Maturation of GABAergic synaptic and intrinsic properties in primary visual cortex.

<p>(A) Stimulus recording configuration for determining the maximal IPSC amplitude prior to (young) and during (mature) the critical period. Upper trace, example of disynaptic IPSC activity in a layer 2/3 pyramidal neuron in response to stimulation of layer 4, 15 µA intensity. IPSC responses are from two separate trials (black), EPSC (gray); scale bar: 50 pA, 10 ms. Lower traces, amplitude of isolated monosynaptic AMPA-mediated current (EPSC, recorded at the empirically determined GABA<sub>A</sub> reversal potential, gray) and compound GABA<sub>A</sub>-mediated current which included both monosynaptic and polysynaptic IPSCs (recorded at 0 mV, black) in layer 2/3 pyramidal neurons in response to stimulation of layer 4. Stimulus input- output curves for a range of stimulation intensities (10–80 µA) were generated (see <a href="" target="_blank">Figure S1</a>). The average current from all trials in which a 350±50 pA EPSC was elicited is shown here. Young: n = 9 (of 14 cells) fulfilled this criteria, mature: n = 8 (of 13 cell) fulfilled this criteria. The normalized IPSC charge (nA*ms) shown here increased 1.7-fold with age (young, 36.5+/−3.6; mature, 62.0+/−9.8 pC, t-test p<0.05). GABA<sub>B</sub> and NMDA-mediated currents were blocked. Scale bar: 250 pA, 10 ms. (B) Unitary IPSCs recorded in layer 2/3 pyramidal neurons in response to stimulation of single Pv+ basket interneurons prior to the critical period (n = 14), and during the critical period (n = 19). Left, averaged current responses across all cells. The average IPSC amplitude increased 1.8-fold with age, while the average synaptic decay time-constant decreased roughly 25%, from 5.8 to 4.3 ms. See <a href="" target="_blank">Table 1</a> for statistics. (C) Input/output curves of instantaneous firing frequency of Pv+ basket interneurons prior to (n = 27, p14–15, triangles), during (n = 21, p19–23, circles), and at the end of the critical period (n = 5, p44–46, squares). Maximal spike output in response to the same input increased by greater than 2-fold during the course of the critical period (right). There was a corresponding decrease in spike half-width during development, p14–15 (0.96+/−0.18 ms), p19–23 (0.56+/−0.14 ms), p44–46 (0.23+/−0.03 ms), example voltage traces of spike shape shown on the left.</p>