Visualizing synaptic disruptions in the release and regulation of dopamine hotspots in Huntington's Disease

Dopamine neuromodulation is a critical process that facilitates learning, motivation, and motor control. Disruption of these processes has been implicated in several neurodegenerative disorders including Huntington's Disease (HD). While dopaminergic signaling is a therapeutic target for treating physical and psychiatric HD symptoms, the mechanism by which dopaminergic dysfunction occurs during HD is unknown. New tools for the visualization of dopamine dynamics at the spatiotemporal resolution of neuromodulator release (ms) and dopaminergic boutons (µm) provide a richer understanding of how dopamine signaling is disrupted in HD. Here we employ near-infrared fluorescent catecholamine nanosensors (nIRCats) to image dopamine release within the striatum of R6/2 Huntington's Disease model mice of either sex. We find that dorsal striatal dopamine release decreases with progressive degeneration and that these deficits are primarily driven by a decrease in the number of nIRCat imaged dopamine release sites, termed dopamine hotspots, combined with decreased release fidelity. Using nIRCat's high spatial resolution, we track individual dopamine hotspots over repeated stimulations and pharmacological applications to measure dopamine release fidelity from individual sites. Compellingly, we found that D2-receptor (D2R) antagonist sulpiride drives increased fidelity of dopamine hotspot activity in wild type striatum but not in late-disease HD striatum, suggesting that D2R regulation of dopamine release is compromised in late HD. These findings, enabled by nIRCats, provide more detailed insights into how dopamine release is disrupted and dysregulated during Huntington's Disease.Significance statement Huntington's Disease (HD) is a neurodegenerative disorder with no cure. Dopamine signaling is known to deteriorate in HD but has not been studied at the level of individual release sites. Here, we image dopamine release from individual dopamine release sites in R6/2 HD mouse brain slices containing the striatum with novel dopamine nanosensors. We find that dopamine release site number and release fidelity are decreased in late HD. Furthermore, we demonstrate that D2-receptor signaling may be altered in late disease R6/2 HD mice, and that these disruptions are likely to drive decreased dopamine release fidelity over multiple stimulations. These findings suggest dopaminergic neurons projecting to the striatum as a potential therapeutic target for HD treatment to complement more commonly targeted medium spiny neurons.