Effect of continuous posttraumatic intrathecal nocistatin on the development of mechanical allodynia.

BACKGROUND AND OBJECTIVES: The neuropeptide nocistatin has a variety of effects on nociception and other central nervous system functions. It has shown to exert diverging effects on nociceptive behavior in various experimental pain models depending on the dose administered. The inhibitory effect of spinal nocistatin on the release of glycine and γ-aminobutyric acid is thought to be responsible for pronociceptive effects, whereas the antinociceptive action of nocistatin can be attributed to diminished glycine-dependent N-methyl-D-aspartate receptor activation. So far, nocistatin has only been investigated in experimental models of already established pain and has been injected as a bolus. METHODS: In the present study, we investigated the effects of continuous intrathecal administration of nocistatin on the development of mechanical allodynia in the chronic constriction injury model of neuropathic pain in rats. The spinal infusion via intrathecal catheters connected to osmotic minipumps was started immediately after the surgical procedure and lasted 24 hrs. The development of mechanical allodynia was assessed with von Frey-type filaments for 2 weeks after chronic constriction injury. RESULTS: Despite a wide range of doses used, the continuous spinal application of nocistatin had no significant effect on the development of pathological nociceptive behavior at any time point, that is, mechanical allodynia developed equally in all groups in the injured paw, whereas nociceptive behavior was unchanged in comparison with baseline values in the uninjured paw in all groups. CONCLUSIONS: Because nocistatin has well-documented effects on established pathological pain, it is conceivable that its effect on nociception is only effective when spinal circuitry is pathologically altered.

Antinociceptive effects of systemic lidocaine: involvement of the spinal glycinergic system.

Beside their action on voltage-gated Na(+) channels, local anesthetics are known to exert a variety of effects via alternative mechanisms. The antinociceptive effect of lidocaine is well documented, yet the exact mechanism is not fully understood. Whether glycinergic mechanisms, which play a pivotal role in pain modulation, are involved in lidocaine-induced antinociception is hitherto unclear. In the present study, lidocaine was injected intravenously in rats using the formalin test for acute pain and the chronic constriction injury model for neuropathic pain. The effect of intrathecally administered d-serine (an agonist at the glycine-binding site at the NMDA-receptor), its inactive isomer l-serine, CGP 78608 (antagonist at the glycineB-site of the NMDA-receptor) and strychnine (antagonist at inhibitory glycine-receptors) on lidocaine-induced antinociception was examined. Systemically administered lidocaine was antinociceptive in both acute and chronic pain model. In the formalin test, the effect of lidocaine was antagonized by d-serine, but not by l-serine or strychnine. In the chronic constriction injury model, antinociception evoked by lidocaine was reduced by d-serine, strychnine and CGP 78608, while l-serine had no effect. These results indicate a modulatory effect of lidocaine on the NMDA-receptor. Additionally, since in our study lidocaine-induced antinociception was antagonized by both glycineB-site modulators and strychnine our results may favor the hypothesis of a general glycine-like action of lidocaine or some of its metabolites on inhibitory strychnine-sensitive receptors and on strychnine-insensitive glycine receptors.

Racemic intrathecal mirtazapine but not its enantiomers acts anti-neuropathic after chronic constriction injury in rats.

The unique noradrenergic and specific serotonergic antidepressant mirtazapine acts antinociceptive. It is optically active and currently marketed as racemate. In an animal model of acute pain it has been shown that the enantiomers exhibit differential effects: the R(-)-enantiomer showed anti-, the S(+)-enantiomer pronociceptive properties while the racemate acted antinociceptive at low doses and profoundly pronociceptive after high-dose application. Aim of the present study was to evaluate potential enantioselective effects of mirtazapine in neuropathic pain. In a chronic constriction injury model of neuropathic pain, Wistar rats were injected (+/-)-mirtazapine and the enantiomers intrathecally. All substances were dosed between 0.001 and 1mg/kg and compared to vehicle in a randomized and blinded approach. Thermal hyperalgesia and mechanical allodynia were assessed. In contrast to the acute pain results, only racemic mirtazapine exerted significant sustained analgesic effects up to 48 h. Antinociception was observed at all dosages with a maximum in the range of 0.01 mg/kg. Surprisingly, neither enantiomer was pro- nor antinociceptive at any dose or time. Our findings suggest that the synergism of both enantiomers is required to evoke a significant analgesic effect for the treatment of neuropathic pain. Our study gained no evidence for the use of either R(-) nor S(+)-mirtazapine alone. Due to the unique characteristics of (+/-)-mirtazapine and its proven efficacy in acute pain our results suggest that racemic mirtazapine may be a particularly useful antidepressant in the adjunctive treatment of chronic neuropathic pain states and could provide additional benefit to current therapeutic options.

Differential effects of spinally applied glycine transporter inhibitors on nociception in a rat model of neuropathic pain.

Changes in glycinergic neurotransmission in the spinal cord dorsal horn are critically involved in the development of pathological pain. Since the concentration of glycine in the synaptic cleft is controlled by specialized proteins, the glycine transporters GlyT1 and GlyT2, manipulation of this system might have significant effects on nociception. In the present study, we investigated the effects of the spinally applied glycine transporter inhibitors ALX 5407 (GlyT1) and ALX 1393 (GlyT2) on nociceptive behavior in the chronic constriction injury model of neuropathic pain in male Wistar rats. After implementation of neuropathy, the animals were injected with three dosages of ALX 5407 and ALX 1393 (10, 50 and 100 microg) via an intrathecal catheter (n = 8 each). Subsequently, nociceptive behavior was evaluated regarding thermal hyperalgesia (Hargreaves method) and mechanical sensitization (von Frey filaments) over 240 min after application. Inhibition of GlyT1 by ALX 5407 had differential dose-dependent effects. While the highest and the lowest concentrations were antinociceptive, the medium dose evoked pronociceptive effects. The GlyT2 inhibitor ALX 1393 was only effective in the highest concentration at which it exerted significant antinociception. However, in the same dose, ALX 1393 caused remarkable side effects such as respiratory depression and motor deficits in three animals. Our findings indicate that inhibition of glycine transporters is capable of evoking significant effects on nociceptive behavior in neuropathic pain. Whether glycine transporter inhibitors have the capability to gain clinical relevance as analgesic compounds on the long run has to be elucidated in further investigations.

Mirtazapine and its enantiomers differentially modulate acute thermal nociception in rats.

The antidepressant mirtazapine is an optically active drug and currently marketed as a racemic compound consisting of its S(+) and R(-)-enantiomers in a 50:50 mixture. As stereochemistry of antidepressants has become increasingly important to consider for the relevance of their analgesic properties, we investigated the effect of (+/-)-mirtazapine and its enantiomers in an animal model of acute thermal nociception. Wistar rats were injected intrathecal with either (+/-)-mirtazapine, R(-)-mirtazapine, S(+)-mirtazapine from 1 to 0.001 mg/kg and vehicle (0.9% NaCl), respectively. The effects on thermal paw withdrawal thresholds were monitored using the Hargreaves test. (+/-)-Mirtazapine exerted pro- and antinociceptive effects in acute thermal nociception, whereas R(-)-mirtazapine showed solely antinociceptive and S(+)-mirtazapine pronociceptive properties. These results clearly demonstrate a differential effect of (+/-)-mirtazapine and its enantiomers on nociception. As R(-)-mirtazapine exerts the antinociceptive activity of the racemic mixture it may be a putative candidate for an enantioselective use as analgesic.

Substance P and opioid peptidergic innervation of the anterior eye segment of the rat: an immunohistochemical study.

Recently discovered endogenous opioid peptides such as nociceptin are known to modulate neurotransmitter release of primary afferent neurons (especially substance P, SP) and they have also been demonstrated in peripheral nerve fibres. The aim of this study was to investigate the opioid peptidergic innervation of the anterior eye segment and to compare it with the innervation pattern of SP in order to shed light on the functional relationship between these peptides. Anterior eye segments of 20 rat eyes were cut in a tangential plane and the sections stained with antibodies against SP, nociceptin, nocistatin, endomorphin 1 and 2, leu-enkephalin and met-enkephalin. Sections of the spinal cord or brain were used as positive controls. Numerous SP-immunoreactive nerve fibres were found in the conjunctiva, cornea, episclera, trabecular meshwork, iris and ciliary body. A weak staining for met-enkephalin and leu-enkephalin could only be found in the iris and anteriormost ciliary body. Nerve fibres immunoreactive for nociceptin, nocistatin, and endomorphin 1 or 2 could not be detected in any part of the anterior eye segment. It is tempting to speculate that the opioid peptidergic innervation of the anterior ciliary body may play a role in the modulation of intraocular inflammation.

The spinal antinociceptive effect of nocistatin in neuropathic rats is blocked by D-serine.

BACKGROUND: The neuropeptide nocistatin (NST) has been implicated in the modulation of nociceptive responses in the spinal cord. Depending on the dose, both pronociceptive and antinociceptive effects have repeatedly been reported. The pronociceptive effect is most likely attributable to inhibition of synaptic glycine and gamma-aminobutyric acid release and a subsequent reduction in the activation of inhibitory glycine and gamma-aminobutyric acid receptors, but the mechanisms of its antinociceptive action have hitherto remained elusive. It has recently been demonstrated that synaptically released glycine contributes to N-methyl-D-aspartate receptor activation. The authors therefore investigated whether a reduction in glycine release might also account for the antinociceptive action of NST in neuropathic rats. METHODS: The authors analyzed the effects of spinally applied NST in the chronic constriction injury model of neuropathic pain. NST was injected intrathecally from nanomolar to picomolar doses and its effects on thermal paw withdrawal latencies were monitored. Furthermore, we tested whether D-serine (100 microg per rat), a full agonist at the glycine binding site of the N-methyl-D-aspartate receptor, would interfere with the effects of NST. RESULTS: At high doses (10 nmol/rat), intrathecally injected NST was pronociceptive, whereas lower doses (1 pmol/rat) elicited antinociception. The antinociceptive, but not the pronociceptive, action was occluded by intrathecal pretreatment with D-serine. L-serine, which does not bind to N-methyl-D-aspartate receptors, affected neither the pronociceptive nor the antinociceptive effect. CONCLUSIONS: These results demonstrate that NST produces a biphasic dose-dependent effect on neuropathic pain. The spinal antinociception by NST is most likely attributable to inhibition of glycine-dependent N-methyl-D-aspartate receptor activation.

Facilitation of spinal NMDA receptor currents by spillover of synaptically released glycine.

In the mammalian CNS, N-methyl-D-aspartate (NMDA) receptors serve prominent roles in many physiological and pathophysiological processes including pain transmission. For full activation, NMDA receptors require the binding of glycine. It is not known whether the brain uses changes in extracellular glycine to modulate synaptic NMDA responses. Here, we show that synaptically released glycine facilitates NMDA receptor currents in the superficial dorsal horn, an area critically involved in pain processing. During high presynaptic activity, glycine released from inhibitory interneurons escapes the synaptic cleft and reaches nearby NMDA receptors by so-called spillover. In vivo, this process may contribute to the development of inflammatory hyperalgesia.