Lack of pre-emptive analgesic effects of local anaesthetics on neuropathic pain

S. Abdi1,*, D. H. Lee2, S. K. Park2 and J. M. Chung2

1Department of Anesthesiology and Critical Care, Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114, USA. 2Marine Biomedical Institute and Departments of Anatomy, Neurosciences, Physiology, and Biophysics, The University of Texas Medical Branch, Galveston, TX 77555, USA

Accepted for publication: April 10, 2000


    Abstract
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 Abstract
 Introduction
 Methods and results
 Comment
 References
 
We investigated the significance of pre-emptive analgesia using a well-known model of neuropathic pain in rats. Lignocaine, bupivacaine or saline was applied locally to the left L5–L6 spinal nerve before or 4 days after nerve injury. Mechanical allodynia was then evaluated before and after injury. Pre- and post-injury treatment with local anaesthetics both resulted in a two- to threefold increase in the pain threshold, as manifested by a significant increase in von Frey measurements. However, this effect lasted only 24 h. Our study in rats questions the beneficial effect of a single dose of local anaesthetic as pre-emptive analgesia.

Br J Anaesth 2000; 85: 620–3.

Keywords: anaesthetics local, bupivacaine; anaesthetics local, lignocaine; pain, neuropathic; rats


    Introduction
 Top
 Abstract
 Introduction
 Methods and results
 Comment
 References
 
Pre-emptive analgesia, the administration of analgesics before tissue damage, aims to reduce both postoperative pain and the subsequent development of chronic pain after surgery. Studies in animals have shown that tissue damage produces significant changes in neuronal processing, including central sensitization, alteration in neuronal phenotype and the loss of neurons in the dorsal root ganglion with associated synaptic plasticity.1 Both basic science and recent clinical studies have shown that such changes may be attenuated to varying degrees if analgesic agents are administered before tissue injury.2

Local anaesthetics are useful in alleviating both acute and chronic pain through their inhibitory actions on sodium channels. The use of local anaesthetic agents in pre-emptive analgesia has been investigated in two inflammatory models of acute pain, the formalin test and the carrageenan test. However, very few investigations have used animal models of neuropathic pain.3

The purpose of our study was to test the efficacy of the direct application of two commonly used local anaesthetics, lignocaine and bupivacaine, onto the nerves before or after injury, using the Chung model of segmental spinal nerve injury.4 In this model, rats develop neuropathic pain with behavioural evidence of hyperalgesia and allodynia within 24 h of nerve ligation, which peaks within 7 days and may persist for several weeks.4


    Methods and results
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 Abstract
 Introduction
 Methods and results
 Comment
 References
 
The experiments were approved by the Animal Care and Use Committee of The University of Texas Medical Branch and performed in accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals.

Thirty male Sprague–Dawley rats (Harlan Sprague–Dawley, Harlan Industries, Indianapolis, IN), weighing 150–200 g, were used. One week after the animals had been acclimatized to the laboratory conditions, baseline (before surgery) quantitative von Frey measurements were recorded and the animals were randomly assigned to one of three experimental groups: group I, saline; group II, 2% lignocaine and group III, 0.5% bupivacaine. Animals in each of the three groups were treated with either of the local anaesthetics or saline before nerve injury. Then the animals were prepared for measurements of chronic neuropathic pain behaviour, as described by Kim and Chung.4 Briefly, the rats were anaesthetized with halothane in oxygen (halothane concentration of 2 vol% for induction and 0.8 vol% for maintenance). Animals were placed in the prone position, skin infiltration was done with saline (group I), 2% lignocaine (group II) or 0.5% bupivacaine (group III), and midline skin incisions at L4–S2 were made, followed by separation of the paraspinal muscles from spinous processes. The transverse process of L6 was then carefully removed. Once the left L5 and L6 spinal nerves had been identified, they were bathed with saline (group I), 2% lignocaine (group II) or 0.5% bupivacaine (group III) for 10 min. The left L5–L6 spinal nerves were then ligated tightly with 6–0 silk thread, and the wound was closed. At the end of surgery, anaesthesia was discontinued and the animals were awakened and placed back into their cages with food pellets and water ad libitum under the same environmental conditions as previously. The mechanical allodynia threshold was measured using von Frey filaments, as described below, every day for 7 days. To avoid the effects of the circadian cycle, all studies were performed during the same period (9 a.m. to 12 p.m.) on each of the experimental days.

Fourteen additional Sprague–Dawley rats were used to study the effect of 0.5% bupivacaine or saline when applied 4 days after injury. These animals were surgically prepared for the study in the same manner as the animals discussed above, except that their ligated L5–L6 spinal nerves had to be exposed again under anaesthesia in order to infiltrate the nerves with saline (group IV) or 0.5% bupivacaine (group V) 4 days after ligation.

Behavioural tests
All animals were tested for quantitative changes in pain threshold, as measured by the use of von Frey filaments. Before the von Frey measurements were made, each rat was placed on a metal mesh floor and covered with a plastic box (8x8x18 cm) and allowed 30 min to acclimatize. Thereafter, light mechanical stimuli using sufficient force to cause light buckling of the filament was applied to the plantar surface of the left foot using different strengths of von Frey filaments (Stoelting, Woodale, IL, USA). In brief, von Frey filaments with incremental stiffness (0.3–15 g) were applied serially to the paw in ascending or descending order of stiffness depending on the foot withdrawal response of the rat. The maximum and minimum cut-offs were at 15 and 0.3 g respectively. A single trial of stimuli consisted of five applications of a von Frey filament every 4 s to the plantar surface of the left hind paw perpendicularly for about 3–4 s. Brisk foot withdrawals (at least three times out of five applications) in response to normally innocuous mechanical stimuli using von Frey filaments were considered positive; no response was considered negative. Depending on the positive or negative response, subsequent filaments were applied in order of descending and ascending intensity respectively.

The baseline threshold value of von Frey measurement was 12.6 (SEM 1.1), 12.8 (0.8), 13.7 (0.7), 15 (0) and 15 (0) g for groups I, II, III, IV and V respectively. All the animals developed significant mechanical allodynia 24 h after spinal nerve ligation (Fig. 1). Pretreatment with either lignocaine or bupivacaine (groups II and III respectively) increased the threshold of mechanical allodynia compared with the control group (group I). However, this effect lasted only 24 h. Similarly, when bupivacaine was used after injury, its effect in alleviating mechanical allodynia lasted only 24 h. Neither pretreatment nor post-treatment with saline altered the development of mechanical allodynia.



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Fig 1 The effects of treatment with 2% lignocaine and 0.5% bupivacaine before injury (A) and 0.5% bupivacaine treatment after injury (B) on mechanical allodynia. Nerve injury was induced by tightly ligating the left L5–L6 spinal nerves in rats. All the data are expressed as mean ± SEM. Statistical analysis was done by analysis of variance followed by Dunnett’s post hoc test and Student’s t-test where appropriate.*Significant difference (P<0.05) compared with the saline control group. BL, Baseline, before surgery.

 

    Comment
 Top
 Abstract
 Introduction
 Methods and results
 Comment
 References
 
Tissue and nerve damage and the subsequent transmission of neural impulses from the surgical site to the spinal cord mark the beginning of a process of central neural sensitization that may be enhanced by other noxious perioperative events. The segmental spinal nerve injury model (Chung’s model) is a well-established model used to induce neuropathic pain behaviour in rats. However, a meticulous surgical technique must be used in order to minimize the damage to the L4 spinal nerve, which can affect locomotor activity and the withdrawal response to various stimuli. Although no animal was excluded from this study because of dragging of the left hind limb (a sign of a damaged L4 spinal nerve), five rats were excluded from the study because of abnormal presurgical baseline von Frey measurements.

Although both pretreatment with 2% lignocaine or 0.5% bupivacaine and post-treatment with 0.5% bupivacaine slightly attenuated mechanical allodynia, all the animals remained significantly allodynic throughout the experimental period. This effect lasted only 24 h, which, although it is a short time, is considerably longer than expected from the half-life of the drugs. These results are consistent with the findings of Dougherty and colleagues,3 who studied the differential influence of local anaesthetic upon two models of experimentally induced peripheral mononeuropathy in the rat, the partial constriction neuropathy (PCN) model and the partial transection neuropathy (PTN) model. They found that the PCN but not the PTN model showed an increase in pain threshold to the local application of anaesthetics at the time of injury. This was consistent with the previous observations made by Gonzales-Darder, whereby autotomy in rats was diminished in magnitude but unchanged in onset after application of anaesthetic to the sciatic nerve before transection.5 Furthermore, Mao and colleagues6 reported that a single perineural injection of bupivacaine (0.5%) on day 3 after nerve ligation reduced thermal hyperalgesia when assessed 24 h after injection, and that this effect lasted for under 48 h. This is consistent with our findings in the present study (Fig. 1B), even though we infiltrated the nerves with local anaesthetic 10 min before injuring (ligating) them in addition to 4 days afterwards. In clinical studies, Nikolajsen and colleagues7 reported that perioperative epidural block that was started about 18 h before the amputation and continued into the postoperative period did not prevent phantom or stump pain.

Although the role of pre-emptive analgesia in neuropathic pain is still not clear, we speculate that it may not prevent the barrage of neural discharges necessary for the development of neuropathic pain at the time of injury. In the present study, although the use of the local anaesthetics before nerve injury seems to have delayed the transmission of pain impulses to the central nervous system, it did not prevent the occurrence of neuropathic pain.

Thus, the important question remains: when is the best time to administer local anaesthetics in order to derive the maximum benefit from their pre-emptive analgesic effects?

In conclusion, our present pilot study suggests that the application of a single dose of local anaesthetic onto an injured nerve may not be a clinically effective means of preventing neuropathic pain. Therefore, further blinded studies are necessary to ascertain the combination of dosage, time frame and route of administration of local anaesthetics that is most effective in preventing neuropathic pain.


    Acknowledgement
 
This research was supported by The National Institutes of Health Grants NS 31680 and NS 11255.


    Footnotes
 
* Corresponding author: Department of Anesthesia and Critical Care, Clinics 3, Massachusetts General Hospital, Harvard Medical School, 32 Fruit Street, Boston, MA 02114, USA Back


    References
 Top
 Abstract
 Introduction
 Methods and results
 Comment
 References
 
1 Woolf CJ, Chong MS. Preemptive analgesia—treating postoperative pain by preventing the establishment of central sensitization. Anesth Analg 1993; 77: 362–79[ISI][Medline]

2 Gottschalk A, Smith DS, Jobes DR, et al. Preemptive epidural analgesia and recovery from radical prostatectomy: a randomized controlled trial. J Am Med Assoc 1998; 279: 1076–82[Abstract/Free Full Text]

3 Dougherty PM, Garrison CJ, Carlton SM. Differential influence of local anaesthetic upon two models of experimentally induced peripheral mononeuropathy in the rat. Pain 1992; 570: 109–15

4 Kim SH, Chung JM. An experimental model for peripheral neuropathy produced by segmental spinal nerve ligation in the rat. Pain 1992; 50: 355–63[ISI][Medline]

5 Gonzales-Darder JM, Barbera J, Abellan MJ. Effects of prior anaesthesia on autotomy following sciatic transection in rats. Pain 1986; 24: 87–91[ISI][Medline]

6 Mao J, Price DD, Mayer DJ, Lu J, Hayes RL. Intrathecal MK-801 and local nerve anesthesia synergistically reduce nociceptive behaviors in rats with experimental peripheral mononeuropathy. Brain Res 1992; 576: 254–62[ISI][Medline]

7 Nikolajsen L, Ilkjaer S, Christensen JH, Kroner K, Jensen TS. Randomised trial of epidural bupivacaine and morphine in prevention of stump and phantom pain in lower-limb amputation. Lancet 1997; 350: 1353–7[ISI][Medline]