Chronic pain (CP) is a global health problem. The incidence of CP in the world ranges from 7 to 40% of the population and, as a consequence, about 50% to 60% of those suffering from it are partially or totally incapacitated, in a transitory or permanent manner significantly compromising the quality of life. The prelimbic (PrL) division of the medial prefrontal cortex (mPFC) is an important region for the elaboration of cognitive and emotional aspects of pain. In addition, chronic neuropathic pain (CNP) can induce morphological changes, resulting in a reorganisation in the mPFC neurons. Moreover, there is an intrinsic relation between CP and anxiety disorder. Our study aims to investigate the effects of a modification of an animal model of CP and evaluate the neuroanatomical and pharmacological bases of neuropathic pain (NP). The role played by PrL cortex in the modulation of CNP was also investigated. Thus, the present work was divided into three steps: 1) Ethological analysis of nociceptive, motor and affective-cognitive aspects of rats submitted to an adapted model of chronic constriction of the ischiadicus nervus (CCI: a simple ligature) compared with the classic CCI model performed by Bennett and Xie (CCI: four ligatures): our results showed that the adapted-CCI model produced cold hypersensitivity and mechanical allodynia similar to those described in laboratory animals submitted to the model with four ligatures of the ischiadicus nervus. Both CCI groups displayed anxiety- and depression-like responses, and cognitive deficits, in the the open field test, forced swim test and object recognition test, respectively. However, the adapted model of CCI used in the present work may be a better choice, since a simple ligature of the ischiadicus nervus cause neither motor deficits, nor autotomy behaviour, unlike the animals with CCI induced by four ligatures of spinal nerves. 2) A- Effect of Indomethacin (2mg/kg) a non-steroidal anti-inflammatory drug peripherally administered (IP) on NP: The peripheral treatment with indomethacin reduced mechanical allodynia on the first, second, and fourth days, but not on the fourteenth, twenty-first, and twenty-eighth days after adapted CCI. These findings suggest that COX-1 and COX-2 are involved in the mediation of NP induction, but not in the maintainance of NP. B- Involvement of the PrL cortex on the generation, potentiation and maintenance of DN, through the microinjection of cobalt chloride (CoCl2: 1mM/200nL), a calcium influx blocker (synapse blocker): CoCl2 attenuated mechanical allodynia at twenty-first and twenty-eighth, but not at seventh and fourteenth days after CCI. Our data also indicate that PrL cortex participates in the elaboration of the chronic phase of mechanical allodynia in our adapted NP model. C- The role of the glutamatergic, endocannabinoid and endovanniloid systems of the PrL cortex on mechanical allodynia 21 days after CCI: The present data showed that microinjection of the N-methyl D-Aspartate agonist (NMDA), in a dose of 1 and 4nmol, was able to increase the mechanical allodynia threshold during mechanical stimulation by von Frey test filaments, whereas the NMDA receptors antagonist LY235959 decreased mechanical allodynia when microinjected at the highest dose (8nmol) in the PrL. The PrL cortex pretreatment with the CB1-cannabinoid receptor antagonist AM251 increased mechanical allodynia at all doses (50, 100 and 200 pmol). Microinjections of anandamide (AEA) at the smaller dose (5pmol) in PrL did not cause influence in the mechanical allodynia. However, the PrL treatment with AEA at the intermediate doses (50 and 100pmol) reduced mechanical allodynia and that effect were blocked by the pretreatment of the PrL cortex with AM251 (200pmol). Interestingly, the higher dose of AEA (200pmol) increased mechanical allodynia. Furthermore, this effect was attenuated by the PrL pretreatment with the transient potential receptor antagonist type 1 (TRPV1) ion channel selective antagonist 6 Iodonordihidrocapsaicin (6-I-CPS) in a dose of 3 pmol. These findings suggest that the PrL cortex is involved in the potentiation and maintenance of CNP through the activation of NMDA receptors and TRPV1 receptors in PrL cortex. The effect of attenuation of mechanical allodynia was caused by the activation of CB1 endocannabinoid receptors in rodents with CNP after 21 days of CCI. 3) Investigation of the comorbidity between CNP with anxiety/panic and the effect of NMDA glutamatergic and CB1 endocannabinoid receptors on PrL cortex after 21 days of CCI in rodents. The confrontation between a constrictor snake and the rodent elicited innate fear-related responses in prey, such as risk assessment, defensive immobility, and escape behaviour that were enhanced in CNP rodents and Sham. Also, after a confrontation with a potential predator, the CNP animals increased their mechanical allodynia thresholds. In adition, the microinjection of NMDA (4nmol) PrL, increased innate fear-related responses, such as risk assessment, and the treatment of PrL with NMDA at 1nmol incresed escape behaviour in rodents with CNP. The treatment of the PrL with NMDA in a dose of 4nmol increased the mechanical allodynia threshold during mechanical stimulation by von Frey test filaments, after confrontantion, whereas PrL pretreatment with LY235959 decreased innate fear-related responses, such risk assessment, defensive immobility, and escape behavior and decreased mechanical allodynia when microinjected (4 and 8nmol). The PrL Pretreatment with the CB1-cannabinoid receptor antagonist AM251 (all doses) increased unconditioned fear-related responses, such as risk assessment. Moreover, AM251 (100 and 200pmol) microinjections in the PrL increased mechanical allodynia after prey versus predator confrontation. The microinjections of AEA (100pmol) in the PrL decreased risk assessment, defensive immobility, and escape behaviour and reduced mechanical allodynia. Interestingly, the pretreatment of the PrL with the higher dose of AEA (200pmol) did not change the fear-induced behaviour elicited by predators, but increased the CNP. There was a classical inverted U-shape curve from the lower to the higher dose of AEA. These data suggest that the anxiety/panic and pain comorbidity increases CNP symptoms. The present findings also indicate that the CCI-adapted model, by ischiadicus nervus ligation with a single ligature is an effective animal model for studying comorbidities between CP and cognitive/emotional disturbances. In conclusion, we observed that nonsteroidal anti-inflammatory drugs are efficient to attenuate the mechanical allodynia only during NP genesis. The PrL cortex is recruited during the maintenance and potentiation of NP. The PrL glutamatergic system via NMDA activation and endovaniloid mechanisms related to TRPV1 ion channel activation potentiate CNP, and the endocannabinoid mechanisms via CB1 receptors recruitment decrease the CNP. Finally, rodents with CNP had their mechanical allodynia thresholds decreased after the confrontation with wild snakes. In addition, their defensive behaviours were itemised, thus showing the anxiety/panic and CNP potential comorbidity and the participation of the neocortex in the elaboration of CP in a model of peripheral neuropathy induced by injury of the ischiadicus nervus through its chronic constriction in Wistar rats. (AU) |
