Effect of tetrahydrocannabinols on brain acetylcholine

Pharmaceutical treatment for cognitive deficits in Alzheimer's disease and other neurodegenerative conditions: exploring new territory using traditional tools and established maps

RATIONALE

Over 30 years ago, we began to develop a nonhuman primate model to study cognitive deficits of age-related neurodegenerative diseases and their neuroanatomical-neurochemical underpinnings for purposes of translating this work toward first pharmacotherapies. This effort produced several notable findings that eventually received consensus support, which we have been asked to review.

OBJECTIVES

A discussion of these findings, in the context of issues and obstacles confronted and principles applied, might facilitate the development of even more effective models and treatments, not only for Alzheimer's disease (AD) but for many other disorders involving cognitive deficits.

RESULTS

Collectively, our research provided first evidence of the following: aged primates can be used as 'models' for human age-related neurodegenerative diseases; key cognitive deficits in early AD share important conceptual similarities to deficits in both aged monkeys as well as non-demented humans (e.g., age-associated memory impairment and mild cognitive impairment); pharmacological intervention can reduce age-related cognitive impairments in animals that are conceptually similar to those seen in human diseases, including AD; cholinergics would likely be the first approved therapeutics for AD; and that many other classes of drugs would not likely succeed.

CONCLUSIONS

Despite the early promise shown by behavioral/functional approaches to develop treatment strategies, the dramatic shift in focus away from behavioral outcomes in animal neurodegenerative research that began 20 years ago has compromised further progress and continues to impede our ability to understand how these diseases impair human cognition and what pathways might lead to effective therapies. Principles applied successfully in the past should provide guidance for facilitating efforts in the future.

On neurodegenerative diseases, models, and treatment strategies: lessons learned and lessons forgotten a generation following the cholinergic hypothesis

Life's Journey If life is indeed a journey, then poetry must be the map that reveals all its topographic possibilitiesellipsis while science is the compass that keeps us from getting lost. -R. T. Bartus, Simple Words for Complex Lives, (c) 1998 In the nearly 20 years since the cholinergic hypothesis was initially formulated, significant progress has been achieved. Initial palliative treatments for Alzheimer's disease (AD) have proven beneficial and have gained FDA approval, the use of animal models for studying AD and other neurodegenerative diseases has achieved wider acceptance, and important insight into the potential causes and pathogenic variables associated with various neurodegenerative diseases continues to increase. This paper reviews the current status of the cholinergic hypothesis in the context of continuing efforts to improve upon existing treatments for AD and explores the role that animal models might continue to play. Using the benefit of hindsight, particular emphasis is placed on an analysis of the approaches, strategies, and assumptions regarding animal models that proved useful in developing the initial treatments and those that did not. Additionally, contemporary issues of AD are discussed within the context of the cholinergic hypothesis, with particular attention given to how they may impact the further refinement of animal models, and the development of even more effective treatments. Finally, arguments are presented that, despite the deserved enthusiasm and optimism for identifying means of halting the pathogenesis of AD, a clear need for more effective palliative treatments will continue, long after successful pathogenic treatments are available. This review, therefore, focuses on issues and experiences intended to: (a) facilitate further development and use of animal models for AD and other neurodegenerative diseases, and (b) accelerate the identification of newer, even more effective treatments.

Effects of acute cannabis use on urinary neurotransmitter metabolites and cyclic nucleotides in man

The noradrenaline, dopamine and serotonin metabolites methoxyhydroxyphenylglycol (MHPG), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA), as well as the cyclic nucleotides c-AMP and c-GMP were estimated in urine samples of five normal volunteers. Ten control samples and two samples after cannabis use were analyzed for each volunteer. Cannabis use caused significant decreases in MHPG and c-AMP, and increases in HVA, while 5-HIAA and c-GMP excretion remained unchanged. The results indicate that cannabis use interferes with catecholaminergic mechanisms in man, decreasing the noradrenaline and increasing dopamine turnover, probably through action on presynaptic receptors.

Tetrahydrocannabinol potentiates reserpine-induced hypokinesia

Delta-9-tetrahydrocannabinol (THC), a substance in marihuana, was found to produce a profound potentiation of reserpine-induced hypokinesia in rats as measured with a bar test. In these experiments, THC had no hypokinetic effect by itself but produced a more than 20-fold increase in the hypokinesia produced by reserpine. Reserpine-induced hypokinesia has been viewed as animal model of Parkinson's Disease. THC potentiation of reserpine-induced hypokinesia was observed to be both time- and dose-dependent (1 to 10 mg/kg THC). When administered by gavage to reserpine-pretreated subjects (7.5 mg/kg IP, 24 hours before), THC produced a potentiation of hypokinesia that developed fully within 1 hour, lasted at least 5 hours, and was absent by 12 hours after THC administration. This THC effect was slightly increased by physostigmine, a cholinesterase inhibitor, relatively unaffected by scopolamine, a muscarinic antagonist, and almost completely blocked by ethopropazine, an anticholinergic antiparkinson drug. The effect was completely unaffected by naloxone. Insofar as reserpine has been used with some clinical efficacy in hyperkinetic movement disorders such as Huntington's disease and tardive dyskinesia, it may be that potentiation of reserpine's hypokinetic effect by a drug such as THC could greatly increase the clinical value of reserpine or related drugs in the treatment of these disorders.

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.

Effect of cannabinoids on the turnover rate of acetylcholine in rat hippocampus, striatum and cortex

The effects of delta9-tetrahydrocannabinol (delta9-THC), the major psychoactive compound of marijuana, and cannabidiol (CBD), a non-psychoactive component, on the acetylcholine (ACh) concentration and the turnover rate of ACh (TRACh) have been studied in various regions of the rat brain. Neither delta9-THC doses from 0.2 to 10 mg/kg nor CBD (10 OR 20 MG/KG) alter the ACh concentration in the brain areas examined 30 min, after the intravenous injection. However, delta9-THC (doses from 0.2 to 10 mg/kg) causes a marked dose-related decrease in the TRACh in hippocampus whereas CBD is without effect in this brain region even when 20 mg/kg is given. Furthermore, high doses of delta9-THC (5 mg/kg) and CBD (20 mg/kg) that produce a significant decrease in the TRACh of striatum fail to change the TRACh in parietal cortex. The low doses of delta9-THC required to reduce hippocampal TRACh suggest that an action on these cholinergic mechanisms may play a role in the psychotomimetic activity of delta9-THC.

Tetrahydrocannabinol and acetylcholinesterase

Recent evidence suggests that the psychoactive effect of delta-9-tetrahydrocannabinol (delta9-THC), the major psychoactive constituent of marihuana, may be mediated through an alteration of cholinergic neurotransmission. One possible mechanism by which delta9-THC could have its effect is by affecting acetylcholinesterase (AChE) and there is evidence that has suggested that this may be an important mechanism. The results reported in the present study have shown that there is no physiologically important interaction between AChE and delta9-THC or its metabolites that could explain its psychoactive effects or the profound clinical depression observed when human marihuana users are administered the cholinesterase inhibitor physostigmine.

Effect of some cannabinoids on naloxone-precipitated abstinence in morphine-dependent mice

Mice were rendered morphine-dependent by the subcutaneous implantation of a pellet containing 75 mg of morphine base; 72 h after the implantation, the animals were injected intraperitoneally either with vehicle or with various doses of delta9-tetrahydrocannabinol, delta8-tetrahydrocannabinol, cannabidiol, cannabinol, or 11-hydroxy-delta8-tetrahydrocannabinol. Thirty minutes after injection of the cannabinoids, the antagonist, naloxone HC1, was administered to induce the stereotyped withdrawal jumping syndrome. The dose of naloxone needed to induce withdrawal jumping in 50% of the animals (ED50) was determined for each dose of the cannabinoids. All of the cannabinoids inhibited the naloxone-precipitated morphine abstinence as evidenced by an increase in the naloxone ED50. Two additional signs of morphine abstinence, defecation and rearing behavior, were also suppressed by the cannabinoids. The relative effectiveness of the cannabinoids in inhibiting morphine abstinence appeared to be in the following order: delta9-tetrahydrocannabinol greater than delta8-tetrahydrocannabinol greater than 11-hydroxy-delta8-tetrahydrocannabinol greater than cannabidiol greater than cannabinol. These data suggest that cannabinoids may be useful in facilitating narcotic detoxification.