The hippocampus plays an important role in learning and memory and is a primary site of pathology in Alzheimer's disease (AD). A large number of animal and human studies suggest that projections of acetylcholine (ACh)-containing neurons to the hippocampus are critical for memory and attentional mechanisms and the degeneration of these neurons plays a critical role in dementia associated with AD. Furthermore, a compound that inhibits Ach metabolism, termed tacrine, is the only approved treatment for AD. However, this compound suffers from low efficacy and a high incidence of side effects. Development of drugs that mimic the actions of Ach by acting as direct agonists of the specific muscarinic acetylcholine receptors (mAChRs) involved in regulating hippocampal physiology could overcome the limitations of tacrine and provide highly efficacious agents for treatment of AD. However, the specific mAChR subtypes that mediate each of the major actions of Ach in the hippocampus are unknown. A series of studies is proposed in which a combination of biophysical, molecular, and pharmacological approaches will be used to determine which of the five MAChR subtypes (m1 - m5) mediate each of the major physiological effects of mAChR activation in the hippocampal formation. Previous studies led us to postulate that 1) m1 mediates mAChR-induced potentiation of NMDA-evoked currents in hippocampal area CA1; 2)reduction of excitatory synaptic transmission is mediated by m4 receptors presynaptically localized on glutamatergic terminals; and 3) m2 mediates MAChR-induced disinhibition by reducing GABA release. In the studies with subtype-specific mAChR toxins, mAChR subtype-specific antibodies, recently developed pharmacological reagents, and antisense oligonucleotides to test these hypotheses.
In aim 4 we will use the same approaches to determine which mAChR subtypes mediate the direct excitatory effects of mAChR activation on CA1 pyramidal cells. mAChR activation increases excitability of hippocampal pyramidal cells by reducing three potassium currents. Thus, voltage clamp techniques will be used with mAChR subtype-selective tools to determine which mAChR subtype mediates mAChR-induced modulation of each of these currents.. These studies will provide significant advance in our understanding of the roles of ACh in agonists that can be used for treatment of AD and related disorders.
