In vivo evidence for multiple opiate receptors mediating analgesia in the rat spinal cord

Neuraxial drug delivery in pain management: An overview of past, present, and future

Activation of neuraxial nociceptive linkages leads to a high level of encoding of the message that is transmitted to the brain and that can initiate a pain state with its attendant emotive covariates. As we review here, the encoding of this message is subject to a profound regulation by pharmacological targeting of dorsal root ganglion and dorsal horn systems. Though first shown with the robust and selective modulation by spinal opiates, subsequent work has revealed the pharmacological and biological complexity of these neuraxial systems and points to several regulatory targets. Novel therapeutic delivery platforms, such as viral transfection, antisense and targeted neurotoxins, point to disease-modifying approaches that can selectively address the acute and chronic pain phenotype. Further developments are called for in delivery devices to enhance local distribution and to minimize concentration gradients, as frequently occurs with the poorly mixed intrathecal space. The field has advanced remarkably since the mid-1970s, but these advances must always address the issues of safety and tolerability of neuraxial therapy.

Interactive Mechanisms of Supraspinal Sites of Opioid Analgesic Action: A Festschrift to Dr. Gavril W. Pasternak

Almost a half century of research has elaborated the discoveries of the central mechanisms governing the analgesic responses of opiates, including their receptors, endogenous peptides, genes and their putative spinal and supraspinal sites of action. One of the central tenets of "gate-control theories of pain" was the activation of descending supraspinal sites by opiate drugs and opioid peptides thereby controlling further noxious input. This review in the Special Issue dedicated to the research of Dr. Gavril Pasternak indicates his contributions to the understanding of supraspinal mediation of opioid analgesic action within the context of the large body of work over this period. This review will examine (a) the relevant supraspinal sites mediating opioid analgesia, (b) the opioid receptor subtypes and opioid peptides involved, (c) supraspinal site analgesic interactions and their underlying neurophysiology, (d) molecular (particularly AS) tools identifying opioid receptor actions, and (e) relevant physiological variables affecting site-specific opioid analgesia. This review will build on classic initial studies, specify the contributions that Gavril Pasternak and his colleagues did in this specific area, and follow through with studies up to the present.

Dopamine and Addiction

Addiction is commonly identified with habitual nonmedical self-administration of drugs. It is usually defined by characteristics of intoxication or by characteristics of withdrawal symptoms. Such addictions can also be defined in terms of the brain mechanisms they activate; most addictive drugs cause elevations in extracellular levels of the neurotransmitter dopamine. Animals unable to synthesize or use dopamine lack the conditioned reflexes discussed by Pavlov or the appetitive behavior discussed by Craig; they have only unconditioned consummatory reflexes. Burst discharges (phasic firing) of dopamine-containing neurons are necessary to establish long-term memories associating predictive stimuli with rewards and punishers. Independent discharges of dopamine neurons (tonic or pacemaker firing) determine the motivation to respond to such cues. As a result of habitual intake of addictive drugs, dopamine receptors expressed in the brain are decreased, thereby reducing interest in activities not already stamped in by habitual rewards.

Preliminary Findings of Structure and Expression of Opioid Receptor Genes in a Peregrine Falcon ( Falco peregrinus), a Snowy Owl ( Bubo scandiacus), and a Blue-fronted Amazon Parrot ( Amazona aestiva)

To further knowledge of the physiology of opioid receptors in birds, the structure and expression of the μ-, δ-, and κ-opioid receptor genes were studied in a peregrine falcon ( Falco peregrinus), a snowy owl ( Bubo scandiacus), and a blue-fronted Amazon parrot ( Amazona aestiva). Tissue samples were obtained from birds that had been euthanatized for poor release prognosis or medical reasons. Samples were taken from the brain (telencephalon, thalamus, pituitary gland, cerebellum, pons, medulla oblongata, mesencephalon), the spinal cord and dorsal root ganglions, and plantar foot skin. Messenger RNA was recovered, and reverse transcription polymerase chain reaction (RT-PCR) was performed to generate complementary DNA (cDNA) sequences. Gene structures were documented by directly comparing cDNA sequences with recently published genomic sequences for the peregrine falcon and the blue-fronted Amazon parrot or by comparisons with genomic sequences of related species for the snowy owl. Structurally, the avian μ-opioid receptor messenger RNA (mRNA) species were complex, displaying differential splicing, alternative stop codons, and multiple polyadenylation signals. In comparison, the structure of the avian κ-receptor mRNA was relatively simple. In contrast to what is seen in humans, the avian δ-receptor mRNA structure was found to be complex, demonstrating novel 3-prime coding and noncoding exons not identified in mammals. The role of the δ-opioid receptor merits further investigation in avian species.

Characterization of the antinociceptive effects of intrathecal DALDA peptides following bolus intrathecal delivery

Background and aims We systematically characterized the potency and side effect profile of a series of small opioid peptides with high affinity for the mu opioid receptor. Methods Male Sprague Dawley rats were prepared with intrathecal (IT) catheters, assessed with hind paw thermal escape and evaluated for side effects including Straub tail, truncal rigidity, and pinnae and corneal reflexes. In these studies, DMT-DALDA (dDAL) (H-Dmt-D-Arg-Phe-Lys-NH2 MW=981), dDALc (H-Dmt-Cit-Phe-Lys-NH2 MW=868), dDALcn (H-Dmt-D-Cit-Phe-Nle-NH2 MW=739), TAPP (H-Tyr-D-Ala-Phe-Phe-NH2 MW=659), dDAL-TICP ([Dmt1]DALDA-(CH2)2-NH-TICP[psi]; MW=1519), and dDAL-TIPP (H-Dmt-D-Arg-Phe-Lys(Nε-TIPP)-NH2 were examined. In separate studies, the effects of approximately equiactive doses of IT DMT DALDA (10 pmol), morphine (30 nmol) and fentanyl (1 nmol) were examined on formalin-induced flinching at different pretreatment intervals (15 min - 24 h). Results (1) All agents resulted in a dose-dependent reversible effect upon motor function (Straub Tail>Truncal rigidity). (2) The ordering of analgesic activity (%MPE) at the highest dose lacking reliable motor signs after bolus delivery was: DMT-DALDA (80%±6/3 pmol); dDALc (75%±8/1 pmol); dDALcn (84%±10/300 pmol); TAPP (56%±12/10 nmol); dDAL-TICP (52%±27/300 pmol). (3) All analgesic effects were reversed by systemic (IP) naloxone (1 mg/kg). Naltrindole (3 mg/kg, IP) had no significant effect upon the maximum usable peptide dose. (4) Tolerance and cross-tolerance development after 5 daily boluses of DMT-DALDA (3 pmol) and morphine (30 nmol) revealed that both agents displayed a progressive decline over 5 days. (5) Cross-tolerance assessed at day 5 revealed a reduction in response to morphine in DMT-DALDA treated animal but not DMT-DALDA in the morphine treated animal, indicating an asymmetric cross-tolerance. (6) IT DMT-DALDA, morphine and fentanyl resulted in significant reductions in phase 1 and phase 2 flinching. With a 15 min pretreatment all drugs resulted in comparable reductions in flinching. However, at 6 h, the reduction in flinching after DMT-DALDA and morphine were comparably reduced while fentanyl was not different from vehicle. All effects on flinching were lost by 24 h. Conclusions These results emphasize the potent mu agonist properties of the DALDA peptidic structure series, their persistence similar to morphine and their propensity to produce tolerance. The asymmetric cross-tolerance between equiactive doses may reflect the relative intrinsic activity of morphine and DMT-DALDA. Implications These results suggest that the DALDA peptides with their potency and duration of action after intrathecal delivery suggest their potential utility for their further development as a spinal therapeutic to manage pain.

AMPAkines and morphine provide complementary analgesia

Glutamate signaling in the central nervous system is known to play a key role in pain regulation. AMPAkines can enhance glutamate signaling through α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. previous studies have shown that AMPAkines are effective analgesic agents, and their site of action is likely in the brain. It is not known, however, if AMPAkines can provide complementary analgesia in combination with opioids, the most commonly used analgesics. Here, we show that the co-administration of an AMPAkine with morphine can provide additional analgesia, both in naïve rats and in rats that experience postoperative pain. Furthermore, we show that this AMPAkine can be administered directly into the prefrontal cortex to provide analgesia, and that prefrontal AMPAkine infusion, similar to systemic administration, can provide added pain relief to complement morphine analgesia.

Current and Future Issues in the Development of Spinal Agents for the Management of Pain

Targeting analgesic drugs for spinal delivery reflects the fact that while the conscious experience of pain is mediated supraspinally, input initiated by high intensity stimuli, tissue injury and/or nerve injury is encoded at the level of the spinal dorsal horn and this output informs the brain as to the peripheral environment. This encoding process is subject to strong upregulation resulting in hyperesthetic states and downregulation reducing the ongoing processing of nociceptive stimuli reversing the hyperesthesia and pain processing. The present review addresses the biology of spinal nociceptive processing as relevant to the effects of intrathecally-delivered drugs in altering pain processing following acute stimulation, tissue inflammation/injury and nerve injury. The review covers i) the major classes of spinal agents currently employed as intrathecal analgesics (opioid agonists, alpha 2 agonists; sodium channel blockers; calcium channel blockers; NMDA blockers; GABA A/B agonists; COX inhibitors; ii) ongoing developments in the pharmacology of spinal therapeutics focusing on less studied agents/targets (cholinesterase inhibition; Adenosine agonists; iii) novel intrathecal targeting methodologies including gene-based approaches (viral vectors, plasmids, interfering RNAs); antisense, and toxins (botulinum toxins; resniferatoxin, substance P Saporin); and iv) issues relevant to intrathecal drug delivery (neuraxial drug distribution), infusate delivery profile, drug dosing, formulation and principals involved in the preclinical evaluation of intrathecal drug safety.

Efficacy of Intrathecal Morphine in a Model of Surgical Pain in Rats

Concerns over interactions between analgesics and experimental outcomes are a major reason for withholding opioids from rats undergoing surgical procedures. Only a fraction of morphine injected intravenously reaches receptors responsible for analgesia in the central nervous system. Intrathecal administration of morphine may represent a way to provide rats with analgesia while minimizing the amount of morphine injected. This study aimed to assess whether morphine injected intrathecally via direct lumbar puncture provides sufficient analgesia to rats exposed to acute surgical pain (caudal laparotomy).In an initial blinded, randomised study, pain-free rats received morphine subcutaneously (MSC, 3mg.kg-1, N = 6), intrathecally (MIT, 0.2mg.kg-1, N = 6); NaCl subcutaneously (NSC, N = 6) or intrathecally (NIT, N = 6). Previously validated pain behaviours, activity and Rat Grimace Scale (RGS) scores were recorded at baseline, 1, 2, 4 and 8h post-injection. Morphine-treated rats had similar behaviours to NaCl rats, but their RGS scores were significantly different over time and between treatments. In a second blinded study, rats (N = 28) were randomly allocated to one of the following four treatments (N = 7): MSC, 3mg.kg-1, surgery; MIT, 0.2mg.kg-1, surgery; NIT, surgery; NSC, sham surgery. Composite Pain Behaviours (CPB) and RGS were recorded as previously. CPB in MIT and MSC groups were not significantly different to NSC group. MSC and MIT rats displayed significantly lower RGS scores than NIT rats at 1 and 8h postoperatively. RGS scores for MIT and MSC rats were not significantly different at 1, 2, and 8h postoperatively. Intraclass correlation value amongst operators involved in RGS scoring (N = 9) was 0.913 for total RGS score. Intrathecal morphine was mostly indistinguishable from its subcutaneous counterpart, providing pain relief lasting up to 8 hours in a rat model of surgical pain. Further studies are warranted to clarify the relevance of the rat grimace scale for assessing pain in rats that have received opioid analgesics.

The Emerging Role of Spinal Dynorphin in Chronic Pain: A Therapeutic Perspective

Notable findings point to the significance of the dynorphin peptide neurotransmitter in chronic pain. Spinal dynorphin neuropeptide levels are elevated during development of chronic pain and sustained during persistent chronic pain. Importantly, knockout of the dynorphin gene prevents development of chronic pain in mice, but acute nociception is unaffected. Intrathecal (IT) administration of opioid and nonopioid dynorphin peptides initiates allodynia through a nonopioid receptor mechanism; furthermore, antidynorphin antibodies administered by the IT route attenuate chronic pain. Thus, this review presents the compelling evidence in the field that supports the role of dynorphin in facilitating the development of a persistent pain state. These observations illustrate the importance of elucidating the control mechanisms responsible for the upregulation of spinal dynorphin in chronic pain. Also, spinal dynorphin regulation of downstream signaling molecules may be implicated in hyperpathic states. Therapeutic strategies to block the upregulation of spinal dynorphin may provide a nonaddictive approach to improve the devastating condition of chronic pain that occurs in numerous human diseases.

Sigma receptors [σRs]: biology in normal and diseased states

This review compares the biological and physiological function of Sigma receptors [σRs] and their potential therapeutic roles. Sigma receptors are widespread in the central nervous system and across multiple peripheral tissues. σRs consist of sigma receptor one (σ₁R) and sigma receptor two (σ₂R) and are expressed in numerous regions of the brain. The sigma receptor was originally proposed as a subtype of opioid receptors and was suggested to contribute to the delusions and psychoses induced by benzomorphans such as SKF-10047 and pentazocine. Later studies confirmed that σRs are non-opioid receptors (not an µ opioid receptor) and play a more diverse role in intracellular signaling, apoptosis and metabolic regulation. σ₁Rs are intracellular receptors acting as chaperone proteins that modulate Ca²⁺ signaling through the IP₃ receptor. They dynamically translocate inside cells, hence are transmembrane proteins. The σ₁R receptor, at the mitochondrial-associated endoplasmic reticulum membrane, is responsible for mitochondrial metabolic regulation and promotes mitochondrial energy depletion and apoptosis. Studies have demonstrated that they play a role as a modulator of ion channels (K⁺ channels; N-methyl-d-aspartate receptors [NMDAR]; inositol 1,3,5 triphosphate receptors) and regulate lipid transport and metabolism, neuritogenesis, cellular differentiation and myelination in the brain. σ₁R modulation of Ca²⁺ release, modulation of cardiac myocyte contractility and may have links to G-proteins. It has been proposed that σ₁Rs are intracellular signal transduction amplifiers. This review of the literature examines the mechanism of action of the σRs, their interaction with neurotransmitters, pharmacology, location and adverse effects mediated through them.

Analgesic synergy between opioid and α2 -adrenoceptors

Opioid and α2 -adrenoceptor agonists are potent analgesic drugs and their analgesic effects can synergize when co-administered. These supra-additive interactions are potentially beneficial clinically; by increasing efficacy and/or reducing the total drug required to produce sufficient pain relief, undesired side effects can be minimized. However, combination therapies of opioids and α2 -adrenoceptor agonists remain underutilized clinically, in spite of a large body of preclinical evidence describing their synergistic interaction. One possible obstacle to the translation of preclinical findings to clinical applications is a lack of understanding of the mechanisms underlying the synergistic interactions between these two drug classes. In this review, we provide a detailed overview of the interactions between different opioid and α2 -adrenoceptor agonist combinations in preclinical studies. These studies have identified the spinal cord as an important site of action of synergistic interactions, provided insights into which receptors mediate these interactions and explored downstream signalling events enabling synergy. It is now well documented that the activation of both μ and δ opioid receptors can produce synergy with α2 -adrenoceptor agonists and that α2 -adrenoceptor agonists can mediate synergy through either the α2A or the α2C adrenoceptor subtypes. Current hypotheses surrounding the cellular mechanisms mediating opioid-adrenoceptor synergy, including PKC signalling and receptor oligomerization, and the evidence supporting them are presented. Finally, the implications of these findings for clinical applications and drug discovery are discussed.

LINKED ARTICLES

This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

Effects of intrathecal SNC80, a delta receptor ligand, on nociceptive threshold and dorsal horn substance p release

Delta-opioid receptors (DOR) are present in the superficial dorsal horn and are believed to regulate the release of small afferent transmitters as evidenced by the effects of spinally delivered delta-opioid preferring peptides. Here we examined the effects of intrathecal SNC80 [(+)-4-[α(R)-α-[(2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl]-3-(methoxybenzyl)-N,N-diethylbenzamide], a selective nonpeptidic DOR agonist, in three preclinical pain models, acute thermal escape, intraplantar carrageenan-tactile allodynia, and intraplantar formalin flinches, and on the evoked release of substance P (SP) from small primary afferents. Rats with chronic intrathecal catheters received intrathecal vehicle or SNC80 (100 or 200 μg). Intrathecal SNC80 did not change acute thermal latencies or carrageenan-induced thermal hyperalgesia. However, SNC80 attenuated carrageenan-induced tactile allodynia and significantly reduced both phase 1 and phase 2 formalin-induced paw flinches, as assessed by an automatic flinch counting device. These effects were abolished by naltrindole (3 mg/kg i.p.), a selective DOR antagonist, but not CTOP (10 µg i.t.), a selective MOR antagonist. Furthermore, intrathecal SNC80 (200 μg) blocked formalin-induced substance P release otherwise evoked in the ispilateral superficial dorsal horn as measured by NK1 receptor internalization. In conclusion, intrathecal SNC80 alleviated pain hypersensitivity after peripheral inflammation in a fashion paralleling its ability to block peptide transmitter release from small peptidergic afferents, which by its pharmacology appears to represent an effect mediated by a spinal DOR.

Analgesia targeting IB4-positive neurons in cancer-induced mechanical hypersensitivity

Cancer patients often suffer from pain and most will be prescribed μ-opioids. μ-opioids are not satisfactory in treating cancer pain and are associated with multiple debilitating side effects. Recent studies show that μ and δ opioid receptors are separately expressed on IB4 (-) and IB4 (+) neurons, which control thermal and mechanical pain, respectively. In this study we investigated IB4 (+) and IB4 (-) neurons in mechanical and thermal hypersensitivity in an orthotopic mouse oral cancer model. We used a δ opioid receptor agonist and a P2X(3) antagonist to target IB4 (+) neurons and to demonstrate that this subset plays a key role in cancer-induced mechanical allodynia, but not in thermal hyperalgesia. Moreover, selective removal of IB4 (+) neurons using IB4-saporin impacts cancer-induced mechanical but not thermal hypersensitivity. Our results demonstrate that peripherally administered pharmacological agents targeting IB4 (+) neurons, such as a selective δ-opioid receptor agonist or P2X(3) antagonist, might be useful in treating oral cancer pain.

PERSPECTIVE

To clarify the mechanisms of oral cancer pain, we examined the differential role of IB4 (+) and IB4 (-) neurons. Characterization of these 2 subsets of putative nociceptors is important for further development of effective clinical cancer pain relief.

Potent in vivo antinociception and opioid receptor preference of the novel analogue [Dmt1]endomorphin-1

[Dmt1]Endomorphin-1 is a novel analogue of the potent mu-opioid agonist endomorphin-1. Given the physiological role of endomorphin-1 in vivo, this compound was investigated to determine if the antinociception occurred through systemic, supraspinal or in a combination of both neuronal pathways. This compound exhibited a potent dose-dependent effect intracerebroventricularly in both spinal and supraspinal regions, and was blocked by opioid antagonist naloxone, which verified the involvement of opioid receptors. Specific opioid antagonists characterized the apparent receptor type: beta-funaltrexamine (mu1/mu2-irreversible antagonist) equally inhibited spinal- and central-mediated antinociception; on the other hand, naloxonazine (mu1-subtype) was ineffective in both neural pathways and naltrindole (delta-selective antagonist) partially (26%), though not significantly, blocked only the spinal-mediated antinociception. Therefore, spinal antinociception was primarily triggered by mu2-subtypes without involvement of mu1-opioid receptors; however, although a slight enhancement of antinociception by delta-receptors cannot be completely ruled out since functional bioactivity indicated mixed mu-agonism/delta-antagonism. In terms of the CNS action, [Dmt1]endomorphin-1 appears to act through mu2-opioid receptor subtypes.

Cancer-related bone pain is attenuated by a systemically available delta-opioid receptor agonist

Patients with bone cancer report severe pain and receive mu-opioids. We developed a family of peptidomimetic delta-agonists, one of which H2N-Tyr-dVal-Gly-Phe-Ala-OH ([dVal(L)2,Ala(L)5]E) binds with a 1700x affinity at the delta versus mu receptor. To examine the systemic analgesic efficacy of this delta-agonist versus morphine in osteosarcoma pain, osteosarcoma cells are injected into one femur of the anesthetized mouse. After 10-18 days, a decalcification of the injected femur occurs along with a pronounced tactile allodynia. IP morphine and [dVal(L)2,Ala(L)5]E produced a dose-dependent reversal of allodynia with the respective ED50 values being 5.3+/-1.9 mg/kg for morphine and 1.3+/-0.3 mg/kg for [dVal(L)2,Ala(L)5]E. Plotting peak effect versus area under the analgesic curve for doses of morphine and [dVal(L)2,Ala(L)5]E revealed overlapping curves suggesting that for a given effect, [dVal(L)2,Ala(L)5]E produced a similar duration of action as morphine. These effects were reversed by IP naloxone (3 mg/kg). IP naltrindole (1 mg/kg) preferentially reversed [dVal(L)2,Ala(L)5]E. The upper dose effects of morphine but not [dVal(L)2,Ala(L)5]E were limited by pronounced hyperactivity. No other effects were noted. These results show that IP [dVal(L)2,Ala(L)5]E through a delta receptor produces analgesia equal in efficacy to that of morphine but with a 4.5-fold greater potency. Over the doses examined, morphine actions were side effect limited. The delta side effects were not so limited, suggesting a favorable therapeutic ratio for delta-agonists in this pain model. These studies suggest that a systemically delivered delta-opioid agonist has pronounced analgesic properties on a preclinical cancer pain model.

Differentiation of opioid receptor preference by [Dmt1]endomorphin-2-mediated antinociception in the mouse

The potent opioid [Dmt1]endomorphin-2 (Dmt-Pro-Phe-Phe-NH2) differentiated between the opioid receptor subtypes responsible for the antinociception elicited by endomorphin-2 in mice. Antinociception, induced by the intracerebroventricular administration of [Dmt1]endomorphin-2 and inhibited by various opioid receptor antagonists [naloxone, naltrindole, beta-funaltrexamine, naloxonazine], was determined by the tail-flick (spinal effect) and hot-plate (supraspinal effect) tests. The opioid receptor subtypes involved in [Dmt1]endomorphin-2-induced antinociception differed between these in vivo model paradigms: naloxone (non-specific opioid receptor antagonist) and beta-funaltrexamine (irreversible mu1/mu2-opioid receptor antagonist) blocked antinociception in both tests, although stronger inhibition occurred in the hot-plate than the tail-flick test suggesting involvement of other opioid receptors. Consequently, we applied naloxonazine (mu1-opioid receptor antagonist) that significantly blocked the effect in the hot-plate test and naltrindole (delta-opioid receptor antagonist), which was only effective in the tail-flick test. The data indicated that [Dmt1]endomorphin-2-induced spinal antinociception was primarily mediated by both mu2- and delta-opioid receptors, while a supraspinal mechanism involved only mu1/mu2-subtypes.

Evidence for a selective role of the delta-opioid agonist [8R-(4bS*,8aalpha,8abeta, 12bbeta)]7,10-Dimethyl-1-methoxy-11-(2-methylpropyl)oxycarbonyl 5,6,7,8,12,12b-hexahydro-(9H)-4,8-methanobenzofuro[3,2-e]pyrrolo[2,3-g]isoquinoline hydrochloride (SB-235863) in blocking hyperalgesia associated with inflammatory and neuropathic pain responses

The specific involvement of the delta-opioid receptor in the control of nociception was explored by investigating the pharmacological activity in vivo of a selective, orally active, and centrally penetrant delta-opioid agonist. [8R-(4bS*,8aalpha,8abeta,12bbeta)]7,10-dimethyl-1-methoxy-11-(2-methylpropyl)oxycarbonyl 5,6,7,8,12,12b-hexahydro-(9H)-4,8-methanobenzofuro[3,2-e]pyrrolo[2,3-g]isoquinoline hydrochloride (SB-235863) is a new pyrrolomorphinan with high affinity (Ki = 4.81 +/- 0.39 nM) for the delta-opioid receptor, full agonist activity, and binding selectivity versus the mu- and kappa-opioid receptors of 189-fold and 52-fold, respectively. Perorally administered SB-236863 was inactive in the rat tail-flick and hot-plate tests of acute pain response, but potently reversed thermal hyperalgesia in rats resulting from a carrageenan-induced inflammatory response. This activity could be blocked by the delta-opioid antagonist naltrindole (3 mg/kg s.c.), but selective mu- and kappa-opioid antagonists were ineffective. Naltrindole (1 microg i.c.v.) also blocked the activity of 10 mg/kg (p.o.) SB-235863, showing that the compound activates delta-opioid receptor sites in the central nervous system. SB-235863 was additionally effective at reversing chronic hyperalgesia in the Seltzer rat model of partial sciatic nerve ligation after peroral administration. These data show that the delta-opioid receptor plays a selective role in regulating evoked and lasting changes in nociceptive pain signaling. Classical side effects of mu- and kappa-opioid receptor activation (slowing of gastrointestinal transit and motor incoordination, respectively) were not observed after administration of 70 mg/kg (p.o.) SB-235863, nor was evoked seizure activity affected. These results suggest a selective and limited role of delta-opioid receptors in the modulation of nociception.

Spinal delivery of analgesics in experimental models of pain and analgesia

Systemic administration of analgesics can lead to serious adverse side effects compromising therapeutic benefit in some patients. Information coding pain transmits along an afferent neuronal network, the first synapses of which reside principally in the spinal cord. Delivery of compounds to spinal cord, the intended site of action for some analgesics, is potentially a more efficient and precise method for inhibiting the pain signal. Activation of specific proteins that reside in spinal neuronal membranes can result in hyperpolarization of secondary neurons, which can prevent transmission of the pain signal. This is one of the mechanisms by which opioids induce analgesia. The spinal cord is enriched in such molecular targets, the activation of which inhibit the transmission of the pain signal early in the afferent neuronal network. This review describes the pre-clinical models that enable new target discovery and development of novel analgesics for site-directed pain management.

Systemic and spinal analgesic activity of a delta-opioid-selective lanthionine enkephalin analog

A lanthionine enkephalin derivative, Tyr-c[D-Val(L)-Gly-Phe-D-Ala(L)]-OH (DV(L)(2)DA(L)(5)LanEnk), where Val(L) and Ala(L) denote the lanthionine amino acid ends linked via a monosulfide bridge to form the lanthionine structure, was synthesized. It was found to possess selectivity for and potency at the delta versus mu opioid receptor as defined by binding studies and by its respective activity on the mouse vas deferens compared with the guinea pig ileum. The agent produced a potent analgesia after intrathecal and intraperitoneal delivery with ED(50) values being, respectively, 0.19 mucrog and 0.49 mg/kg. The effects of the agent were reversed by the delta-selective antagonist naltrindole. These analgesic actions occurred at doses that had no effect upon general behavior or motor function. These results suggest a potent delta-preferring agent suitable for development as a systemic delta opioid analgesic.

Stress-induced analgesia in mu-opioid receptor knockout mice reveals normal function of the delta-opioid receptor system

Stress-induced analgesia (SIA) was examined in wildtype and mu-opioid receptor knockout mice. We used thermal paw withdrawal (TPW) latency following a continuous 3-min swim in 20 degrees C water, and found a significant increase in TPW latency in both wild-type and knockout mice. Pre-treatment prior to the swim with naltrindole, a selective delta-opioid receptor antagonist, blocked the increase in TPW latency in knockout mice. These results demonstrate an intact delta-receptor-mediated function of a physiologically-released endogenous agonist in the mu-opioid receptor knockout mouse. The present findings are in contrast with previous reports that analgesia induced by exogenous delta agonists is reduced in the knockout mice.

Characterization of mechanical withdrawal responses and effects of mu-, delta- and kappa-opioid agonists in normal and mu-opioid receptor knockout mice

Clinical and experimental observations suggest that opiates can exert different influences on the perception of stimuli from distinct sensory modalities. Thermally-induced nociception is classically responsive to opiate agonists. mu-Opioid receptor-deficient transgenic mice are more sensitive to thermal nociceptive stimuli and morphine fails to attenuate the nociceptive responses to thermal stimuli in these animals. To enhance our understanding of opiate influences on mechanical sensitivity, we have examined withdrawal responses to a sequence of ascending forces of mechanical stimuli in mice with normal (wild type), half-normal (heterozygous) and absent (homozygous) mu-opioid receptor levels. We report data from mice examined without drug pretreatment or following pretreatment with morphine, the selective kappa-opioid agonist, U50488H, and the selective delta-opioid agonist, DPDPE. Saline-pretreated mice of each genotype displayed similar, monotonically increasing frequency of withdrawal responses to the graded stimuli. Subcutaneously administered morphine produced a dose-dependent reduction in withdrawal responses in wild type and heterozygous mice, but had no significant effect in homozygous mice. Intraventricular administration of DPDPE also reduced the frequency of paw withdrawal (FPW) in wild type mice, but not in homozygous mice. In contrast, systemic U50488H produced a dose-dependent attenuation of paw withdrawal in both wild type and homozygous mice. These findings suggest that (1) interactions of endogenous peptides with mu-opioid receptors may not play a significant role in the response to mechanical stimuli in drug-free animals, and (2) deficiency of mu-opioid receptors has no functional consequence on the response to the prototypical kappa-opioid receptor agonist, but decreases responses to the prototypical mu- and delta-opioid receptor agonists.

Visceral analgesic tolerance to intrathecal butorphanol in rats

PURPOSE

Recent experimental data suggest that intrathecal (it) kappa-opioid agonists produce profound visceral analgesia. This study investigated the development of visceral analgesic tolerance to it butorphanol, a potent kappa-agonist that has fewer side effects than commonly used it opioids. Understanding of this tolerance could make it butorphanol more effective in treating chronic visceral pain.

METHODS

This was a randomized, controlled animal study involving 80 Sprague-Dawley rats. Rats implanted with lumbar it catheters were infused either with it saline or butorphanol (52 nmol.hr-1) for 96 hr. Six hours afterwards, each rat was challenged once with one of the differing it butorphanol doses to construct dose-response curves. Visceral analgesia was evaluated by the abdominal writhing responses to the acetic acid injected intraperitoneally. The time of the first writhe and the total number of writhes were recorded.

RESULTS

For both the saline- and butorphanol-infused groups, a higher challenge dose of it butorphanol produced longer time for the first writhe to occur (P < 0.01, one-way ANOVA), and fewer writhes occurring within 30 min (P < 0.01, one-way ANOVA). However, the dose response curves of the butorphanol-infused groups were shifted rightward (P < 0.001, partial F test).

CONCLUSION

The challenge doses of it butorphanol produced dose-dependent visceral analgesia in both the saline- and butorphanol-infused groups, confirming its efficacy. The butorphanol-infused groups showed dose-response shifts, demonstrating the development of tolerance to this visceral analgesia.

Modulation of enkephalin release by nociceptin (orphanin FQ)

Nociceptin (orphanin FQ) is an endogenous peptide agonist for the newly discovered receptor (opioid receptor-like 1 receptor, ORL1) that bears striking homology to opioid receptors. Initial reports claimed that this peptide had hypoalgesic effects following i.c.v. or i.t. administration. The present study demonstrates that, in the presence of opioid receptor blockade, nociceptin can substantially alter the magnitude of the stimulated release of methionine-enkephalin from the guinea pig myenteric plexus. This effect is concentration dependent. Low doses (1 or 10 nM) inhibit whereas higher concentrations (100 or 1000 nM) enhance evoked enkephalin release. In contrast, in the absence of opioid receptor blockade, a statistically significant inhibition of stimulated enkephalin release is observed in response to 1, 100 or 1000 nM nociceptin. However, the magnitude of this effect did not differ among these concentrations. Furthermore, at 10 nM nociceptin, either an inhibition or enhancement of stimulated enkephalin release is manifest. The ability of naloxone to alter the nociceptin modulation of enkephalin release suggests that a component of the nociceptin modulation of enkephalin release is mediated via opioid receptors. This is consistent with the observation that this peptide has modest affinity for opioid receptors (L > K > 8) which, under appropriate conditions, should be sufficient to permit interactions with multiple opioid receptor types. This complicates dose responsiveness for nociceptin since both the naloxone-resistant (ORL1-mediated) and naloxone-sensitive (opioid receptor-mediated) component exhibit a concentration-dependent bimodality (albeit in opposite directions). Determination of i.c.v. or i.t. nociceptin dose responsiveness over several orders of magnitude is suggested before concluding the physiological effects of this peptide.

Opiate receptor knockout mice define mu receptor roles in endogenous nociceptive responses and morphine-induced analgesia

Morphine produces analgesia at opiate receptors expressed in nociceptive circuits. mu, delta, and kappa opiate receptor subtypes are expressed in circuits that can modulate nociception and receive inputs from endogenous opioid neuropeptide ligands. The roles played by each receptor subtype in nociceptive processing in drug-free and morphine-treated states have not been clear, however. We produced homologous, recombinant mu, opiate receptor, heterozygous and homozygous knockout animals that displayed approximately 54% and 0% of wild-type levels of mu receptor expression, respectively. These mice expressed kappa receptors and delta receptors at near wild-type levels. Untreated knockout mice displayed shorter latencies on tail flick and hot plate tests for spinal and supraspinal nociceptive responses than wild-type mice. These findings support a significant role for endogenous opioid-peptide interactions with mu opiate receptors in normal nociceptive processing. Morphine failed to significantly reduce nociceptive responses in hot plate or tail flick tests of homozygous mu receptor knockout mice, and heterozygote mice displayed right and downward shifts in morphine analgesia dose-effect relationships. These results implicate endogenous opioid-peptide actions at mu opiate receptors in several tests of nociceptive responsiveness and support mu receptor mediation of morphine-induced analgesia in tests of spinal and supraspinal analgesia.

Role of opioid receptors in the spinal antinociceptive effects of neuropeptide FF analogues

1. Neuropeptide FF (NPFF) has been shown to produce antinociceptive effects and enhance morphine-induced antinociception after intrathecal (i.t.) injection. In this study, the spinal effects of two NPFF analogues, -D-Tyr1,(NMe)Phe3-NPFF (1DMe) and [D-Tyr1,D-Leu2,D-Phe3]NPFF (3D), which are resistant to degradation and exhibit a high affinity for NPFF binding sites, were examined in tests of thermal and mechanical nociception. 2. 1DMe and 3D produced potent dose-dependent spinal antinociception in the tail-flick test. On a molar basis, 1DMe was 20 and 50 times more potent than 3D and morphine, respectively, and high doses of 1DMe and 3D produced a sustained antinociceptive effect without visible signs of motor impairment. 3. Spinal antinociceptive effects produced by 1DMe (0.86 nmol) or 3D (8.6 nmol) were significantly reduced by i.t. co-administration of naloxone (11 nmol) or i.t. pre-administration of D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP, 9.25 nmol) or beta-funaltrexamine (beta-FNA, 2 nmol) or naltrindole (2.2 nmol). The doses of the mu-antagonists (CTOP and beta-FNA) or the delta-antagonist (naltrindole) used in 1DMe and 3D experiments blocked the antinociceptive effects of mu- or delta-receptor-selective agonists. 4. When administered in combination with antinociceptive doses of the mu-receptor agonist, morphine (13.2 nmol) or the delta-receptor agonist, [D-Ala2]deltorphin I (20 nmol), sub-effective dose of 1DMe or 3D (0.009 nmol) enhanced and prolonged the spinal effects of these opioid agonists. 5. The results of this study show that spinal mu- and delta-opioid receptors play a role in antinociception produced by NPFF analogues. These results also suggest a role for NPFF in modulation of nociceptive signals at the spinal level.

Comparative study of the analgesic and paralytic effects induced by intrathecal dynorphin a in rats

Intrathecal injection of dynorphin A produced dual effects on sensory and motor functions in the spinal cord of the rat. At a dose of 5 nmol, dynorphin A produced an increase in tail flick latency (TFL) as well as a reversible motor paralysis as assessed by change in the angle of inclined plane. At a dose of 10 or 20 nmol, dynorphin produced a motor paralysis lasting for up to 24 hours. The effect of dynorphin A on the sensory function of the spinal cord was shown by an increase in the vocalization threshold induced by electrical stimulation of the tail, at dose range of 1.25-10 nmol, with a quick onset (5 min) and relatively short duration (within 60 min). Unlike tail flick reaction which involves spinal motor function, tail stimulation-induced vocalization threshold is a relatively pure index for spinal nociceptive activities. The differential effect of dynorphin on sensory and motor function was supported by the evidence that (1) dynorphin-induced analgesic effect (increase in vocalization threshold) was naloxone reversible, whereas dynorphin-induced motor paralysis was naloxone resistant. (2) Nor-BNI, a specific antagonist for kappa opioid receptor, blocked the sensory effect of dynorphin, but had no influence on motor effect of dynorphin. It is thus concluded that dynorphin has both analgesic and paralytic effects in spinal cord. The analgesia shown by an increase of vocalization threshold is an opioid effect, most probably mediated by kappa opioid receptor; the paralytic effect, however, is a non-opioid effect. The increase of TFL induced by dynorphin involves both sensory (analgesia) and motor (paralysis) effects.

A lack of supersensitivity to opioid receptor agonists following chronic spinal opioid receptor antagonist administration in the rat

1. Male Sprague-Dawley rats were chronically tested with intrathecal (i.t.) receptor selective opioid antagonists to determine if antinociceptive supersensitivity developed to selective i.t. opioid receptor agonists. 2. A subcutaneously implanted osmotic minipump was used to deliver the mu-opioid receptor antagonist CTOP (0.3 nmol) or the delta-opioid receptor antagonist naltrindole (5.5 nmol) for 7 days. 3. Following a 24 hr washout period, rats received a single i.t. dose (ED50) of either DAMPGO (for CTOP-treated animals) or DPDPE (for naltrindole-treated animals) and the antinociceptive effects of the agents were tested on the tail-flick test. 4. Our findings revealed that chronic spinal treatment with selective opioid receptor antagonists did not induce an antinociceptive supersensitivity to selective opioid receptor agonists. 5. Perhaps this lack of supersensitivity is reflective of difficulties inherent to opioid receptor antagonists that do not possess negative intrinsic activity.

Antisense oligodeoxynucleotide to a delta-opioid receptor selectively blocks the spinal antinociception induced by delta-, but not mu- or kappa-opioid receptor agonists in the mouse

An antisense oligodeoxynucleotide (A-oligo) to delta-opioid receptor mRNA was utilized to block the expression of mouse delta-opioid receptor in the spinal cord of male ICR mice. Intrathecal treatment with A-oligo (1.6-163 pmol) dose-dependently attenuated the antinociception induced by i.t. administered DPDPE ([D-Pen2,5]enkephalin) or [D-Ala2]deltorphin II, delta-opioid receptor agonist, without affecting the antinociception induced by DAMGO ([D-Ala2-MePhe4,Gly(ol)5]enkephalin) or U50,488H, respective mu- or kappa-opioid receptor agonists. Scrambled sense oligodeoxynucleotide (163 pmol) was ineffective against the tail-flick inhibition induced by DPDPE,[D-Ala2]deltorphin, DAMGO or U50,488H. The studies confirm previous pharmacological studies at the molecular level indicating a distinct delta-opioid receptor for antinociception in the spinal cord.

Receptor selectivity of icv morphine in the rat cold water tail-flick test

The cold water tail-flick test in the rat is somewhat unique in that it is sensitive to the analgesic effects of delta- and kappa- in addition to mu-opioid agonists. The present study was designed to test whether a component of morphine-induced analgesia in this test might be mediated by delta- or kappa-opioid receptors. Morphine was administered icv in combination with the non-selective opioid antagonist naloxone (NLX), as well as the mu-, delta- and kappa-selective antagonists, D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr (CTAP), naltrindole (NTI) and norbinaltorphimine (norBNI), respectively. Morphine induced analgesia in a dose related manner. Administration of NLX (1-10 micrograms) or CTAP (1 microgram) antagonized morphine in a competitive fashion. Neither NTI (1-10 micrograms) nor norBNI (0.1 microgram) had any effect on the morphine dose-effect curve. Thus, morphine appeared to be a selective mu agonist in the cold water tail-flick test, at least by the icv route.

Antinociceptive effects of intrathecally administered F8Famide and FMRFamide in the rat

The effects of intrathecal injections of F8Famide (Phe-Leu-Phe-Gln-Pro-Gln-Arg-Phe-NH2, 0.05-17.5 nmol) and FMRF-amide (Phe-Met-Arg-Phe-NH2, 0.002-25 nmol), known as anti-opioid agents, were investigated by using noxious thermal (tail flick) and mechanical (paw pressure) tests in the rat. Both peptides produced significant long-lasting (24-48 h) analgesia in both tests without causing detectable motor dysfunction. Pretreatment with systemic naloxone (5.5 mumol/kg i.p.) attenuated the initial antinociceptive effects (first hour) induced by both peptides (8.8 nmol) in the tail flick test and only by FMRFamide in the paw pressure test. A subeffective dose of F8Famide (0.05 nmol) enhanced both the intensity and the duration of spinal morphine (6.6 nmol) analgesia in both tests. In contrast, a subanalgesic dose of FMRFamide (0.002 nmol) decreased the intensity and enhanced the duration of the effect of morphine. These results show that, besides acting as antinociceptive agents in the spinal cord, F8Famide and FMRFamide could differentially modulate spinal opioid functions.

Dose-dependent antagonism of spinal opioid receptor agonists by naloxone and naltrindole: additional evidence for delta-opioid receptor subtypes in the rat

Intrathecally administered mu-opioid (morphine; DAMGO ([D-Ala2,N-MePhe4,Gly5-ol]enkephalin)) and delta-opioid (DPDPE ([D-Pen2,D-Pen5] enkephalin); DADLE ([D-Ala2,D-Leu5]enkephalin)) receptor preferring agonists were systematically challenged with the competitive opiate antagonists naloxone or naltrindole in the rat. Naloxone produced a dose-dependent reduction in agonist effect with the intrathecal IC50 being similar for all agonists (2.1-5.4 micrograms). In contrast, the naltrindole antagonist profile was (IC50 in micrograms) DPDPE (4.0); morphine (23.5); DADLE (> 30) and DAMGO (> 30). Three points are emphasized: (1) antagonism of DPDPE and not DAMGO by naltrindole suggests two distinct opioid sites; (2) a similar potency for naloxone against these agonists suggests that the agonists may act upon spinal sites for which naloxone has comparable affinity or that they may act upon separate sites which are functionally coupled and that the action of naloxone on one or the other site is responsible for the antagonism; and (3) given the modest cross-tolerance between DADLE and mu agonists, the failure of naltrindole to antagonize DADLE suggests that in the rat this peptide acts through a delta site different from that acted upon by DPDPE.

Spinal antinociceptive effects of [D-Ala2]deltorphin II, a novel and highly selective delta-opioid receptor agonist

Pharmacological assays in isolated tissues and binding tests have recently shown that two peptides, with the sequence Tyr-D-Ala-Phe-Asp-(or Glu)- Val-Val-Gly-NH2, isolated from skin extracts of Phyllomedusa bicolor and named [D-Ala2]deltorphin I and II, respectively, possess a higher affinity and selectivity for delta-opioid receptors than any other known natural compound. Since much evidence supports the role of spinal delta-opioid sites in producing antinociceptive effects, we investigated whether analgesia might be detected by direct spinal cord administration of [D-Ala2]deltorphin II (DADELT II) in the rat. The thermal antinociceptive effects of intrathecal DADELT II and dermorphin, a potent mu-selective agonist, were compared at different postinjection times by means of the tail-flick test. The DADELT II produced a dose-related inhibition of the tail-flick response, which lasted 10-60 min depending on the dose and appeared to be of shorter duration than the analgesia produced in rats after intrathecal injection of dermorphin (20-120 min). The analgesic effect of infused or injected DADELT II was completely abolished by naltrindole, the highly selective delta antagonist. These results confirm the involvement of delta receptors in spinal analgesic activity in the rat.

Beta-endorphin processing and cellular origins in rat spinal cord

While enkephalin and dynorphin peptides have been well characterized in the spinal cord, the cellular localization of beta-endorphin (beta E) and the processing of pro-opiomelanocortin (POMC) to beta E and other non-opioid peptides in the cord have not been extensively investigated. Other investigators have characterized the various beta E forms present in rat spinal cord regions. Previous studies have also suggested that spinal POMC content is entirely derived from supraspinal sources. However, high proportions of beta E precursors present in spinal cord sieving profiles led us to suspect the presence of POMC cell bodies intrinsic to the cord. In this study, we performed thoracic spinal cord lesions on a group of animals and demonstrated the persistence of about one-third of control levels of beta E immunoreactivity (beta E-IR) below the level of the lesions. We also characterized POMC processing in various regions of the spinal cord both before and after lesioning. These data suggested that there may be intrinsic POMC/endorphinergic neuronal systems in the spinal cord.

Parallel activation of multiple spinal opiate systems appears to mediate 'non-opiate' stress-induced analgesias

Pain is powerfully modulated by circuitries within the CNS. Two major types of pain inhibitory systems are commonly believed to exist: opiate (those that are blocked by systemic opiate antagonists and by systemic morphine tolerance) and non-opiate (those that are not). We used intrathecal delivery of mu, delta, and kappa opiate receptor antagonists to examine 3 well-accepted non-opiate stress-induced analgesias. Combined blockade of all 3 classes of opiate receptors antagonized all of the 'non-opiate' analgesias. Further experiments demonstrated that blocking mu and delta or mu and kappa was sufficient to abolish 'non-opiate' analgesias. Combined blockade of kappa and delta receptors was without effect. The clear conclusion is that all endogenous analgesia systems may in fact be opiate at the level of the spinal cord. Phenomena previously thought to be non-opiate appear to involve parallel activation of multiple spinal opiate processes. These findings suggest the need for a fundamental shift in conceptualizations regarding the organization and function of pain modulatory systems in particular, and opiate systems in general.

Spinal antinociception by Tyr-D-Ser(otbu)-Gly-Phe-Leu-Thr, a selective delta-opioid receptor agonist

The spinal antinociceptive potency of the delta-opioid receptor agonist, Tyr-D-Ser(otbu)-Gly-Phe-Leu-Thr (DSTBULET), was studied in rats. The tail flick test was used as nociceptive stimulus and the rotarod test was used to detect any motor or sedative effects. A dose-response curve was also made for the mu-opioid receptor agonist, morphine. The ED50 for DSTBULET was 0.3 micrograms (0.4 nmol) and a near 100% maximum effect was achieved with 5 micrograms (7.5 nmol). No motor or sedative effects were detected. Antinociception by DSTBULET was antagonized by s.c. naltrindole (1 mg/kg), a selective delta-opioid receptor antagonist, and naloxone (1 mg/kg), a non-selective opioid receptor antagonist. The ED50 for morphine was 0.5 micrograms (1.0 nmol) and the antinociceptive effects were not antagonized by naltrindole (1 mg/kg). The results evidence further the important role of the delta-opioid receptor in spinal nociceptive processing.

Behavioural effects of deltorphins in rats

When given i.c.v. in rats deltorphins induced a syndrome of behavioural stimulation consisting of increased locomotion rearing and sniffing. The increased locomotor activity and rearing were dose-related over the range of 0.13 to 3.8 nmol/rat for [D-Ala2]deltorphin II (DADELT II) and 1.04 to 20.8 nmol/rat for deltorphin. The delta-selective antagonist, naltrindole (10 mg/kg i.p.), completely abolished the behavioural stimulation induced by 1.3 nmol/rat of DADELT II and shifted the dose-response curve to the right, without decreasing the maximum effect. The mu-preferring antagonist, naloxone, was able to antagonize the DADELT II-induced locomotor activity but only at very high doses (10 and 20 mg/kg i.p.). The i.v. administration of a large dose (10 mg/kg) of the mu 1-selective antagonist, naloxonazine, did not affect the DADELT II response. At doses up to 38 nmol/rat, the i.c.v. injection of DADELT II never induced analgesia. At doses over 20.8 nmol/rat, deltorphin always induced spontaneous controlateral barrel rotations and circling, responses which were not blocked by prior administration of naloxone or haloperidol. In studies performed on the social behaviour of rats, i.c.v. administration of 0.38 nmol/rat of DADELT II was ineffective, while 1.3 nmol/rat increased the number of social contacts. Regression analysis showed that the increase in social contacts was a primary effect of the peptide, not correlated with the increased locomotor activity.

Placental opioid-enhancing factor (POEF): generalizability of effects

A substance in amniotic fluid and placenta (POEF for Placental Opioid-Enhancing Factor) has been shown to enhance opiate- or opioid-mediated analgesia in rats. Recent studies have only touched on the generalizability of the phenomenon. The present studies further tested the generalizability of the POEF effect: they examined sex specificity of the mechanism; whether POEF activity exists in afterbirth material of species other than the rat; whether POEF activity exists in tissue other than afterbirth material; whether POEF activity could be demonstrated after injection rather than ingestion of afterbirth material; and whether POEF enhances all opioid-mediated phenomena. We found that (a) POEF is effective in male rats as well as in female rats; (b) POEF activity exists in human and dolphin afterbirth material; (c) ingestion of pregnant-rat liver does not produce enhancement of opioid-mediated analgesia; (d) POEF does not seem to be effective when amniotic fluid is injected either IP or SC; and (e) POEF does not modify morphine-induced hyperthermia.

Synergistic analgesic interactions between the periaqueductal gray and the locus coeruleus

Opiates modulate pain perception at a number of different levels within the central nervous system and the importance of synergistic spinal and supraspinal influences have been well documented. In the present study we demonstrate synergistic interactions between the periaqueductal gray and locus coeruleus. Administered either systemically or intracerebroventricularly (i.c.v.), ethylketocyclazocine elicits a potent naloxonazine-sensitive analgesia, indicating a mu 1 action. mu 1 Receptors also play a major role in opioid analgesic mechanisms in the periaqueductal gray and the locus coeruleus. However, microinjection of EKC into either the periaqueductal gray or locus coeruleus failed to elicit an analgesic response at any dose tested (0.1-20 micrograms) and, in additional studies, antagonized the analgesic actions of coadministered morphine or [D-Ser2,Leu5]enkephalin-Thr6 (DSLET). However, the simultaneous administration of EKC into both the periaqueductal gray (10 micrograms) and the locus coeruleus (10 micrograms; total combined dose 20 micrograms) produced a potent naloxonazine-sensitive analgesia greater than that observed with 50 micrograms i.c.v. These results suggest that EKC is a partial mu 1 agonist which lacks the efficacy to elicit analgesia when microinjected into either of the two brain regions alone. However, when exposed to several regions at once, either through simultaneous microinjections into the periaqueductal gray and locus coeruleus or by injection into the ventricle, EKC is a potent mu 1 analgesic. These results point out the existence of synergistic supraspinal interactions between the periaqueductal gray and the locus coeruleus, similar to the spinal/supraspinal interactions observed previously.

Comparison of the antinociceptive and motor effects of intrathecal opioid agonists in the rat

This study compared the antinociceptive and motor effects produced by intrathecal administration of selective mu-, delta-, and kappa-opioid receptor agonists in the rat. Changes in nociceptive threshold were measured using the Randall-Selitto paw-withdrawal test and changes in motor coordination were evaluated using the rotarod treadmill test. Each opioid agonist produced statistically significant, dose-dependent increases in mechanical nociceptive thresholds compared to vehicle controls. In the motor coordination studies, DAMGO and DPDPE, but not U50,488H, produced statistically significant decreases in rotarod performance scores compared to vehicle controls. The results of these studies suggest that motor side-effects produced by opioid agonists need to be considered when interpreting the results of antinociceptive tests that are dependent on a normally functioning motor system.

Increased calcitonin gene-related peptide (CGRP), substance P, and enkephalin immunoreactivities in dorsal spinal cord and loss of CGRP-immunoreactive motoneurons in arthritic rats depend on intact peripheral nerve supply

The distribution of peptides thought to be involved in pain modulation--substance P, calcitonin gene-related peptide (CGRP), and enkephalin--were studied in the spinal cord and dorsal root ganglia of polyarthritic rats and in rats with one sciatic nerve sectioned prior to induction of arthritis. In arthritic rats there was a bilateral increase of CGRP- and substance P-immunoreactive fibers and appearance of enkephalin-immunoreactive cell bodies in the dorsal horn of the lumbar (L4) spinal cord when compared to controls. In the corresponding dorsal root ganglia there were significant increases of CGRP- (P less than 0.02) and substance P- (P less than 0.001) immunoreactive cell bodies compared to controls. In the ventral horn of the control rats CGRP-immunoreactive motoneurons were abundant but were significantly (P less than 0.001) reduced in the arthritic spinal cord. Less pronounced changes were seen in the contralateral L4 spinal cord of arthritic rats with unilateral sciatic nerve section. In the ipsilateral dorsal horn, however, CGRP- and substance P-immunoreactive fibers were markedly depleted, and no enkephalin cell bodies were present. Furthermore, a number of CGRP-immunoreactive motoneurons were observed. In the ipsilateral L4 ganglia CGRP- (P less than 0.02) and substance P- (P less than 0.02) immunoreactive cells were significantly decreased compared to the contralateral side. The data suggest that pain perception is linked to complex interactions between CGRP, substance P, and enkephalin in sensory pathways and an intact peripheral input. The loss of CGRP-immunoreactive motoneurons may reflect muscular dysfunction associated with the arthritic condition.

Effects of receptor-selective opioids on operant behavior in morphine-treated and untreated rats

Selective cross-tolerance is often used as evidence to differentiate opioid receptor subtypes. We used this strategy to study operant behavioral effects of the opioid peptides, [D-Ala2, NMePhe4, Gly-ol5]enkephalin (DAGO), [D-Pen2.5]-enkephalin (DPDPE) and dynorphin, agonists highly selective for mu, delta, and kappa receptors, respectively. Food-deprived rats were trained to lever-press on a fixed-interval 3-min schedule of food-reinforcement. Time-effect and dose-effect curves were generated for each of the peptides, as well as for morphine, administered ICV, 5 min prior to the 1-h operant session. Experiments were performed on untreated subjects and on subjects receiving chronic infusion of morphine (10 mg/kg/day) from osmotic pumps. In untreated animals, morphine and the mu-selective peptide, DAGO, induced relatively long-lasting dose-related decreases in responding, whereas the non-mu agonists, DPDPE and dynorphin, induced only transient effects: response rates increased at low doses and decreased at high doses. Animals receiving chronic morphine infusion were tolerant to the rate-decreasing effects of morphine and DAGO; ED50s increased by factors of 8 and 6, respectively. There was some evidence of cross-tolerance to DPDPE and of sensitization to dynorphin in the morphine-maintained animals.

The utility of 2-hydroxypropyl-beta-cyclodextrin as a vehicle for the intracerebral and intrathecal administration of drugs

The substituted glucopyranose ring structure 2-hydroxypropyl-beta-cyclodextrin (CDEX) increases the solubility of molecules by inclusion of the agent in the lipophilic interior of the ring. This property is of particular use for the administration of molecules by the intracerebral (ICV) or intrathecal (IT) routes. In concentrations up to 40% w/v (isotonic), this agent (10 microliters) effect upon nociceptive or motor function after IT injection or on EEG and general behavior after ICV injection in rats. Using 20% CDEX, there is no change in the ED50 as compared to saline on the hot plate (HP) after IT injection of morphine, D-Ala2-D-Leu5 enkephalin or Tyr-Aib-Gly-gPhe-mAib-NH2, (Aib: alpha-aminoisobutyric acid) although there is an increase in their respective durations of effect. Cyclic peptide opioids: Tyr-c[D-A2bu-Gly-D-beta Nal(1)-D-Leu] (A2bu: alpha, gamma-diaminobutyric acid; beta-Nal(1): beta-naphthylalanine(1)) or Tyr-c[DA2bu-Gly-beta Nal(1)-D-Leu] are insoluble in saline but are readily dissolved in CDEX, and display a naloxone-sensitive antinociception following spinal administration. In other studies, saline insoluble capsaicin is administered in 25% dimethylsulfoxide (DMSO) or 20% CDEX (15 microliters; 5 mg/ml) which result in a significant reduction in the spinal levels of substance P and calcitonin gene related peptide and an increase in the HP latency. DMSO alone, but not CDEX alone, reduces the levels of the two peptides. These data emphasize the utility of complexation with CDEX for intracerebral drug delivery and compatibility with brain and spinal tissue.

The antinociceptive action of supraspinal opioids results from an increase in descending inhibitory control: correlation of nociceptive behavior and c-fos expression

In an earlier report, we demonstrated that subcutaneous injection of formalin in the rat hindpaw evokes a characteristic pattern of expression of the fos protein product of the c-fos protooncogene in spinal cord neurons, and that systemic morphine reversed the fos-like immunoreactivity in a dose-dependent, naloxone-reversible manner. The present study compared the effects of intracerebroventricular administration of the mu-selective opioid ligand [D-Ala2, NMe-Phe4, Gly-ol5] enkephalin, on the pain behavior and spinal cord fos-like immunoreactivity produced by subcutaneous formalin. Formalin injection produced a biphasic pain behavioral response which lasted about 1 h. There was a significant correlation between the formalin pain score and overall fos-like immunoreactivity in the lumbar enlargement. The greatest numbers of labeled cells and most intense fos-like immunoreactivity were found in laminae I, IIo and V of the L4-5 segments, ipsilateral to the formalin-injected paw. Considerable staining was also found in the ipsilateral ventral horn laminae VII and VIII. [D-Ala2, NMe-Phe4, Gly-ol5]enkephalin produced a dose-related, naloxone-reversible inhibition of both the formalin-evoked pain behavior and fos expression in the cord. The behavioral response to formalin, however, could be completely blocked without eliminating the expression of fos in spinal neurons. Moreover, subpopulations of neurons were differentially regulated. Thus, 100% inhibition of pain behavior was produced at a dose of [D-Ala2, NMe-Phe4, Gly-ol5]enkephalin which reduced fos-like immunoreactivity in the superficial laminae by only 64% and in the neck and ventral cord by 85%. Furthermore, the dose of [D-Ala2, NMe-Phe4, Gly-ol5]enkephalin which produced approximately 50% inhibition of fos-like immunoreactivity in the neck and ventral regions of the spinal cord was without effect in the superficial dorsal horn. Since the potencies for inhibition of pain behavior and fos-like immunoreactivity in the neck and ventral horn were comparable, these data suggest that the activity of neurons in these regions is directly related to the pain behavior produced by nociceptive inputs. Finally, we found that bilateral, midthoracic lesions of the dorsal part of the lateral funiculus blocked both the antinociception and fos suppression produced by intracerebroventricular [D-Ala2, NMe-Phe4, Gly-ol5]enkephalin. These results are consistent with the hypothesis that the analgesic action of supraspinally administered opiates results from an increase in descending inhibitory controls that regulate the firing of subpopulations of spinal cord nociresponsive neurons.

Effects of hypophysectomy and adrenalectomy on naloxone-induced analgesia

Experiment 1 demonstrated that pairings of the opiate antagonist, naloxone, with a heated floor came to induce analgesia, as indexed by the latencies with which rats licked their paws. This analgesia appears to be neurally mediated because it is unaffected by either hypophysectomy (experiment 2) or adrenalectomy (experiment 3). However, there was evidence for a pituitary involvement, as its removal potentiated the analgesic effect accruing from naloxone-stressor pairings.

Analgesic synergy and improved motor function produced by combinations of mu-delta- and mu-kappa-opioids

This study evaluated the effects of intrathecal administration of a low-analgesic dose of the selective mu-agonist DAMGO co-administered with sequentially increasing doses of either the selective delta-agonist DPDPE or the selective kappa-agonist, U50,488H on mechanical nociceptive thresholds in the rat. Potent analgesic synergy was observed with both combinations. Since an elevation in nociceptive threshold can result from motor deficits, as well as true analgesia, we also evaluated the effects of the combination regimens on motor coordination using a rotarod apparatus. The combination regimens produced significantly less motor deficits than those observed when DPDPE and U50,488H were administered as single agents. These findings of enhanced analgesia with decreased motor side-effects associated with administration of fixed mu/delta or mu/kappa combinations suggest that co-administration of opiates that act at different receptors may constitute a superior approach to the treatment of pain.

Delta-opioid receptor binding sites in rodent spinal cord

1. The delta-opioid receptor agonist [D-Pen2,D-Pen5]enkephalin showed an antinociceptive effect in the mouse tail-flick test, following intrathecal administration. This action was reversed by naloxone and by the selective delta-opioid receptor antagonist ICI 174864. 2. High affinity, saturable binding of [3H]-[D-Pen2,D-Pen5]enkephalin has been demonstrated in spinal cord homogenates from guinea-pig, hamster, rat and both adult and young (18-20 g) mice. The binding was shown by autoradiography to be concentrated in the superficial laminae of the dorsal horn. 3. Competition studies confirmed that the binding of [3H]-[D-Pen2,D-Pen5]enkephalin was to the delta-opioid site. However, anomalies were seen with displacement assays using mu-ligands, which may suggest some common high affinity site for delta- and mu-opioid receptor agonists in the spinal cord. 4. The results add further evidence for a role of the delta-opioid receptor in spinally-mediated antinociception.

Tolerance to delta- but not mu-opioid receptors in the spinal cord attenuates inhibition of the tail-flick response induced by beta-endorphin administered intracerebroventricularly in mice

Male ICR mice were rendered tolerant by intrathecal (IT) injection once a day with either mu-agonist, D-Ala2-NMePhe4-Gly-ol-enkephalin (DAMGO) or delta-agonist, D-Pen2-D-Pen5-enkephalin (DPDPE) (toleragen) by doubling the dose each day starting from 0.125 and 1 microgram for DAMGO and DPDPE, respectively, for 6 days. On day 6, the magnitude of tolerance was assessed by establishing IT dose-response lines for the effect of the chronic drug given as bolus injections (probe). The antinociception was assessed by the tail-flick and hot-plate test. Repeated IT injections of DPDPE reduced inhibition of the tail-flick and hot-plate response induced by DPDPE (ED50 values for DPDPE increase 10-fold) but not DAMGO. Repeated IT injections of DAMGO reduced inhibition of the tail-flick and hot-plate response induced by DAMGO (ED50 value for DAMGO increase 7- to 10-fold) but not DPDPE. The effects of the tolerance to mu- and delta-opioid receptor activity in the spinal cord on inhibition of the tail-flick and hot-plate response induced by intracerebroventricularly (ICV) administered beta-endorphin and morphine were then studied. beta-Endorphin or morphine at different doses were injected ICV 4 hr after the last IT injection of DPDPE or DAMGO. Repeated IT bolus injections of DPDPE reduced inhibition of the tail-flick response but not the hot-plate response induced by beta-endorphin. On the other hand, repeated IT bolus injections of DAMGO did not affect inhibition of the tail-flick and hot-plate response induced by beta-endorphin.(ABSTRACT TRUNCATED AT 250 WORDS)

Different spinal effects of opioid agonists on spinal and spino-bulbo-spinal reflexes in rats

The effects of morphine-HCl (MOR), methionine-enkephalin (ME) and dynorphin (DYN) on spinal and spino-bulbo-spinal (SBS) reflexes were studied. Although spinal intrathecal administration of MOR (15 micrograms) did not produce any apparent effect on these reflexes, systemically administered MOR (3 mg/kg i.v.) reduced the electrical toe stimulation-induced SBS reflex. Furthermore, MOR (3 mg/kg i.v.) increased the polysynaptic reflex induced by electrical stimulation of low-threshold dorsal root afferents in intact (non-spinal) rats, but not in spinal rats. Intrathecally administered DYN (0.5 and 5 micrograms) reduced both the electrical toe stimulation-induced spinal and SBS reflexes, while ME (15 micrograms) only reduced the SBS reflex. These results indicate the physiological multiplicity of spinal opioid receptors. MOR may affect supraspinal nuclei but not the spinal pathway which possesses MOR-sensitive opioid receptors, whereas ME and DYN affect spinal opioid peptide receptors and modulate the reflex activities in which they participate.