Abstracts

Rationale: Decreased release probability (Pr) and increased monosynaptic IPSC failure rate are present in presynaptic inhibitory terminals of the undercut (UC) model of posttraumatic epileptogenesis. These indices of pyramidal (P) cell inhibition are normalized in high [Ca++] ACSF, suggesting dysfunction of Ca2+ channels in terminals (Faria and Prince, 2008). Activation of P/Q and N-type Ca2+ channels underlie transmitter release in cortical inhibitory terminals. We tested the hypothesis that there are functional abnormalities in these channels in this epilepsy model. Methods: Pharmacologically isolated monosynaptic IPSCs were evoked by extracellular stimuli in voltage-clamped layer V P cells of in vitro adult rat sensorimotor cortical slices. Local perfusion of 0.2µM ω-agatoxin IVa and/or 1µM ω-conotoxin GVIA was used to block P/Q and N-type calcium channels, respectively. The peak amplitude of evoked IPSCs and paired pulse ratio (PPR) were indices of presynaptic changes. Results: In control cells, blockade of N-type channels with conotoxin decreased peak IPSC amplitude from 174.3±42.4pA to 77±20.1pA (n=8; P<0.01) and increased PPR from 0.8±0.1 to 0.9±0.1 (P<0.05). In contrast, there were no significant changes in IPSC amplitude or PPR in P cells of slices from UC cortex. Normalized results confirmed that the sensitivity of peak amplitude to ω-conotoxin was decreased significantly in cells from UC slices (control=0.49±0.1, UC=0.9±0.1; P<0.05). Blockade of P/Q-channels significantly decreased IPSC peak amplitude of both controls (127.2±18.6pA to 59.8±9.6pA, n=7; P<0.01) and UCs (190±79.1pA to 125.4±84.3pA, n=6; P<0.05), without a change in PPR. Application of both calcium blockers induced a total and irreversible suppression of the eIPSC in 8/10 control cells (80%) and 4/6 (67%) injured P neurons. Rise time (10 - 90%) was similar in control and UC after treatment with calcium channel blockers. However, after ω-conotoxin GVIA treatment, decay time decreased from 21.9±2.9ms to 12.3±2.1ms in control (n=8; P<0.05) and in UC from 20.2±2.6ms to 12±1.2ms (n=6; P<0.05).