Spiral fiber neuron activity is modulated by NMDA receptor blockade in setting the startle threshold and habituation. (A) Representative image of the transgenic line pSAM. Grey boxes indicate the two clusters of SFNs. The green box shows the axon cap, a bundle of spiral fiber neuron axons surrounding the M-cell axon initial segment. (B) SFN activity does not significantly influence short-latency escape directionality after non-directional acoustic stimulation using our speaker system (n = 6 fish; five responses per fish). (C) Representative fluorescent trace indicating that delivering an acoustic stimulus below the startle threshold at 89.8 dB results in approximately 60% of the fluorescence level measured above the threshold at 91.7 dB. (D) Representative fluorescent trace indicating that administration of 50 μM l-701 increases fluorescence values at 89.8 dB to values measured at 91.7 dB. (E) Significant increase in fluorescence peak values in response to treatment with 50 μM l-701. Peak fluorescence values at 89.8 dB were normalized to values after stimulation at 91.7 dB (n = 8, 13 for vehicle, treated respectively, p = 0.0029, two-sample t-test). Lines represent the mean ± SD. (F) The probability of SFN activation at 89.8 dB in both vehicles and treated animals. The threshold for cell activation was defined as the mean value of baseline fluorescence of the cell +2 times the standard deviation of the baseline. At 89.8 dB control conditions, SFNs are active only 60% of the time. Treatment with l-701 results in an increase in probability of cell activation at 89.8 dB (n = 13 for vehicle and treated, respectively; p < 0.05, one-way ANOVA). (G) The activity of SFNs to repetitive acoustic stimulation at 1 Hz. Fluorescence peak values from every five stimuli were averaged and normalized to the mean value coming from the first five stimuli. Treatment with 50 μM l-701 significantly decreased the depression of cell activity (n = 6 and 8 for vehicle and treated, respectively; p < 0.05, two-sample t-test). *p < 0.05, **p < 0.01.
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