Altered Vesicular Acetylcholine Transporter Expression Regulates Acetylcholine Abundance in the Brain of Drosophila melanogaster

Abstract

The neurotransmitter acetylcholine (ACh) plays a central role in the regulation of vital neurological processes like cognition. As a result, increases or decreases in neuronal cholinergic signaling lead to an impairment in learning and memory. Although much about how acetylcholine is regulated is known, the mechanism through which alterations in cholinergic signaling affect changes in ACh-linked behavior is not fully understood. Importantly, the nature of the relationship between vesicular acetylcholine packaging and its release at the synaptic cleft is also unclear. We are interested in using the vesicular acetylcholine transporter (VAChT), which mediates the packaging and transport of acetylcholine (ACh) for exocytotic release, to elucidate the ways in which ACh loading alters its release and metabolism. We used both an overexpression of VAChT and two Vacht mutant lines, Vacht² and Vacht⁸, to increase or decrease the gene's expression respectively. Taking a combined immunohistochemical and biochemical approach, we measured the expression of ACh in VAChT overexpressors, and followed up those studies with an assay for ACh levels as a means of quantifying ACh storage and those of choline, its degradation product. We report that consistent with its well-known role in mediating ACh release, the VAChT overexpression line has elevated total head ACh levels in females but not males while Vacht mutants show strong reductions. By contrast, choline levels are elevated in VAChT overexpressors but unchanged in Vacht mutants. These findings are supported by immunolocalization studies in the fly brain in which the VAChT overexpressors show both elevated ACh in and around ACh neurons and an increased localization to synaptic release sites. When we analyzed the immunostaining studies by sex, we found that in contrast to the neurochemical studies, both males and females had elevated ACh levels, while the two Vacht mutants had reductions in that neurotransmitter's levels. By our estimation, these data represent the first time that acetylcholine has been measured directly in VAChT overexpression and Vacht mutants in Drosophila and demonstrate a consistency with findings from mammalian studies.