Introduction

Cholinergic receptors function in signal transduction of the somatic and autonomic nervous systems. The receptors are named because they become activated by the ligand acetylcholine. These receptors subdivide into nicotinic and muscarinic receptors, which are named secondary to separate activating ligands that contributed to their study. Nicotinic receptors are responsive to the agonist nicotine, while muscarinic receptors are responsive to muscarine. 

The two receptors differ in function as ionotropic ligand-gated and G-protein coupled receptors, respectively. Nicotinic receptors function within the central nervous system and at the neuromuscular junction. While muscarinic receptors function in both the peripheral and central nervous systems, mediating innervation to visceral organs. The difference in signal transduction of the two receptor types confers separate physiological functions upon receptor activation. Furthermore, differences in receptor subtypes create unique implications for pharmacologic targets and pathogenesis of the disease.

Issues of Concern

Due to the diffuse presence of cholinergic receptors, dysfunction can yield various outcomes. Muscarinic receptors mediate autonomic function in all major organ systems; however, receptors are also present throughout the central nervous system. Abnormal muscarinic receptor function has shown correlations in diseases such as Alzheimer disease, Parkinson disease, schizophrenia, and epilepsy. Furthermore, research shows both nicotinic and muscarinic receptors play a role in the dopamine reward system pathway.[1] Due to receptor involvement in a wide and varied range of diseases, medical therapies targeting these receptors continue to be an area of prominent investigation.

At the neuromuscular junction, nicotinic receptors function in signal transduction of voluntary movement. In myasthenia gravis, competitive receptor inhibition secondary to autoimmune dysfunction can lead to the life-threatening loss of function at the neuromuscular junction. This life-sustaining function works to the clinicians’ advantage during surgical cases where a patient requires paralysis with pharmacologic interference at the neuromuscular junction. However, great care must be taken in patients with disorders such as lower or upper motor neuron denervation, major trauma, severe infection, or burn injuries, as these may result in the upregulation of nicotinic receptors at the neuromuscular junction, which contributes to an overabundance of receptors.[2] Use of the neuromuscular-blocking drug succinylcholine to induce paralysis may potentially contribute to life-threatening electrolyte abnormalities.[2] This condition is secondary to both the function of the nicotinic receptor as an ionotropic channel and succinylcholine activation of the receptor.

Cellular Level

Cholinergic receptors perform major roles in neural transmission within the somatic and autonomic nervous systems. The nicotinic receptor subdivides into two subtypes, N1 and N2. N1 may also be referred to as the peripheral or muscle receptor type, while N2 is known as the central or neuronal receptor subtype.[3] The designation of the two receptors is primarily due to their distinctive locations within the autonomic and somatic nervous systems. The N1 receptor is present on skeletal muscle at the neuromuscular junction. N2 is within the peripheral and central nervous systems. N2 receptors are on the cell bodies of postganglionic neurons within the parasympathetic and sympathetic nervous systems. They are also on the adrenal medulla as a component of the sympathetic nervous system. The distribution of nicotinic receptors differs from that of muscarinic receptors, which primarily function within the autonomic nervous system, mediating the function of the parasympathetic subdivision.

Muscarinic receptors are divided into five main subtypes M1, M2, M3, M4, and M5.[4] While each subtype exists within the central nervous system, they are encoded by separate genes and localized to different tissue types. The M1 receptor is primarily found in the cerebral cortex, gastric, and salivary glands.[5] M2 receptors are diffusely located in smooth muscle and cardiac tissue.[6] M3 receptors are also present on smooth muscle, gastric, and salivary glands.[7] M4 and M5 receptors are not as well characterized but appear within the hippocampus and substantia nigra.[5] The wide distribution of receptors functions to mediate the parasympathetic division of the autonomic nervous system, maintaining internal homeostasis.

Development

In the development process of the central nervous system, cholinergic receptors influence neuronal cell growth and survival, cell differentiation, and synapse formation. Nicotinic receptors compose some of the first receptor proteins observed in CNS development. Expression of various nicotinic receptor subtypes in the brain influences cell migration, neuronal outgrowth, and signaling pathways. The varied expression of receptor subtypes can confer different development pathways in the brain. Receptor subtypes may exhibit separate ionic permeability. Receptors that are permeable to Ca++ exhibit a regional variance in activating second messenger systems, stimulating the growth of neuronal progenitor cells or the activation of gene expression through indirect phosphorylation of the cyclic adenosine monophosphate (cAMP) response element-binding protein (CREB). 

The nicotinic receptors perform a wide range of functions due to small changes in the overall subunit structure. Furthermore, nicotinic receptors influence the release of multiple neurotransmitters such as dopamine, noradrenaline, acetylcholine, glutamate, and GABA. Similarly, muscarinic receptors display regional specificity within the brain, contributing to development. These receptors have also been shown to play a role in the growth of neuronal cells as well as astrocytes and oligodendrocytes; however, they also exhibit specialized function by mediating the development of spatial memory and long-term potentiation through contributions to neuronal plasticity. Cholinergic receptors play an early and essential role in brain development. The many functions of these receptors in development have long-term implications for dysfunction due to their presence in areas of high neuronal plasticity throughout adulthood, like the hippocampus.[8]

Function

The autonomic nervous system is responsible for maintaining the homeostatic environment of the body with adjustments affecting major organ systems such as neuronal, circulatory, respiratory, integumentary, digestive, and urinary. The autonomic nervous system divides into sympathetic, parasympathetic, and enteric divisions.[9] Within the parasympathetic and sympathetic nervous system, neurons are categorized as preganglionic and postganglionic, depending on the location of their cell bodies within the central or peripheral nervous systems. The N2 or neuronal nicotinic receptor subtype exists on all postganglionic cell bodies. The N2 receptors are responsive to acetylcholine and transmit signals from the preganglionic to the postganglionic cell.[9] The ionic flux generated at the postganglionic cell is responsible for excitatory signal transduction to effector organs of the autonomic nervous system. Separately, the N1 or muscle nicotinic receptor is at the neuromuscular junction on muscle cells generating voluntary muscle movement.[10]  An excitatory signal may become generated through N1 receptor activation. Depending on the strength of the signal, receptor activation may result in membrane depolarization with subsequent muscle contraction.

Muscarinic receptors mediate many functions of the parasympathetic nervous system. The muscarinic receptors are on various organs throughout the body. Receptors diffusely express on organs of the neuronal, cardiac, musculoskeletal, pulmonary, digestive, and urinary systems. As mentioned, different receptor subtypes exist on different organs, producing various effects. The overall function of the receptors aims to achieve the “rest and digest” function of the parasympathetic nervous system. While the sympathetic nervous system readies the body for “fight or flight,” the parasympathetic nervous system functions as the unconscious restorative and energy-conserving system. Therefore, many functions of muscarinic receptors may be referred to as opposing the action of the sympathetic nervous system. Receptors present on cardiac muscle cells receive innervation from the vagus nerve and act to slow the heart rate and decrease the force of contraction. Receptors function at the SA node, AV node, atria, and within the ventricles, resulting in a slowed heart rate, decreased conduction velocity, and a prolonged cardiac muscle refractory period.[11] 

Within the digestive system, receptor activation stimulates intestinal motility and digestive enzyme secretion. Receptor activation in the lungs leads to smooth muscle contraction, narrowing the airways, and increasing secretion production. Furthermore, muscarinic receptors are present throughout the central nervous system and have demonstrated important functions in both learning and memory. Animal models lacking the M1 receptor develop deficiencies in both cognition and long-term potentiation.[12] Therefore the activation of M1 receptors serves to maintain synaptic plasticity and neuronal differentiation.[5] The wide range of actions mediated by muscarinic receptors highlights their critical role within the autonomic nervous system. With such a diffuse presence of receptors, medical therapies activating or blocking the receptor can cause a range of effects beyond the targeted use.

Mechanism

While both nicotinic and muscarinic receptors become activated in response to the ligand-binding of acetylcholine, their mechanism of activation differs significantly. As mentioned, nicotinic receptors are ionotropic; this means activation of the receptor leads to the formation of an ion channel within the cell membrane, known as a ligand-gated ion channel.[10] The channel consists of five homologous subunits that form a central pore in the membrane upon activation, which cations may pass through.[10] Depending on the strength of signals, the influx of cations into the cell can cause depolarization, generating an excitatory action potential. This activity differs from the activation of muscarinic receptors, which are G-protein coupled receptors (GPCRs). GPCRs constitute a family of receptors that generate a second messenger system when activated via ligand binding. A second messenger system utilizes the activation of intracellular signaling molecules to produce the excitatory or inhibitory response. Muscarinic receptors generate separate second messenger systems. The M1 and M3 receptors are characterized as excitatory GPCRs known as Gq GPCRs.[13] Upon their activation, they generate phospholipase C, producing the second messenger's inositol triphosphate (IP3) and diacylglycerol (DAG) and leading to an increase in intracellular calcium and protein kinase.[14] 

The generation of calcium and protein kinase C is responsible for further activation of downstream events, which produce the overall effect of receptor activation. Alternatively, the M2 receptor is an inhibitory g-protein (Gi) GPCR, which upon ligand binding, leads to the inhibition of adenylate cyclase, resulting in a decrease in the second messenger molecule cAMP.[13]. The downregulation of cAMP levels in the cell opposes the activation of a separate stimulatory GPCR known as Gs. cAMP production occurs within the cell through activation of the stimulatory g-protein (G) GPCR, which constitutes the mechanism of beta-adrenergic receptor stimulation of the cell, a component of the sympathetic nervous system.[13] As a result of separate receptor physiology, cholinergic receptors produce vastly different chemical messaging systems in the cells of their effector organs due to differences in signal transduction. Knowing these differences helps one understand the roles each receptor plays within its specific tissue type. At the neuromuscular junction, rapid signal transduction is necessary, while within the neuronal tissue, activation of the M1 receptor may lead to a longer, more sustained response through activation of gene transcription.

Pathophysiology

Due to the diffuse presence of cholinergic receptors throughout the body, their dysfunction affects both the peripheral and central nervous systems. At the somatic neuromuscular junction, nicotinic acetylcholine receptors are at the center of the pathophysiology of antibody-mediated myasthenia gravis and congenital myasthenic syndromes. Within the central nervous system, there is evidence of dysfunctional muscarinic and nicotinic receptors playing a role in the development of Alzheimer disease, Parkinson disease, schizophrenia, epilepsy, and addiction.

The neuromuscular junction works to convey the electrical signal of voluntary movement to mechanical action. The dysfunctional states of myasthenia gravis and congenital myasthenia syndromes disrupt transmission at the neuromuscular junction. Myasthenia gravis is an autoimmune disease that generates antibodies against the nicotinic receptor at the neuromuscular junction. The binding of pathologic antibodies to the receptor results in its loss of function and recycling back within the cell.[15] The loss of receptors at the cell surface results in fewer receptors being able to respond to a chemical stimulus and generate an appropriate electrical stimulus at the surface of the muscle cell. Congenital myasthenic syndromes are similar; however, they are not a result of immune dysfunction. There is most often a gene mutation in congenital syndromes, resulting in a dysfunctional nicotinic acetylcholine receptor.[16] Similarly, due to the loss or decreased function of the receptor at the membrane, the ability to generate the appropriate electrical impulse is impaired. Both mechanisms result in muscle weakness and fatigue, as the ability to produce the excitatory stimulus at the neuromuscular junction decreases.

Within the central nervous system, cholinergic receptors play a role in developing Alzheimer, Parkinson, schizophrenia, epilepsy, and addiction. Alzheimer disease is debilitating; progressive dementia mostly affecting individuals over the age of 65. Early disease research found a common disruption of M1 receptor signaling. Researchers have found this to play an important role in cognitive function in animal models, with M1 gene knockouts demonstrating memory decline and accumulation of the pathologic Alzheimer protein, amyloid-beta.[5][4] These findings have contributed to the use of cholinesterase inhibitors as one of the central treatments in Alzheimer disease to delay the onset of memory decline. However, current therapies do not delay the progression of dementia, acknowledging a complex disease process. Additionally, with knockout animal studies, M1, M4, and M5 receptors are believed to play a role in the development of the psychiatric illness of schizophrenia and addiction; this is mainly due to M4 receptor knockout mice demonstrating hypersensitivity to dopamine signaling within the brain, a central hypothesis in the development of schizophrenia.[4] The M1, M4, and M5 receptor knockout animal models also demonstrate a disrupted response in the acetylcholine-mediated dopaminergic reward system.[4][1] Together these receptors with the N2 receptor are believed to play roles in neuronal pathways contributing to the development of addiction.

Clinical Significance

The wide distribution of cholinergic receptors in the peripheral nervous system mediating the activity of visceral organs and skeletal muscle creates a unique pharmacologic niche. When activated, muscarinic receptors can produce bradycardia, bronchoconstriction, increased GI motility, emptying of the bladder, gland secretion, and pupillary constriction for near vision.[11][9] Therefore, care is necessary when using pharmaceutical agents that affect the concentration of acetylcholine. Due to the wide dispersion of receptors, an abundance of acetylcholine may result in diarrhea, diaphoresis, urination, salivation, lacrimation, miosis, bronchospasm, and bradycardia; while agents which inhibit or decrease acetylcholine binding may result in tachycardia, dry mouth, dry eyes, mydriasis, reduced sweating, urinary retention, sedation, hallucinations, or agitation. These effects highlight the wide range of clinical options with agonist and antagonist pharmaceuticals.

At the neuromuscular junction, nicotinic receptor agonists are used to induce a state of paralysis. Nicotinic agonists do this by binding to the receptor, occupying the acetylcholine binding domain. Two popular classes of drugs are succinylcholine and tubocurarine. Both of these drugs exhibit specificity for nicotinic receptors at the neuromuscular junction but differ in their mechanism for receptor inactivation. Succinylcholine binds and activates the nicotinic receptor but remains bound to the active site of the recept.[17] This binding prevents subsequent activation of the receptor while succinylcholine is bound; it is commonly referred to as a “depolarizing neuromuscular blocker” due to initial receptor activation and subsequent membrane depolarization. On the other hand, the tubocurarine class of drugs such as rocuronium, vecuronium, and atracurium falls in the category of “non-depolarizing agents.” These agents act via competitive inhibition, occupying the active receptor site and preventing acetylcholine binding and activation.[18]