The Spared Nerve Injury animal model is described here as a mouse model of peripheral neuropathic pain following partial denervation of the sciatic nerve by lesioning the tibial and common peroneal nerve branches, leaving the remaining sural nerve intact. Behavioral modification resulting from mechanical allodynia is quantified by von Frey filaments.
Peripheral neuropathic pain is a severe chronic pain condition which may result from trauma to sensory nerves in the peripheral nervous system. The spared nerve injury (SNI) model induces symptoms of neuropathic pain such as mechanical allodynia i.e. pain due to tactile stimuli that do not normally provoke a painful response [1].
The SNI mouse model involves ligation of two of the three branches of the sciatic nerve (the tibial nerve and the common peroneal nerve), while the sural nerve is left intact [2]. The lesion results in marked hypersensitivity in the lateral area of the paw, which is innervated by the spared sural nerve. The non-operated side of the mouse can be used as a control. The advantages of the SNI model are the robustness of the response and that it doesn’t require expert microsurgical skills.
The threshold for mechanical pain response is determined by testing with von Frey filaments of increasing bending force, which are repetitively pressed against the lateral area of the paw [3], [4]. A positive pain reaction is defined as sudden paw withdrawal, flinching and/or paw licking induced by the filament. A positive response in three out of five repetitive stimuli is defined as the pain threshold.
As demonstrated in the video protocol, C57BL/6 mice experience profound allodynia as early as the day following surgery and maintain this for several weeks.
Critical steps
Damage to the sural nerve should be avoided in order to study pathological changes in the intact sural nerve following injury to the tibial and common peroneal nerves. Collateral damage to the sural nerve may lead to paralysis and can thus be visualized as dragging of the operated hind limb.
Only the lateral side of the paw is innervated by the spared sural nerve and, hence, only this area develops neuropathy. Testing other areas, innervated by the transected nerves, can strongly bias the evaluation of altered mechanical threshold.
Possible modifications
Other types of ligations may be supplementary in terms of studying pathological pain conditions following peripheral nerve injury, such as chronic constriction injury [5] or partial nerve ligation [6]. Each experimental procedure results in distinct phenotypic changes which should be considered prior to post-surgery testing.
Furthermore, other sensory tests such as thermal hyperalgesia may be applied [7], although this phenotype is less robust following SNI.
Future applications
This technique can be used for testing of drugs altering the development or maintenance of mechanical allodynia [8]. Analysis of the sciatic nerve, the dorsal root ganglia (DRG) and/or the lumbar spinal cord allow research of the molecular mechanisms responsible for the induced phenotype.
Animal experimentation was performed according to good laboratory practice in full compliance with Danish and European regulations. All experiments were approved by the Danish Animal Experiments Inspectorate under the Ministry of Justice (permission number 2006/561-1206).
The authors have nothing to disclose.
This work was supported by the Lundbeck Foundation, the Carlsberg Foundation and Dagmar Marshall Foundation.
Anesthesia: Ketamine and Xylazine | Scalpel |
Ophthalmic ointment | Pair of tweezers nr. 2 |
Electric razor | Pair of tweezers nr. 5 |
Gloves | Pair of scissors for blunt dissection |
70% (v/v) ethanol | Micro scissors |
Cotton-wool buds/gaze | 6.0 Prolene non-absorbable suture |
Stereo microscope | 2 x needle holders |
Adhesive tape | Heat pad |
von Frey filaments (0.02 – 2.0 g for mice) (Stoelting) |