FACTORS CONTRIBUTING TO THE DECAY OF THE STIMULUS-EVOKED IPSC IN RAT HIPPOCAMPAL CA1 NEURONS

1. Monosynaptic gamma-aminobutyric acid-A (GABA(A)-mediated inhibitory postsynaptic currents (IPSCs) were evoked in CA1 pyramidal neurons in the hippocampal slice preparation by direct stimulation of the intern eurons in the presence of glutamatergic blockers and intracellular QX- 314 to block GABA(B)-mediated postsynaptic inhibition. 2. Paired-pulse stimulation was used to activate presynaptic GABA(B) autoreceptors an d thereby reduce the amount of transmitter release. This caused paired -pulse depression, persisting for >3 s, and maximal at interpulse inte rvals between 100 and 250 ms where peak current (I-peak) was decreased by 29.7% and decay time (t(1/2)) was decreased by 44.7%. There was cl ear correlation between changes in I-peak and t(1/2) at all interpulse intervals. 3. With paired-pulse stimulation, the decay of the second IPSC in most cells (12/18) could be resolved into two components, I-fa st and I-slow, each decaying monoexponentially with tau(fast) = 14.10 ms and tau(slow) = 58.87 ms. The faster decay during paired-pulse depr ession was predominantly caused by a larger I-fast fraction, which acc ounted for 27.5% of I-peak in a single control IPSC and 79.3% at an in terpulse interval of 250 ms. 4. Reducing the probability for transmitt er release at all active sites by reducing [Ca2+](o) from 2 mM to 1 mM decreased I-peak by 49.7%, reduced paired-pulse depression, and partl y mimicked the changes in decay kinetics seen after activation of pres ynaptic GABA(B) receptors. Lowering the stimulating intensity to 10% o f the maximal value decreased I-peak by 73.8%, but hardly affected the decay of the IPSC and the paired-pulse depression. 5. Application of the selective blocker of GABA uptake, tiagabine (20-50 mu M), increase d t(1/2) of a single IPSC by 114% without affecting I-peak. The increa se was caused solely by an increase in tau(slow) of 141%. On the other hand, the benzodiazepine agonist midazolam (2 mu M), selectively incr eased tau(fast). It therefore is suggested that tau(fast) reflects the kinetics of the GABA(A) receptor/ionophore complex and tau(slow) the efficiency of the GABA uptake system. 6. The findings show that GABA a ctivates postsynaptic receptors throughout the tail of a single IPSC. This could be caused by reactivation of synaptic receptors or activati on of extrasynaptic receptors. The decay therefore is limited mainly b y the efficiency of the uptake system. An IPSC that is maximally depre ssed by paired-pulse stimulation is mediated primarily by a single act ivation of synaptic receptors, and the decay is limited mainly by the kinetics of the GABA(A) receptor/ionophore complex. 7. The results for a single IPSC are not in accordance with a model for synaptic activat ion where each synapse releases a single vesicle in an all-or-none fas hion and GABA disappears quickly from the synaptic cleft. It is sugges ted that there are multiple active release sites in each synapse and/o r that GABA can activate receptors extrasynaptically or in neighboring synapses. The consequences of the results for the proposed models of synaptic transmission are discussed.