The action of some analgesic drugs in intact and chronic spinal rats

The inhibition of enkephalin catabolism by dual enkephalinase inhibitor: A novel possible therapeutic approach for opioid use disorders

Despite the increasing impact of opioid use disorders on society, there is a disturbing lack of effective medications for their clinical management. An interesting innovative strategy to treat these disorders consists in the protection of endogenous opioid peptides to activate opioid receptors, avoiding the classical opioid-like side effects. Dual enkephalinase inhibitors (DENKIs) physiologically activate the endogenous opioid system by inhibiting the enzymes responsible for the breakdown of enkephalins, protecting endogenous enkephalins and increasing their half-lives and physiological actions. The activation of opioid receptors by the increased enkephalin levels, and their well-demonstrated safety, suggests that DENKIs could represent a novel analgesic therapy and a possible effective treatment for acute opioid withdrawal, as well as a promising alternative to opioid substitution therapy minimizing side effects. This new pharmacological class of compounds could bring effective and safe medications avoiding the major limitations of exogenous opioids, representing a novel approach to overcome the problem of opioid use disorders. LINKED ARTICLES: This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

Entanglement between thermoregulation and nociception in the rat: the case of morphine

In thermoneutral conditions, rats display cyclic variations of the vasomotion of the tail and paws, the most widely used target organs in current acute or chronic animal models of pain. Systemic morphine elicits their vasoconstriction followed by hyperthermia in a naloxone-reversible and dose-dependent fashion. The dose-response curves were steep with ED50 in the 0.5-1 mg/kg range. Given the pivotal functional role of the rostral ventromedial medulla (RVM) in nociception and the rostral medullary raphe (rMR) in thermoregulation, two largely overlapping brain regions, the RVM/rMR was blocked by muscimol: it suppressed the effects of morphine. "On-" and "off-" neurons recorded in the RVM/rMR are activated and inhibited by thermal nociceptive stimuli, respectively. They are also implicated in regulating the cyclic variations of the vasomotion of the tail and paws seen in thermoneutral conditions. Morphine elicited abrupt inhibition and activation of the firing of on- and off-cells recorded in the RVM/rMR. By using a model that takes into account the power of the radiant heat source, initial skin temperature, core body temperature, and peripheral nerve conduction distance, one can argue that the morphine-induced increase of reaction time is mainly related to the morphine-induced vasoconstriction. This statement was confirmed by analyzing in psychophysical terms the tail-flick response to random variations of noxious radiant heat. Although the increase of a reaction time to radiant heat is generally interpreted in terms of analgesia, the present data question the validity of using such an approach to build a pain index.

Loss of neurons in rostral ventromedial medulla that express neurokinin-1 receptors decreases the development of hyperalgesia

It is well known that neurons in the rostral ventromedial medulla (RVM) are involved in descending modulation of nociceptive transmission in the spinal cord. It has been shown that activation of neurokinin-1 receptors (NK-1Rs) in the RVM, which are presumably located on pain facilitating ON cells, produces hyperalgesia whereas blockade of NK-1Rs attenuates hyperalgesia. To obtain a better understanding of the functions of NK-1R expressing neurons in the RVM, we selectively ablated these neurons by injecting the stable analog of substance P (SP), Sar(9),Met(O2)(11)-Substance P, conjugated to the ribosomal toxin saporin (SSP-SAP) into the RVM. Rats received injections of SSP-SAP (1 μM) or an equal volume of 1 μM of saporin conjugated to artificial peptide (Blank-SAP). Stereological analysis of NK-1R- and NeuN-labeled neurons in the RVM was determined 21-24 days after treatment. Withdrawal responses to mechanical and heat stimuli applied to the plantar hindpaw were determined 5-28 days after treatment. Withdrawal responses were also determined before and after intraplantar injection of capsaicin (acute hyperalgesia) or complete Freund's adjuvant (CFA) (prolonged hyperalgesia). The proportion of NK-1R-labeled neurons in the RVM was 8.8 ± 1.3% in naïve rats and 8.1 ± 0.8% in rats treated with Blank-SAP. However, injection of SSP-SAP into the RVM resulted in a 90% decrease in NK-1R-labeled neurons. SSP-SAP did not alter withdrawal responses to mechanical or heat stimuli under normal conditions, and did not alter analgesia produced by morphine administered into the RVM. In contrast, the duration of nocifensive behaviors produced by capsaicin and mechanical and heat hyperalgesia produced by capsaicin and CFA were decreased in rats pretreated with SSP-SAP as compared to those that received Blank-SAP. These data support our earlier studies using NK-1R antagonists in the RVM and demonstrate that RVM neurons that possess the NK-1R do not play a significant role in modulating acute pain or morphine analgesia, but rather are involved in pain facilitation and the development and maintenance of hyperalgesia.

Dissection of placebo analgesia in mice: the conditions for activation of opioid and non-opioid systems

Amanzio and Benedetti (J Neurosci 1999; 19: 484-494) first addressed the conditions necessary for the activation of opioid and non-opioid placebo responses in human. Here, we investigated whether placebo analgesia is subdivided into opioid and non-opioid components in mice by using the model of hot-plate test. Drug conditioning was performed by the combination of the conditioned cue stimulus with the unconditioned drug stimulus, either opioid agonist morphine hydrochloride or non-opioid aspirin. Placebo analgesic responses were evoked by an exposure to a conditioned cue previously paired with drug conditioning. Morphine conditioning produced placebo responses that were completely antagonised by naloxone. By contrast, the conditioned cue after aspirin conditioning elicited a placebo effect that was not blocked by naloxone. Therefore, we first evoked opioid and non-opioid placebo responses in mice that were either naloxone-reversible or naloxone-insensitive, depending on the drug used in conditioning procedure. These findings support that the mechanisms underlying placebo analgesia may depend on the drug conditioning that was originally performed. The present procedure of mice may serve as a model for further understanding of the opioid and non-opioid mechanisms underlying placebo responses.

The role of descending fibers from the rostral ventromedial medulla in opioid analgesia in rats

There has been controversy as to whether the contribution of descending fibers from the rostral ventromedial medulla to opioid analgesia depends on the nature of the noxious stimulus eliciting pain. In the present study, inactivation of descending fibers by microinjection of muscimol (50 ng) in the rostral ventromedial medulla abolished morphine analgesia in the tail immersion and hot plate tests but decreased morphine analgesia by 60% in the formalin test. Analysis of the dose-response relation for morphine after inactivation of descending fibers revealed that, except for the tail immersion test, high doses of morphine could not overcome the block induced by muscimol. Also, morphine analgesia elicited supraspinally was not detectable when descending fibers were inactivated, suggesting that the analgesic effect of morphine in the brain requires a relay via the rostral ventromedial medulla. The analgesic effect of buprenorphine also depends on the integrity of descending fibers from the rostral ventromedial medulla. The results indicate that descending fibers from the rostral ventromedial medulla are critically important to the analgesic effect of opioids, regardless of the type of noxious stimulation eliciting pain. Residual analgesic effects of opioids after inactivation of descending fibers may be due to peripheral effects in the presence of inflammation.

Morphine and dextrorphan lose antinociceptive activity but exhibit an antispastic action in chronic spinal rats

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

Comparison of the antinociceptive and antispastic action of (-)-baclofen after systemic and intrathecal administration in intact, acute and chronic spinal rats

Baclofen is particularly effective in treating spasticity of spinal origin in humans. However, most investigations of this drug in animals have only assessed its antinociceptive effect, presumably because of the difficulty in developing animal models of spasticity. This study attempted to evaluate both, the antinociceptive and antispastic action of (-)-baclofen (the more active enantiomer) by incorporating the chronic spinal preparation, in which spasticity gradually develops following spinal transection. Separate groups of intact, acute (1 day) or chronic (20-25 days) spinal rats were pretested on the nociceptive tail-flick (TF) assay prior to either subcutaneous (SC; 1-30 mg/kg) or intrathecal (IT; 0.1-12 micrograms) injection of (-)-baclofen and retested at specific post-injection intervals. Hindlimb spasticity was elicited in chronic spinal rats by mechanical stimulation to the abdomen. Because the clinical use of baclofen generally involves chronic administration, both responses were tested for 3 successive days to assess tolerance. Results confirmed the analgesic effect of SC and IT (-)-baclofen in intact rats. As previously reported, the antinociceptive effect of IT (-)-baclofen was increased in acute spinal rats. However, three weeks after spinalization there was a profound decrease in this response. In contrast, antinociception produced by SC (-)-baclofen was reduced in acute and chronic spinal rats compared to intact animals; but there was no difference between the acute and chronic conditions. In spite of this differential decrease in antinociception after IT, relative to SC, administration, both routes of administration produced an antispastic effect in chronic spinal rats. There was no antinociceptive tolerance to SC administration and only minimal tolerance to IT (-)-baclofen (in intact rats); the antispastic effect did not become tolerant. A peripheral action might explain the dichotomy between SC and IT (-)-baclofen in regard to antinociception. However, further research is needed to determine why both routes of administration were effective against spasticity while only SC (-)-baclofen retained an antinociceptive action in chronic spinal rats.

Characterization of tailshock elicited withdrawal reflexes in intact and spinal rats

The tail flick withdrawal reflex (TFR) was generated by applying graded electric current to the tail of intact and spinally transected rats. In Experiment 1, separate groups of rats were tested 1, 3, 7, 10, 14, or 21 days after spinal transection. The latency, amplitude, and magnitude of the TFR was highly related to current intensity in both intact and spinal animals. However, the TFR changed dramatically as a function of the number of days between spinalization and TFR measurement. Compared to intact controls, the current intensity at which TFR was initiated (threshold) in spinal rats was elevated 1 and 3 days after transection, did not differ at 7 and 10 days, and was reduced at 14 and 21 days. Latency of TFR in spinal rats did not differ from controls 1 day after transection, but decreased steadily thereafter. Amplitude and magnitude of TFR in spinal rats remained depressed, but did show recovery toward control levels as the interval between transection and testing increased. Changes in the TFR of spinal rats was correlated with recovery of tailpinch-elicited hindlimb withdrawal. Experiment 2 demonstrated that the dose-response curve relating systemic morphine treatment to increases in TFR thresholds was shifted to the right in chronic spinal rats. Threshold increases in both spinal and intact rats were not necessarily accompanied by changes in TFR performance. These experiments establish the segmental organization of tailshock-elicited TFR and supports its use as a measure of nociceptive transmission at spinal levels.

Spinal transection reduces both spinal antinociception and CNS concentration of systemically administered morphine in rats

Within one day after spinal transection, the antinociceptive effect of systemically administered morphine on the spinal withdrawal reflex is significantly reduced. This observation has provided important empirical support for the present model of opiate-induced analgesia. One prediction from this model is that the antinociceptive effect of intrathecal (spinal) morphine injections should not be reduced by spinalization. When examined experimentally, this prediction was not supported; the antinociceptive effect of intrathecally administered morphine was significantly enhanced after acute spinalization. This result suggested an alternate hypothesis of morphine-induced analgesia. One prediction from this new hypothesis is that the decreased behavioral response to systemic morphine in spinal rats is due to a decrease in the spinal concentration of morphine produced by spinal transection. To test this prediction separate groups of intact rats and acute (one day) spinal rats, were assessed with the tail-flick (TF) procedure 60 min after subcutaneous injection of various doses of morphine (0.75, 1.5, 3.0, 4.5, 6.0 or 9.0 mg/kg) or at different time points (30, 60, 90, 150 or 240 min) after a single injection of 9.0 mg/kg. Immediately after behavioral testing, the rats were killed and brains, spinal cords and blood samples were collected and subsequently analyzed with a morphine radioimmunoassay. The results show that the concentration of morphine in the brain and spinal cords of acute spinal rats is significantly lower than that of intact rats, whereas morphine levels in the blood do not differ. These data suggest that the decreased antinociceptive effect of subcutaneous morphine in acute rats is due to a decrease in the concentration of the opiate in the central nervous system.(ABSTRACT TRUNCATED AT 250 WORDS)

Depression of the flexor reflex by systemic morphine increases in chronically spinalized rats

The effect of i.v. morphine on the spinal nociceptive flexor reflex was examined in decerebrate, unanesthetized rats in which the spinal cord was acutely or chronically transected. In acutely spinalized rats i.v. morphine dose dependently depressed the flexor reflex with an ED50 of 1.4 mg/kg. In rats in which the spinal cord was transected 1-3 days prior to the electrophysiological experiments, i.v. morphine had a similar depressive effect (ED50 = 1.1 mg/kg). However, 4-6 days after spinalization there was a 4-fold increase in morphine's depressive effect (ED50 = 0.35 mg/kg). The sensitivity to morphine was further increased 7-10 days post-spinalization with ED50 of 0.17 mg/kg. This supersensitivity was maintained up to 30-60 days. The reflex depression caused by morphine in both acutely and chronically spinalized rats could be reversed by naloxone. Morphine i.v. did not influence the monosynaptic reflex in either acutely or chronically spinalized rats. The present results support the clinical finding that patients with spinal lesions have an increased sensitivity to the antinociceptive and reflex depressive effects of systemic and intrathecal morphine.

Hepatobiliary effects of morphine are mediated in the brain

Morphine slows hepatobiliary elimination of sulfobromophthalein in rodents, raising dye levels in plasma and liver. Earlier studies showed these effects to be independent of other opiate effects such as bile duct spasm, hypothermia or blood gas changes resulting from respiratory depression. Because opiate receptors are distributed throughout the body, within the central nervous system and at peripheral sites including the gastrointestinal tract, experiments were performed to ascertain whether central or peripheral sites mediate the hepatobiliary effects of morphine. Sulfobromophthalein was administered intravenously to mice and its levels were measured in plasma and liver. Tail-flick latency indicated centrally mediated analgesia. Inhibited intestinal transit of India ink reflected an opiate effect with a significant peripheral component. When injected into a cerebral ventricle morphine was much more potent in producing analgesia and raising sulfobromophthalein levels than when administered intravenously or intraperitoneally. An intravenous dose of naloxone that reversed morphine analgesia also prevented sulfobromophthalein elevation but did not prevent gut slowing. Naltrexone injected in a cerebral ventricle also reversed analgesia and sulfobromophthalein elevation but not intestinal slowing. The polar opiate agonist N-methylmorphine did not cause analgesia or raise sulfobromophthalein levels at peripheral intraperitoneal doses to 100 mg/kg. When given in a central ventricle at 4 x 10(-3) mg/kg, this agent produced analgesia and raised sulfobromophthalein but did not slow intestinal transit. After spinal cord transection, intravenous morphine did not retard the tail-flick response or affect sulfobromophthalein disposition, but peripherally mediated intestinal transit was slowed as it was in intact mice. These experiments demonstrate parallel opiate effects on analgesia and on BSP disposition but not on intestinal transit.(ABSTRACT TRUNCATED AT 250 WORDS)

Antinociceptive effect of intrathecal morphine in tolerant and nontolerant spinal rats

The antinociceptive effect of intrathecal morphine on the tail-flick (TF) reflex of rats was significantly enhanced within one day after spinal transection (ED50 = 0.125 microgram) relative to the effect obtained in intact rats (ED50 = 5.9 micrograms). By 20-30 days after spinalization the potency of intrathecally administered morphine had substantially declined. Intact rats, made tolerant to the antinociceptive effect of systemic morphine (3.0 mg/kg, SC on each of seven consecutive days), were not tolerant to intrathecal morphine (ED50 = 6.5 micrograms). In contrast, rats that were pretreated with either morphine alone, repeated TF tests alone, or both of these treatments, were tolerant to intrathecal morphine when tested one day after spinal transection. The results suggest first, that the antinociceptive effect of intrathecal morphine in intact rats is tonically inhibited by descending supraspinal input and that removal of this input is responsible for the enhanced antinociceptive effect of intrathecal morphine in spinal rats. Second, the data suggest that tolerance to the antinociceptive effect of intrathecal morphine in intact rats may also be tonically inhibited by supraspinal input, because spinal opiate tolerance is expressed after spinal transection.

Spinal vs. supraspinal actions of morphine on the rat tail-flick reflex

A controversy exists as to whether morphine attenuates spinal cord nociceptive transmission through a supraspinal site of action. The approach of examining the effects of morphine on spinal cord nociceptive transmission in the presence and absence of spinal cord conduction has led to conflicting conclusions. We have compared the effects of morphine on the rat tail-flick reflex (TFR) in lightly anaesthetized animals in the presence and absence of a spinal cord cold-block. Morphine, administered systemically, was found to be more potent in increasing the latency of the reflex when the spinal cord conduction was present. However, when low doses of morphine were injected intrathecally, morphine was more potent when spinal cord conduction was blocked. These data indicate that systematically administered morphine, at low doses, has a supraspinal site of action in prolonging the onset of the TFR. Conflicting results on this issue do not appear to be due to plasticity changes following spinal cord section or lesions, psychological stress in conscious animals or the presence of tonic bulbospinal inhibition.

Antinociceptive effect of systemic and intrathecal morphine in spinally transected rats

The antinociceptive effect of morphine on the tail withdrawal reflex was examined in spinally transected rats. The efficacy of systemically administered morphine was significantly reduced within 24 h after transection, and continued to decline during the first three posttransection weeks. In contrast to the diminished effect of systemic morphine, the efficacy of intrathecal morphine was not reduced during the first three weeks after a spinal transection. These data demonstrate a significant difference in the functional effect of systemic and spinal morphine in spinally transected rats. The results indicate that the direct antinociceptive effect of morphine on the spinal cord is not reduced after spinal transection.

A quantitative study on the tail flick test in the rat

Tail flick latency and response temperature were measured in awake rats. Both of these parameters varied with the change in rate of increase of radiant heat stimulus and the area of skin irradiated. The caloric factor applied to the tail before the response occurred was calculated approximately from the time-temperature curves monitored over the tail and the response latencies. The magnitude of the caloric factor over a tentative threshold temperature was the crucial factor for the induction of tail flick response, because it was similar for the various irradiating conditions. It was proposed that analgesic effects in the tail flick test may be expressed using this caloric factor which allows direct comparison of studies in which different base-line latencies are used.

Kappa opioid receptor-mediated analgesia in the developing rat

The prototypic kappa opiate ketocyclazocine produced robust analgesia in 10-day-old rats in the tail-flick nociceptive test. The kappa-opiate behavioral response coincided with the onset of a rapid rise to adult levels in brain kappa receptor site density. In contrast, morphine (prototypic mu opiate) was without marked effect until 14 days of age. The period of rapid mu receptor increase did not take place until days 14-16, which was after kappa receptor levels had already plateaued. Further, there was no or incomplete cross-tolerance between ketocyclazocine and morphine at 14 days of age. The present study, therefore, establishes a role for the kappa binding site in thermal analgesia in the tail flick test and differentiates its ontogenetic pattern from that of the mu receptor.

Antinociceptive effects of orphenadrine citrate in mice

The antinociceptive effect of orphenadrine citrate, a muscle relaxant, was investigated in mice. Four different pain tests were selected to involve different noxious stimuli. Clear antinociceptive effects were found in the formalin test. The increasing temperature hot-plate test showed a biphasic dose-response relationship with slight hyperalgesia after low doses and hypoalgesia after higher doses. No significant effects of orphenadrine (0-25 mg/kg) were found in the tail flick and constant temperature hot-plate tests. The data suggest that orphenadrine may reduce, enhance or leave unaffected different types of nociceptive transmission. Orphenadrine may also possess analgesic properties in conditions not involving muscle spasm.

Central and peripheral antialgesic action of aspirin-like drugs

The peripheral and central effects of some non-steroid anti-inflammatory drugs, aspirin, indomethacin, paracetamol and phenacetin were studied by comparing their intraplantar and intracerebroventricular effects on hyperalgesia induced by carrageenin injected into the rat paw. Hyperalgesia was measured by a modification of the Randall-Selitto test. The agents tested had antialgesic effects when given by any route. Their intraventricular administration enhanced the antialgesic effect of anti-inflammatory drugs administered into the paw. Previous treatment of one paw with carrageenin reduced the oedema caused by a second injection of carrageenin in the contralateral paw. In contrast, it had no effect on the intensity of hyperalgesia but shortened the time necessary for it to reach a plateau. Administration of a prostaglandin antagonist (SC-19220) in the cerebral ventricles, in the rat paw or in both sites, significantly inhibited the hyperalgesia evoked by carrageenin. The maximal hyperalgesic effect of intraplantar injections of prostaglandin E2 could be further enhanced by its cerebroventricular administration. It was suggested that carrageenin hyperalgesia has a peripheral and a central component and that the cyclo-oxygenase inhibitors used may exert an antialgesic effect by preventing the hyperalgesia induced by a peripheral and/or central release of prostaglandins.