Synthesized in the endoplasmic reticulum, a fraction of the subunits of this receptor assemble into homo and hetero-pentameric complexes. The only complexes that reach the cell surface, after exiting from endoplasmic reticulum are the pentameric complexes. nAChRs have been identified as crucial elements in central nervous system functions such as consciousness, attention, and memory; and participate in numerous cerebral circuits. (Bocquet, Carvalho, Cartaud, Neyton Poupon, Taly, Grutter, Changeux, Corringer 2007)
The opening of the channels of these receptors is triggered by the neurotransmitter acetylcholine. Nicotine also has the same effect. A diagram of the chemical structure of acetylcholine is given below.
Nicotinic acetylcholine receptors are located mainly in the postsynaptic membrane under the motor nerve terminal at the neuromuscular junction. Nicotinic receptors can also be found in different synaptic locations; e.g. the muscle nicotinic receptor always functions post-synaptically. The nerve cell forms of the receptor can be found both post-synaptically (performing classical neurotransmission) and pre-synaptically (causing the release of other neurotransmitters). In contrast to the muscarinic acetylcholine receptors, Nicotinic Acetylcholine receptors do not operate with the help of a second messenger. Instead they open themselves forming an ion channel. Curare causes inhibition of these receptors.
Nicotinic acetylcholine receptors have a very wide distribution in the various body tissues. Nerve cell receptors are found in the central nervous system and also in the peripheral nervous system. The neuromuscular receptors, on the other hand are found in the neuromuscular junctions of somatic muscles.
The neuromuscular receptors, on the other hand are found in the neuromuscular junctions of somatic muscles. The stimulation of these receptors causes muscular contraction.(Barrantes 1998)
Structure and function of Nicotinic Acetylcholine Receptors
The nicotinic acetylcholine receptor protein includes one or many sites which can bind the neurotransmitter ACh and also a specific channel for cat ions which is also known as the intrinsic cationic channel. These ions can be Na+, Ca2+ or K+ and the channel is specific in nature. The process of the opening of ion channel is linked to the binding of ACh. (Maelicke 1986)
Protein chemistry and sequence analysis of polypeptide has revealed a general scheme for each subunit which consists of :
1. A globular extra cellular N-terminal domain (ECD);
2. A trans-membrane domain (TMD)
3. A cytoplasmic domain
There are 2 to 5 Acetylcholine binding sites on the ECD. Nicotinic receptors have many subunits and these subunits belong There are 2 to 5 Acetylcholine binding sites on the ECD. Nicotinic receptors have many subunits and these subunits fit in a multigene family (seventeen members in the human being) When many of the subunits combine they form a great numbers of receptors. Every subunit gives three parts or "loops" to the site of binding. The sites which bind ACh are located on the outer side of the subunits. And as the agonist binds, the subunits happen to be more alike to the other subunits. And the channel gets more proportioned and a hole of about 0.65nm in diameter opens up. This channel helps the nicotinic acetylcholine receptor to adhere to its role by many ways. Firstly it encloses a mechanism of gating which is closed in the inactive states of the channel but it is open in the active