The Effect Of Inhibitory Input Activated By Synchronized Nonisofrequency Sound Stimulus On The Duration Selectivity In The Inferior Collicular Neurons

The interaction of excitatory and inhibitory inputs affects the response of inferior collicular (IC) neurons during acoustic signal processing. Duration selectivity which is an important property of IC neurons is the resulted of the intergration of excitatory and inhibitory inputs evoked by sound signal. We examined the duration selectivity in the IC neurons of Kunming mouse (Mus musculus, Km) by extra-cellular recording in the free-field condition with a single BF tone stimulus and a two-tone stimulus which is composed of a BF tone and a synchronized nonisofrequency tone, and then analyzed the effects of the inhibitory input excited by synchronized different frequency tone on the duration selectivity of IC neurons.Totally,117 neurons were recorded, and 88 (75%,88/117) of them displayed duration selectivity. According to duration tuning curves, they were divided into different types, such as long-pass (3%,3/88), short-pass (80%,70/88) and band pass (17%,15/88). Others 29 neurons (25%,29/117) didn't show duration selective features, including two types neurons of all-pass (83%,24/29) and irregular (17%,5/29). During the synchronized nonisofrequency tone was added, that is, under two-tone stimulation model, the neurons'response at each duration showed a reduction in the spike number. Meanwhile, we analyzed the duration-inhibition rate curves of each unit which showed different inhibition types:short-duration inhibition (12%,14/117), long-duration inhibition (9%,11/117), medium-duration inhibition (14%,16/117), sides-inhibition (10%,12/117), all-duration inhibition (48%,56/117) and irregular-inhibition (6%,8/117), the all-duration inhibition type neurons were in majority.Among 88 duration neurons under two-tone stimulation,16 (18%,16/88) short-pass neurons changed their selective type,5 units changed to band-pass and 11 units changed to non-selective type. Other 72 (82%,72/88) duration selective neurons kept their selective type same with before. However, the best duration of some neurons (13%,9/72) shifted, and most (76%,55/72) of these neurons' 50% cut off duration shifted. On the other hand, among 29 non-selective units,6 (20%,6/29) all-pass neurons changed their types,3 of them changed to short-pass,2 of them changed to band-pass and 1 of them changed to irregular type. Except the 6 units, other non-selective units (80%,23/29) kept their types same with before.Fixing the BF sound at 40 ms, we compared the response latency of each neuron under the two sound stimulus model (single BF tone stimulation; two-tone stimulation). The result showed that the latency of most units (75%,88/117) did not change; however, some neurons'(25%,29/117) latency increased. In 88 duration selective neurons,22 (25%,22/88) neurons'latency increased, other 66 (75%,66/88) neurons'latency unchanged; In 29 non-duration selective neurons,7 (24%,7/29) increased, other 22 (76%, 22/29) unchanged. On the other hand, neurons showed different suppressed patterns which were divided into three types according to the difference of inhibition degree between two half PSTHs divided by middle point of total response time. Three types were early-inhibition (19%,22/117), late-inhibition (23%,27/117) and proportional-inhibition (58%,68/117), respectively. The ratio of different inhibitional type neurons'number is different between latency increased and unchanged neurons33 neurons were examined in theγ-aminobutyric acid drug injection experiments for studying the underlying mechanism of inhibitory effect. After ion electrophoresis of Bicuculine, the number of spikes of IC neurons increased at different durations but the increased degrees were different. Among these 33 neurons,23 units were duration selective neurons,19 of them showed the same selective type under all the three conditions (single BF tone; two-tone; two tone and drug injection),3 short-pass neurons of them showed irregular type under two-tone stimulus condition and then changed back to short-pass type during drug injection,1 short-pass neuron showed band-pass type under the lately two condition (two-tone; two tone and drug injection). Besides 23 selective neurons, other 10 neurons were all-pass type neurons which did not show duration selectivity,8 of them showed the same selective type under all the three different conditions,1 neuron showed band-pass type under two-tone stimulus condition and then changed back to all-pass type during drug injection, what's more,1 neuron showed the same type of band-pass under the lately two conditions. The results indicated that the inhibitory input elicited by synchronized nonisofrequency sound stimulus could affect the duration selectivity of IC neurons and the neurons under two-tone stimulation condition also have the ability to reintegrate the excitatory and inhibitory neural input for adapting to the different acoustic environment.