Involvement of supraspinal alpha-adrenergic receptors in tonic pain

The involvement of supraspinal alpha-adrenergic receptors in tonic pain was assessed in formalin-induced pain in rats. The alpha [2] adrenoceptor agonist clonidine, along with yohimbine and prazosin, Alpha2 and Alpha1 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 alpha [2] role in this kind of pain, 2] icv yohimbine did not change the rat's nociception, and 3] icv 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 Alpha1 receptors contribute significantly to adrenergic analgesia in the formalin test in supraspinal structures, by undefined nature and site[s]