Stimulus intensity and the comparative efficacy of mu- and kappa-opioid agonists on nociceptive spinal reflexes in the rat

Self-injurious behavior in neurodevelopmental disorders: relevance of nociceptive and immune mechanisms

Self-injurious behavior (SIB) among individuals with intellectual and related neurodevelopmental disorders (IDD) is a clinical challenge and scientific puzzle. The physiological mechanisms regulating the sensory components of SIB remain a mystery with no clear understanding of the underlying pathophysiology. The central dogma regarding sensory processing in general and pain in particular among individuals with IDD and chronic SIB is that sensory processing is reduced and pain is absent or blunted. In this paper, recent findings challenging some of the conventional wisdom regarding pain and sensory function among individuals with IDD and SIB are reviewed. It seems that at least a subgroup of individuals with IDD and chronic SIB may be in a physiological state similar to neuropathic pain in which hyperalgesia is mediated by plasticity mechanisms regulating inflammatory, immune, and nociceptive systems. In response to repeated tissue damage associated with chronic self-injury, innate immune cells may be producing pro-inflammatory and pro-nociceptive cytokines that act on the brain to cause sickness-like behavior and sensitize primary sensory nerve afferents contributing to pain hypersensitivity (i.e., hyperalgesia).

NCX-701 (nitroparacetamol) is an effective antinociceptive agent in rat withdrawal reflexes and wind-up

1. Non-steroidal anti-inflammatory drugs (NSAIDs) are effective anti-inflammatory and analgesic drugs although they also induce unwanted side effects due to the inhibition of the physiological effects regulated by prostaglandins. This has led to the search for new compounds with fewer side effects, such as the nitro-NSAIDs (NO-NSAIDs). Paracetamol is an analgesic drug devoid of some of the side effect of the NSAIDs but without anti-inflammatory activity. NCX-701 is a nitric oxide releasing version of paracetamol with anti-inflammatory and analgesic properties. 2. We have tested, in the single motor unit technique, the antinociceptive actions of intravenous cumulative doses of NCX-701 vs paracetamol, studying their antinociceptive effects in responses to noxious mechanical and electrical stimulation (wind-up). 3. Paracetamol did not induce any significant effect at the doses tested (maximum of 480 micromol kg(-1), 72.5 mg kg(-1)). NCX-701 however was very effective in reducing responses to noxious mechanical stimulation (32+/-10% of control response) and wind-up (ED(50) of 147+/-1 micromol kg(-1), 41.5+/-0.3 mg kg(-1)). The inhibition was not reversed by 1 mg kg(-1) of the opioid antagonist naloxone. In control experiments performed with either the vehicle or the NO donor NOC-18, no significant changes were observed in the nociceptive responses studied. 4. We conclude that NCX-701 is a very effective non-opioid antinociceptive agent in normal animals and its action is located mainly at central areas. The antinociceptive effect was not due solely to the release of NO.

Subeffective doses of dexketoprofen trometamol enhance the potency and duration of fentanyl antinociception

The combination of classic non-steroidal antiinflammatory drugs (NSAIDs) with opiates induces more analgesia than the summed effect of each drug given separately. No studies have been performed using new generation NSAIDs and fentanyl nor on the duration of this effect. We have studied the analgesic effect of fentanyl alone and after the administration of subeffective doses of dexketoprofen trometamol in rat nociceptive responses. The responses were evoked by noxious mechanical stimulation and were recorded as single motor units in male Wistar rats anaesthetized with alpha-chloralose. The effective dose 50 (ED(50)) observed with fentanyl was 22.4 +/- 1.5 microg kg(-1) and full recovery was apparent 20 min later. The administration of a total dose of 40 microg kg(-1) of dexketoprofen trometamol did not induce any significant effect on the nociceptive responses. In the presence of dexketoprofen trometamol, the ED(50) for fentanyl was 5 fold lower than before: 3.8 +/- 1.1 microg kg(-1) and no significant recovery was observed 45 min later. The opioid antagonist naloxone (200 microg kg(-1)) did not reverse the effect, although in control experiments the same dose was able to prevent any action of fentanyl given alone. We conclude that the combination of fentanyl and subeffective doses of dexketoprofen trometamol induces a more potent and longer lasting analgesic effect than that observed with fentanyl alone, and that this is not an opioid mediated action.

The effect of fentanyl, DNQX and MK-801 on dorsal horn neurones responsive to colorectal distension in the anaesthetized rat

Certain dorsal horn neurones respond in a graded manner to noxious colorectal distension (CRD). Morphine inhibits these responses in the spinalized rat, but the role of excitatory amino acids in baseline visceral nociceptive transmission is less clear. This study examines the effect of the mu-opiate receptor agonist fentanyl, and the non-NMDA and NMDA antagonists DNQX and MK-801, respectively, on such responses to CRD in the sodium pentobarbitone-anaesthetized rat. Male rats were prepared for extracellular recording from the lumbosacral spinal cord. 90 neurones responsive to CRD, located throughout the dorsal horn, were classified according to their response duration and latency to 60 mmHg distension, as SL-A (short latency-abrupt; 59%), SL-S (short latency-sustained; 23%), L-L (long-latency; 10%) and Inhib (inhibited; 8%). Convergent cutaneous receptive fields were mapped for 79/90 neurones and classified as LT (low threshold), WDR (wide dynamic range) or HT (high threshold). CRD (20-100 mm Hg) elicited graded responses in most neurones. In 6/6 SL-S neurones, fentanyl (1-8 microg kg-1) dose-dependently inhibited the response to 60 mm Hg CRD, in a naloxone-sensitive manner, with an ID50 value (+/-95% confidence limits) of 2.48 (1.7-3. 7) microg kg-1. In 6/6 SL-A neurones, fentanyl had no significant effect on the response to CRD. DNQX (0.03-3 mg kg-1) produced a dose-dependent inhibition of the response to CRD in 5/5 SL-A neurones, with an ID50 value of 0.32 (0.01-41.1) mg kg-1. MK-801 (0. 03-0.3 mg kg-1) had no significant effect on responses to CRD in 6/6 SL-A neurones. The differential inhibitory effects of fentanyl on two neuronal subtypes may indicate functional differences. In SL-A neurones AMPA/kainate, but not NMDA receptors are involved in mediating baseline nociceptive neurotransmission.

Effects of U-69,593, a kappa-opioid receptor agonist, on carrageenin-induced peripheral oedema and Fos expression in the rat spinal cord

In an attempt to study the anti-inflammatory and the antinociceptive effects of a kappa1-opioid receptor agonist (U-69,593: trans-3,4-dichloro-N-methyl-N-[7-(1-pyrrolidinyl)cycloexil]benzene acetamide methanesulfonate), we used a combination of the measurement of peripheral oedema (with a calliper) and Fos immunodetection in the carrageenin model of inflammation. The intraplantar injection of carrageenin-induced the development of a peripheral oedema, associated with an increase in Fos-like immunoreactivity at the level of the dorsal horn of the spinal cord. U-69,593 administered intravenously (i.v.) 10 min before carrageenin administration over the dose range 0.75, 1.5 and 3 mg/kg, reduced both paw and ankle oedema in a non dose-dependent manner. The maximal decrease was observed at the highest dose and did not exceed 21% and 20% for the paw and the ankle respectively. These effects were kappa-opioid receptor specific since the anti-inflammatory effect of 1.5 mg/kg i.v. of U-69,593 was antagonised by a specific kappa-opioid receptor antagonist nor-binaltorphimine. Pre-treatment with U-69,593 strongly decreased the number of Fos-like Immunoreactive neurones of the spinal cord in a dose-dependent, antagonist reversible manner; maximal effect was 65%. The disparate results between the anti-inflammatory effects and the depressive effects on Fos expression suggest that anti-inflammatory effects of kappa-opioid receptor agonist are of minor importance for the antinociceptive effects of this compound.

The NSAID dexketoprofen trometamol is as potent as mu-opioids in the depression of wind-up and spinal cord nociceptive reflexes in normal rats

The aim of this study was to examine the potency of the antinociceptive effects of the non-steroidal antiinflammatory drug (NSAID), Dexketoprofen Trometamol (the active enantiomer of ketoprofen) on spinal cord nociceptive reflexes. These effects were compared with those of the mu-opioid receptor agonist fentanyl in normal animals. The experiments were performed in male Wistar rats anaesthetised with alpha-chloralose. The nociceptive reflexes were recorded as single motor units in peripheral muscles, activated by mechanical and electrical stimulation. Both dexketoprofen and fentanyl inhibited responses evoked by mechanical and electrical stimulation with doses in the same nanomolar range (dexketoprofen ID50s: 100 and 762 nmol kg-1 and fentanyl: 40 and 51 nmol kg-1, respectively). Dexketoprofen and fentanyl also significantly inhibited wind-up. Since fentanyl has been shown to be some 1000 times more potent than morphine in this type of experiments, we conclude that dexketoprofen has central analgesic actions in normal animals and depresses nociceptive responses with a potency similar to that of mu-opioid agonists.

Cutaneous responsiveness of rat single motor units activated by natural stimulation

Recordings of withdrawal reflexes have been used extensively to study sensory-motor integration and processing of nociceptive information in the spinal cord. We describe here a new technique for the manufacture of improved EMG electrodes that permit the characterisation of the physiological properties of single motor units as well as the easy location of the muscles studied. Individual motor units from three rat hind-limb muscles: peroneus longus, tibialis cranialis and extensor digitorum longus, were activated by thermal and mechanical stimulation applied to their cutaneous receptive fields, which were located mainly on the 4th and 5th toes. Thresholds for thermal and mechanical (Von Frey hairs) stimulation were similar in the three muscles studied, with a value of 44 +/- 1 degrees C and 100 mN (median), respectively. However, when a mechanical pincher with a stimulus area of 14 mm2 was used, the values seen were similar for peroneus longus and tibialis cranialis (342 +/- 23 and 330 +/- 71 mN, respectively, mean +/- S.E.M.) but lower for extensor digitorum longus (220 +/- 37 mN, mean +/- S.E.M.). The firing rate of the single motor units was similar for all types of stimulation at threshold intensity, and showed a linear relationship with stimulus intensity, except for units of the tibialis cranialis, which showed a greater degree of adaptation.

Reversal by naloxone of the spinal antinociceptive actions of a systemically-administered NSAID

1. Possible interactions between non-steroidal anti-inflammatory drugs (NSAIDs) and endogenous opioids were examined in electrophysiological experiments in alpha-chloralose anaesthetized spinalized rats without or with carrageenan-induced acute inflammation of one hindpaw. Spinal reflex responses, monitored as single motor unit discharges, were elicited by noxious pinch and electrical stimuli. 2. The mu-opioid agonist, fentanyl, was an effective depressant of reflexes under all conditions (ED50 6-14 micrograms kg-1, i.v.). In rats without peripheral inflammation the NSAID, flunixin, a niflumic acid derivative, had only a small effect that was not dose-dependent. However, in animals with unilateral inflammation, flunixin reduced spinal reflexes evoked both by noxious pinch stimuli (that activate peripheral nociceptors; ID50 4 mg kg-1, i.v.) and by electrical stimuli (that bypass nociceptor endings; ID50 6.5- 11 mg kg-1, i.v.), indicating that it has a central site of action at doses comparable to those used clinically. 3. The opioid antagonist, naloxone (1 mg kg-1, i.v.), reversed all actions of fentanyl. It did not reverse the small effects that flunixin had in rats without inflammation, showing that the NSAID is not a direct opioid agonist. In rats with carrageenan-induced inflammation of the hindpaw, however, naloxone fully reversed or prevented the antinociception by flunixin, but not that by the alpha 2-adrenoceptor agonist, medetomidine. 4. We conclude that under conditions of peripheral inflammation and the resultant central changes, the NSAID, flunixin, has antinociceptive actions that are mediated by endogenous opioids acting within the spinal cord.