Comparative analyses of rodent and human brain in ageing and disease using novel NMDAR-2B selective probes

Abstract

The NMDA subtype of excitatory ionotropic glutamate receptors is a hetero-oligomeric ligand-gated ion channel. Molecular cloning techniques have identified three types of NMDA receptor subunits, the NR 1, NR2A-D, and NR3A-B. NR2B-containing receptors are the focus of this thesis and are believed to play an important role in learning and memory, as well as being involved in neuronal cell death accompanying cerebral ischeamia, seizures, and degenerative neurological disorders such as Alzheimer’s and Parkinson’s disease. The main aim of this thesis was to investigate the comparative abundance and distribution of NR2B receptors in the rodent and human brain using an anti-NR2B selective antibody, and two NMDAR-2B subtype-selective antagonist radioligands, [(^3)H]Ro 25,6981 and [(^3)H] CP-101,606, with a variety of experimental techniques. This thesis provides new evidence that Ro 25,6981 and CP-101,606 label distinct NMDA NR2B subtype populations in both the rodent and human brain. Ligand autoradiographical and immunohistochemical studies of the mouse and rat brain brain revealed high NR2B receptor expression levels in the CAI and CA2 hippocampal formation stratum oriens and radiatorn, striatum, and outer layers of the frontal and entorhinal cortex in both species, with higher expression levels present in the 3-week compared to 3 month adult rat brain. Radioligand binding studies of native NMDA receptors with a range of NR2B-selective antagonists provided pharmacological evidence for heterogeneity in NR2B-containing receptor populations in the two age groups indicating a developmental switch in early adulthood. Ligand autoradiography of human control and disease state brain sections revealed an overall preservation of NR2B receptors in the 60-92 year age range. Control female cases showed a significantly lower level of NR2B containing receptors compared to age-matched control male cases, in the anterior cingulate cortex (ACC). Interestingly, higher NR2B- containing receptor levels were observed in earlier age cases of AD compared to controls, and a weak correlation overall of increasing NR2B expression with decreasing scores in dementia and psychiatric tests. Overall, this study suggests that the NR2B subtype is not the major underlying receptor to explain memory deficits or psychotic symptoms in the common human dementias.