| Summary: | Adenosine receptors are members of the G protein coupled receptor (GPCR) superfamily. Neurochemical and electrophysiological experiments point to a neuromodulatory role for adenosine. Adenosine inhibits the release of several neurotransmitters in brain by pre- and postsynaptic actions at adenosine A1 receptors. In this thesis the pharmacology and modulation of adenosine A1 receptors in the central nervous system (CNS) of different species was characterised. Rat renal adenosine A1 receptors were also studied. In addition the central penetration by adenosine A1 receptor antagonists following peripheral administration was investigated. An in vitro [3H]DPCPX ligand binding assay utilising a range of receptor agonists and antagonists, were used to compare the pharmacology of rat, mouse, guinea pig and human adenosine A1 receptors. Standard xanthine-based adenosine antagonists had 10-fold lower affinity in human and guinea pig in comparison with rat and mouse. Several novel nonxanthine A1 antagonists from the Fujisawa Pharmaceutical Co. Ltd., retained high affinity for adenosine A1 receptors in all species, including human. Modulation of ligand binding to adenosine A1 and A2a receptors by Gpp(NH)p and magnesium was examined. In common with other GPCRs, agonists bind to high and low affinity states, with the equilibrium modified by GTP analogues and magnesium, whereas effects on antagonist affinity are minimal. Agonist radioligands under the conditions used labelled predominantly the high affinity state of adenosine A1 and A2a receptors. Gpp(NH)p and magnesium have essentially inverse effects on radioligand (agonists and antagonists) binding to both adenosine A1 and A2a receptors. In addition their effects on [3H]agonist and [3H]antagonist binding are generally opposite, which is consistent with effects observed for other GPCRs. To act centrally, compounds must cross the blood brain barrier (BBB). A modified radioreceptor assay, involving a denaturation step to overcome the lipophilic nature of adenosine A1 receptor antagonists was developed. This assay accurately measures the central penetration of adenosine A1 receptor antagonists. Adenosine A1 antagonists administered peripherally at equipotent behavioural doses, were found to be present in brain, at concentrations between 50-500 fold greater than in vitro Ki values for adenosine A1 receptors. In addition, there was an excellent association between antagonist affinity in vitro and antagonist brain concentrations at the equipotent behavioural dose. Adenosine and its receptors are widely distributed throughout the body. It is important to determine if peripheral adenosine A1 receptors are identical to those in the CNS. Renal adenosine A1 receptors were examined using radioligand binding, in vitro autoradiography and in situ hybridisation. Even using this combined approach, identification of adenosine A1 receptors in rat kidney was not possible. Alzheimer's disease involves a myriad of neurochemical changes and neurotransmitter deficits. By antagonising the actions of endogenous adenosine, A1 receptor antagonists could compensate for some of these deficits, by enhancing synaptic facilitation and increasing neurotransmitter release. Analogues of FK453, a non-xanthine adenosine A1 antagonist, which are BBB permeable and potent and selective for human adenosine A1 receptors, could prove to be clinically useful compounds, by enhancing cognition, in conditions such as Alzheimer's disease.
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