Alpha 2-adrenoceptor agonists for the treatment of chronic pain

The antinociceptive effect of alpha(2)-adrenoceptor agonists is mediated by activation of descending inhibiting noradrenergic systems, which modulates 'wide-dynamic-range' neurones. Furthermore, they inhibit the liberation of substance P and endorphines and activate serotoninergic neurones. Despite this variety of antinociceptive actions, there is still little experience with alpha(2)-adrenoceptor agonists as therapeutic agents for use in chronic pain syndromes. Studies in animals and patients have shown that the transdermal, epidural and intravenous administration of the alpha(2)-adrenoceptor agonist clonidine reduces pain intensity in neuropathic pain syndromes for periods varying from some hours up to 1 month. Patients suffering from lancinating or sharp pain respond best to this therapy. Topically applied clonidine (200-300 microg) relieves hyperalgesia in sympathetically maintained pain. Epidural administration of 300 microg clonidine dissolved in 5 ml NaCl 0.9 % has also been shown to be effective. In patients suffering from cancer pain tolerant to opioids, pain control has proved possible again with combinations of opioids and clonidine. In isolated cases clonidine has been administered epidurally at a dose of 1500 microg/day for almost 5 months without evidence for any histotoxic property of clonidine. Side effects often observed during administration of alpha(2)-adrenoceptor agonists are dry mouth, sedation, hypotension and bradycardia. Therapeutic interventions are usually not required.