The Study Of Synaptic Mechanism Underlying FM Direction Selectivity Of Inferior Collicular Neurons

As a broadband signal, Frequency modulation (FM) component exists widely in animal communication signal and human speech sounds. Previous studies on FM direction selectivity of central auditory neurons were performed almost by extracellular recording and neuropharmacological methods in bats and other animals. But it is difficult to observe how the sideband inhibition at synapses shapes FM direction selectivity of the neurons in the central auditory system. Therefore, the present study was carried out on the inferior colliculus (IC) of light anesthetized mice (Mus musculus, Km) with nembutal (60mg/kg. b.wt) and recorded the IC neuron responses to pure tones (1～48kHz), up sweep and down sweep FM (FMup and FMdn) sounds (modulation range:1～48kHz/48-1kHz) with a fixed sound amplitude using in vivo intracellular recording.Total of80IC neurons were obtained, and FM modulation or sweep direction selectivity of66IC neurons among them was analyzed. The ranges of the recording depth, best frequency(BF), and resting potential(RP) of66neurons were438～1837(1172.9±316.5)μm.2～46(18.4±10.7) kHz and-88～0(-25.5±21.3) mV, respectively.The results and findings demonstrated as following:1. According to directional selectivity index(DSI) of neurons responses to FMup and FMdn stimuli, they could be classified into three types, i.e.(1) up-selective neurons (DSI＞0.33, n=20,30.3%),(2) down-selective neurons (DSI＜-0.33, n=19,28.8%) and (3) non-selective neurons (-0.33＜DSI＜0.33, n=27,40.9%). The any one among three types of the IC neurons with FM sweep direction selectivity could be divided into three subtypes, which they were corresponding to IPSP, EPSP and AP, respectively.2. Under two-tone stimulus condition, we studied that the effect of on-BF inhibition on the FM sweep direction selectivity of down-selective neurons (n=2). The results demonstrated that the down-selective neueons could not generate any response to FMdn stimulus when at small gap (gap=0,5and10ms), and then EPSPs of the neurons were gradually changed into action potentials with IPI increasing (gap=40,80ms).3. There was a significant diference (p<0.05) among the number of APs, amplitude, duration, rise time and decay time of PST induced by FMup and FMdn stimuli in the FM direction selective neurons, while there was not a significant difference (p＞0.05) among the number of APs, amplitude and duration of PSP evoked by FMup and FMdn stimuli in the neurons with FM sweep direction selectivity. The responses of non-selective neurons to BF stimulus were better than that to any modulation direction FM stimulus. We calculated the neurons’ direction selectivity index (DSI) according to the amplitude of post-synaptic potential (PSP) and AP counts. By coparison of AP-DSI and PSP-DSI, the results showed that the averaged AP-DSI value was nearly twice as large as the averaged PSP-DSI value of up-selective (or down-selective) neurons, while the averaged absolute AP-DSI value was equal with the averaged absolute PSP-DSI value of non-selective neurons.These results suggested that IC neurons could use the PSP and number of the APs to show their FM direction selectivity, side-band inhibition acdtivated by sound stimuli directly shaped the FM sweep direction selectivity of IC neurons and on-BF inhibition evoked by sound stimulus could affect the FM sweep direction selectivity of IC neurons. Thus, these two inhibitory inputs were possible the mechanism of the FM sweep direction selectivity of IC neurons.