PRESYNAPTIC NEUROCHEMICAL MARKERS

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Some markers of presynaptic cholinergic nerve terminals such as the enzyme choline acetyltransferase (ChAT) are reduced in the cortex of Alzheimer's disease (AD) patients. Similarly, reductions in dopaminergic indices in the basal ganglia are recognized in Parkinson's disease (PD). These neurochemical changes correlate with the severity of cognitive impairment in AD and with extrapyramidal features in PD. Such marker losses are widely assumed to reflect losses of basal forebrain cholinergic neurons and their terminal projects in AD and of substantial nigra dopaminergic neurons and their striatal terminals in PD. Results obtained in our laboratories indicate that a relatively new class of synaptic markers, vesicular neurotransmitter transporters, are quantitative markers of synaptic terminal losses in neuropathological conditions and are not affected by regulatory changes in response to altered synaptic activity or drug treatments. These transporters can now be measured in vitro with ligand binding and immunohistochemistry and in vivo with positron- or single photon emission computer tomography. We recently determined that the vesicular acetylcholine transporter (VAChT) in differentially preserved in AD cortex and hippocampus in comparison to reductions in ChAT activity. This contrasts with results in experimental animals where septo-hippocampal axotomy results in a complete loss of VAChT from the hippocampal formation. Combined, these results suggest that basal forebrain cholinergic neurons in AD are not as depleted as ChAT measures suggest, and that cortical cholinergic innervation may be better structural preserved than previously appreciated. Experiments in the current proposal will characterize further the postmortem neurochemical phenotypes of subcortical cholinergic and dopaminergic projection system in neurodegenerative dementing disorders. We will determine the relationships between cell body and nerve terminal losses in the basal forebrain-to-cerebral cortex cholinergic system and in the midbrain-to-striatum dopaminergic system, employing a combination of histological, ligand binding, immunohistochemical, and mRNA hybridization techniques. We anticipate that distinct, specific and inter-related patterns of cholinergic and dopaminergic pathologies will be found in AD, Lewy body dementias, and PD. These patterns may account for some apparent discrepancies among prior reports of neurochemical pathology in AD and related dementias, and may provide important insights into possible common versus differing pathophysiological mechanisms of dementing diseases.