Cholinergic Neuron Degeneration in Mn Neurotoxicity

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4.2. Project Summary (Abstract): The long-term goal of the proposed research is to understand the role of the cholinergic system in manganese (Mn)-induced neurological dysfunction. Today, millions of welders, smelters, and miners in the United States (US) and throughout the world are chronically exposed to Mn- containing fumes, aerosols, and particles on a regular basis. Furthermore, drinking water with naturally high Mn concentrations is now recognized as an important source of chronic Mn exposure to large segments of the population in the US and globally. Therefore, the number of humans that are potentially exposed to neurotoxic levels of Mn worldwide are much larger than previously recognized, making it a public health problem of global proportion. Exposure to contemporary levels of Mn results in impairments in working memory and executive function and produces deficits in fine motor control and postural stability. These neurological effects of chronic Mn exposure are likely to have a pathophysiology that involves multiple neuronal systems. Previous studies from our laboratory have shown that chronic exposure to moderate levels of Mn in non-human primates produces dysfunction of nigrostriatal dopaminergic (DAergic) neurons by inhibiting striatal dopamine release. We now find a marked loss of striatal cholinergic interneurons (ChI) and these findings challenge the current dogma of Mn-induced pathophysiology from a solely DAergic perspective to one in which there is disruption of the DAergic-Cholinergic balance in the basal ganglia. Cholinergic neurons are important in the physiology of cognition, emotion, compulsive behavior, locomotion, and gait, domains that are affected in Mn-induced neurological dysfunction. Here, we also provide initial evidence that chronic Mn exposure in non-human primates results in an apparent basal forebrain cholinergic neuron loss or injury similar to what is found in Alzheimer's disease and other neurodegenerative disorders. Thus, we propose to rigorously characterize the effect of chronic Mn exposure on choline acetyltransferase (ChAT)-positive cholinergic neurons in the caudate/putamen/nucleus accumbens as well as in the basal forebrain and pedunculopontine nucleus in the non-human primate brain (specific aim 1). These studies will use rigorous unbiased stereological cell counting and soma size determination methods. We will also determine the effect of chronic Mn exposure on vesicular acetylcholine transporter (vAChT) in cholinergic axon terminals and varicosities (specific aim 2) to assess if chronic Mn exposure produces cholinergic neuron axonopathy. Finally, we will examine the role of neurotrophic factors on the Mn-induced loss of cholinergic neurons (specific aim 3) by measuring concentrations of Brain-Derived Neurotrophic Factor and Nerve Growth Factor in relevant brain regions. The proposed studies will provide a more precise mechanistic understanding of Mn-induced pathophysiology that can lead to the development of cholinergic- and/or neurotrophic factor- based therapies for the treatment of Mn-induced neurological dysfunction.