“Recently, the nature of these receptors has come into … insofar as the presynaptic inhibitory effects of the alpha 2-adrenoceptor agonist , clonidine, in the guinea-pig trachea have been shown to be antagonized by the alpha 2-adrenoceptor antagonist, yohimbine , as well as the alpha 1-adrenoceptor antagonist, prazosin.”