Inhibitory Synaptic Input Activated By Side-band And Center Frequencies Mediates The Forward Masking Of Inferior Collicular Neurons

Forward masking is a general auditory phenomenon and it plays an important role in the sound communication and localization. In order to investigate the synaptic mechanism underlying the forward masking, we probed the role and mechanism of inhibitory synaptic input activated by side-band and center frequencies (SBF and CF) in the forward masking of inferior collicular neurons using in vivo intracellular recording under free field and paired soud stimulation conditions. The results obtained in the experiment are as following:1. Total of113neurons was obtained in the experiment and43neurons among these neurons could be classified into two types, which type one (n=21) had inhibitory areas (IA) at low or/and high limbs of their excitatory frequency tuning curve (EFTC) and type two (n=22) had no inhibitory areas at both sides of EFTC. The neurons (n=21) with IA at low or/and high limbs of EFTC could be as well as categorized into three subtypes, i.e. subtype one (n=12/21) with IA at low limb of EFTC, subtype two (n=6/21) with IA at high limb of EFTC and subtype three (n=3/21) with IAs at both limbs of EFTC. By comparison of forward masking induced by SBF and CF, the result showed that the forward masking caused by CF was stronger (P<0.05) than that by SBF and also the duration of inhibitory postsynaptic potential (IPSP) evoked by CF was longer (P<0.05) than that by SBF in spite of the neurons with IA and without IA at any one or both sides of EFTC. These results suggested that the inhibitory synaptic inputs activated by SBF and CF mediated the forward masking and became one of the synaptic mechanisms underlying forward masking.2. In order to further understand the relation between response pattern and forward masking of the neurons,50among recorded neurons were analyzed and they were categorized into two types, onset (O)(n=37) bursting action potential (AP) at onset of sound stimulus and sustained (S)(n=13) having similar duration of bursting AP with duration of sound stimulus. By comparison of forward masking the neurons of these two types, the results showed that the response of O neurons to probe could be masked completely when a small inter-stimulus interval (ISI) between masker and probe was presented while S neurons had forward masking weaker than O neurons. In the experiment, the IPSP following the end of AP firing was also observed and the duration of IPSP following the end of AP firing of O neurons was longer than S neurons, but there was no significant difference (P>0.05) between the durations of O and S neurons. In addition, when ISI was less than10ms for O neurons, two excitatory postsynaptic potential (EPSP) severally evoked by masker and probe could be temporally summated into AP firing. For S neurons, they need longer ISI (≤34ms) than O neurons to summate two EPSPs evoked by masker and probe into AP firing. These results suggested that O neurons have stronger forward masking than S neurons because they accept more inhibitory synaptic inputs and have stronger temporary property than S neurons. Contrarily, S neurons just have less inhibitory synaptic input so that their response minimum thresholds to sound stimulus are lower and burst more easily AP than O neurons. Therefore, these results show that inhibitory synaptic input takes part in the formation of forward masking in IC neurons.