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Uncovering salt’s addictive nature

Uncovering salt’s addictive nature

Dr Craig Smith

The Florey Institute of Neuroscience and Mental Health | 2015 | $562,864

‘Over consumption of salt is responsible for five times more deaths than the Australian road toll1

Ever wonder why you crave that salty packet of chips or go on a desperate search for a fry-up after a big night out?

Dr Craig Smith and a team of scientists at the Florey Institute of Neuroscience and Mental Health’s Addiction Neuroscience Laboratory are investigating one of the receptors in the brain they think are responsible for those seriously rewarding feelings.  Not only does this have the potential to help with obesity but it is closely linked with addictions to opioids such as heroin and could lead to a new group of targeted drugs.

The team received an NHRMC Project Grant to discover how a specific pathway in the brain is responsible for making us seek those delicious salty snacks.

‘We find salt very palatable when we taste it. We seek salt, crave salt even before we’ve started to eat a meal. These two things collectively are called salt appetite,’ Dr Smith explained.

‘This particular study found that there is an emotional area of the brain called the central amygdala and this part of the brain is involved in a whole range of emotions like fear but also positive emotions like eating or socialising which are beneficial to our survival. We found that opioid signalling within this area is very important for driving this salt appetite.

‘A crucial feature of salt appetite is that when you are salt depleted—such as when you are hungover—the craving for salt increases quite strongly. It’s kind of a battle between what we consciously know is right versus what emotionally we are craving. This circuit is involved in our craving of salt,’ he added.

With humans having evolved to find salt very, very tasty due to its scarcity, its availability is now no longer a problem in the modern world.

‘These genes, these evolutionary mechanisms that have been honed over millions of years are actually betraying us in our modern society,’ he said.

‘We over-consume salt at two to three times the recommended intake. Over-consumption of salt contributes to hypertension, which is one of the main drivers of heart attacks, strokes and kidney disease.

‘We are trying to correct for the fact that we have these genes that used to be beneficial to us millions of years ago but now they’re not beneficial. They are actually driving behaviour which isn’t appropriate in our modern society,’ he added.

After finding the area of the brain responsible for salt appetite, Dr Smith and the team injected three different drugs into this area in mice. One of these drugs—naloxonazine—drastically reduced the amount of salty water that the mice consumed.

‘Although a GP can inform a person with hypertension and high salt diet to reduce their salt intake, we know that doesn’t often change people’s behaviours. Sometimes you need to intervene with some kind of drug,’ he said.

‘It wouldn’t be a drug that everyone would get. It would be a drug that perhaps obese people and [people with] salt-induced hypertension might receive,’ he added.

Those same drugs—and future drugs that might reduce salt craving—could also potentially reduce heroin craving. ‘If we can understand how this natural salt craving and salt reward pathway works it might be applicable to how heroin reward and how heroin craving also works,’ he said.

‘We have opioid receptors in our brain which normally reward us for doing things like eating or socialising which are beneficial to our survival. Heroin and other opiate drugs of abuse hijack these natural reward pathways,’ he explained.

Dr Smith also describes how important NHMRC funding is not only for scientific investigations but for career development and progression. 

‘When you talk about career development and progression as a post-doc they’re [NHMRC grants] really what you aspire to attain,’ he said.

‘For anyone who is aspiring to apply I would suggest that they do give it a go but note they are also not trivial things to do. So do so with correspondence with your fellow peers and hopefully with support from your institute,’ he added.

In 2016 NHMRC funded 539 projects worth more than $451 million to investigate new research ideas.

1Extrapolated from: Victorian Government (2015) The State of Salt: the case for salt reduction in Victoria, VicHealth