Nicotinic Acetylcholine Receptor

Nicotinic Acetylcholine Receptor

The nicotinic acetylcholine receptor is a key protein in neuronal communication. This protein effectively converts neurotransmitter binding into a membrane depolarization event. The protein combines neurotransmitter binding sites, specifically acetylcholine, with a cationic ion channel, specifically sodium (Na).

Structure

The receptor is a transmembrane pentameric glycoprotein. It has a weight of approximately 300,000 Daltons. It cylindrical in appearance by electron microscopy approximately 16nm in length and 8nm in diameter. The main ion channel is composed of a water pore that runs through the entire length of the protein. If viewed from the synaptic cleft, the protein will look like a pseudo-symmetrical rosette shown in the picture below composed of 10 different alpha and 4 different beta subunits.

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ACh Binding Sites

This protein carries anywhere from 2 to 5 acetylcholine binding sites which are located at the interface between two subunits. Each subunit contributes 3 loops to the binding site. There is also a "principle" side and a "complimentary" side of the subunits. The principle side binding nicotine with a high degree of specificity and the complimentary side binding a wide variety of acetylcholine like molecules.

See also Binding site of AChR.

Ion Channel

The ion channel of the protein is a 20 membered alpha helix bundle. This channel also contributes to the receptor function in three critical aspects: it contains a gating mechanism, it contains a water pore to stabilize ions, and it has a selectivity filter for ion charge. As with most transmembrane alpha helix bundles, it is hydrophobic around the edges to effectively be supported within the membrane, and it is hydrophilic on the inner portion to transport charged ions.

Physiology

The receptor is a cylindrically-shaped protein. It is embedded in the cell wall of post synaptic nerves at the skeletal neuromuscular junction. The receptor acts as a chemically controlled sodium (Na) channel also known as a ligand gated channel.

When in the presence of acetylcholine, the receptor undergoes a conformational change opening up the channel to an influx of sodium (Na) within the cell. When this happens the cell undergoes a depolarization event that triggers an action potential to propagate along the rest of the cell stimulating, for example, a muscle response.

The opening of these channels only lasts for a millisecond due to cholinesterase being present and breaking down acetylcholine attached to the receptor causing the receptor to close again. Introduction of cholinesterase inhibitors can cause a depolarization block. It works by creating a prolonged refractory period in the depolarization event promoted by the opening of the receptor channel.

Locations

•skeletal neuromuscular junctions

•sympathetic and parasympathetic nervous system

•autonomic ganglia

•central nervous system

Clinical Findings

Findings due to nicotinic stimulation from Acetylcholinesterase inhibitors

• Neuromuscular junctions of skeletal muscles

• • Fasciculations and myotonic jerks
  • Weakness and paralysis of the muscle

• Sympathetic nervous system

• • Hyperglycemia and ketosis
  • Hypertension
  • Leukocytosis
  • Pupillary dilation
  • Sweating
  • tachycardia
  • Urinary retention

Below is a mnemonic for remembering nicotinic signs of cholinesterase inhibitor toxicity

Monday Mydriasis (pupil dilation)

Tuesday Tachycardia

Wednesday Weakness

Thursday Hypertension

Friday Fasciculations