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The primary role of acetylcholinesterase (AChE) is hydrolysis of acetylcholine (ACh). However, observations by numerous groups have suggested that AChE may have non-cholinergic functions. Furthermore, developmental roles for AChE and its related enzyme, butyrylcholinesterase (BuChE), which is also capable of ACh hydrolysis, have been postulated. One line of evidence to support a non-cholinergic role for AChE is the apparent disparity in several brain areas between the distribution of AChE and the cholinergic marker choline acetyltransferase. The substantia nigra (SN), an area of the ventral mesencephalon (VM), which contains the dopaminergic cells that degenerate in Parkinson's disease (PD), is an area that displays such a disparity. One approach to treating PD involves implantation of embryonic dopaminergic VM cells into the parkinsonian brain. This procedure, known as foetal transplantation, has met with limited success, in part due to degeneration of dopaminergic cells within the donor preparation. It is known that incorporation of trophic factors into the preparation for grafting improves dopaminergic cell survival. It has previously been shown that AChE enhances survival and neurite outgrowth of postnatal dopaminergic cells in organotypic cultures of the VM. The aim of the studies in this thesis was to establish the effects of BuChE and AChE on embryonic dopaminergic neurons in a preparation analogous to that used in the animal model of foetal transplantation as a treatment for PD. Addition of BuChE and monomeric (G_1-) and tetrameric (G_4-) forms of AChE enhanced dopaminergic neurite outgrowth. Inhibition of the active site of BuChE and AChE by echothiophate had no effect upon neurite outgrowth or cell survival, demonstrating that the trophic effects of BuChE and AChE on neurite outgrowth were not dependent upon ACh hydrolysis. In contrast, inhibition of the peripheral anionic site (PAS) of AChE by BW284c51 markedly decreased cell survival and neurite outgrowth. The mechanism of action of BW284c51 toxicity was subsequently investigated using a mixture of nicotinic ACh receptor antagonists in order to demonstrate that the chronic toxic effects of BW284c51 were not a consequence of elevated ACh resulting from inhibition of AChE. Finally, the technique of whole-cell patch-clamp electrophysiology revealed a novel inhibitory effect of BuChE and G_1- and G_4-AChE on voltage-dependent calcium currents. It was postulated that these actions underlie the trophic effects of BuChE and AChE on embryonic dopaminergic neurons, a suggestion that was supported by the findings that established inhibitors of voltage-dependent calcium currents enhanced dopaminergic neurite outgrowth. The findings of this thesis are discussed in the context of other studies and are related to both physiological and pathological functions of the central nervous system.

Myelination of axons in the CNS by oligodendrocytes (OLs) is critical for the rapid and reliable conduction of action potentials down neuronal axons, as evidenced by the severe disabilities associated with myelin loss in multiple sclerosis and other diseases of myelin. The specification, differentiation, and maturation of OLs along with myelin formation by OLs have been thoroughly characterized. How myelin is turned over, however remains unclear. It is unsurprising that little is known about myelin turnover considering that for decades following their discovery, myelin and OLs were considered static elements in the adult nervous system. Recent evidence, however, shows that myelin in the CNS is actually plastic. Moreover, myelin remodeling in humans has been suggested to be mediated by mature OLs. As mature OLs have limited capacity to generate new myelin sheaths, we must ask whether mature OLs can remodel the myelin at preexisting myelin sheaths. One intriguing but unproven possibility is that myelin at individual internodes may be remodeled cell-autonomously by mature OLs to modulate neuronal circuit function. Macroautophagy (MA) is responsible for the lysosome-mediated elimination of cytosolic proteins, lipids, and organelles. MA achieves this by capturing cargo in bulk or selectively in a transient, multilamellar structure known as an autophagosome (AP). In this study, we used a combination of in vivo and cellular approaches to test the hypothesis that MA in OLs may be important for myelin remodeling in the adult CNS. We establish that myelin of individual internodes is remodeled, and does so through the coordinated efforts of endocytosis and MA. We found that autophagy protein Atg7 is essential for myelin remodeling in vivo: loss of Atg7 in OLs leads to an age-dependent increase of myelin at the internode and the formation of aberrant myelin structures, most notably myelin outfoldings. In addition, we find that MA has the potential to occur throughout the mature OL, and examination of OLs in culture suggests that formation of a mature AP structure, the amphisome, is required to facilitate the efficient degradation of myelin-containing endocytic structures. Together, we propose that myelin is a dynamic structure that is regularly remodeled through the cooperative efforts of MA and endocytosis. These findings raise the possibility that myelin remodeling is involved in neural plasticity and the tuning of neural circuits.