K +- and temperature-evoked taurine efflux from hypothalamic astrocytes

Abstract

Hypothalamic astrocytes in culture released taurine, a suspected inhibitory amino acid neurotransmitter/neuromodulator/osmoregulator, in response to isoosmotically increasing extracellular K + in a dose-dependent fashion. In the absence of added Ca 2+, basal release levels rose to approach those obtained after exposure to 60 mM K + in the presence of 2.5 mM Ca 2+, and were only partially lowered by the addition of 10 mM Mg 2+. Stimulation with K + (60 mM) did not further increase taurine efflux above the high basal levels seen in the absence of Ca 2+. Under standard conditions complete replacement of Na + with choline Cl had little effect on basal taurine release, but reduced K +-evoked (60 mM) efflux by 60%. The temperature dependence of the basal levels of taurine released from hypothalamic astrocytes was similar to that seen for cultured cerebellar astrocytes and neurons over the range 5–50°C. Taurine release increased from 5 to 15°C, remained constant between 15 and 33°C, decreased between 33 and 37°C and increased thereafter. The infection point of increased basal taurine release seen around 37°C (most prominent in astrocytes), may be of physiological significance. Results presented also show that the ion (Na +, Ca 2+ and K +) sensitivities of taurine efflux for cultured hypothalamic astrocytes are similar to those previously reported for cultured astrocytes from the cerebellum.