Stimulus‐coupled taurine efflux from cerebellar neuronal cultures: On the roles of Ca++ and Na+

Primary cultures of cerebellar neurons obtained from 7–9‐day‐old rats and grown 7–9 days in vitro (DIV) were used to study the effects of Na+ and Ca++ on K+‐evoked taurine release. These cultures, made up largely of granule neurons (90%) and inhibitory interneurons (5–7%), produced a dose‐dependent, depolarization‐evoked taurine release that was Ca++‐dependent at 40 mM K+, and Ca++‐independent at K+ concentrations above 40 mM. The dihydropyridine Ca++ channel agonist BAY K 8644 (1 μM) augmented 30 mM K+‐evoked release, while the antagonist nifedipine (5 μM) abolished both the BAY K 8644‐ and K+‐enhanced release. Depolarization with the Na+ channel agonist veratridine (50 μM) stimulated taurine efflux, which was completely blocked by pretreatment with tetrodotoxin (2 μM). However, 50 mM K+‐evoked taurine release was not affected by tetrodotoxin pretreatment. Substitution of choline Cl for NaCl partially antagonized 50 mM K+‐evoked release, and by itself, the Na+ ionophore monensin (50 μM) stimulated release. These results suggest that both K+‐evoked and basal taurine release from primary cerebellar neuronal cultures are sensitive to the levels of both intracellular and extracellular Na+ and Ca++ In contrast to previous findings using cerebellar astrocytes, neuronal L‐type Ca++ channels, but not voltage‐dependent Na+ channels, also appear to be necessary. The implications of these results on taurine's status as a putative neurotransmitter are discussed.