It's widely accepted that our perception of medicine plays a part in the influence a treatment has on our health. A new study reveals that our brains' reaction to a medication differs based on whether the drug is injected or taken orally.
The study, led by a team from the US National Institutes of Health (NIH), was put together to investigate another interesting phenomenon: drugs that reach the brain more quickly are more addictive.
The addictive quality partly has to do with dopamine release, but here the researchers discovered that a brain region known as the salience network is activated when drugs are taken intravenously, but not when taken orally.
"We've known for a long time that the faster a drug enters the brain, the more addictive it is – but we haven't known exactly why," says psychiatrist Nora Volkow, from NIH.
"Now, using one of the newest and most sophisticated imaging technologies, we have some insight."
The prescription stimulant methylphenidate was used for the study. Ordinarily used for conditions such as ADHD, the 20 participants involved in the study didn't have a relevant diagnosis. In addition to having the drug administered intravenously and orally, they were also asked about how they felt after taking it.
As well as self-reported observations, the researchers also used PET scans to monitor dopamine levels in the brain and fMRI scans to monitor overall brain activity. As expected, dopamine levels shot up faster when injections were used.
It was in the fMRI scans that the differences showed up in two major regions of the salience network, the dorsal anterior cingulate cortex and the insula cortex. These areas were only activated after injections, the more addictive method of drug administration.
The salience network activity was consistent with higher feelings of euphoria felt by the participants. This part of the brain has previously been linked to drug addiction, though before now substantial solid evidence has been lacking.
This is all invaluable information when it comes to figuring out how treatments should be given to patients, and how addictions might be tackled. The salience network is known to be important for interpreting internal sensations, and for assigning value externally.
One of the next steps could be to run experiments where activity in the salience network is deliberately blocked – and then to see whether the study volunteers get the same feeling of being high.
"Understanding the brain mechanisms that underlie addiction is crucial for informing prevention interventions, developing new therapies for substance use disorders, and addressing the overdose crisis," says Volkow.
The research has been published in Nature Communications.