Stimulus Timing-Dependent Plasticity in Dorsal Cochlear Nucleus Is Altered in Tinnitus

A, Schematic describing the startle-based gap-detection assay for tinnitus. No gap (top row) and gap trials (50 ms gap, 50 ms before the startle sound; bottom two rows) are presented to the animal. Each trial consists of a continuous 60 dB background sound (gray bar) with a 10 ms, 115 dB startle pulse embedded (black bar). The guinea pig startles in response to the startle stimulus, with the amplitude of the response shown by the height of each arrow. In animals without tinnitus, the gap introduces a suppression of the startle response (middle row). In animals with tinnitus, the gap is filled by the tinnitus (pink), and the startle response shows less reduction relative to the no gap startle response (white arrow). B–F, Gaussian mixture model analysis partitioning the normalized startle distribution into normal and tinnitus distributions. B, Histogram of the normalized startle distribution (white line) partitioned into two distributions: no evidence for tinnitus (black bars) and evidence for tinnitus (red bars). C, The posterior probabilities that normalized startle values belong to the tinnitus or nontinnitus distributions. D, Histogram of the partitioned distribution of postexposure normalized startle observations for Sham animals. E, Histogram of the partitioned distribution of normalized startle observations for baseline (preexposure) observations from Sham and exposed animals. F, Histogram of the partitioned distribution of postexposure normalized startle observations from exposed animals. D–F, Percentages of observations placed into the TG is shown on each panel.

ABR thresholds are temporarily elevated after noise exposure. ABR threshold shifts for noise-exposed guinea pigs in the exposed (A) and unexposed (B) ear and Sham guinea pigs in both ears (C, D). Thresholds were measured after the first exposure (green), the second exposure (pink), and before the acute unit recordings (gray). Dashed lines indicate thresholds measured immediately after the noise exposure. Solid lines indicate threshold shifts measured after recovering 1 or more weeks after noise exposure. Shaded bands represent 95% confidence intervals.

Elevated normalized startle responses demonstrate tinnitus in the 4 to 16 kHz bands. A, Percentage of Sham (white bars) and exposed (black bars) guinea pigs that show evidence for tinnitus in different frequency bands. B, Normalized startle response amplitudes in each frequency band for exposed animals (black bars) compared with Sham animals (white bars). C, Normalized startle response amplitudes in each frequency band for tinnitus animals (ET, red bars) compared with animals without tinnitus (ENT, gray bars) and Sham animals (white bars). B, C, Error bars indicate 95% confidence intervals. ★Significantly different from other bars within the same frequency band.

Bimodal plasticity timing rules are significantly different between Sham and noise-exposed guinea pigs. A, Diagram representing the bimodal plasticity measurement protocol. Spontaneous activity (Spont) and responses to tones (Tone) were recorded before and 3 and 15 min after the bimodal pairing protocol. The bimodal pairing protocol consisted of repeated presentations of an Sp5 stimulus (brown line) and a short tone burst (black sinusoid) with varied BIs. BI is defined below the diagram, with t_Tone representing the onset time of the tone stimulus and t_Sp5 representing the onset time of the Sp5 stimulus. B, Two examples of single-unit Hebbian timing rules: one from a Sham (gray) and one from a noise-exposed (pink) guinea pig. A diagram at the top of the panel demonstrates the relative order of Sp5 and sound stimuli (also in E). The brown vertical line represents the Sp5 stimulus and the sinusoid represents the tone stimulus. C, Two examples of single-unit anti-Hebbian timing rules: one from a Sham and one from a noise-exposed guinea pig. B, C, Filled circles represent significant changes in sound-evoked firing rates. D, The percentage of principal units that showed Hebbian-like (H), anti-Hebbian-like (aH), enhancing (E), and suppressing (S) timing rules from Sham (left) and noise-exposed (right) animals. Stacked bars indicate units from below (black), within (white), and above (gray) the damaged frequency region. Data were obtained at the 15 min time point. ★Significant differences in the distribution of timing rules between Sham and exposed animals. E, Timing rules shifted from Hebbian in Sham animals to anti-Hebbian in exposed animals. Mean timing rules showing bimodal plasticity of sound-evoked firing rates for units from Sham (gray) and exposed (pink) guinea pigs. Mean timing rules were computed for all measurements from principal cell units. Error bars indicate SEM. ★Significant differences (p < 0.05; Tukey–Kramer's post hoc test).

Bimodal plasticity shifts from predominantly Hebbian to broader anti-Hebbian rules in guinea pigs with tinnitus and predominantly suppressive in guinea pigs without tinnitus. A, Mean timing rules showing bimodal plasticity of sound-evoked firing rates for units from Sham (gray), ENT (pink), and ET (red) guinea pigs. Diagram at top represents the relative order of Sp5 and sound stimuli. The brown vertical line indicates the Sp5 stimulation, and the sinusoid represents the tone stimulus. Mean timing rules were computed for all measurements from presumed principal cell units. Error bars indicate SEM. B, The percentage of units that showed Hebbian-like (H), anti-Hebbian-like (aH), enhancing (E), and suppressing (S) timing rules from Sham (left), ENT (middle), and ET (right) animals. Within the ET group of animals, stacked bars indicate units outside (white) and within (gray) identified tinnitus bands. ★Significant differences between Sham, ENT, and ET groups (p < 0.05; Tukey–Kramer's post hoc test).

Spontaneous firing rates show hyperactivity in ET but not ENT guinea pigs. A, Spontaneous firing rates before any bimodal stimulation for each unit as a function of each unit's best frequency for Sham (gray), ENT (pink), and ET (red) units. B, Mean spontaneous rates for units with best frequencies <12 kHz. C, Mean spontaneous rates for units with best frequencies >12 kHz. SR, Spontaneous rates. ★Significant differences (p = 0.003). D, Mean timing rules show bimodal plasticity of spontaneous firing rates for units from Sham (gray), ENT (pink), and ET (red) guinea pigs. Mean timing rules were computed for all measurements from presumed principal cell units. Error bars indicate SEM.

Responses to Sp5 stimulation shift from predominantly excitatory to predominantly suppressive in tinnitus animals. The percentage of units showing excitatory (green), complex (blue), and inhibitory (red) responses to unimodal Sp5 stimulation.