TONIC GABA SECRETION OF CULTURED RAT HIPPOCAMPAL-NEURONS RAPIDLY TRANSFORMED BY ZN-2+ INTO QUANTAL RELEASE

Different mechanisms of neurotransmitter secretion at synapses have be en ascribed to quantal, transient signals [4-9] and to continuous, ton ic activity [11, 21, 25]. Quantal transmission is imputed to be the re lease of transmitter packets, or quanta [6], assembled and stored in v esicles present in presynaptic terminals [2, 8]. We report that 300 mu M Zn2+ rapidly transformed tonic Cl- conductance mediated by the trans mitter gamma-aminobutyric acid (GABA) into transient synaptic-like sig nals. After addition of Zn2+, the size of the elementary fluctuations in the Cl- current progressively increased while randomly activated Cl - channel kinetics remained unchanged and accounted for the bi-exponen tial time course of average transient decay. The results suggest that Zn2+ rapidly transforms tonic GABA secretion into quantal-like transie nt release.