**1. Introduction**

It is well established that mammals have got multiple functional receptors for neurotransmitters, and their role in the physiology of lower vertebrates is still less explored. Neurotransmitters are signal molecules with a confirmed neuronal release [1]. Among the different types of neurotransmitters, the major neurotransmitter released is acetylcholine (ACh) [2]. There are two types of acetylcholine receptors (AChRs) which are the nicotinic acetylcholine receptors (nAChRs) and the muscarinic acetylcholine receptors (mAChRs). Of these, the muscarinic receptors, are membrane proteins that belong to the superfamily of G-protein coupled receptors (GPCRs) that transmit their signals into the cell through heterotrimeric GTP-binding proteins (G-proteins), having seven transmembrane domains [3]. mAChRs are the most predominant cholinergic receptors in the central and peripheral nervous systems which plays an important role in modulating cell activity and function [4].

Moreover, muscarinic receptors are present in virtually all organs with a predominance of individual subtypes in various tissues and organs [5]. There are five genetically distinct subtypes of mammalian mAChRs (M1-M5) [6] in neurons and other cell types. M1, M4, and M5 receptors are most abundant in the central nervous system (CNS), while M2 and M3 receptors are widely distributed in both central and peripheral tissues [7]. The structural diversity between the five different mAChR subtypes is attributed to the presence of the residues in the third intracellular loop of the protein [8]. Experiments like Northern blot and *in situ* hybridization have revealed that there is some tissue specificity in the distribution of receptor subtype mRNAs. Muscarinic subtypes have also been distinguished based on tissue-specific antagonists and have been developed for therapeutic purposes [5].

The M1 mAChR subtype is abundant in the brain and enteric nervous system while the M2 mAChR subtype is mainly expressed in the heart. Peripheral M3 mAChRs are found extensively in smooth muscles of the gastrointestinal and urinary tracts, exocrine glands, and the eye [9]. In the periphery, M4 mAChRs have been found in relatively higher concentrations in the lungs and in lower concentrations in the salivary glands and ileum. Various peripheral and cerebral blood vessels have been used for the study and identification of mRNA for M5 mAChRs [7]. The M1, M3, and M5 subtypes are tightly coupled to the phosphoinositide system, whereas the M2 and M4 subtypes are closely linked to the mechanism of adenylate cyclase inhibition [10].

Many biochemical and histochemical studies have illustrated evolutionary changes in terms of the concentration of acetylcholine in different parts of the brains of the lower vertebrates [11]. Another study [12] has proposed an increasing functional role for acetylcholine in some telencephalic structures, particularly in the basal ganglia and the tuberculum olfactorium in lower vertebrates. The basal ganglia in nonmammalian brains are extremely enriched in AChE (acetylcholine esterase) activity like that in mammals, while differences in the concentration and distribution of this enzyme occur in the cerebellum of the vertebrate brain [13]. Some studies report chicken may have a cholinergic habenulointerpeduncular pathway system identical to that reported in the rat [14] which suggests the existence of cholinergic cell bodies and fibers in the avian brain. The study has confirmed the above results using immunohistochemical localization of choline acetyltransferase in the chicken mesencephalon, which is part of the avian midbrain.

However, despite the characterization and cloning of all muscarinic receptor subtypes in different species, to date, there are no reports of a functional M1 muscarinic receptor in chicken. This article focuses on the avian muscarinic receptors, their subtypes, distribution, expression as well as characterization and reviews the available information about the various research held in this field so far.
