Abstracts

Rationale: Partially isolated “undercut” (UC) neocortex is a well-established model of post traumatic epilepsy. After a 2 week latent period, epileptiform activity, originating in cortical layer V pyramidal (P) cells, can be recorded (Prince and Tseng 1993). A decreased frequency of spontaneous and miniature (m) IPSCs in P neurons may contribute to epileptogenesis (Li and Prince, 2002). We tested the hypothesis that a decreased probability of GABA release contributes to the decreased mIPSC frequency Methods: Using standard techniques, we evoked pharmacologically isolated whole cell monosynaptic IPSCs in layer V P neurons of control and UC neocortical slices with pairs or trains of 1.5 x threshold extracellular stimuli (interstimulus intervals (ISIs) 20, 50 and 75 ms) and calculated the paired pulse ratio (PPR; amplitude of response (R)2/R1 or R8/R1). Two sample T test was used throughout to compare averaged amplitudes and PPRs. Results: The threshold stimulus intensity required to evoke IPSCs was higher in UCs (0.27±0.02 mA, n = 16,) than in control group (0.11±0.01mA, n = 18; p<0.001). At threshold, eIPSCs from control and UC groups showed similar amplitudes (control eIPSC = 52.1±8.3 pA, UC eIPSC = 44.6±8.9 pA; p>0.05). Input-output slopes of IPSC amplitudes were not significantly different (p>0.05). At ISIs of 50 and 75 ms, but not at 20 ms, the PPR in neurons from UC slices was significantly larger than control (control PPR50 = 0.77± 0.04, n = 17 and PPR75 = 0.81±0.03, n = 18; UC PPR50 = 1.03±0.05, n = 11, and PPR75 = 1.01±0.05, n=9; p<0.001 for both ISIs). There were no failures in controls for R1 or R2 (ISI 50ms), however the failure rate in UC cells was 8.2±2.3%; p = 0.007 for R1 and 9.09±2.50% for R2; p = 0.006. Slice incubation in 4mM [Ca++]o for 1 hr significantly decreased the PPR (increased PPD) in both groups at ISIs of 50 and 75 ms, however the extent of this effect was not significantly difference in control vs UC neurons. Incubation in 4mM [Ca++]o prevented failures of synaptic responses in UC rats. A potential effect of GABAB receptor activation on the amplitude of IPSCs evoked at ISIs of 50 and 75ms was assessed in perfusates containing 10 nM CGP54626. Trains of 8 stimuli were applied in the control group in regular ACSF and in the presence of the GABAB antagonist. PPR2/1 and PPR8/1 were not affected by GABAB receptor blockade, nor was there a significant difference in the slopes of exponential fits for peak response amplitudes vs time (p>