How to keep track of mouse urine
Eight hours is a long time without a trip to the bathroom when awake, yet most of us can sleep through the night without peeing. And no, it’s not just because you (presumably) stop drinking coffee in your sleep: even when food and drink are factored out, you both make less urine and have better bladder capacity during the night. As with most behaviors that change from day to night, it does indeed have everything to do with the circadian rhythm.
In a new study published in Nature Communications, researchers compared normal mice with mice whose circadian rhythms were disrupted by genetic mutations. To keep track of mice urination over time, they used a rather charming contraption that slowly unspooled urine paper under the cages (see image). Urine spots on the paper were counted up and, sure enough, urination in the normal mice showed 24-hour patterns while the mutant mice did not.
The study also identified a molecular mechanism that lets bladders hold more urine during sleep. Concentrations of the bladder protein Cx43 goes up and down over the course of 24 hours. It makes bladder muscles more sensitize to stimuli, so more of it means more frequent urges to pee. Further molecular work identified pathways showing how Cx43 expression is linked to circadian clock genes. So people who wake up in the middle of the night with the urge to pee—mostly kids and the elderly—it’s not just the bladder’s fault. The blame may actually fall to people’s circadian rhythms.