PAIRED-PULSE FACILITATION IN THE DENTATE GYRUS - A PATCH-CLAMP STUDY IN RAT HIPPOCAMPUS IN-VITRO

1. Whole-cell patch-clamp recordings were used to study paired-pulse facilitation (PPF) of the lateral perforant path input to the dentate gyrus in thin hippocampal slices. 2. Orthodromic stimulation of the lateral perforant pathway evoked a excitatory postsynaptic current(EPSC) with a latency of 3.3 +/- 0.1 ms(mean +/- SE)that fluctuated in amplitude. The EPSC had a rise time (10-90%) of 2.79 +/- 0.06 ms (n = 35) and decayed with a single exponential time course with a time-constant of 9.14 +/- 0.24 ms (n = 35). No correlation was found between the amplitude of the EPSC and the rise time or decay time-constant. The non-N-methyl-D-aspartate (NMDA) antagonist 6-cyano-7-nitro-quinoxaline-2,3-dione completely blocked the EPSC whereas the NMDA antagonist D-aminophosphonovaleric acid (APV) had modest effects. 3. When a test(T-) EPSC was preceded at an interval of 100 ms by a conditioning (C-) EPSC, a significant increase in the amplitude of the T-EPSC was seen in 38 out of 44 trials analyzed from a total of 27 granule cells. The average amount of PPF was 35.7 +/- 2.1%. There was no apparent correlation between the amount of PPF and the stimulation intensity or mean amplitude of the C-EPSC. The time course of the facilitated T-EPSC was not significantly different from that of the C-EPSC. 4. No correlation was found between the amplitude of the C-EPSC and that of the T-EPSC. Estimates of quantal content (m(cv)) were determined by calculating the ratio of the squared averaged EPSC amplitude (from 48 responses) to the variance of these responses (M(2)/sigma(2)) whereas quantal amplitudes (q(cv)) were estimated by calculating the ratio of the response variance to average EPSC amplitude (sigma(2)/M). PPF was found to be associated with an average increase in mcv of 64.8 +/- 7.2% (n = 38) whereas q(cv) was decreased by 12.1 +/- 3.8%. 5. The time course of PPF was studied by varying the interval between the C- and T-pulse from 10 to 400 ms while keeping the stimulation intensity constant. Maximal facilitation of the T-EPSC was obtained with interpulse intervals less than or equal to 25 ms where the average facilitation amounted to similar to 70% (n = 6). the decline of facilitation was nearly exponential and was no longer evident with intervals >350 ms. 6. Blocking NMDA-receptors with APV had no effect on the amount of PPF indicating that only non-NMDA-receptors were involved. Bicuculline also was without effect on both the amount and the time course of PPF indicating that PPF is not because of a concomitant paired-pulse depression of gamma-aminobuturic acid (GABA)ergic inhibition. Baclofen, a GABA(B)-receptor agonist, reduced the amplitude of both the C-EPSC and the T-EPSC by a similar amount. 7. When extracellular calcium was lowered, C- and T-EPSC amplitudes were reduced by an average of 62 and 55% (n = 3), respectively. This was followed by a decrease in the amplitude fluctuation of both EPSCs. The differential effect on the C-EPSC and T-EPSC resulted in an average increase in PPF of 24.6%.