Much has been learned since the publication of the 1982 Institute of Medicine (IOM) report Marijuana and Health.* Although it was clear then that most of the effects of marijuana were due to its actions on the brain, there was little information about how THC acted on brain cells (neurons), which cells were affected by THC, or even what general areas of the brain were most affected by THC. Too little was known about cannabinoid physiology to offer any scientific insights into the harmful or therapeutic effects of marijuana. That is no longer true. During the past 16 years, there have been major advances in what basic science discloses about the potential medical benefits of cannabinoids, the group of compounds related to THC. Many variants are found in the marijuana plant, and other cannabinoids not found in the plant have been chemically synthesized. Sixteen years ago it was still a matter of debate as to whether THC acted nonspecifically by affecting the fluidity of cell membranes or whether a specific pathway of action was mediated by a receptor that responded selectively to THC (Table 2.1).
*The field of neuroscience has grown substantially since the publication of the 1982 IOM report. The number of members in the Society for Neuroscience provides a rough measure of the growth in research and knowledge about the brain: as of the middle of 1998, there were over 27,000 members, more than triple the number in 1982.
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TABLE 2.1 Landmark Discoveries Since the 1982 IOM Report
Year
- Discovery
Primary Investigators
1986
- Potent cannabinoid agonists are developed; they are the key to discovering the receptor.
M. R. Johnson and L. S. Melvin75
1988
- First conclusive evidence of specific cannabinoid receptors.
A. Howlett and W. Devaneh36
1990
- The cannabinoid brain receptor (CB,) is cloned, its DNA sequence is identified, and its location in the brain is determined.
L. Matsuda107 and M. Herkenham et al60
1992
- Anandamide is discovered—a naturally occurring substance in the brain that acts on cannabinoid receptors.
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Basic science is the wellspring for developing new medications and is particularly important for understanding a drug that has as many effects as marijuana. Even committed advocates of the medical use of marijuana do not claim that all the effects of marijuana are desirable for every medical use. But they do claim that the combination of specific effects of marijuana enhances its medical value. An understanding of those specific effects is what basic science can provide. The multiple effects of marijuana can be singled out and studied with the goals of evaluating the medical value of marijuana and cannabinoids in specific medical conditions, as well as minimizing unwanted side effects. An understanding of the basic mechanisms through which cannabinoids affect physiology permits more strategic development of new drugs and designs for clinical trials that are most likely to yield conclusive results.
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Basic science has made it clear that cannabinoids can affect pain transmission and, specifically, that cannabinoids interact with the brain's endogenous opioid system, an important system for the medical treatment of pain (see chapter 4).
The cellular machinery that underlies the response of the body and brain to cannabinoids involves an intricate interplay of different systems. This chapter reviews the components of that machinery with enough detail to permit the reader to compare what is known about basic biology with the medical uses proposed for marijuana. For some readers that will be too much detail. Those readers who do not wish to read the entire chapter should, nonetheless, be mindful of the following key points in this chapter:
· The most far reaching of the recent advances in cannabinoid biology are the identification of two types of cannabinoid receptors (CB1 and CB2) and of anandamide, a substance naturally produced by the body that acts at the cannabinoid receptor and has effects similar to those of THC. The CB1 receptor is found primarily in the brain and mediates the psychological effects of THC. The CB2 receptor is associated with the immune system; its role remains unclear.
· The physiological roles of the brain cannabinoid system in humans are the subject of much active research and are not fully known; however, cannabinoids likely have a natural role in pain modulation, control of movement, and memory.
· Basic research in cannabinoid biology has revealed a variety of cellular pathways through which potentially therapeutic drugs could act on the cannabinoid system. In addition to the known cannabinoids, such drugs might include chemical derivatives of plantderived cannabinoids or of endogenous cannabinoids such as anandamide but would also include noncannabinoid drugs that act on the cannabinoid system.
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Many speakers at the public workshops associated with this study argued that animal studies of marijuana are not relevant to humans. Animal studies are not a substitute for clinical trials, but they are a necessary complement. Ultimately, every biologically active substance exerts its effects at the cellular and molecular levels, and the evidence has shown that this is remarkably consistent among mammals, even those as different in body and mind as rats and humans.
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The importance of knowing specific brain circuits that involve anandamide (and other endogenous cannabinoid ligands) is that such circuits are the pivotal elements for regulating specific brain functions, such as mood, memory, and cognition. Anandamide has been found in numerous regions of the human brain.
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Brain Region
- Functions Associated with Region
Brain regions in which cannabinoid receptors are abundant:
Basal ganglia
- Movement control
Substantia nigra pars reticulata
Entopeduncular nucleus
Globus pallidus
Putamen
Cerebellum
- Body movement coordination
Hippocampus
- Learning and memory, stress
Cerebral cortex, especially cingulate, frontal, and parietal regions
- Higher cognitive functions
Nucleus accumbens
- Reward center
Brain regions in which cannabinoid brain receptors are moderately concentrated:
Hypothalamus
- Body housekeeping functions (body temperature regulation, salt and water balance, reproductive function)
Amygdala
- Emotional response, fear
Spinal cord
- Peripheral sensation, including pain
Brain stem
- Sleep and arousal, temperature regulation, motor control
Central gray
Analgesia
Nucleus of the solitary tract
- Visceral sensation, nausea and vomiting
SOURCES: Based on reviews by Pertwee (1997b)124 and Herkenham (1995).57