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Even moderate pot use tied to clear brain changes: study

April 16, 2014
Courtesy of Northwestern University
and World Science staff

Young adults who used ma­ri­jua­na only recrea­t­ionally showed clear ab­nor­mal­i­ties in two key brain re­gions im­por­tant for emo­tion and mo­tiva­t­ion, sci­en­tists re­port. 

It’s the first study to tie cas­u­al ma­ri­jua­na use to ma­jor brain changes, the re­search­ers said: it showed the de­gree of ab­nor­mal­i­ties is di­rectly re­lat­ed to smok­ing fre­quen­cy. More fre­quent smok­ing was linked to more ab­nor­mal shape, vol­ume and dens­ity, or com­pact­ness, of the brain re­gions.

Young adults who used ma­ri­jua­na only recrea­t­ionally showed clear ab­nor­mal­i­ties in two key brain re­gions im­por­tant for emo­tion and mo­tiva­t­ion, sci­en­tists re­port. These regions are the nu­cle­us ac­cum­bens and the amyg­dala—key re­gions for emo­tion and mo­tiva­t­ion, and as­so­ci­at­ed with ad­dic­tion. (Im­age cour­tesy NIH/NIAAA)


This “raises a strong chal­lenge to the idea that cas­u­al ma­ri­jua­na use is­n’t as­so­ci­at­ed with bad con­se­quences,” said study co-au­thor Hans Bre­iter, a psy­chi­a­trist and be­hav­ior­al sci­ent­ist at North­west­ern Uni­vers­ity and a psy­chi­a­trist at North­west­ern Me­mo­ri­al Hos­pi­tal, both in Chi­ca­go.

“Some of these peo­ple only used ma­ri­jua­na to get high once or twice a week,” he added. “Peo­ple think a lit­tle recrea­t­ional use should­n’t cause a prob­lem, if some­one is do­ing OK with work or school. Our da­ta di­rectly says this is not the case.”

“I’ve de­vel­oped a se­vere wor­ry about wheth­er we should be al­low­ing an­y­body un­der age 30 to use pot un­less they have a ter­mi­nal ill­ness and need it for pain,” he added.

The study was pub­lished April 16 in the Jour­nal of Neu­ro­sci­ence.

The sci­en­tists ex­am­ined the nu­cle­us ac­cum­bens and the amyg­dala—key re­gions for emo­tion and mo­tiva­t­ion, and as­so­ci­at­ed with ad­dic­tion—in the brains of cas­u­al ma­ri­jua­na users and non-users. Re­search­ers an­a­lyzed three meas­ures: vol­ume, shape and dens­ity of grey mat­ter (that is, where most cells are lo­cat­ed in brain tis­sue).

Both re­gions in recrea­t­ional pot users were ab­nor­mally al­tered for at least two of these meas­ures, the in­ves­ti­ga­tors said. The nu­cle­us ac­cum­bens was found to be ab­nor­mally large, and its al­tera­t­ion in size, shape and dens­ity was found to be di­rectly re­lat­ed to smok­ing fre­quen­cy.

“We looked at the nu­cle­us ac­cum­bens in three dif­fer­ent ways to get a de­tailed and con­sist­ent pic­ture of the prob­lem,” said lead au­thor Jo­di Gil­man, a re­search­er in the Mas­sa­chu­setts Gen­er­al Cen­ter for Ad­dic­tion Med­i­cine and an in­struc­tor in psy­chol­o­gy at Har­vard Med­i­cal School.

Ex­am­in­ing the three meas­ures al­so was im­por­tant be­cause no sin­gle meas­ure is the gold stand­ard, the re­search­ers added. But “these are co­re, fun­da­men­tal struc­tures of the brain,” said co-senior study au­thor Anne Blood, di­rector of the Mood and Mo­tor Con­trol Lab­o­r­a­to­ry at Mas­sa­chu­setts Gen­er­al and a psy­chi­a­trist at Har­vard Med­i­cal School. “They form the ba­sis for how you as­sess pos­i­tive and neg­a­tive fea­tures about things in the en­vi­ron­ment and make de­ci­sions about them.”

Through dif­fer­ent brain im­ag­ing meth­ods, the sci­en­tists ex­am­ined the brains of young adults, ages 18 to 25, from Boston-area col­leges; 20 who smoked ma­ri­jua­na and 20 who did­n’t. Each group had nine males and 11 fe­males. The users un­der­went a psy­chi­at­ric in­ter­view to con­firm they weren’t de­pend­ent on ma­ri­jua­na. They did­n’t meet cri­te­ria for abuse of oth­er drugs.

“It may be that we’re see­ing a type of drug learn­ing in the brain,” Gil­man said. “We think when peo­ple are in the pro­cess of be­com­ing ad­dict­ed, their brains form these new con­nec­tions.” In an­i­mals, these new con­nec­tions are thought to in­di­cate that the brain is adapt­ing to the un­nat­u­ral lev­el of re­ward and stimula­t­ion from ma­ri­jua­na, mak­ing oth­er, nat­u­ral re­wards less sat­is­fy­ing.

Drugs of abuse can cause greater re­lease of dopamine, a brain chem­ical asso­ciated with a pleas­ur­able sen­sation, “than nat­u­ral re­wards like food, sex and so­cial in­ter­ac­tion,” Gil­man said. “In those you al­so get a burst of dopamine but not as much as in many drugs of abuse. That is why drugs take on so much sa­li­ence, and eve­ry­thing else loses its im­por­tance.”

Ma­ri­jua­na is the most com­monly used il­lic­it drug in the U.S. with an es­ti­mat­ed 15.2 mil­lion users, the study re­ports, based on the Na­t­ional Sur­vey on Drug Use and Health in 2008. The drug’s use is in­creas­ing among ado­les­cents and young adults, par­tially due to so­ci­ety’s chang­ing be­liefs about can­na­bis use and its le­gal sta­tus.

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Young adults who used marijuana only recreationally showed clear abnormalities in two key brain regions important for emotion and motivation, scientists report. It’s the first study to tie casual marijuana use to major brain changes, the researchers said: it showed the degree of abnormalities is directly related to smoking frequency. More frequent smoking was linked to more abnormal shape, volume and density, or compactness, of the brain regions. This “raises a strong challenge to the idea that casual marijuana use isn’t associated with bad consequences,” said study co-author Hans Breiter, a psychiatrist and behavioral scientist Northwestern University’s Feinberg School of Medicine and a psychiatrist at Northwestern Memorial Hospital in Chicago. “Some of these people only used marijuana to get high once or twice a week,” he added. “People think a little recreational use shouldn’t cause a problem, if someone is doing OK with work or school. Our data directly said this is not the case.” “I’ve developed a severe worry about whether we should be allowing anybody under age 30 to use pot unless they have a terminal illness and need it for pain,” he added. The study was published April 16 in the Journal of Neuroscience. The scientists examined the nucleus accumbens and the amygdala—key regions for emotion and motivation, and associated with addiction—in the brains of casual marijuana users and non-users. Researchers analyzed three measures: volume, shape and density of grey matter (that is, where most cells are located in brain tissue). Both regions in recreational pot users were abnormally altered for at least two of these measures, the investigators said. The nucleus accumbens was found to be abnormally large, and its alteration in size, shape and density was found to be directly related to smoking frequency. “We looked at the nucleus accumbens in three different ways to get a detailed and consistent picture of the problem,” said lead author Jodi Gilman, a researcher in the Massachusetts General Center for Addiction Medicine and an instructor in psychology at Harvard Medical School. Examining the three measures also was important because no single measure is the gold standard, the researchers added. But “these are core, fundamental structures of the brain,” said co-senior study author Anne Blood, director of the Mood and Motor Control Laboratory at Massachusetts General and a psychiatrist at Harvard Medical School. “They form the basis for how you assess positive and negative features about things in the environment and make decisions about them.” Through different brain imaging methods, the scientists examined the brains of young adults, ages 18 to 25, from Boston-area colleges; 20 who smoked marijuana and 20 who didn’t. Each group had nine males and 11 females. The users underwent a psychiatric interview to confirm they weren’t dependent on marijuana. They didn’t meet criteria for abuse of other drugs. “It may be that we’re seeing a type of drug learning in the brain,” Gilman said. “We think when people are in the process of becoming addicted, their brains form these new connections.” In animals, these new connections are thought to indicate that the brain is adapting to the unnatural level of reward and stimulation from marijuana, making other, natural rewards less satisfying. “Drugs of abuse can cause more dopamine release than natural rewards like food, sex and social interaction,” Gilman said. “In those you also get a burst of dopamine but not as much as in many drugs of abuse. That is why drugs take on so much salience, and everything else loses its importance.” Marijuana is the most commonly used illicit drug in the U.S. with an estimated 15.2 million users, the study reports, based on the National Survey on Drug Use and Health in 2008. The drug’s use is increasing among adolescents and young adults, partially due to society’s changing beliefs about cannabis use and its legal status.