The effects of botulinum toxin A on mechanical and cold allodynia in a rat model of neuropathic pain

PURPOSE

Botulinum toxin type A (BoNT-A) has been used to treat many disorders related to excessive muscle contraction, but there are few studies evaluating its effects on neuropathic pain. The aim of this study was to evaluate the analgesic effects of BoNT-A in a rat model of neuropathic pain.

METHODS

Male Sprague-Dawley rats were prepared by ligating the left L5 and L6 spinal nerves to produce neuropathic pain. Seventy neuropathic rats were randomly assigned into seven groups. Either normal saline or BoNT-A (10, 20, 30 and 40 U.kg(-1)) was administered to the plantar surface of the affected left hind paw, and BoNT-A (30 and 40 U.kg(-1)) was administered into the unaffected right paw in order to determine the drug-induced systemic effect. Mechanical and cold allodynia were observed at pre-administration, one, three, five, seven and 15 days after drug administration, and were quantified by measuring withdrawal frequencies to stimuli with von Frey filament and 100% acetone, respectively. Rotarod performance was measured to detect drug-induced adverse motor effects.

RESULTS

The mean minimum withdrawal frequencies to mechanical and cold stimuli were 77 +/- 11 and 90 +/- 4.5%, 46 +/- 5 and 66 +/- 7%, 33 +/- 7 and 62 +/- 7%, 12 +/- 2.9 and 54 +/- 7.3% with 10, 20, 30 and 40 U.kg(-1) BoNT-A respectively (P < 0.05). Doses of 30 and 40 U.kg(-1) BoNT-A resulted in reduced rotarod performance time.

CONCLUSION

We conclude that peripherally administered BoNT-A reduces mechanical and cold allodynia in a rat model of neuropathic pain.

Motor cortex stimulation in a patient with intractable complex regional pain syndrome type II with hemibody involvement. Case report

The authors describe the effectiveness of motor cortex stimulation (MCS) in a patient with complex regional pain syndrome (CRPS) Type II, formerly known as causalgia, with hemibody allodynia. During MCS, a subjective sensation of warm paresthesia developed in the painful hand and forearm and spread toward the trunk. Pain and allodynia in the areas associated with this sensation were alleviated significantly. The analgesic effect of stimulation proved to be long lasting and was still present at the 12-month follow up. The authors speculate that MCS might exert its effect through the modulation of thalamic activity in this particular case of CRPS with hemisensory deficit. A central mechanism associated with functional disturbance in noxious-event processing in the thalamus might have an important role in the pathogenesis of the condition.

Epidural naloxone reduces intestinal hypomotility but not analgesia of epidural morphine

PURPOSE

Epidural morphine is associated with decreased bowel motility and increased transit time. Low doses of intravenous naloxone reduce morphine-induced pruritus without reversing analgesia, but the effect of epidural naloxone on bowel motility has not been studied. Therefore we evaluated bowel motility and analgesia when naloxone was co-administered with morphine into the epidural space.

METHODS

Forty-three patients having combined thoracic epidural and general anesthesia for subtotal gastrectomy were randomly assigned to one of two study groups. All received a bolus dose of 3 mg epidural morphine at the beginning of surgery, followed by a continuous epidural infusion containing 3 mg morphine in 100 ml bupivacaine 0.125% with either no naloxone (control group, n = 18) or a calculated dose of 0.208 microg x kg(-1) x hr(-1) of naloxone (experimental group, n = 25) for 48 hr. We measured the time to the first postoperative passage of flatus and feces to evaluate the restoration of bowel function, and visual analog scales (VAS) for pain during rest and movement. Scores were assessed at 2, 4, 8, 16, 24, 36 and 48 hr postoperatively.

RESULTS

The experimental group had a shorter time to the first postoperative passage of flatus (5 1.9 +/- 1 6.6 hr vs 87.0 +/- 19.5 hr, P < 0.001 ) and feces (95.3 +/- 25.0 hr vs 132.9 +/- 29.4 hr, P < 0.001). No differences were found in either resting or active VAS between the two groups.

CONCLUSION

Epidural naloxone reduces epidural morphine-induced intestinal hypomotility without reversing its analgesic effects.

Adrenergic sensitivity of the sensory receptors modulating mechanical allodynia in a rat neuropathic pain model

This study focuses on changes in adrenergic sensitivity in untransected sensory axons that innervate an area of skin made neuropathic by transection of neighboring nerves. The segmental nerve injury model is favorable for this since all axons in the L5 and L6 nerves are transected whereas the L4 axons are intact. Earlier findings are that pain behaviors develop after this injury and that these behaviors are ameliorated by sympathectomy. The present study shows that behavior indicating mechanical allodynia can be rekindled after sympathectomy by intradermal norepinephrine and alpha-2 but not alpha-1 adrenergic ligands and the rekindling can be blocked by alpha-2 but not alpha-1 adrenergic antagonists. By contrast neither intradermal norepinephrine nor other adrenergic agonists or antagonists have any demonstrable effects in the normal or after either neuropathic surgery or sympathectomy alone. These data suggest that the combination of neuropathic surgery and sympathectomy results in an upregulation of active alpha-2 adrenergic receptors on the undamaged sensory axons that provide the remaining sensory innervation to a neuropathic area partially denervated by segmental nerve lesions. These changes on undamaged axons presumably compliment similar changes on the transected axons and, thus play a role in the development of neuropathic pain.