Whenever
I talk to people about what worries them when it comes to pot, inevitably they
bring up addiction. And let me tell you, addiction is a tragic condition that I
wish on no one and that needs to be taken seriously. But are we sure cannabis addiction is even a thing? Here’s what I
think.
From
a neuroscience perspective, we often measure “addictive potential” of a drug as
its ability to activate the brain’s reward system. This system
consists of a certain group of brain cells releasing the brain chemical dopamine when activated,
and is in place to help us pursue natural rewards (food, company, a mate,
etc.). The dopamine release is basically the brain’s way of saying “this is
good, go after this, get more of it.” What all addictive drugs have in common
is that they manage to manipulate the
brain into activating
this system.
But
whether cannabis activates this system is under heated debate. Animal models are certainly
looking like it’s the case -- dopamine release left and right. And not only that,
but animals are also behaving in ways that suggest reward: There is plenty of evidence for
“self-administration” (pressing a lever to receive the drug, sometimes hundreds
of times for a single dose -- a measure of how badly an animal wants it and how
hard it is willing to work for it), and animals will prefer places where they previously
had the drug (a measure of how much they like it). But little-known fact: it
actually takes all sorts of tricks to get animals
to start self-administering THC, and under certain conditions, they actually avoid places where it was previously
given. So yes, there is a convincing case that animals will come to want and
like and release dopamine for it -- IF the conditions are made just right.
But
when it comes to humans, the picture gets blurry, and inconsistencies abound. I
mean, there is no doubt – judging from both the real world and human
self-administration studies – that people
WILL “press the lever” and like it. But here it isn’t so easily linked to
reward system activation. Some studies find that it releases dopamine. Others do not. Or maybe it does, if you happen
to be at risk for psychosis. Or if it does, the effect is
ever-so-slight, making the clinical relevance questionable. Or maybe there is no effect after all. Nope,
looks like there isn't. And what causes
this mess is anyone’s guess: Is dopamine
release a real phenomenon that is masked
in some studies by “suboptimal experimental design?” Or is there no actual effect, but some
studies find something that looks
like dopamine release by chance fluctuations in the imaging signal? Either way,
a recent review
paper concedes that “in man, there is as yet little direct evidence to
suggest that cannabis use affects dopamine release or dopamine receptor status
in healthy human volunteers.”
So
what are we to believe? The animal models, which may or may not apply to actual
human experience, and need certain conditions (which may or may not mimic
actual human conditions) to work? Or the human brain images, which rely on indirect
measures and statistical calculations and can’t seem to agree on the evidence?
And does a lack of human dopamine evidence mean that cannabis isn’t addictive,
or can addiction exist in the absence of a clean dopamine story? Clearly, my
point is that there are more questions than answers. But while I shrug my
agnostic shoulders, the National Institute on Drug Abuse sounds pretty
convinced that “contrary to common belief, marijuana can be addictive” – a
claim they support using dopamine
logic…
To
me, the evidence here is a draw, and strong assertions may be based on political
motivation over evidence. Luckily (for the debate anyway), there is more to
addiction than dopamine; for example, the trademark phenomena of tolerance and
withdrawal. How’s the evidence shake out on that one? Stay tuned for part II.
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