Cholinergic receptors types

Organ System Sympathetic Effect d Adrenergic Receptor Type Parasympathetic Effect d Cholinergic Receptor Type ... 

Acetylcholine Binds to cholinergic receptor, which are of two types— muscarinic and nicotinic. 

Abstract The discovery that mammalian brain expresses the mRNAs for nine different nicotinic cholinergic receptor subunits (a2-a7, p2-p4) that form functional receptors when expressed in Xenopus laevis oocytes suggests that many different types of nicotinic cholinergic receptors (nAChRs) might be expressed in the mammalian brain,. Using an historical approach, this chapter reviews some of the... 

Schechter MD (1995) Scopolamine-physostigmine combination does not substitute for nicotine. Prog Neuropsychopharmacol Biol Psychiatr 19 499-508 Schechter MD, Meehan SM (1992) Further evidence for the mechanisms that may mediate nicotine discrimination, Pharmacol Biochem Behav 41 807-812 Schechter MD, Rosecrans JA (197 la) Behavioral evidence for two types of cholinergic receptor in the C,N,S, Eur J Pharmacol 15 375-378... 

Figure 5. Cartoon of a cholinergic synapse showing major steps in the synthesis of acetylcholine. The two major receptor types, the ionotropic nicotinic receptor and the metabotropic muscarinic receptor, are shown (see also Chapter 1). Presynaptic muscarinic (M2) and nicotinic receptors are also depicted. Drugs which have been widely used to manipulate the cholinergic systems, and which are mentioned in the text, include the muscarinic receptor antagonists scopolamine and atropine and the nicotinic receptor agonist nicotine. Anticholinesterases (discussed elsewhere in this volume) include drugs such as physostigmine, rivastigmine, donepezil, and galanthamine.

Unlike the receptors at postganglionic neuroeffector junctions or at skeletal neuromuscular junctions, both types of cholinergic receptors, that is, nicotinic and mus-... 

Acetylcholine acts at two different types of cholinergic receptors [see (1) and (2) in Fig. 2.5]. Muscarinic receptors bind ACh as well as other agonists (muscarine, pilocarpine, bethanechol) and antagonists (atropine, scopolamine). There are at least five different types of muscarinic receptors (M1-M5). All have slow response times. They are coupled to G proteins and a variety of second messenger systems. When activated, the final effect can be to open or close channels for K, Ca ", or CL (Bonner, 1989). Nicotinic receptors are less abundant than the muscarinic type in the CNS. They bind ACh as well as agonists such as nicotine or an-... 

As do most neuronal systems, cholinergic receptors show multiplicity, and we distinguish between nicotinic and muscarinic receptors, which differ in many respects. Whereas acetylcholine (4.1) binds to both types of receptors, the plant alkaloids nicotine (4.2) and muscarine (4.3) trigger a response only from nicotinic or muscarinic cholinergic receptors, respectively. Nicotinic receptors are found in all autonomic ganglia (i.e., in the sympathetic system as well as the parasympathetic) and at the neuromuscular endplate of striated muscle. Muscarinic receptors occur at postganglionic... 

Acetylcholine (ACh) is an example of an endogenous neurotransmitter that binds to more than one receptor type, the nicotinic acetylcholine receptor (nAChR) which preferentially binds nicotine and the muscarinic receptor which binds muscarine, a mushroom alkaloid. The latter is a G protein-coupled receptor while the nACh receptor is an excitatory ligand-gated ion channel that transports Na-i- ions. Nicotinic cholinergic receptors are found in the CNS, autonomic ganglia, and at the neuromuscular junction of skeletal muscles. They are a possible target for anaesthetics. 

Four different receptor types have been the main focus of interest. These are muscarinic cholinergic, histamine HI, dopamine D2 (different from the vascular dopamine receptor), and serotonin 5-HT3. In addition, neurokinin NKl-receptor antagonists are undergoing early clinical trials. 

There are a host of receptors for each neurotransmitter with varying types of responses. For example, the four subtypes of adrenergic receptors which we saw in the PNS are also present in the CNS (ai, a2, Pl, P )-Cholinergic receptors come in two flavors, Mi and M2, while dopaminergic receptors show an elevation of cAMP levels upon stimulation of Di types and a decrease in cAMP after D2 stimulation. Opiate receptors (n,m, , k, ct) are the subject of intensive study as are 5-HT receptors. Drawn below are a few of the known CNS neurotransmitters and their activities as postulated. 

Two general types of neuromuscular blockers are discussed here. They are classified according to those that depolarize the skeletal muscle cell when binding to the cholinergic receptor and those that do not.37... 

Since there are two primary neurotransmitters involved in autonomic discharge, there are two primary classifications of postsynaptic receptors. Cholinergic receptors are located at acetylcholine synapses, and adrenergic receptors are located at norepinephrine synapses. As indicated in Figure 18-2, each type of receptor has several subclassifications. The location and functional significance of these classifications and subclassifications are presented here. 

Acetylcholine (9.12) binds both types of cholinergic receptors muscarinic and nicotinic (Figure 9.9). The names of these receptor types were based on muscarine (9.13) and nicotine (9.14), selective agonists for each receptor. A muscarinic response is characterized by nausea, salivation, and tearing. Nicotinic responses are noted by an acceleration of the heart rate. Acetylcholine s ability to elicit a response from both subtypes of receptors may imply that different conformations of acetylcholine may be responsible for binding to each receptor. 

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