Taurine Transporter Regulation in Hippocampal Neurons

Taurine, one of the most prevalent free amino acids in the brain and other tissues (Awapara et al., 1950), has been widely studied as an osmolyte used for regulation of brain cell volume (Pasantes-Morales et al., 2000b). Taurine is produced metabolically from cystine in brain cells (Peck and Awapara, 1967); however, this is a slow process, requiring approximately 48 h to replace the total cellular content of cerebral astrocytes (Beetsch and Olson, 1998). In contrast, active taurine accumulation via a plasma mem- brane transporter in these cells is approximately 30 times faster than the metabolic pathway (Beetsch and Olson, 1996). Furthermore, passive efflux of taurine via plasma membrane channels can reduce cellular taurine contents by as much as 5% per min in osmoti- cally swollen cells and can deplete cells of their taurine contents in only 20-30 min (Kimelberg et al., 1990; Pasantes-Morales et al., 1990). Thus, minute to minute regulation of cellular taurine content is achieved by the balance of these passive and active membrane transport systems.