Multiple sites of action in the inhibitory effect of nociceptin on the micturition reflex

Purpose: Nociceptin, the endogenous peptide ligand for the opioid receptor- like(1) (ORL1) receptors, exerts a naloxone-resistant suppressant effect on micturition reflex after intravenous administration. This work aims to elu cidate the mechanism and the site of action of the inhibitory effect of noc iceptin on the micturition reflex. Materials and Methods: The bladder of urethane-anesthetized rats was cannul ated through the dome (cystometries) or the urethra in isovolumetric condit ions (distension-induced reflex contractions, DIRCs). In this latter model, the effect of the application of nociceptin onto the serosal surface of th e urinary bladder was determined. The effect of intravenous, intrathecal an d intracerebroventricular administration of nociceptin on ongoing cystometr ies at two different infusion rates (50 and 250 mu L/min.) was assessed. Th e effect of the intravenous administration of nociceptin on cystometries wa s also studied in capsaicin-pretreated animals. Results: When cystometric recordings were obtained at a low infusion-rate ( 50 mu L/min.), the intravenous administration of nociceptin (10 to 100 nmol ./kg.) induced a dose-dependent reduction in the micturition frequency asso ciated to an increase of the pressure threshold for activating the micturit ion reflex, whereas the amplitude of micturition contractions was unaffecte d. These effects faded within 60 minutes. The intracerebroventricular admin istration of nociceptin (0.3 nmol./rat) produced urodynamic changes similar to those observed after the intravenous route and, in addition, also reduc ed the amplitude of micturition contractions. The intrathecal administratio n of nociceptin up to 1 nmol./rat was ineffective. Capsaicin pretreatment ( 164 mu mol./kg., s.c, 5 to 6 days before) significantly reduced the micturi tion frequency as compared with controls. In capsaicin pretreated animals i ntravenous nociceptin was ineffective. When cystometries were recorded at a high infusion-rate (250 mu L/min.) either intravenous (100 nmol./kg.), i.t . (1 nmol./rat) nociceptin or capsaicin pretreatment had no effect. In cont rast, intracerebroventricular nociceptin (0.3 and 1 nmol./rat) inhibited th e micturition reflex by reducing both the frequency and the amplitude of mi cturition contractions: these effect were not modified by naloxone (0.5 mu mol./kg., i.v.). The topical application of nociceptin (5 and 50 nmol./rat) caused a dose-dependent inhibition of DIRCs. Conclusion: Nociceptin inhibits the micturition reflex at a peripheral and at a supraspinal site. The effects observed after the intravenous administr ation of nociceptin indicate that the functional integrity of capsaicin-sen sitive bladder afferents is required for exerting its inhibitory activity a t the peripheral level. In contrast, the supraspinal effect of nociceptin i nvolves both the afferent and the efferent pathways of the micturition refl ex, possibly through a direct effect on ORL1 receptors located in the ponti ne micturition center.