Cellular Mechanism Of MGB Activation Modulating The Responses Of Neurons In Rat Primary Auditory Cortex

The environmental sound signals are changed from mechanical wave into electroneurographic signal through the ear, which has the functions of transduction and transformation. Then they are mainly transmitted via the auditory nuclei and pathways of central auditory system to the primary auditory cortex (AⅠ). AⅠis the major superior center in the system of acoustic information processing system. AⅠplays a dominant role in sound perception, voice discrimination, sound source location, adaption of sound, as well as learning and memory. Thus it was the focus of the investigation of auditory neuroscience ever since a long time.In the auditory information sending and processing system, medial geniculate body (MGB) locates at the top of the subcortical relay nuclei and has been extensively proved to be in charge of gating and driving all the auditory signals delivered from periphery to auditory cortex (AC). There are profuse bidirectional projection fibers between AⅠand MGB, which make them a both compacted and complex neuronal circuitry. They are congenerous and accordant in function.Previous experiments in vivo and in vitro indicated that the bidirectional thalamocortical and corticothalamic projection system acted important modulation effects on the responses of cortical neurons and plasticity of receptive field (RF). It had been documented that the thalamocortical synapses undergo long-term strengthening or weakening following high- or low-frequency electrical stimulation of thalamocortical inputs. This provided a solid synaptic basis for the centripetal plasticity of cortical RFs. Yan et al. had also proved that local electric stimulus in MGB caused the parameters of the neuronal RF in AC faithfully tending toward the RF of the electrically stimulated auditory thalamic neurons, which suggested that the thalamocortical system possesses intrinsic mechanisms that underlay the input specificity of learning induced or experience-dependent cortical plasticity. A following report demonstrated that ventral division of medial geniculate body (MGBv) placed in the core of the specific sensory pathway, and its activation could evoke tone-specific plasticity in AⅠfor adjusting different auditory signal processing. However, there is still a distant way to know how the thalamocortical projection contributes to the modulatory mechanisms of various properties of neural reactions and plasticity of cortical neurons. This may partly result from the complex connections, changeable states and diverse characteristics of responses in cortical neurons. Moreover, those previous investigations all adopted extracelluar recording method. For these reasons, they could only identify the existence of the effects of thalamic activations and the tending direction of the cortical plasticity, but couldn't reveal the electrophysiological mechanism of the contribution of thalamic activation to the auditory information processing in AⅠfrom the level of cellular membrane potential. The clarity of this mechanism will provide theoretical support for our further studies on learning induced or experience-dependent cortical plasticity and will advance our understanding of the biological nature of the information processing in sensory cortex.Therefore, we used intracellular recording technique with sharp glass electrode to make further demonstration. We recorded the membrane potentials of neurons during autonomous state in different cortical layers of rats in vivo, as well as the intracellular membrane characteristics of the excitatory and inhibitory responses to acoustic stimuli of the AⅠneurons. We also focused on the changes of the excitatory and inhibitory responses to paired sound stimuli of the AⅠneurons induced by thalamic activation, and analyzed its effects on the excitation and inhibition of AⅠneurons.The results were as follow:1.The neurons in different layers of rat AⅠexhibited significant differences in intracellular membrane potential characteristics of responses to sound stimuli. The amplitudes of both the excitatory auditory responses and the inhibitory auditory responses depended on their onset membrane potentials.2.The magnitudes of excitatory and inhibitory inputs reached to the balance gradually with the projections between different layers.3.The activations of MGBv with train stimuli of various frequencies had different mechanism of effects on the membrane potential characteristics of excitatory responses of AⅠneurons. The high-frequency stimuli would induce monotonic changes to it, i.e. the amplitudes of excitatory responses reduced in prior and then rose up to the level before thalamic activation gradually. The low-frequency stimuli would induce non-monotonic changes to the amplitudes of the excitatory auditory responses, i.e. the amplitudes of excitatory responses increased in prior, then got down sharply, and increased to the level before thalamic activation at last.4.The effects of MGB activation on the membrane potential characteristics of inhibitory responses of AⅠneurons could be in the same or in adverse to the mechanisms of the excitatory responses, which depended on the local circuit that the neurons located in.Conclusion:MGB activation contributed to the modulation of the membrane potential characteristics of auditory responses of AⅠneurons. The degree of this modulation depended on the frequencies and intensities of train stimuli, the property of the modulation depended on the frequency of the train, and the way of this modulation depended on the natural characteristics of AⅠneurons.