Adrenoceptor agonists

General information

Subtypes of adrenoceptors were first described by Ahlquist in 1948 [ , ]. Adrenoceptor agonists evoke physiological responses similar to those produced by stimulation of adrenergic nerves or the physiological release of adrenaline (see Table 1 ). For many of these responses it is currently possible to conclude that only an alpha-adrenoceptor or a beta-adrenoceptor is involved, and in some cases one can distinguish a beta ₁ from a beta ₂ response. In some cases, however, the distinction is not clear: most adrenoceptor agonists, however specific to a particular receptor type they are claimed to be, will for example on occasion stimulate central nervous functions, resulting in nervousness, insomnia, tremors, dizziness, or headache. In some organ systems both alpha-adrenoceptors and beta-adrenoceptors are present; thus, the nature of the response produced will depend either on the concentrations achieved or on other factors; whether, for example, the uterus contracts or relaxes in response to an adrenergic drug depends in part on the hormonal balance in the system at that moment. Alpha-adrenoceptor agonists, such as clonidine, are little used nowadays in the treatment of hypertension or migraine. Clonidine is used epidurally, in combination with opioids, neostigmine, and anesthetic and analgesic agents, to produce segmental analgesia, particularly for postoperative relief of pain after obstetrical and surgical procedures. Apraclonidine is available for the short-term reduction of intraocular pressure.

The drugs that were originally developed as general beta-adrenoceptor agonists have largely fallen into disuse with the development of more selective beta ₁ -adrenoceptor agonists (for use in cardiac failure) and beta ₂ -adrenoceptor agonists (for use in airways disease and threatened premature labor).