From an all-time high to an all-time low

Cannabis is the most commonly used illegal substance amongst teenagers. For the overly suspicious psychonaut, it may involve a hasty lung-full at a house party, followed by nauseating paranoia that your own skin is trying to digest you. The more mentally durable teenager might enter a hazy smog for a few years, before emerging seemingly unblemished. However, despite the herb’s affable image, a growing body of evidence suggests that cannabis may contribute to serious psychiatric problems.

Recently, a group of Canadian scientists found that frequent use of the drug during adolescence might increase the likelihood of developing mood disorders, such as anxiety and depression, in later life. The adolescent brain is in a state of development and reorganisation. The surly teenage caricature, with emotional eruptions, body insecurities and a proclivity for slamming the door, actually reflects major underlying changes in the structure and chemistry of the brain. A significant outcome of these changes is the development of healthy emotional control.

Emotional stability is thought to depend upon establishing a delicate balance of signalling chemicals, called monoamines, at synapses (the junctions between neurons). Low levels of certain monoamines at these synapses are implicated in mood disorders, such as anxiety and depression, and many antidepressant drugs work by increasing these levels.

Cannabis contains a molecule called THC (Tetrahydrocannabinol), which enters the brain and activates CB1 receptors, specialised receptor sites on neurons. While the activation of such receptors usually causes transient changes in the behaviour of neurons (such as the temporary sensation of being ‘high’), repeated exposure to a specific molecule, such as THC, can cause permanent alterations in the behaviour of neurons and their receptors. Could prolonged exposure to THC during adolescence cause such permanent changes in monoaminergic neurons, leading to emotional disturbances in adulthood?

To explore this possibility, the team of researchers developed a rat-based model of adolescent cannabis abuse. For three weeks, rats were dosed with a chemical, which has the same effects as THC. The animals were split into two groups: one group of adolescents and one group of adults. Once the adolescents reached adulthood, both groups entered behavioural testing. These tests looked for signs of anxiety, such as the decreased exploration of unfamiliar environments, and symptoms of depression like a reduced interest in a sugar-water reward. The results were interesting. Rats exposed to the drug during adolescence demonstrated symptoms of anxiety and depression in three out of five behavioural tests. No such effects were observed in the rats given the drug during adulthood. Furthermore, measurements with electrodes revealed abnormal electrical activity in the monoaminergic neurons of rats dosed during adolescence, but not adulthood. It seemed that, in adolescent rats at least, cannabis use did interfere with monoaminergic neurons and emotional control. So how does this effect on our whiskered, little cousins translate to us?

Population studies have established a firm association between heavy cannabis use and depression. Furthermore, a number of studies have identified associations between early cannabis use, and subsequent depression in later life. However, humans are complex creatures, and such causative links are often hard to establish; in this case, somebody might be likely to both use cannabis and suffer from depression. It is also possible that the coincidence of depression and cannabis-use stems from self-medication; people who are already depressed might seek relief in the drug. Unfortunately, some proponents of cannabis exploit this lack of causation to declare the drug innocuous.

Nevertheless, a number of animal studies directly compliment the research team’s findings. Furthermore, animal work is allowing neuroscientists to elucidate possible mechanisms by which cannabis may be affecting the adolescent brain on a cellular level, as exemplified by the unusual monoaminergic neuron activity observed in the current study. There is little to suggest that these effects cannot be seen in humans. Teenagers getting high today should know that they might be risking painful lows in the future.

Niall Hill


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