Role of mu- and delta-opiate receptors in realization of the autonomic effects of opioid peptides

Mechanisms involved in the cardiovascular responses to opioid products of proenkephalin in the anaesthetised rat

1. Cardiovascular effects of opioid peptide products of proenkephalin, [Met] enkephalin (ME), [Leu] enkephalin (LE), [Met] enkephalyl Arg6-Phe7 (MEAP) and [Met] enkephalyl Arg6-Gly7-Leu8 (MEAGL) have been studied in urethane-anaesthetised rats. 2. ME, LE, MEAP and MEAGL produced vasodepression and bradycardia mediated by mu-opioid receptors. 3. Atypical responses to MEAP were observed in a quarter of the animals studied showing tachycardia and pressor effects. This response was probably due to the release of the dipeptide Arg-Phe which exerted its effects at sympathetic ganglia. 4. Studies with the peptidase inhibitors captopril and bestatin showed a differential potentiation of the cardiovascular effects of the proenkephalin products by inhibition of angiotensin converting enzyme and aminopeptidase. 5. The effects of vagotomy, pithing and studies with atropine, and N-methyl levallorphan were used to demonstrate that, for all four proenkephalin peptides, cardiovascular effects were mediated by peripheral opioid receptors and transmission to the CNS via vagal afferents.

Multiple opioid receptors mediate the respiratory depressant effects of fentanyl-like drugs in the rat

1. Respiratory depressant effects of five drugs of the fentanyl series have been studied in anaesthetised rats. 2. The potency ratios of the fentanyl drugs to produce apnea and depress minute volume were dissimilar. Further, in vivo naloxone pA2 values were identical for blockade of apnea for the fentanyl drugs but different for antagonism of minute volume. 3. Differences in agonist potency and in naloxone pA2 values were also seen in vagotomised rats where only depression of minute volume is observed. 4. The data suggests that multiple receptor interactions are involved in the respiratory depressant effects of these drugs; the apnea response is primarily mediated through peripheral mu receptors but minute volume depression involves both mu receptors and non-mu sites.