Coronaridine congeners potentiate GABAA receptors and induce sedative activity in mice in a benzodiazepine-insensitive manner

Catharanthine Modulates Mesolimbic Dopamine Transmission and Nicotine Psychomotor Effects via Inhibition of α6-Nicotinic Receptors and Dopamine Transporters

Iboga alkaloids, also known as coronaridine congeners, have shown promise in the treatment of alcohol and opioid use disorders. The objective of this study was to evaluate the effects of catharanthine and 18-methoxycoronaridine (18-MC) on dopamine (DA) transmission and cholinergic interneurons in the mesolimbic DA system, nicotine-induced locomotor activity, and nicotine-taking behavior. Utilizing ex vivo fast-scan cyclic voltammetry (FSCV) in the nucleus accumbens core of male mice, we found that catharanthine or 18-MC differentially inhibited evoked DA release. Catharanthine inhibition of evoked DA release was significantly reduced by both α4 and α6 nicotinic acetylcholine receptors (nAChRs) antagonists. Additionally, catharanthine substantially increased DA release more than vehicle during high-frequency stimulation, although less potently than an α4 nAChR antagonist, which confirms previous work with nAChR antagonists. Interestingly, while catharanthine slowed DA reuptake measured via FSCV ex vivo, it also increased extracellular DA in striatal dialysate from anesthetized mice in vivo in a dose-dependent manner. Superfusion of catharanthine or 18-MC inhibited the firing rate of striatal cholinergic interneurons in a concentration dependent manner, which are known to potently modulate presynaptic DA release. Catharanthine or 18-MC suppressed acetylcholine currents in oocytes expressing recombinant rat α6/α3β2β3 or α6/α3β4 nAChRs. In behavioral experiments using male Sprague-Dawley rats, systemic administration of catharanthine or 18-MC blocked nicotine enhancement of locomotor activity. Importantly, catharanthine attenuated nicotine self-administration in a dose-dependent manner while having no effect on food reinforcement. Lastly, administration of catharanthine and nicotine together greatly increased head twitch responses, indicating a potential synergistic hallucinogenic effect. These findings demonstrate that catharanthine and 18-MC have similar, but not identical effects on striatal DA dynamics, striatal cholinergic interneuron activity and nicotine psychomotor effects.

Tabernanthalog and ibogainalog inhibit the α7 and α9α10 nicotinic acetylcholine receptors via different mechanisms and with higher potency than the GABAA receptor and CaV2.2 channel

In this study, we have investigated the pharmacological activity and structural interaction of two novel psychoplastogens, tabernanthalog (TBG) and ibogainalog (IBG) at heterologously-expressed rat (r) and human (h) nicotinic acetylcholine receptors (nAChRs), the rα1β2γ2L γ-aminobutyric acid type A receptor (GABAAR), and the human voltage-gated N-type calcium channel (CaV2.2 channel). Both compounds inhibited the nAChRs with the following receptor selectivity: α9α10 > α7 > α3β2 ≅ α3β4, indicating that β2/β4 subunits are relatively less important for their activity. The potencies of TBG and IBG were comparable at hα7 and hα9α10 subtypes, and comparable to their rat counterparts. TBG- and IBG-induced inhibition of rα7 was ACh concentration-independent and voltage-dependent, whereas rα9α10 inhibition was ACh concentration-dependent and voltage-independent, suggesting that they interact with the α7 ion channel pore and α9α10 orthosteric ligand binding site, respectively. These results were supported by molecular docking studies showing that at the α7 model TBG forms stable interactions with luminal rings at 9', 13', and 16', whereas IBG mostly interacts with the extracellular-transmembrane junction. In the α9α10 model, however, these compounds interacted with several residues from the principal (+) and complementary (-) sides in the transmitter binding site. Ibogaminalog (DM506) also interacted with a non-luminal site at α7, and one α9α10 orthosteric site. TBG and IBG inhibited the GABAAR and CaV2.2 channels with 10 to 30-fold lower potencies. In sum, we show that TBG and IBG inhibit the α7 and α9α10 nAChRs by noncompetitive and competitive mechanisms, respectively, and with higher potency than the GABAAR and CaV2.2 channel.

The novel non-hallucinogenic compound DM506 (3-methyl-1,2,3,4,5,6-hexahydroazepino[4,5-b]indole) induces sedative- and anxiolytic-like activity in mice by a mechanism involving 5-HT2A receptor activation

The anxiolytic and sedative-like effects of 3-methyl-1,2,3,4,5,6-hexahydroazepino[4,5-b]indole (DM506), a non-hallucinogenic compound derived from ibogamine, were studied in mice. The behavioral effects were examined using Elevated O-maze and novelty suppressed feeding (NSFT) tests, open field test, and loss of righting reflex (LORR) test. The results showed that 15 mg/kg DM506 induced acute and long-lasting anxiolytic-like activity in naive and stressed/anxious mice, respectively. Repeated administration of 5 mg/kg DM506 did not cause cumulative anxiolytic activity or any side effects. Higher doses of DM506 (40 mg/kg) induced sedative-like activity, which was inhibited by a selective 5-HT2A receptor antagonist, volinanserin. Electroencephalography results showed that 15 mg/kg DM506 fumarate increased the transition from a highly alert state (fast γ wavelength) to a more synchronized deep-sleeping activity (δ wavelength), which is reflected in the sedative/anxiolytic activity in mice but without the head-twitch response observed in hallucinogens. The functional, radioligand binding, and molecular docking results showed that DM506 binds to the agonist sites of human 5-HT2A (Ki = 24 nM) and 5-HT2B (Ki = 16 nM) receptors and activates them with a potency (EC50) of 9 nM and 3 nM, respectively. DM506 was relatively less potent and behaved as a partial agonist (efficacy <80%) for both receptor subtypes compared to the full agonist DOI (2,5-dimethoxy-4-iodoamphetamine). Our study showed for the first time that the non-hallucinogenic compound DM506 induces anxiolytic- and sedative-like activities in naïve and stressed/anxious mice in a dose-, time-, and volinanserin-sensitive manner, likely through mechanisms involving 5-HT2A receptor activation.

Coronaridine congeners induce sedative and anxiolytic-like activity in naïve and stressed/anxious mice by allosteric mechanisms involving increased GABAA receptor affinity for GABA

The sedative and anxiolytic-like activity of two coronaridine congeners, (+)-catharanthine and (-)-18-methoxycoronaridine (18-MC), was studied in male and female mice. The underlying molecular mechanism was subsequently determined by fluorescence imaging and radioligand binding experiments. The loss of righting reflex and locomotor activity results showed that both (+)-catharanthine and (-)-18-MC induce sedative effects at doses of 63 and 72 mg/kg in a sex-independent manner. At a lower dose (40 mg/kg), only (-)-18-MC induced anxiolytic-like activity in naïve mice (elevated O-maze test), whereas both congeners were effective in mice under stressful/anxiogenic conditions (light/dark transition test) and in stressed/anxious mice (novelty-suppressed feeding test), where the latter effect lasted for 24 h. Coronaridine congeners did not block pentylenetetrazole-induced anxiogenic-like activity in mice. Considering that pentylenetetrazole inhibits GABA_A receptors, this result supports a role for this receptor in the activity mediated by coronaridine congeners. Functional and radioligand binding results showed that coronaridine congeners interact with a site different from that for benzodiazepines, increasing GABA_A receptor affinity for GABA. Our study showed that coronaridine congeners induce sedative and anxiolytic-like activity in naïve and stressed/anxious mice in a sex-independent fashion, likely by a benzodiazepine-independent allosteric mechanism that increases GABA_A receptor affinity for GABA.

(+)-Catharanthine and (-)-18-methoxycoronaridine induce antidepressant-like activity in mice by differently recruiting serotonergic and norepinephrinergic neurotransmission

The antidepressant-like activity of (+)-catharanthine and (-)-18-methoxycoronaridine [(-)-18-MC] was studied in male and female mice using forced swim (FST) and tail suspension tests (TST). The underlying molecular mechanism was assessed by electrophysiological, radioligand, and functional experiments. The FST results showed that acute administration (40 mg/kg) of (+)-catharanthine or (-)-18-MC induces similar antidepressant-like activity in male and female mice at 1 h and 24 h, whereas the TST results showed a lower effect for (-)-18-MC at 24 h. Repeated treatment at lower dose (20 mg/kg) augmented the efficacy of both congeners. The FST results showed that (-)-18-MC reduces immobility and increases swimming times without changing climbing behavior, whereas (+)-catharanthine reduces immobility time, increases swimming times more markedly, and increases climbing behavior. To investigate the contribution of the serotonin and norepinephrine transporters in the antidepressant effects of (+)-catharanthine and (-)-18-MC, we conducted in vitro radioligand and functional studies. Results obtained demonstrated that (+)-catharanthine inhibits norepinephrine transporter with higher potency/affinity than that for (-)-18-MC, whereas both congeners inhibit serotonin transporter with similar potency/affinity. Moreover, whereas no congener activated/inhibited/potentiated the function of serotonin receptor 3A or serotonin receptor 3AB, both increased serotonin receptor 3A receptor desensitization. Depletion of serotonin decreased the antidepressant-like activity of both congeners, whereas norepinephrine depletion only decreased (+)-catharanthine's activity. Our study shows that coronaridine congeners induce antidepressant-like activity in a dose- and time-dependent, and sex-independent, manner. The antidepressant-like property of both compounds involves serotonin transporter inhibition, without directly activating/inhibiting serotonin receptors 3, while (+)-catharanthine also mobilizes norepinephrinergic neurotransmission.

Virtual Screening and Hit Selection of Natural Compounds as Acetylcholinesterase Inhibitors

Acetylcholinesterase (AChE) is one of the classical targets in the treatment of Alzheimer's disease (AD). Inhibition of AChE slows down the hydrolysis of acetycholine and increases choline levels, improving the cognitive function. The achieved success of plant-based natural drugs acting as AChE inhibitors, such as galantamine (GAL) from Galanthus genus and huperzine A from Huperzia serrate (approved drug in China), in the treatment of AD, and the fact that natural compounds (NCs) are considered as safer and less toxic compared to synthetic drugs, led us to screen the available NCs (almost 150,000) in the ZINC12 database for AChE inhibitory activity. The compounds were screened virtually by molecular docking, filtered for suitable ADME properties, and 32 ligands from 23 structural groups were selected. The stability of the complexes was estimated via 1 μs molecular dynamics simulation. Ten compounds formed stable complexes with the enzyme and had a vendor and a reasonable price per mg. They were tested for AChE inhibitory and antioxidant activity. Five compounds showed weak AChE inhibition and three of them exhibited high antioxidant activity.

(+)-Catharanthine potentiates the GABAA receptor by binding to a transmembrane site at the β(+)/α(-) interface near the TM2-TM3 loop

(+)-Catharanthine, a coronaridine congener, potentiates the γ-aminobutyric acid type A receptor (GABA_AR) and induces sedation through a non-benzodiazepine mechanism, but the specific site of action and intrinsic mechanism have not beendefined. Here, we describe GABA_AR subtype selectivity and location of the putative binding site for (+)-catharanthine using electrophysiological, site-directed mutagenesis, functional competition, and molecular docking experiments. Electrophysiological and in silico experiments showed that (+)-catharanthine potentiates the responses to low, subsaturating GABA at β2/3-containing GABA_ARs 2.4-3.5 times more efficaciously than at β1-containing GABA_ARs. The activity of (+)-catharanthine is reduced by the β2(N265S) mutation that decreases GABA_AR potentiation by loreclezole, but not by the β3(M286C) or α1(Q241L) mutations that reduce receptor potentiation by R(+)-etomidate or neurosteroids, respectively. Competitive functional experiments indicated that the binding site for (+)-catharanthine overlaps that for loreclezole, but not those for R(+)-etomidate or potentiating neurosteroids. Molecular docking experiments suggested that (+)-catharanthine binds at the β(+)/α(-) intersubunit interface near the TM2-TM3 loop, where it forms H-bonds with β2-D282 (TM3), β2-K279 (TM2-TM3 loop), and β2-N265 and β2-R269 (TM2). Site-directed mutagenesis experiments supported the in silico results, demonstrating that the K279A and D282A substitutions, that lead to a loss of H-bonding ability of the mutated residue, and the N265S mutation, impair the gating efficacy of (+)-catharanthine. We infer that (+)-catharanthine potentiates the GABA_AR through several H-bond interactions with a binding site located in the β(+)/α(-) interface in the transmembrane domain, near the TM2-TM3 loop, where it overlaps with loreclezole binding site.

Coronaridine congeners decrease neuropathic pain in mice and inhibit α9α10 nicotinic acetylcholine receptors and CaV2.2 channels

The primary aim of this study was to determine the anti-neuropathic activity of (±)-18-methoxycoronaridine [(±)-18-MC] and (+)-catharanthine in mice by using the oxaliplatin-induced neuropathic pain paradigm and cold plate test. The results showed that both coronaridine congeners induce anti-neuropathic pain activity at a dose of 72 mg/kg (per os), whereas a lower dose (36 mg/kg) of (+)-catharanthine decreased the progress of oxaliplatin-induced neuropathic pain. To determine the underlying molecular mechanism, electrophysiological recordings were performed on α9α10, α3β4, and α4β2 nAChRs as well as voltage-gated calcium (Ca_V2.2) channels modulated by G protein-coupled γ-aminobutyric acid type B receptors (GABA_BRs). The results showed that (±)-18-MC and (+)-catharanthine competitively inhibit α9α10 nAChRs with potencies higher than that at α3β4 and α4β2 nAChRs and directly block Ca_V2.2 channels without activating GABA_BRs. Considering the potency of the coronaridine congeners at Cav2.2 channels and α9α10 nAChRs, and the calculated brain concentration of (+)-catharanthine, it is plausible that the observed anti-neuropathic pain effects are mediated by peripheral and central mechanisms involving the inhibition of α9α10 nAChRs and/or Ca_V2.2 channels.