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Lecture 2

PSYC62H3 Lecture Notes - Lecture 2: Inverse Agonist, Partial Agonist, Intrinsic Activity

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Suzanne Erb

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Lecture 2: Principles of Pharmacology
1. Drug-Receptor Interactions
- Important basic concepts:
Receptors: large protein molecules that drugs and naturally occurring chemicals of the body act on
to exert their effects.
-Receptors: drugs in central nervous system synapse with the terminal adjacent to the postsynaptic
neuron. Receptor molecules are embedded in the postsynaptic neuron where neurotransmitter that
are released will bind
Ligands: Biologically active chemicals in the body (e.g., hormones, neurotransmitters,
neurohormones, etc).
- Ligands: the chemicals themselves are also attracted to bind to the neurotransmitter
Affinity: The ability of a compound to bind to or maintain contact with a receptor.
-Does the ligand fit the receptor? This is known as Affinity (eg: key and lock)
- Affinity: the drug will attach to receptor and have the effect, then disassociate
Intrinsic activity: The relative capability of a compound to activate a receptor after binding with it.
-Intrinsic Activity: Does the drug/ligand activate the drug after binding (eg: wrong key in lock has no
intrinsic activity)
-there isn’t a direct relationship between intrinsic activity and affinity -->as affinity increase intrinsic
activity can increase or decrease
Drugs act at receptors as either agonists or antagonists

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-Agonist: produce some sort of biological activity (both affinity + intrinsic activity)
-Antagonist: fit the “lock” (receptor) but no activation (affinity but no intrinsic activity)
Indirect agonist: a drug that does not interact directly with a receptor but enhances the amount of
endogenous ligand available for the receptor.
-by increasing the natural ability or amount available for the receptor
-example: dopamine and cocaine the amount of dopamine increases in the synapse
Partial agonist: a drug that is not as effective as a full agonist but is more effective than an antagonist.
-not as effective as a full agonist but more effective than an antagonist
-the full agonist, when competing with the partial agonist would then have an antagonist effect it
decreases the effect because not all the receptors are activated with full agonist
Inverse agonist: a drug that acts at the same receptor as an agonist but that decreases basal activity at
the receptor and, thereby, produces effects opposite those of the agonist.
agonist because it produces a change in the body however it is inverse because it has the opposite
effect of the full agonist

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Graph of all of the Agonists
Competitive antagonist: an antagonist that is capable of dissociating from the receptor, allowing for
‘competition’ between the agonist and antagonist for the receptor.
- eventually, given an appropriate number of agonist and antagonist, the agonist can eventually win
over the antagonist (which disassociates and allows the agonist to bind).
Non-competitive antagonist: an antagonist that is not capable of dissociating from the receptor.
-the non-competitive antagonist cannot be overcome by increasing the number of agonists added
-does not disassociate from receptor.
Mixed agonist-antagonist: a drug that acts as an agonist itself but blocks the activity of another agonist
in the same system.
-acts as agonist at some receptors and antagonist at other receptors
-eg: opium acts as an antagonist at mu opioid receptors and the same drug at kappa and delta opioid
receptors acts as partial agonist
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