Question

With the aid of diagrams, compare and contrast the muscarinic (Gq-coupled) and nicotinic receptors with respect to location, structure, signalling and timescale of response.

Answer 1

Nicotinic receptors are ionotropic, meaning that when when acetylcholine binds to it, ions flow through it. It acts as a channel for positively charged ions, mainly sodium which depolarizes the cell. While the N1 nicotinic receptors present at neuromuscular junctions, plays an integral part in allowing muscles to move. N2 nicotinic receptors are found in the brain, plus the autonomic and parasympathetic nervous systems . Different subunits make up these two types of nicotinic receptor, so they are considered separately. Collectively, nicotinic receptors can be found in both the sympathetic and parasympathetic nervous systems on post-ganglionic neurons.

Nicotinic receptors types :

N1 or Nm present at neuromuscular junction,

N2or Nn present at the autonomic ganglia ,CNS and adrenal medulla   

for both receptors the same signalling is occurs via the and the signalling needs the opening of cation (Na+ ,K+ ) channles .

response: due to the presence as the ionic channels the response time is high in the nicotinic receptors .

Muscarinic receptors have a different mechanism of action. Instead of becoming an ion channel for sodium, they use a G-protein. When ACh binds to the receptor, this special protein changes shape, which then allows it to phosphorylate various second messengers. There   are five different types of muscarinic receptors. M1, M3 & M5 are excitatory receptors because their G-protein stimulates phospholipase C, which then activates IP3 and DAG. The two others, M2 and M4, are inhibitory.. Like nicotinic receptors, they are found in both the parasympathetic and sympathetic nervous systems.

M1,M4 and M5 Receptors involved in complex CNS responses like memory, arousal , attention and analgesia.

M2receptors present at Heart , main function in the heart rate regulation generally lower the heart rate

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M3 present at smooth muscles they produce responses in variety of organs

In the M1 and M3 Muscuranic receptor the signalling occurs via the IP3/DAG which causes increase in cytosolic Ca ++ also PLA2 also increases which helps in prostaglandin synthesis while in M3 Muscuranic receptors the signalling occurs due to k+ channels opening which causes decrease in c AMP production .

Response :The effects of muscuranic receptors are slow and it may be inhibitory or excitatory.