall point to alternative treatment options modulating the 5-HT(3) receptor system and open new possibilities for drug development in the future.

Interaction of lipids and ligands with nicotinic acetylcholine receptor vesicles assessed by electron paramagnetic resonance spectroscopy

Electron paramagnetic resonance (EPR) spectroscopy is a powerful technique that permits the study of membrane-embedded proteins in its lipid environment by assessing the interaction of spin labels with the protein in its natural environment (i.e., native membranes) or in reconstituted systems prepared with exogenous lipid species. Nicotinic acetylcholine receptors (AChRs) contain a large surface in intimate contact with the lipid membrane. AChRs, members of the Cys-loop receptor superfamily, have essential functional roles in the nervous system and its malfunctioning has been considered as the origin of several neurological diseases including Alzheimer's disease, drug addiction, depression, and schizophrenia. In this regard, these receptors have been extensively studied as therapeutic targets for the action of several drugs. The majority of the marketed medications bind to the neurotransmitter sites, the so-called agonists. However, several drugs, some of them still in clinical trials, interact with non-competitive antagonist (NCA) binding sites. A potential location for these binding sites is the proper ion channel, blocking ion flux and thus, inhibiting membrane depolarization. However, several NCAs also bind to the lipid-protein interface, modulating the AChR functional properties. The best known examples of these NCAs are local and general anesthetics. Several endogenous molecules such as free fatty acids and neurosteroids also bind to the lipid-protein interface, probably mediating important physiological functions. Phospholipids, natural components of lipid membranes interacting with the AChR, are also essential to maintain the structural and functional properties of the AChR. EPR studies showed that local anesthetics bind to the lipid-protein interface by essentially the same dynamic mechanisms found in lipids, and that local and general anesthetics preferably decrease the phospholipid but not the fatty acid interactions with the AChR. This is consistent with the existence of annular and non-annular lipid domains on the AChR.

Modulation of major voltage- and ligand-gated ion channels in cultured neurons of the rat inferior colliculus by lidocaine

AIM

The purpose of the present study was to explore how lidocaine as a therapeutic drug for tinnitus targets voltage- and ligand-gated ion channels and changes the excitability of central auditory neurons.

METHODS

Membrane currents mediated by major voltage- and ligand-gated channels were recorded from primary cultured neurons of the inferior colliculus (IC) in rats with whole-cell patch-clamp techniques in the presence and absence of lidocaine. The effects of lidocaine on the current-evoked firing of action potentials were also examined.

RESULTS

Lidocaine at 100 micromol/L significantly suppressed voltage-gated sodium currents, transient outward potassium currents, and the glycine-induced chloride currents to 87.66%+/-2.12%, 96.33%+/-0.35%, and 91.46%+/-2.69% of that of the control level, respectively. At 1 mmol/L, lidocaine further suppressed the 3 currents to 70.26%+/-4.69%, 62.80%+/-2.61%, and 89.11%+/-3.17% of that of the control level, respectively. However, lidocaine at concentrations lower than 1 mmol/L did not significantly affect GABA- or aspartate-induced currents. At a higher concentration (3 mmol/L), lidocaine slightly depressed the GABA-induced current to 87.70%+/-1.87% of that of the control level. Finally, lidocaine at 100 mumol/L was shown to significantly suppress the current-evoked firing of IC neurons to 58.62%+/-11.22% of that of the control level, indicating that lidocaine decreases neuronal excitability.

CONCLUSION

Although the action of lidocaine on the ion channels and receptors is complex and non-specific, it has an overall inhibitory effect on IC neurons at a clinically-relevant concentration, suggesting a central mechanism for lidocaine to suppress tinnitus.

Molecular properties of local anesthetics as predictors of affinity for nicotinic acetylcholine receptors

In spinal anesthesia, the effects of local anesthetics (LAs) are not completely explained by sodium channel inhibition. Other targets include neuronal nicotinic acetylcholine receptors (nAChRs). LA affinities for the Torpedo californica nAChR were measured by inhibition of [(3)H]TCP binding and correlated with molecular volume, surface area, molecular weight, and log of the octanol-water partition coefficients (P and D). To understand the molecular determinants important for interaction with the nAChR, ester and amide LAs were compared separately. Also, correlations with the aromatic/linker half and the hydrophilic half of the LA molecules were considered individually. The IC(50)s of the ester LAs correlated better with the molecular volume, surface area, molecular weight, and log P of the aromatic/linker half of the molecules; whereas the IC(50)s for amide LAs correlated better with the four parameters based on the hydrophilic half. These correlations were used to predict the IC(50) of various LAs (including several not studied here) and to compare these values with the published values. The predicted values of IC(50) correlated well with the published results both for neuronal and for electroplaque-desensitized nAChR, suggesting that the results can be generalized to include neuronal nAChRs.

Luminal hypotonicity increases duodenal mucosal permeability by a mechanism involving 5-hydroxytryptamine

AIM

To investigate whether 5-hydroxytryptamine (5-HT) participates in the mediation of the hypotonicity-induced increase in duodenal mucosal permeability.

METHODS

Proximal duodenum in anaesthetized rats was perfused in situ with a hypotonic NaCl solution and effects on duodenal motility, net fluid flux, mucosal permeability [blood-to-lumen clearance of (51)Cr-ethylenediaminetetraacetic acid (EDTA)] and the release of 5-HT into the luminal solution studied in the presence of the cyclooxygenase inhibitor indomethacin.

RESULTS

Perfusion of the duodenum with 50 mm NaCl increased mucosal permeability eightfold, increased the luminal output of 5-HT twofold and induced net fluid absorption. This rise in permeability was enhanced 25% by 5-HT (3 x 10(-3) m), reduced by the 5-HT(3)-receptor antagonists granisetron (10(-4)-3 x 10(-4) m) or ondansetron (10(-5)-10(-4) m) or by the 5-HT(4) receptor antagonist SB 203186 (10(-4) m). The 5-HT(3/4) receptor antagonist tropisetron, at 10(-4) m, did not affect while 3 x 10(-4) and 3 x 10(-3) m augmented the hypotonicity-induced increase in mucosal permeability. Lidocaine (1.1 x 10(-3) m) similarly potentiated while tetrodotoxin (TTX) (5 x 10(-5) m) inhibited the hypotonicity-induced increase in mucosal permeability. Compared with animals treated with indomethacin alone ondansetron and granisetron augmented (by 30-40%) while tropisetron and lidocaine reduced (by 60-70%) the hypotonicity-induced net fluid absorption. Tetrodotoxin and all 5-HT receptor antagonists, except tropisetron, depressed duodenal motility.

CONCLUSIONS

Luminal hypotonicity increases duodenal mucosal permeability by a neural mechanism involving 5-HT acting on 5-HT(3) and 5-HT(4) receptors. 5-HT also appears to participate in the regulation of the hypotonicity-induced fluid flux.

Molecular mechanisms and binding site locations for noncompetitive antagonists of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors are pentameric proteins that belong to the Cys-loop receptor superfamily. Their essential mechanism of functioning is to couple neurotransmitter binding, which occurs at the extracellular domain, to the opening of the membrane-spanning cation channel. The function of these receptors can be modulated by structurally different compounds called noncompetitive antagonists. Noncompetitive antagonists may act at least by two different mechanisms: a steric and/or an allosteric mechanism. The simplest idea representing a steric mechanism is that the antagonist molecule physically blocks the ion channel. On the other hand, there exist distinct allosteric mechanisms. For example, noncompetitive antagonists may bind to the receptor and stabilize a nonconducting conformational state (e.g., resting or desensitized state), and/or increase the receptor desensitization rate. Barbiturates, dissociative anesthetics, antidepressants, and neurosteroids have been shown to inhibit nicotinic receptors by allosteric mechanisms and/or by open- and closed-channel blockade. Receptor modulation has proved to be highly complex for most noncompetitive antagonists. Noncompetitive antagonists may act by more than one mechanism and at distinct sites in the same receptor subtype. The binding site location for one particular molecule depends on the conformational state of the receptor. The mechanisms of action and binding affinities of noncompetitive antagonists differ among nicotinic receptor subtypes. Knowledge of the structure of the nicotinic acetylcholine receptor, the location of its noncompetitive antagonist binding sites, and the mechanisms of inhibition will aid the design of new and more efficacious drugs for treatment of neurological diseases.

A gating mechanism proposed from a simulation of a human alpha7 nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor is a well characterized ligand-gated ion channel, yet a proper description of the mechanisms involved in gating, opening, closing, ligand binding, and desensitization does not exist. Until recently, atomic-resolution structural information on the protein was limited, but with the production of the x-ray crystal structure of the Lymnea stagnalis acetylcholine binding protein and the EM image of the transmembrane domain of the torpedo electric ray nicotinic channel, we were provided with a window to examine the mechanism by which this channel operates. A 15-ns all-atom simulation of a homology model of the homomeric human alpha7 form of the receptor was conducted in a solvated palmitoyl-2-oleoyl-sn-glycerol-phosphatidylcholine bilayer and examined in detail. The receptor was unliganded. The structure undergoes a twist-to-close motion that correlates movements of the C loop in the ligand binding domain, via the beta10-strand that connects the two, with the 10 degrees rotation and inward movement of two nonadjacent subunits. The Cys loop appears to act as a stator around which the alpha-helical transmembrane domain can pivot and rotate relative to the rigid beta-sheet binding domain. The M2-M3 loop may have a role in controlling the extent or kinetics of these relative movements. All of this motion, along with essential dynamics analysis, is suggestive of the direction of larger motions involved in gating of the channel.

Mechanism-Based Approach to the Successful Prevention of Cocaine Inhibition of the Neuronal (α3β4) Nicotinic Acetylcholine Receptor

The nicotinic acetylcholine receptor (nAChR) belongs to a family of five channel-forming proteins that regulate communication between the approximately 10(12) cells of the nervous system. A minimum mechanism of inhibition of the muscle-type nAChR (1) by the noncompetitive inhibitors cocaine and MK-801 [(+)-dizocilpine, an anticonvulsant] indicated they bind to a regulatory site, with higher affinity for the closed-channel form than for the open-channel form, thus shifting the equilibrium toward the closed-channel form and inhibiting receptor function. The mechanism predicts that compounds that bind to this regulatory site with equal or higher affinity for the open-channel conformation than for the closed-channel conformation will prevent receptor inhibition (1). Does a neuronal form of the receptor behave similarly? The mechanism of inhibition of the neuronal nAChR by cocaine and MK-801 using rapid chemical kinetic techniques was investigated. The alpha3beta4 nAChR stably expressed in HEK 293 cells was used in these investigations. Whole-cell currents originated from a major and minor nAChR isoform. Only the major isoform has been characterized. For the dominant, rapidly desensitizing isoform, the carbamoylcholine dissociation constant for the site controlling receptor activation, Kd, is 2 mM; the channel-opening equilibrium constant, Phi(-1), is 4; and the dominant desensitization rate constant, k34, is 20 s(-1). Cocaine inhibits the receptor noncompetitively, with an apparent KI of 84 and 26 microM at high and low carbamoylcholine concentrations, at which concentrations the receptor is mainly in the open- or closed-channel form, respectively. Similar results were obtained with MK-801. A combinatorially synthesized RNA ligand and a cocaine analogue alleviated cocaine inhibition of this neuronal receptor.

Rapid chemical reaction techniques developed for use in investigations of membrane-bound proteins (neurotransmitter receptors)

New techniques for investigating chemical reactions on cell surfaces in the microsecond-to-millisecond time region are described. Reactions mediated by membrane-bound neurotransmitter receptors that control signal transmission between approximately 10(12) cells of the nervous system are taken as an example. Cells with receptors on their plasma membrane are equilibrated with photolabile, biologically inactive precursors of the neurotransmitters. Photolysis of these compounds releases free neurotransmitter that interacts with the receptors, leading to the transient opening of transmembrane receptor-formed channels that are permeant to small inorganic ions. The current thus induced can be measured. The technique can be used to measure the elementary steps of the receptor-mediated reactions. To illustrate the approach it was shown that an understanding of the mechanism of inhibition of the nicotinic acetylcholine receptor by the drug cocaine was obtained and led to the first proof that compounds exist that alleviate the inhibition.

Unique general anesthetic binding sites within distinct conformational states of the nicotinic acetylcholine receptor

General anesthesia is a complex behavioral state provoked by the pharmacological action of a broad range of structurally different hydrophobic molecules called general anesthetics (GAs) on receptor members of the genetically linked ligand-gated ion channel (LGIC) superfamily. This superfamily includes nicotinic acetylcholine (AChRs), type A and C gamma-aminobutyric acid (GABAAR and GABACR), glycine (GlyR), and type 3 5-hydroxytryptamine (5-HT3R) receptors. This review focuses on recent advances in the localization of GA binding sites on conformationally and compositionally distinct AChRs. The experimental evidence outlined in this review suggests that: 1. Several neuronal-type AChRs might be targets for the pharmacological action of distinct GAs. 2. The molecular components of a specific GA binding site on a certain receptor subtype are different from the structural determinants of the locus for the same GA on a different receptor subtype. 3. There are unique binding sites for distinct GAs in the same receptor protein. 4. A GA can activate, potentiate, or inhibit an ion channel, indicating the existence of more than one binding site for the same GA. 5. The affinity of a specific GA depends on the conformational state of the receptor. 6. GAs inhibition channels by at least two mechanisms, an open-channel-blocking and/or an allosteric mechanism. 7. Certain GAs may inhibit AChR function by competing for the agonist binding sites or by augmenting the desensitization rate.

Cholinergic nicotinic receptors: competitive ligands, allosteric modulators, and their potential applications

Discovery of the important role played by nicotinic acetylcholine receptors (nAChRs) in several CNS disorders has called attention to these membrane proteins and to ligands able to modulate their functions. The existence of different subtypes at multiple levels has complicated the understanding of this receptor's physiological role, but at the same time has increased the efforts to discover selective compounds in order to improve the pharmacological characterization of this kind of receptor and to make the possible therapeutical use of its modulators safer. This review focuses on the structure of new ligands for nAChRs, agonists, antagonists and allosteric modulators, and on their possible applications.

Molecular modelling of the interactions of carbamazepine and a nicotinic receptor involved in the autosomal dominant nocturnal frontal lobe epilepsy

1. The normal and a mutant (S248F) human neuronal alpha4beta2 nicotinic receptors, and their interaction with the channel blocker carbamazepine (CBZ) have been modelled. The mutant, responsible for the autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE), has an enhanced sensitivity to and a slower recovery from desensitization, a lower conductance, short open times, reduced calcium permeability, and is 3 fold more sensitive to CBZ, a drug used in the treatment of partial epilepsies. 2. Mutant channel properties are explained by the physicochemical properties of the two Phe248 side chains, including size and cation-pi interaction, and their dynamic behaviour. A defective mechanism of dehydration might be responsible for the reduced calcium influx. 3. Phe248 residues are the main component of CBZ binding sites in the mutant, while this is not true for Ser248 in the normal receptor. 4. A higher number of blocking binding sites and a predicted higher affinity found for CBZ in the mutant account for its differential sensitivity to CBZ. 5. Aromatic-aromatic interactions between CBZ and the two Phe248 account for the difference in affinity, which is at least 12 times higher for the mutant, depending on the method used for calculating K(i). 6. Normal vs mutant differences in K(i), enhanced by the higher number of blocking binding sites in the mutant, seem excessive compared to the differential sensitivities to CBZ experimentally found. The negative cooperativity suggested by a predicted overlapping of blocking and non-blocking binding sites gives an explanation, as overlapping is higher in the mutant. 7. For both types of receptors we found that the carbamyl group of the best blocking conformers of CBZ forms hydrogen bonds with serine residues, which may explain the fundamental role of that moiety for this molecule to act as antiepileptic drug.

Intravenously administered lidocaine in therapeutic doses increases the intraspinal release of acetylcholine in rats

The local anesthetic lidocaine suppresses different pain conditions when administered systemically. Part of the antinociceptive effect appears to be mediated via receptor mechanisms. We have previously shown that muscarinic and nicotinic agonists that produce antinociception increase the intraspinal release of acetylcholine. In the present study it was hypothesized that systemically administered lidocaine is acting through the same mechanisms as cholinergic agonists and affects the intraspinal release of acetylcholine. Microdialysis probes were placed in anesthetized rats for sampling of acetylcholine. Ten and 30 mg/kg lidocaine injected intravenously significantly increased the intraspinal release of acetylcholine. The effect of lidocaine could be reduced by pretreatment with intraspinally administered atropine or mecamylamine. Our results suggest that the antinociceptive effect produced by systemically administered lidocaine is mediated through an action on muscarinic and nicotinic receptors.

Characterization of the dizocilpine binding site on the nicotinic acetylcholine receptor

Although the dissociative anesthetic dizocilpine [(+)-MK-801] inhibits nicotinic acetylcholine receptor (AChR) function in a noncompetitive manner, the location of the dizocilpine binding site(s) has yet to be clearly established. Thus, to characterize the binding site for dizocilpine on the AChR we examined 1) the dissociation constant (K(d)) and stoichiometry of [(3)H]dizocilpine binding; 2) the displacement of dizocilpine radioligand binding by noncompetitive inhibitors (NCIs) and conversely dizocilpine displacement of fluorescent and radiolabeled NCIs from their respective high-affinity binding sites on the AChR; and 3) photoaffinity labeling of the AChR using (125)I-dizocilpine. The results establish that one high-affinity (K(d) = 4.8 microM) and several (3-6) low-affinity (K(d) = approximately 140 microM) binding sites exist for dizocilpine on the desensitized and resting AChR, respectively. The binding of the fluorescent NCIs ethidium, quinacrine, and crystal violet as well as [(3)H]thienylcyclohexylpiperidine was inhibited by dizocilpine on desensitized AChRs. However, Schild-type analyses indicate that only the inhibition of quinacrine in the desensitized state seems to be mediated by a mutually exclusive action. Photoaffinity labeling of the AChR by (125)I-dizocilpine was primarily restricted to the alpha1 subunit and subsequent mapping revealed that the principal sites of labeling are localized to the M4 (approximately 70%) and M1 (30%) transmembrane domains. Collectively, the data indicate that the high-affinity dizocilpine binding site is not located in the lumen of the ion channel but probably near the quinacrine binding locus at a nonluminal domain in the AChR desensitized state.

Localization of agonist and competitive antagonist binding sites on nicotinic acetylcholine receptors

Identification of all residues involved in the recognition and binding of cholinergic ligands (e.g. agonists, competitive antagonists, and noncompetitive agonists) is a primary objective to understand which structural components are related to the physiological function of the nicotinic acetylcholine receptor (AChR). The picture for the localization of the agonist/competitive antagonist binding sites is now clearer in the light of newer and better experimental evidence. These sites are located mainly on both alpha subunits in a pocket approximately 30-35 A above the surface membrane. Since both alpha subunits are identical, the observed high and low affinity for different ligands on the receptor is conditioned by the interaction of the alpha subunit with other non-alpha subunits. This molecular interaction takes place at the interface formed by the different subunits. For example, the high-affinity acetylcholine (ACh) binding site of the muscle-type AChR is located on the alphadelta subunit interface, whereas the low-affinity ACh binding site is located on the alphagamma subunit interface. Regarding homomeric AChRs (e.g. alpha7, alpha8, and alpha9), up to five binding sites may be located on the alphaalpha subunit interfaces. From the point of view of subunit arrangement, the gamma subunit is in between both alpha subunits and the delta subunit follows the alpha aligned in a clockwise manner from the gamma. Although some competitive antagonists such as lophotoxin and alpha-bungarotoxin bind to the same high- and low-affinity sites as ACh, other cholinergic drugs may bind with opposite specificity. For instance, the location of the high- and the low-affinity binding site for curare-related drugs as well as for agonists such as the alkaloid nicotine and the potent analgesic epibatidine (only when the AChR is in the desensitized state) is determined by the alphagamma and the alphadelta subunit interface, respectively. The case of alpha-conotoxins (alpha-CoTxs) is unique since each alpha-CoTx from different species is recognized by a specific AChR type. In addition, the specificity of alpha-CoTxs for each subunit interface is species-dependent. In general terms we may state that both alpha subunits carry the principal component for the agonist/competitive antagonist binding sites, whereas the non-alpha subunits bear the complementary component. Concerning homomeric AChRs, both the principal and the complementary component exist on the alpha subunit. The principal component on the muscle-type AChR involves three loops-forming binding domains (loops A-C). Loop A (from mouse sequence) is mainly formed by residue Y(93), loop B is molded by amino acids W(149), Y(152), and probably G(153), while loop C is shaped by residues Y(190), C(192), C(193), and Y(198). The complementary component corresponding to each non-alpha subunit probably contributes with at least four loops. More specifically, the loops at the gamma subunit are: loop D which is formed by residue K(34), loop E that is designed by W(55) and E(57), loop F which is built by a stretch of amino acids comprising L(109), S(111), C(115), I(116), and Y(117), and finally loop G that is shaped by F(172) and by the negatively-charged amino acids D(174) and E(183). The complementary component on the delta subunit, which corresponds to the high-affinity ACh binding site, is formed by homologous loops. Regarding alpha-neurotoxins, several snake and alpha-CoTxs bear specific residues that are energetically coupled with their corresponding pairs on the AChR binding site. The principal component for snake alpha-neurotoxins is located on the residue sequence alpha1W(184)-D(200), which includes loop C. In addition, amino acid sequence 55-74 from the alpha1 subunit (which includes loop E), and residues gammaL(119) (close to loop F) and gammaE(176) (close to loop G) at the low-affinity binding site, or deltaL(121) (close to the homologous region of loop G) at the high-affinity binding site, are i