Slow IPSC kinetics, low levels of alpha 1 subunit expression and paired-pulse depression are distinct properties of neonatal inhibitory GABAergic synaptic connections in the mouse superior colliculus

Remodelling of visual maps in the superior colliculus (SC) depends on neuro nal activity. Synaptic inhibition could contribute to this process because spontaneous spike discharge in the SC was modulated by GABA(A) receptor act ivation at postnatal days (P) 1-3. To investigate the functional capacity o f GABAergic synaptic transmission at this early stage of development, whole -cell patch-clamp recordings were made from wide field neurons (WFNs) in ho rizontal slices comprising the superficial grey layer of the SC. Focal stim ulation in the vicinity of WFNs evoked tetrodotoxin-sensitive stimulus-lock ed inhibitory postsynaptic currents (eIPSCs). The failure rate of eIPSCs wa s low (approximate to 0.2), and the maximal amplitude of evoked unitary eIP SCs exceeded the amplitude of average miniature IPSCs (mIPSCs) by a factor of 4-5, suggesting that action potential-mediated GABA release was more eff ective than spontaneous release. Some of the properties of GABAergjc synapt ic transmission in the neonatal SC were age-specific. In contrast with eIPS Cs in the more mature SC at P20-22, neonatal eIPSCs decayed more slowly, pr eferentially fluctuated in duration, not amplitude, and mostly lacked tempo ral summation, due to depression at shorter intervals. The paired-pulse rat io (eIPSC(2):eIPSC(1)) was inversely related to the duration of eIPSCs. PCR analysis showed, in addition, that the ratio of alpha1: alpha3 subunit exp ression was lower in the neonatal SC. Together, these results suggest that, at a young age, efficacy of GABAergic synaptic transmission is primarily c onstrained by the slow kinetics and the saturation of postsynaptic GABA(A) receptors.