Differential association of spinal mu, delta and kappa opioid receptors with cutaneous thermal and visceral chemical nociceptive stimuli in the rat

Antinociceptive Effects of Kappa-Opioid Receptor Agonists

Preclinical models that assess "pain" in rodents typically measure increases in behaviors produced by a "pain stimulus." A large literature exists showing that kappa opioid receptor (KOR) agonists can decrease these "pain-stimulated behaviors" following many different pain stimuli. Despite showing apparent antinociceptive properties in these preclinical models, KOR agonists failed as analgesics in clinical trials. Recent studies that assessed decreases in behavior due to a pain stimulus show that KOR agonists are not effective in restoring these "pain-depressed behaviors" to normal levels, which agrees with the lack of effectiveness for KOR agonists in clinical trials. One current explanation for the failure of previous KOR agonists in clinical trials is that those agonists activated beta-arrestin signaling and that KOR agonists with a greater bias for G protein signaling will be more successful. However, neither G protein-biased agonists nor beta-arrestin-biased agonists are very effective in assays of pain-depressed behavior, which suggests that novel biased agonists may still not be effective analgesics. This review provides a concise account of the effectiveness of KOR agonists in preclinical models of pain-stimulated and pain-depressed behaviors following the administration of different pain stimuli. Based on the previous results, it may be appropriate to include both behaviors when testing the analgesic potential of KOR agonists.

Pentazocine-induced antinociception is mediated mainly by μ-opioid receptors and compromised by κ-opioid receptors in mice

Pentazocine is a widely used mixed agonist-antagonist opioid. Previous animal studies have demonstrated that pentazocine-induced antinociception displayed a ceiling effect characterized by biphasic dose response with a increasing and then descending analgesia like a bell-shaped curve. This study attempted to clarify the mechanisms underlying such dose-response relationships. ddY and C57BL/6J mice received subcutaneous injection of saline or pentazocine (3, 10, 30, 56, or 100 mg · kg(-1)), at 120 min after subcutaneous injection of saline, a μ-opioid receptor antagonist clocinnamox mesylate (C-CAM) (5 mg · kg(-1)), a κ-opioid receptor antagonist nor-binaltorphimine (nor-BNI) (10 mg · kg(-1)), or the combination of C-CAM and nor-BNI. The antinociceptive effects of pentazocine were evaluated using tail pressure, hot plate, tail flick, and acetic acid writhing tests. Without pretreatment with an opioid receptor antagonist, the antinociceptive effects of pentazocine exhibited biphasic bell-shaped dose-response curves peaking at 30 mg · kg(-1). C-CAM completely and partly antagonized the antinociception induced by pentazocine at low (3-30 mg · kg(-1)) and high (56-100 mg · kg(-1)) doses, respectively. nor-BNI enhanced the antinociception by pentazocine at high doses and turned the later descending portion of the biphasic dose-response curves into a sigmoid curve. The combination of C-CAM and nor-BNI completely abolished the antinociception by pentazocine at all doses. Our results suggest pentazocine produces antinociception primarily via activation of μ-opioid receptors, but at high doses, this μ-opioid receptor-mediated antinociception is antagonized by concomitant activation of κ-opioid receptors. This provides the first reasonable hypothesis to explain the ceiling effects of pentazocine analgesia characterized by a biphasic dose response.

Buprenorphine with bupivacaine for intraoral nerve blocks to provide postoperative analgesia in outpatients after minor oral surgery

PURPOSE

The demonstration that opioid receptors exist in the peripheral nervous system offers the possibility of providing postoperative analgesia in the ambulatory surgical patient. Over the previous decade, many investigators have studied this approach and have compared the efficacy of various opioids added to the local anesthetic near the brachial plexus; and it appears from several of these studies that buprenorphine provides the longest duration of analgesia, the most important parameter of postoperative analgesia in outpatients. One of these studies indicated that the agonist-antagonist, buprenorphine, added to bupivacaine provided a longer period of postoperative analgesia than the traditional opiates, but none of the studies was performed in patients undergoing minor oral surgery to check the efficacy of buprenorphine to provide postoperative analgesia in dental patients. The present study was undertaken to ascertain the efficacy of buprenorphine in providing prolonged postoperative analgesia when added to 0.5% bupivacaine with epinephrine 1:200,000.

PATIENTS AND METHODS

Fifty healthy, consenting adult patients scheduled for upper extremity surgery were enrolled in the study. Patients were assigned randomly to 1 of 2 equal groups based on the agents used for the blocks. Patients in group I received 40 mL of a local anesthetic alone, and those in group II received the same local anesthetic plus buprenorphine 0.3 mg. The study was kept double-blind by having one dentist prepare the solutions, a second dentist perform the blocks, and a third dentist monitor the anesthesia and analgesia thereafter, up to and including the time of the first request for an analgesic medication. The data were reported as means +/- standard errors of the mean, and differences between groups were determined using t test. A P value less than .01 was considered statistically significant.

RESULTS

The mean duration of postoperative pain relief after injection of the local anesthetic alone was 8.34 +/- 0.11 hours compared with 28.18 +/- 1.02 hours after buprenorphine was added, a difference that was statistically (and clinically) significant (P < .001).

CONCLUSION

The addition of buprenorphine to the local anesthetic used for intraoral nerve blocks in the present study provided a 3-fold increase in the duration of postoperative analgesia, with complete analgesia persisting 30 hours beyond the duration provided by the local anesthetic alone in 75% of patients. This practice can be of particular benefit to patients undergoing minor oral surgery by providing prolonged analgesia after discharge from the hospital.

High doses of processed Aconiti tuber inhibit the acute but potentiate the chronic antinociception of morphine

AIM OF THE STUDY

In this study, we investigated the effects of processed Aconiti tuber (PAT), an oriental herbal medicine, at analgesic doses on acute morphine antinociception in morphine-naïve mice and morphine tolerance in morphine-tolerant mice.

MATERIALS AND METHODS

In acute experiments, mice received subcutaneous (s.c.) morphine (2, 5, or 10 mg/kg) and oral distilled water or PAT (0.3, 1.0, or 3.0 g/kg). The mechanical nociceptive threshold (MNT) and thermal nociceptive latency (TNL) were measured with the tail pressure test and tail flick test, respectively, before, and at 30, 60, 90, and 120 min after s.c. morphine injection. In chronic experiments, mice received s.c. morphine (10 mg/kg) and oral distilled water or PAT (0.3, 1.0, or 3.0 g/kg) once daily for 11 days. MNT was measured before, and at 60 min after, and TNL was measured before, and at 30 min after, daily morphine injections on days 1-11.

RESULTS

PAT at analgesic doses inhibited the acute antinociceptive effect of morphine dose-dependently in morphine-naïve mice. In contrast, PAT at analgesic doses potentiated the chronic antinociceptive effect of morphine dose-dependently by inhibiting the development of morphine tolerance dose-dependently. These effects of PAT on acute and chronic morphine antinociception were mediated through activation of kappa-opioid receptors.

CONCLUSIONS

These results indicated that chronic co-administration of PAT at analgesic doses with morphine could provide better-maintained morphine analgesia in a long-term morphine treatment after initial inhibition of acute morphine antinociception for a brief period of time.

Inhibitory effect of processed Aconiti tuber on the development of antinociceptive tolerance to morphine: evaluation with a thermal assay

In the previous studies, we demonstrated that an oriental herbal medicine processed Aconiti tuber (PAT) at subanalgesic doses could inhibit the development of mechanical antinociceptive tolerance to morphine using the tail pressure test. In the present study, we evaluated whether PAT could inhibit thermal antinociceptive tolerance to morphine using the high temperature (55 degrees C) hot plate test. Mice received subcutaneous morphine (10mg/kg), and oral PAT at doses that did not inhibit the hot plate response (0.3, 0.5, 1.0, and 2.0 g/kg), once daily for 14 days. The thermal nociceptive latency was measured at 30 min after daily morphine injections. Compared with placebo, oral PAT partially and dose-dependently inhibited the development of morphine tolerance in morphine-naïve mice, and reversed already-developed morphine tolerance in morphine-tolerant mice. These data suggested that PAT at subanalgesic doses could dose-dependently inhibit and reverse thermal antinociceptive tolerance to morphine.

A Comparison of the Antinociceptive and Adverse Effects of the mu-Opioid Agonist Morphine and the delta-Opioid Agonist SNC80

delta-Opioid receptor agonists have been postulated to induce analgesia without the adverse effects commonly associated with mu-opioids e.g. morphine. In the present study, we evaluated the occurrence of antinociceptive and opioid-like side effects in rats (n=5-7) treated with a single dose of subcutaneous morphine (0.01 to 40 mg/kg) or SNC80 (0.63 to 80 mg/kg). The antinociceptive effects of morphine and SNC80 were compared using a range of nociceptive tests including the tail withdrawal test, the acetic acid-induced abdominal constriction (writhing) assay, the automated formalin test and a model of inflammation-induced thermal hyperalgesia. The adverse effects of both drugs were examined in animal models for gastrointestinal (GI) inhibition (charcoal test; ricinus oil test), respiratory depression (blood-gas analysis), motor disturbances (automated rotarod model) and abuse liability (drug discrimination learning). Morphine displayed significant antinociceptive and adverse effects in all the animal models employed. SNC80 exhibited a significant effect in the writhing test and limited efficacy in a model of inflammation-induced thermal hypersensitivity. A delay in the occurrence of diarrhoea and some limited increases in PCO(2) were observed with the higher doses of SNC80 (> or =40 mg/kg). In conclusion, the delta-opioid agonist SNC80 lacks both the analgesic efficacy and adverse effects of mu-opioids. However, the activity of SNC80 in the inflammatory model suggests delta-opioid agonists may be useful analgesics in the treatment of some forms of inflammatory pain.

The delta agonists DPDPE and deltorphin II recruit predominantly mu receptors to produce thermal analgesia: a parallel study of mu, delta and combinatorial opioid receptor knockout mice

Delta-selective agonists have been developed to produce potent analgesic compounds with limited side-effects. DPDPE and deltorphin II are considered prototypes, but their delta-selectivity in vivo and the true ability of delta receptors to produce analgesia remain to be demonstrated. Here we have performed a parallel analysis of mu, delta and combinatorial opioid receptor knockout mice, in which we found no obvious alteration of G-protein coupling for remaining opioid receptors. We compared behavioural responses in two models of acute thermal pain following DPDPE and deltorphin II administration by intracerebroventricular route. In the tail-immersion test, both compounds were fully analgesic in delta knockout mice and totally inactive in mu knockout mice. In the hotplate test, the two compounds again produced full analgesia in delta knockout mice. In mu knockout mice, there was significant, although much lower, analgesia. Furthermore, DPDPE analgesia in the delta knockout mice was fully reversed by the mu selective antagonist CTOP in both tests. Together, this suggests that mu rather than delta receptors are recruited by the two agonists for the tail withdrawal and the hotplate responses. Finally, deltorphin II slightly prolonged jump latencies in double mu/kappa knockout mice (delta receptors only) and this response was abolished in the triple knockout mice, demonstrating that the activation of delta receptors alone can produce weak but significant mu-independent thermal antinociception.

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.

Limbic postictal events: anatomical substrates and opioid receptor involvement

1. Amygdaloid kindled seizures in rats produce postictal motor deficits, disruption of affective responding to sensory input, postictal explosiveness, and seizure suppression that may be similar to events following complex partial seizures in humans. 2. Preliminary 2DG studies in kindled rats indicate that postictal motor deficits may be mediated by the substantia nigra. Disruption of affective responding and postictal seizure suppression may be mediated by the hippocampus. 3. Data from the literature indicates that postictal motor deficits may be mediated by mu and kappa opioid receptors. The disruption of affective responding may be mediated primarily be delta and maybe also by kappa receptors. Postictal explosiveness may involve either a non-mu receptor or it may be a non-opioid effect. Kindling-induced postictal seizure suppression may be mediated by kappa receptors and perhaps also by mu receptors. 4. Mechanisms underlying postictal effects of complex partial seizures in humans are suggested by the data in this manuscript. New approaches to the treatment of these postictal events are also proposed.

Disruption of the kappa-opioid receptor gene in mice enhances sensitivity to chemical visceral pain, impairs pharmacological actions of the selective kappa-agonist U-50,488H and attenuates morphine withdrawal

***micro***-, delta- and kappa-opioid receptors are widely expressed in the central nervous system where they mediate the strong analgesic and mood-altering actions of opioids, and modulate numerous endogenous functions. To investigate the contribution of the kappa-opioid receptor (KOR) to opioid function in vivo, we have generated KOR-deficient mice by gene targeting. We show that absence of KOR does not modify expression of the other components of the opioid system, and behavioural tests indicate that spontaneous activity is not altered in mutant mice. The analysis of responses to various nociceptive stimuli suggests that the KOR gene product is implicated in the perception of visceral chemical pain. We further demonstrate that KOR is critical to mediate the hypolocomotor, analgesic and aversive actions of the prototypic kappa-agonist U-50, 488H. Finally, our results indicate that this receptor does not contribute to morphine analgesia and reward, but participates in the expression of morphine abstinence. Together, our data demonstrate that the KOR-encoded receptor plays a modulatory role in specific aspects of opioid function.

Cross-tolerance to acute administration of mu and kappa opioid agonists at the spinal cord level in the rat

Development of tolerance and cross-tolerance after acute administration of the mu agonist morphine and the kappa agonist U-50,488H was assessed in rats, through recording of a C-fiber-evoked spinal nociceptive reflex. Rats rendered tolerant to morphine (a single dose of 1 mg/kg i.p.) showed, after a 5-hour period, tolerance to morphine and cross-tolerance to the kappa-opioid receptor agonist U-50,488H, as revealed by depressed C-reflex responsiveness. In contrast, pretreatment with U-50,488H (a single dose of 1 mg/kg i.p.) rendered tolerant the rats to U-50,488H, but the animals did not develop cross-tolerance to morphine. Results indicate that acute administration of mu and kappa ligands leads to development of unidirectional cross-tolerance in rat spinal cord. This points to limitations in using alternated mu and kappa opioid agonists to bypass the problem of development of opioid tolerance in chronic pain complaints.

Spinal mu and delta, but not kappa, opioid-receptor agonists attenuate responses to noxious colorectal distension in the rat

The antinociceptive efficacy of different opioid-receptor agonists following their intrathecal (i.t.) administration was examined in awake, unanesthetized rats in a model of visceral pain. Cumulative i.t. doses of the mu-preferring opioid-receptor agonist morphine produced dose-dependent attenuation of the change (increase) in mean arterial pressure (delta MAP) and elevation of the visceromotor threshold to colorectal distension (CRD). Similar dose-dependent antinociceptive effects were produced after i.t. administration of the mu opioid-receptor-selective agonist DAMPGO. Morphine and DAMPGO were equipotent against the delta MAP to phasic CRD (80 mm Hg, 20 sec), but DAMPGO was more than 6 times more potent than morphine in elevating the visceromotor threshold to an incrementing CRD. Intrathecal administration of the delta opioid-receptor-selective agonist DPDPE produced, like morphine and DAMPGO, a dose-dependent attenuation of the delta MAP to CRD; DPDPE was one-tenth as potent as morphine or DAMPGO. DPDPE also dose-dependently elevated the visceromotor threshold to CRD, but its efficacy was only half that of morphine or DAMPGO. The kappa opioid-receptor-selective agonist U 50488H was without antinociceptive efficacy after i.t. administration, but did attenuate responses to CRD after systemic administration. The antinociceptive effects produced by morphine and DAMPGO were antagonized by i.t. pretreatment with naloxone and the effects produced by DPDPE were antagonized by i.t. pretreatment with the delta opioid-receptor-selective antagonist naltrindole. These data indicate that local mu and delta, but not kappa, opioid receptors can modulate visceral nociceptive transmission in the spinal cord.

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.

Neurophysiological evidence for increased kappa opioidergic control of spinal cord neurons in rats with unilateral inflammation at the ankle

The role of the endogenous kappa opioid system in the control of neuronal activity has been studied in the spinal cord of normal rats and in rats with Freund's adjuvant induced unilateral inflammation of the ankle under barbiturate anaesthesia. During recordings from neurons with ankle input the kappa receptor agonist U50,488H and/or the kappa antagonist nor-binaltorphamine were administered ionophoretically using multibarrel electrodes. In most neurons tested U50,488H reduced the responses evoked by pressure applied across the ankle whereas smaller proportions of neurons showed increased activity or were not affected. The kappa opioid antagonist nor-binaltorphamine affected more neurons in rats with inflammation than in control rats. Ongoing activity was increased in 7 of 19 (37%) neurons in control rats, in 16 of 24 (67%) neurons in the acute phase of inflammation (2 days post inoculation) and in 15 of 23 (65%) neurons in the chronic phase of inflammation (16-20 days post inoculation). During application of nor-binaltorphamine in control rats, the responses to pressure were increased in 9 cells (36%), reduced in 7 cells (28%) and unaffected in 9 cells (36%). In the acute phase of inflammation significantly more neurons (11 of 15, 73%) showed enhanced responses to pressure during ionophoresis of nor-binaltorphamine but not in the chronic phase. These results show that spinal cord neurons with ankle input are influenced by the endogenous kappa opioid system particularly under inflammatory conditions. The upregulation of this system under inflammatory conditions may serve to counteract inflammation-induced hyperexcitability.

Protection by U-50,488H against beta-chlornaltrexamine antagonism of nitrous oxide antinociception in mice

Nitrous oxide antinociception in the abdominal constriction test was significantly reduced in mice pretreated intracerebroventricularly (i.c.v.) with beta-chlornaltrexamine (beta-CNA). However, this antagonism was reversed when the beta-CNA was co-administered i.c.v. with the kappa-opioid ligand U-50,488H but not the mu-opioid ligand CTOP. These findings further demonstrate that nitrous oxide antinociception in the mouse abdominal constriction paradigm is mediated by kappa- but not mu-opioid receptors.

Standardization of the rat paw formalin test for the evaluation of analgesics

Administration of 5% formalin into the rat or guinea pig hind paw evokes two spontaneous responses: flinching/shaking and licking/biting of the injected paw. The temporal and behavioral characteristics of these objective endpoints are described. Additionally, several practical suggestions aimed at standardizing this test for the evaluation of analgesics are presented. The early/acute and late/tonic (0-10 and 20-35 min post-formalin, respectively) phases of flinching were used to quantitate antinociception in the rat. PD 117302, the kappa selective agonist, was three times more potent than morphine against tonic flinching after SC administration. Formalin may therefore be a noxious stimulus of choice in the evaluation of kappa agonists. Morphine was only twice as potent against tonic flinching as against acute flinching or the tail-dip reflex to water (50 degrees C). In contrast, PD 117302 was 27 times less potent on early phase and was inactive in the tail-dip test. Thus, while morphine is essentially equipotent across tests, PD 117302 shows a spectrum of activity with impressive potency and efficacy being obtained against tonic pain. Kappa receptors may therefore be prominently involved in tonic pain states. Aspirin given orally was not consistently antinociceptive in either phase of the formalin test. Spinal transection completely abolished late phase responding but only partly attenuated flinching in the early phase. This suggests that the relative involvement of spinal (as opposed to supraspinal) processing of noxious inputs may, at least in part, be a function of stimulus intensity and underlie the differences in antinociceptive potency observed in this work.

Analgesia produced by intrathecal administration of the kappa opioid agonist, U-50,488H, on formalin-evoked cutaneous pain in the rat

The antinociceptive activity of the selective kappa opioid agonist U-50,488H, given intrathecally (i.t.) against chemically induced cutaneous pain in rats, was assessed from cumulative dose-response experiments and the formalin test. Three successive i.t. doses of 5, 10 and 35 nmol of U-50,488H produced a gradual reduction of pain scores which was statistically significant at all observation periods. This effect was antagonized significantly by 3 mg/kg i.p. of the opiate antagonists, naloxone and WIN 44,441-3. The analgesia profile showed a clear dose-response relationship. A dose producing 50% 'maximum possible analgesia' of 6.20 nmol (95% confidence interval: 3.05-12.59 nmol) was calculated. The results indicated that cutaneous pain of a chemical/inflammatory nature is highly sensitive to activation of kappa receptors of the spinal cord dorsal horn.

Effect of subtype-selective opioid receptor blockers on nitrous oxide antinociception in rats

Nitrous oxide antinociception in rats was evaluated by the warm water tail withdrawal test following central pretreatment with blockers of various opioid receptor subtypes. The analgesic dose, 50% (AD50) value for nitrous oxide antinociception was significantly elevated by MR-2266 (which is relatively selective for kappa-opioid receptors) and increased to a lesser degree by ICI-174,864 (which is selective for delta-opioid receptors). However, pretreatment with beta-funaltrexamine (which is selective for mu-opioid receptors), even at extremely high doses, was ineffective in altering the AD50 for nitrous oxide antinociception. According to these findings, nitrous oxide antinociception, as evaluated in this paradigm, appears to be mediated by kappa- and possibly delta- but not mu-opioid receptors.

Intrathecal administration of delta receptor agonists in the urethane anesthetized rat provokes an increase in arterial pressure via a non-opioid mechanism

Intrathecal administration of the delta receptor specific agonists Leu5-enkephalin (Leu-Enk; 300 nmol), Met5-enkephalin (Met-Enk; 300 nmol) and [D-Pen2,D-Pen5]enkephalin (DPDPE; 100 nmol) to the T2 or the T9 segment of the rat spinal cord provoked a transient (less than 5 min) increase (15-20 mm Hg) in arterial pressure. DPDPE, but not Leu-Enk or Met-Enk, also significantly increased heart rate by 30-35 bpm. Intravenous administration of 300 nmol of Leu-Enk mimicked the effects observed following intrathecal administration. The hypertensive effect of intrathecal and intravenous Leu-Enk administration was blocked by prior systemic administration (10 mg/kg) of the nicotinic ganglion blocker hexamethonium, suggesting that the effect was mediated via sympathetic activation. The increase in arterial pressure observed following intrathecal Leu-Enk administration was not blocked by either intrathecal (305 nmol) or intravenous (10 mg/kg) administration of the opiate receptor blocker naloxone, although naloxone did block the hypertension provoked by intravenous Leu-Enk administration. Moreover, intrathecal administration of Des-Tyr1-Leu-Enk (300 nmol), an enkephalin fragment devoid of opiate receptor activity, also increased arterial pressure. These results suggest that the hypertension elicited by intrathecal delta agonist administration was not mediated via an opioid mechanism.

A comparative analysis of monoaminergic involvement in the spinal antinociceptive action of DAMPGO and DPDPE

The antinociceptive properties of intrathecally (i.t.) administered [D-Ala2, N-methyl-Phe4, Gly5-ol]enkephalin (DAMPGO) and [D-Pen2, D-Pen5]enkephalin (DPDPE), selective opioid agonists for mu (mu) and delta (delta) sites, respectively, were compared in rats. DAMPGO and DPDPE elevated tail-flick latency (TFL) in a dose-dependent manner, and the spinal antinociceptive actions of both drugs were reversed by the opiate antagonist naloxone. These findings suggest that both DAMPGO and DPDPE interact with spinal opiate receptors to elevate TFL. Another set of experiments was done to determine the involvement of local spinal serotonin (5-HT) or norepinephrine (NE) in DAMPGO and DPDPE-induced spinal analgesia. Both the alpha 1 noradrenergic receptor antagonist WB-4101 and the alpha 2 blocker yohimbine failed to alter the antinociceptive actions of DAMPGO and DPDPE. Similarly, the 5-HT receptor antagonists pindolol, ritanserin and ICS 205-930 (selective for 5-HT1, 5-HT2 and 5-HT3 sites, respectively) failed to inhibit opioid-induced spinal analgesia. Thus, while DAMPGO and DPDPE produce antinociception via an interaction with spinal opioid receptors, apparently neither drug activates endogenous monoaminergic systems.

Correlation of ontogeny with function of [3H]U69593 labelled kappa opioid binding sites in the rat spinal cord

In this study, we have used a variety of in vitro and in vivo techniques to demonstrate the presence, and examine the function, of [3H]U69593 binding sites in the spinal cord of the 9-16-day-old rat in comparison to the adult. Equilibrium binding of [3H]U69593 to homogenates of adult rat spinal cord revealed a single population of non-interacting sites with a maximum binding capacity of 10.4 +/- 1.4 fmol/mg protein and an apparent equilibrium dissociation constant of 2.31 +/- 0.47 nM while in 9-16-day-old cord these parameters were 57.0 +/- 9.4 fmol/mg protein and 2.28 +/- 0.22 nM, respectively. The total binding capacity per cord was 95.8 +/- 8.3 and 121.8 +/- 7.7 fmol/cord for adult and immature rat, respectively. Competition studies using receptor-selective opioid ligands showed that these sites were kappa opioid in nature. Autoradiographical techniques demonstrated a uniform distribution of these sites over transverse sections of 9-16-day-old rat cord. In vitro electrophysiology was performed on spinal cord slice preparations from the 9-16-day-old rat. U69593 (100 nM-1 microM) had no effect on passive membrane properties but produced a naloxone-reversible depression of both spontaneous and electrically evoked activity in dorsal horn neurones. Direct intrathecal injection of U69593 (0.3-10.0 micrograms/animal) into 9-16-day-old rats produced a dose-dependent, naloxone-reversible, antinociception when measured using the paw-pressure test. In conclusion, we have shown that, in contrast to the adult, the spinal cord of the 9-16-day-old rat has a significantly higher concentration of [3H]U69593 binding sites which have functional in vitro and in vivo correlates.

Spinal antinociceptive actions of mu- and kappa-opioids: the importance of stimulus intensity in determining 'selectivity' between reflexes to different modalities of noxious stimulus

1. In electrophysiological experiments in spinalized rats, mu- and kappa-opioids were tested intravenously on the responses of single motoneurones to electronically controlled, alternating noxious heat and noxious pinch stimuli. The effects of mu- and kappa-opioids were compared with those of the general anaesthetic alpha-chloralose and the dissociative anaesthetic/PCP ligand ketamine. 2. The kappa-opioids U-50,488 (0.5-16 mgkg-1 i.v.) and tifluadom (0.05-1.6 mgkg-1 i.v.) had very similar actions to the mu-opioid fentanyl (0.5-16 micrograms kg-1 i.v.). Thus all three agonists reduced thermal and mechanical nociceptive reflexes in parallel and in a dose-dependent manner, but only so long as neuronal responses to the alternating stimuli elicited similar excitability levels in the neurone under study. Ketamine (0.5-16 mgkg-1 i.v.) had similar actions to the opioids whereas alpha-chloralose (20 mgkg-1 i.v.) had very little effect on neuronal responsiveness. 3. Apparently 'selective' depressions by both mu- and kappa-opioids could be orchestrated by a deliberate mismatch of the intensities of alternating noxious heat and pinch stimuli; as measured by neuronal firing rate, the weaker of the responses to either type of stimulus was invariably reduced to a greater degree. 4. Similar 'selectivity' could be demonstrated for both mu- and kappa-ligands when the weaker and stronger responses were of the same modality, being applied by the same pincher device but with alternating applied force. 5. It is concluded that the 'selective' spinal actions of kappa-opioids seen in non-thermal over thermal behavioural models of nociception is likely to be related to the relative intensities, rather than the modalities, of the noxious stimuli used. The validity of the interpretation of results obtained in such behavioural studies is discussed.

Spinal antinociceptive actions and naloxone reversibility of intravenous mu- and kappa-opioids in spinalized rats: potency mismatch with values reported for spinal administration

1. The relative spinal effectiveness of mu- and kappa-opioids has been assessed by their intravenous potencies on nociceptive responses (heat and/or pinch) of single motoneurones recorded in alpha-chloralose anaesthetized, spinalized rats. 2. The depressant actions of both mu- and kappa-opioids were reversed by low intravenous doses of naloxone (10 to 100 micrograms kg-1). When tested at a dose of 1 microgram kg-1 i.v., naloxone antagonized the effects of the mu-agonist morphine but had no effect on the kappa-opioid U-50,488. This provides further support for the theory that the actions of mu- and kappa-ligands were mediated at different subclasses of opioid receptor but highlights the difficulties in using antagonists with poor receptor selectivity to differentiate between mu- and kappa-receptor-mediated effects in vivo. 3. The molar potency rations of fentanyl: morphine:U-50,488: tifluadom for thermal and mechanical nociceptive responses were 620: 1.0:0.74:5.7 and 520:1.0:0.56:7.7 respectively. These potency ratios, as well as the absolute potencies, agree well with those reported in several behavioural studies in which systemic administration of agonists was used in non-thermal tests. 4. The agonist potency values obtained in this study contrast with those reported for local spinal administration. By this route, the potency of lipophilic opioids (e.g. fentanyl, U-50,488 and tifluadom) relative to hydrophilic opioids (e.g. morphine) is much reduced, implying that activity of intrathecally administered opioids is more dependent on the physico-chemical properties of the agonists used than on the relative abundance in the spinal cord of functional opioid receptors of the mu- and kappa-subtypes. This conclusion indicates that the results with locally applied opioids should not be used to assess spinal opioid receptor function.

Differential sensitivity of antinociceptive tests to opioid agonists and partial agonists

1. The antinociceptive activity of a range of opioid agonists and agonist-antagonist analgesics was determined in mice by use of the 55 degrees C hot plate and abdominal constriction assays. 2. Opioid agonists were approximately 10 times more effective in the abdominal constriction assay. 3. The agonist-antagonists produced analgesia only in the abdominal constriction assay, and antagonized the antinociceptive action of opioid agonists in the 55 degrees C hot plate test. 4. These differences were shown to be attributable to the different levels of stimulus employed in the two tests. 5. By comparing the antagonist potencies of the agonist-antagonists in the 55 degrees C hot plate test with their antinociceptive ED50 values in the abdominal constriction assay, an index of intrinsic activity was calculated.

Peripheral and central antinociceptive actions of ethylketocyclazocine in the formalin test

The antinociceptive actions of ethylketocyclazocine and morphine were examined in rats in a thermal nociceptive test (tail-immersion) and a test involving minor tissue injury (formalin). In the formalin test, the antinociceptive effects of high doses of ethylketocyclazocine, but not morphine, were attenuated by the peripherally acting antagonist naloxone methylbromide. Naloxone methylbromide had no effect on antinociception produced by ethylketocyclazocine in the tail-immersion test. When ethylketocyclazocine was injected intraventricularly, only partial antinociception was observed in the formalin test. Conversely, naloxone given intraventricularly only partially attenuated the antinociception produced by ethylketocyclazocine given systemically. The data indicate that the antinociceptive effects of ethylketocyclazocine in the tissue injury-induced nociception are a result of summation of central and peripheral actions. Morphine antinociception reaches ceiling at doses that are devoid of such peripheral actions. The data imply that it may be possible to develop a new class of peripherally acting analgesics that are effective in acute inflammatory pain.

Influence of narcotic antagonist drugs upon nitrous oxide analgesia in mice

Nitrous oxide produced a concentration-related suppression of phenylquinone-induced abdominal constriction in mice. This analgesic effect was significantly reduced (but not abolished) by systemic pretreatment with (-)-naloxone or naltrexone but not (+)-naloxone. Systemic pretreatment with methylnaltrexone failed to appreciably influence nitrous oxide analgesia; however, methylnatrexone, administered centrally, significantly attenuated the drug effect. Furthermore, nitrous oxide analgesia was significantly reduced by MR-2266 (which is relatively selective for kappa-opioid receptors) but not by beta-funaltrexamine (which is selective for mu-opioid receptors) at the doses employed in this study. These findings suggest that nitrous oxide analgesia might involve an activation of kappa-opioid receptors in the central nervous system; however, a possible involvement of mu-opioid receptors is not absolutely precluded by this study.

Morphine-6-glucuronide: analgesic effects and receptor binding profile in rats

The antinociceptive effects of morphine-6-glucuronide (M6G) were examined in two animal models of pain, the tail immersion test (reflex withdrawal to noxious heat) and the formalin test (behavioral response to minor tissue injury). In the tail immersion test, M6G produced an increase in withdrawal latency that rose rapidly between 0.01 and 0.025 ug ICV or 1 and 2 mg/kg SC. A further increase occurred at doses greater than 0.2 ug ICV or 4 mg/kg SC and was associated with marked catalepsy and cyanosis. Naloxone, 0.1 mg/kg SC, shifted the lower component of the dose-effect relation by a factor of 24. In the formalin test, 0.01 ug M6G ICV produced hyperalgesia, while between 0.05 and 0.2 ug ICV, antinociception increased rapidly without toxicity. The dose effect relations for hyperalgesia and antinociception were shifted to the right by factors of 20- and 3-fold, respectively. By comparison, ICV morphine was 60 (formalin test) to 145-200 (tail immersion test) times less potent than M6G. At sub-nanomolar concentrations, M6G enhanced the binding of [3H]-etorphine, [3H]-dihydromorphine and [3H]-naloxone to rat brain membrane receptors by 20-40%. At higher concentrations, M6G displaced each ligand from binding sites, with Ki values of about 30 nM, as compared to morphine Ki values of about 3 nM. The data indicate that the in vivo and in vitro effects of M6G are complex and that M6G may play an important role in analgesia in experimental animals, and by implication, in man.

Intraspinal opiates for treatment of intractable pain in the terminally ill cancer patient

The discovery of opiate receptors and then their endogenous ligands in 1974 (Snyder et al., 1974) has elucidated a vast pharmacology of opiates providing a basis for their diverse clinical applications. With the awareness of quality of life as a primary goal in terminal cancer patients, widespread attention has been drawn to the direct delivery of long-term intraspinal analgesics to cancer patients for who all medical pain control regimens have failed (Coombs & Saunders, 1974). Intraspinal administration of opiates and nonopiate analgesics is not only appealing on theoretical grounds but provides a minimally invasive method to insure otherwise unobtainable pain relief while eliminating obtundation and systemic side-effects associated with conventional therapy (Cobb et al., 1984; Harbaugh et al., 1982; Leavens et al., 1982; Malone et al., 1985; Onofrie et al., 1981; Poletti et al., 1981). Although intraspinal opiates have been used in the treatment of postoperative and benign-pain syndromes (Asari et al., 1981; Cousins & Mather, 1984), in our discussion we review the basic science, current techniques and possible future improvements in spinal analgesia in the control of chronic cancer pain.

A dose-ratio comparison of mu and kappa agonists in formalin and thermal pain

The effects of putative mu and kappa agonists, with and without naloxone, were compared in the formalin and tail flick tests in rats. The mu agonist sufentanil was more potent in the tail flick test than the formalin test while the opposite was true for the kappa agonist ethylketocyclazocine (EKC). MR2034 was equipotent in the two tests and in the tail flick test, analgesia decreased at high doses. The naloxone (0.1 mg/kg) dose-ratios (DR) for sufentanil and EKC were 3 to 7 times larger for the tail flick test than the formalin test. From this and other DR studies it is argued that in thermal pain tests, opioid analgesia is mediated primarily by mu receptors while in non thermal tests kappa effects predominate.

Qualitative differences in effects of opioids in man: preliminary evidence for multiple mechanisms of analgesic action

The analgesic effects of meperidine, anileridine, codeine and codeine + acetominophen on surgical and non-surgical pain in 101 patients were assessed using the McGill Pain Questionnaire. The quality of analgesia was determined by analyzing the changes in the pain descriptors chosen 1 hour after medication. Meperidine and anileridine differentially reduced pain qualities rated as "bright-phasic" by a student sample. Codeine and codeine + acetominophen produced similar patterns of analgesia that were homogeneous across "bright-phasic" and "dull-tonic" types of pain. The data suggest the possibility that opioids may differ in the quality of analgesia produced either as dose increases or different opioid receptor types are recruited.

Autoradiographic localization of mu, delta and kappa opioid receptor binding sites in rat and guinea pig spinal cord

The autoradiographic distribution of mu, delta and kappa opioid binding sites was evaluated in various segments of the rat and guinea pig spinal cord. Mu opioid receptor binding sites are highly concentrated in the superficial layers of the dorsal horn (laminae II and III) in both species, without any marked gradient along the cord. Delta binding sites are somewhat concentrated in the superficial layers of the dorsal horn. However, delta binding sites are also present and evenly distributed in other areas of the gray matter. The highest density of delta sites is found in the cervical segment with only low levels in the lumbo-sacral region of the rat and guinea pig spinal cord. Kappa opioid binding sites are highly concentrated in the superficial layers of the dorsal horn of the spinal cord. Lower levels are seen in the rest of the gray matter with some enrichment in lamina X. Moreover, the lumbo-sacral portion of the spinal cord is enriched in kappa sites as compared to the cervical and thoracic segments. These data demonstrate the differential laminar distribution of mu, delta and kappa opioid binding sites in rat and guinea pig spinal cord.

Characterization of dynorphin A-induced antinociception at spinal level

Dynorphin A (DYN A) injected intrathecally in the rat produced a significant elevation of the nociceptive threshold, measured by the tail flick test. The highest dose of DYN A (25 nmol) produced maximal elevation of tail flick latency to radiant heat together with hindlimb paralysis and tail flaccidity lasting several hours, thus confirming several previous reports. A lower dose of DYN A (12.5 nmol) produced only a smaller, not constant, short-lasting change in the nociceptive threshold. The vocalization test (electrical stimulation of the tail) gave a different result: the time course curve showed that the antinociceptive effect had worn off 60 min after DYN A 25 nmol. Thus it can be assumed that the prolonged depression of the tail flick reflex was related to motor dysfunction and did not completely reflect the animal's response to painful stimuli. Tolerance to the antinociceptive and motor effects developed after the chronic intrathecal infusion of DYN A with osmotic minipumps. Intrathecal MR 1452 (30 nmol), a purported kappa-receptor blocker, fully prevented the effects of DYN A but not morphine-induced antinociception. Naloxone antagonized DYN A only at a 4 fold higher dose. MR 1452 (90 nmol) administered after DYN A reversed the elevation of the vocalization threshold while tail flick latency remained unmodified. Analysis by high performance liquid chromatography of intrathecally injected radiolabelled DYN A revealed that DYN A was largely broken down about 10 min after its administration. Our results seem to indicate that DYN A in the spinal cord causes alterations in nociception and motor function, clearly distinguishable in time and both mediated by an opioid receptor, probably of the kappa type. However, different mechanism(s), possibly non-opioid in nature, may contribute to the prolonged depression of the tail flick.

Anatomical relationship between opioid peptides and receptors in rhesus monkey brain

To determine whether opioid peptide-receptor pharmacological association found in vitro (e.g., enkephalin-delta, dynorphin-kappa) predict anatomical relationships in situ, immunocytochemical and receptor autoradiographic studies were carried out on adjacent sections from the same brains of formaldehyde-perfused rhesus monkeys. Apparent mu and kappa opioid receptors (labeled, respectively, by [3H] naloxone and [3H]bremazocine under different incubation conditions), but not delta opioid receptors (labeled by [3H]D-Ala2, D-Leu5-enkephalin), survived the fixation procedure, and were found to be colocalized throughout the brain. We have observed complex associations between these binding sites and one, two, or all three opioid peptide systems (i.e., proopiomelanocortin, proenkephalin, and prodynorphin) in different brain regions. These multiple opioid peptide-receptor subtype associations are apparent, for example, in neural systems involved in the processing of pain stimuli, and may be important for mediating different types of analgesia. Since differential processing of proenkephalin and prodynorphin can give rise to opioids of varying receptor selectivities, the colocalization of opioid receptor subtypes may signify that such processing is a key regulatory event in determining which receptor subtype is activated and, thus, the physiological consequences of opioid neurotransmission.

Pain, nociception and spinal opioid receptors

Opioid peptides derived from proenkephalin and prodynorphin are differentially distributed in the spinal cord. Proenkephalin peptides are preferentially located in the sacral portion of the cord while prodynorphin peptides are concentrated in the cervical spinal cord. Mu opioid receptor are highly concentrated in superficial layers of the dorsal horn in all the spinal cord. Delta opioid receptor are more diffusely distributed in the gray matter of the spinal cord. These sites are principally located in cervical and thoracic portions of the spinal cord. Kappa opioid receptors are highly concentrated in the superficial layers of the lumbo-sacral spinal cord. Its density decreased in the upper levels of the spinal cord. It appears that mu opioid receptors are indifferentially activated by thermal, pressure and visceral nociceptive inputs. Delta receptors are more likely to be involved in thermal nociception while kappa opioid binding sites are associated to visceral pain nociceptive inputs.