Sulforhodamine 101, a widely used astrocyte marker, can induce cortical seizure-like activity at concentrations commonly used

Sulforhodamine 101 (SR101) is a preferential astrocyte marker widely used in 2-photon microscopy experiments. Here we show, that topical loading of two commonly used SR101 concentrations, 100 mu M and 250 mu M when incubated for 10 min, can induce seizure-like local field potential (LFP) activity in both anaesthetized and awake mouse sensori-motor cortex. This cortical seizure-like activity develops in less than ten minutes following topical loading, and when applied longer, these neuronal discharges reliably evoke contra-lateral hindlimb muscle contractions. Short duration (< 1 min) incubation of 100 mu M and 250 mu M SR101 or application of lower concentrations 25 mu M and 50 mu M of SR101, incubated for 30 and 20 min, respectively, did not induce abnormal LFP activity in sensori-motor cortex, but did label astrocytes, and may thus be considered more appropriate concentrations for in vivo astrocyte labeling. In addition to label astrocytes SR101 may, at 100 mu M and 250 mu M, induce abnormal neuronal activity and interfere with cortical circuit activity. SR101 concentration of 50 mu M or lower did not induce abnormal neuronal activity. We advocate that, to label astrocytes with SR101, concentrations no higher than 50 mu M should be used for in vivo experiments.