Figure 7. Disynaptic inhibitory transmission between VBN neurons.

A–D, disynaptic inhibition in a pair of VBN cells from a P13 rat. Spikes were evoked in cell 1 (with intracellular current steps) while disynaptic IPSPs were recorded in cell 2 and in cell 1. Cell 2 was maintained at a resting potential of −66 mV. A, single spikes in cell 1 evoked 3–4 mV IPSPs in cell 2 (filled arrowhead), although failures occurred occasionally (2 failures in 15 trials shown; open arrowhead). Spikes in cell 1 also evoked IPSPs in cell 1 (filled arrowhead); these are evident by comparison with trials where the IPSP failed (open arrowhead). The IPSP failures in cell 1 occurred on the same trials as those in cell 2. B, trains of spikes at 40 Hz resulted in IPSP summation. Two example trains are shown. IPSPs were evoked in cell 2 following most spikes in cell 1 (IPSP failures are evident after spikes 5, 7 and 8: open arrowheads). The individual IPSPs depressed in amplitude during the train, but summation caused peak voltages to be nearly 2-fold larger than single spike-evoked IPSPs. C, bursts were most effective at evoking disynaptic inhibition. Two bursts are shown (4 spikes per burst). Cell 1 was first stepped to −90 mV with hyperpolarizing current (>500 ms), then current was removed, allowing for depolarization and bursting. Peak burst-evoked IPSPs recorded in cell 2 were larger than peak train-evoked IPSPs in B. Note the presence of IPSPs also in cell 1 following the burst (arrow). D, schematic of the presumed connections underlying the example of A–C. Cell 1 (c1) in VBN makes excitatory synapses with at least one inhibitory cell in TRN (not recorded). The TRN cell makes inhibitory synapses with cells 1 and 2 in the VBN, leading to IPSPs. E, the probability of observing disynaptic inhibition between adjacent VBN cells across early postnatal development in rats. Values in parentheses are the number of connections tested for the respective age groups (2 connections tested per pair).