A recent study has shown that male fruit flies experience sex – and more precisely, ejaculation – as pleasurable. Depriving them of it appears to affect their interest in consuming alcohol.
Nobody ever said that science was going to be glamorous. This study however is a serious one: compared to a ‘frustrated’ control group, the insects who could ejaculate showed less interest in alcohol, highlighting the neurological circuits that could be implicated in addictions. “Successful mating is naturally rewarding to male flies and increases the levels of a small peptide in the brain called neuropeptide F”, explains Galit Shohat-Ophir from Bar-Ilan University in Israel. “Male flies that are sexually deprived have increased motivation to consume alcohol as an alternative reward”.
The researchers used optogenetic instruments in this instance. These tools allowed the researchers to genetically breed fruit flies, such that it was possible to activate particular neurons with light. In their studies, they used flies in whom the neurons expressing the neuropeptide corazonin (CRZ) could be activated through exposure to red light. Previous research had shown that in a fly’s abdomen, CRZ neurons triggered the release of sperm and semen. By examining the insects in this way, the researchers were able to explore the reward systems linked with ejaculation outside of the other aspects of sexual encounters.
“We wanted to know which part of the mating process entails the rewarding value for flies”, said Shohat-Ophir. “The actions that males perform during courtship? A female’s pheromones? The last step of mating which is sperm and seminal fluid release?” In order to find out whether ejaculation produced an immediate and pleasurable response in the flies, the researchers used a container in which one side emitted red light, to trigger optogenetic activation of the CRZ neurones. They then observed the males to see where they chose to spend their time. These experiments showed that the flies showed a strong preference for the red light, which implies that ejaculation in itself is a pleasurable experience for them.
The researchers then led the flies to associate the red light and ejaculation with a particular smell. They then tested to see if the flies preferred the smell that reminded them of the past experience of ejaculation. And this was equally the case.
After a few days of repeated activation of the CRZ neurons, the male fruit flies had elevated levels of neuropeptide F in their brains, similar to in males who breed with female partners. While these males had the choice between liquid foods and liquid foods enriched with alcohol, they preferred the non-alcoholic option. On the other hand, the control group flies and artificial males who weren’t exposed to red light preferred the alcohol.
“The principles by which the brain processes reward are extremely conserved in all animals; this is a really basic everyday machinery that helps animals survive”, continues the researcher. “”Drugs of abuse use the same systems in the brain that are used to process natural rewards. This allows us to use simple model organisms to study aspects of drug addiction, including the interplay between natural and drug rewards and the connection between experience and the mechanisms that underlie the risk to develop drug addiction.”
On the whole, the results of the experiment indicate that when pleasure responses in the nervous system are not adequate, an animal may turn to alternative solutions to make up the deficit. By understanding what makes us happy, we could thus find better ways of treating other more dangerous addictive behaviours.
You can find all the details of this study in the journal Current Biology.