This blog is currently on hiatus owing to work commitments. Whilst I still keep an eye on the goings on at RiAus, and contribute to the work of the good folks at eLife, little will be added to this blog for the foreseeable future. Simon Says remains open for business, albeit at a reduced capacity. Thanks for stopping by, and I hope the archive of content found here will prove to be of interest.

Monday, 20 May 2013

Monday Science: Brains and Blue Icing

It is said that when a sense is lost, particularly that of sight, remaining senses are heightened. This is true not only in those individuals who could once see — although they do have better spatial awareness — but also in people who were born blind, since these individuals have been shown to have accelerated brain processing ability in the regions responsible for the remaining senses. But how can the brain do this? How can it intrinsically know things it can no longer directly determine in the ways it was built to do?

Fruit flies similarly have the ability to continue to perceive their environment in times of sensory loss. For example, they can determine the nutritional value of food, even when their ability to taste has been impaired. Now, a study in Nature Neuroscience has uncovered details as to how this might be possible.

Flies that are incapable of tasting — by means of genetic mutation in the taste receptors around its mouth and proboscis — actively select easily digested sugars over harder to stomach compounds. After all, they still need to eat well. This ability is particularly strong during times of starvation, so the researchers looked deeper to see if this was to do with sugar levels in the haemolymph, the liquid that circulates around the insects’ body, something akin to blood plasma. They found that starved flies were no longer able to choose between glucose and agar, or different kinds of glucose, if their food was supplemented with a chemical that prevents the passing of glucose from the gut to the haemolymph. Just to check that this wasn’t because the chemical was messing everything up and the flies were just off their food completely (and who can blame them, on a diet of agar and chemicals), they repeated the experiment using agar and fructose, which differs chemically and isn’t prevented from passing into the haemolymph. This time the flies chose the fructose.

On top of this, flies that have had their ability to taste restored, but are still chemically unable to absorb glucose into their haemolymph, actively choose the sweetest foods they can find, even if those sugars are the hardest to digest and therefore the worst nutritional option. They’re like toddlers presented with the option of cake with blue icing, or Ryvita and hummus - sweetest wins, regardless of whether it's any good for them.

If they can taste, but not absorb sugar into the blood, flies make bad food decisions. If they can't taste, and can't absorb sugar into the blood, flies can't decide anything, and eat whatever they can find. Clearly, there’s something about the presence of glucose in the haemolymph that activates a taste-independent pathway that selects nutritional food.

To try to work out the molecular causes of this, the researchers looked for mutations in genes that caused normal, happy healthy flies to suddenly lose the ability to distinguish different sugars. They found one and, obviously, named the gene it was in ‘cupcake’. Cupcake is similar to a human gene called, boringly, SGLT1, which is responsible for the absorption of glucose into the blood from the small intestine. So is cupcake responsible for glucose movement into the haemolymph, and therefore subsequent food selection based on nutritional value?

The answer, confusingly, is no.

The curiously beautiful fruit fly
ellipsoid body (right)
Oddly, cupcake isn’t used in the gut or the blood vessels. It’s only expressed in a highly specific set of nerves in the fly brain, right in the middle of it, in a circular structure called the ellipsoid body. The ellipsoid body is part of the brain responsible for movement, although it is part of a larger structure called the central complex, which is more widely responsible for behaviour and, neatly, processing of the senses. Restoration of cupcake gene activity in these nerves returned the ability to make good food choices in mutant flies.

Deep in the fly brain, a tiny bundle of cupcake nerves somehow fire electrical impulses when, after hours of starvation, sugar reaches the gut and passes into the haemolymph. This then changes the fly's behaviour to seek out more of the good quality food that has triggered this reaction. Whether the sugar directly triggers the nerves or other, indirect processes are at play remains to be seen.

All of this makes me wonder — what processes, decisions and impulses are going on in our brains, independently of the information we feed them? Could we make good food choices even without the ability to taste? If people can achieve spatial awareness without the ability to see, perhaps this is not so far fetched. Brains are the most remarkable of things.

Dus, M., Ai, M. & Suh, G. S. Taste-independent nutrient selection is mediated by a brain-specific Na(+)/solute co-transporter in Drosophila. Nature Neuroscience 16, 526-528 (2013)

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