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α-blockers (alpha-blockers) or α-adrenergic-antagonists are pharmacological agents that act as receptor antagonists of α-adrenergic receptors (α-adrenoceptors).

Classification

• α₁-blockers or antagonists act at α₁-adrenoceptors
• α₂-blockers or antagonists act at α₂-adrenoceptors

When the term "alpha blocker" is used without further qualification, it sometimes refers to α₁-blockers, and sometimes refers to agents that act at both types of receptors.

Examples of non-selective α-adrenergic blockers include:

• Phenoxybenzamine
• Phentolamine
• Tolazoline

Selective α₁-adrenergic blockers include:

• Alfuzosin
• Prazosin
• Doxazosin
• Tamsulosin
• Terazosin

Selective α₂-adrenergic blockers include:

• Atipamezole
• Idazoxan
• Yohimbine

The agents carvedilol and labetalol are both α- and β-blockers.

Uses

α-blockers are used in the treatment of several conditions, such as Raynaud's disease, hypertension, and scleroderma.

α-blockers can also be used to treat anxiety and panic disorders, such as generalized anxiety disorder, panic disorder or posttraumatic stress disorder (PTSD). Alpha2-adrenergic receptor agonists, such as clonidine and guanfacine, act at noradrenergic autoreceptors to inhibit the firing of cells in the locus ceruleus, effectively reducing the release of brain norepinephrine (3). Clonidine has shown promise among patients with Anxiety, Panic and PTSD in clinical trials and was used to treat severely and chronically abused and neglected preschool children. It improved disturbed behavior by reducing aggression, impulsivity, emotional outbursts, and oppositionality (4). Insomnia and nightmares were also reported to be reduced.

Kinzie and Leung (5) prescribed the combination of clonidine and imipramine to severely traumatized Cambodian refugees with Anxiety, Panic and PTSD. Global symptoms of PTSD were reduced among sixty-six percent and nightmares among seventy-seven percent. Guanfacine produces less sedation than clonidine and thus may be better tolerated. Guanfacine reduced the trauma-related nightmares (6). A recently completed randomized double-blind trial among veteran patients with chronic PTSD showed that augmentation with guanfacine was associated with improvement in anxiety and PTSD.

Prazosin is an alpha1-receptor antagonist. Raskin and colleagues (7) studied the efficacy of prazosin for PTSD among Vietnam combat veterans in a 20-week double-blind crossover protocol with a two-week drug washout to allow for return to baseline (7). The CAPS and the Clinical Global Impressions-Change scale (CGI-C) were the primary outcome measures. Patients who were taking prazosin had a robust improvement in overall sleep quality (effect size, 1.6) and recurrent distressing dreams (effect size, 1.9). In each of the PTSD symptom clusters the effect size was medium to large: .7 for reexperiencing or intrusion, .6 for avoidance and numbing, and .9 for hyperarousal. The reduction in CGI-C scores (overall PTSD severity and function at endpoint) also reflected a large effect size (1.4). Prazosin appears to have promise as an effective treatment for PTSD-related sleep disturbance, including trauma-related nightmares, as well as overall Anxiety and PTSD symptoms.

References

3 Kaplan HI, Sadock B: Synopsis of Psychiatry, 8th ed. Baltimore, Lippincott Williams & Wilkins, 1998

4 Harmon RJ, Riggs P: Clonidine for posttraumatic stress disorder in preschool children. Journal of the American Academy of Child and Adolescent Psychiatry 35:1247–1249, 1996[CrossRef][Medline]

5 Kinzie JD, Leung P: Clonidine in Cambodian patients with posttraumatic stress disorder. Journal of Nervous and Mental Disease 177:546–550, 1989[Medline]

6 Horrigan JP, Barnhill LJ: The suppression of nightmares with guanfacine. Journal of Clinical Psychiatry 57:371, 1996

7 Raskind MA, Peskind ER, Kanter ED, et al: Reduction of nightmares and other PTSD symptoms in combat veterans by prazosin: a placebo controlled study. American Journal of Psychiatry 160:371–373, 2003[Abstract/Free Full Text]

See also

• Beta blocker

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