Properties of spontaneous giant IPSPs.

(A) Raw trace illustrating a sample period (dashed line) during which spontaneous giant IPSPs (arrows) occurred. (B) Close-up of another period with spontaneous giant IPSPs. (C) Raw traces of IPSPs (arrows, middle panel) and histogram of IPSP amplitudes (right panel) evoked by threshold IO stimulation. For comparison, the averaged maximal IO evoked IPSP is illustrated to the left. Dashed lines indicate onset of stimulation and onset latency time of the maximal IPSP. Note the later, variable response latencies (summarized in histogram at bottom right) of the threshold IPSPs, which are likely due to that the direct neuronal activation is replaced with a more indirect, synaptic activation of the IO neurons as the stimulation intensity is reduced [65]. All recordings were made at −50 mV. Only IPSP events with an absolute amplitude >0.7 mV were analyzed. (D) Histogram of intervals between sequential giant IPSPs recorded during 3 minutes. The coefficient of variation of the intervals was 104%. (E) Average of the largest spontaneous giant IPSPs (N = 15) recorded from one DCN neuron and average of submaximal IO-evoked IPSPs (N = 10) recorded in the same neuron. Dashed lines indicate start of excitatory component, start of inhibitory component and peak of inhibitory component, respectively. (F) Peri-IPSP histogram of spike discharge relative to the onset of spontaneous giant IPSPs. Average from 9 DCN neurons. Bin width 5 ms.