In Vivo Intracellular Recording Reveals The Relationship Between FM Direction Selectivity And Forward Masking Of Inferior Collicular Neurons

FM sound component and auditory forward masking exist widely in the real environment. It was thought that the FM or FM direction sensitivity and forward masking of central auditory neurons were related with the neural inhibition, but there were some arguments because of no direct evidence of inhibitory synaptic input obtained in previous studies of extracellular recording.In present study, we study the relation between FM direction sensitivity and forward masking of inferior collicular (IC) neurons with in vivo intracellular recording in20mice (Mus muculus, Km). Total93IC neurons were recorded and58with complete data among these neurons were analyzed and discussed. The results showed as follow.(1) Three types of FM direction selectivity in58neurons, i.e.①FMU selective neurons (N=18,31.0%), which the majority of neurons generated action potential (AP)(n=12,66.7%), the remaining generated excitatory post-synaptic potential (EPSP)(n=2,11.1%) and inhibitory post-synaptic potential (IPSP)(n=2,10.5%);(2) FMD selective neurons (N=15,25.9%), which the majority of neurons generated AP (n=8,53.3%) and EPSP (n=5,33.3%), the remaining neurons generated IPSP (n=2,13.3%); and②non-FM (FMN) selective neurons (N=25,43.1%), which the majority of neurons generated AP (n=16,64.0%), the remaining neurons generated EPSP (n=8,32.0%) and IPSP (n=1,4.0%).(2) The distribution range of best frequency (BF) in the neurons of three types, i.e. the ranges of their BF distribution were2～18kHz,11.4±5.3kHz for FMU selective neurons;16～46kHz,30.0±8.4kHz for FMD selective neurons; and9～35kHz,17.4±6.3kHz for FMN selective neurons, respectively.(3) The relationship between FM direction selectivity and forward masking,①FMU sound stimulus in the FMU selective neurons might generate stronger forward masking and need longer interpulse interval (IPI)(5-40,19.9±12.7ms) for50%recovery than in FMD selective neurons;②FMD sound stimulus in the FMD selective neurons also might generate stronger forward masking and need longer IPI (5～40,19.7±13.0ms) for50%recovery than in FMU selective neurons; and③FMU and FMD sound stimuli in the FMN selective neurons might basically generate similar forward masking and need similar IPI for50%recovery.Therefore, these results show that the inhibitory area at low or high frequency side of frequency tuning is one of the reasons forming FM sweep direction sensitivity of IC neurons. By comparison of forward masking produced by FMU and FMD sound stimuli in FMU, FMD and FMN selective neurons, the preferred FM sounds could produce stronger forward masking than the non-preferred in FMU and FMD selective neurons while there was no forward masking difference between FMU and FMD stimuli in the FMN selective neurons. The reason that produce strong forward making in FMU and FMD selective neurons is the IPSP (behind AP/EPSP).