In vitro action of mescaline. Possible mode of action

Mescaline: The forgotten psychedelic

INTRODUCTION

Mescaline (3,4,5-trimethoxyphenethylamine) is one of the oldest hallucinogens, with evidence of use dating back 5700 years. Mescaline is a naturally occurring alkaloid found in cacti, mainly in the peyote cactus (Lophophora williamsii) and in the cacti of the Echinopsis genus. Since the prohibition of psychoactive substances in the early 70s, research on mescaline and other classical psychedelics has been limited.

OBJECTIVES

This article aims to review the pharmacology and behavioural effects of mescaline, focusing on preclinical and clinical research.

FINDINGS

Mescaline is a serotonin 5HT2A/2C receptor agonist, with its main hallucinogenic effects being mediated via its 5HT2A receptor agonist action. It also exerts effects via agonist binding at α1A/2A noradrenaline and D1/2/3 dopamine receptors. Overall, mescaline has anxiolytic-like effects in animals and increases prosocial behaviour, locomotion, and response reactivity. In humans, mescaline can induce euphoria, hallucinations, improvements in well-being and mental health conditions, and psychotomimetic effects in a naturalistic or religious setting.

CONCLUSION

The pharmacological mechanisms of mescaline are similar to those of other classical psychedelics, like psilocybin and lysergic acid diethylamide (LSD). Mescaline appears to be safe to consume, with most intoxications being mild and easily treatable. Improvement in mental well-being and its ability to overcome alcoholism render mescaline potentially beneficial in clinical settings. This article is part of the Special Issue on 'Psilocybin Research'.

Dark Classics in Chemical Neuroscience: Mescaline

Archeological studies in the United States, Mexico, and Peru suggest that mescaline, as a cactus constituent, has been used for more than 6000 years. Although it is a widespread cactus alkaloid, it is present in high concentrations in few species, notably the North American peyote ( Lophophora williamsii) and the South American wachuma ( Trichocereus pachanoi, T. peruvianus, and T. bridgesii). Spanish 16th century chroniclers considered these cacti "diabolic", leading to their prohibition, but their use persisted to our days and has been spreading for the last 150 years. In the late 1800s, peyote attracted scientific attention; mescaline was isolated, and its role in the psychedelic effects of peyote tops or "mescal buttons" was demonstrated. Its structure was established by synthesis in 1929, and alternative routes were developed, providing larger amounts for pharmacological and biosynthetic research. Although its effects are attributed mainly to its action as a 5-HT_2A serotonin receptor agonist, mescaline binds in a similar concentration range to 5-HT_1A and α_2A receptors. It is largely excreted unchanged in human urine, and its metabolic products are apparently unrelated to its psychedelic properties. Its low potency is probably responsible for its relative neglect by recreational substance users, as the successful search for structure-activity relationships in the hallucinogen field focused largely on finding more potent analogues. Renewed interest in the possible therapeutic applications of psychedelic drugs may hopefully lead to novel insights regarding the commonalities and differences between the actions of individual classic hallucinogens.

Actions of mescaline on isolated rat atria

Mescaline, in concentrations of 5 x 10(-4) and 1 x 10(-3) M, produced negative chronotropic and positive inotropic responses in isolated, spontaneously beating rat atria. In tissues driven at a constant rate, the inotropic response was diminished greatly, indicating that the increment in the force of contraction was secondary to the reduction in rate. These chronotropic and inotropic responses were not altered consistently by pretreatment with the histamine antagonists chlorpheniramine and metiamide.

Studies on mode of antagonism between adrenergic beta-mimetics and beta-blocking agents (I). Beta-blocking action of mescaline and its derivatives

In order to clarify whether or not trimetoquinol (TMQ) and isoproterenol (ISO) interact with the same receptor, the pA2 values of propranolol (PR) and certain trimethoxybenzene derivatives were measured, using isolated guinea pig tracheal chains. Each of PR, mescaline (MES) and its derivatives gave almost the same pA2 values for TMQ and ISO. Introduction of an alkyl group into the N atom of MES increased the affinity to the receptor in the order of methyl and isopropyl as well as the structureactivity relationship of catecholamines, while that of hydroxyl group in the beta-position of the side chain decreased pA2 values. The slopes of the regression lines for anti-TMQ action of MES derivatives as well as PR were almost one, but those for their anti-ISO action were less than 0.3. 3,4,5-Trimethoxyaniline and 3,4,5-trimethoxybenzoic acid had little activity as beta-blocking agents. These results suggest the possibility that TMQ and ISO would interact with the same receptor sites. The importance of the trimethoxybenzene and the phenethylamine moieties in the MES-derivatives for anti-TMQ action is discussed.

Role of serotonin in the discriminative stimulus properties of mescaline

Rats were trained to discriminate intraperitoneally administered mescaline from saline in a two-lever operant chamber for food reinforcement. Reward was contingent upon responses made greater than 15 sec apart (DRL-15) on the appropriate lever paired with either drug or saline administration. Following the establishment of discriminative response control by mescaline, the animals were tested for stimulus generalization produced by mescaline after: (a) blockade of periphreral and central serotonin (5-HT) receptors with cinanserin, methysergide, or cyproheptadine; (b) blockade of peripheral 5-HT receptors with xylamidine tosylate; and (c) depletion of brain 5-HT with the tryptophan hydroxylase inhibitor p-chlorophenylalanine (PCPA). The results show that all three central 5-HT antagonists greatly reduced the discriminability of mescaline while the peripheral antagonist, xylamidine tosylate, was without effect. Furthermore, these agents at the doses employed did not effect the discriminability of saline. Depletion of 5-HT with PCPA potentiated the effects of a sub-threshold dose of mescaline and slightly reduced the discriminability of saline. The results indicate that mescaline produces its discriminative stimulus properties by directly stimulating central serotonergic receptors.