Drug Agonists and Antagonists

Pilocarpine | Nicotine | Atracurium | Atropine | Benzhexol | Salbutamol | Adrenaline | Propranolol | Morphine | Pethidine | Naloxone | Bromocriptine | Thioridazine | Haloperidol

 

Pilocarpine

Pilocarpine is an acetylcholine agonist. It mimics the action of acetylcholine on muscarinic receptors, which are found at the synapse between the post-ganglionic fibres of the parasympathetic nervous system and their effector organs. Pilocarpine therefore enhances the parasympathetic effects of acetylcholine.

It is used for the treatment of open angle glaucoma, in which there is raised intra-ocular pressure due to reduced flow of aqueous humour out of the posterior chamber of the eye. Pilocarpine binds to muscarinic recptors on the ciliary muscle of the eye, causing it to contract. As well as making the lens thicker (for near vision), this also facilitates the outflow of aqueous humour and so reduces intra-ocular pressure.


Neal, M (1991) Medical pharmacology at a glance Oxford, Blackwell Scientific Publications

 

Nicotine

Nicotine, in low doses, is an acetylcholine agonist. It mimics the action of acetylcholine on nicotinic receptors, which are found in the autonomic ganglia (ie the between pre-and post-synaptic neurones of the sympathetic and parasympathetic nervous system), in the adrenal medulla (where stimulation leads to the release of adrenaline), at neuromuscular junctions and in the central nervous system. A cigarette contains approximately 1-6 mg nicotine, but an oral dose of more than approximately 60mg of nicotine could be fatal, as in these concentrations nicotine blocks, rather than stimulates receptors.

Neal, M (1991) Medical pharmacology at a glance Oxford, Blackwell Scientific Publications

BNF, www.bnf.org

 

Atracurium

Atracurium is an acetylcholine antagonist, which selectively blocks the nicotinic receptors present at neuromuscular junctions. It competes with acetylcholine at this site and the effect is a paralysis of the skeletal muscle. Pancuronium is used to produce muscle relaxation for surgery. Its duration of action is about 15 - 40 minutes.


Neal, M (1991) Medical pharmacology at a glance Oxford, Blackwell Scientific Publications

BNF, www.bnf.org

 


Atropine

Atropine is an acetylcholine antagonist, blocking its effects at muscarinic receptors ie blocking the parasympathetic effects of acetylcholine. It therefore causes dilated pupils, urinary retention, and dry mouth and tachycardia. Clinically, it may be used in patients with bradycardia and as an eye drop to dilate the pupils.

Atropine is found in the deadly nightshade plant (Atropa belladona). It is called the belladonna because of the effects on the pupils - dilated pupils were thought to be more attractive.


Neal, M (1991) Medical pharmacology at a glance Oxford, Blackwell Scientific Publications

BNF, www.bnf.org

 

Benzhexol

Benzhexol is an acetylcholine antagonist, which is sometimes used in the drug treatment of Parkinson's disease. It blocks central muscarinic receptors, including those in the basal ganglia. This can be beneficial in Parkinson's disease where there is a relative overactivity of cholinergic neurones (ie those which release acetylcholine) compared with dopaminergic neurones (ie those which release dopamine) in this area of the brain.


Neal, M (1991) Medical pharmacology at a glance Oxford, Blackwell Scientific Publications

BNF, www.bnf.org

 


Salbutamol

Salbutamol is a beta- receptor agonist. This means that it mimics the effects of noradrenaline on beta receptors. It also has specificity for the B2 class of receptors found in the airway. Salbutamol is therefore used in the treatment of asthma, enhancing the sympathetic effects of noradrenaline on the smooth muscle of the airways and causing bronchodilatation.

Although salbutamol is specific for B2 receptors, it may also have some effect on the B1 receptors in the heart, especially in high doses. This explains the potential side effects of tachycardia and arrhythmias.


BNF, www.bnf.org

 

 

Adrenaline

Adrenaline is a naturally occurring neurotransmitter and hormone released in the sympathetic nervous system, causing the 'fight or flight' response. However, it may also be administered clinically to produce sympathetic effects, acting non-selectively at alpha and beta- receptors. For example, it may be given in anaphylactic shock, in order to reverse the massive vasodilatation that occurs, and to increase cardiac output. It is also used in the treatment of cardiac arrests due to asystole and ventricular fibrillation.


BNF, www.bnf.org

 

Propranolol

Propranolol is a beta-receptor blocker ie it antagonises the effect of adrenaline and noradrenaline at beta receptors. Propranolol is used in the treatment of hypertension and may work by decreasing the sympathetic nervous system action on the heart ie by decreasing heart rate and force of contraction. Side effects may include bronchoconstriction due to the blocking of beta receptors in the airways and so propranolol and other beta-blockers need to be used cautiously in people with asthma and chronic obstructive pulmonary disease.


Neal, M (1991) Medical pharmacology at a glance Oxford, Blackwell Scientific Publications

BNF, www.bnf.org

 

Morphine

Morphine is an opiate, which mimics the effect of endogenous opioids such as b-endoprhin. Morphine therefore modulates the transmission of pain in the spinal cord by interfering with the release and actions of substance P in the substantia gelatinosa. It is particularly useful for the relief of visceral pain. (For more information about pain perception and the neurotransmitters involved, refer to the section on pain in this WebCT course.)

Morphine also has a number of other central effects, including the depression of the respiratory centre.


Neal, M (1991) Medical pharmacology at a glance Oxford, Blackwell Scientific Publications

 

Pethidine

Pethidine is an opiate, similar in action to morphine ie it mimics the effect of the naturally occurring opioids. However, it is more lipid soluble that morphine and so has a quicker onset of action (takes effect more quickly) and is used for pain relief in labour. (For more information about pain perception and the neurotransmitters involved, refer to the section on pain in this WebCT course.)


Neal, M (1991) Medical pharmacology at a glance Oxford, Blackwell Scientific Publications

 

Naloxone

Naloxone is an opiate receptor antagonist and competes with opiates such as morphine. It is therefore used to produce a rapid reversal of respiratory depression caused by an overdose of such drugs.


BNF, www.bnf.org

 

Bromocriptine

Bromocriptine is a dopamine agonist, which has effects on the D2 class of receptor. Clinically, it may be used to inhibit prolactin secretion and so suppress lactation. (Dopamine usually inhibits the release of prolactin). It may also be useful in the treatment of Parkinson's disease, mimicking the actions of dopamine, which is deficient in this condition. In Parkinson's disease, it may be given alone, or in combination with levodopa (a precursor of dopamine).


Neal, M (1991) Medical pharmacology at a glance Oxford, Blackwell Scientific Publications

BNF, www.bnf.org

 


Thioridazine

Thioridazine is a member of the phenothiazine family, all of which block dopamine receptors. They are used to treat psychosis, for example in people who have schizophrenia. However, the way in which they produce an anti-psychotic effect is not clearly understood. Although drugs such as thioridazine block dopamine receptors immediately, they do not seem to have any clinical effect for several weeks. One possibility is that the drugs lead to a change in the number or sensitivity of dopamine receptors in the brain.

Side effects of these drugs occur because they are not specific for dopamine receptors in the mesolimbic system (their target site in psychosis), but also affect dopamine receptors in the basal ganglia, causing Parkinson-like symptoms, as well as some blocking action on histamine and muscarinic receptors.


Neal, M (1991) Medical pharmacology at a glance Oxford, Blackwell Scientific Publications

BNF, www.bnf.org

 


Haloperidol

Haloperidol is a butyrophenone and a dopamine receptor antagonist. Like the phenothiazines (eg thioridazine), haloperidol is used in the treatment of psychosis, particularly acute psychosis. Haloperidol has less effect on muscarinic receptors (compared with the phenothiazines), but is associated with a high incidence of Parkinson-like symptoms, due to its action on the dopamine receptors in the basal ganglia.


Neal, M (1991) Medical pharmacology at a glance Oxford, Blackwell Scientific Publications

BNF, www.bnf.org