Abstracts

Postnatal development of synaptic connections in the neocortex is critical for normal brain function and abnormalities in this process may play a critical role in some types of epilepsy. While extensive work has been done in the excitatory pyramidal neurons, very little is known about development of synaptic currents in cortical inhibitory interneurons. We have studied the postnatal development of intrinsic properties and excitatory synaptic activity in transgenic mice that express green fluorescent protein (GFP) in somatostatin-containing (SS) interneurons. Coronal cortical brain slice were obtained from P7 to P45 mice. Slices were continuously perfused with ACSF containing (in mM) 124 NaCl, 26 NaHCO[sub]2[/sub], 1.25 NaH[sub]2[/sub]PO[sub]4[/sub], 2.5 KCl, 2 CaCl[sub]2[/sub], 2 MgCl[sub]2[/sub], 10 D gassed with 95% O[sub]2[/sub]-5% CO[sub]2[/sub] giving pH 7.4. Whole cell recordings were obtained from SS interneurons which were identified using fluorescence microscopy. Recording pipettes were filled with an internal solution consisting of: 125 K-gluconate, 8 NaCl, 10 HEPES, 4 MgATP, 0.3 Na3GTP, 0.2 EGTA, 0.1% biocytin (pH 7.3 with KOH, 290-300 mOsM).Intrinsic firing patterns were evaluated in current clamp configuration and synaptic activity was recorded in voltage clamp mode. Short-term plasticity (STP) was studied using a 5-pulse stimulation train. All excitatory currents were recorded in the presence of picrotoxin (50 [micro]M). We recorded spontaneous excitatory postsynaptic currents (sEPSCs) from P7 to P45 mice that were divided into 4 groups. We found that the frequency of sEPSCs undergoes a dramatic increase during development. At P7 to P12 (n = 12), the average frequency was 0.5 [plusmn] 0.1 Hz, it significantly increased to 2.4 [plusmn] 0.5 Hz at P13-18 (n = 18) and 4.3 [plusmn] 0.7Hz at P19-25 (n = 21). No significant increase was found after P25 (4.7[plusmn]1.3 Hz, n = 13). Amplitude of sEPSCs and STP of evoked EPSCs did not differ between the different age groups. The frequency of firing with depolarization also showed a developmental increase. The average firing frequency was 49 [plusmn] 3 Hz at P7-12, 58 [plusmn] 3 Hz at P13-18, 75 [plusmn] 4 Hz at P19-25 and 72 [plusmn] 3Hz at [gt] P25. Excitatory synaptic activity undergoes a developmental increase in SS interneurons. This increase may be largely due to a developmental increase in synapse formation. The ability to rapidly fire action potentials in these interneurons is also enhanced with maturation. Thus, the first three weeks after birth is critical for the normal functional maturation of cortical SS interneurons. This data may help explain why immature cortex has an increased propensity for some types of epileptiform activity and will serve as a foundation for comparison in models of developmental epilepsy where interneuron function is abnormal. (Supported by NIH (5R01 NS35651) and Partnership for Pediatric Epilepsy Research to SNR.)