agonist and antagonist

1. Rajkumari Lodhi
Assistant Professor
Department of Pharmacology
Bhopal Madhya Pradesh
AGONISTS AND ANTAGONISTS(COMPETITIVE
AND NON-COMPETITIVE)
Subject:-pharmacology-I
Semester:- 4th

2. Agonists and antagonists(competitive and non-competitive)
on the basis of these receptors, drugs can be divided into four groups,
a. agonists
b. antagonists
c. agonist-antagonists
d. partial agonists
Agent Class Example Action
Agonist Morphine ,Fentanyl ,pethidine activation of all receptor subclasses,
though, with different affinities
Antagonist naloxone devoid of activity at all receptor classes
Agonist-Antagonist nalorphine
pentazocine
agonist activity at one type and antagonist
activity at another
Partial Agonist buprenorphine activity at one or more, but not all
receptor types

3. with regard to partial agonists, receptor theory states that drugs have two independent
properties at receptor sites,
a. affinity
 The ability, to bind to the receptor
b. efficacy
• or, intrinsic activity, and is the ability of the D-R complex to initiate a
pharmacological effect
Difference Between Agonist and AntagonistDrugs
Agonist vs Antagonist Drugs
Agonist drugs are the drugs which are capableof
activating.
Antagonist drugs are the drugs which bind to the receptors
block the binding of ligands to the receptors thereby
inhibiting the effect of the ligand.
Effects
Agonist drugs stimulate the action. Antagonist drugs inhibit the action.
Response
The response is caused when the agonist bindsto the
binding site.
The response is prevented when the antagonistbinds to the
binding site.
Types
There are two types of agonist drugs; Directbinding
agonist drugs and Indirect binding agonist drugs.
There are three types of antagonist drugs; Competitive
antagonist drugs, Non – competitive antagonist drugs,
and Irreversibleantagonist drugs.

4. Agonist Drugs:
An agonist drug is a chemical that mimics the natural ligand of the specific brain receptor. Thus the
binding of the agonist drug results in similar biological effect as the natural ligand. The agonist binds to
the same binding site as that of the natural ligand. Thus in the absence of the natural ligand, agonist drugs
are capable of providing the full or the partial response.
Examples of agonist drugs include oxycodone, methadone, hydrocodone, morphine, and opium. These
drugs bring relief of pain. Strong doses can have many side effects related to respiration, organ failure etc.
Figure 01: Mechanism of Agonist and Antagonist drugs

5. There are two main types of agonist drugs;
 Direct binding agonist drugs
 Indirect binding agonist drugs
Direct binding agonist drugs or complete agonists are capable of directly binding to the specific
binding site of the receptor. This binding site is the site in which the natural ligand binds under
normal conditions. This brings about a faster response as it directly binds to the receptor and
activates the brain signaling. Examples are morphine and nicotine.
Indirect binding agonist drugs are also termed as partial agonists, are drugs which enhance the
binding of the natural ligand to the receptor to bring about an effect. These drugs give delayed
responses. An example of an indirect binding agonist is cocaine.

6. Competitive antagonist drugs are drugs which have the ability to bind at the original binding site and inhibit the
binding of the natural ligand. This is due to the shape of the antagonist which mimics the natural ligand. Increasing the
ligand concentration can suppress the effect of the competitive antagonist.
Noncompetitive antagonist drugs binds to another site other than the true binding site. The binding of the non –
competitive antagonist will cause a conformational change in the receptor which will inhibit the binding of the true
ligand.
Irreversible agonist drugs bind strongly to the receptor through covalent linkages. This will permanently modify the
receptor preventing the binding of the ligand.
Examples of antagonist drugs include naltrexone and naloxone. Most often these drugs are used to inhibit the effects of
harmful drugs such as cocaine and heroin which are agonist drugs.
Antagonist Drugs:
Antagonist drugs are drugs which inhibit the effects of the natural ligand. The natural ligand can be a hormone,
neurotransmitter or an agonist.
Antagonist drugs can be of three main types.
 Competitive antagonists
 Non - competitive antagonists
 Irreversible antagonists

7. Agonist?
Agonist (sometimes called full agonist) is a molecule or chemical compound that can bind to
a receptor and activates the receptor, thus producing a biological response. It may be drugs or
endogenous substances such as neurotransmitters, hormones, etc. Agonist (full agonist)
shows intrinsic activity of
1. For example, Morphine mimics the action of endorphins at opioid receptors, here
Morphine is an Agonist.
Simply, an agonist is a drug that binds to and activates a receptor and produces a biologic
effect .
Partial Agonist?
Partial Agonist is a molecule or chemical compound that can bind to a receptor and weakly
activates the receptor, thus producing a submaximal biological response. It has the property
of affinity but has less intrinsic efficacy than a full agonist. Partial agonist shows intrinsic
activity greater than 0 but less than 1.
For example, Buprenorphine is a partial agonist at the μ receptor.

8. Inverse agonist:
Inverse agonist is a molecule or chemical compound that can bind to the same receptor
site as an agonist and produces a biological response opposite to that of the agonist
i.e. exhibits negative efficacy. For example, Flumazenil produces an anxiogenic effect at
the GABA (Gamma-Aminobutyric Acid) receptor.
 Here, Flumazenil is an inverse agonist.
 In other words, an Inverse agonist is a molecule or chemical compound that can bind
to a receptor but rather than producing an activation, it will lead to a deactivation that
means decrease the baseline activity of the receptor.
 Inverse agonist is a molecule or chemical agent that binds with a receptor and
decreases the baseline activity of the receptor, which in turn produces a reverse effect
that of binding of the agonist.

9. Antagonist:
Antagonist is a molecule or chemical compound that can bind to a receptor but unable to activate the receptor, thus
produce no biological response. It just occupies the receptor site and preventing binding agonist, thus blocks the
action of an agonist
example:
Atropine blocks the action of acetylcholine (Ach) in mAChR (Muscarinic Acetylcholine Receptor). Here, Atropine
is an Antagonist. Also, Antagonist shows zero intrinsic activity.
Antagonists bind with the receptor but produce no activation of the receptor and hence block responses from the
tissue .

10. Receptor Agonists Major Actions
mu1 morphine
phenylpiperidines
analgesia
sedation
mu2 morphine phenylpiperidines respiratory depression
delta d-Ala-d-Leu-Enk "DADLE" analgesia-
Weak respiratory depression
kappa ketocyclazocine
dynorphin
nalbuphine
butorphanol
analgesia-weak
respiratory depression
sedation
sigma SKF-10,047 pentazocine Delerium,hallucinations
Tachycardia,hypertension
epsilon b-endorphin stress response acupuncture
Characteristics of Opioid Receptors

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