Peripheral Antagonists

Category Histamine H I receptor antagonist Muscarinic antagonist Peripheral vasodilator Half-life 4 hours... 

Peripheral Antagonists. The relatively long duration of action of the thyroid hormones makes it desirable to have compounds capable of blocking them competitively at their site of action. This is desirable in the treatment of thyroid storm where the reduction of circulating hormone levels brought about by the inhibition of their synthesis is too slow. 

A large number of thyroid hormone analogues have been tested for this effect (6). Among others, i-T (3) and 3,3 -T2 (5) and their propionic acid side-chain analogues decrease oxygen consumption at molar ratios of 50—200 1 of T. Nevertheless, no potent or clinically usehil peripheral antagonists have been found. 

Antidepressants are used in the treatment of neuropathic pain and headache. They include the classic tricyclic compounds and are divided into nonselective nor-adrenaline/5-HT reuptake inhibitors (e.g., amitriptyline, imipramine, clomipramine, venlafaxine), preferential noradrenaline reuptake inhibitors (e.g., desipramine, nortriptyline) and selective 5-HT reuptake inhibitors (e.g., citalopram, paroxetine, fluoxetine). The reuptake block leads to a stimulation of endogenous monoaminer-gic pain inhibition in the spinal cord and brain. In addition, tricyclics have NMDA receptor antagonist, endogenous opioid enhancing, Na+ channel blocking, and K+ channel opening effects which can suppress peripheral and central sensitization. Block of cardiac ion channels by tricyclics can lead to life-threatening arrhythmias. The selective 5-HT transporter inhibitors have a different side effect profile and are safer in cases of overdose [3]. 

Class II drugs are classical (3-adrenoceptor antagonists such as propranolol, atenolol, metoprolol or the short-acting substance esmolol. These drugs reduce sinus rate, exert negative inotropic effects and slow atrioventricular conduction. Automaticity, membrane responsiveness and effective refractory period of Purkinje fibres are also reduced. The typical extracardiac side effects are due to (3-adrenoceptor blockade in other organs and include bronchospasm, hypoglycemia, increase in peripheral vascular resistance, depressions, nausea and impotence. 

C-fibre afferents from the aitways contain peptide tachykinin transmitters such as substance P (SP) and neurokinins A and B (NKA and NKB). Stimulation of these nerves can also cause local release of these mediators at their peripheral terminal, allowing them to enhance the activity of the RARs. SP, NKA and NKB act at the tachykinin receptors (NK4-NK3), and so understandably, antagonists for NK2 in particular appear promising in cough. 

Several nonpeptidic, orally active vasopressin receptor antagonists have been developed. The dual V1A/V2R antagonist conivaptan is used in the treatment of hyponatraemia and could also become useful for diseases such as congestive heart failure, in which increased peripheral resistance and dilutional hyponatremia both are present [4]. Side effects of conivaptan include headache, injection site reactions, vomiting, diarrhoea, constipation and thirst. 

ATC A03FA06 A04AD Use anti-emetic, specific antagonist of peripheral and central dopamine receptors, reversible MAO-inhibitor... 

It is perhaps easier to identify some of the central functions of DA than that of the other monoamines because not only does it have distinctive central pathways associated with particular brain areas, but it has few peripheral actions. Also the actions of its antagonists reveal its central effects. These are summarised in Table 7.4. 

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