Table 2 Main limitations of the literature of experimental studies assessing the central cholinergic system in schizophrenia and future directions.

Lack of data in early stages of the disorder.

In vivo studies in clinical and familial high-risk as well as first-episode psychosis; and the use of longitudinal designs.

Difficulty disentangling medication and illness effects.

Investigate cholinergic function in antipsychotic-naïve patients with schizophrenia.

Report and account for lifetime and current antipsychotic/medication use as well as illness duration.

Difficulty distinguishing nicotine and illness effects.

Investigate cholinergic function in patients and healthy individuals before and after smoking cessation.

Report and account for smoking/vaping with nicotine and metabolites analyses; and, if feasible, conduct subgroup analyses.

Limited number of tools to assess specific receptor subtypes and pre-synaptic markers of the cholinergic system.

Developing more specific radioligands and assessing comprehensively pre-, post-synaptic markers in the same samples.

Lack of useful clinical biomarkers validated in vivo.

Development of biomarkers and characterization of biotypes in vivo.

Small sample sizes and lack of power analyses.

Replications with larger sample sizes, effect size reporting, and power analyses.