Figure 4.

Altered synaptic integration within mature spinal DYN interneurons after early surgical injury. A, Representative synaptic currents (top; from a holding potential of −60 mV) and corresponding AP discharge (bottom) evoked by dorsal root stimulation (100 μA, 100 μs) in an adult DYN-tdTOM DH neuron from a naive mouse, which lacks evidence of FFI (No FFI; black arrow). Gray triangle represents stimulus onset in all panels. B, Representative voltage-clamp (top; from a holding potential of −60 mV) and current-clamp (bottom) traces illustrating evidence of FFI (top, black arrow) and a corresponding absence of AP firing (bottom) following dorsal root stimulation (300 μA, 100 μs) in a mature DYN interneuron after P3 incision. Bottom, FFI appears as a hyperpolarizing inhibitory postsynaptic potential (IPSP) that precedes a larger depolarizing excitatory postsynaptic potential (EPSP). C, The appearance of FFI in adult DYN interneurons is significantly more frequent after neonatal incision (right; naive: n = 44 neurons; incision: n = 33; p = 0.012; Fisher's exact test). D, Primary afferent-evoked AP discharge was significantly less common in those DYN interneurons with measurable FFI (left) compared with DYN interneurons without signs of FFI (right; n = 21–24 neurons; Fisher's exact test, p < 0.0001).