Fig. 8.

Distinct properties of mature intralaminar and interlaminar excitatory inputs. Schematic shows the sources of glutamatergic inputs onto layer 2/3 parvalbumin (PV)-expressing interneurons and their synaptic properties in juvenile and adult stages. Middle, the most prominent sources of glutamatergic drive to layer 2/3 PV interneurons are intralaminar (layers 2/3) and interlaminar (layers 4/5) pyramidal neurons (Pyr). Vertical ticks underneath Pyr neurons represent the rate of action potential firing and the sparse coding of superficial layers compared with deeper layers that accentuates with development. Left, representative normalized AMPA receptor (AMPAR)- and NMDA receptor (NMDAR)-mediated currents for intra- and interlaminar juvenile inputs; both inputs display similar properties and therefore would have similar drive onto PV interneuron activity. Right, representative normalized AMPAR- and NMDAR-mediated currents for intra- and interlaminar adult inputs. A minimal contribution of NMDARs for intralaminar connections would ensure precise timing and engagement of feedback inhibition in the sparsely firing superficial pyramidal neurons. A substantial NMDAR component and facilitating dynamics would ensure temporal integration of interlaminar inputs and efficient recruitment of feedforward inhibition.