Regulation of GABA release by depolarisation-evoked Ca2+ transients at a single hippocampal terminal.
We correlated dynamic changes in free cytosolic [Ca2+] ([Ca2+]i) within single presynaptic terminals of cultured hippocampal neurones with the postsynaptic GABA-mediated currents. The local changes in [Ca2+]i and evoked inhibitory postsynaptic currents (eIPSCs) were recorded simultaneously using Fura-2 fluorescence and whole-cell patch-clamp respectively. The Ca2+ signals and eIPSCs were evoked by direct extracellular electrical stimulation of a single presynaptic terminal by short depolarising pulses. The presynaptic Ca2+ transient was graded by varying the amplitude of extracellular stimulating pulses. The probability of the release event, P, estimated for each stimulation strength, reached a maximum (P=1) when the Ca2+ signal became maximal and remained at this level at higher stimulation strength, despite the subsequent decrease in the amplitude of the Ca2+ transient. A gradual, linear increase in stimulation amplitude (Vstim) resulted in a bell-shaped dependence of the averaged amplitudes of Ca2+ signals and corresponding averaged amplitudes of eIPSCs. Analysis of the eIPSC demonstrated that the decrease in both the mean eIPSC amplitude and the mean quantal content of release resulted from a reduction in the probability of multivesicular release, i.e. in the disappearance of failures and in the decrease of individual eIPSC amplitude. The Ca2+ signals of similar amplitude resulted in both random and determinate (non-random) neurotransmitter release. We conclude that depolarisation-induced elevation of [Ca2+]i within the terminal is necessary but not sufficient for activation of vesicular release of neurotransmitter.
Fedulova
S A
SA
Laboratory of Physiology of Synaptic Transmission, Bogomoletz Institute of Physiology, National Academy of Science, Bogomoletz St. 4, 01024, Kiev-24, The Ukraine.
Verkhratsky
A
A
Veselovsky
N S
NS
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D005071:Evoked Potentials / Q000502:physiology; D006624:Hippocampus / Q000502:physiology*; D008564:Membrane Potentials / Q000502:physiology; D009433:Neural Inhibition / Q000502:physiology*; D017729:Presynaptic Terminals / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
D005680:gamma-Aminobutyric Acid; D002118:Calcium