BRISBANE: Frustrated by their unsuccessful attempts at mating, spurned male fruit flies turn to alcohol, say researchers, which highlights the relationship between different reward systems in the brain.
A paper published in Science today has compared the rewards of sex and alcohol consumption in fruit flies (Drosophila melanogaster), which could help researchers to understand a specific ‘reward’ pathway in the brain which has implications for human addiction.
“We looked for ways to assess the mechanism by which the brain perceives social reward and what social interactions are rewarding to animals,” said lead author and geneticist Galit Shohat-Ophir from the Howard Hughes Medical Institute at the Janelia Farm Research Centre in Virginia, U.S.
Alcohol a substitute for sex
Previous studies by the same research group have shown that Drosophila flies like to consume alcoholic food, and are physically capable of detoxifying the alcohol in rotting fruits as a food source.
The new study examined the reactions of male fruit flies going through two distinct sexual experiences. They found that spurned flies and those denied access to females turned to food mixed with 15% alcohol, while the flies that had mated successfully for several days – enjoying six-hour-long mating sessions with multiple virgin females – weren’t interested in the alcoholic food.
“We can reverse these affects by taking the rejected males and giving them the opportunity to mate. The ones that were rejected that get to mate rapidly reverses the behaviour, and they don’t take alcohol,” said Shohat-Ophir.
Human reward systems
There are reward systems in our brains that determine which experiences are good or bad. Scientists think that when these reward systems are out of balance, they play a role in certain addictions in humans. Neuropeptide Y (NPY), a neurotransmitter found in the brain and nervous systems of humans, is affected by stressful experiences and regulated by environments.
Previous studies have shown that mice injected with NPY decreased their intake of ethanol. Lower levels of NPY have also been shown in people suffering from depression and post-traumatic stress disorder.
Shohat-Ophir’s team found that rejected males had low levels of the fly equivalent of NPY – called neuropeptide F (NPF) – and the mated males had high levels of NPF. They were able to genetically manipulate the levels of NPF expressed in the flies to discover, “We can take the males and inhibit or lower the levels of the receptor to NPF and [found that] even if these males were mated, alcohol-wise they behave as if they were rejected,” said Shohat-Ophir.
The researchers suggest that levels of NPF determine the reward-seeking behaviour of the flies. When they are low due to a lack of natural rewards (from mating), they seek the external rewards from alcohol to boost their levels of NPF. “So the reward from mating inhibits alcohol as being perceived as rewarding,” said Shohat-Ophir.
From flies to humans
The author of a “Perspective” on the study published in the same issue of Science, behavioural geneticist Troy Zars from the University of Missouri in the U.S., said that this study “should not be taken lightly”. He said, “Identifying NPF as a signalling mechanism that links together this one social behaviour and the different rewarding behaviours provides the first molecular underpinnings for a better understanding that these two different sorts of behaviours are linked,” he said.
Shohat-Ophir added that one of the strengths of their study is that it shows the system of rewards (between natural and external) being observed through NPF levels in flies, and can be translated into the similar NPY levels in humans. “Understanding this system in flies might tell us [why the] human brain perceives social interactions as rewarding, and how systems and situations in which the reward systems don’t function properly like in addiction arise. This may give us more tools in the future to develop better therapies,” she said.
Evolutionary biologist Mark Elgar from the University of Melbourne found the neurological experiments of the study “impressive”, but he said they were rather difficult to interpret within an evolutionary perspective. He said that the data could be interpreted differently with the rejected males ‘hanging out’ at the ethanol food feeders as a strategy to seek more sexual encounters with females. “I’d be rather more cautious than the authors about making the links between the rewards of sexual activity and ethanol consumption,” he said.