Central and peripheral motor effects of morphine on the rat urinary bladder

Intravesical pressure recordings were performed in anaesthetized rats, and the effect of morphine on urinary bladder function was studied. The action of morphine was registered as its influence on bladder hyperactivity induced by central catecholaminergic stimulation with 1-dihydroxyphenylalanine (L-DOPA) after peripheral decarboxylase inhibition, and as its action on the response to regional injection of receptor agonists (acetylcholine (ACh), substance P (SP)) and peripheral motor nerve stimulation (PNS). The bladder response to L-DOPA was inhibited by intracerebroventricularly (i.c.v., fourth ventricle, 10 micrograms), as well as by systemically administered (1-5 mg kg-1 i.p.), morphine. Intravenous and i.c.v. naloxone antagonised the inhibitory actions of i.v. and i.c.v. morphine, respectively. Regional intra-arterial administration of morphine (0.01-5 mg) induced a weak bladder contraction per se, with a subsequent slight depression of bladder reactivity to ACh, SP and PNS. It is suggested that the inhibitory effect of morphine on bladder motility in the rat, is mainly mediated by central opioid-receptors. The direct peripheral effects on the detrusor muscle are weak, with an initial contraction followed by slight depression of the reactivity.