Pass the salt — How pregnancy drives food cravings
Researchers in Lisbon, Portugal have discovered fruit flies share our craving for salt during pregnancy.
Pregnancy is famous for inspiring relentless food cravings in women, who allegedly become consumed by a desire to eat odd foods. It is commonly believed that such cravings are not just arbitrary, but are linked to the nutritional needs of the fetus they carry. This is particularly true for nutrients such as proteins and salt.
Results of this study are published in the September 24th edition of the journal Current Biology.
What changes in the brain during pregnancy to cause food cravings? More specifically, how does the brain know what the body needs, and what changes occur in the brain to lead women to seek out certain foods during pregnancy? Pursuing answers to these questions, Carlos Ribeiro PhD and his team at the Champalimaud Centre for the Unknown in Lisbon, studied neural changes in fruit flies after eating following mating.
"Nutrition is a highly complex topic. To understand how the brain regulates nutrient intake, you need to work in an organism that gives you access to a lot of diverse technologies. In that respect, the fruit fly is unbeatable," says Dr. Ribeiro. "We wanted to exploit these tools to uncover how the female fly's food preferences changed after mating. Many mammalian species increase their preference for salt during pregnancy; but until now it was not known if the fruit fly shared this behavior."
Following a series of experiments, researchers revealed for the first time, that not only do flies indeed share mammals' inclination towards salt during pregnancy, but also that higher salt levels in their diet resulted in a higher production of offspring.
"We found that there is a direct correlation between the amount of salt in the diet and the amount of eggs they were able to produce," explains Dr. Ribeiro. "It seems that salt is important everywhere, from flies, to elephants, to humans. It also suggests the existence of unifying biological principles underlying this behavior that could be traced across species."
Surprisingly, even though different levels of salt directly influenced offspring production, the researchers discovered that the salt craving was not based on the precise needs of the body. On the contrary, "even if their egg production was disabled, mated females showed increased salt preference, demonstrating that salt-craving was independent of the actual needs of the body," says Dr. Ribeiro.
It appears that the brain of the female knows she will need more salt to produce eggs and so automatically changes the way the animal perceives salt, allowing it to ingest higher amounts of this important nutrient.
Just as in humans, the "tongue" of the fly becomes much more responsive to the taste of salt, leading her to prefer saltier food. The key question became: "What is the biological mechanism that leads to this change in salt sensation in pregnant animals?"
During mating the male injects a molecule called Sex Peptide into the female which manipulates her taste perception.
"The molecule activates neurons in the uterus of the female. From there, we found that a short chain of neuronal interactions signals the brain to ´dial up her salt preference.'"
Samuel Walker, PhD candidate, Ribeiro lab, research leader.
By piecing together this complicated puzzle, researchers were able to  demonstrate that salt-craving exists in flies much as it does in mammals. And this craving  plays an important role in the fly's reproduction. The researchers were also able to identify the trigger for salt-craving and map several steps in it's neural circuitry that brings about this behavioural change.
"Now we move on to the next question, which is to identify how the brain's response to salt changes after mating to bring about this cross-species behavior." says Dr. Rebeiro.
•Dietary salt supports egg production
•Mating induces a salt appetite by increasing the gustatory response to sodium
•Salt appetite is independent of egg production rate
•Sex Peptide Receptor circuitry drives postmating appetites for both salt and yeast
To optimize survival and reproduction, animals must match their nutrient intake to their current needs. Reproduction profoundly changes nutritional requirements, with many species showing an appetite for sodium during reproductive periods. How this internal state modifies neuronal information processing to ensure homeostasis is not understood. Here, we show that dietary sodium levels positively affect reproductive output in Drosophila melanogaster; to satisfy this requirement, females develop a strong, specific appetite for sodium following mating. We show that mating modulates gustatory processing to increase the probability of initiating feeding on salt. This postmating effect is not due to salt depletion by egg production, since abolishing egg production leaves the sodium appetite intact. Rather, the salt appetite is induced need-independently by male-derived Sex Peptide acting on the Sex Peptide Receptor in female reproductive tract neurons. We further demonstrate that postmating appetites for both salt and yeast are driven by the resultant silencing of downstream SAG neurons. Surprisingly, unlike the postmating yeast appetite, the salt appetite does not require octopamine, suggesting a divergence in the postmating circuitry. These findings demonstrate that the postmating circuit supports reproduction by increasing the palatability of specific nutrients. Such a feedforward regulation of sensory processing may represent a common mechanism through which reproductive state-sensitive circuits modify complex behaviors across species.
This study was published today (September 24) in the scientific journal Current Biology. See: Walker SJ, Corrales-Carvajal VM, Ribeiro C. (2015). Postmating circuitry modulates salt taste processing to increase reproductive output in Drosophila. Current Biology. DOI: 10.1016/j.cub.2015.08.043.
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