Teresa Pelissier

A method to perform direct transcutaneous intrathecal injection in rats.

This work describes a method for performing direct intrathecal (i.t.) injections in rats without introducing a spinal catheter. Its ease of use in awake animals yields rapid and reproducible results with no sign of motor impairment. The quality of each injection was ensured by the observation of an injection-induced tail-flick. A 10-microL injection of methylene blue was well localized yielding a very limited diffusion along the spinal cord. The method was validated by demonstrating that morphine (i.t.) had a marked antinociceptive effect and that naloxone (i.t.) blocked the effect of systemic (s.c.) morphine in mononeuropathic rat.

Antinociceptive effects of Ca2+ channel blockers

The antinociceptive action of four Ca2+ channel blockers, nifedipine, nimodipine, verapamil and diltiazem, was evaluated and compared to that of morphine using three algesiometric tests in mice and rats, namely, formalin, writhing and modified hot-plate test. Dose-response curves for all the drugs tested were similar and a significant dose-dependent antinociceptive action was evident in the formalin and writhing tests. However, in the hot-plate test, only nimodipine exhibited a significant analgesic effect, confirming the misleading results previously reported for this test. The findings suggest a pharmacological role of Ca2+ channel blockers in the modulation of antinociception under acute conditions. The analgesic action of Ca2+ channel blockers could be mediated by an increase in the nociceptive threshold resulting from interference with Ca2+ influx at opioid receptors, because Ca2+ influx is critical for the release of neurotransmitters and other substances implicated in nociception and inflammation. It is suggested that if a substance has a Ca2+ channel blocking effect, it should probably have some antinociceptive properties.

The orofacial capsaicin test in rats: effects of different capsaicin concentrations and morphine.

&NA; The aim of this study was to develop a rat model of capsaicin‐induced pain in the orofacial region. We examined the effects of subcutaneous injection of different doses of capsaicin (0.25, 0.4, 0.8, 1.5, 2.5, 25, 50, 100, 500 &mgr;g) on the face‐grooming response. Injection of capsaicin into the vibrissa pad produced an immediate grooming of the injected area with ipsilateral fore‐ or hindpaw. A positive relationship between the amplitude of the grooming response and the capsaicin dose was observed until 1.5 &mgr;g, but with the highest concentrations (ranging from 25 to 500 &mgr;g) the amplitude of the response decreased. Morphine administered either systemically (in the neck, 0.5–4.0 mg/kg) or locally (0.25–1.0 mg/kg) reduced in a dose‐dependent fashion the face grooming provoked by subcutaneous capsaicin (1.5 &mgr;g). The systemic and local morphine effects could be reversed by systemic (0.1 mg/kg) and local (0.05 mg/kg) administration of naloxone, respectively. The local administration of morphine (ED50: 0.65 mg/kg) was more potent than systemic injection (ED50: 2.54 mg/kg) in reducing the grooming behavior. In conclusion, the orofacial capsaicin test appears to be a valid and reliable method for studying trigeminal pain mechanisms and testing analgesic drugs. The results of the present study also support the clinical use of peripheral opioid administration for the treatment of orofacial painful conditions.

Evidence for an exclusive antinociceptive effect of nociceptin/orphanin FQ, an endogenous ligand for the ORL1 receptor, in two animal models of neuropathic pain

&NA; Nociceptin/orphanin FQ (noci/OFQ), the endogenous ligand for the orphan ORL1 (opioid receptor‐like1), has been shown to be anti‐ or pronociceptive and modify morphine analgesia in rats after central administration. We comparatively examined the effect of noci/OFQ on hyperalgesia and morphine analgesia in two experimental models of neuropathic pain: diabetic (D) and mononeuropathic (MN) rats. Noci/OFQ, when intrathecally (i.t.) injected (0.1, 0.3, or 1, to 10 &mgr;g/rat) was ineffective in normal rats, but reduced and suppressed mechanical hyperalgesia (paw‐pressure test) in D and MN rats, respectively. This spinal inhibitory effect was suppressed by naloxone (10 &mgr;g/rat, i.t.) in both models. Combinations of systemic morphine with spinal noci/OFQ resulted in a strong potentiation of analgesia in D rats. In MN rats, an isobolographic analysis showed that the morphine+noci/OFQ association (i.t.) suppressed mechanical hyperalgesia in a superadditive manner. In summary, the present findings reveal that spinal noci/OFQ produces a differential antinociception in diabetic and traumatic neuropathic pain according to the etiology of neuropathy, an effect possibly mediated by opioid receptors. Moreover, noci/OFQ combined with morphine produces antinociceptive synergy in experimental neuropathy, opening new opportunites in the treatment of neuropathic pain.

Evidence of a central antinociceptive effect of paracetamol involving spinal 5HT3 receptors.

THIS study was carried out to determine both the effect of systemic paracetamol on the C-fibre evoked reflex activity, a test sensitive to centrally acting analgesic drugs, and the influence of an intrathecally administered 5HT3 receptor antagonist, tropisetron. Paracetamol (200, 300, 400mgkg-1, i.v.) dose-dependently decreased (maximal effects −60±8%) the C-evoked responses for a duration of 90 min (for the lowest dose). This effect was totally suppressed by tropisetron (1μg, i.t.). These data confirm previous studies suggesting a central effect of this drug and demonstrate the involvement of a spinal 5HT3 mediated serotonergic mechanism.

Evidence for a monoamine mediated, opioid-independent, antihyperalgesic effect of venlafaxine, a non-tricyclic antidepressant, in a neurogenic pain model in rats

Antidepressants are the first choice in the treatment of neuropathic pain but the mechanism of their analgesic effect is still unclear. Venlafaxine (VFX) is a structurally new antidepressant which inhibits reuptake of 5-hydroxytryptamine and noradrenaline, but unlike tricyclic antidepressants, has few side effects. Using hind paw pressure testing, this study investigated the antinociceptive effect of VFX and its mechanism of action in healthy rats and in a model of neuropathic pain, chronic constriction injury of the sciatic nerve (CCI model). In healthy rats and the CCI model, the antinociceptive effect of VFX was studied after subcutaneous (s.c.) acute injection (10, 20 and 40 mg/kg, s.c.) and repeated administration (five successive injections of 2.5, 5 and 10 mg/kg, s.c, every half-life). While no effect was observed in healthy rats, a single injection of VFX significantly increased, in a dose-dependent manner, the vocalisation threshold in the CCI model. VFX, repeatedly administered, had a dose-dependent antinociceptive effect in healthy rats and a better efficacy in CCI model. The effect was reversed by intraperitoneal (i.p.) α-methyl-p-tyrosine, an inhibitor of noradrenaline synthesis (100 mg/kg/day for 2 days), and parachlorophenylalanine, an inhibitor of serotonin synthesis (300 mg/kg/day for 2 days). In contrast, naloxone (1 mg/kg), an antagonist of opioid receptors, was ineffective. These results demonstrate a clear antihyperalgesic effect of VFX, particularly after repeated administrations. To be effective, VFX requires functional noradrenergic and serotonergic systems, but unlike most mixed tricyclic antidepressants does not involve the endogenous opioid system. Further experimental work is needed to clarify the specific effect of VFX. This knowledge could be of clinical relevance if it enabled us to distinguish between different classes of antidepressants used to treat neuropathic pain.

Antinociceptive interactions of ketamine with morphine or methadone in mononeuropathic rats

To study the antinociceptive synergy resulting from the combination of opioid receptor agonists and N-methyl-D-aspartate (NMDA) receptor antagonists on neuropathic pain, an isobolographic analysis of equianalgesic combinations of ketamine with methadone or morphine was performed in rats with mononeuropathy produced by placing four constrictive ligatures around the common sciatic nerve. Two weeks later, the antinociceptive effect of subcutaneous administration of the drugs alone or combined was evaluated by using the paw pressure test. Drugs and their combinations produced dose-dependent antinociception. Combinations produced synergy of a supra-additive nature in the neuropathic paw, but only additive antinociception in the normal paw. The ketamine/methadone combination was more effective to produce antinociception in the neuropathic paw than was the ketamine/morphine association, as revealed by the lower ED25. The results indicate supra-additive synergy between NMDA receptor antagonists and opioids, especially methadone, to produce antinociception in experimental neuropathy.

Evidence for an antihyperalgesic effect of venlafaxine in vincristine-induced neuropathy in rat.

Venlafaxine, a new antidepressant with fewer side effects, could be of interest to reduce neuropathic pain following antineoplasic drug treatment. In the present study, we demonstrated that venlafaxine inhibits hyperalgesia in a new rat model of neuropathy induced by the antineoplasic drug vincristine, and exerts its effect preferentially via supraspinal and spinal mechanisms.

Effect of melatonin on rat spinal cord nociceptive transmission.

Melatonin has been shown to exert potent antinociception but the sites and mechanisms of action underlying this effect have not yet been clarified. The effect of melatonin on spinal cord nociceptive transmission was studied in rats by assessing wid-up activity in a C-fiber reflex responses paradigm evoked by repetitive (0.6 Hz) electric stimulation. Intraperitoneal administration of 1.25, 2.5, 5.0 and 10.0 mg/kg melatonin induced a dose-dependent inhibition of spinal wind-up activity, the higher dose of the drug used being able to depress completely the C reflex gain. Results indicate that melatonin markedly depresses spinal wind-up in rats, probably through hyperpolarization of dorsal horn neurons consecutive to melatonin binding to membrane receptors, and/or via intracellular interference with a NMDA receptor-dependent nitric oxide generating pathway.

Involvement of spinal cord BDNF in the generation and maintenance of chronic neuropathic pain in rats

Brain-derived neurotrophic factor (BDNF) is involved in neuronal survival and synaptic plasticity of the central and peripheral nervous system. In chronic pain, plastic changes in dorsal horn neurons contribute to a phenomenon of hypersensitivity to pain sensation that is maintained over time, known as central sensitization. This process is accompanied by BDNF overexpression, but the role of BDNF in the generation and maintenance of the hyperalgesic phenomenon is still unclear. The present study was aimed to investigate if exogenous BDNF administered to the rat spinal cord, in addition to trigger pain, participates in the maintenance of the central sensitization process (i.e., pain persistence) and to determine if the pain generated is comparable to that observed in a neuropathic pain model. Results showed that a single intrathecal injection of 0.003 ng of BDNF was able to decrease the nociceptive threshold (Randall-Selitto test) in normal rats, for at least a 42-day period. Furthermore, the hyperalgesia generated was comparable to that observed in rats with a 42-day history of mononeuropathy. Increasing the dose or administering additional doses of BDNF resulted neither in additional effectiveness in reducing the pain threshold nor in the prolongation of the hyperalgesic effect, thus showing that central sensitization induced by BDNF is a dose-independent, all-or-none process. It is concluded that BDNF alone is sufficient for generating a long-lasting neural excitability change in the spinal cord via tyrosine kinase B receptor signaling, similar to that observed in chronic pain models such as neuropathy.