BARIUM IONS FAIL TO SUPPORT NEUROTRANSMISSION AT A CENTRAL SYNAPSE

Synaptic transmission in the CA1 area of the hippocampal slice prepara tion in vitro was studied in bathing media containing different levels of divalent cations. Transmission was abolished by replacing the norm al levels (2.5 mM) of Ca2+ with 3 mM Mg. Transmission was not permanen tly restored by subsequent addition of Ba2+ but added Ca2+ was effecti ve. Transient reappearance of synaptic currents were seen when Ba2+ wa s added at a time when contaminating levels of Ca2+ were still present , but neurotransmission waned as [Ca2+], declined with protracted wash out. In accordance with this interpretation, Ba2+ potentiated the tran smission observed in the presence of low concentrations (0.25 mM) of C a2+. Little evidence was found for Ba2+ effects at axonal sites but th e potentiation of synaptic transmission by Ba2+ could be accounted for in terms of a blockade of terminal K-channels.