To determine the experimental response parameters for each cell, we performed an input-output protocol consisting of a range of current stimulations (typically between 50 and 250 mA; 0.125 Hz), starting at the threshold current required to elicit an IPSP up to the strength required to elicit the maximum amplitude. The stimulus strength eliciting 50% of the maximum response was chosen to conduct experiments and maintained throughout the entire duration of the experiment. Stability of IPSPs was established by stimulating for at least 15 min prior to beginning experiments. The synaptic responses were quantified by averaging two consecutive responses (30 sec apart, i.e. 1 data point/min) and calculating the IPSP amplitude with Clampfit software (Axon Instruments). We examined paired-pulse facilitation (PPF), a phenomenon whereby a secondary evoked synaptic response is increased by a preceding primary stimulation of equal intensity (Siggins et al., 2005; Thomson, 2000), in each neuron using 100 msec inter-stimulus intervals; we calculated the paired-pulse ratio as the second IPSP amplitude over that of the first IPSP.
