Facilitation of benzodiazepine binding by levonantradol

A review of the direct targets of the cannabinoids cannabidiol, Δ9-tetrahydrocannabinol, N-arachidonoylethanolamine and 2-arachidonoylglycerol

Marijuana has been used by humans for thousands of years for both medicinal and recreational purposes. This included the treatment of pain, inflammation, seizures, and nausea. In the 1960s, the structure of the principal psychoactive ingredient Δ9-tetrahydrocannabinol was determined, and over the next few decades, two cannabinoid receptors were characterized along with the human endocannabinoid system and what it affects. This includes metabolism, the cardiovascular and reproductive systems, and it is involved in such conditions as inflammation, cancer, glaucoma, and liver and musculoskeletal disorders. In the central nervous system, the endocannabinoid system has been linked to appetite, learning, memory, and conditions such as depression, anxiety, schizophrenia, stroke, multiple sclerosis, neurodegeneration, addiction, and epilepsy. It was the profound effectiveness of cannabidiol, a non-psychoactive ingredient of marijuana, to relieve the symptoms of Dravet syndrome, a severe form of childhood epilepsy, that recently helped spur marijuana research. This has helped substantially to change society's attitude towards this potential source of useful drugs. However, research has also revealed that the actions of endocannabinoids, such as anandamide and 2-arachidonoylglycerol, and the phytocannabinoids, tetrahydrocannabinol and cannabidiol, were not just due to interactions with the two cannabinoid receptors but by acting directly on many other targets including various G-protein receptors and cation channels, such as the transient receptor potential channels for example. This mini-review attempts to survey the effects of these 4 important cannabinoids on these currently identified targets.

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.

Medicinal Chemistry Endeavors around the Phytocannabinoids

Over the past 50 years, a considerable research in medicinal chemistry has been carried out around the natural constituents of Cannabis sativa L. Following the identification of Delta9-tetrahydrocannabinol (Delta9-THC) in 1964, critical chemical modifications, e.g., variation of the side chain at C3 and the opening of the tricyclic scaffold, have led to the characterization of potent and cannabinoid receptor subtype-selective ligands. Those ligands that demonstrate high affinity for the cannabinoid receptors and good biological efficacy are still used as powerful pharmacological tools. This review summarizes past as well as recent developments in the structure-activity relationships of phytocannabinoids.

[3H]Ro 15-1788 binding to benzodiazepine receptors in mouse brain in vivo: marked enhancement by GABA agonists and other CNS drugs

Administration of the benzodiazepine receptor antagonist, [3H]Ro 15-1788, to mice intravenously was found to label these receptors in brain. Binding of [3H]Ro 15-1788 in vivo was strongly blocked by pretreating mice with clonazepam or diazepam. Marked enhancement of [3H]Ro 15-1788 binding in vivo was induced by progabide or sodium valproate. This effect was greater than a similar enhancement of [3H]flunitrazepam binding. The increased membrane-bound [3H]Ro 15-1788 elicited by progabide was completely dissociated on subsequent incubation with Ro 15-1788, diazepam or clobazam, indicating that the enhanced binding occurred at benzodiazepine receptors. Compounds that exert diazepam-like actions and/or indirect GABAergic activity (cartazolate, pentobarbital, methaqualone, levonantradol, phenytoin) elicited enhancement of [3H]Ro 15-1788 in vivo. Other CNS agents (atypical neuroleptics, GABA antagonists, baclofen, some 5-HT1 agonists) also induced elevation of [3H]Ro 15-1788 binding in vivo, as did drugs exerting vasodilatatory effects (papaverine, nimodipine, verapamil, prazosin, N6-cyclohexyladenosine). Possible explanations for enhancement of [3H]Ro 15-1788 binding in vivo include increase in the number of benzodiazepine receptors induced by GABA or GABAergic drugs or effects of binding enhancers that elevate brain levels of [3H]Ro 15-1788, such as accelerating cerebral blood flow, competing for radioligand binding sites in plasma or increasing metabolic stability of the radioligand.

Interaction of a water-soluble derivative of delta-9-tetrahydrocannabinol with [3H]diazepam and [3H]flunitrazepam binding to rat brain membranes

We have tested the effect of a psychoactive water-soluble derivative of delta-9-tetrahydrocannabinol, SP-111A, on the binding of [3H]diazepam and [3H]flunitrazepam to rat brain membranes. It was found that SP-111A reduced the specific binding of [3H]diazepam and [3H]flunitrazepam. The inhibition by SP-111A was dependent not only on the concentration of the ligand but also on the protein content of membrane preparations. The inhibition of the specific binding of [3H]diazepam by SP-111A was found to be competitive with Ki value of 3.1 microM. In the presence of 7.5 microM SP111A the apparent Kd of [3H]diazepam binding increased from 4.3 nM to 12.5 nM, without affecting the Bmax. The inhibition of the specific binding of [3H]flunitrazepam by SP-111A was also competitive, however, the IC50 was higher than with [3H]diazepam. The inhibition by SP-111A appeared to be caused by its tight binding to the benzodiazepine binding sites of brain membranes.

Enhancement of brain [3H]flunitrazepam binding and analgesic activity of synthetic cannabimimetics

Novel, synthetic cannabimimetics and delta 9-tetrahydrocannabinol were found to enhance the binding of [3H]flunitrazepam to mouse brain in vivo. This property, suggestive of facilitation of binding to benzodiazepine receptors, is consistent with the potentiation of the anticonvulsant activity of diazepam against pentylenetetrazol by these compounds. The relative potencies of delta 9-tetrahydrocannabinol and the new cannabimimetics for enhancing [3H]flunitrazepam binding in vivo could also be correlated with their relative analgesic efficacies. Similar pharmacological stereospecificity was displayed for both binding enhancement and analgesic effects. The following order of decreasing potency was observed: N-methyllevonantradol and (-)-CP-55,244 greater than levonantradol, canbisol, CP-42,096 and (-)-CP-55,940 greater than 9-beta-normethyl-9-beta-hydroxyhexahydrocannabinol, nabilone and CP-47,497 greater than delta 9-tetrahydrocannabinol. Dextronantradol, (+)-CP-55,940 and (+)-CP-55,244 were considerably less active than the respective (-)-enantiomers; cannabidiol was inactive. Extensive investigation of structure versus activity led to N-methyllevonantradol and the 3-(2-hydroxyphenyl)cyclohexanols derivative, (-)-CP-55,244, which are approximately 1000-fold more potent than delta 9-tetrahydrocannabinol.

Enhancement of diazepam activities induced by denzimol in mice

Denzimol, a new anticonvulsant drug, enhances the depressant and antimetrazol activities of diazepam in mice, in a dose and time-dependent fashion. The depressant and anticonvulsant activities of phenobarbital were not affected by Denzimol. It is suggested that Denzimol induces an increase in the number of the benzodiazepine receptors, and that such increase might be responsible for the enhancement of the diazepam's activities.

Levonantradol: a role for central prostanoid mechanisms?

Although delta 9-tetrahydrocannabinol (THC) possesses many pharmacologic activities, attempts to define sites of biochemical action for the natural cannabinoids have been hampered by their low solubility, their low potency, and their relative lack of biologic selectivity. We have recently described a potent, cannabinoid-related analgetic, levonantradol, which acts stereospecifically in animals to produce analgesia qualitatively similar to morphine but at 1/9 to 1/34 the dose. While levonantradol does not act at or through the opiate receptor, the finding of one-way cross tolerance in animals suggests that morphine and levonantradol influence common nociceptive pathways. This report describes a striking structural homology between PGE1 and levonantradol as elucidated by x-ray and conformational studies. This observation is consistent with the generally recognized involvement of prostaglandins in pain and emesis and may have relevance to the site of levonantradol's analgetic actions. More importantly, it provides an ongoing, heuristic basis for exploring, in depth, the role of prostaglandins in the action of levonantradol and cannabinoids.

Cannabinoids and the cholinergic system

delta 9-Tetrahydrocannabinol (THC) decreases EEG activation and causes slow waves in the cat. The EEG slow-wave activity is accompanied by a concomitant decrease in acetylcholine release from the neocortex. The findings suggest that THC depresses the brain stem activating system. Large doses of delta 8- and delta 9-THC increase brain acetylcholine levels in rodents such as the mouse and rat, but this effect is not seen with minimal doses of the cannabinoids which show behavioral effects. The most dramatic change produced by THC is that brain acetylcholine utilization is reduced primarily in the hippocampus.