Opioid receptor types on adrenergic nerve terminals of rabbit ear artery

1 Methionine enkephalin, leucine enkephalin, [D‐Ala 2 , D‐Leu 5 ] enkephalin, α‐neoendorphin, β‐endorphin, dynorphin (1–13) and ethylketocyclazocine inhibited the contractions of rabbit ear artery ring segments elicited by transmural nerve stimulation at 8 Hz. 2 Ethylketocyclazocine, dynorphin (1–13) and leucine enkephalin produced partial inhibition, their apparent intrinsic activities (α) being 0.57, 0.75 and 0.66, respectively. 3 Morphine and normorphine, which are agonists at μ‐receptors, did not inhibit the response of the artery. 4 Naloxone antagonized the actions of opioids and ethylketocyclazocine, and was more effective against methionine enkephalin, leucine enkephalin and [D‐Ala 2 , D‐Leu 5 ] enkephalin than against α‐neoendorphin, ethylketocyclazocine and dynorphin (1–13). The pA 2 values of naloxone against so‐called δ‐agonists were approx. 8.5, and against so‐called k ‐agonists were approx. 7.7. 5 The supposed k ‐antagonist, Mr 2266, was more effective than naloxone in antagonizing the actions of α‐neoendorphin, and the k ‐agonists dynorphin (1–13) and ethylketocyclazocine. The pA 2 values of Mr 2266 against k ‐agonists were 8.5–9.0, and against δ‐agonists were 7.8 or less. 6 The opioid peptides and opioids tested did not cause dilatation of the artery previously contracted with histamine. 7 These results suggest that the opioid peptides and ethylketocyclazocine acted on opioid receptors at adrenergic nerve terminals in the ear artery. 8 The opioid receptors appear to be of the δ‐ and k ‐types, not the μ‐type.