Temporal And Spatial Characteristics Of Excitatory And Inhibitory Synaptic Inputs Of SOM+ Interneurons In Mice Primary Auditory Cortex

The auditory cortex is the advanced center responsible for the perception of acoustic cues.Different types of cortical neurons can work together to encode and transfer various kinds of auditory information.The interplay between excitatory and inhibitory synaptic inputs have found to be prominent in deciding the functions of pyramidal cells.Balanced excitation and inhibition can shape the tuning properties of neurons in auditory cortex,while unbalanced excitation and inhibition can generate novel functions.Based on the functional role,cortical neurons can be classified into two groups: excitatory neurons and inhibitory neurons.While majority of cortical neurons are excitatory,the inhibitory neurons can play essential roles in generating and modulating cortical functions.Inhibitory neuron is important for the regulation of the cortex function,through release GABA neurotransmitter to evoke other neurons response.By the feedforward and feedback mediated inhibitory interneurons in the cortex can play a balance role in the regulation network.SOM+ neuron is an important kind of inhibitory neurons,but the number is sparse,so is difficult for single cell recording in the cortex.As a special kind of inhibitory neuron,the SOM+(Somatostatin expressing)neuron is known for its role in suppressing the activity of other inhibitory neurons which is known as "disinhibition".This kind of disinhibition is considered to be critical in the laminar transformation of sensory information.Yet it is still largely unclear if SOM+ neurons also take inhibitory modulation from other inhibitory neurons,and if so,what kind of inhibitory inputs are received by SOM+ neurons.For excitatory pyramidal cells,many previous studies have identified the roles of excitatory and inhibitory synaptic inputs by using in-vivo whole-cell patch clamp recording from the cortex of living animal.However,it is much more challenging to perform intracellular recording from SOM+ interneurons in vivo because the size of cell body is too small and the number of cells are so rare.In my thesis work,I tried to record different types of synaptic inputs directly from SOM+ neurons in the primary auditory cortex(A1)of juvenile mice.Instead of performing patch clamp recording in living brain,we tried to perform patch clamp recording in specially prepared cortical slices where the thalamo-cortical projection from MGB to A1 can be preserved.Under the guidance of fluorescent microscopy,the SOM+ interneurons can be easily identified.By recording the excitatory(EPSCs)and inhibitory(IPSCs)synaptic currents evoked by electrical stimulation in MGB,investigated the spatial-temporal characteristics of excitatory and inhibitory inputs of nearby SOM+ interneurons and excitatory pyramidal cells.Two major questions were studied in this work.Part ?: The temporal and spatial characteristics of the synaptic inputs of SOM+ neurons in the mouse auditory cortexAs mentioned above,very little is known about the characteristics of synaptic inputs received by SOM+ interneurons.The synaptic inputs(EPSCs &IPSCs)received by SOM+ interneuron and pyramidal cells were recorded using voltage-clamp whole-cell recording.I tried to collect and compare some very fundamental properties such as membrane capacitance,resistance,onset latency,peak latency,conductance of excitatory and inhibitory synaptic inputs.In addition,we compared these properties of SOM+ interneurons with that of nearby pyramidal cells which have been well studied before.Part ?: The relationship of excitatory and inhibitory synaptic inputs received by Somatostatin-expressing neuronsIn this experiment,we adopted whole-cell patch clamp recording to observe EPSCs and IPSCs of SOM+ neurons and pyramidal neurons under different stimulus intensities.And compared rise time,decay time and excitatory and inhibitory current amplitude ratio of EPSCs and IPSCs between SOM+ neurons and pyramidal neurons in different stimulation intensity.The relationship of synaptic inputs between SOM+ interneuron and nearby pyramidal cells would also provide helpful insights to understand the possible circuit of SOM+ neurons.Result:Part ?:The temporal and spatial characteristics of the synaptic inputs of SOM+ neurons in the mouse auditory cortex1.There was no significant difference in the onset latency of EPSCs and IPSCs between SOM+ neurons and pyramidal neurons.2.The peak latency of the postsynaptic currents of SOM+ neurons was significantly shorter than that of excitatory pyramidal neurons.3.There was no significant difference in the amplitude of postsynaptic currents between SOM+ neurons and pyramidal neurons,but the amplitude of IPSCs was greater than that of EPSCs.Part ?: The relationship of excitatory and inhibitory synaptic inputs received by Somatostatin-expressing neurons1.The rise time of SOM+ neuron IPSCs was longer than that of EPSCs,however,the result of pyramidal neurons was just the opposite.2 The decay time of EPSCs in SOM+ neurons and pyramidal neurons was always shorter than that of IPSCs.3.The amplitude of the postsynaptic current of SOM+ neurons and pyramidal neurons changed in accordance when the stimulation increased,and the conductance of excitatory and inhibitory inputs ratio were relatively balanced.Conclusion:In the primary auditory cortex of mice,SOM+ neurons and pyramidal neurons can receive stable excitatory and inhibitory synaptic input.Some SOM+ neurons can receive direct projections from the thalamus similar to that of pyramidal neurons.The differences of peak latency of synaptic input between the two types of neurons suggested that the synaptic input of SOM+ neurons and pyramidal cells may come from different presynaptic neurons.Both SOM+ neurons and pyramidal neurons receive strong inhibitory inputs in amplitude.When given stimulus with different intensities in the MGB,the amplitude of postsynaptic current received by SOM+ neurons and pyramidal neurons changes in accordance,shows that the two kinds of neurons receive relatively balanced excitatory and inhibitory synaptic input.Through the study of SOM+ neurons and pyramidal neurons in temporal and spatial characteristics,to reveal the functional properties of SOM+ neurons receive synaptic input in the cortical microcirculation,thus it can provide research basis to study why SOM+ neurons can play different functions compare to excitatory neuronsin the cortex.