Abstracts

Rationale: Human cortical dysplasia (CD) has a strong clinical association with intractable epilepsy. It is thought that neuronal networks of CD are hyperexcitable, which may cause seizures. The underlying mechanisms of this hyperexcitability remain unclear. GABAergic neurons provide major inhibition in the CNS and play a critical role in constraining excitation of the cortex. We have established a mouse model of CD, in utero irradiation, in transgenic mice (GadGFP45704Swn/J = GIN mice). In these mice, a subtype of cortical GABAergic interneuron, somatostatin-containing (SS) interneuron, expresses green fluorescent protein (GFP). We have studied the excitatory synaptic activity in GFP interneuron in this model. Methods: GIN mice were purchased from Jackson laboratory and bred in house. Pregnant mice were irradiated on E14, at dosage of 225 cGy. Offspring were used for experiments. Cresyl violet staining was performed to examine histological changes of the cortex. For electrophysiology, coronal cortical brain slices were obtained from the sensorimotor cortex of mice aged P10-12 and P28-30. Slices were continuously perfused with artifical CSF containing (in mM) 124 NaCl, 26 NaHCO₃, 1.25 NaH₂PO₄, 2.5 KCl, 2 CaCl₂, 2 MgCl₂, 10 D-glucose gassed with 95% O₂-5% CO₂ giving pH 7.4. Whole cell recording was performed from GFP neurons which were identified using fluorescent microscopy. Recording pipettes were filled with an internal solution consisting of (in mM): 125 K-gluconate, 8 NaCl, 10 HEPES, 4 MgATP, 0.3 Na₃GTP, 0.2 EGTA, 0.1% biocytin (pH 7.3; 290-300 mOsM). Spontaneous excitatory postsynaptic currents (sEPSCs) were recorded on GFP interneurons at a holding potential of -65 mV. Results: Compared to control, the cortex of irradiated mice was thinner. Microscopically, there was disturbed lamination of the neocortex and hippocampal heterotopia. We recorded sEPSCs from P10-12 and P28-30 mice. We found that the frequency and amplitude of sEPSCs from both age groups showed a significant increase in CD compared with controls. At P10-12, the average frequency and amplitude were 1.8 ± 0.13 Hz and 16.46 ± 0.72 pA in CD (n =10) and 0.55 ± 0.1 Hz and 13.57 ± 0.77 pA in controls (n = 19; p = 0.02 and p<0.00001, respectively; t-test). At P28-30, the average frequency and amplitude were 7.31 ± 0.76 Hz and 21.37±1.4 pA in CD (n = 8) and 3.5 ± 0.36 Hz and 13.34 ± 0.67 pA in controls (n = 15; p = 0.0001 and p = 0.0001, respectively). Conclusions: The results show that excitatory activity in SS interneuron is significantly enhanced in CD compared with controls. Our present results could suggest an increased number of excitatory synapses on SS-interneuron or increased frequency of firing of excitatory cells that synapse on these interneurons. Previous results in irradiated rats showed a decreased frequency of miniature EPSCs in two subtypes of interneuron defined by physiological properties. This suggests that excitation of interneuron may differ between subtype or that there is interspecies variability in the effect of in utero irradiation on innervation of cortical interneuron.