CHOLINERGIC REPLACEMENT THERAPY

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The objective of the proposed work is to provide a rational scientific basis for cholinergic replacement therapy for memory disorders, specifically Alzheimer's disease (AD). This objective is based upon two closely related themes. The first is that the M1 muscarine receptor mechanism is worthy of intense study in its own right, because M1 receptors are primarily localized in the forebrain, appear essential for normal cerebral excitation and short-term memory, and have not been well studied. The second is that cholinergic therapy with M1 agonists is one of the best hopes for treating AD. The starting point for most of the experiments is a new binding assay which permits detailed studies of the interaction of agonists with the high and low affinity states (KH, KL) of M1 receptors, and measurements of the nature of the M1 receptor mechanism in normal, mammalian nerve cell membranes. It is clear that the KL/KH ratio for different agonists correlates closely with the biochemical activity of the same agonists in promoting the breakdown of intramembranous phosphoinositides (PI) and the physiological activity of these agonists in exciting cortical cells. These measurements show that none of the five agonists yet tested for the Rx of AD is a good M1 agonist. Three series of experiments are planned, based on extensive preliminary data. Studies of agonists will be with rabbit hippocampal, brainstem and submaxillary gland membranes for M1, M2h (heart type) and M2g (gland type) receptors, respectively. Measurements of KL/KH and PI breakdown will define the structure of good M1 agonists, and permit the selection of new agonists with high M1 efficacy and M1/M2 selectivity. Additional studies probe the nature of the primary binding site and hypothetical secondary binding sites for bifunctional agonists. Studies of the M1 receptor mechanism in rabbit hippocampal membranes will test our working hypothesis that M1 receptors work in pairs, show which transition metal ion(s) best support(s) the KH state, demonstrate whether allosteric agents can improve the binding or efficacy of M1 agonists, and show whether A1+++, somatostatin etc. directly interfere with the M1 mechanism. The status of the M1 mechanism with interventions in vivo will be assessed in rat brains. Studies of chronic denervation and receptor blockage will show how the mechanism responds to prolonged inactivity. Studies with infused tetrahydroaminoacridine and the M2 agonist, oxotremorine, will show whether either distorts beneficial responses. Finally replacement therapy with the M1 agonist AF102B will be examined for chronic effects on normal and cholinergically- denervated receptors.