Whether rightward or leftward, the classic libertarian argument favoring legalization of America’s most popular illicit drug, marijuana, hinges on individual choice. But a new study suggests a greater complexity as adolescent exposure to the drug affects epigenetic change altering the expression of specific genes within the next generation.

Yasmin Hurd, a professor of psychiatry and neuroscience at Mount Sinai in New York, says the male offspring of rats exposed to delta-9-tetrahydrocannabinol (THC) — the psychoactive ingredient of marijuana — experienced neurological and behavioral changes. With no individual history of marijuana exposure, these rats showed a stronger urge to self-administer heroin in the experiment, as opposed to a control group that showed less interest. Moreover, the animals experienced molecular changes in the brain’s glutamatergic system, the primary excitatory pathway for neurotransmission. Previous study has shown that damage to this area, which regulates synaptic plasticity, has been associated with malfunctioning in goal-directed behavior and the formation of habit.

"Our study emphasizes that cannabis [marijuana] affects not just those exposed, but has adverse effects on future generations," Hurd said in a statement. "Finding increased vulnerability to drug addiction and compulsive behavior in generations not directly exposed is an important consideration for legislators considering legalizing marijuana."

In the experiment, Hurd and his colleagues gave adolescent male rats a 1.5 mg/kg of THC, a dosage roughly analogous to a human smoking a joint. After one generation, male offspring from rats exposed to the drug showed lowered motivation and weight gain, though the metabolic effect disappeared by the third generation.

Previously, Hurd and her colleagues showed that THC exposure during adolescence increased a later desire for heroin during adulthood. Now, they’re trying to explain the neurological repercussions of exposure to cannabis. "We're nowhere close to figuring out how it is that these mechanisms are passed on across generations," Hurd said. "What this opens up are many questions regarding the epigenetic mechanisms that mediate cross-generational brain effects."

Next, the investigators hope to explore the possible multi-generational effects of THC and whether science may develop therapies to reverse epigenetic change in a given patient.

Source: Hurd Y, Szutorisz H, DiNieri JA, Sweet E, Egervari G, Michaelides M. Parental Exposure To THC Linked To Drug Addiction, Compulsive Behavior In Unexposed Offspring. Neuropsychopharmacology. 2014.