Synaptic Mechanism Of Corticofugal Modulation Of Frequency Tuning In Inferior Colliculus Of Hipposideros Pratti

The auditory cortex (AC) projects to a wide range of subcortical nucleus in the auditory pathway, which forms the corticofugal system. The corticofugal system plays an important role in the modulation of auditory signal processing in the subcortical nucleus. As to frequency analysis, AC widens and sharpens the frequency tuning (FT) of subcortical neurons by facilitatory and inhibitory modulation. However, most of researches on corticofugal modulation were done by in vivo extracellular recordings and the underlying mechanisms are not very clear. We, therefore, used the method of free-field sound stimulation combined with focal electrical stimulation of AC (ACES) and recorded the response of IC neuron in 16 Hipposideros pratti by in vivo intracellular recordings to explore the synaptic mechanisms of the corticofugal modulation on FT. The results are demonstrated as follows:1、We have obtained the data about FT from 104 neurons. In 11 of the recorded IC neurons, ACES widened its FT (5.87±3.46 kHz vs 6.11±3.52 kHz;p<0.01) and shortened its response latency (15.12±3.93 ms vs 14.61±3.94 ms;p<0.01), while ACES sharpened the FT (4.73±3.72 kHz vs 4.18±3.47 kHz;p<0.01) and lengthened the response latency (14.01±4.33 ms vs 14.57±4.40 ms;p<0.01) of 27 inhibited neurons. The other 66 neurons were not affected by the activation of AC.2、ACES generated local depolarization potential in low- and high- frequency-tail of FT in facilitated IC neurons, which increased the number of firing spikes, widened the FT and shortened the response latency. In the inhibited IC neurons, ACES generated local depolarization potential in low- and high- frequency-tail of FT and reverse results were obtained.3、The pre-spike sub-depolarization (the onset of depolarization of cell membrane to the threshold level of bursting AP) was also under the modulation of corticofugal system. The facilitatory and inhibitory effects of AC shortened and prolonged the sub-depolarization time of facilitated IC neurons (3.31±1.76 ms vs 3.07±1.79 ms; p<0.01) and inhibited IC neurons (2.77±1.36 ms vs 3.03±1.25 ms;p<0.01) respectively.4、The change in latency of IC neurons and change in sub-depolarization time showed significant correlation, which suggested that the effects of corticofugal system on response latency of IC neurons was based on the modulation of sub-depolarization.