Discovery clarifies role of peptide in biological clock
Uncoupling the clock
A biologist at Washington University in St. Louis is giving the VIP treatment to laboratory mice in hopes of unraveling more clues about our biological clock. VIP is not "very important person," but vasoactive intestinal polypeptide (VIP), a neuropeptide originally found in the gut, that is also made by a specialized group of neurons in the brain.
Erik Herzog, Ph.D., Washington University assistant professor of Biology in Arts & Sciences, has discovered that VIP is needed by the brain’s biological clock to coordinate daily rhythms in behavior and physiology. Neurons in the biological clock, an area called the suprachiasmatic nucleus (SCN), keep 24-hour time and are normally synchronized as a well-oiled marching band coming onto the field at half time. Herzog and graduate student, Sara Aton, found that mice lacking the gene that makes VIP or lacking the receptor molecule for VIP suffer from internal de-synchrony. When they recorded the electrical activity of SCN neurons from these mice, they found that many had lost their beat while others were cycling but unable to synch to each other.
But when Herzog and Aton added VIP to the mice cells, the synchronicity was restored, showing that VIP couples pacemaker cells and drives rhythms in slave cells.
"VIP between SCN neurons is like a rubber band between the pendulums of two grandfather clocks, helping to synchronize their timing. Some researchers had proposed that knocking out VIP or the receptor for it stopped the clock," Herzog said. "We’ve found that the biological clock is still running, but its internal synchrony is uncoordinated. This causes irregular patterns of sleep and wake, for example."
The study was published on-line in Nature Neuroscience on March 6, 2005. Herzog’s work is funded by the National Institutes of Health.
"In a light-dark schedule, these mice looked normal, but as soon as you leave off the lights, they reveal their internal de-synchrony," he said. "The mice showed multiple rhythms, getting up both earlier and earlier and later and later on subsequent days so that their daily activity patterns were splitting apart."
Herzog and Aton recorded neuron activity from the SCN using a multielectrode array with 60 electrodes upon which they place SCN cells, a "clock in a dish." This enabled them to record data from many cells for many days.
"We found that the VIP mutants, indeed, can generate circadian rhythms, but the neurons can’t synchronize to each other," Herzog said. "We showed that we could restore rhythms to the arrhythmic neurons and synchrony to the SCN by providing VIP once a day."
The SCN is a part of the hypothalamus that can be found on the bottom of the brain just above the roof of your mouth where your optic nerves cross. There are roughly 10,000 neurons in this nucleus on either side of your brain. The timekeeping mechanism in these cells depends on daily cycles in gene activity.
Herzog found in his latest study that the percentage of rhythmic cells in the mutant SCN was very low, and he believes these rhythmic neurons are specialized circadian pacemakers.
"We suspect that at least some of the pace making cells in the SCN are VIP cells, and one of the things we’ll try to do next is confirm this. We will also try to understand better how VIP synchronizes pacemakers," he said.
It is surprising that the process is regulated by a peptide, usually a slow signaling agent, rather than a neurotransmitter, associated with fast events, Herzog said.
"We’re trying to understand the mechanics of how the system synchronizes and the secondary messenger systems as well," Herzog said. "We’re getting closer to the heart and soul of circadian rhythmicity by uncoupling the (biological) clock."
Tony Fitzpatrick | EurekAlert!