If you happen to watch any survival-based reality series, such as the Canadian Survivorman series, you’ll come to realize starvation has a dire effect on the body. A person becomes weak, disoriented, and begins to crave protein. In humans, this is considered to be normal as we are considered omnivores. Yet, this effect also can be seen in other species, including one usually considered to be herbivorous.
The common fruit fly, Drosophila melanogaster, primarily feeds, as the name implies, on decaying fruit and the microorganisms inhabiting it Yet, when this insect undergoes starvation, its tastes change. After several days with no food, they turn carnivorous and even cannibalistic. This dramatic change in food choice, while observed, still has yet to be fully understood.
One theory suggests the fly’s neurological taste mechanisms change during starvation. The insect somehow is able to desensitize to non-traditional flavours of different nutrient sources. Although several different taste types exist, for the fruit fly, the main deciding factor is bitterness. If a food is bitter, it is normally avoided. However, in times of starvation, this awareness of bitter flavour may be turned off.
Last month, a team of researchers from the University of British Columbia investigated this theory in the hopes of finding an answer. Their work, which was published in the journal, Current Biology focused on those bitter taste receptors during starvation. Their results confirm the theory although the actual mechanism behind this switch is far more complicated than once believed.
The team focused on a group of neurons known as gustatory receptor neurons, or GRNs. These are involved in taste and can differentiate between sweet and bitter. These neurons connect to an area of the brain called the subesophageal zone, or the SEZ. When a fly detects a flavour, the GRNs send a signal to the SEZ at which point the fly can determine whether the nutrient is worth eating.
However, during starvation, a change occurs in how taste is transmitted. Sweet GRNs are more active while bitter GRNs are suppressed. In essence, the fly becomes less likely to notice a bitter taste during starvation. This in turn, may allow flies to consume unconventional foods, perhaps even leading to carnivorous activity.
The team first wanted to find the neurons responsible for this change in taste sensation. Using fluorescent markers, they were able to identify a group of suspect neurons lying near the bitter GRNs. This cluster, known as the OA-VLs (for ventrolateral cluster of octopaminergic neurons), produced two neurotransmitters called octopamine and tyramine. Last year, both chemicals had been implicated in fruit fly starvation and starvation resistance and appeared to be an excellent starting point.
At this point, the group needed to find out if the OA-VLs had any effect bitter taste sensation. To accomplish this, the team altered the ability of these neurons to send signals to other cells, such as the GRNs. When signals were inhibited, the flies lost their aversion to bitter chemicals, just like flies that had been starved. This suggested there was indeed a link between the cluster and the GRNs.
While the experiments offered visual evidence, the team still wanted to find out whether this was an indirect or a direct effect. In other words, they wanted to find out if the lynchpins of this switch in taste were octopamine and tyramine. They did this by adding these chemicals directly to the brain to see if there would be any effect. As expected, bitter sensitivity returned during starvation. However, this did not increase sensitivity in normally fed flies. This suggested the effect of these neurotransmitters was reflective of the starvation state.
The results of this study reveal a remarkable process of bitter taste desensitization. Instead of being controlled by the GRNs themselves, the effect is governed by the OA-VLs through the secretion of octopamine and tyramine. During starvation, the effect of the neurotransmitters – bitter sensitivity – is dampened. This enables the fly to seek out any source, regardless of taste, in the hopes of gaining sufficient amounts of nutrients to survive. Once the fly has been properly fed, bitter sensitivity returns to normal.
The effects of starvation are obviously intense and this study offers perspective on how the brain attempts to deal with the condition. Depending on the severity of nutrient loss, the brain may even cause a fruit fly to eat unconventional foods. But this shouldn’t come as a surprise. Whether you happen to be a human or a fly, when the situation is dire, there is every reason to believe the individual will eat just about anything to survive.
Read the original research article in Current Biology:
LeDue EE, Mann K, Koch E, Chu B, Dakin R, Gordon MD. Starvation-Induced Depotentiation of Bitter Taste in Drosophila. Curr Biol. 2016 Nov
7;26(21):2854-2861. doi: 10.1016/j.cub.2016.08.028.