Receptors are the sensing molecules or elements which communicate the signal from a ligand to the cell to elicit specific physiological change.
Ligand is the one which binds to the receptor and brings in conformational changes so as to produce a definite effect on the body’s physiology.
In general, most receptors have an endogenous ligands or biomolecules which can trigger a response.
Most receptors are named based on their endogenous ligands like serotonin receptors, acetylcholine receptors, opioid receptors.
As per IUPHAR, there are some hundreds of receptors in the body. They vary with each other in terms of the changes they bring about in the body when ligand binds.
Types of receptors
Based on Their molecular structure and mechanism:
- Ligand-gated ion channel receptors
- G-protein coupled receptors
- Kinase-linked receptors
- Nuclear receptors.
Based on their location in the body:
- Cell surface receptors
- Cytoplasmic receptors
- Intra-Nuclear receptors
- Floating receptors
Based on the physiological effect in the body
- Pharmacological receptors
- Silent receptors
- Orphan receptors
- Synaptic receptors.
Ligand gated ion channel receptors: These receptors are the ones which have a channel through which ions can move inside and outside the cells. They are located on the cell membrane. The response through these receptors is the fastest and is in few seconds to microseconds/ milliseconds. These types of receptors are few in number than other types
Ions like Sodium, chloride, calcium and potassium move into the cell or outside through them
Body responses like reflex action, sense of pain, touch; movements are coordinated by these receptors.
Ex: Nicoinic acetyl choline receptors, GABAA receptors
G-protein coupled receptors: This is the largest class of receptors. These receptors are of 3 types as family-A, Family-B, Family-C. These receptors act through both ligand gated channel and also enzyme linked pathways.
The response through these receptors takes in few seconds.
Ex: Muscarnic acetylcholine receptors
Kinase linked or enzymatic receptors:
These receptors are cytoplasmic receptors. Their action involves enzymatic activation by phosphorylation. Then there can be gene transcription and protein synthesis to produce effect.
The response time is few hours.
Ex: Cytokine receptors.
As the name indicates these receptors are located in the nucleus of cells. They get activated when ligand molecules enter the nuclear membrane and binds with them.
Their response time is in hours.
Ex: Estrogen and other steroid receptors.
Silent receptors: Silent receptors are those receptors to which ligands bind with high affinity but interestingly no pharmacological effect is produced.
Ex: Plasma proteins.
Orphan receptors: These are the receptors which do not have well defined endogenous ligands. But however, they can produce a response when other ligands bind to them. In other words, their actual endogenous ligands have not been discovered yet. Once the endogenous ligand is identified, they will be given a specific name.