Autonomic receptors are grouped into two categories, namely cholinergic receptors and adrenergic receptors/adrenoceptors, depending on their sensitivity to specific agonist and antagonist drugs. Cholinergic receptors mediate the neural actions of the acetylcholine (ACh) at the postsynaptic neuroeffector junctions of both the sympathetic and parasympathetic nervous systems. On the other hand, adrenoceptors, also called adrenergic receptors, mediate the actions of the epinephrine and other groups of adrenaline related compounds at the presynaptic neuroeffector junctions.
v Cholinergic receptors use acetylcholine as the only neurotransmitter to relay impulses from one neuron to another through the postsynaptic neuroeffector junctions.
v Cholinergic receptors are common in parasympathetic nervous system.
v Cholinergic receptors are further classified into two categories: muscarinic and nicotinic receptors.
Types of Cholinergic Receptors
i) Muscarinic receptors
- Ganglionic and other classes of neural muscarinic receptors (M1) are involved in the neural transmission of the Central Nervous System (CNS). Their actions are blocked by atropine, but selectively activated by McN-A-343 and Oxotremorine.
- Classical muscarinic receptors (M2 and M3) are known to sub-serve effects such as cardiac slowing, vasodilation, papillary constrictions, defecation, urination, salivation and bronchoconstriction.
- The muscarinic receptors are located at the neuroeffector junctions of the postganglionic parasympathetic neurons of the exocrine glands, heart and smooth muscles.
- They are also found on effectors cells with or without cholinergic innervations.
- The agonists involved in muscarinic receptors include ACh and muscarine.
ii) Nicotinic receptors
- These are cholinergic receptors which mediate the actions of nicotine and acetylcholine (ACh) upon activation.
- Some of the effects mediated by nicotine include the release ganglionic transmission, discharge of catecholamines from adrenal medulla, and conduction of NMJ (somatic neuromuscular junction).
v Adrenergic receptors (adrenoreceptor) are solely responsible for the mediation of action of epinephrine and similar compounds.
v A wide range of beta-phenylethylamine related agonists and antagonists are known to act upon adrenergic receptors.
v Sympathetic neuroeffector junction is the usual site of adrenergic receptor.
v In some cases, adrenergic receptors can be found in neurons of the effector organs without any sympathetic innervations. Therefore, some of them are not innervated.
v Adrenergic receptors are subdivided into two major types – Alpha and Beta receptors – depending on the selectivity of drug on the receptors. On activation, Alpha receptors produce excitatory response on the smooth muscles where they are found. In sharp contrast, Beta receptors evoke inhibitory responses of the smooth muscles.
i) Alpha receptors
- Alpha1 receptors are postsynaptic; they particularly respond to the blockade by prazosin while Alpha2 is sensitive to a neural blockade of the agonist yohimbine.
- Alpha2 are mainly presynaptic; they are found on the terminals of the sympathetic postganglion nerve. Presynaptic Alpha2 receptors selectively inhibit the release of epinephrine.
ii) Beta receptors
- Upon activation, Beta receptors produce inhibitory responses in the smooth muscles (like heart) where they are found. Activation of the Beta-receptors results into vasodilation and bronchodilation can be caused in some vascular smooth muscles.
- Activation of Beta 1 receptors produces lipolysis in the body adipose tissues and positive excitation of the heart.
- Beta 2 receptors produce uterine relaxation, vasodilation and bronchodilation. Beta 2 agonist: terbutaline is used to treat asthma (bronchial asthma).
Differences between Cholinergic and Adrenergic Receptors
v Cholinergic receptors use acetylcholine as the only neurotransmitter while adrenergic receptors utilize epinephrine.
v Cholinergic receptors are located at the presynaptic neuroeffector junctions of the sympathetic and parasympathetic nervous systems while adrenergic receptors are presynaptically located on the neuroeffector junctions of the ganglion terminals of the parasympathetic nervous system.