receptor interactions

English: A general CB1 receptor inverse agonis...

English: A general CB1 receptor inverse agonist pharmacophore model. Putative CB1 receptor amino acid side chain residues in receptor-ligand interaction are shown. A and B both constitute an aromatic ring connected to a central core unit C. A hydrogen bond acceptor unit D interconnects unit C with a lipophilic moiety E. Rimonabant is taken as a representative example below. The applied colors indicate the mutual properties with the general CB1 pharmacophore. (Photo credit: Wikipedia)

_

English: Created myself using PowerPoint based...

English: Created myself using PowerPoint based on a figure produced by User:Shao (http://uk.wikipedia.org/wiki/Зображення:GABAA-rec1.jpg) (Photo credit: Wikipedia)

_

Fig 4. Palonosetron: Second generation 5-HT 3 ...

Fig 4. Palonosetron: Second generation 5-HT 3 receptor antagonist (Photo credit: Wikipedia)

_

Selective non-peptide vasopressin receptor ant...

Selective non-peptide vasopressin receptor antagonist. (Photo credit: Wikipedia)

_

see name

see name (Photo credit: Wikipedia)

_

English: Hypothetical model for the metabolic ...

English: Hypothetical model for the metabolic effects of CB1 receptor antagonism. ECS stands for endocannabinoid system. (Photo credit: Wikipedia)

_

Receptors are macromolecules involved in chemical signaling between and within cells; they may be located on the cell surface membrane or within the cytoplasm (see Table 1: Pharmacodynamics: Some Types of Physiologic and Drug-Receptor Proteins). Activated receptors directly or indirectly regulate cellular biochemical processes (eg, ion conductance, protein phosphorylation, DNA transcription, enzymatic activity). Molecules (eg, drugs, hormones, neurotransmitters) that bind to a receptor are called ligands. A ligand may activate or inactivate a receptor; activation may increase or decrease a particular cell function. Each ligand may interact with multiple receptor subtypes. Few if any drugs are absolutely specific for one receptor or subtype, but most have relative selectivity. Selectivity is the degree to which a drug acts on a given site relative to other sites; selectivity relates largely to physicochemical binding of the drug to cellular receptors.

Table 1

Some Types of Physiologic and Drug-Receptor Proteins

Type
Structure
Cellular Location
Examples

Multisubunit ion channels
Cell surface transmembrane
Acetylcholine (nicotinic)

GABAA

Glutamate

Glycine

G-protein– coupled receptors
Cell surface transmembrane
Acetylcholine (muscarinic)

α- and β-adrenergic receptor proteins

Eicosanoids

Protein kinases
Cell surface transmembrane
Growth factors

Insulin

Peptide hormones

Transcription factors
Cytoplasm
Steroid hormones

Thyroid hormone

Vitamin D

GABA = γ-aminobutyric acid; GDP = guanosine diphosphate; GTP = guanosine triphosphate.

A drug’s ability to affect a given receptor is related to the drug’s affinity (probability of the drug occupying a receptor at any given instant) and intrinsic efficacy (intrinsic activity—degree to which a ligand activates receptors and leads to cellular response). A drug’s affinity and activity are determined by its chemical structure.

Physiologic functions (eg, contraction, secretion) are usually regulated by multiple receptor-mediated mechanisms, and several steps (eg, receptor-coupling, multiple intracellular 2nd messenger substances) may be interposed between the initial molecular drug-receptor interaction and ultimate tissue or organ response. Thus, several dissimilar drug molecules can often be used to produce a desired response.

Ability to bind to a receptor is influenced by external factors as well as by intracellular regulatory mechanisms. Baseline receptor density and the efficiency of stimulus-response mechanisms vary from tissue to tissue. Drugs, aging, genetic mutations, and disorders can increase (up-regulate) or decrease (down-regulate) the number and binding affinity of receptors. For example, clonidine

down-regulates α2-receptors; thus, rapid withdrawal of
clonidine

can cause hypertensive crisis. Chronic therapy with β-blockers up-regulates β-
receptor density; thus, severe hypertension or tachycardia can result from abrupt withdrawal. Receptor up-regulation and down-regulation affect adaptation to drugs (eg, desensitization, tachyphylaxis, tolerance, acquired resistance, postwithdrawal supersensitivity).

Ligands bind to precise molecular regions, called recognition sites, on receptor macromolecules. The binding site for a drug may be the same as or different from that of an endogenous agonist (hormone or neurotransmitter). Agonists that bind to an adjacent site or a different site on a receptor are sometimes called allosteric agonists. Nonspecific drug binding also occurs—ie, at molecular sites not designated as receptors (eg, plasma proteins). Drug binding to such nonspecific sites prohibits the drug from binding to the receptor and thus inactivates the drug. Unbound drug is available to bind to receptors and thus have an effect.

Agonists and antagonists: Agonist drugs activate receptors to produce the desired response. Conventional agonists increase the proportion of activated receptors. Inverse agonists stabilize the receptor in its inactive conformation and act similarly to competitive antagonists (see Pharmacodynamics: Agonists and antagonists). Many hormones, neurotransmitters (eg, acetylcholine, histamine, norepinephrine), and drugs (eg, morphine

, phenylephrine

,
isoproterenol

) act as agonists.

Antagonists prevent receptor activation. Preventing activation has many effects. Antagonist drugs increase cellular function if they block the action of a substance that normally decreases cellular function. Antagonist drugs decrease cellular function if they block the action of a substance that normally increases cellular function.

Receptor antagonists can be classified as reversible or irreversible. Reversible antagonists readily dissociate from their receptor; irreversible antagonists form a stable, permanent or nearly permanent chemical bond with their receptor (eg, by alkylation). Pseudo-irreversible antagonists slowly dissociate from their receptor.

In competitive antagonism, binding of the antagonist to the receptor prevents binding of the agonist to the receptor. In noncompetitive antagonism, agonist and antagonist can be bound simultaneously, but antagonist binding reduces or prevents the action of the agonist. In reversible competitive antagonism, agonist and antagonist form short-lasting bonds with the receptor, and a steady state among agonist, antagonist, and receptor is reached. Such antagonism can be overcome by increasing the concentration of the agonist. For example, naloxone

(an opioid receptor antagonist that is structurally similar to morphine

), when
given shortly before or after morphine

, blocks morphine

‘s effects. However, competitive
antagonism by naloxone

can be overcome by giving more morphine

.

Structural analogs of agonist molecules frequently have agonist and antagonist properties; such drugs are called partial (low-efficacy) agonists, or agonist-antagonists. For example, pentazocine

activates opioid receptors but blocks their activation by other opioids. Thus,
pentazocine

provides opioid effects but blunts the effects of another opioid if the opioid is
given while pentazocine

is still bound. A drug that acts as a partial agonist in one tissue
may act as a full agonist in another.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: