Cerebral Effects

Cerebral Effects of cannabis use, such as biochemical addiction and the potential for cerebral damage, have been studied at some length. Current advances have discredited earlier claims and offered new insights into natural cannabinoids found in the brain.

Interference with dopamine production is the chemical basis of addiction in the use of alcohol, amphetamines, cocaine, nicotine, opiate compounds, and other powerful addictive substances. Clinical studies show that cannabis is dissimilar to classic addictive drugs in that it does not significantly interfere with dopamine production in the “pleasure centers” of the brain, according to the Office of Technology Assessment after ten years of research. [1]

Although one Italian researcher has reported a minor increase in dopamine activity (DA levels) associated with THC administration, [2] follow-up studies at the Brookhaven National Laboratories disagree. Examining the effects of cannabis on dopamine activity in rats, the Brookhaven Medical Department determined that:

"…unlike a number of drugs of abuse, THC does not alter the activity of A10 DA neurons and that the previously reported THC-induced increase in brain DA levels is not due to its action on firing rate or pattern in A10 DA neurons." [3]

The 1988 discovery of the CB1 receptor, found abundant in the brain, and the 1992 discovery of the CB2 receptor found throughout the body, clearly distinguish cannabinoid compounds from other substances [4] Cannabinoids bind to anandamide receptors in the frontal lobes of the brain, according o research published by the American Association for the Advancement of Science.[5] Researchers from the Duke University Medical Center in North Carolina agree:

Behavioral manifestations of marijuana intoxication may be associated with increased functional activity of the brain especially in the frontal cortex… and regression analysis indicated it correlated most markedly with the right frontal region. [6]

Anandamide is a natural neurotransmitter found in the brain that binds to the same neuroreceptors as do cannabinoids. [7], [8] Current research suggests that cannabinoids are very similar to anandamide, which regulates mood, memory, pain, movement, and appetite.[9] Although these recent findings are not thoroughly understood, it is clear that cannabis affects the brain in ways completely dissimilar to other drugs, in accordance with natural neurochemical pathways.

One decades-old study continues to fuel claims that cannabis use causes cerebral damage. Two unwitting rhesus monkeys were exposed to 200 times the normal human dose of THC, administered through a constant cloud of smoke. However, in a more recent study, rhesus monkeys exposed to the equivalent of five cannabis cigarettes per day for seven months (what would be termed heavy chronic use in an adult human) showed no signs of cerebral abnormalities, discrediting claims attached to the older study.[10]

A 1988 study of rhesus monkeys showed that high doses of THC impairs visual recognition memory, but not discrimination learning, even at very high does. [11] While clinical testing of human subjects indicates that cannabis use mildly interferes with short-term memory functions in novice users, long-term memory and learning skills are not affected. Minor short-term memory impairment induced by cannabis intoxication is temporary. Experienced users who have developed a tolerance to the psychoactive drug frequently display no signs of cerebral impairment while under the influence of cannabis. The natural development of a tolerance to the cerebral effects of cannabinoids is well established and was clearly recognized by the Institute of Medicine in 1999. [12]

During Britain’s recent movement toward reform of marijuana laws, hundreds of doctors and scientists have supplied evidence to the government detailing the therapeutic value and non-addictive properties of cannabis. Professor Colin Blakemore, chairman of the British Neuroscience Association, told the members of a 1997 conference, “Efforts to prove the damaging effects of cannabis have produced little evidence of any harm to the brain and central nervous system.”[13]

Jack Fletcher of the University of Texas has been testing the mental skills of heavy long-term cannabis users in Costa Rica for the past 25 years. Studying those who have smoked in the range of ten joints per day for more than thirty years, Fletcher has detected only minor cognitive impairments that fall well within normal ranges. Brian page, an anthropologist from the University of Miami who also took part in the study notes that, “The effects are subtle and sub-clinical.” [14]

According to a study published by the prestigious American Association for the Advancement of Science, cognition is definitely not impaired by the chronic use of cannabis. A battery of scientific tests, including the Wechsler Adult Intelligence Scale, the Benton Visual Retention Test, and the Rey Auditory-Verbal Learning Test, in conjunction with urine analysis by the enzyme immunoassay method, were used to determine that heavy, long-term cannabis users showed no cognitive impairment compared to standardized norms. [15]

Dr. John P. Morgan of the City University of New York has said, “There is no convincing evidence that heavy long-term marijuana use impairs memory or other cognitive functions. During the past 30 years, researchers have found, at most, minor cognitive differences between chronic marijuana users and nonusers, and the results differ substantially from one study to another.” [16]

The first extensive US study of long-term cognitive performance in a large population group was published in the May 1st 1999 issue of the American Journal of Epidemiology. 1,218 participants, including long-term marijuana smokers, were given standard cognitive tests, called Mini-Mental-State Examinations, over a twelve year period. The research team from John Hopkins Hospital in Baltimore detected no abnormal decline in thinking skills among marijuana users. The study determined that:

There were no significant difference in cognitive decline between heavy users, light users, and nonusers of cannabis. There were also no male-female differences in cognitive decline in relation to cannabis use. The authors conclude that over long time periods, in persons under age 65 years, cognitive decline occurs in all age groups. This decline is closely associated with aging and educational level but does not appear to be associated with cannabis use.[17]

A similar finding is reported by the United Nations World Health Organization, “The weight of the available evidence suggests that even the long-term heavy use of cannabis does not produce any severe or grossly debilitating impairment of cognitive function.” [18]

“Neuropsychological studies have indicated that chronic heavy users may, depending on intensity and duration of use, show subtle impairments of memory, attention, and ability to organize and integrate complex information. The first large-scale longitudinal study in 1999 on the influence of marijuana use on decrease of cognitive performance did not find any such effect of cannabis.” [19]

Research from the US National Institute for Mental Health has indicated that THC and another cannabis compound, cannabidiol, actually protect the brain from cellular damage caused by stroke, blood clots, and head injury. [20], [21]

Related sections: Addiction, Dependence, Psychoactivity, Stroke and Head Trauma, Tolerance.

[1] Gettman, “Drug Abuse, Cannabis and the Brain.” High Times, 1997, http://www.hightimes.com/ See also: Gettman, 1995

[2] Maugh, “Similar effects found for pot, harder drugs.” Los Angeles Times: Science Focus, June 27, 1997

[3] Gifford, Gardner, and Ashby, “The effects of intravenous administration of delta-9-tetrhydrocannabinol on the activity of A10 dopamine neurons recorded in vivo anesthetized rats.” Neuropsychopharmacology, Vol. 36, No. 2, pp. 96-99, 1997

[4] Finn, “Cannabinoid Investigations Entering The Mainstream.” The Scientist, Vol. 12, No. 3, pp. 1-8, February 2, 1998

[5] Beltramo and Piomelli, “Functional role of high-affinity anandamide transport, as revealed by selective inhibition,” Science, Vol. 277, No. 5329, p. 1094(4), August 22, 1997

[6] Mathew, Wilson, Coleman, Turkington, and DeGrado, “Marijuana intoxication and brain activation in marijuana smokers.” Life Science, Vol. 60, No. 23, pp. 2075-2089, 1997

[7] Devane, et al. Science, Vol. 258, pp. 1946-1949, 1992

[8] Axelrod, “Enzymatic synthesis of anandamide, an endogenous ligand for the cannabinoid receptor, by brain membranes.” Laboratory of Cell Biology, National Institute of Mental Health, Bethesda MD, 1997

[9] Fackelman, “Marijuana and the brain: scientists discover the brain’s own THC.” Science News, Vol. 143, No. 6, p. 88, February 6, 1993

[10] Zimmer and Morgan, Marijuana Myths: Marijuana Facts. New York: The Lindesmith Center, 1997

[11] Aigner, “Delta-9-tertrahydrocannabinol impairs visual recognition memory but not discrimination learning in rhesus monkeys.” Psychopharmacology, Vol. 95, No. 4, pp. 507-511, 1988

[12] Institute of Medicine, Marijuana and Medicine: Assessing the Science Base. Washington DC: National Academy Press, 1999

[13] “Cannabis: Look, Listen, Learn.” Independent (UK), cannabis@independentco.org, 1998, Source: Media Awareness Project, http://www.mpp.org/

[14] “Planet Science: Marijuana special report.” New Scientist, February 21, 1998

[15] Schaeffer, Andrysiak, and Ungerleider, “Cognition and long term use of ganja.” Science, Vol. 213, pp. 465-466, July 24, 1991

[16] Armentano, “Pot Doesn’t Rot Your Brain.” High Times, No. 290, October, 1999

[17] Lyketsos et al., “Cannabis use and cognitive decline in persons under 65 years of age.” American Journal of Epidemiology, Vol. 149, pp. 794-800 May 1, 1999

[18] World Health Organization Project on Health Implications of Cannabis Use, 1997

[19] Mikuriya, “Dependency and Cannabis.” Chapter 20

[20] “U.S. Study: Marijuana might protect brain.” Reuters, july 6, 1998

[21] Radford, “Cannabis is stroke hope.” The Guardian (UK), July 4, 1998