By Julia Duszczyszyn
From an evolutionary standpoint, the brain’s reward system is there to reinforce behaviours and actions that are advantageous for one’s fitness. In reality though, the things that are reinforced are not always beneficial.
As you might have guessed, food, social interaction and sexual intercourse are all linked to the brain’s reward system. Many drugs, including alcohol, are also linked to the reward system and therefore addictions to food, social interaction, sex and drugs are common in nature.
Galit Shohat-Ophir and colleagues at the University of California used fruit flies, Drosophila melanogaster, to study ethanol consumption as a response to sexual deprivation. Like humans, fruit flies exhibit addiction-like behaviours. For example, some have a preference for food soaked with ethanol, even if it is made abhorrent. In other words, even fruit flies enjoy the occasional shot of alcohol… some more than others.
They found male fruit flies that had frequent sex exhibited higher levels of the hormone neuropeptide F (NPF). NPF is a hormone that acts on the brain’s reward system, which can affect ethanol preference.
Male flies that have fun in the sack prefer lower-ethanol meals and males that are sexually deprived (and socially rejected) prefer as much ethanol they can get their proboscises on.
This experiment was rather simple but holds compelling results. Drosophila are easy to manipulate, and are therefore common biology subjects. For example, to tease apart the effects of social rejection from sexual deprivation, males were exposed to decapitated virgin females. The females did not show obvious signs of rejection, but also did not allow the males to mate… well, because they were dead. The sexually frustrated and socially rejected males had lower levels of NPF in their brains and showed significant preference for high-ethanol food.
Males that were given normal virgin females (virgin females were chosen because they are eager to mate) usually were able to ‘woo’ the ladies into mating with them, using intricate courtship techniques. These satisfied males had higher levels of NPF in their brains and showed a preference toward food with low-levels of ethanol. Rejected flies, on the other hand, had lower levels of NPF in their brains, and sought an alternative reward — drinking to the point of intoxication.
The team also performed a number of additional tests to see if the environment, the number of females or virginity played a role in the discovered ethanol preference. The results all pointed to sexual activity being correlated with ethanol preference and fluctuations in NPF levels. This result is enhanced when the male is isolated.
So, sexual experience governs NPF levels, which dictates ethanol preference.
Galit Shohat-Ophir and colleagues genetically manipulated the NPF levels in Drosophila brains and showed that males who have preset low NPF levels but are sexually satisfied, still prefer high ethanol food. This confirms the NPF-ethanol interaction.
This experiment is significant because for the first time ever, scientists have linked sex, the NPF system and ethanol consumption. Although there is no obvious adaptive benefit to drinking high-ethanol food, the sexually frustrated flies seem to be acting as a result of their discontent. This study may prove to help other scientists understand human coping mechanisms and addiction. •