IJUP08 - Universidade do Porto

A new method to evaluate spontaneous pain in neuropathic pain
models: a study in rats with spared nerve injury
A. Sequeira 1, 2 , I. Martins 1, 2 , S. Araújo 1, 2 , D. Lima 2 and I. Tavares 1
1 Institute of Histology and Embryology, Faculty of Medicine, University of Porto and IBMC,
Portugal.
2 Laboratory of Molecular Cell Biology, Faculty of Medicine, University of Porto and IBMC,
Portugal.
Patients with neuropathic pain report spontaneous pain along with evoked allodynia and
hyperalgesia. However, only evoked reactions have been used to study the magnitude of
neuropathic pain, since models to evaluate spontaneous pain are scarce.
Based on the relevance of the spared nerve injury (SNI) model in long-term studies of
neuropathic pain, we performed an overtime evaluation of spontaneous pain in SNI rats in
comparison with evoked pain.
Male Wistar rats were submitted to SNI induction (n=8) or sham-operation (n=6). Evoked
and spontaneous pain reactions were evaluated immediately after surgery and at 1, 3 and 6
months later. Evoked pain was studied using the von Frey, acetone and pin-prick tests. In
spontaneous pain, animal behavior was evaluated during 10 minutes using Etholog2.2 ® and
two types of reactions were analyzed: pain-traducing and physiological behaviors. Paintraducing
behaviors included several reactions directed to the hindpaw ipsilateral to the
injured nerve, such as freezing and shaking, scratching, lifting and licking. Physiological
reactions included exploring, rearing, washing and all routine animal behaviors.
Before SNI induction and in sham-operated rats, evoked pain behaviors were reduced and
spontaneous pain reactions were absent. In SNI animals, the increase in evoked pain
behavior was more evident one month after surgery. These animals also presented a
decrease in physiological behavior. Spontaneous pain-traducing actions remained elevated
during all analyzed time points.
Based on the relation between evoked and spontaneous pain detected in SNI rats, the
present study shows that the method of spontaneous pain analysis is a reliable protocol to
fully behaviorally access animals with chronic pain.
KEY WORDS: neuropathic pain, spontaneous pain, evoked pain, allodynia, hyperalgesia.
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