Male fruit flies enjoy ejaculation

Moody red lighting in a lab is helping researchers figure out what fruit flies like best about sex.

The question has arisen as scientists try to tease out the neurobiological steps in how the brain’s natural reward system can get hijacked in alcoholism, says neuroscientist Galit Shohat-Ophir of Bar-Ilan University in Ramat Gan, Israel.

Male fruit flies (Drosophila melanogaster) were genetically engineered to ejaculate when exposed to a red light. Ejaculation increased signs in the insects’ brains of a rewarding experience and decreased the lure of alcohol, researchers found. After several days in this red-light district, the flies tended to prefer a plain sugary beverage over one spiked with ethanol. Males not exposed to the red light went for the boozier drink, Shohat-Ophir and colleagues report April 19 in Current Biology.

Earlier lab research has shown that male flies repeatedly rejected by females are more likely to get drunk. Those with happy fly sex lives don’t show much interest in alcohol. Shohat-Ophir wondered what aspect of sex, or lack thereof, had such a profound effect on the brain’s reward system.

The answer wasn’t that obvious. In rats, for instance, the brains of first-timer males light up with intense biochemical signs of reward just from rodent intercourse, regardless of whether ejaculation occurs. In female rats, copulation needs the right circumstances to evoke reward chemistry.

The red-light system let researchers remove the possibly confounding factor of female presence and see that male flies find ejaculation itself rewarding. (Among the evidence: pairing the red light with an odor cue, which males eagerly sought out afterward.) The red light triggers what are called Crz nerve cells in the abdomen, which cause sperm release and a surge of neuropeptide F, a cousin of a human brain reward compound called neuropeptide Y.

Male flies’ bedazzlement with the right light or drinking binges after rejection may be easy for humans to understand. Shohat-Ophir says that’s because brain reward chemistry is so ancient that parts of it have been inherited by creatures with six legs as well as two.