Abstract
The major receptor for excitatory neurotransmission in the central nervous system is the
n-methyl-D-aspartate (NMDA) receptor. Previous work has shown this receptor to be
important in development, synaptic plasticity, learning and memory, and ischaemia.
Studies have shown that this receptor can be linked to networks of signalling proteins
through binding to a scaffolding protein Postsynaptic Density Protein 95 (PSD-95). This
complex of proteins is termed the NMDA receptor complex (NRC). This thesis
examined the role of PSD-95 in excitatory signalling and synaptic plasticity in several
areas of the brain, with particular focus on the corticostriatal system. A combination of
anatomical, biochemical and electrophysiological techniques were used to address this
central question. Previous work in the laboratory had reported a localised increase in
dendritic spine density in mice with a mutation in PSD-95. Anatomical analysis of
striatal neurones revealed a decrease in spine density along the dendrites in mice with
mutated PSD-95. Further to this, experiments addressed the role of PSD-95 in
corticostriatal synaptic plasticity. Electrophysiological recordings from striatal spiny
cells revealed no strong phenotype in the PSD-95 mutant mouse with regard to
alterations in excitatory postsynaptic potential (EPSP) amplitude post trains of cortical
stimulation. However, there was significant increase in the duration of the EPSPs in the
PSD-95 mutants with respect to wild type. In the final set of experiments, a proteomic
approach was used to assess the expression levels of proteins in the NRC in specific
regions of the mouse brain in wild type and PSD-95 mutants. Analysis revealed there to
be no difference in expression levels of associated proteins within the forebrain
(striatum, hippocampus and cortex) of wild type animals, but that the cerebellum
showed expression levels that differed to the other areas. However, analysis of mice with
a mutation in PSD-95 revealed there to be alterations in the expression levels and
phosphorylation states of NRC associated proteins. These proteins were altered
throughout the forebrain regions analysed, along with the cerebellum. They included
proteins known to be important in NMDA receptor dependant signalling and synaptic
plasticity. Moreover, analysis of expression levels of specific NRC associated proteins
believed to have roles in global ischaemia, revealed further alterations in PSD-95 mutant
mice that had been subjected to global ischaemia.
The data reported in this thesis address the roles of PSD-95 in excitatory signalling. The
data reveal that PSD-95 is important in several aspects of neuronal connectivity, but that
its effects can be specific to different areas of the brain. Moreover, the data suggest that
the removal of an important protein from a signalling network can affect other signalling
molecules within the same network.
