The involvement of supraspinal alpha-adrenergic receptors in tonic pain was assessed in formalin-induced pain in rats. The alpha2 adrenoceptor agonist clonidine, along with yohimbine and prazosin, α2 and α1 receptor antagonists, were introduced intracerebroventrically (icv) and/or systemically in different doses. The data show that 1) clonidine exerts an alpha adrenergic analgesic effect, in addition to its known alpha2 role in this kind of pain, 2) icv yohimbine did not change the rat's nociception, and 3) icy prazocin also failed to alleviate the animal's nociception although both the latter drugs show analgesic activity in the formalin test when injected systemically. It can be concluded that α1 receptors contribute significantly to adrenergic analgesia in the formalin test in supraspinal structures, by undefined nature and site(s).

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