Sulforhodamine 101 is widely used as a specific marker of astrocytes in the neocortex and hippocampus of rodents. Using transgenic mice expressing EGFP under control of the astrocyte-specific GFAP promoter, we compared the labeling of astrocytes by SR101 in acute slices of the ventrolateral medulla (VLM) and the hippocampus. In contrast to the specific SR101-labeling of astrocytes in the stratum radiatum of the hippocampus, SR101-labeling of VLM astrocytes was not sufficient for reliable identification of astrocytes. To confirm selective labeling of astrocytes by SR101, we performed whole-cell voltage-clamp recordings of SR101+ and SR101- EGFP-labeled astrocytes in the hippocampus and found no significant differences regarding membrane potential, input resistance and I-V relationship between SR101+ and SR101- astrocytes.

To reveal the reasons for the differences in SR101-labeling, we first used blockers of gap junction and pannexin hemichannels. However, the results excluded hemichannels as a route for SR101-uptake. When we used substrates (estron-3-sulfate and dehydroepiandrosterone sulfate) or blockers (rifampicin) of organic anion transporting polypeptides (OATP/SLCO), SR101-uptake of hippocampal astrocytes was significantly reduced compared to CTRL conditions. In conclusion our results show, that SR101 is not passively diffusing into hippocampal astrocytes but actively transported via OATPs. Currently, we are performing next-generation sequencing analysis of brainstem and hippocampus astrocytes to find the transporter responsible for SR101-uptake and to reveal the physiological role of this transporter in the hippocampus.