Reduction Of Morphine Analgesia Research Articles

Neuron-restrictive silencer factor-mediated downregulation of μ-opioid receptor contributes to the reduced morphine analgesia in bone cancer pain.

Bone cancer pain has been reported to have unique mechanisms and is resistant to morphine treatment. Recent studies have indicated that neuron-restrictive silencer factor (NRSF) plays a crucial role in modulating the expression of the μ-opioid receptor (MOR) gene. The present study elucidates the regulatory mechanisms of MOR and its ability to affect bone cancer pain. Using a sarcoma-inoculated murine model, pain behaviors that represent continuous or breakthrough pain were evaluated. Expression of NRSF in the dorsal root ganglion (DRG) and spinal dorsal horn was quantified at the transcriptional and translational levels, respectively. Additionally, chromatin immunoprecipitation assays were used to detect NRSF binding to the promoter of MOR. Furthermore, NRSF was genetically knocked out by antisense oligodeoxynucleotide, and the expression of MOR and the effect of morphine were subsequently analyzed. Our results indicated that in a sarcoma murine model, NRSF expression is upregulated in dorsal root ganglion neurons, and the expression of NRSF mRNA is significantly negatively correlated with MOR mRNA expression. Additionally, chromatin immunoprecipitation analysis revealed that NRSF binding to the neuron-restrictive silencer element within the promoter area of the MOR gene is promoted with a hypoacetylation state of histone H3 and H4. Furthermore, genetically knocking down NRSF with antisense oligodeoxynucleotide rescued the expression of MOR and potentiated the systemic morphine analgesia. The present results suggest that in sarcoma-induced bone cancer pain, NRSF-induced downregulation of MOR is involved in the reduction of morphine analgesia. Epigenetically, up-regulation of MOR could substantially improve the effect of system delivery of morphine.

Reversal of morphine tolerance by a compound with NPFF receptor subtype-selective actions

Neuropeptide FF (NPFF) modulates opiate actions. It has pro-nociceptive effects, primarily through the NPFF receptor 1 subtype, and anti-nociceptive effects, primarily through the NPFFR2 subtype. AC-263093 is a small l, organic, systemically active molecule that was previously shown to functionally activate NPFFR2, but not NPFFR1. It was hypothesized that AC-263093 would attenuate morphine tolerance. Rats were tested for radiant heat tail-flick latency before and after 5mg/kg morphine sulfate s.c. They were then rendered morphine-tolerant by continuous subcutaneous infusion of 17.52mg/kg/day morphine sulfate. On the seventh day of infusion, they were retested for analgesia 10 and 20min after 5mg/kg morphine sulfate s.c. Tolerance was indicated by reduction of morphine analgesia from the pre-infusion test. Fifty minutes prior to morphine challenge, rats received either 10mg/kg i.p. AC-263093 or injection vehicle alone. AC-2623093-treated rats had far smaller tolerance scores than control rats. This drug effect was significant, p=0.015. The same dose of AC-263093 had almost no analgesic effect in non-tolerant, saline-infused rats. In vitro experiments revealed that AC-263093 had equal affinity for NPFFR1 and NPFFR2, and functionally inactivated NPFFR1, in addition to its previously shown ability to activate NPFFR2. Thus, altering the balance between activation of NPFF receptor subtypes may provide one approach to reversing opiate tolerance.

Inhibition of morphine analgesia by lithium: role of peripheral and central opioid receptors

Intraperitoneal (i.p.) administration of lithium chloride (LiCl) has two effects on pain sensation: (1) it induces a transient hyperalgesia that is reversed by intracerebroventricular (i.c.v.) or intrathecal (i.t.) administration of the opioid receptor antagonist naloxone or by peripheral administration of the quaternary compound naloxone methiodide [Behav. Neurosci. 114 (2000) 1183]; (2) it produces a long-lasting (24 h) reduction in morphine analgesia and does so in the absence of hyperalgesia [Behav. Brain Res. 142 (2003) 89]. We confirmed that rats administered with LiCl showed a reduction in analgesia when administered morphine 24 h later. We also found that morphine analgesia was restored if LiCl had been preceded by i.p. or i.c.v. administration of naloxone or by i.p. administration of naloxone methiodide. However, i.p. administration of naloxone methiodide prior to testing 24 h after an injection with LiCl did not restore morphine analgesia. Thus, activity at peripheral and central opioid receptors is necessary for the inhibition of morphine analgesia by LiCl, but peripheral opioid receptors are not critical for the expression of this inhibition.

Reversal of sex differences in morphine analgesia elicited from the ventrolateral periaqueductal gray in rats by neonatal hormone manipulations

Male rats display significantly greater analgesic responses than female rats following systemic, ventricular and intracerebral morphine administration into either the ventrolateral periaqueductal gray (vlPAG) or the rostral ventromedial medulla, and following beta-endorphin administration into the vlPAG. Although adult gonadectomy severely reduces nonopioid forms of swim stress-induced analgesia, the marked sex differences in morphine analgesia were minimally affected by either male or female adult gonadectomy. Since very little is known about neonatal effects of gonadal hormones upon sex differences in morphine analgesia elicited from the vlPAG, the present study evaluated the effects of neonatal (within 1 day of birth) castration in male rat pups relative to sham-operated controls, and systemic androgenization with testosterone propionate in female rat pups relative to vehicle-injected controls upon baseline nociceptive thresholds and morphine analgesia elicited from the vlPAG in rats tested as adults. Significant sex differences in morphine analgesia elicited from the vlPAG were observed with adult males receiving neonatal sham surgeries displaying significantly greater morphine analgesia on two nociceptive measures than adult females tested during the estrous phase and receiving neonatal vehicle injections. Neonatal gonadectomy essentially reversed the pattern of sex difference effects upon morphine analgesia elicited from the vlPAG. Neonatally-castrated male rats tested in adulthood displayed dramatic reductions in morphine analgesia elicited from the vlPAG on both the tail-flick (∼15-fold rightward shift) and jump (6-fold rightward shift) tests relative to sham-operated males, and essentially mirrored those of vehicle-treated females. Conversely, neonatally-androgenized female rats tested in adulthood displayed dramatic increases in morphine analgesia elicited from the vlPAG on the tail-flick (5-fold leftward shift) and jump (12-fold leftward shift) tests relative to vehicle-treated females, and approximated those observed in sham-operated males. The potent differences between neonatally-castrated and sham-operated male rats and between neonatally-androgenized and vehicle-treated female rats suggest a possible ‘organizational’ role of gonadal hormones in mediating sex differences in morphine analgesia elicited from the vlPAG.

Lack of tolerance in peripheral opioid analgesia in mice

We recently developed a sensitive peripheral analgesic test in mice. Bradykinin, a representative pain-producing substance, when given subcutaneously through a polyethylene tube into the plantar of the limb connected to a transducer, induced a flexor reflex response, in a dose dependent manner. When morphine, a μ-opioid receptor agonist, was added to the plantar through another polyethylene tube, bradykinin-induced responses were completely abolished in a naloxone-reversible manner. These peripheral analgesic effects were also observed with DAM GO, another μ-opioid receptor agonist, and U-69,593, a κ-opioid receptor agonist, but not DSLET, a δ-opioid receptor agonist. When morphine was given subcutaneously to the back, a potent analgesia in the tail pinch test was observed. Repeated administrations of morphine once per day for 5 days showed a marked tolerance or reduction in morphine analgesia on the 6th day, while there was no significant reduction in the peripheral analgesia of morphine. These findings suggest that tolerance to morphine analgesia is mediated through synaptic plasticity in the central nervous system, but not through a receptor desensitization at the level of the single cell.

Tolerance to morphine analgesia is reduced by the novel addition or omission of an alcohol cue.

A recent study demonstrated that ethanol tolerance was reduced by the presentation of a novel extraneous stimulus at the time of test. In Pavlovian terms, this phenomenon is known as external inhibition. The present study sought to determine whether a drug cue could act as an external inhibitor of tolerance. Theoretically, either the occurrence of an unexpected stimulus or the nonoccurrence of an expected one can operate to disrupt already established conditioned responses. This prediction was assessed in the present study by the novel presentation or the novel omission of a drug cue at test. Two groups of rats were made completely tolerant to the analgesic effects of morphine. During tolerance acquisition the groups were treated identically except that one group always received a dose of alcohol 15 min following morphine. At test, animals experienced either the novel introduction or the novel omission of the alcohol cue. Both manipulations led to a reduction of morphine analgesia. Beyond their theoretical importance, these results have clinical implications in view of the frequency of multiple concurrent drug abuse.

Antagonism of morphine analgesia by intracerebroventricular naloxonazine.

Intravenous pretreatment with naloxonazine, an irreversible and selective antagonist of mu-1 sites for over 24 hr, reduces analgesia induced by morphine as well as a series of opiates and enkephalins. The present study evaluated whether intracerebroventricular (ICV) administration of naloxonazine produces similar long-term (24 hr) reductions in morphine analgesia on the tail-flick and jump tests. Naloxonazine failed to alter baseline tail-flick latencies or jump thresholds, but antagonized in a dose-dependent manner morphine analgesia for 24 hr. Naloxone had no effect at 24 hr. Morphine actions in the jump test were quite sensitive to doses of naloxonazine as low as 1 microgram/rat. Although tail-flick assays also revealed naloxonazine effects, far greater doses (30 micrograms/rat) were needed. Naloxonazine also shifted full morphine dose-response curves to the right. Again, naloxonazine antagonized morphine in the jump test more effectively than in the tail-flick assay. These data provide support for the involvement of the mu-1 opioid binding site in the central mediation of morphine analgesia and point out the differing sensitivities of two analgesiometric assay systems to naloxonazine.

Age-related decrements in morphine analgesia: A parametric analysis

Although age-related reductions in levels of opiate receptors and endogenous opioid peptides have been observed in rats, effects of aging upon basal pain thresholds and morphine analgesia have not indicated clear results. The present study evaluated the dose-dependent (1, 2.5, 5 and 10 mg/kg, SC) and time-dependent (30, 60, 90, 180 min) properties of morphine analgesia on two nociceptive measures (tail-flick latencies and jump thresholds) across five age cohorts of rats (4, 9, 14, 19 and 24-months of age). To ascertain whether any changes were the result of specific alterations in pain inhibition or an overall shift in opiate responses, effects upon morphine-induced hyperthermia were also evaluated. Age-related effects upon morphine analgesia on the tail-flick test exhibited a biphasic pattern with the three older age groups displaying significant decreases in morphine analgesia 30 min after injection and significant increases in morphine analgesia 180 min after injection. Age-related effects upon morphine analgesia on the jump test revealed equivilent reductions in analgesic magnitude across doses for the three older cohorts. In contrast, morphine hyperthermia displayed small inconsistent changes across cohorts. The reductions in morphine analgesia in older animals could not be attributed to changes in either baseline pain thresholds or delayed peripheral absorption and/or clearance of the drug. Rather, the reductions in morphine analgesia in older animals complement the recent findings of similar age-related reductions in the analgesic responses induced by exposure to several environmental stressors.

Chronic administration of clomipramine inhibits morphine hot plate analgesia

Clomipramine, chronically administered in mice, for 3 days, inhibits partially but significantly morphine analgesia in the hot plate test, when used at dose of 10 mg/kg/day, i.p. ; 2.5 and 5 mg/kg/day were ineffective. Neither higher doses (20 and 40 mg/kg/day) nor longer duration of pretreatment (8 and 16 days) modified the intensity of this inhibition. Reduction in morphine analgesia was obtained after a 24h delay between the last injection of clomipramine and that of morphine (30 min before testing), while clomipramine did not induce any antinociceptive effect and clomipramine and desmethylclomipramine plasma and brain levels were low or undetectable. These results provide new evidence for the interaction between clomipramine and the endogenous opiate system. A pharmacokinetic interaction between clomipramin and morphine was excluded ; involvement of change in opiate and 5 HT 2 receptors by chronic administration of clomipramine is discussed.

INVOLVEMENT OF THE PERIAQUEDUCTAL GREY MATTER AND SPINAL 5‐HYDROXYTRYPTAMINERGIC PATHWAYS IN MORPHINE ANALGESIA: EFFECTS OF LESIONS AND 5‐HYDROXYTRYPTAMINE DEPLETION

1 Electrolytic lesions of the periaqueductal grey matter (PAG) were made in rats. The analgesia produced by intraperitoneal injection of morphine (10 and 20 mg/kg), tested by the tail flick and hot plate methods, was substantially reduced in the lesioned rats. Baseline pain thresholds were unaffected by the lesions.2 The lesion effects were not due to damage to the dorsal raphé nucleus. The extent of histologically determined damage to the dorsal raphé and the resulting decrease in striatal 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) concentrations did not correlate with the reduction in morphine analgesia produced by the lesion. Furthermore, microinjections of 5, 6-dihydroxytryptamine (5,6-DHT) into the dorsal raphé nucleus produced a similar fall in 5-HIAA levels but had no effect on morphine analgesia.3 Selective destruction of the periventricular catecholamine system produced by microinjection of 6-hydroxydopamine (6-OHDA) caused a slight decrease in morphine analgesia, thus raising the possibility that catecholamines may be involved in the action of morphine in the PAG.4 5,7-Dihydroxytryptamine-induced lesions of the spinal cord 5-hydroxytryptaminergic pathways reduced cord 5-HT concentration by 70% and markedly attenuated morphine analgesia as determined by the tail flick test.5 These experiments provide additional evidence that the PAG is a major site of action of opiates in producing analgesia. Furthermore, they have demonstrated the probable involvement of spinal 5-hydroxytryptaminergic pathways in the mediation of opiate analgesic effects.

Anatomical specificity as the critical determinant of the interrelationship between raphe lesions and morphine analgesia

Electrolytic lesions were produced in three separate parts of the raphe mesencephalic area: the nucleus raphe medianus, the nucleus raphe dorsalis and an area between these two nuclei. Seven days after the surgery, the animals were tested for morphine analgesia using the tail compression method and then sacrificed for the estimation of brain serotonin. It was found that the analgesic effect of morphine was significantly reduced in the rats lesioned in the nucleus raphe medianus but not in the animals lesioned either in the nucleus raphe dorsalis or in the ‘intermediate raphe area’. Since a decrease of forebrain serotonin was observed in each experimental group, the reduction of morphine analgesia does not involve a simple direct correlation with a decrease of serotonin in the forebrain. The results are discussed in view of the possibility that the reduction of morphine analgesia after lesions of the nucleus raphe medianus is due to a disruption of a specific brain serotonergic system.