Cys-loop receptors on cannabinoids: All high?

Endocannabinoids (eCBS) are endogenously derived lipid signaling molecules that serve as tissue hormones and interact with multiple targets, mostly within the endocannabinoid system (ECS). The ECS is a highly conserved regulatory system involved in homeostatic regulation, organ formation, and immunomodulation of chordates. The term “cannabinoid” evolved from the distinctive class of plant compounds found in Cannabis sativa, an ancient herb, due to their action on CB1 and CB2 receptors. CB1/2 receptors are the primary targets for eCBs, but their effects are not limited to the ECS. Due to the high interest and extensive research on the ECS, knowledge on its constituents and physiological role is substantial and still growing. Crosstalk and multiple targeting of molecules are common features of endogenous and plant compounds. Cannabimimetic molecules can be divided according to their origin, natural or synthetic, including phytocannabinoids (pCB’s) or synthetic cannabinoids (sCB’s). The endocannabinoid system (ECS) consists of receptors, transporters, enzymes, and signaling molecules. In this review, we focus on the effects of cannabinoids on Cys-loop receptors. Cys-loop receptors belong to the class of membrane-bound pentameric ligand gated ion channels, each family comprising multiple subunits. Mammalians possess GABA type A receptors (GABAAR), glycine receptors (GlyR), serotonin receptors type 3 (5-HT3R), and nicotinic acetylcholine receptors (nAChR). Several studies have shown different modulatory effects of CBs on multiple members of the Cys-loop receptor family. We highlight the existing knowledge, especially on subunits and protein domains with conserved binding sites for CBs and their possible pharmacological and physiological role in epilepsy and in chronic pain. We further discuss the potential for cannabinoids as first line treatments in epilepsy, chronic pain and other neuropsychiatric conditions, indicated by their polypharmacology and therapeutic profile.

Effects of cannabinoids on ligand-gated ion channels

Phytocannabinoids such as Δ⁹-tetrahydrocannabinol and cannabidiol, endocannabinoids such as N-arachidonoylethanolamine (anandamide) and 2-arachidonoylglycerol, and synthetic cannabinoids such as CP47,497 and JWH-018 constitute major groups of structurally diverse cannabinoids. Along with these cannabinoids, CB1 and CB2 cannabinoid receptors and enzymes involved in synthesis and degradation of endocannabinoids comprise the major components of the cannabinoid system. Although, cannabinoid receptors are known to be involved in anti-convulsant, anti-nociceptive, anti-psychotic, anti-emetic, and anti-oxidant effects of cannabinoids, in recent years, an increasing number of studies suggest that, at pharmacologically relevant concentrations, these compounds interact with several molecular targets including G-protein coupled receptors, ion channels, and enzymes in a cannabinoid-receptor independent manner. In this report, the direct actions of endo-, phyto-, and synthetic cannabinoids on the functional properties of ligand-gated ion channels and the plausible mechanisms mediating these effects were reviewed and discussed.

Why Do Marijuana and Synthetic Cannabimimetics Induce Acute Myocardial Infarction in Healthy Young People?

The use of cannabis preparations has steadily increased. Although cannabis was traditionally assumed to only have mild vegetative side effects, it has become evident in recent years that severe cardiovascular complications can occur. Cannabis use has recently even been added to the risk factors for myocardial infarction. This review is dedicated to pathogenetic factors contributing to cannabis-related myocardial infarction. Tachycardia is highly important in this respect, and we provide evidence that activation of CB₁ receptors in brain regions important for cardiovascular regulation and of presynaptic CB₁ receptors on sympathetic and/or parasympathetic nerve fibers are involved. The prototypical factors for myocardial infarction, i.e., thrombus formation and coronary constriction, have also been considered, but there is little evidence that they play a decisive role. On the other hand, an increase in the formation of carboxyhemoglobin, impaired mitochondrial respiration, cardiotoxic reactions and tachyarrhythmias associated with the increased sympathetic tone are factors possibly intensifying myocardial infarction. A particularly important factor is that cannabis use is frequently accompanied by tobacco smoking. In conclusion, additional research is warranted to decipher the mechanisms involved, since cannabis use is being legalized increasingly and Δ⁹-tetrahydrocannabinol and its synthetic analogue nabilone are indicated for the treatment of various disease states.

Serotonin and beyond-a tribute to Manfred Göthert (1939-2019)

Manfred Göthert, who had served Naunyn-Schmiedeberg's Arch Pharmacol as Managing Editor from 1998 to 2005, deceased in June 2019. His scientific oeuvre encompasses more than 20 types of presynaptic receptors, mostly on serotoninergic and noradrenergic neurones. He was the first to identify presynaptic receptors for somatostatin and ACTH and described many presynaptic receptors, known from animal preparations, also in human tissue. In particular, he elucidated the pharmacology of presynaptic 5-HT receptors. A second field of interest included ligand-gated and voltage-dependent channels. The negative allosteric effect of anesthetics at peripheral nACh receptors is relevant for the peripheral clinical effects of these drugs and modified the Meyer-Overton hypothesis. The negative allosteric effect of ethanol at NMDA receptors in human brain tissue occurred at concentrations found in the range of clinical ethanol intoxication. Moreover, the inhibitory effect of gabapentinoids on P/Q Ca2+ channels and the subsequent decrease in AMPA-induced noradrenaline release may contribute to their clinical effect. Another ligand-gated ion channel, the 5-HT3 receptor, attracted the interest of Manfred Göthert from the whole animal via isolated preparations down to the cellular level. He contributed to that molecular study in which 5-HT3 receptor subtypes were disclosed. Finally, he found altered pharmacological properties of 5-HT receptor variants like the Arg219Leu 5-HT1A receptor (which was also shown to be associated with major depression) and the Phe124Cys 5-HT1B receptor (which may be related to sumatriptan-induced vasospasm). Manfred Göthert was a brilliant scientist and his papers have a major impact on today's pharmacology.

Serotonin discovery and stepwise disclosure of 5-HT receptor complexity over four decades. Part II. Some contributions of Manfred Göthert

About 40% of the papers within the scientific oeuvre of Manfred Göthert (1939-2019) were dedicated to serotonin (5-hydroxytryptamine, 5-HT). He was not only the witness of the gradual definition of the fourteen 5-HT receptor subtypes but also was involved directly by identifying 5-HT_1B, 5-HT_1D and 5-HT₃ receptors. Moreover, he identified presynaptic 5-HT receptors on central and/or peripheral serotoninergic, noradrenergic and/or cholinergic neurones. Two inhibitory (5-HT_1B, 5-HT_1D) and two facilitatory (5-HT₃, 5-HT₄) receptors were found, the 5-HT_1B receptor representing a possible target for antidepressant drugs. Ten years earlier than electrophysiologists, he identified ligand-gated receptors like the 5-HT₃ and the nicotinic acetylcholine (nACh) receptor as targets of halothane. Simultaneously with, but independent of, other authors he found that ethanol allosterically inhibits N-methyl-D-aspartate (NMDA) receptors, which are affected at an even lower concentration than 5-HT₃ and nACh receptors. The latter two receptors were shown to be subject to allosteric inhibition also by cannabinoids via a mechanism unrelated to cannabinoid CB₁ or CB₂ receptors; cannabinoid inhibition of 5-HT₃ receptors may represent a new target for the treatment of neuropathic pain.

Interaction between urethane and cannabinoid CB1 receptor agonist and antagonist in penicillin-induced epileptiform activity

Previous experimental studies have shown that various anesthetics alter the effects of cannabinoid agonists and antagonists on the cardiac response to different stimuli. Since no data have shown an interaction between urethane and cannabinoid signaling in epilepsy, we examined the suitability of urethane with regard to testing the effects of a cannabinoid CB1 receptor agonist and an antagonist on penicillin-induced epileptiform activity in rats. Permanent screw electrodes for electrocorticographic (ECoG) recordings, and a permanent cannula for administration of the substances to the brain ventricles, were placed into the cranium of rats. Epileptiform activity was induced by injection of penicillin through the cannula in conscious animal. The CB1 receptor agonist arachidonyl-2-chloroethylamide (ACEA; 7.5 μg) and the CB1 receptor antagonist [N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3 carboxamide] (AM-251; 0.25 μg) were administered intracerebroventricularly 30 minutes after the penicillin application in urethane-anesthetized and conscious animals. Urethane completely eliminated spontaneous ictal events in ECoG recordings and reduced the frequency and total amount of epileptiform activity. It did not alter either the proconvulsant effects of AM-251 or the anticonvulsant effects of ACEA on penicillin-induced epileptiform activity. The electrophysiological evidence suggests that there is no possible interaction between urethane and cannabinoid CB1 receptors in this experimental model of epilepsy.

Orally administered nicotine effects on rat urinary bladder proliferation and carcinogenesis

Tobacco smoking is a major risk factor for human cancers including urinary bladder carcinoma. Cigarette smoke inhalation in mice and orally administered nicotine in rats and mice increased urothelial cell proliferation. Nicotine, a major component of smoke, induced cell proliferation in multiple cell types in vitro. In the present study, the enhancing effects of nicotine on F344 rat bladder carcinogenesis induced by N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) were examined. Nicotine administered in drinking water for 32 weeks following 4 weeks of BBN treatment significantly increased the incidence and number of urothelial carcinomas dose-dependently. Ki67 and pSTAT3 labeling indices and expression of nicotinic acetylcholine receptor alpha 7 (nAChRα7) in non-tumor bladder urothelial lesions were significantly increased by nicotine, but the TUNEL assay for apoptosis showed no increase. In a 4 week study, inhibitors of nicotinic acetylcholine receptor decreased nicotine-induced urothelial simple hyperplasia and Ki67 labeling index in the bladder and kidney pelvis at a single cytotoxic dose of nicotine (40 ppm). Urothelial cytotoxicity with regenerative proliferation was observed by light and scanning electron microscopy. In vitro, nicotine was not cytotoxic to rat or human immortalized urothelial cells (do not express nicotine receptors) below millimolar concentrations, nor in human RT4, T24 or UMUC3 urothelial carcinoma cells (express nicotine receptors). However, nicotine slightly, but statistically significantly, increased cell proliferation at micromolar concentrations in human urothelial carcinoma cells. These data suggest that nicotine enhances urinary bladder carcinogenesis by inducing cytotoxicity with regenerative proliferation. The possible role of direct mitogenesis, involving nAChR and STAT3 signaling and of nicotine receptors requires further investigation at non-cytotoxic doses of nicotine.

Cannabinoids Modulate Neuronal Activity and Cancer by CB1 and CB2 Receptor-Independent Mechanisms

Cannabinoids include the active constituents of Cannabis or are molecules that mimic the structure and/or function of these Cannabis-derived molecules. Cannabinoids produce many of their cellular and organ system effects by interacting with the well-characterized CB1 and CB2 receptors. However, it has become clear that not all effects of cannabinoid drugs are attributable to their interaction with CB1 and CB2 receptors. Evidence now demonstrates that cannabinoid agents produce effects by modulating activity of the entire array of cellular macromolecules targeted by other drug classes, including: other receptor types; ion channels; transporters; enzymes, and protein- and non-protein cellular structures. This review summarizes evidence for these interactions in the CNS and in cancer, and is organized according to the cellular targets involved. The CNS represents a well-studied area and cancer is emerging in terms of understanding mechanisms by which cannabinoids modulate their activity. Considering the CNS and cancer together allow identification of non-cannabinoid receptor targets that are shared and divergent in both systems. This comparative approach allows the identified targets to be compared and contrasted, suggesting potential new areas of investigation. It also provides insight into the diverse sources of efficacy employed by this interesting class of drugs. Obtaining a comprehensive understanding of the diverse mechanisms of cannabinoid action may lead to the design and development of therapeutic agents with greater efficacy and specificity for their cellular targets.

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.

Triphasic blood pressure responses to cannabinoids: do we understand the mechanism?

The cannabinoids comprise three major classes of substances, including compounds derived from the cannabis plant (e.g. Δ(9) -tetrahydrocannabinol and the chemically related substances CP55940 and HU210), endogenously formed (e.g. anandamide) and synthetic compounds (e.g. WIN55212-2). Beyond their psychotropic effects, cannabinoids have complex effects on blood pressure, including biphasic changes of Δ(9) -tetrahydrocannabinol and WIN55212-2 and an even triphasic effect of anandamide. The differing pattern of blood pressure changes displayed by the three types of compounds is not really surprising since, although they share an agonistic effect at cannabinoid CB(1) and CB(2) receptors, some compounds have additional effects. In particular, anandamide is known for its pleiotropic effects, and there is overwhelming evidence that anandamide influences blood pressure via (i) CB(1) receptors, (ii) TRPV1 receptors, (iii) endothelial cannabinoid receptors and (iv) degradation products. This review is dedicated to the description of the effects of externally added cannabinoids on cardiovascular parameters in vivo. First, the cardiovascular effects of cannabinoids in anaesthetized animals will be highlighted since most data have been generated in experiments of that type. The text will follow the three phases of anandamide on blood pressure, and we will check to which extent cardiovascular changes elicited by other cannabinoids show overlap with those effects or differ. The second part will be dedicated to the cardiovascular effects of the cannabinoids in conscious animals. In the third part, cardiovascular effects in humans will be discussed, and similarities and differences with respect to the data from animals will be examined.

Urethane, but not pentobarbitone, attenuates presynaptic receptor function in rats: a contribution to the choice of anaesthetic

BACKGROUND AND PURPOSE

We examined whether cannabinoid CB(1) and histamine H(3) receptors resemble alpha(2)-adrenoceptors in that their presynaptically mediated cardiovascular effects are less marked in urethane- than in pentobarbitone-anaesthetized pithed rats.

EXPERIMENTAL APPROACH

Effects of the cannabinoid agonist CP-55,940 and the H(3) receptor agonist imetit on electrically induced tachycardic and vasopressor responses, respectively, was compared in pithed rats anaesthetized with urethane or pentobarbitone. The affinity of urethane for the three receptors was measured by radioligand binding studies in rat brain cortex membranes and its potency assessed in superfused mouse tissues preincubated with (3)H-noradrenaline.

KEY RESULTS

The neurogenic tachycardic response was less markedly inhibited by CP-55,940 in urethane- than in pentobarbitone-anaesthetized pithed rats. Imetit inhibited the neurogenic vasopressor response after pentobarbitone but not after urethane. The catecholamine-induced tachycardic and vasopressor response did not differ between rats anaesthetized with either compound. Urethane 10 mM (plasma concentration reached under anaesthesia) did not affect binding to CB(1) or H(3) receptors and alpha(2) adrenoceptors, nor did it alter the inhibitory effect of agonists at the three receptors on electrically evoked (3)H-noradrenaline release.

CONCLUSIONS AND IMPLICATIONS

Urethane, but not pentobarbitone, abolished the H(3) receptor-mediated vascular response in pithed rats and attenuated the CB(1) receptor-mediated cardiac response much more than pentobarbitone. The weaker effects of CB(1), H(3) and alpha(2) receptor agonists cannot be explained by antagonism by urethane at the three receptors in vitro. Pentobarbitone, but not urethane, is suitable as an anaesthetic for investigations of inhibitory presynaptic receptor function in pithed and anaesthetized rats.

Methanandamide blocks amphetamine-induced behavioral sensitization in rats

Methanandamide acts at targets which modulate amphetamine-induced behaviors. Therefore, we investigated methanandamide effects on the acute hyperactivity produced by a single injection of amphetamine and behavioral sensitization induced by repeated amphetamine exposure in rats. Methanandamide (5mg/kg, i.p.) did not affect basal locomotor or stereotypical activity. Methanandamide (5mg/kg, i.p.) pretreatment did not alter the acute increase in locomotor or stereotypical activities produced by acute amphetamine (2mg/kg, i.p.). For chronic studies, rats injected with amphetamine (2mg/kg, i.p.) once daily for 3 consecutive days were then challenged with amphetamine (2mg/kg, i.p.) 5 days later. Expression of locomotor sensitization was blocked when methanandamide (5mg/kg, i.p.) was given once, just prior to amphetamine (2mg/kg, i.p.) challenge. In rats co-exposed to methanandamide (5mg/kg, i.p.) and amphetamine (2mg/kg, i.p.) on days 1-3 and then challenged with amphetamine (2mg/kg, i.p.) following 5 days of drug absence, the development of both locomotor and stereotypical sensitization was blocked. The ability of methanandamide to block amphetamine-sensitized behaviors suggests that this pharmacologically diverse lipid regulates signaling events impacted by repeated psychostimulant exposure.