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).