Figure 3.

Changing patterns of inhibitory inputs and relationships between inhibitory and excitatory inputs at different postnatal ages. (A) Map of inhibitory inputs onto a P20 pyramidal neuron. The number of inputs is represented by the colored squares (scale bar at right); black indicates that no inputs were generated at that location (same scale applies for (C, E, G)). Relatively few sites generate inputs to this cell, and all are located within 500 μm of the cell body (indicated by the white cross). We stimulated 644 sites, mapping an area of approximately 1 mm². PSC, post-synaptic current. (B) Map of the same cell in (A), but indicating sites that, when stimulated, generated excitatory (red), inhibitory (blue), or both (green) types of synaptic inputs (same convention applies to (B, D, F, H)). The number of events is not coded in these plots. Excitation dominates locally, as indicated by the red regions close to the cell. Inhibition dominates with increasing distance as indicated by the abundance of blue squares. (C) Example of the pattern of inhibitory inputs onto a P34 pyramidal neuron. Both the number of inputs and the area from which they arise have increased substantially compared to P20 to 22. The map consisted of 1,174 points covering 1.7 mm². (D) At P34, inhibitory inputs predominate; close to the cell body stimulation evokes a mixture of excitatory and inhibitory inputs whereas at earlier ages (B) excitation dominated. (E) Map of inhibitory inputs into a P38 pyramidal neuron from 1,029 stimulation sites, covering an area of 1.4 mm². At this age, the overall number of sites generating inhibitory inputs has declined; the decline is especially pronounced at longer ranges (>500 μm). (F) At P38, the pattern of excitatory and inhibitory inputs is also markedly different. Fewer sites generate exclusively inhibitory inputs, and longer range inputs are exclusively excitatory. (G) Map of a mature (P56) pyramidal neuron (1,019 points covering 2.3 mm²). The pattern of inhibitory inputs is similar to that seen at P38: most inhibitory post-synaptic currents are generated close to the neuron cell body. (H) The plot of excitatory and inhibitory inputs reveals significant changes in the pattern of excitatory inputs: far fewer are present than at P38 and those that are form occasional clusters (arrowhead). Scale bar (A-H), 250 μm.