Hyperactive Striatal Dopamine Signaling Observed in Huntington's Disease Mouse Models Prior to Motor Symptom Onset Can be Attenuated via Activation of M. Receptors

Almost all Huntington's Disease (HD) patients die within 10-30 years after symptom onset and currently there are no available treatments to slow, stop, or reverse its progression. HD is a neurodegenerative disease that causes a biphasic signaling pattern in the striatum in which there is hyperactive signaling during the early stages, followed by hypoactive signaling later on. Among the many neurotransmitters shown to follow the biphasic signaling pattern found in HD, dopamine (DA) and its inputs from the mid-brain to the striatum are thought to be critical for motor and action planning. However, there is currently a lack of studies that directly examine DA signaling in animal models before symptom onset, leaving room for investigation in a critical area of the disease. Here we use a combination of techniques to assess dopamine signaling and its regulation during pre-symptomatic disease stages. We hypothesized that we would observe increased dopamine transients in YAC128 positive mice when compared to littermate controls at this time point, and that activation of the M4 receptor with the M4 selective positive allosteric modulator (PAM) VU'154 would normalize transient amplitude in YAC128 mice. Using fast scanning cyclic voltammetry (FSCV), we recorded dopamine signaling in pre-symptomatic YAC128 mice via ex vivo preparations. We also used, fiber photometry to record dopamine signaling in vivo, in the same mouse strain. We found that indeed, DA transient amplitudes are increased in pre-symptomatic YAC128 mice when compared to littermate controls in both ex vivo and in vivo settings. Additionally, we report that activation of the M4 receptor is able to normalize the enhanced DA signaling in HD mice. These findings are some of the first to directly show increased DA signaling in the basal ganglia in early stage HD rodent models and support previous evidence that M4 -------is potentially an exciting target for normalizing aberrant neurotransmission in HD.