Sodium deprivation increases the antinociceptive activity of morphine

Interplay between RAS and opioids: opening the Pandora of complexities

Angiotensin and endogenous opioids are important bioactive neuropeptides, which are widely distributed in the brain and peripheral regions to produce diverse biological and neurobiological activities. An endogenous opioid system includes proopiomelanocortin-derived enkephalin, dynorphin and endorphin that act on their specific receptors such as delta (δ), kappa (κ) and mu (μ) receptors. Research evidence demonstrates significant positive as well as negative interactions between renin angiotensin system (RAS) and endogenous opioids in the brain and periphery. The diverse actions of Ang II are possibly mediated indirectly through endogenous opioids, while opioids are also shown to activate RAS components suggesting the up-regulation of each system in concern with each other. On the contrary, there are reports suggesting a negative correlation between RAS and opioid system. Research evidence also supports the notion that Ang II acts as anti-opioid peptide to decrease the actions of opioids. Moreover, opioids-induced decline in angiotensin release and functioning has also been reported. Co-administration of ACE inhibitors with opioids exhibits significant interactions possibly due to decreased metabolism of opioids leading to potentiation of their actions. The present review describes the complexities of positive and negative interactions between RAS and opioids along with possible mechanisms responsible for these interactions.

Is intracellular sodium involved in the mechanism of tolerance to opioid drugs?

Recent studies indicate that one of the more likely mechanisms of opioid tolerance could involve a decrease in the efficiency with which agonists can induce coupling of their specific binding sites in neuronal membranes to the activation (or deactivation) of an effector system. Reports of sodium-induced decreases in opioid receptor agonist binding and in the size of ligand/receptor complexes, as well as modulation of opioid activity by manipulation of sodium in vivo, indicate that sodium might play a physiological role in modulating opioid receptor function. Reports of morphine-induced systemic sodium retention in animals, as well as morphine-induced increases in brain intracellular sodium and decreases in brain Na+, K(+)-ATPase activity, indicate that the development of tolerance may be accompanied by changes in the disposition of sodium. The direction of these sodium- and morphine-induced changes is consistent with the hypothesis that an increase in intracellular sodium could participate in the mechanism(s) of opioid tolerance development.