Central delta-opioid receptor interactions and the inhibition of reflex urinary bladder contractions in the rat

The in vivo effects of a number of opioid agonists and antagonists were studied on the spontaneous reflex contractions of the urinary bladder recorded isometrically in the rat anesthetized with urethane. All substances were administered into the central nervous system by the intracereboventricular (i.c.v.) or spinal intrathecal (i.t.) route. The conformationally restricted enkephalin analogues [2-D-penicillamine, 5-L-cysteine] enkephalin (DPLCE), [2-D-penicillamine, 5-L-penicillamine] enkephalin (DPLPE) and [2-D-penicillamine, 5-D-penicillamine] enkephalin (DPDPE) produced dose-related inhibition of reflex bladder contractions when administered by the i.c.v. or i.t. route. Both the novel delta-opioid receptor antagonist ICI 154,129 (200-600 micrograms) [N,N-bisallyl-Tyr-Gly-Gly-Psi-(CH2S)-Phe-Leu-OH) and ICI 174,864 (1-3 micrograms) [N,N-dially-Tyr-Aib-Aib-Phe-Leu-OH: Aib = alpha-aminoisobutyric acid] attenuated or abolished the effects of DPLCE, DPLPE and DPDPE when administered by the i.c.v. or i.t. route. The antagonism observed was selective since the equipotent inhibition produced by the mu-opioid receptor agonist [D-Ala2, Me-Phe4, Gly(ol)5] enkephalin (DAGO) was unaffected. Overall, ICI 154,129 was considerably weaker than ICI 174,864 and both antagonists inhibited bladder activity at doses higher than those required to demonstrate delta-receptor antagonism. Further studies of the agonistic effect of ICI 174,864 showed that it was insensitive to low doses of naloxone (2 micrograms, i.c.v. or i.t.) but could be abolished by higher (10-15 micrograms) doses of naloxone. These observations suggested that the agonistic effect of ICI 174,864 was not mediated by mu-opioid receptor. beta-Endorphin (0.2-1.0 micrograms, i.c.v.) inhibited bladder contractions but following recovery from this effect, appeared to prevent the expression of delta-receptor antagonism by ICI 174,864. In addition a previously subthreshold dose of ICI 174,864 now exhibited marked agonistic activity. The inhibitory effect of a submaximal dose of DPDPE was also potentiated by beta-endorphin under these circumstances. These observations suggest that supra-spinal and spinal delta-opioid receptors are involved in the opioid-mediated inhibition of reflex bladder contractions in the rat. Moreover beta-endorphin may be important in regulating central delta-opioid receptors.