What Can Fruit Fly Courtship Tell Us About the Neural Basis of Motivation?

Why study the love lives of fruit flies? According to scientists at Harvard Medical School including NYSCF – Robertson Investigator and Assistant Professor of Neurobiology Dragana Rogulja, PhD, because they can tell us about the neural basis behind motivation and potentially, how these circuits are affected in disorders like addiction.

Fruit fly courtship is an ideal subject for examining motivation circuits, as the area in charge of courtship in the fly brain (a group of cells called P1 neurons) is simple and easy to manipulate. The team of researchers monitored activity in the brains of male flies as they courted a female and found that two factors influenced the male flies’ decision to court or not court: their internal state (libido) and the quality of their mating target.

If a male hadn’t mated in a while, he was more likely to initiate courtship than if he had engaged in mating recently (44% of the time versus 6.5%). And if a female had mated recently, the chances that a male will court her is just 7%.

Previous studies have shown that levels of the neurotransmitter dopamine are higher in males that haven’t mated recently, but it was unknown how dopamine acts on P1 neurons to direct courtship.

The team found that dopamine helped prevent inhibitory signals that would typically prevent mating by acting on P1 neurons, resulting in increased mating behavior. Dopamine also helped flies maintain courtship behavior up until mating.

But even in ideal cases where a motivated male is presented with a suitable female, he still only courts 44% of the time. And sometimes a male who has recently mated will still pursue a non-ideal female. The researchers think this is because the decision to court contains an element of chance.

Unlike motor circuits that help an animal walk with relative consistency (we don’t often mess up which foot to put in front of the other), motivation circuits don’t always lead to consistent behavior. Sometimes they result in illogical choices. This is reminiscent of behavior seen in addiction—when someone actively seeks out harmful substances or activities even though it is not logical. If we can better understand the factors that influence motivation circuits and cause individuals to make irrational choices, then we will have a better idea of how to intervene in cases of addiction.

For more information on this study, check out the paper in Neuron or the press release from Harvard.