Inhibition of N-methyl-D-aspartate receptors by straight-chain diols: implications for the mechanism of the alcohol cutoff effect

Physically Acting Treatments for Head Lice—Can We Still Claim They Are ‘Resistance Proof’?

Head lice worldwide have developed resistance to insecticides, prompting the introduction of a range of alternative treatments including plant extracts and natural and synthetic oils. Clinical studies of physically acting treatments showed them to be highly effective when first introduced, and a widely held, but unsubstantiated, belief is that lice are unlikely to develop resistance to them. However, this ignores possibilities for natural selection of traits enabling lice to survive exposure. More recent investigations of some physically acting products have shown reduced efficacy, suggesting either changes of behavior, physical structure, or physiology of some louse populations. In addition, the activity of surfactants and similar compounds, acting as solubilizing agents of insect cuticular lipids, can be compromised by the widespread use of toiletry products containing similar substances. Hitherto, most clinical investigations have provided “best case” data resulting from investigator application of treatments. In the few studies involving participant application, the effectiveness was reduced, suggesting that consumer use allows some insects to survive, which could then be selected for tolerance. Unlike neurotoxic insecticides, there is no straightforward method to test for the activity of physically acting chemicals other than by clinical investigations, which need to be rigorous to eliminate poorly effective products as a way of ensuring the continued effectiveness of those treatments that are successful in eliminating infestation.

Escaping from the Cutoff Paradox by Accumulating Long-Chain Alcohols in the Cell Membrane

The mechanism for the cutoff, an activity cliff at which long-chain alcohols lose their biological effects, has not been elucidated. Highly hydrophobic oleyl alcohol (C_18:1) exists as a mixture of monomers and aggregated droplets in water. C_18:1 did not inhibit the yeast growth but inhibited the growth of the slime mold without a cell wall. C_18:1 exhibited toxicity to the yeast protoplast, which was enhanced by polyethylene glycol, a fusogen. Therefore, direct interactions of C_18:1 with the membrane are crucial for the toxicity. The cutoff alcohols, C₁₄ and C₁₆, also exhibited strong toxicity obeying the Meyer-Overton correlation, in intact yeast cells whose membrane growth was suppressed in water. Taken together, the cutoff is avoidable by securing sufficient accumulation of the wall-permeable monomers in the membrane, which supports the lipid theory. It would be important to distinguish the effective drug structure localizing in the membrane and deal with the amount in the membrane.

Decane-1,2-diol derivatives as potential antitumor agents for the treatment of glioblastoma

Glioblastoma remains the most common and aggressive type of malignant brain tumor among adults thus, considerable attention has been given to discovery of novel anti-tumor drugs for its treatment. This study reports the synthesis of a series of twelve novel decane-1,2-diol derivatives and evaluation of its anti-tumor activity in mammalian glioblastoma cell lines, U87 and LN229. Starting from decane-1,2-diol, several derivatives were prepared using a diversity oriented synthesis approach through which a small library composed of esters, silyl ethers, sulfonates, sulfites, sulfates, ketals, and phosphonates was built. The decane-1,2-diol ditosylated derivative, DBT, found to have higher cytotoxicity than the standard drug cisplatin, has IC₅₀ value of 52 µM in U87 and 270 µM in LN229. Migration analysis of U87 cell line treated with the DBT indicated its ability to effectively suppress proliferation during initial hours of treatment and decrease anti-proliferative property over time. Additionally, DBT was assessed for its role in apoptosis, oxidative stress and caspase 3/7 activation in U87. Interestingly, our experiments indicated that its cytotoxicity is independent of Reactive oxygen species induced caspase 3/7 activity. The compound also exhibited caspase independent apoptosis activity in U87. DBT treatment led to G1/S cell cycle arrest and apoptosis induction of glioma cell lines. In addition, we identified 1533 genes with significant changes at the transcriptional level, in response to DBT. A molecular docking study accounting for the interaction of DBT with NMDA receptor disclosed several hydrogen bonds and charged residue interactions with 17 amino acids, which might be the basis of the DBT cytotoxicity observed. We conclude that this molecule exerts its cytotoxicity via caspase 3/7 independent pathways in glioblastoma cells. Concisely, simple decane-1,2-diol derivatives might serve as scaffolds for the development of effective anti-glioblastoma agents.

The effect of alcohol on recombinant proteins derived from mammalian adenylyl cyclase

The cyclic AMP (cAMP) signaling pathway is implicated in the development of alcohol use disorder. Previous studies have demonstrated that ethanol enhances the activity of adenylyl cyclase (AC) in an isoform specific manner; AC7 is most enhanced by ethanol, and regions responsible for enhancement by ethanol are located in the cytoplasmic domains of the AC7 protein. We hypothesize that ethanol modulates AC activity by directly interacting with the protein and that ethanol effects on AC can be studied using recombinant AC in vitro. AC recombinant proteins containing only the C_1a or C₂ domains of AC7 and AC9 individually were expressed in bacteria, and purified. The purified recombinant AC proteins retained enzymatic activity and isoform specific alcohol responsiveness. The combination of the C_1a or C₂ domains of AC7 maintained the same alcohol cutoff point as full-length AC7. We also find that the recombinant AC7 responds to alcohol differently in the presence of different combinations of activators including MnCl₂, forskolin, and Gsα. Through a series of concentration-response experiments and curve fitting, the values for maximum activities, Hill coefficients, and EC₅₀ were determined in the absence and presence of butanol as a surrogate of ethanol. The results suggest that alcohol modulates AC activity by directly interacting with the AC protein and that the alcohol interaction with the AC protein occurs at multiple sites with positive cooperativity. This study indicates that the recombinant AC proteins expressed in bacteria can provide a useful model system to investigate the mechanism of alcohol action on their activity.

γ-Aminobutyric Acid Type A Receptor Modulation by Etomidate Analogs

BACKGROUND

Etomidate is a highly potent anesthetic agent that is believed to produce hypnosis by enhancing γ-aminobutyric acid type A (GABAA) receptor function. The authors characterized the GABAA receptor and hypnotic potencies of etomidate analogs. The authors then used computational techniques to build statistical and graphical models that relate the potencies of these etomidate analogs to their structures to identify the specific molecular determinants of potency.

METHODS

GABAA receptor potencies were defined with voltage clamp electrophysiology using α1β3γ2 receptors harboring a channel mutation (α1[L264T]) that enhances anesthetic sensitivity (n = 36 to 60 measurements per concentration-response curve). The hypnotic potencies of etomidate analogs were defined using a loss of righting reflexes assay in Sprague Dawley rats (n = 9 to 21 measurements per dose-response curve). Three-dimensional quantitative structure-activity relationships were determined in silico using comparative molecular field analysis.

RESULTS

The GABAA receptor and hypnotic potencies of etomidate and the etomidate analogs ranged by 91- and 53-fold, respectively. These potency measurements were significantly correlated (r = 0.72), but neither measurement correlated with drug hydrophobicity (r = 0.019 and 0.005, respectively). Statistically significant and predictive comparative molecular field analysis models were generated, and a pharmacophore model was built that revealed both the structural elements in etomidate analogs associated with high potency and the interactions that these elements make with the etomidate-binding site.

CONCLUSIONS

There are multiple specific structural elements in etomidate and etomidate analogs that mediate GABAA receptor modulation. Modifying any one element can alter receptor potency by an order of magnitude or more.

Hydrocarbon molar water solubility predicts NMDA vs. GABAA receptor modulation

Background

Many anesthetics modulate 3-transmembrane (such as NMDA) and 4-transmembrane (such as GABA_A) receptors. Clinical and experimental anesthetics exhibiting receptor family specificity often have low water solubility. We hypothesized that the molar water solubility of a hydrocarbon could be used to predict receptor modulation in vitro.

Methods

GABA_A (α₁β₂γ_2s) or NMDA (NR1/NR2A) receptors were expressed in oocytes and studied using standard two-electrode voltage clamp techniques. Hydrocarbons from 14 different organic functional groups were studied at saturated concentrations, and compounds within each group differed only by the carbon number at the ω-position or within a saturated ring. An effect on GABA_A or NMDA receptors was defined as a 10% or greater reversible current change from baseline that was statistically different from zero.

Results

Hydrocarbon moieties potentiated GABA_A and inhibited NMDA receptor currents with at least some members from each functional group modulating both receptor types. A water solubility cut-off for NMDA receptors occurred at 1.1 mM with a 95% CI = 0.45 to 2.8 mM. NMDA receptor cut-off effects were not well correlated with hydrocarbon chain length or molecular volume. No cut-off was observed for GABA_A receptors within the solubility range of hydrocarbons studied.

Conclusions

Hydrocarbon modulation of NMDA receptor function exhibits a molar water solubility cut-off. Differences between unrelated receptor cut-off values suggest that the number, affinity, or efficacy of protein-hydrocarbon interactions at these sites likely differ.

Effects of straight chain alcohols on specific isoforms of adenylyl cyclase

BACKGROUND

Our previous studies showed that the activity of adenylyl cyclase (AC) was enhanced by pharmacologically relevant concentrations of ethanol, that this enhancing effect of ethanol on AC activity was AC isoform specific, and that the alcohol cutoff effect for n-alkanol potentiation of AC activity was also AC isoform specific. Therefore, we hypothesized that within the cyclic AMP-generating system, AC is the target of ethanol's action and that alcohols interact directly with the AC molecules. To characterize the interaction between alcohols and AC proteins, the effects of a series of straight chain alcohols would be very valuable in understanding alcohol action at the molecular level. To our knowledge, straight chain alcohols other than n-alkanols and 1,Omega-diols have not been used extensively to study alcohol effects on the activity of AC or other proteins important in the alcohol research field.

METHODS

The effects of a series of straight chain alcohols on D1A dopamine receptor-stimulated activity of AC isoforms type 6, 7, and 9 (AC6, AC7, and AC9) were examined in transfected Hela cells by a cAMP accumulation assay.

RESULTS

In general, all 3 AC isoforms responded to a series of straight chain alcohols in a similar manner. The order of responsiveness is as follows: monoalcohol > diol > triol and tetraol. Within monoalcohols, 1-alcohols had larger effects than 2-alcohols. Two of 3 stereoisomers of 2,3-butanediol, [D-(-)-2,3-butanediol and meso-2,3-butanediol] showed similar enhancing effects on all 3 AC isoforms. However, the third stereoisomer, L-(+)-2,3-butanediol, inhibited AC7 activity, while it stimulated AC6 and AC9.

CONCLUSION

The number and the position of hydroxyl groups in straight chain alcohols play an important role in the magnitude of the enhancement on AC activity. Regardless of AC isoforms, the most effective of the straight chain alcohols seems to be the 1-alcohol (n-alkanol) for a given chain length. We found that one of the stereoisomers of 2,3-butanediol had opposite effects on AC activity depending on the AC isoform. Overall, the results are consistent with the hypotheses and demonstrate that a series of straight chain alcohols can be a valuable tool to study AC-alcohol interactions.

A discrete alcohol pocket involved in GIRK channel activation

Ethanol modifies neural activity in the brain by modulating ion channels. Ethanol activates G protein-gated inwardly rectifying K⁺ channels, but the molecular mechanism is not well understood. Here, we used a crystal structure of a mouse inward rectifier containing a bound alcohol and structure-based mutagenesis to probe a putative alcohol-binding pocket located in the cytoplasmic domains of GIRK channels. Substitutions with bulkier side-chains in the alcohol-binding pocket reduced or eliminated activation by alcohols. By contrast, alcohols inhibited constitutively open channels, such as IRK1 or GIRK2 that binds PIP₂ strongly. Mutations in the hydrophobic alcohol-binding pocket of these channels had no effect on alcohol-dependent inhibition, suggesting an alternate site is involved in inhibition. Comparison of high-resolution structures of inwardly rectifying K⁺ channels suggests a model for activation of GIRK channels utilizing this hydrophobic alcohol-binding pocket. These results provide a tool for developing therapeutic compounds that could mitigate the effects of alcohol.

A feed-forward artificial neural network for prediction of the aquatic ecotoxicity of alcohol ethoxylate

A feed-forward artificial neural network (ANN) has been developed for predicting the aquatic ecotoxicity of alcohol ethoxylate (AE), a non-ionic surfactant comprising a variety of homologues. Trained with previously reported ecotoxicity data, the ANN utilizes both molecular characteristics (alkyl chain length, branching extent in alkyl chain, and ethoxylate (EO) number) and exposure features (effect endpoint, test duration, test type, and species taxon) as inputs to predict the ecotoxicity. The ANN predicted an increase in ecotoxicity for homologues with a longer or less-branched alkyl chain, or those with fewer EO units. But for long alkyl chain (>14) homologues, the ecotoxicity increase was predicted by the ANN to level off, which is obscured by existing quantitative structure-activity relationships (QSARs). A "leave-one-out" cross-validation process indicated that the prediction accuracy was within a factor of 5 with 90% probability. This research demonstrated that the current ANN covers a wider application domain with respect to the homologue range and a variety of exposure features without compromising on predictive accuracy.

Interaction of anesthetics with the Rho GTPase regulator Rho GDP dissociation inhibitor

The physiological effects of anesthetics have been ascribed to their interaction with hydrophobic sites within functionally relevant CNS proteins. Studies have shown that volatile anesthetics compete for luciferin binding to the hydrophobic substrate binding site within firefly luciferase and inhibit its activity (Franks, N. P., and Lieb, W. R. (1984) Nature 310, 599-601). To assess whether anesthetics also compete for ligand binding to a mammalian signal transduction protein, we investigated the interaction of the volatile anesthetic, halothane, with the Rho GDP dissociation inhibitor (RhoGDIalpha), which binds the geranylgeranyl moiety of GDP-bound Rho GTPases. Consistent with the existence of a discrete halothane binding site, the intrinsic tryptophan fluorescence of RhoGDIalpha was quenched by halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) in a saturable, concentration-dependent manner. Bromine quenching of tryptophan fluorescence is short-range and W192 and W194 of the RhoGDIalpha are located within the geranylgeranyl binding pocket, suggesting that halothane binds within this region. Supporting this, N-acetyl-geranylgeranyl cysteine reversed tryptophan quenching by halothane. Short chain n-alcohols ( n < 6) also reversed tryptophan quenching, suggesting that RhoGDIalpha may also bind n-alkanols. Consistent with this, E193 was photolabeled by 3-azibutanol. This residue is located in the vicinity of, but outside, the geranylgeranyl chain binding pocket, suggesting that the alcohol binding site is distinct from that occupied by halothane. Supporting this, N-acetyl-geranylgeranyl cysteine enhanced E193 photolabeling by 3-azibutanol. Overall, the results suggest that halothane binds to a site within the geranylgeranyl chain binding pocket of RhoGDIalpha, whereas alcohols bind to a distal site that interacts allosterically with this pocket.

Evidence that ethanol acts on a target in Loop 2 of the extracellular domain of alpha1 glycine receptors

Considerable evidence indicates that ethanol acts on specific residues in the transmembrane domains of glycine receptors (GlyRs). In this study, we tested the hypothesis that the extracellular domain is also a target for ethanol action by investigating the effect of cysteine substitutions at positions 52 (extracellular domain) and 267 (transmembrane domain) on responses to n-alcohols and propyl methanethiosulfonate (PMTS) in alpha1GlyRs expressed in Xenopus oocytes. In support of the hypothesis: (i) The A52C mutation changed ethanol sensitivity compared to WT GlyRs; (ii) PMTS produced irreversible alcohol-like potentiation in A52C GlyRs; and (iii) PMTS binding reduced the n-chain alcohol cutoff in A52C GlyRs. Further studies used PMTS binding to cysteines at positions 52 or 267 to block ethanol action at one site in order to determine its effect at other site(s). In these situations, ethanol caused negative modulation when acting at position 52 and positive modulation when acting at position 267. Collectively, these findings parallel the evidence that established the TM domain as a target for ethanol, suggest that positions 52 and 267 are part of the same alcohol pocket and indicate that the net effect of ethanol on GlyR function reflects the summation of its positive and negative modulatory effects on different targets.

Interactions of anesthetics with their targets: non-specific, specific or both?

What makes a general anesthetic a general anesthetic? We shall review first what general anesthesia is all about and which drugs are being used as anesthetics. There is neither a unique definition of general anesthesia nor any consensus on how to measure it. Diverse drugs and combinations of drugs generate general anesthetic states of sometimes very different clinical quality. Yet the principal drugs are still considered to belong to the same class of 'general anesthetics'. Effective concentrations of inhalation anesthetics are in the high micromolar range and above, and even for intravenous anesthetics they do not go below the micromolar range. At these concentrations, many molecular and higher level targets are affected by inhalation anesthetics, fewer probably by intravenous anesthetics. The only physicochemical characteristic shared by anesthetics is the correlation of their anesthetic potencies with hydrophobicity. These correlations depend on the group of general anesthetics considered. In this review, anesthetic potencies for many different targets are plotted against octanol/water partition coefficients as measure of hydrophobicity. Qualitatively, similar correlations result, suggesting several but weak interactions with proteins as being characteristic of anesthetic actions. The polar interactions involved are weak, being roughly equal in magnitude to hydrophobic interactions. Generally, intravenous anesthetics are noticeably more potent than inhalation anesthetics. They differ considerably more between each other in their interactions with various targets than inhalation anesthetics do, making it difficult to come to a decision which of these should be used in future studies as representative 'prototypical general anesthetics'.

Anesthetic potency of two novel synthetic polyhydric alkanols longer than the n-alkanol cutoff: evidence for a bilayer-mediated mechanism of anesthesia?

The polyhydroxyalkanes 1,6,11,16-hexadecanetetraol (1) and 2,7,12,17-octadecanetetraol (2) were synthesized utilizing the thiophene ring as a scaffold to affix the hydroxyalkyl chains by lithiation of the acidic alpha-hydrogens and subsequent desulfurization. Both compounds exhibited significant anesthetic potency, individually and in additivity studies with hexanol, using immobility in tadpoles as the phenotypic endpoint. These results, which contradict a protein-binding mechanism in which cutoff results from steric hindrance, are consistent with recent predictions of a membrane-mediated mechanism involving the lateral pressure profile.

The N-Methyl-d-aspartate Receptor Inhibitory Potencies of Aromatic Inhaled Drugs of Abuse: Evidence for Modulation by Cation-π Interactions

Benzene and several close structural analogs are inhaled drugs of abuse with general anesthetic activity. By virtue of their pi electron clouds, they may engage in attractive electrostatic interactions with cationic atomic charges on protein targets. In this study, we tested the hypothesis that inhaled drugs of abuse inhibit human N-methyl-D-aspartate (NMDA) receptors with potencies that correlate with their abilities to engage in cation-pi interactions. Electrophysiological techniques were used to define the NR1/NR2B NMDA receptor inhibitory concentrations of volatile benzene analogs, and computer modeling was used to quantify their abilities to engage in cation-pi interactions and their molecular volumes. In addition, each compound's octanol/gas partition coefficient (a measure of hydrophobicity) was quantified. All 18 compounds inhibited human NR1/NR2B NMDA receptors reversibly and in a concentration-dependent manner. NMDA receptor inhibitory potency correlated strongly with the ability to engage in cation-pi interactions, weakly with hydrophobicity, and was independent of molecular volume. This is consistent with the hypothesis that cation-pi interactions enhance the binding of inhaled drugs of abuse to the NMDA receptor and suggests that the receptor binding site(s) for these drugs possesses significant cationic character.

Serotonin-3 receptors in the actions of alcohol, alcohol reinforcement, and alcoholism

This article represents the proceedings of a symposium at the 2003 annual meeting of the Research Society on Alcoholism in Fort Lauderdale, FL. The organizers and chairs were William J. McBride and David M. Lovinger. The presentations were (1) Mechanisms of alcohol potentiation of 5-HT3 receptor function, by David M. Lovinger and Tina Machu; (2) Chronic alcohol drinking alters 5-HT3 receptors regulating the mesolimbic dopamine system, by Richard J. Thielen; (3) 5-HT3 receptors in the VTA regulate alcohol drinking and the reinforcing effects of alcohol, by Zachary A. Rodd and James M. Murphy; and (4) Ondansetron as a treatment for "biological" alcoholism, by John D. Roache and Bankole A. Johnson.

A site of alcohol action in the fourth membrane-associated domain of the N-methyl-D-aspartate receptor

The N-methyl-d-aspartate (NMDA) subtype of ionotropic glutamate receptor is an important mediator of the behavioral effects of ethanol in the central nervous system. Although ethanol is known to inhibit NMDA receptors by influencing ion-channel gating, its molecular site of action and the mechanism underlying this effect have not been established. We have previously identified a conserved methionine residue in the fourth membrane-associated domain of the NMDA receptor NR2A subunit (Met823) that influences desensitization and gating of the ion channel. Here we report that this residue plays an important role in mediating the effect of ethanol on the NMDA receptor. Ethanol IC50 values among functional substitution mutants at this position varied over the range approximately 130-225 mm. There was a weak correlation between ethanol IC50 and mean open time of NR2A(Met823) mutants that was dependent on inclusion of the value for the tryptophan mutant. In the absence of this value, there was no trend toward a correlation among the remaining mutants. Desensitization appeared to influence the action of ethanol, because ethanol IC50 of the mutants was correlated with the steadystate to peak current ratio. With the exception of tryptophan, ethanol sensitivity was significantly related to the molecular volume and hydrophobicity of the substituent. The relation between ethanol sensitivity and the molecular volume and hydrophobicity at this position suggests that this residue interacts with or forms part of a site of ethanol action and that the presence of a tryptophan residue in this site disrupts its ability to interact with ethanol.

Inhibition of glycine receptor function of native neurons by aliphatic n-alcohols

The inhibitory effects of n-alcohols (methanol to dodecanol) on glycine-activated currents were studied in neurons freshly dissociated from the ventral tegmental area of neonatal rats using whole-cell patch-clamp recording technique. Ethanol enhanced and depressed glycine-activated currents in 35% and 45%, respectively, of neurons of ventral tegmental area of neonatal rats. In this report, we extended our focus of ethanol-induced inhibition of glycine currents to other straight-chain alcohols. Aliphatic n-alcohols, which have carbon numbers less than nine, suppressed glycine currents in 45% (71/158) of the neurons. All results from this study are obtained from the 45% of cells displaying inhibition; the other 55% of the neurons were not studied. Alcohol potency increased as the number of carbon atoms increased from one to five, and was at a maximal plateau from five to nine; alcohols with 10 or more carbons did not inhibit glycine-activated currents. Thus, a 'cutoff' point in their potency for inhibition of glycine receptor function occurred at about decanol. A coapplication of dodecanol with ethanol eliminated the inhibition resulting from ethanol. Thus, dodecanol may bind to the receptor silently and compete with ethanol. These observations indicate that straight-chain n-alcohols exhibit a 'cutoff' point in their potency for inhibition of the glycine receptor function between nine and 10 carbon atoms. The inability of longer alcohols to change the activation properties of the receptors may contribute to the cutoff effect.