Aims and Objectives
The term autacoid is used for a group of
hormone like substances, which
originate from tissues, and produce
effects at the site of release.
Important members of this group are: I.
5 Hydroxytryptamine (5HT)
Autacoids play an important role in
the causation of inflammatory
reactions, and a variety of other
The agonists and antagonists belonging to this
group of substances are of considerable clinical
importance, and will, therefore, be discussed in
This lecture deals with Histamine and
1. Smooth Muscles
i. Intestine: - Antagonism of Histamine
ii. Bronchi: - Histamine is blocked
Guinea-pig can tolerate 100 times lethal
dose of Histamine in the presence of a
suitable dose of Antihistamine.
2. Blood Pressure
The initial rapid hypotensive effect is
antagonized by H1 blockers. The hypotensive
effect in the second phase is antagonized by
3. HCl secretion: No effect
The first generation induces sedation.
Central excitation is the striking effect of
Atropine like actions. The second
generation has no such effects.
6. Local anesthetic action
However, large doses are required.
7. Blockade of α 1 Adrenergic receptors
i. Capillary Permeability and Edema
H1 antagonists strongly block the action
of Histamine, that results in increased
capillary permeability and formation of
edema and wheal.
ii. Flare and Itch:
The flare component of the triple response
and the itching caused by intradermal
injection of histamine are two different
manifestations of the action of histamine on
nerve endings. H1 antagonists suppress
iii. Serotonin (5HT) Blocking Action
Strong blocking effects at serotonin
receptors have been demonstrated for H1
antagonist cyproheptadine. This drug is
promoted as an anti serotonin agent.
I. ALLERGIC DISORDERS
1. Pruritis (Itching)
3. Vasomotor Rhinitis
4. Allergic responses to insect bites, chemicals
5. Allergic responses to Drugs
6. Common cold
MOTION SICKNESS (Prophylaxis)
Promethazine strongest in the group,
because of anticholinergic effect (See
NAUSEA & VOMITING
Cyclizine HCl (Marezine)
Vestibular Disturbance; Meniere’s Disease
IV. SEDATION (CNS DEPRESSION)
1. Edema & Itch Blocked
2. Hypotension Controlled to a lesser
3. Bronchospasm No Effect
Note: - Other substances involved.
1. ANTIMUSCARINIC EFFECTS
i) Dryness of mouth
ii) Retention of urine
iii) Dryness of respiratory passages.
These effects are not seen with Second
2. CENTRAL ACTIONS (CNS)
i) Sedation (not 2nd generation). May be
useful effect in some cases.
ii) Other central actions.
Dizziness; Tinnitus; Lassitude*; Euphoria;
* Laziness, Sluggishness
3. DIGESTIVE TRACT
Loss of Appetite; Epigastric distress
4. POLYMORPHIC VENTRICULAR
5. OTHER EFFECTS
Mutagenecity; Leukopenia; Agranulocytosis
• Effects strikingly resemble acute
• Central excitatory effect
• Dilated pupils
• Flushed face
• Dry mouth
• Sinus tachycardia
• Retention of urine
• Finally coma with cardio-respiratory
collapse, and death with in 2-18 hours.
1. An antihistamine which has comparatively
lower potential to induce drowsiness /
2. A drug which belongs to second
generation of antihistamines:
3. A second generation antihistamine which
has been withdrawn from the market:
4. Many antihistamines (H1 Blockers) have
additional non-histamine related effects:
These are likely to include which one of
Muscarinic increase in the bladder tone
General anesthetic effects if the drug is injected
Antimotion sickness effect
Increase in total peripheral resistance