Information Coding And Its Synaptic Mechanism Of Neurons In The Auditory Core Areas

The auditory cortex is a critical center for auditory information processing,and its function is important for animal to perceive dangerous sound signals,as well as for human auditory cognition and language communication.Bilateral auditory cortical dysfunction can seriously impair auditory cognition,clinically manifested as central deafness and aphasia.Damage to this area in animals can also deprive animals of auditory cognitive ability.The auditory cortex can be divided into several functional subareas according to their response properties,and the number of subareas increases with the complexity of the brain.The primary auditory cortex(A1)and the anterior auditory field(AAF)are two major auditory cortex regions,which together constitute the core region of the auditory cortex.The core area receives information from the thalamus.Previous studies have shown that the auditory cortex core region plays an important role in auditory information processing and participates in auditory processes such as sound source localization and acoustic context extraction.Therefore,the study of the basic information coding characteristics of neurons in the auditory cortex core area is the basis for our understanding of the processing of auditory information in the cortex.With the development of in vivo patch-clamp technology in recent years,we have been able to apply loose-patch cell-attached recording to investigate the response of a single neuron,and to obtain a number of adjacent neurons in a same cortical region.By applying whole-cell patch-clamp technique,we can obtain the excitatory and the inhibitory synaptic input,as well as the spike responses from the same neuron,bridging the input and output of the single neuron.The coding of information for neurons in the auditory core region depends on the firing of action potentials.In this study,we explored the information coding in the auditory cortex from three perspectives: single neuronal responses,populational level and interaction between cortical regions.We intend to answer the following scientific questions: 1.The contributing factors at the synaptic level that determine the firng stability and the temporal precision of an auditory cortical neuron.2.The effect of sound pressure level on the frequency coding for an auditory cortical neuron.3.The synaptic mechanism that generate local functional heterogeneity in the auditory cortex.4.Connections between A1 and AAF,and the interactions between these two sub-regions.We used the extracellular multi-unit recording,the in vivo loose patch-clamp recording,the whole-cell patch-clamp recording and the immunohistochemical technique and the optogenetic technique,to answer these questions.1.The stability of auditory cortical neurons and its synaptic mechanismAuditory cortical neurons have a response property termed as “binary coding”,that is,a simple acoustic stimulus usually evokes only 0 or 1 action potential.The ability to reliably evoke this "1" action potential determines whether the sound signal can be successfully transferred.However,it has been found that the spiking responses of one neuron to a same sound stimulus are not stable: the neuron fail to fire an action potential at some trials and the onset latency can be variaded,which can affect the accuracy of neuronal coding.The stability of auditory cortical neurons has not been systematically evaluated before.More importantly,the synaptic basis contributing to neuronal variation is largely ambiguious.It has been suggested both internal and external factors can lead to neuronal variation,but intrinsic factors,like fluctuations in firing threshold and resting membrane potential fluctuation are not sufficient to explain the generation of discharge variation.Recent studies have shown that manipulation of presynaptic neurons can affect the responses of cortical neurons,implying that synaptic input may have a significant effect on response variation.Results:(1)A consideribley level of variation of spiking can be observed in A1 neurons even when the most preferred tone stimuli were given.(2)By comparing the contribution of firing threshold,the resting membrane potential and the evoked postsynaptic potential(PSP)to the generation of variation,we found that the amplitude of PSP primarily determins the variation of a spike.(3)Synaptic input can be evoked for each trail,but the amplitude can be highly unstable,and the amplitude variation of the excitatory input is much higher than the inhibitory input.(4)The temporal jitter of the synaptic input onset latency is much smaller than that of spike integration time,and the integration time jitter positively correlate to the temporal jitter of spking.(5)Using mathematical modeling,we further confirm that the amplitude variation of the excitatory input can largely affect the variation of the action potential,and the temporal jitter of the excitatory input can greatly affect the temporal fluctuation of the spike.(6)The inhibitory synaptic input is highly stable and poorly correlated to frequency,which is consistent with the observation that local inhibitory interneuron neurons show stable firing properties.2,Sound intensity bidirectionally shifts the best frequency of auditory cortical neurons.In the study of psychoacoustics,it has long been found that the increase of sound intensity can make a pure tone sounds higher or lower,suggestting that the perception of frequency and sound intensity is not independent of each other.There are also studies on the basilar membrane suggesting that the effect of sound intensity on the frequency is generated from the peripheral auditory system.Recent studies have also observed similar psychological phenomenon in patients with hearing aids,in whom the peak responses in the basilar membrane are determined by the implanted electrodes and should not be biased in theory,suggesting that illusion of frequency perception may not only be caused by the basilar membrane.At the same time,in the auditory accending pathway,multi-level subcortical nuclei are involved in the processing of sound information.It is unclear whether the effect of sound intensity on frequency preferences is also present in the auditory cortex core region and how this effect is manifested in the response properties of cortical neurons.Results:(1)We observed that the best frequency of auditory cortical neurons was significantly shifted with the increase in sound intensity in awake and anesthetized state.(2)This shifting effect was bidirectional: if the neuron's characteristic frequency(CF)is less than 32 kHz,then its best frequency would shift to lower frequency with the increase in sound intensity;if the neuron's characteristic frequency is higher than 32 kHz,then the best frequency would shift to higher frequency with the increase in sound intensity.(3)This systematic bidirectional shifting effect can affect the distribution of auditory tonotopic map of the auditory cortex: at a higher sound intensity(30 dB above the threshold),the area of high frequency region and low frequency region both enlarge.(4)The best frequency of excitatory and inhibitory synaptic input shift in a coordinate pattern,suggesting that this shift is inherited from the thalamus,rather than generated in the intracortical circuits.3,Local functional diversity of auditory cortical neurons and the corresponding synaptic mechanism.It is well studied that in large-scale,neurons with similar frequency preferrance are located adjacently due to the similarity of thalamic inputs,which constitutes the tonotopic map of the auditory cortex.However,in a recent study,by using two-photon calcium imaging techniques and two-photon-guided loose-patch recordings,it was found that even the adjacent neurons in the same layer had different frequency preferrance,and that the responsive diversity is more pronounced in layer 2/3 neurons than in layer 4 neurons,suggesting that the level of preference diversity may be different in neurons in different laminar.At present,most of the studies on the functional diversity of auditory cortical neurons are performed in A1,and it is unclear whether or not AAF,another component of the auditory cortex core region,shows functional diversity.More importantly,the synaptic mechanism that primarily determined the local functional diversity is largely unknown.Results:(1)We found that the response properties for adjacent neurons in layer 4 are highly homogeneous,while that for adjacent neurons in layer 5 are considerably heterogeneous.(2)We used in vivo whole-cell patch-clamp technique,finding that the layer 5 neurons received mismatched excitatory and inhibitory synaptic input in the frequency domain,and the degrees of mismatch varied between cells.(3)This mismatch is due to the varying degrees of skewness in the frequency tuning profiles of the excitatory input,whereas the inhibitory input is always normally distributed.(4)For layer 4 neurons,the excitatory and inhibitory tuning profiels are generally balanced and are normally distributed.(5)Finally,we found that the degree of mismatch between excitation and inhibition is highly correlated with the diversity of frequency preferrence.4,Excitatory modulation of A1 on ipsilateral AAF neuronsIn the auditory cortex core area,considerable researches are focused on A1,and the research on AAF is still relatively inadequate.Previous studies have found that A1 and AAF are tonotopically organized,and the frequency gradient of the two areas is arranged in a mirror.Previous studies have suggested that these two regions are parallel auditory pathways.Many studies have found that in other sensory systems,such as the visual cortex and somatosensory cortex,there are intensive interactions between sub-regions.However,the functional relationship between A1 and AAF is poorly understood.It has been reported that using the reversible cooling method to inactivate AAF can inhibit the auditory cortical response of ipsilateral A1;auditory cortical(including A1 and AAF)inactivation can regulate the response of contralateral AAF.However,there is little report on how A1 affects the neuronal response in the ipsilateral AAF.With the development of optogenetics in recent years,we can more easily and reversibly manipulate the activities of A1 neurons,and can use adeno-associated virus to specifically infect a certain type of neurons,which permit us to more accurately explore the relationship between A1 and AAF.Results:(1)We functionally located the A1 and AAF in rats,and used the optogenetic method to manipulate the activity of A1 excitatory neurons.We found that activation of A1 can increase the sound induced activity of AAF neurons,lower the intensity threshold and broaden the frequency response bandwidth,but do not change its BF,and inhibition of A1 can lead to the opposite effect.(2)Based on the precise functional localization of A1 and AAF,we used the anterograde and retrograde tracing methods to find the direct projection from A1 excitatory neurons to ipsilateral AAF.(3)The use of optogenetics to activate the A1 axon terminals in AAF did not change the excitability of AAF,suggesting that this excitatory effect may not depend on the direct ipsilateral cotico-cortical projections.In summary,our study explored the information coding properties of auditory cortical neurons from three perspectives: single cellular level,populational level and interacions between sub-regions.We reported for the first time the synaptic mechanism contributing to response variation and local functional diversity,which provide a useful experimental basis for deepening the understanding of neuronal functions in the auditory core area.