A. Dray

Opioid receptor subtypes involved in the central inhibition of urinary bladder motility.

Intracerebroventricular morphine consistently inhibited spontaneous urinary bladder contractions recorded from the anesthetized rat. This effect was reversed by naloxone and appeared to be exerted within the forebrain. Neither dynorphin-(1-13) nor U-50, 488 (kappa-agonists) affected bladder motility. Ethylketocyclazocine inhibited contractions only at higher doses, possibly due to mu-receptor interactions. Bladder activity was consistently inhibited by the mu-agonists morphiceptin and [D-Ala2, MePhe4, Gly-(ol)5]enkephalin (DAGO) and by [D-Ala2, D-Leu5]enkephalin (DADLE, delta-agonist). DAGO was the most potent compound tested. These observations support the involvement of mu- and possibly delta-receptors in the centrally mediated inhibition of urinary bladder motility by opioids.

Central δ‐opioid receptor interactions and the inhibition of reflex urinary bladder contractions in the rat

1 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. 2 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. 3 Both the novel δ‐opioid receptor antagonist ICI 154,129 (200–600 μg) [N,N‐bisallyl‐Tyr‐Gly‐Gly‐ψ‐(CH2S)‐Phe‐Leu‐OH) and ICI 174,864 (1–3 μg) [N,N‐diallyl‐Tyr‐Aib‐Aib‐Phe‐Leu‐OH: Aib = α‐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 μ‐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 δ‐receptor antagonism. 4 Further studies of the agonistic effect of ICI 174,864 showed that it was insensitive to low doses of naloxone (2 μg, i.c.v. or i.t.) but could be abolished by higher (10–15 μg) doses of naloxone. These observations suggested that the agonistic effect of ICI 174,864 was not mediated by μ‐opioid receptor. 5 β‐Endorphin (0.2‐1.0 μg, i.c.v.) inhibited bladder contractions but following recovery from this effect, appeared to prevent the expression of δ‐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 β‐endorphin under these circumstances. 6 These observations suggest that supra‐spinal and spinal δ‐opioid receptors are involved in the opioid‐mediated inhibition of reflex bladder contractions in the rat. Moreover β‐endorphin may be important in regulating central δ‐opioid receptors.

Selective δ-opioid receptor antagonism by ICI 174,864 in the central nervous system

The effects of the novel gamma-opioid receptor antagonist ICI 174,864 (N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH: Aib = alpha-aminoisobutyric acid) have been examined in the CNS in vivo using spontaneous reflex contractions of the rat urinary bladder as an index of activity. Bladder contractions were inhibited by equipotent intracerebroventricular (ICV) doses of the selective mu-agonist DAGO [D-Ala2, MePhe4,Gly-(ol)5]enkephalin and the delta-agonist DPDPE[D-Pen2, D-Pen5]enkephalin. ICI 174,864 (1-3 micrograms) administered by the same route produce a selective and reversible antagonism of DPDPE effects. At higher doses (6-15 micrograms, ICV) ICI 174,864 exhibited marked agonistic activity, producing inhibition of bladder contractions that were resistant to ICV naloxone (1-2 micrograms). Thus ICI 174,864 was considered a selective central delta-opioid receptor antagonist but its usefulness was limited by additional agonistic properties.

The proenkephalin a fragment, peptide E: Central processing and CNS activity in vivo☆

The proenkephalin A derivative, peptide E, delayed gastrointestinal transit in mice and inhibited the micturition reflex in anesthetized rats after intracerebroventricular (i.c.v.) administration. BAM22P, BAM12P and [Met5]enkephalin, possible processing fragments of peptide E, were also compared in the two test systems. Of these peptides, peptide E and BAM 22P were found to have the greatest potency and activity. Studies in vitro of peptide E metabolism by enzyme homogenates of mouse brain using HPLC techniques revealed that peptide E is bound to the membrane homogenate avidly for an extended period of time. The total formation of BAM22P, BAM12P, [Met5]enkephalin and all other peptide fragments during a 40 min incubation period accounted for only 8% of the total peptide E added to the homogenates. Thus, peptide E, rather than one of its known metabolites, appears to be of primary importance in the initiation of CNS-mediated effects. Further, these effects are probably the result of mu-opioid receptor activation.

Vascular effects of substance P change synaptic responsiveness of cat dorsal horn neurons

Abstract The effects of systemically administered substance P were examined on the responses induced by noxious (radiant heat) and non-noxious (air jets) peripheral stimuli in dorsal horn neurons of the feline spinal cord. Substance P produced a significant fall in arterial blood pressure and selectively enhanced responses to noxious heat stimulation. Other vasoactive substances administered systemically caused either selective increases or decreases in noxious heat induced responses which coincided with decreases or increases in systemic arterial blood pressure respectively. It was concluded that the selective neuronal effects of substance P were secondary to changes in vascular perfusion in the area stimulated by the radiant heat source.

Naloxonazine and opioid-induced inhibition of reflex urinary bladder contractions.

Spontaneous volume-induced contractions of the urinary bladder were recorded isometrically in urethane-anesthetized rats. Contractions were inhibited by alternate submaximal but equieffective doses of the selective mu and delta-opioid ligands [D-Ala2-Me-Phe4,Gly(ol)5] enkephaline (DAGO) and [2-D-penicillamine, 5-D-penicillamine] enkephalin (DPDPE), respectively, administered by the intracerebroventricular (i.c.v.) or spinal intrathecal (i.t.) route. Naloxonazine, postulated to be an irreversible mu 1-opioid receptor antagonist, administered by the same route, antagonized the effects of both DAGO and DPDPE. The antagonism of the effect of DAGO was reversed 3-4 hr later but that of DPDPE was more prolonged. Recovery of the effect of DPDPE was observed some 24 hr later. A similar pattern of activity against DAGO and DPDPE given intraventricularly or intrathecally was observed following intravenous injection of naloxonazine (10 mg/kg). Also naloxonazine (i.c.v., i.t. or i.v.) antagonized the effect of morphine given intraventricularly or intrathecally, but antagonism was not observed when morphine was retested 3-4 hr and 24 hr later. Naloxonazine increased the frequency of contraction of the bladder after each route of administration. This effect lasted 1-3 hr and was not seen 24 hr later. Systemic administration of naloxone (10 mg/kg, i.v) also increased the frequency of bladder contraction and attenuated or abolished the effect of DAGO given intraventricularly or intrathecally and the delta-receptor agonist [2-D-penicillamine, 5-L-penicillamine] enkephaline (DPLPE).(ABSTRACT TRUNCATED AT 250 WORDS)

Neuropeptide Y depresses reflex urinary bladder contractions in rats and modifies central activity of opioid agonists

The 36 amino acid peptide neuropeptide Y (NPY) has been found distributed in central structures associated with nociception and the actions of opioid analgesics. We therefore studied its central actions on reflex bladder contractions which we have shown to be inhibited by supraspinal and spinal opioid administrations in urethane anesthetized rats. Neuropeptide Y produced a dose related (0.5-2 micrograms per rat) inhibition of bladder contractions following intracerebroventricular (ICV) and spinal intrathecal (IT) administrations. These effects could not be antagonized by naloxone (2 micrograms, ICV or IT) or by ICI 174,864 [N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH: Aib = alpha-aminoisobutyric acid] (3 micrograms, ICV or IT). NPY (0.5-1 micrograms) reduced the ICV and IT effects of morphine but potentiated the action of the selective delta-receptor ligand [2-D-penicillamine, 5-L-penicillamine] enkephalin (DPLPE). The effect of the mu-selective opioid ligand [D-Ala2, Me-Phe4, Gly(ol)5] enkephalin (DAGO) were unaffected as were the submaximal ICV and IT actions of noradrenaline. It was concluded that NPY-induced inhibition of bladder activity was not due to a direct opioid receptor interaction. However since NPY consistently changed the activity of opioids (morphine and DPLPE), this suggested a possible physiological role in the regulation of opioid receptors, central neural excitability and thereby visceral activity.

Meptazinol: Unusual in vivo opioid receptor activity at supraspinal and spinal sites

Systemic (1-10 mg/kg, s.c.), intracerebroventricular (i.c.v. 20-80 micrograms) and spinal intrathecal (i.t., 5-20 micrograms) administration of meptazinol hydrochloride produced dose-related inhibition of reflex contractions of the urinary bladder, recorded isometrically in urethane-anesthetized rats. The effects of meptazinol were reversed by naloxone administered by the same route. Indeed, this was achieved with intracerebroventricular or intrathecal administration of naloxone (2 micrograms), which also selectively antagonized the mu-receptor ligand [D-Ala2, MePhe4, Gly(ol)5]enkephalin (DAGO). However ICI 174,864 (3 micrograms, i.c.v. or i.t.), a delta-opioid receptor antagonist, did not affect the actions of meptazinol given intracerebroventricularly or intrathecally though it consistently abolished the equieffective actions of a selective delta-receptor ligand (2-D-penicillamine, 5-L-penicillamine) enkephalin (DPLPE). Naloxonazine (5 micrograms, i.c.v. or i.t.), an irreversible mu 1-opioid receptor antagonist, produced prolonged antagonism of the effects of DPLPE and meptazinol. The effects of DPLPE partially or completely recovered by 24 hr, indicating that naloxonazine produced prolonged antagonism of delta-opioid receptors. The effects of maptazinol however only recovered after 72 hr, suggesting that antagonism by naloxonazine of this ligand was irreversible and was mediated through a unique opioid receptor interaction. Subthreshold doses of meptazinol (10 micrograms, i.c.v.; 3 micrograms, i.t.) consistently antagonized the effects of morphine given intracerebroventricularly or intrathecally but not the equieffective doses of DPLPE or DAGO. These observations suggest that meptazinol inhibited reflex contractions of the bladder by supraspinal and spinal mu-opioid receptor activation. Furthermore, its agonistic effect and its antagonistic actions were compatible with interactions at a subpopulation of opioid receptors, possibly mu 1-receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Prolonged invivo antagonism of central Mu- and delta-opioid receptor activity by β-funaltrexamine

Abstract β-Funaltrexamine (β-FNA) was tested in the spinal cord and supraspinally against inhibition of reflex bladder contractions produced in the anesthetized rat by the opioid-receptor selective agonists [D-Ala2, MePhe4, Gly (ol)5] enkephalin (DAGO, μ-agonnst) and [D-Pen2, D-Pen5] enkephalin (DPDPE, δ-agonist). All agents were microinjected either intracerebroventricularly (i.c.v.) or intrathecally (i.t.). β-FNA (1–8 μg) produced long-lasting antagonism of both DAGO and DPDPE. Complete recovery from its effects was only observed some 24–32 h later. Higher doses of β-FNA (4 and 8 μg i.t.) produced short-lived agonistic activity though the selectivity of this was not determined. It was concluded that β-FNA was a potent, long-lasting antagonist at central opioid receptors in vivo but was unselective for the μ and δ opioid receptor.

Endorphins and the central inhibition of urinary bladder motility

The involvement of endogenous opioid mechanisms in the central neurogenic control of urinary bladder function has been examined in anesthetized rats. Intracerebroventricular (ICV) microinjections of beta-endorphin (0.5-2.0 micrograms) produced powerful inhibition of rhythmic bladder contractions initiated by central reflex activity. The peptide fragments gamma-endorphin and alpha-endorphin (4-16 micrograms), formed by the processing of beta-endorphin by membrane homogenates of brain, were less active than the parent compound. The inhibitory effects of beta-endorphin was reversed by ICV naloxone (1-2 micrograms) but higher doses were required to reverse gamma- or alpha-endorphin effects. ICV naloxone administered alone increased intravesicular pressure and bladder contraction frequency. These observations support the hypothesis that the endorphins have a physiological role in the central regulation of urinary bladder activity.