Agonists and antagonists

Question 1

Affinity

Answer 1

the strength of interaction between the ligand (or drug) and receptor (i.e., how easily or readily they bind together)
agonists and antagonists = high

Question 2

Intrinsic activity

Answer 2

the ability of the ligand (or drug)-receptor interaction to cause a change in function (i.e., to produce an effect) e.g. to set off a second messenger system/effect
agonists = high antagonists = low

Question 3

Competitive antagonism

Answer 3

Most agonists and antagonists will ‘compete’ for a particular receptor type – the agent with the highest concentration will occupy the receptor

Question 4

Partial agonists

Answer 4

In small concentrations they act as ‘agonists’ i.e. trigger a response but their intrinsic activity (efficacy) is less than a full agonist (i.e. there is a ‘ceiling effect’)
In higher concentrations they can act as ‘antagonists’ blocking further access to the receptors for other agonists or endogenous substances

E.g. buprenorphine – used as an opioid analgesic and in opioid dependence (advantage – difficult to overdose because of ceiling effect)

Question 5

Inverse agonist

Answer 5

binds to the same receptor as an endogenous agonist but it induces a pharmacological response opposite to that agonist
e.g. loratadine is an inverse agonist which has the opposite effect to histamine