Classification and mechanism of action of M-cholinergic receptors
M-cholinergic receptors represent a crucial target for pharmacological intervention. Their primary site of action is the cell membranes of effector organs that receive parasympathetic innervation. In modern science, it is customary to distinguish five different subtypes of these receptors, designated by indices from $M_1$ to $M_5$. The greatest practical interest lies in the $M_1$, $M_2$, and $M_3$ subtypes, as they have been studied in the greatest detail.
A fundamental feature of all M-cholinergic receptors is that they are metabotropic. This means that signal transmission is carried out not through ion channels, but via coupling with G-proteins.
To understand the mechanism of signal transduction, it is helpful to examine this process step-by-step using the classic example of $M_2$-receptors located on the membranes of cardiomyocytes (heart cells):
- First, the activated receptor interacts with inhibitory $G_i$-proteins.
- This interaction leads to the inhibition of a specific enzyme, adenylyl cyclase.
- As a result of enzyme blockade, there is a predictable decrease in the intracellular synthesis of cyclic adenosine monophosphate (cAMP).
- The drop in cAMP levels causes a decrease in the activity of cAMP-dependent protein kinases.
- As a result, the phosphorylation processes of intracellular proteins are disrupted.
- The final stage of this biochemical cascade is a cellular response manifested by the inhibition of cardiac function.
N-cholinergic receptors and their stimulation
N-cholinergic receptors have a different localization and response mechanisms. A striking example of a substance acting on these structures is lobeline. By origin, it is an alkaloid isolated from the plant Lobelia inflata, and from a chemical perspective, it is a tertiary amine.
The mechanism of action of lobeline is based on the stimulation of N-cholinergic receptors located in the carotid bodies (sinocarotid zone). This stimulation triggers the reflex excitation of the vital centers of the medulla oblongata—the respiratory and vasomotor centers.
Of particular note is lobeline's effect on the cardiovascular system, which is distinctly biphasic in nature:
- Primary effect (inhibitory phase): the drug causes excitation of the vagus nerve center in the medulla oblongata. Clinically, this manifests as the development of bradycardia (decreased heart rate) and a drop in blood pressure.
- Secondary effect (stimulatory phase): activation of N-cholinergic receptors localized in the sympathetic ganglia and adrenal medulla occurs. The consequence of this process is an increase in blood pressure.
Pharmacological stimulation of cholinergic synapses
Agents that stimulate cholinergic synapses are classified into two main groups based on their mechanism of action on synaptic transmission.
The first group consists of cholinomimetics (direct stimulants). Their mechanism of action is as simple as possible: they work similarly to the natural neurotransmitter. The drug molecule directly binds to the cholinergic receptor and activates it. A typical representative of a direct M-cholinomimetic is pilocarpine. Historically, this alkaloid was extracted from the leaves of the South American shrub Pilocarpus pinnatifolius Jaborandi. Interestingly, in the context of studying plant alkaloids, the use of a mixture of Piper betle (betel) leaves as a mild narcotic substance is often mentioned. However, in modern medical practice, exclusively synthetic pilocarpine is used, which exerts a direct stimulatory effect on M-cholinergic receptors.
The second group consists of anticholinesterase agents (indirect stimulants). Their fundamental difference is that they do not act directly on the receptors. Instead, they block the breakdown of endogenous acetylcholine. Through this mechanism, the concentration of the natural neurotransmitter in the synaptic cleft increases significantly. As a result, the effects of endogenous acetylcholine are repeatedly enhanced and prolonged in time.
To remember the biphasic effect of lobeline on blood pressure, use the "Down-Up" rule: first, the vagus nerve center is activated (pressure drops down), then the ganglia and adrenal glands are activated (pressure rises up).
Class and exam questions
What is the difference between direct and indirect stimulants of cholinergic synapses?
Direct cholinomimetics (e.g., pilocarpine) act like the neurotransmitter by directly binding to the receptor and activating it. Indirect agents (anticholinesterase drugs) block the enzymatic degradation of acetylcholine, increasing its concentration in the synaptic cleft and thereby enhancing the effects of the endogenous neurotransmitter.
How exactly do M2-cholinergic receptors transmit an inhibitory signal in heart cells?
They interact with inhibitory Gi-proteins, leading to the inhibition of adenylyl cyclase and a decrease in cAMP synthesis. This reduces the activity of cAMP-dependent protein kinases, disrupts protein phosphorylation, and ultimately inhibits cardiac function.
Why does lobeline cause a biphasic change in blood pressure?
The primary effect is associated with the excitation of the vagus nerve center in the medulla oblongata, which causes bradycardia and a drop in blood pressure. The secondary effect is due to the stimulation of N-cholinergic receptors in the sympathetic ganglia and adrenal medulla, leading to a subsequent increase in blood pressure.
What else to study on this topic
- Detailed characteristics of the M1, M3, M4, and M5 receptor subtypes and their localization in the body.
- Chemical properties of tertiary amines using the alkaloid lobeline as an example.
- The historical context of using herbal mixtures (Piper betle leaves) and the transition to synthetic drugs.
- The mechanisms of the respiratory and vasomotor centers of the medulla oblongata during reflex stimulation.
- Comparative analysis of the effects of endogenous acetylcholine and exogenous direct cholinomimetics.
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