Abstracts

Rationale: Early-life seizures are often refractory to conventional antiepileptic drugs and can result in chronic later-life epilepsy and long-term cognitive deficits including autism. Our previous studies have shown that early life seizures increase AMPAR function at least in part due to reduction of NMDAR-only silent synapses in hippocampal pyramidal neurons (Rakhade et al., 2008; Zhou et al., 2011). To determine whether early life seizures also affect other developing synaptic networks, we examined the effects of early life seizures on AMPAR function and silent synapses in developing auditory cortex. Methods: Early life seizures from P9-11 were induced by daily pentylenetetrazol (PTZ) injections (50mg/kg, i.p.). Whole-cell patch-clamp recordings were performed from Layer IV (L4) A1 pyramidal neurons in auditory thalamocortical slices from P12-13 post-seizure mice and littermate controls. Results: We found that L4 pyramidal neurons in slices from post-seizure pups showed a significant increase in the amplitude of AMPAR sEPSCs (?'13.620.82pA; n=10; p=0.005) compared with slices from control pups (?'10.170.40 pA; n=13), and these events occurred at a frequency (0.490.10Hz; n=10; p=0.31) comparable to controls (0.410.07Hz; n=13). In addition, minimally evoked AMPAR-mediated EPSCs through stimulating single fibers of thalamocortical circuits showed significantly higher amplitudes in neurons from post-seizure mice (17.971.61pA; n=10, p=0.017) compared to neurons from controls (12.971.01pA; n=10). We next measured the changes in the ratio of NMDAR-only silent thalamocortical synapses in P12-13 L4 A1 pyramidal neurons following early life seizures at P9-11. Silent synapses are determined by the difference of failure rates of evoked eEPSCs at negative membrane potentials (-60 mV) and positive potentials (+40mV). L4 pyramidal neurons from P12-13 controls exhibited a failure rate of 51.21 4.35% at -60mV and a failure rate of 17.94 2.05% at +40mV (n=8, p < 0.001) when stimulating thalamocortical projection fibers, yielding a fraction of silent synapses of 60.71 4.32% (n=8). In contrast, L4 pyramidal neurons from P12-13 mice following PTZ-induced seizures at P9-11 showed a failure rate of 52.58 4.14% at -60mV and a failure rate of 43.66 2.65% at +40mV (n=8 cells, p>0.05), yielding a significant decreases in the NMDAR-only silent synapses (21.77 6.61%, n=8, p < 0.001) compared to controls. In addition to thalamocortical synapses, when stimulating the layer 6, L4 pyramidal cells from P12-13 mice following PTZ-induced seizures at P9-11 exhibited a significant decreases in the NMDAR-only silent synapses (13.62 5.55%, n=10, p < 0.001) compared to controls (55.024.80%, n=9). Conclusions: These data demonstrated NMDAR-only silent synapses in primary auditory cortex are modifiable and converted to functional synapses by early-life seizures, suggesting the potential effects of early-life seizures on function and synaptic plasticity of auditory cortex. Funding: R01-NS031718-17, 1DP10D00347, IDDRC P30 HD 18655, MFE-115462.