To study how scopolamine affects acetylcholine dynamics within the medial prefrontal cortex (MFC) during an interval timing task.

Cholinergic deficits are a major feature in patients with Alzheimer’s disease (AD) and Parkinson’s disease (PD). AD and PD patients can have impairments in interval timing, the ability to control movements in time. Timing is highly conserved across mammalian species and involves the MFC, which receives prominent cholinergic input. Our recent work showed that muscarinic antagonist scopolamine impairs interval timing by disrupting stimulus-related, rather than timing-related neurons, in the MFC. However, it is unclear how scopolamine affects MFC acetylcholine dynamics during interval timing.

We employed the acetylcholine fluorescent sensor iAchSnFR for fiber photometry recording of acetylcholine dynamics, and leveraged ChAT-Cre transgenic mice for optogenetic activation of cholinergic neurons. We performed bilateral basal forebrain implantation (AAV-DIO-ChrimsonR; AP-0.7/ML±1.8/DV AAV injection -4.2/-5.2; optic canulae -4.4) and unilateral MFC implantation (AAV-hSyn-iAchSnFR; AP+1.8/ML+0.5/DV AAV injection -1.7/-1.9; optic canulae -1.8). MFC acetylcholine dynamics were recorded while mice performed a timing task. For basal forebrain optogenetic stimulation with simultaneous MFC iAchSnFR recordings, laser (638nm) stimulation was assigned on/off randomly across trials, and interval timing sessions were performed 30 min after intraperitoneal scopolamine (1.5mg/kg) or normal saline injections.

MFC acetylcholine release, as recorded through iAchSnFR fiber photometry, was strongly modulated at the beginning of the switch timing task, responding to the stimulus indicating the start of each trial. The MFC release of acetylcholine was verified with simultaneous basal forebrain optogenetic stimulation. After scopolamine treatment, we observed a decrease in acetylcholine dynamics during interval timing trials.

The muscarinic antagonist scopolamine disrupts MFC acetylcholine dynamics, this extends its established role of direct antagonism downstream of muscarinic receptors and may have relevance for the detailed mechanisms underlying clinical side effects of muscarinic antagonists, such as delirium and confusion, especially in AD/PD.