Investigation Of The Neural Response To Somatosensory Stimulation In Hippocampal CA1Region

Processing of somatosensory signal has always been a hot spot in neuroscience. It receives more attention due to the numerous receptors spreading all over the body and the ever-growing applications in haptic technology. However, the basic mechanism of this process is largely undiscovered. As a well-known structure for learning and memory, hippocampus receives somatosensory input from entorhinal cortex, and encodes them to certain pattern for communication and storage within the brain. Previous studies found multiple unit activity (MUA) of the hippocampal CA1region responded to somatosensory stimuli, yet the explicit effect of the stimulation as well as the mechanism is not clear. Since the somatosensory system is crucial for animal’s survival and daily life, it is of great significance to study the hippocampal response to somatosensory stimulation and the underlying mechanism.In this dissertation, we investigated the hippocampal response to somatosensory input by applying tail-clamping on anesthetized rats and recorded the neural signals in CA1region with a microelectrode array. We analyzed the local field potential (LFP), unit spike, as well as the orthodromic population spike (OPS) evoked by Schaffer Collateral stimulation. Besides, a computer model with somatosensory input and local inhibitory circuits was built to explain the result in in-vivo experiment and the possible mechanism behind the observation. Lastly, we studied the mechanism of processing and encoding somatosensory information in the hippocampus.The major findings are:(1) Somatosensory stimulation could suppress the pricipal neurons’ activities in hippocampal CA1regionWe recorded hippocampal CA1neural activity before and during somatosensory stimulation, analyzing the response of LFP, unit spike and OPS. Results showed that during somatosensory stimulation:θ rhythm became the major power component of LFP, and mixed MUA responses (suppression or no change) appeared in CA1region; CA1pyramidal neurons showed firing decrease, while the interneurons showed firing increase; number of the burst of the pyramidal neurons decreased while the number of spikes within each burst did not show obvious change; the amplitude of OPS decreased significantly; the firing of MUA and pyramidal cells exhibited adaptation to the repetitive stimulation. These results demonstrated a strong suppressive effect on the hippocampal CA1neuronal activity, especially the excitability of pyramidal cells by somatosensory stimulation.(2) Computer model showed the somatosensory stimulation could enhance the effect of local inhibitory circuits in hippocampal CA1regionWe developed a NEURON model with somatosensory input and local inhibitory circuits, and the simulation result was in accordance to the experimental observation. Therefore it successfully demonstrated that the interneurons in the local circuits were responsible for the decreasing excitability of the pyramidal cells, and the suppressive effect by the somatosensory stimulation relied on both the feed-forward inhibitory circuit and the feedback inhibitory circuit, and the feed-forward inhibitory circuit plays a major role. Besides, we used an epileptic model to study the effect of somatosensory stimulation on over-excited neurons. Results showed that somatosensory stimulation could alleviate unilateral seizure-like activity, but not bilateral seizure-like activity.(3) Somatosensory information could be encoded by the phase-locking relationship between spikes and LFP in the hippocampal CA1regionWe analyzed the phasic relationship between unit spike and the θ cycles with Rayleigh test, showing that:a few neurons (3/22) exhibited phase-locking characteristic during spontaneous activity; tail-clamping stimulation increased the number of the phase-locking neurons significantly (10/22), and the spikes concentrated on the negative phase of the0cycle. These results suggested a possible mechanism of processing and encoding somatosensory information in the hippocampal CA1region.In brief, this dissertation combined in-vivo experiment and computer modeling to study the neural response to the somatosensory stimulation in hippocampal CA1region. It demonstrated the suppressive effect of the somatosensory stimuli, and revealed the underlying mechanism. This study was significant for studying the processing of somatosensory information, it also help understand the neural encoding and learning function in the hippocampus.