**6. Conclusions and future directions**

As the distinction between the roles of tonic and phasic ACh signaling becomes clearer, it is important to understand the specific dysfunction that is occurring in both of these types of signaling in the cortex in disorders such as AD and AUD. It is likely that these two disorders have effects on both muscarinic and nicotinic ACh receptors and that these deficits are part of what is driving the dysregulation of the two types of ACh signaling in the cortex. Additionally, the role of AChE in this distinction cannot be overstated, as its role in hydrolyzing synaptic ACh may be one of the most important regulators of phasic ACh, with its inhibition by high ACh concentrations possibly leading to spillover into the extracellular space and therefore augmenting ACh tone that is usually otherwise due to a different set of basal forebrain cholinergic cells.

The availability of fluorescent ACh sensors for fiber photometry allows for the measurement of ACh activity in vivo on a second timescale. Studies in which changes to ACh signaling following alterations to normal AChE activity can be assessed and are needed to determine the exact role of this enzyme in cortical cholinergic synapses, and how its dysfunction relates to some of the deficits seen in common cholinergic disorders. Likewise, the contribution of alterations to typical nAChR and mAChR function in the PFC to phasic ACh signaling could be investigated using photometric methods. Studies looking directly into how tonic and phasic ACh utilize different cellular machinery seem to be elusive for now, but further breakthroughs in the field could lead to new avenues to address this gap in the literature.
