The process of aging is known to be accompanied by loss of memory. Due to the importance of central muscarinic receptors in memory, the present proposal aims at investigating the possible changes in brain muscarinic receptors that take place during aging. Dissociated rat brain cell aggregates and brain slices will be used to study the distribution and subtypes of muscarinic receptors in animals of different ages. In preliminary studies, the binding properties of muscarinic receptors in these preparations will be characterized, including tissue linearity, regional distribution and the kinetics of association and dissociation. Moreover, saturation and displacement experiments will be conducted using different radioactive muscarinic receptor ligands and displacers to assess the contribution of receptor subtypes and affinity states to total binding. These results will be compared with those obtained in brain homogenates. Subsequently, these experiments will be repeated in intact brain tissue prepared from rats of different ages. Consequently, the effect of age on the rates of synthesis and degradation of the receptors, the rate of agonist-induced receptor down-regulation, and antagonist-induced up-regulation, will be studied. Intact brain cells and slices will also be used to study muscarinic receptor-mediated responses, such as increases in cyclic GMP formation and phosphoinositide hydrolysis, in addition to decreases in cyclic AMP synthesis, since the first two responses can be elicited only in intact cells. Moreover, different agonist-mediated biochemical responses will be measured in brain cells and slices obtained from young and aged animals. Furthermore, the ability of high agonist concentrations to desensitize these responses will be assessed in animals of the different age groups. The proposed work is expected to increase our knowledge regarding the changes in the properties of brain muscarinic receptors which accompany aging.
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