Effects Of Acoustic Trauma On The Activity Balance Of Excitatory And Inhibitory Neurons In The Inferior Colliculus Of Mice

Objective:Acoustic trauma is a common cause of sensorineural hearing loss.Many studies have shown that acoustic trauma induces plastic changes of neuronal activities in the auditory pathway.In particular,an aberrant increase of spontaneous activity is often observed in different levels of the auditory pathway,which is thought to predicate tinnitus.This plastic change after the acoustic trauma has been supposed to be relevant to the modification of the balance of excitation and inhibition in the neural circuits.In the dorsal cochlear nucleus,inferior colliculus(IC),medial geniculate body and auditory cortex,acoustic trauma was shown to modify synaptic inhibition.Furthermore,in these structures,the balance of the excitation and inhibition was shown to be disrupted.However,little is known about how acoustic trauma affects the activities of excitatory and inhibitory neurons in the auditory pathway.IC is the auditory center of the midbrain and the main subcortical auditory integration center,consisting of excitatory glutamatergic(75%)and inhibitory GABA(25%)neurons.IC receives ascending inputs from almost all auditory brainstem nuclei,as well as descending inputs from the thalamus and cortex.Besides these external inputs,the IC contains a dense network of local connections that are considered to provide gain control and contribute to selectivity for complex acoustic features.To study the effect of acoustic trauma on the spontaneous activity of glutamatergic and GABA neurons in the IC of mice.A study of the changes in inhibitory and excitatory neuronal activity in IC after noise exposure will help to reveal the mechanism of acoustic trauma.Therefore,in this study,we have created a method that can effectively distinguish glutamatergic and GABAergic neurons in vivo to study the effect of acoustic trauma on the balance of electrical activity of excitatory and inhibitory neurons in the inferior colliculus.Methods:(1)To distinguish neurons types in in vivo unit recordings,we assembled an electrode system that can transmit light stimuli and record electrical signals.Using this set we identified the glutamatergic and inhibitory GABA neurons in the IC of VGAT–ChR2 mice in which the inhibitory neurons expressed channelrhodopsin–2.(2)Animals were unilaterally exposed to an octave–band noise of 115 dB SPL centered at 16 k Hz for 1 h at the age of 1 month.The noise was delivered to the right ear canal through a plastic tube attached to the tweeter,while the left ear canal was plugged with a silicone adhesive.(3)For each animal exposed to noise,auditory brainstem response(ABR)thresholds were measured immediately before NE(day zero)and one day after NE(8,12,16,24,and 32 k Hz).ABR thresholds were measured two months after NE.(4)Before in vivo single–unit recording,the experimental animals received 1w of adaptive training to be familiar with recording environments and be adapt to head-fixed state.(5)Idetified the electrical activity of glutamatergic and GABAergic neurons in the IC by the electrical activity of single units in response to light and sound stimulation.(6)To explore the difference of glutamatergic and GABAergic neurons electrical activites in the IC between NE mice and the control mice(7)Multiclamp 700 B amplifiers,Digidata1440 A digitizers and Clampex10.5 systems(Molecular Devices)were used to record electricity and to produce sound stimuli.The electrical signal sampling rate was 20 k Hz,and filtered from 300 Hz to 4000 Hz.Results:(1)Through repeated tests,we found that the optical fiber wrapped with zirconia is integrated with the glass electrode drawn using a silicate capillary with an outer diameter of 1.5mm.The use of blue light with a power of 45 m W and a wavelength of 465 nm can effectively trigger the firing activity of GABAergic neurons in the VGAT-Ch R2 mice IC.These firing activities can be synchronously collected by glass electrodes.Using this method,we successfully collected 60 glutamatergic and 124 GABAergics in the IC of 48 awak mice.These confirms that this optic fiber compound electrode can effectively identify the electrical activities of different neurons types in vivo.(2)By detecting the ABR thresholds of NE mice at different times,we found that the ABR thresholds of the noise-exposed ears of 11 NE mice were all increased,and this acoustic damage effect lasted at least 2 months.The ABR thresholds of the contralateral ears which had not been exposed to noise did not change significantly over time.Comparing the two ears,the ABR thresholds of noise-exposed ears for sound stimulation of various frequencies were significantly higher than those of the contralateral ear,and the ABR threshold of 32 k Hz increased the most(P <0.05,post-hoc Tukey test).(3)In the IC of the control animals,the GABAergic neurons had higher spontaneous firing rates(SRs)than the glutamatergic neurons(GABA,10.5 ± 15.3 Hz;GLU,4.2 ± 8.1 Hz,P = 0.0025,Steel–Dwass test).But the SRs of NE–glutamatergic neurons were much higher than those of NE–GABAergic neurons(NE–GABA,14.0 ± 17.8 Hz;NE–GLU,64.4 ± 32.7 Hz,P < 0.001,Steel–Dwass test).Through NE,the SRsof glutamatergic neurons were enhanced(GLU vs.NE–GLU,P < 0.001,Steel–Dwass test),whereas those of the GABAergic neurons did not change significantly(GABAvs.NE–GABA,P = 0.603,Steel–Dwass test).(4)Furthermore,we examined the effects of NE on spike shapes.The spike half–width was reduced in NE–GABAergic neurons.In contrast,the ratio of the antipeak amplitude to the peak amplitude was lower in NE–glutamatergic neurons.These changes in the spike shape suggested that NE might induce the changes in the postsynaptic membrane properties.(5)Next,we analyzed the burst events in a series of spontaneous firings.Burst events rates were higher in NE–glutamatergic neurons than others.Conclusions:(1)Optic fiber compound electrode can effectively identify the electrical activities of different neurons types in vivo.(2)Unilateral noise exposure can cause unilateral chronic hearing loss in a wide frequency range.(3)Noise exposure can selectively enhance the spontaneous firings and bursting events,aand change the action potential waveform of glutamatergic and GABAergic neurons,which may be one of the neuropathological mechanisms of the auditory function damage induced by noise exposure.