The human blood meal is a favorite recipe for female mosquitoes. So attracted to their taste, they can’t help but bite – spreading diseases that claim 500,000 lives every year.
Scientists, however, aren’t sure how the insects can even sense the complex taste of blood, or how they know this is something they can feed on. Nothing else, not even sweet nectar, makes them pump so hard as when they empty our veins. “When a female mosquito pierces the skin, it sucks so hard that the capillaries sometimes collapse. It’s behavior that she reserves specifically for blood, ”says Leslie Vosshall, Rockefeller neuroscientist.
Now Vosshall and her colleagues have found what turns this powerful blood pump on. In a study published in Neuron, the team describes a unique group of neurons in the syringe-like stylet of the female mosquito that don’t seem to care about simple tastes like sweet or salty. Rather, they are only activated when sugar, salt and other blood components are present at the same time.
“These neurons violate the rules of traditional taste coding, which is believed to persist from flies to humans,” says Veronica Jové, a PhD student in Vosshall’s lab who led the study. “We knew the female stiletto was unique, but no one had ever asked what their neurons taste like.”
Homemade blood recipe
Vosshall, Jové and colleagues first perfected their blood recipe. They started with sheep’s blood, which the mosquitoes swarmed to suck up. When they swapped the blood for sugar and salt solutions, the mosquitoes were no longer interested – even in the presence of heat and carbon dioxide, two cues that tell mosquitoes that people are nearby. The scientists then served their picky eaters a mixture of glucose, salt, sodium bicarbonate, and adenosine triphosphate. The mosquitoes ate with enthusiasm.
To find out what is special about the combination of these four ingredients, the researchers first tested how mosquito pens reacted to the individual components. “There are two neat rows of sensory neurons on either side of the stylet,” says Jové. “We just delivered the smallest microfluidic drop to the tip of the stylet and recorded which neurons responded.”
Glucose, a sugar that plays a prominent role in both nectar and blood, did not consistently activate stiletto neurons. The other three components, which occur only in the blood, each individually activated a specific group of neurons. However, a prominent cluster of neurons did not respond to a single component. It was only activated when the entire blood prescription was delivered at once – almost like you couldn’t taste coffee, milk, or sugar individually unless all of the ingredients were mixed together.
The scientists call this group the integrator neurons because they appear to combine signals from multiple taste components and act as the final decision-makers in deciding whether to activate the pump for blood instead of nectar or water.
A syringe that can taste blood
Of the forty neurons that line a mosquito stylet, only half appear to be activated by blood. Future studies will examine the function of these remaining neurons, which may be involved in recognizing unique flavors that only appear when the stiletto pierces a capillary.
The results have far-reaching effects on basic research. The study opens the door to further investigation of a novel form of taste recognition and offers fascinating insights into the development of unique feeding strategies by specialized species such as mosquitoes. “The real beauty of this study is, how cool is it that we found a syringe that can taste blood ?!” says Vosshall.