Presynaptic and postsynaptic mechanisms underlie paired pulse depression at single GABAergic boutons in rat collicular cultures.
Paired pulse depression (PPD) is a common form of short-term synaptic plasticity. The aim of this study was to characterise PPD at the level of a single inhibitory bouton. Low-density collicular cultures were loaded with the Ca2+ indicator Oregon Green-1, active boutons were stained with RH414, and action potentials were blocked with TTX. Evoked IPSCs (eIPSCs) and presynaptic Ca2+ transients were recorded in response to direct presynaptic depolarisation of an individual bouton. The single bouton eIPSCs had a low failure rate (< 0.1), large average quantal content (3-6) and slow decay (tau(1) = 15 ms, tau(2) = 81 ms). The PPD of eIPSCs had two distinct components: PPD(fast) and PPD(slow) (tau = 86 ms and 2 s). PPD(slow) showed no dependence on extracellular Ca2+ concentration, or on the first eIPSC's failure rate or amplitude. Most probably, it reflects a release-independent inhibition of exocytosis. PPD(fast) was only observed in normal or elevated Ca2+. It decreased with the failure rate and increased with the amplitude of the first eIPSC. It coincided with paired pulse depression of the presynaptic Ca2+ transients (tau = 120 ms). The decay of the latter was accelerated by EGTA, which also reduced PPD(fast). Therefore, a suppressive effect of residual presynaptic Ca2+ on subsequent Ca2+ influx is considered the most likely cause of PPD(fast). PPD(fast) may also have a postsynaptic component, because exposure to a low-affinity GABA(A) receptor antagonist (TPMPA; 300 microM) counteracted PPD(fast), and asynchronous IPSC amplitudes were depressed for a short interval following an eIPSC. Thus, at these synapses, PPD is produced by at least two release-independent presynaptic mechanisms and one release-dependent postsynaptic mechanism.
Kirischuk
Sergei
S
Developmental Physiology, Johannes Müller Institute of Physiology, Humboldt University Medical School (Charité), 10117 Berlin, Germany.
Clements
John D
JD
Grantyn
Rosemarie
R
D000818:Animals; D000831:Animals, Newborn; D002118:Calcium / Q000378:metabolism; D002478:Cells, Cultured; D005071:Evoked Potentials / Q000502:physiology; D008564:Membrane Potentials / Q000502:physiology; D009433:Neural Inhibition / Q000502:physiology*; D009474:Neurons / Q000166:cytology / Q000502:physiology*; D017729:Presynaptic Terminals / Q000502:physiology; D051381:Rats; D018080:Receptors, GABA-B / Q000502:physiology; D013477:Superior Colliculi / Q000166:cytology / Q000502:physiology*; D005680:gamma-Aminobutyric Acid / Q000502:physiology*
D018080:Receptors, GABA-B; D005680:gamma-Aminobutyric Acid; D002118:Calcium