PER expression in glia of the medulla. A prominent group of GFP-positive giant glial cells in the medulla of NP6520-GAL4>GFP brains. (A) PER protein was detected in young brains and at a lower level in old brains at ZT22 but was absent in both at ZT10. Scale bars equal 20 μm. (B) Brain outline indicating location of cells shown in (A) in the region of the medulla. (C) Graph showing average relative PER fluorescence of in giant glia at ZT22. PER level is significantly lower in old brains (**p = 0.0100, t = 3.056, df = 12). Number of brains analyzed are shown within each bar; error bars indicate SEM. Arrowheads indicate GFP labeled glial cell nuclei.