The effects of naloxone on the neural control of the urinary bladder of the cat.

Naloxone in doses ranging from 0.5 to 512 micrograms/kg i.v., enhanced reflex contractions of the urinary bladder of the cat. At the lowest doses (threshold, 0.5-5 micrograms/kg) the drug increased the frequency of spontaneous bladder contractions. In large doses (10-100 micrograms/kg) the drug produced an initial tonic contraction of bladder lasting 15-40 min followed by a period of high frequency rhythmic activity. Multiunit firing in parasympathetic postganglionic nerves on the surface of the urinary bladder was also enhanced. Bursts of firing which in untreated animals occurred during large bladder contractions occurred continuously during the entire sustained contraction of the bladder following large doses of naloxone. Various evidence indicates that the site of action of naloxone is in the central nervous system. These findings suggest that the parasympathetic reflex pathway to the urinary bladder may be subject to tonic enkephalinergic inhibitory control.