Acetylcholine is synthesized in cholinergic neurons by a reaction catalyzed by the cytosolic enzyme choline acetyltransferase (CAT). This linkage of choline and an acetate group provided by acetyl‐CoA is not, however, the rate‐limiting step in the biosynthesis of acetylcholine (ACh); rather, the high‐affinity transport of choline from the extracellular medium by a specific transporter protein represents the rate‐limiting step. ACh is stored in vesicles in cholinergic terminals and released into the synaptic cleft by exocytosis (Figure 4.1). The proteins involved in this exocytotic release of ACh are the targets for botulinum toxin; the therapeutic actions of Botox in the treatment of neuromuscular disorders derive from the ability of this toxin to block the release of ACh from cholinergic neuron terminals (Coffield 2003). Acetylcholine released from neurons into the synaptic cleft is hydrolyzed by the membrane bound enzyme, acetylcholinesterase (ACHE), which represents the mechanism for termination of the signal in cholinergic neurotransmission. ACHE has one of the highest catalytic powers ever reported for an enzyme and is therefore capable of a rapid clearance of ACh from the synaptic cleft. This distinguishes cholinergic neurons from those of other biogenic amines where the synaptic signal is terminated by the high‐affinity reuptake of the transmitter. Acetylcholine is the neurotransmitter of all autonomic ganglia, postganglionic parasympathetic synapses, the neuromuscular junction, and cholinergic neurons in the central nervous system.

Cholinergic pathways in the brain have now been identified by histochemical and neurochemical methods (Figure 4.2). The majority of cholinergic neurons in the mammalian brain are found in four regions. These include: (i) the brainstem pedunculo‐pontine and lateral dorsal tegmental nuclei; (ii) a subset of the thalamic nuclei; (iii) the striatum, where cholinergic neurons serve as local interneurons; and (iv) the basal forebrain nuclei, which collectively serve as the major sources of cholinergic projection neurons to the cerebral cortex, hippocampus, and amygdala (Ballinger et al. 2016). The cholinergic neurons originating in the nucleus basalis of Meynert, substantia innominata, and the diagonal band (basal forebrain) project to virtually all cerebral cortical areas and layers (Sarter et al. 2016). These projections terminating throughout the cerebral cortex degenerate in senile dementia and Alzheimer’s disease and this cholinergic neuron loss has been correlated with the degree of cognitive decline. The ACh content of the brain in Alzheimer’s patients at postmortem is profoundly reduced. These observations have led to the cholinergic hypothesis of cognitive decline in senile dementia that has served as the basis for pharmacological attempts to ameliorate learning and memory deficits by restoration of cholinergic neurotransmission. One pharmacologic strategy for augmenting cholinergic transmission has been the use of inhibitors of ACHE. Inhibition of this enzyme prolongs the half‐life of ACh in the synapse and therefore enhances cholinergic transmission. Drugs such as tacrine, donepezil (Aricept^®) and rivastigmine (Exelon^®) are ACHE inhibitors (ACHEI) that have been used in the treatment of cognitive dysfunction associated with Alzheimer’s disease. These drugs produce modest beneficial effects for periods of approximately one year, but do not prevent the progressive deterioration of mental function in Alzheimer’s patients. Inhibition of ACHE has moreover a high propensity for adverse effects and a significant fraction of patients withdraw from treatment due to side effects.

Cholinergic receptors that mediate the synaptic actions of ACh are either muscarinic or nicotinic cholinergic receptors. The muscarinic receptors are members of the G‐protein coupled receptor family and five subtypes of muscarinic cholinergic receptors (M1–M5) have been characterized. Muscarinic receptor activation therefore trigger their signaling through activation of heterotrimeric G proteins that in turn affect the opening, closing, and kinetics of K⁺, Ca²⁺, and non‐selective cation channels. The alkaloids, atropine and scopolamine, act as nonselective, competitive antagonists of these muscarinic receptors. Both atropine and scopolamine are capable of disrupting working memory in humans and animals through their antimuscarinic actions in the cerebral cortex and hippocampus. High doses of either drug can also produce delirium, hyperactivity, visual hallucinations, and disorientation.

The M₂ muscarinic receptor predominates in hindbrain regions while the M₁ receptor is expressed at high levels in the cerebral cortex and hippocampus which are brain regions involved with memory and cognition. The M₁ muscarinic receptor has accordingly generated considerable recent interest as a target for drug discovery of agents that might facilitate learning and memory. In animal models, M₁ muscarinic receptor agonists restore cognitive impairment associated with damage to cholinergic pathways and may exert additional disease‐modifying effects beneficial to Alzheimer’s disease (Fisher et al. 2003).

The other class of cholinergic receptors that exist in the central nervous system are nicotinic receptors. These ACh receptors are members of the ligand‐gated ion channel superfamily and are composed of multiple homologous subunits oriented around a central cation channel. Twelve different types of nicotinic receptor subunits have been identified in the brain (α2–10 and β2–β4) and each nicotinic cholinergic receptor is composed of five of these subunits. The combination of these subunits into a pentameric structure determines its electrophysiological and pharmacological properties. It takes the binding of two molecules of ACh to open the cation channel and allow the Na⁺ influx and depolarization of neuronal membranes. The most common nAChR subtypes in the brain are α4β2 and α7 receptors.

In addition to responding to ACh, these receptors, as their name implies, are also activated by the alkaloid nicotine found in tobacco leaves. Nicotine shares with other drugs of abuse the ability to activate the mesocortical limbic dopaminergic pathways that represent the reward circuitry of mammalian brain. The interaction with nicotinic cholinergic receptors in this pathway underlies the rewarding and dependence‐related actions of nicotine. In addition to the adverse addictive properties of nicotine, this naturally occurring product increases mental alertness and improves memory in humans and animals. Chronic transdermal nicotine has indeed been found to improve attentional performance in Alzheimer’s disease patients and in individuals with the precursor age‐associated memory impairment syndrome (White and Levin 2004). There is therefore a great deal of interest in the pharmaceutical industry directed toward the development of synthetic nicotinic receptor agonists for the treatment of memory deficits and cognitive dysfunction. There are currently several nicotine analogs in various stages of preclinical and clinical development for use in neurodegenerative and Alzheimer’s disease to mitigate the cognitive effects of the disease and delay clinical cognitive manifestations.