Hippocampal output to neocortex: Examination of the electrophysiological and plastic properties of CA1 projections to the perirhinal cortex
Kealy, John (2009) Hippocampal output to neocortex: Examination of the electrophysiological and plastic properties of CA1 projections to the perirhinal cortex. PhD thesis, National University of Ireland Maynooth.
The hippocampal formation is an important structure in learning and memory that is required for the transfer of sensory information into long-term storage. This long-term storage is believed to occur in the neocortex and the physiological mechanism underpinning this transfer of information is believed to be long-term changes in synaptic plasticity, namely long-term potentiation (LTP) and depression (LTD). The aim of this thesis is to characterise synaptic plasticity in a particular hippocampal-neocortical projection. The CA1 to perirhinal cortex projection has been previously shown to sustain LTP; by stimulating the area CA1 and recording in the perirhinal cortex, we show that it can sustain short- and long-term changes in synaptic plasticity. Additionally we demonstrate that multiple frequencies of high-frequency stimulation can induce LTP in this projection and that LTP-induction may require AMPA/kainate receptor activation but not NMDA receptor activation; indicating that glutamatergic signalling underlies synaptic plasticity in this projection. We also determine the role of the CA1 to perirhinal cortex projection in a model of electrophysiologically excitatory and inhibitory hippocampal projections to the parahippocampal region of the neocortex. We propose that this projection forms part of an electrophysiologically excitatory circuit from the distal CA1 and proximal subiculum along with the lateral entorhinal cortex. Moreover, we investigate the roles of the hippocampus and perirhinal cortex in recognition and spatial memory. Utilising an object recognition task (a recognition memory task) and an object displacement task (a spatial memory task), we show that there are increased levels of hippocampal brain-derived neurotrophic factor (BDNF) following the spatial task. Furthermore, we demonstrate that AMPA/kainate glutamate receptors are necessary for performance in the object recognition task whereas both NMDA and AMPA/kainate receptors are required for the object displacement task. These findings suggest that glutamatergic signalling not only underlies synaptic plasticity in the CA1 to perirhinal cortex projection but that it is also required for learning and memory in recognition and spatial tasks.
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