Types, Mechanism of action and more of: 5-Hydroxytryptamine receptors, Opioid receptors, GABA receptors - ePharmacology

Read about other receptors:

In my other hub I've discussed about:

1. Cholinergic receptors
2. Adrenergic receptors
3. Histamine receptors
4. Dopamine receptors

Also read about:

• Methods of studying receptors
• Number of receptors in a cell
• Receptor Regulation
• How drugs act?
• Life cycle of receptors
• G proteins
• Second messengers (cAMP, calcium, cGMP, IP3, DAG and calmodulin)

Today on ePharmacology we will discuss about the following receptors:

1. 5 Hydroxytryptamine (Serotonin) receptors
2. Opioid receptors
3. GABA receptors

Before starting you should review your knowledge of receptors in general.

5-Hydroxytryptamine (Serotonin) receptors

5-Hydroxytryptamine (5-HT) / Serotonin receptors are of seven types. The seven Serotonin receptors are further divided into a total of 14 discovered serotonin receptors.

1) 5HT₁ receptor

a) 5HT_1A

b) 5HT_1B

c) 5HT_1D

d) 5HT_1E

e) 5HT_1F

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2) 5HT₂ receptors

a) 5HT_2A

b) 5HT_2B

c) 5HT_2C

3) 5HT₃ receptors

4) 5HT₄ receptors

5) 5HT₅ receptors

a) 5HT_5A

b) 5HT_5B

6) 5HT₆ receptors

7) 5HT₇ receptors

5-HT 1 and 2 receptor agonists and antagonists

5-HT 3, 4, 5, 6 and 7 receptor agonists and antagonists

Type 1, 2, 4, 5, 6, and 7 are G-coupled receptors and type 3 is a Ligand-gated Na⁺ and K⁺ cation channel. These receptors are present in the CNS and vascular smooth muscle.

Specific agonists for 5HT₁ receptors are sumatriptan and buspirone. Spiperone, methiothepin and quipazine are the 5HT₁ receptor antagonists.

Methysergide, ketanserin and cyproheptadine are the examples of 5HT₂ receptor antagonists.

Specific agonist for 5HT₃ receptor is 2-methyl-5-HT. Ondansetron and tropisetron are the 5HT₃ receptor antagonists.

Metoclopramide is the specific agonist for 5HT₄ receptor.

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Opioid receptors

Morphine and related drugs produce their responses by acting on their specific receptors which are called opioid receptors. There are 3 well defined or classical subtypes of opioid receptor:

• μ (Mu) opioid receptor
• κ (Kappa) opioid receptor
• δ (Delta) opioid receptor

More recently, cDNA encoding an orphan receptor was identified which has a high degree of homology to the classical opioid receptors. On structural grounds this receptor is an opioid receptor and has been named ORL₁ (opioid receptor-like). Later an endogenous ligand was discovered for this receptor which is called Nociceptin. So ORL1 is now an adopted receptor.

Opioid receptors are G-protein coupled receptors. All of the opioid receptors possess the same general structure of an extracellular N-terminal region, seven transmem­brane domains and intracellular C-terminal tail structure.

Different agonists act on different subtypes of receptor. For example, morphine predominantly acts on μ and κ opioid receptors. Enkephalins mainly bind with δ opioid receptors but have weak affinity to μ opioid receptors. Dynorphin preferentially binds with κ opioid receptors.

Selective and non-selective antagonists for opioid receptors:

GABA receptors

GABA receptors are of two types:

• GABA_A
• GABA_B

Both types of recep­tors are present in the CNS. However, the concentration of GABA _A receptors is more than that of GABA_B receptors. GABA_A receptors are widely distributed in the granular cell layers of the cerebellum and are directly associated with a Cl^- channel (pore size ~5 Å). The concentration of GABA_B receptor in the interpeduncular nucleus is far greater than the concentration of GABA_A receptors and it appears to be linked to calcium and potassium channels via GTP-binding proteins.

GABA receptors appear to exist as pentamers of three distinct subunits in the arrangement α_mβ_nγ. The β subunit appears to have the recognition capacity for GABA and related agonists. GABA_A receptor agonists are GABA, muscimol, and isoguvacine. GABA_B receptor agonists are GABA and baclofen. GABA_A receptor antagonists are bicuculline and picrotoxin. GABA_B receptor antagonist is phaclofen.

Recently a third type of GABA receptor, named GABA_C is reported to be distrib­uted in the mammalian CNS which has central chloride pore and is insensitive to both bicuculline and baclofen. GABA_C receptors are composed of p-subunits. cis-4-aminocrotonic acid (CACA) is the selective agonist for GABA_C receptor. Although the term GABA_С receptor is frequently used, GABA_С may be viewed as a variant within the GABA_A receptor family

GABA receptor agonists and antagonists

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