Opiate Antinociception Research Articles

Mu Opioid Receptor Modulation of Dopamine Neurons in the Periaqueductal Gray/Dorsal Raphe: A Role in Regulation of Pain.

The periaqueductal gray (PAG) is a brain region involved in nociception modulation, and an important relay center for the descending nociceptive pathway through the rostral ventral lateral medulla. Given the dense expression of mu opioid receptors and the role of dopamine in pain, the recently characterized dopamine neurons in the ventral PAG (vPAG)/dorsal raphe (DR) region are a potentially critical site for the antinociceptive actions of opioids. The objectives of this study were to (1) evaluate synaptic modulation of the vPAG/DR dopamine neurons by mu opioid receptors and to (2) dissect the anatomy and neurochemistry of these neurons, in order to assess the downstream loci and functions of their activation. Using a mouse line that expresses eGFP under control of the tyrosine hydroxylase (TH) promoter, we found that mu opioid receptor activation led to a decrease in inhibitory inputs onto the vPAG/DR dopamine neurons. Furthermore, combining immunohistochemistry, optogenetics, electrophysiology, and fast-scan cyclic voltammetry in a TH-cre mouse line, we demonstrated that these neurons also express the vesicular glutamate type 2 transporter and co-release dopamine and glutamate in a major downstream projection structure-the bed nucleus of the stria terminalis. Finally, activation of TH-positive neurons in the vPAG/DR using Gq designer receptors exclusively activated by designer drugs displayed a supraspinal, but not spinal, antinociceptive effect. These results indicate that vPAG/DR dopamine neurons likely play a key role in opiate antinociception, potentially via the activation of downstream structures through dopamine and glutamate release.

P.4.001 Influence of single and chronic intranasal injections of the arginine-vasopressin fragment Ac-D-MPRG on the ontogeny of the behavioural reactions in white rats

KAVALIERS, M., E. CHOLERIS AND D. M. SAUCIER.The NMDA receptor antagonist, NPC 12626, reduces the pronociceptive effects of orphanin FQ, and kappa opiate antinociception in the land snail, Cepaea nemoralis. PEPTIDES 18(7) 943–947, 1997.—The heptadecapeptide, orphanin FQ or nociceptin (Phe-Gly-Phe-Thr-Gly-Ala-Arg-Lys-Ser-Ala-Arg-Lys-Leu-Ala-Asn-Gln), originally isolated from rat brain has been identified as an endogenous ligand for the orphan opioid-like receptor. Although orphanin FQ shares some sequence and structural homology with kappa-opioid peptides, it has been speculated to exert its effects through novel nonopioid mechanisms. Kappa opioids have also been suggested to have nonopioid actions in rodents involving the N-methyl-D-aspartate (NMDA) receptor. The present study examined the effects of the competitive NMDA antagonist, NPC 12626, on the antinociceptive effects of the specific kappa-opiate receptor agonist, U69,593, and the pronociceptive effects of orphanin FQ in an invertebrate system, the land snail, Cepaea nemoralis. NPC 12626 had no effect on the basal nociceptive sensitivity of snails, as measured by the latency of response to a thermal (40°C) surface. As reported for rodents, NPC 12626 dose-dependently reduced U69,593-induced antinociception in a manner comparable to that produced by the specific kappa-opiate antagonist, nor-binaltorphimine, while slightly enhancing the antinociceptive effects of the predominately mu-opiate agonist, morphine. Similarly, NPC 12626 dose-dependently reduced the pronociceptive effects of orphanin FQ. These findings with the snail, Cepaea, indicate that NMDA systems/receptors are associated with the mediation of the nociceptive effects of both kappa opioids and orphanin FQ. They suggest an early evolutionary development and phylogenetic continuity of NMDA opioid and related neuropeptide interactions in the mediation of nociception.

Dopamine Receptors are Involved in the Control of Opioid Antinociception in Mice

The present investigation was focused on the effects induced by haloperidol and apomorphine on the antinociception induced by DAMGO (highly selective μ-agonist), U50-488H (highly selective k-agonist) and deltorphin II (highly selective δ-agonists). Haloperidol (a dopamine receptor antagonist) and apomorphine (a dopamine receptor agonist) (0.1-1.0-2.0 mg/kg/i.p.) per se did not change the pain threshold of mice both in the hot plate and in the tail flick test. The antinociception effects of DAMGO (5 mg/kg/i.p.), U50-488H (5 mg/kg/i.p.) and deltorphin II (10 ng/i.c.v./mouse) peaked by 15 min after treatment and was antagonized by haloperidol in a dose dependent manner. By contrast, the dopamine receptor agonist, apomorphine, depending on the doses used, exhibited opposite effects on opiate antinociception. Treatment of mice with low doses of apomorphine (0.1-1.0 mg/kg/i.p.) reduces the antinociception induced by μ, k and δ receptor agonists whereas a high dose (2 mg/kg/i.p.) potentiates it. Our results indicate that dopamine receptors are involved in the control of antinociception at both the μ, k and δ receptor level.

Opiate anti-nociception is attenuated following lesion of large dopamine neurons of the periaqueductal grey: critical role for D 1 (not D 2) dopamine receptors

The periaqueductal grey (PAG) area is involved in pain modulation as well as in opiate-induced anti-nociceptive effects. The PAG possess dopamine neurons, and it is likely that this dopaminergic network participates in anti-nociception. The objective was to further study the morphology of the PAG dopaminergic network, along with its role in nociception and opiate-induced analgesia in rats, following either dopamine depletion with the toxin 6-hydroxydopamine or local injection of dopaminergic antagonists. Nociceptive responses were studied through the tail-immersion (spinal reflex) and the hot-plate tests (integrated supraspinal response), establishing a cut-off time to further minimize animal suffering. Heroin and morphine were employed as opiates. Histological data indicated that the dopaminergic network of the PAG is composed of two types of neurons: small rounded cells, and large multipolar neurons. Following dopamine depletion of the PAG, large neurons (not small ones) were selectively affected by the toxin (61.9% dopamine cell loss, 80.7% reduction of in vitro dopaminergic peak), and opiate-induced analgesia in the hot-plate test (not the tail-immersion test) was reliably attenuated in lesioned rats ( P<0.01). After infusions of dopaminergic ligands into the PAG, D 1 (not D 2) receptor antagonism attenuated opiate-induced analgesia in a dose-dependent manner in the hot-plate test. The present study provides evidence that large neurons of the dopaminergic network of the PAG participate in supraspinal (not spinal) nociceptive responses after opiates through the involvement of D 1 dopamine receptors. This dopaminergic system should be included as another network within the PAG involved in opiate-induced anti-nociception.

Long-term alterations in opiate antinociception resulting from infant fentanyl tolerance and dependence

Postnatal day-14 (P14) infant rats remained naive or were implanted with osmotic minipumps infusing saline or fentanyl (50 μg kg −1 h −1). Fentanyl was administered 72 h later for measurement of antinociception in the tail-flick test. The potency of fentanyl was 3.0-fold lower in fentanyl-infused compared to saline-infused P17 rats. Fentanyl-infused P17 rats injected with naloxone underwent withdrawal characterized by increases in spontaneous activity, wall climbing, diarrhea, abdominal stretching, forepaw treading/tremors, wet-dog shakes, jumping, ptosis, rhinorrhea and hypothermia. Other naive, saline-infused and fentanyl-infused P17 rats not challenged with fentanyl or naloxone were housed until maturing into P42 juveniles. Fentanyl's potency was equal among each treatment group. However, morphine's potency was reduced in juveniles tolerant to fentanyl as infants. Morphine was also less potent in P90 adults tolerant to fentanyl as infants. Thus, chronic opiate exposure during infancy may affect the developing central nervous system, and desensitize animals and humans to opiate analgesia throughout life.

Morphine and Dextrorphan Lose Antinociceptive Activity but Exhibit an Antispastic Action in Chronic Spinal Rats

Within 3–4 weeks after spinal transection, morphine-induced antinociception, assessed with the tail flick reflex in rats, is profoundly reduced. The cause of this decrement is unknown. The present studies were conducted to determine whether this phenomenon reflects a general loss in opiate activity or a selective decline in opiate antinociception. This was accomplished by assessing the effect of morphine on two different responses, the tail flick reflex and the hindlimb spasticity that develops in chronic spinal rats. Because excitatory amino acid antagonists are also antinociceptive in acute spinal rats, the effect of one such drug, dextrorphan, on these two behaviors was also evaluated in chronic spinal animals. The antinociceptive and antispastic effect of subcutaneous (6 mg/kg) and intrathecal (5 μg) morphine injections were assessed in intact and chronic (21–28 days) spinal rats, whereas the effect of subcutaneous (25 and 40 mg/kg) and intrathecal (350 μg) dextrorphan was assessed in acute (1 day) and chronic spinal rats. The antinociceptive effect of both drugs was significantly reduced in chronic spinal animals, relative to saline controls. However, each drug treatment produced a significant antispastic effect in the same animals, indicating a selective decline in opiate action. This outcome also suggests that excitatory amino acid antagonists may be useful as adjunct antispastic agents.

The NMDA Receptor Antagonist, NPC 12626, Reduces the Pronociceptive Effects of Orphanin FQ and Kappa Opiate Antinociception in the Land Snail, Cepaea nemoralis

KAVALIERS, M., E. CHOLERIS AND D. M. SAUCIER.The NMDA receptor antagonist, NPC 12626, reduces the pronociceptive effects of orphanin FQ, and kappa opiate antinociception in the land snail, Cepaea nemoralis. PEPTIDES 18(7) 943–947, 1997.—The heptadecapeptide, orphanin FQ or nociceptin (Phe-Gly-Phe-Thr-Gly-Ala-Arg-Lys-Ser-Ala-Arg-Lys-Leu-Ala-Asn-Gln), originally isolated from rat brain has been identified as an endogenous ligand for the orphan opioid-like receptor. Although orphanin FQ shares some sequence and structural homology with kappa-opioid peptides, it has been speculated to exert its effects through novel nonopioid mechanisms. Kappa opioids have also been suggested to have nonopioid actions in rodents involving the N-methyl-D-aspartate (NMDA) receptor. The present study examined the effects of the competitive NMDA antagonist, NPC 12626, on the antinociceptive effects of the specific kappa-opiate receptor agonist, U69,593, and the pronociceptive effects of orphanin FQ in an invertebrate system, the land snail, Cepaea nemoralis. NPC 12626 had no effect on the basal nociceptive sensitivity of snails, as measured by the latency of response to a thermal (40°C) surface. As reported for rodents, NPC 12626 dose-dependently reduced U69,593-induced antinociception in a manner comparable to that produced by the specific kappa-opiate antagonist, nor-binaltorphimine, while slightly enhancing the antinociceptive effects of the predominately mu-opiate agonist, morphine. Similarly, NPC 12626 dose-dependently reduced the pronociceptive effects of orphanin FQ. These findings with the snail, Cepaea, indicate that NMDA systems/receptors are associated with the mediation of the nociceptive effects of both kappa opioids and orphanin FQ. They suggest an early evolutionary development and phylogenetic continuity of NMDA opioid and related neuropeptide interactions in the mediation of nociception.

The specific [formula omitted] (NMDA) receptor antagonist MK-801 blocks U-50,488, but not morphine antinociception

The effect of the specific NMDA receptor antagonist MK-801 on antinociception produced by the κ opiate receptor agonist U-50,488 and the μ receptor agonist morphine was assessed using the tail-flick test in rats. MK-801 (0.05 and 0.1 mg/kg) antagonized antinociception induced by all three doses of U-50,488 (7.5, 15 and 30 mg/kg), and potentiated antinocicepgion induced by the lower (1 mg/kg) but not higher (5 mg/kg) dose of morphine. Naloxone at a dose of 1.0 but not 0.1 mg/kg blocked U-50,488 antinociception, indicating that MK-801 affects opiate antinociception. The present results are the first to suggest a critical role for the NMDA receptor in opiate antinociception involving the κ receptor.

Environmental modulation of behavioral tolerance in spinal rats

We previously reported that the antinociceptive effect of morphine on the thermal tail withdrawal reflex (tail flick, TF) was reduced in rats that had practiced the TF response prior to morphine administration. This phenomenon, termed behavioral tolerance, was observed even when rats were spinally transected following the TF tests. The present experiments were conducted to determine whether such retention of behavioral tolerance after spinal transection was dependent on prior experience with the TF procedure, or whether other nociceptive pretreatments would also reduce opiate antinociception on the TF test in spinal rats. Separate groups of rats were pretreated with either nociceptive thermal TF or Hot Plate (HP), or mechanical Tail Pinch (TP) stimuli, as well as non-nociceptive exposure to the experimental context (Habituation) or No Pretreatment. Approximately half of the rats were spinally transected after their respective pretreatment, while the other half were left intact. When Intact rats were subsequently tested on the TF at 30, 60, 120 and 180 min after morphine administration, only the TF pretreated group was tolerant, relative to the Non-pretreated control group. However, this was a transient effect, and was only significant at the first, 30-min test; there was no difference among the groups in the total duration of antinociception across all the time points. When spinal rats were tested after morphine administration, there was also an overall significant difference among the groups at 30 min. In this case, the differences were maintained across all time points. All three nociceptive pretreatments (TF, HP and TP) significantly reduced the total duration of opiate antinociception in acute spinal rats, relative to the Non-pretreatment condition, whereas the Habituation treatment did not. These data reveal a differential effect of behavioral pretreatment on morphine-induced antinociception in Intact vs. Spinal rats. In Intact rats, behavioral tolerance on the TF test was transient and only observed in animals that had prior experience with the TF procedure. In contrast, when rats were spinally transected after nociceptive pretreatment, behavioral tolerance on the TF test was maintained throughout the test session and was observed in all of the nociceptive pretreatment groups. These data indicate first that nociceptive stimulation can produce a persistent change in the spinal cord which significantly influences spinal reflex function and second that when the neuraxis is intact, the effect of such stimulation may be modulated by descending supraspinal input.

Differential effects of morphine on histamine metabolism in brain and spinal cord of mice

The effects of morphine on the levels of histamine (HA), its metabolite tele-methylhistamine (t-MH) and on t-MH synthesis rates (thought to be indicative of neuronal HA release) were investigated in brain regions and spinal cords of DBA/2J (DBA) and C57/BL6 (C57) mice, two strains known to differ in their sensitivity to morphine. In DBA (a strain highly sensitive to morphine antinociception), morphine (10 mg/kg, s.c.) had no effect on brain regional t-MH or HA levels, but produced a generalized inhibition of regional t-MH synthesis rates ranging from 11 to 53%. The monoamine oxidase (MAO) inhibitor pargyline (used to estimate t-MH synthesis rates) had no effect on HA or t-MH levels in the DBA or C57 spinal cord, indicating the absence of detectable spinal HA turnover. Morphine (10 mg/kg) had no effect on DBA or C57 spinal cord HA or t-MH levels, but significantly increased t-MH synthesis in the DBA but not in the C57 spinal cord. These results suggest that in DBA mice, antinociceptive doses of morphine inhibit HA release in brain, and promote the release of HA from spinal cord. Neither effect was found in C57 mice, which are resistant to morphine antinociception. The relevance of these actions to previous studies showing the blockade of opiate antinociception by H2 antagonists remains to be established.

Differential actions of central alloxan upon opioid and nonopioid antinociception in rats.

Spontaneous or induced diabetes, as well as glucose loading, reduce opiate antinociception, presumably through induction of hyperglycemia. While peripheral administration of alloxan is a potent pancreatic beta-cell toxin, intracerebroventricular (ICV) alloxan reduces glucoprivic feeding in the absence of hyperglycemia, presumably through interactions with specific brain glucoreceptors. Our laboratory demonstrated that opioid-mediated 2-deoxy-D-glucose (2DG) antinociception is significantly reduced by central pretreatment with alloxan, and that this deficit is reversed by coadministration with 3M-D-glucose. The present study compared ICV and intravenous (IV) routes of alloxan (200 micrograms) upon morphine (1-10 mg/kg, SC) analgesia on the tail-flick and jump tests in rats, and evaluated these effects in terms of concomitant changes induced by ICV alloxan upon nonopioid-mediated continuous cold-water swim (CCWS: 2 degrees C for 3.5 min) antinociception. Two weeks following central, but not peripheral pretreatment with alloxan, morphine (2.5 and 5.0 mg/kg, SC) antinociception was markedly (30-56%) reduced on both nociceptive tests. In contrast, central pretreatment with alloxan respectively reduced (30 min) and subsequently potentiated (60 and 90 min) CCWS antinociception on the jump test. Alterations in antinociception by central alloxan occurred in the absence of changes in basal nociceptive thresholds, hypothermia or hyperglycemia. These data suggest that central alloxan may be acting upon either specific, but unidentified brain glucoreceptors and/or a glucoprivic control mechanism.

Opioid peptides (DAGO-enkephalin, dynorphin A(1–13), BAM 22P) microinjected into the rat brainstem: comparison of their antinociceptive effect and their effect on neuronal firing in the rostral ventromedial medulla

The highly μ-selective agonist Tyr- d-Ala-Gly-MePhe-Gly-ol-enkephalin (DAGO) produces potent, dose-dependent naloxone-reversible antinociception when microinjected into the ventrolateral periaqueductal gray (PAG) (ED50 = 0.72 nmol) or rostral ventromedial medulla (RVM) (ED50 = 0.50 nmol) as measured on the ratr tail flick (TF) assay. In single-unit recording experiments, DAGO microinjected into the PAG also affected On- and Off-Cell firing in the RVM in the same way as previously demonstrated by our group for morphine. PAG-microinjected DAGO inhibits spontaneous and noxious-evoked On-Cell firing (attenuating the characteristic On-Cell burst) ( n=19), and excites spontaneous Off-Cell firing, preventing the characteristic Off-Cell pause ( n=12) at doses which suppress the TF. These results support a major role for the μ receptor in PAG and RVM mechanisms of opiate antinociception. In our experiments using BAM22P, an endogenous weakly μ-selective opioid peptide, we could not demonstrate a dose-dependent antinociceptive effect, whether the peptide was microinjected supraspinally into the PAG ( n=9) or RVM ( n=11), or intrathecally at the lumbar cord ( n=4). In two animals, a naloxone-reversible antinociceptive effect was observed following the microinjection of 10 nmol BAM 22P into the RVM; however, no effect was seen in 3 animals microinjected with 20 nmol. Dyn A(1–13), a putative endogenous ligand for the κ receptor, had no antinociceptive effect when microinjected into the ventrolateral PAG, and no effect on the firing (spontaneous or noxious-evoked) of RVM On ( n=3)- or Off ( n=2)-Cells.

Opiate antinociception is altered by immunemodification: the effect of inteferon, cyclosporine and radiation-induced immune suppression upon acute and long-term morphine activity

It has recently been demonstrated that various forms of immune modification result in a profound attenuation of the opiate withdrawal syndrome. Herein we investigate the extent to which some of the immune modifiers active in withdrawal attenuation affect other opiate related behaviors, namely antinociception and the development of tolerance to this effect. The observations demonstrate that immune modification by cyclosporine and irradiation exposure result in an alteration of the acute antinociceptive effect of morphine; while none of these treatments modify the development of tolerance to this property of morphine.

Role of spinal cord neuropeptides in pain sensitivity and analgesia: Thyrotropin releasing hormone and vasopressin

The existence of a wide variety of neuropeptides within the spinal cord dorsal horn raises the question of their possible roles in sensory processing. The present series of behavioral experiments examined the effects of intrathecal (IT) administration of two such neuropeptides, thyrotropin-releasing hormone (THR) and vasopressin (VAS), on pain sensitivity and antinociception. TRH exerted no marked effect on basal pain sensitivity over the dose range examined (0.25 ng–2.5 μg). However, a U-shaped dose-response effect on morphine antinociception (3 μg, IT) was observed, wherein potent attenuation, moderate attenuation, or enhancement of morphine-induced anticiception was observed following the various doses tested. In contrast, VAS produced non-opiate antiniception at the highest doses tested (25 ng and 250 ng) and none of the VAS doses (0.25ng–250 ng) appeared to interact with IT morphine (3 μg) antinociception. Lastly, IT TRH was not observed to interact with IT VAS antinociception. These data provide evidence that these neuropeptides exert strikingly different effects on pain sensitivity and opiate antinociception, and provide initial evidence that TRH may be included in the growing list of neuropeptides that can act endogenous opiate antagonists within the central nervous system.

Potentiation of morphine analgesia by subanesthetic doses of pentobarbital

Pentobarbital pretreatment reportedly either inhibits, enhances or has no effect on morphine analgesia. The effect of subanesthetic doses of sodium pentobarbital (8–12 mg kg −1, SC) delivered via a delivery system on analgesia of morphine (5 mg kg −1, SC or 1 mg kg −1, IV) acutely administered 45 min after the sodium pentobarbital pellet implantation was assessed using the warm water (55°C)-induced tail-withdrawal reflex in male Wistar rats. Significant potentiation of morphine analgesia was observed in sodium pentobarbital as compared to the placebo-pelleted animals. Pharmacokinetic or dispositional factors were not involved in this potentiation, which was possibly due to the activation of the descending inhibitory control pathways of nociceptive spinal tail-withdrawal reflex by a combined interaction of two drugs at spinal and supraspinal sites of action, that mediate opiate antinociception.

Effects of some GABA-mimetic drugs on the antinociceptive activity of morphine and beta-endorphin in rats.

Intracerebroventricular administration of muscimol, a potent GABA-receptor agonist, counteracted the antinociceptive effect of morphine or beta-endorphin in rats as measured by the "tail flick" method. Muscimol's activity was reversed by bicuculline. Isoguvacine, another GABA agonist, as well as nipecotic acid and guvacine, two inhibitors of neuronal and glial uptake of GABA, also antagonized morphine's antinociceptive effect. A role of the central GABA-ergic system in mediating opiate antinociception is proposed.