Effects of adrenergic depletion with guanethidine before and after the induction of a peripheral neuropathy on subsequent mechanical-, heat- and cold sensitivities in rats

Sympathectomy alleviates mechanical allodynia in an experimental animal model for neuropathy in the rat

We attempted to determine the effects of surgical sympathectomy on an animal model for neuropathic pain. The L5 and L6 spinal nerves on one side were tightly ligated in anesthetized rats. Mechanical sensitivity of the affected hind paw was significantly elevated from the first day after the surgery as evidenced by the increased occurrence of foot withdrawal to innocuous mechanical stimulation applied with von Frey filaments to the hind paw. The increased mechanical sensitivity continued for three weeks, at which time surgical sympathectomy was performed by removing the L2-L6 sympathetic chain. The sympathectomy produced an immediate and almost complete reversal of the increased mechanical sensitivity, whereas sham sympathectomy had no effect. The data suggest that sympathectomy alleviates mechanical allodynia in this experimental animal model.

Adjacent neuropathic hyperalgesia in rats: a model for sympathetic independent pain

Following sciatic nerve section in rats a saphenous nerve mediated adjacent neuropathic hyperalgesia (ANH) develops in response to pressure. This study demonstrates that a similar ANH to heat stimuli develops over the medial dorsum of the hindpaw (MDH) following sciatic section. A similar neuropathic hyperalgesia, with an associated pain syndrome, is seen in man following peripheral nerve section. When chronic bilateral ANH rats were unilaterally injected intradermally over the MDH with noradrenaline (NA), and with vehicle over the contralateral MDH, no side to side pressure or heat withdrawal threshold differences were observed. Likewise, no changes in pressure or heat withdrawal thresholds occurred in chronic ANH rats following chemical sympathectomy with 6-hydroxydopamine (6-OHDA). A similar pattern was seen in control rats treated with NA and 6-OHDA. The pressure and heat ANH seen in the rat was not dependent on sympathetic outflow, and thus provides an animal model for sympathetic independent pain (SIP). Sympathetic modulation was also ineffective on control rat nociceptive thresholds, which is in agreement with prior behavioral findings (Nature, 323 (1986) 158-160).

Effects of guanethidine on sensitization to natural stimuli and self-mutilating behaviour in rats with a peripheral neuropathy

In the present study we have used a recent rat model for neuropathic pain to investigate the effect of the sympatholytic drug guanethidine on changes in behavioural responses evoked by mechanical, heat and cold stimuli and on self-mutilating behaviour. After a unilateral peripheral mononeuropathy induced by ligatures around the right common sciatic nerve, the left side receiving just a sham wound, lesioned and non-operated control animals were treated with saline or guanethidine (30 mg/kg) for 4 consecutive days commencing 5 days before or 10 days after surgery. Behavioural parameters were followed for 4 weeks after drug treatment. Lesioned rats were found to be sensitized to the otherwise innocuous cold stimulus and showed decreased response thresholds to noxious heat and mechanical stimuli. Some lesioned animals self-mutilated. Treatment with guanethidine diminished heat and cold sensitization considerably, but had less effect on mechanical sensitization and, if administered before surgery, rather increased the severity of self-mutilating behaviour. While these results are in agreement with clinical observations on the prominence of sensitization to evoked stimuli, especially cold, and the effectiveness of guanethidine in sympathetically maintained neuropathic pain, they indicate that the mechanisms involved in sensitization to different stimuli and self mutilating behaviour differ.

Thermographic observations on rats with experimental neuropathic pain

Infrared thermographic images were obtained from the plantar hind paws of rats with an experimental nerve injury that produces signs of neuropathic pain. Thermograms confirmed that the experimental neuropathy produces signs resembling those of patients with neuropathic pain. The hind paws on the nerve-damaged side were abnormally hot, abnormally cold, or apparently normal 8-16 days post injury, a variability that is seen clinically in neuropathic pain patients. Abnormally cold hind paws became warm as soon as the injured sciatic nerve was transected, indicating that the underlying vasoconstriction was mediated by neural impulse activity. Xylazine (Rompun), a sympatho-inhibitory alpha 2-adrenoceptor agonist that normally increases cutaneous temperature, caused the hind paw on the control side to warm, as anticipated, while causing paradoxical cooling of abnormally hot hind paws, and even of 'normal temperature' paws on the nerve-injured side. These findings shed light on possible mechanisms underlying abnormal deviations of skin temperature as a symptom of nerve injury. The findings also attest to the usefulness of the experimental animal model of neuropathic pain and of the thermographic method.