en Physiology Physiology Original Research: Basic Science in Medicine Original Research: Basic Science in Medicine 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). 279 284 http://mjiri.iums.ac.ir/browse.php?a_code=A-10-1-413&slc_lang=en&sid=1 S SEMNANIAN 20031947532846005279 20031947532846005279 Yes From the Department of Physiology, Shaheed Beheshti University of Medical Sciences S HAJISAYAH 20031947532846005280 20031947532846005280 No the Department of Physiology, Tarbiat Modarres University MR ZARRINDAST 20031947532846005281 20031947532846005281 No the Department of Pharmacology, Tehran University of Medical Sciences, Tehran, Islamic Republic of Iran.