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GPCRs are known to form dimeric structures, and this affects their pharmacological properties. The ?2AR and AT1aR are GPCRs that are involved the regulation of the adrenergic and renin-angiotensin systems. The ?2AR is polymorphic at position 164, affecting its responsiveness to adrenergic ligands. Both receptors have been shown to form dimers, but little is known on how dimerization affects their trafficking and signalling following ligand treatments. Plasma membrane localization, arrestin-2 recruitment, and G-protein interactions were determined between receptor dimers using molecular biological techniques. This study demonstrates that the formation of heterodimers can change the expected response to ligand treatments, along with associated trafficking events. It was determined that ligands bind to dimers, resulting in conformational changes to the dimeric complexes. Both the ?2AR and AT1aR are targeted in cardiovascular disease and this research demonstrates the importance of dimerization when prescribing drug therapies to avoid potential unwanted drug side effects.

To establish comprehensive pharmacology of P2X₇ receptors, membrane current recording, intracellular calcium transient recording and ethidium bromide uptake were carried out to examine several selective (A-740003, A- 438079) and non-selective (suramin) P2X₇ antagonists across mammalian P2X₇ receptors (human, mouse and rat). These P2X₇ receptors demonstrated species-dependent sensitivities to antagonists. In each species, A-740003 revealed variant IC50 values with different assays, indicating the assay- dependent pharmacology of P2X₇ receptors. Conventionally, pharmacology can be used to define a native current but not in the case of the human breast cancer cell line, Hs578T. It is found that P2X₇ was expressed at both mRNA and protein level. The ATP-evoked currents recorded from Hs578T cells were P2X₇-like with distinctive electrophysiological features. But the pharmacology profile of the currents did not fit with P2X₇ receptor. Further experiments are needed to either include or exclude the existence of functional P2X₇ receptors in Hs578T. Transmembrane domain 2 (TM2) is known as the pore-forming region for P2X receptors. TM2 of P2X₇ receptor was investigated with cysteine substitution scanning. The predicted α-helix structure of the TM2 segment was in good agreement with the results from the substituted cysteine accessibility method (SCAM). Thr336, Ser339, Tyr343, Phe344 and Thr348 were found important for both channel dilation and aqueous pore formation. Ser339 was further studied. Various substitutions at Ser339 were explored. The results suggest that the polarity of the side chain at Ser339 is essential for the channel dilation. Furthermore, disulfide bond formation was identified between S339C in the trimeric receptor, implying that the side chains of Ser339 might turn very close to each other during the channel opening and dilation.

NMDARs are ligand-gated cation channels which are activated by the neurotransmitter glutamate. NMDARs are essential in coupling electrical activity to biochemical signalling as a consequence of their high Ca2+ permeability. This Ca2+ influx acts as a secondary messenger to mediate neurodevelopment, synaptic plasticity, neuroprotection and neurodegeneration. The biological outcome of NMDAR activation is determined by a complicated interrelationship between the concentration of Ca2+ influx, NMDAR location (synaptic vs. extrasynaptic) as well as the subtype of the GluN2 subunit. Despite the recognition that NMDAR mediated physiology is multifaceted, tools used to study subunit and location dependent signalling are poorly characterized and in other cases, non-existent. Therefore, the aim of this thesis is to address this issue. Firstly, I assessed the current pharmacological approach used to selectively activate extrasynaptic NMDARs. Here, synaptic NMDARs are first blocked with MK-801 during phasic activation and then extrasynaptic NMDARs are tonically activated. This approach relies on the continual irreversible blockade of synaptic NMDARs by MK-801 yet contrary to the current dogma, I demonstrate this blockade is unstable during tonic agonist exposure and even more so when physiologically relevant concentrations of Mg2+ are present. This confines a temporal limit in which selective activation of extrasynaptic NMDARs can occur with significant consequences for studying synaptic vs. extrasynaptic NMDAR signalling. Dissecting subunit-dependent signalling mediated by the two major GluN2 subunits in the forebrain, GluN2A and GluN2B, has been advanced significantly by selective GluN2B antagonism yet a reciprocal GluN2A selective antagonist has been lacking. Utilizing novel GluN2A-specific antagonists, I demonstrate a developmental upregulation of GluN2A-mediated NMDA currents which concurrently dilutes the contribution of GluN2B-mediated currents. Moreover, I tested the hypothesis that the Cterminus of GluN2A and GluN2B are essential in controlling the developmental switch of GluN2 subunits utilizing knock-in mice whereby the C-terminus of GluN2A is replaced with that of GluN2B. Surprisingly, the exchange of the C-terminus does not impede the developmental switch in subunits nor the proportion of NMDARs at synaptic vs extrasynaptic sites. However, replacing the C-terminus of GluN2A with that of GluN2B induces a greater neuronal vulnerability to NMDA-dependent excitotoxicity. Collectively, this work enhances our understanding of the complex physiology mediated by the NMDAR by determining how pharmacological tools are best utilized to study the roles of NMDAR location and subunit composition in addition to revealing the importance of the GluN2 C-terminus in development and excitotoxicity.

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