Cholinergic system uses acetylcholine (ACh) as the neurotransmitter. Choline O-acetyltransferase (ChAT) is a marker of cholinergic neurons. Parasympathetic nervous system is almost exclusively cholinergic. Pre- and postganglionic neurons of the sympathetic nervous system are also cholinergic. Sympathetic ganglion neuron bodies are located in the bilateral chain of sympathetic ganglia, while the terminal ganglia of the parasympathetic nervous system are close or within the innervated organ.
Cholinergic system includes the muscarinic and nicotinic acetylcholine receptors (mAChR and nAChR), vesicular acetylcholine transporter (VAChT), acetylcholinesterase (AChE), butyrylcholinesterase (BuChE), and choline acetyltransferase (ChAT).
Central nervous system
In the CNS the major cholinergic projection systems are (Roy et al., 2016):
- the nucleus basalis of Meynert, supplying cholinergic projections throughout the cerebral cortex and hippocampus
- pedunculopontine nucleus pars compacta, projecting to the forebrain and subcortical structures such as thalamus
- striatal cholinergic neurons.
Muscarinic acetylcholine receptors (mAChRs) are highly expressed in caudate nucleus and nucleus accumbens, less in somatosensory, primary motor, and temporal cortices, and very little in the cerebellar cortex (Roy et al., 2016). Scopolamine is a muscarinic antagonist.
Nicotinic acetylcholine receptors (nAChRs) are highly expressed in the entorhinal, temporal, and primary motor cortices, and in the hippocampus and thalamus (Roy et al., 2016). nAChRs can be composed of various subunit combinations, leading to large differences in their affinity to ACh and other ligands (including nicotine), and Ca2+ permeability.
Dementia is associated with loss of cholinergic neurotransmission; therefore AChE inhibitors (donepezil, rivastigmine) have been used as treatment for the dementia symptoms in AD and PD. AChE inhibitors have been labelled with positron emitting isotopes to be used in PET imaging of dementia as markers of the integrity of the cortical cholinergic system; these tracers include [11C]MP4A and [11C]PMP. The effect of AChE inhibitors can be studied using FDG. Cholinergic perturbation is also apparent in traumatic brain injury (TBI), which can be seen as decreased AChE activity using [11C]MP4A PET (Östberg et al., 2011); in TBI patients with treatment response to rivastigmine the baseline AChE activity is lower than in non-responders (Östberg et al., 2018). [11C]Donepezil is a reversible antagonist radioligand of AChE (inhibitor, not a substrate), and has been used for imaging cholinergic mechanisms also in peripheral organs (Gjerløff et al., 2014). However, donepezil also binds to σ1 receptors with comparable affinity to that of AChE (Kato et al., 1999; Horsager et al., 2019).
nAChRs have been targeted with [11C]nicotine, α4β2-nAChRs specifically with 2-[18F]fluoro-A85380, [18F]AZAN, and (+)- and (-)-[18F]flubatine, and α7-nAChRs with [11C]CHIBA1001 and [18F]ASEM. PET tracers for mAChRs include [11C]NMPB and [11C]LSN3172176. Also PET tracers for VAChT have been introduced, including [18F]FEOBV.
PET can be used to estimate regional acetylcholine concentration variation in human brain, utilizing mAChR or nAChR radioligands (Smart et al., 2021).
Components of cholinergic system are upregulated in active inflammatory cells, which use ACh as a paracrine signalling molecule (Kawashima et al., 2012; Fujii et al., 2017) and therefore the tracers of the cholinergic system may be useful in research and diagnosis of infection and inflammation and cancer (Stokholm et al., 2016; Jørgensen et al., 2017; Boswijk et al., 2017). In white matter lesions of MS patients, BuChE activity correlates with microglial activation and is absent within demyelinated lesions (Thorner et al., 2021).
Nicotine increases the release of many neurotransmitters, including ACh. It has neuroprotective actions, possibly via α7-nAChRs. Continuous exposure to nicotine causes upregulation of nAChRs in smokers, and nAChRs are thought to have essential role in nicotine addiction.
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Updated at: 2022-01-26
Created at: 2016-05-28
Written by: Vesa Oikonen