Biologists already knew that one set of neurons play a big role in triggering puberty. A new study shows that these neurons don't stop working once puberty ends, but keep running through adulthood, serving as a sort of reproductive timer.
The neurons are found inside the arcuate nucleus, a part of the brain that also regulates hormones controlling appetite and growth. At puberty, they start releasing a protein called kisspeptin, which excites neurons attached to the pituitary gland, triggering the release of reproductive hormones that turn on testes and ovaries and start the cascade that changes children to adults. In a study appearing in PNAS this week, researchers at the University of Otago used optogenetic techniques to show that the same neurons probably control the hourly release of reproductive hormones from the pituitary that continue through adulthood.
They added light-sensitive molecules to kisspeptin-secreting neurons in adult mice, and then activated the neurons by pulsing light at them. That's how the researchers were able to turn on the neurons and measure whether levels of one pituitary reproductive hormone, called luteinizing hormone (LH), changed. They found that turning on kisspeptin neurons at regular intervals produced similar pulses of LH. This was true in both male and female mice.
But this particular region of the brain makes a lot of different proteins, any of which could be making the timer tick. So the team repeated the experiment in mutant mice whose neurons couldn't grab on to the kisspeptin protein, making their cells blind to the signal. This time, when they pulsed the light, the hormones didn't respond, showing that kisspeptin was the important link for the timer.
Kisspeptin neurons seem to be marking off time for the pituitary, telling it when to release the pulses of hormones that in turn tell testes and ovaries to make sperm or mature an egg. But at this point we still don't know what's timing the timer.