Morphine receptor action/selectivity

The relative extent of the unwanted effects caused by selective agonists at the different opioid receptors is of great importance in determining if non-mu opioids will have better spectra of actions as compared to morphine. However, there are good indications that the kappa and delta receptor agonists cause less respiratory depression than mu... 

It is N-allyl analogue of oxymorphone, have a high affinity for mu receptor and lower affinity at delta and kappa sites. It selectively antagonizes the respiratory depression produced by opioids. After intravenous administration, it antagonizes all actions of morphine. It also blocks the actions of endogenous opioid peptides. 

On the other hand, a number of studies have shown that morphine and opioid peptides could have cardioprotective effects toward ischemic processes and may be able to reduce the size of infarct [72,73]. These effects seem to involve the activation of delta opioid receptors, the localization of which remains unknown. In addition, enkephalin-degrading enzyme inhibitors, such as acetorphan and, particularly, RB 101, have also been demonstrated to decrease the susceptibility to the arrhythmogenic action of epinephrine. Thus RB 101 completely prevented the ventricular tachycardia, fibrillation, and repetitive ventricular extrasystoles induced by epinephrine (Maslov et al., unpublished data). These effects were reversed by the selective delta antagonist ICI 174,864. 

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