Victoria Chapman

Electrophysiological characterization of spinal neuronal response properties in anaesthetized rats after ligation of spinal nerves L5-L6

1 Despite a number of models of nerve injury, few studies have examined how peripheral nerve injury influences spinal somatosensory processing. 2 Ligation of two (L5‐L6) of the three spinal nerves that form the sciatic nerve produces a partial denervation of the hindlimb. Following ligation, rats exhibited withdrawal responses to normally innocuous punctate mechanical and cooling stimuli (acetone) applied to the lesioned hindpaw. Such mechanical and cooling allodynia was not observed in sham‐operated rats. 3 A significantly greater proportion of spinal neurones of ligated rats exhibited spontaneous activity at post‐operative (PO) days 7–10 (P= 0.03) and 14–17 (P= 0.0001), compared with sham controls. The frequency of the spontaneous activity was significantly higher than that of the sham controls (P= 0.03 and P= 0.02 for days 7–10 and days 14–17, respectively). 4 At the earlier PO period, significantly (P= 0.02) more neurones of spinal nerve‐ligated (SNL) rats responded to brush compared with the sham controls; at the later PO period the proportion of neurones of SNL rats responsive to prod was significantly (P= 0.007) reduced compared with the sham controls. The magnitude of the evoked neuronal response of SNL rats at PO days 7–10 was comparable to that of the sham controls. The magnitudes of brush‐ and prod‐evoked neuronal responses of SNL rats were significantly smaller (P= 0.05 and P= 0.002, respectively) than the sham controls at PO days 14–17. In addition, neuronal responses of SNL rats to mechanical punctate stimuli and the C fibre‐evoked neuronal responses were significantly reduced at the later PO period, compared with sham controls. Aβ‐fibre‐induced wind‐up was not observed under any conditions. 5 These complex changes in neuronal responses are both time and modality dependent. The plasticity of some of the neuronal and behavioural responses following nerve injury was difficult to reconcile. We suggest that an interplay between pathological peripheral and central mechanisms may account for some of the changes that could contribute to allodynia and hyperalgesia.

The spinal and peripheral roles of bradykinin and prostaglandins in nociceptive processing in the rat

The effect of peripheral and intrathecal administration of the bradykinin B2 receptor antagonist HOE140 and the non-steroidal anti-inflammatory drug indomethacin were studied on the response of dorsal horn nociceptive neurones to formalin in the anaesthetized rat. Peripheral pretreatment with HOE140 reduced both phases of the formalin response whereas subcutaneous administration of indomethacin (5 mg/kg) reduced only the second phase. Pre-treatment with intrathecal HOE140 resulted in a dose-dependent reduction of the second phase whereas a high dose (500 micrograms) intrathecal indomethacin reduced both phases of the response. Bradykinin plays a role in peripheral nociception during the first and second phase of the response whereas the prostaglandins are only involved during the second phase. The converse is true centrally, the prostaglandins appear to be involved in both phases of the formalin response whereas bradykinin only plays a central role during the second phase of the response.

Inhibitory action of nociceptin on spinal dorsal horn neurones of the rat, in vivo

Intrathecally administered nociceptin (5, 50, 225 μg) dose‐relatedly inhibited the C‐fibre evoked wind‐up and post‐discharge of dorsal horn neurones, but not the baseline C‐fibre evoked responses. Spinal naloxone 50 μg, but not 10 μg, reversed the effects of nociceptin. Thus the antinociceptive role of nociceptin in the spinal cord differs from that of classical opioids.

Effects of systemic carbamazepine and gabapentin on spinal neuronal responses in spinal nerve ligated rats

&NA; There are few pharmacological studies of central neuronal measures in animal models of neuropathic pain. In the present study we have compared the effects of two anticonvulsants, carbamazepine and gabapentin, on spinal neuronal responses of nerve injured rats (selective ligation of spinal nerves L5 and L6, SNL) and sham‐operated rats. The development and maintenance of cooling and mechanical allodynia of the lesioned hindlimb of SNL rats was followed with behavioural indices. The contralateral hindlimb of SNL rats and the ipsilateral hindlimb of sham‐operated rats did not develop allodynia. Electrophysiological studies of SNL rats were then performed at two post‐operative (PO) time points (PO days 7–10 and PO days 14–17). Spinal neurones of SNL rats, but not sham‐operated rats, exhibited spontaneous activity at both PO days 7–10 and 14–17 (1±0.4 and 3±1 Hz, respectively). Paradoxically, the magnitude of electrical (C‐fibre) and natural (mechanical and thermal) evoked neuronal responses of SNL rats at PO days 14–17 were smaller than the evoked neuronal responses of SNL rats at PO days 7–10 and sham‐operated rats. The electrical evoked A‐fibre responses of neurones were comparable for the three groups of rats. Both subcutaneous carbamazepine (0.5–22.5 mg/kg) and gabapentin (10–100 mg/kg) significantly reduced the spontaneous activity of spinal neurones of SNL rats at both PO time points. Carbamazepine had inhibitory effects on electrical C‐ and A‐fibre and mechanical punctate (9 and 50 g) evoked neuronal responses of SNL rats which were significantly different to the lack of effect of carbamazepine on these measures in sham‐operated rats. Gabapentin had comparable effects as carbamazepine on the electrical C‐and A‐fibre and mechanical punctate (9 and 50 g) evoked neuronal responses of SNL rats. In contrast to carbamazepine, gabapentin also reduced evoked neuronal responses of sham‐operated rats and there was no difference between the effects of gabapentin in SNL and sham‐operated rats. Robust behavioural changes in the SNL model of neuropathy are paralleled by a temporal increase in spontaneous activity and a paradoxical decrease in evoked spinal neuronal responses. The peripheral nerve dysfunction reveals an effect of carbamazepine which is maintained throughout the observation period, validating this experimental approach. Gabapentin, a novel treatment for neuropathic pain states, also reduced neuronal responses, but the actions of the drug were not dependent on nerve injury. Further studies at the spinal level may shed light on the physiology and pharmacology of the aberrant processes associated with neuropathic pain.

The effectiveness of spinal and systemic morphine on rat dorsal horn neuronal responses in the spinal nerve ligation model of neuropathic pain

The treatment of pain arising from nerve injury can be difficult and the opioid sensitivity of neuropathic pain remains debatable. Clinical and animal studies report a wide range in the effectiveness of morphine, ranging from inadequate to potent analgesia. In this electrophysiological study we compare the effectiveness of spinal versus systemic administration of morphine on the natural and electrically evoked responses of spinal neurones of rats with a selective spinal nerve (L5/6) ligation. Recordings were made 1 week and/or 2 weeks after ligation. We have also compared the effects of morphine, by the two routes, on normal and sham operated animals. In spinal nerve ligated rats, morphine (0.1-5 microg) administered via the intrathecal route produced greater dose-dependent inhibitions of the neuronal responses compared with those produced by the systemic route (1-6 mg/kg). The dose response curves for intrathecal morphine on the C-fibre evoked and noxious natural stimuli evoked neuronal responses (mechanical and thermal) of spinal nerve ligated rats were to the left of those of sham operated and normal rats, suggesting an enhanced potency of intrathecal morphine after nerve injury. This was clearest for the lower doses of the opioid. The effects of spinal morphine on the responses to low intensity stimuli were similar in all groups of rats. In contrast to the spinal route, systemic morphine was less effective in inhibiting the evoked neuronal responses of spinal nerve ligated rats. This was especially clear for the C-fibre evoked and noxious natural stimuli evoked responses (mechanical and thermal) of spine nerve ligated rats. Our results suggest that the effectiveness of morphine may be partly related to the timing of the treatment relative to the duration of the neuropathy, the route of administration and also the neuropathic symptom. Spinal opioids may be a useful approach to pain control in neuropathic pain states where systemic routes produce inadequate analgesia.

The effect of intrathecal administration of RP67580, a potent neurokinin 1 antagonist on nociceptive transmission in the rat spinal cord

The effect of intrathecal application of the selective neurokinin 1 (NK1) receptor antagonist RP67580 and its enantiomer RP68651 was studied on the responses of dorsal horn nociceptive neurones to formalin in the rat. The first and second phases of the formalin response were inhibited by RP67580 in a dose-related manner (1-10 micrograms), whereas RP68651 (5 micrograms) facilitated the second phase of the response. The same doses of RP67580 had minimal effects on the acute C-fibre responses. The NK1 receptor appears to play a role in prolonged nociceptive transmission in the spinal cord.

Electrophysiologic Analysis of Preemptive Effects of Spinal Opioids on N-methyl-D-aspartate Receptor--mediated Events

Background:Spinal N-methyl-D-aspartate (NMDA) receptormediated mechanisms may contribute to reduced opiold sensitivity in conditions of pain. The effectiveness of spinal opioids in inhibiting NMDA-mediated nociceptive events was assessed with two models. In addition, oploid dose-response curves with preemptive administration were compared with early and late postadministrations. Methods:Dorsal horn nociceptive neuronal responses were recorded in the Intact halothane anesthetized rat to acute repetitive C-fiber electrical stimulation (0.1 and 0.5 Hz) and to the peripheral injection of 5% formalin. At 0.5 Hz but not at 0.1 Hz, there was an enhanced C-fiber evoked response of dorsal horn neurons elicited by repetitive C-fiber stimulation (wind-up), which is mediated by the NMDA receptor. Formalin produced a biphasic response; the late protracted inflammatory phase was NMDA receptor-mediated. Results:With 0.5-Hz stimulation a large degree of wind-up was elicited; it was less sensitive to 5 µg morphine compared with the effect of the same dose on the residual wind-up elicited at 0.1 Hz. Preadministration and early postadminlstration of morphine were equieffective at inhibiting the second-phase formalin response. In contrast, administration of the fast-acting µ opioid, D-Ala-Gly-MePHe-Gly-o1, given late postadminlstration (during the second phase) was less effective than preadministration. Increasing the dose of D-Ala-Gly-MePHe- Gly-ol produced complete Inhibitions. Conclusions:NMDA receptor-mediated neuronal responses, such as wind-up and the established second phase of the formalin response, are poorly responsive to opioids. Dose increases and preemptive opioids effectively inhibit these NMDA receptor-mediated events.

Distinct inhibitory effects of spinal endomorphin‐1 and endomorphin‐2 on evoked dorsal horn neuronal responses in the rat

Intrathecal endomorphin‐1 and endomorphin‐2 (0.25–50 μg) dose‐relatedly reduced all components of electrical evoked C‐fibre responses of spinal neurones. These effects were partially reversed by naloxone. Endomorphin‐1, but not endomorphin‐2, dose‐relatedly reduced the Aβ‐fibre evoked responses. Peak inhibitory effects of endomorphin‐1 and −2 were at 15–20 min post‐administration. Thus spinal endomorphin‐2 had selective effects on noxious responses, whereas endomorphin‐1 was non‐selective.

Spinal local anaesthetic actions on afferent evoked responses and wind-up of nociceptive neurones in the rat spinal cord: combination with morphine produces marked potentiation of antinociception

&NA; Lignocaine was tested either alone or in combination with a low dose of morphine by intrathecal administration on the C‐ and A‐beta evoked responses of nociceptive neurones in the dorsal horn of the halothane‐anaesthetized rat. In addition the effect of prilocaine was compared to lignocaine. The effects of lignocaine on wind‐up, a frequency‐dependent increase in the responses of the cells produced by repeated C‐fibre stimulation was also tested. Lignocaine produced dose‐dependent inhibitions of the C‐, A‐delta and A‐beta evoked responses of the cells which became more selective for the noxious evoked responses as the dose increased. The effective doses corresponded well to those used clinically. Wind‐up was also decreased by lignocaine. In combination with a low dose of morphine, threshold doses of lignocaine produced a highly marked potentiation of the inhibitions of the C‐fibre evoked responses compared to either agent alone. No potentiation of the inhibitions of the A‐beta responses was observed. The potentiated inhibitory effects on the C‐fibre responses were rapidly reversed by intrathecal naloxone. The finding that spinal local anaesthetic and morphine potentiate markedly to reduce spinal nociception is discussed both in terms of mechanisms of action of the agents and their clinical application.

A novel spinal action of mexiletine in spinal somatosensory transmission of nerve injured rats

Abstract Mexiletine is widely used for the treatment of neuropathic pain although its site(s) of action remain unclear. Here we have studied the effect of spinal administration of mexiletine (10–1000 &mgr;g) on the spontaneous and peripherally evoked responses of spinal neurones of nerve injured (selective ligation of spinal nerves L5‐L6; SNL) rats. Sham controls for the surgical intervention were performed. A high proportion of the spinal neurones of SNL rats exhibited de novo spontaneous activity (mean frequency of firing 4±1 Hz), this activity was highly sensitive to spinal mexiletine (F5,55=2.5, P≤0.05). The spinal neurones of the sham operated rats exhibited negligible spontaneous activity. The electrically evoked A&bgr;‐fibre neuronal responses of SNL and sham operated rats were not significantly influenced by spinal mexiletine. In contrast, the A&dgr;‐fibre and C‐fibre evoked neuronal responses of the SNL rats, but not sham operated rats, were significantly reduced by spinal mexiletine (F5,52=4.9, P≤0.001 and F5,48=12, P≤0.0001, respectively). In addition, the mechanical punctate von Frey 9 and 50 g evoked neuronal responses of the SNL rats, but not sham operated rats, were significantly reduced by spinal mexiletine (F5,57=4.3, P≤0.002 and F5,52=6.1, P≤0.001). This pharmacological study suggests that following nerve injury there is a novel mexiletine sensitive spinal substrate which contributes to A&dgr;‐fibre and C‐fibre, but not A&bgr;‐fibre, somatosensory transmission. This central action may underlie some of the clinical efficacy of mexiletine in the treatment of neuropathic pain states.