The novel nicotinic receptor antagonist, N,N'-dodecane-1,12-diyl-bis-3-picolinium dibromide (bPiDDB), inhibits nicotine-evoked [(3)H]norepinephrine overflow from rat hippocampal slices

Anti-depressant activity of Erythrina variegata bark extract and regulation of monoamine oxidase activities in mice

ETHNOPHARMACOLOGICAL RELEVANCE

Erythrina variegata, commonly referred to as 'Indian coral tree' belongs to the Fabaceae family. It is a plant native to the coast of India, China and is distributed in tropical and subtropical regions worldwide. In traditional medicine, E. variegata is known to exhibit anxiolytic and anti-convulsant activities and has been used as a nervine sedative. As per the Indian Materia Medica, E. variegata barks have been traditionally known to act on the central nervous system. However, there is a lack of data demonstrating this.

AIM OF THE STUDY

Our study focuses on previously unreported anti-depressant activity of E. variegata bark ethanolic extract (EBE) and determination of its mechanism of action possibly through regulation of monoamine oxidase activity in mouse brain homogenates.

MATERIALS AND METHODS

EBE was characterized using standard protocols for phytochemical analysis, followed by liquid chromatography-mass spectrometry (LC-MS) and gas chromatography-mass spectrometry (GC-MS) analysis. The compounds in EBE (previously reported for anti-depressant activity), were further studied by LC-MS/MS and GC-MS/MS analysis. Anti-depressant activity of EBE (50, 100, 200 and 500 mg/kg) was evaluated in Swiss albino mice using acute and chronic tail suspension test (TST) and forced swim test (FST) models. Furthermore, the possible mechanism of action of EBE was evaluated using the chronic unpredictable mild stress (CUMS) model, wherein inhibitory effects on monoamine oxidase (MAO) A and B were assessed by spectrophotometric-chemical analysis in mouse whole brain homogenates.

RESULTS

EBE showed significant reduction in immobility time periods in both TST (acute: 50, 100 and 200 mg/kg and chronic: 100 and 200 mg/kg) and FST (acute: 200 mg/kg and chronic: 100, 200 and 500 mg/kg) models. Moreover, the locomotor activity test confirmed that acute and chronic administration of EBE did not significantly affect the motor activity of mice. In the CUMS model, EBE when administered alone (100 and 200 mg/kg) and in combination (50, 100 and 200 mg/kg) with escitalopram (15 mg/kg), showed significant reductions in immobility time periods compared to the control group, in the acute FST performed on 22nd day of CUMS. Furthermore, when administered alone (50, 100 and 200 mg/kg), EBE showed significant inhibition in MAO-A and B activities compared to the control group. When used in combination, EBE (50, 100 and 200 mg/kg) showed synergistic action with escitalopram (15 mg/kg), resulting in significantly greater inhibition of MAO-A and B activities, compared to both EBE alone and escitalopram alone. Phytochemical analysis of EBE revealed presence of sugars, steroids, glycosides, alkaloids and tannins. LC-MS, LC-MS/MS, GC-MS and GC-MS/MS analysis identified components in EBE, previously reported for their anti-depressant activity.

CONCLUSIONS

The study thus concluded the anti-depressant like activity of EBE. The study identified components present in EBE that may be responsible for its anti-depressant activity. The possible mechanism of action of EBE was also investigated in the CUMS model, wherein inhibitory effects of EBE on MAO-A and B activities in the mouse brain were demonstrated. Furthermore, the study confirms the traditional use of E. variegata barks in CNS related activities through its anti-depressant like activity.

Nicotinic receptor antagonists as treatments for nicotine abuse

Despite the proven efficacy of current pharmacotherapies for tobacco dependence, relapse rates continue to be high, indicating that novel medications are needed. Currently, several smoking cessation agents are available, including varenicline (Chantix®), bupropion (Zyban®), and cytisine (Tabex®). Varenicline and cytisine are partial agonists at the α4β2* nicotinic acetylcholine receptor (nAChR). Bupropion is an antidepressant but is also an antagonist at α3β2* ganglionic nAChRs. The rewarding effects of nicotine are mediated, in part, by nicotine-evoked dopamine (DA) release leading to sensitization, which is associated with repeated nicotine administration and nicotine addiction. Receptor antagonists that selectivity target central nAChR subtypes mediating nicotine-evoked DA release should have efficacy as tobacco use cessation agents with the therapeutic advantage of a limited side-effect profile. While α-conotoxin MII (α-CtxMII)-insensitive nAChRs (e.g., α4β2*) contribute to nicotine-evoked DA release, these nAChRs are widely distributed in the brain, and inhibition of these receptors may lead to nonselective and untoward effects. In contrast, α-CtxMII-sensitive nAChRs mediating nicotine-evoked DA release offer an advantage as targets for smoking cessation, due to their more restricted localization primarily to dopaminergic neurons. Small drug-like molecules that are selective antagonists at α-CtxMII-sensitive nAChR subtypes that contain α6 and β2 subunits have now been identified. Early research identified a variety of quaternary ammonium analogs that were potent and selective antagonists at nAChRs mediating nicotine-evoked DA release. More recent data have shown that novel, nonquaternary bis-1,2,5,6-tetrahydropyridine analogs potently inhibit (IC50<1nM) nicotine-evoked DA release in vitro by acting as antagonists at α-CtxMII-sensitive nAChR subtypes; these compounds also decrease NIC self-administration in rats.

Effects of cannabidiol on the function of α7-nicotinic acetylcholine receptors

The effects of cannabidiol (CBD), a non-psychoactive ingredient of cannabis plant, on the function of the cloned α7 subunit of the human nicotinic acetylcholine (α7 nACh) receptor expressed in Xenopus oocytes were tested using the two-electrode voltage-clamp technique. CBD reversibly inhibited ACh (100 μM)-induced currents with an IC50 value of 11.3 µM. Other phytocannabinoids such as cannabinol and Δ(9)-tetrahydrocannabinol did not affect ACh-induced currents. CBD inhibition was not altered by pertussis toxin treatment. In addition, CBD did not change GTP-γ-S binding to the membranes of oocytes injected with α7 nACh receptor cRNA. The effect of CBD was not dependent on the membrane potential. CBD (10 µM) did not affect the activity of endogenous Ca(2+)-dependent Cl(-) channels, since the extent of inhibition by CBD was unaltered by intracellular injection of the Ca(2+) chelator BAPTA and perfusion with Ca(2+)-free bathing solution containing 2mM Ba(2+). Inhibition by CBD was not reversed by increasing ACh concentrations. Furthermore, specific binding of [(125)I] α-bungarotoxin was not inhibited by CBD (10 µM) in oocytes membranes. Using whole cell patch clamp technique in CA1 stratum radiatum interneurons of rat hippocampal slices, currents induced by choline, a selective-agonist of α7-receptor induced currents were also recoded. Bath application of CBD (10 µM) for 10 min caused a significant inhibition of choline induced currents. Finally, in hippocampal slices, [(3)H] norepinephrine release evoked by nicotine (30 µM) was also inhibited by 10 µM CBD. Our results indicate that CBD inhibits the function of the α7-nACh receptor.

Bis-azaaromatic quaternary ammonium salts as ligands for the blood-brain barrier choline transporter

A series of bis-azaaromatic quaternary ammonium compounds containing flexible polymethylenic linkers as well as conformationally restricted linkers were evaluated for their affinity for the blood-brain barrier choline transporter (BBB-ChT). The preliminary structure-activity relationships obtained from this study suggest that incorporating a linear, conformationally restricted linker into the molecule improves affinity for the BBB-ChT.

Predictive screening model for potential vector-mediated transport of cationic substrates at the blood-brain barrier choline transporter

A set of semi-rigid cyclic and acyclic bis-quaternary ammonium analogs, which were part of a drug discovery program aimed at identifying antagonists at neuronal nicotinic acetylcholine receptors, were investigated to determine structural requirements for affinity at the blood-brain barrier choline transporter (BBB CHT). This transporter may have utility as a drug delivery vector for cationic molecules to access the central nervous system. In the current study, a virtual screening model was developed to aid in rational drug design/ADME of cationic nicotinic antagonists as BBB CHT ligands. Four 3D-QSAR comparative molecular field analysis (CoMFA) models were built which could predict the BBB CHT affinity for a test set with an r(2) <0.5 and cross-validated q(2) of 0.60, suggesting good predictive capability for these models. These models will allow the rapid in silico screening of binding affinity at the BBB CHT of both known nicotinic receptor antagonists and virtual compound libraries with the goal of informing the design of brain bioavailable quaternary ammonium analogs that are high affinity selective nicotinic receptor antagonists.