Structure and Composition of Postsynaptic Densities

Communication between neurons within the brain occurs at chemical synapses and is fundamental for all brain functions. Modulation of the strength of communication is controlled by both presynaptic and postsynaptic mechanisms and is termed synaptic plasticity. One postsynaptic structure postulated to regulate synaptic strength is the postsynaptic density (PSD), a large electron dense protein complex located just below the synaptic membrane. The PSD, which is composed of signaling, scaffold and cytoskeletal proteins, supports and organizes neurotransmitter receptors within the synaptic membrane in addition to bridging signaling with the actin cytoskeletal network. The protein composition and structure of PSDs is known to change in response to synaptic activity and several PSD proteins are implicated in neurological conditions characterized by synaptic dysfunction. However, there is a lack of information regarding the variability of PSD structure and composition from individual PSDs across the brain. In order to address this deficiency, PSDs were isolated from adult rat cerebella, hippocampi and cerebral cortices three brain regions with unique neuronal populations. The structure and composition of morphologically identified PSDs from these regions was then compared through immunogold analysis and electron tomography. Tomographic reconstructions revealed that while the majority of PSDs shared a similar dense protein organization, there were cerebellar PSDs which displayed a latticelike protein organization. PSDs from cortices and cerebellar were