Determination of basal synaptic strength and paired-pulse ratio
A series of input-output (I-O) curves of current vs. synaptic responses was constructed by delivering increasing current pulses at 0.067 Hz from 0 to 800 µA (100 µA steeps, 100 µs duration) and measuring the evoked fEPSP amplitude (in mV). From the I-O curves, we also analyzed the Fiber volley (FV) amplitude in response to current injection to determine presynaptic excitability. The basal synaptic strength was determined by analyses of the FV amplitude vs. fEPSP slope fitted with a Boltzmann function, and the differences between control and MK-801-treated slices were compared using the resulting slope of the Boltzmann equation. For short-term plasticity determined by paired-pulse ratio (PPR) analysis, a paired stimulation at 0.067 Hz (15-20% of evoked maximal fEPSP amplitude) was obtained, with inter-stimulus intervals (ISI) of 40, 60, 100, 200, and 500 ms. The PPR was calculated as the ratio between the amplitude of the second response (S2) and the amplitude of the first response (S1) (PPR = S2/S1). The input-specific origins of synaptic responses from the MPP and the LPP were corroborated by the preferential pharmacological sensitivity to DCG-IV (5 µM) and L-AP4 (20 µM), respectively (Macek et al., 1996).