Neuropathic pain models in the development of analgesic drugs.

Introduction Animal disease models are predictive for signs seen in disease. They may rarely mimic all signs in a specific disease in humans with respect to etiology, cause or development. Several models have been developed for different pain states and the alteration of behavior has been interpreted as a response to external stimulus or expression of pain or discomfort. Considerable attention must be paid not to interpret other effects such as somnolence or motor impairment as a pain response and similarly not to misinterpret the response of analgesics. Neuropathic pain is caused by injury or disease of the somatosensory system. The clinical manifestations of neuropathic pain vary including both stimulus-evoked and non-stimulus evoked (spontaneous) symptoms. By pharmacological intervention, the threshold for allodynia and hyperalgesia in the various pain modalities can be modulated and measured in animals and humans. Animal models have been found most valuable in studies on neuropathic pain and its treatment. Aim of the study With these interpretation problems in mind, the present text aims to describe the most frequently used animal models of neuropathic pain induced by mechanical nerve injury. Methods The technical surgical performance of these models is described as well as pain behavior based on the authors own experience and from a literature survey. Results Nerve injury in the hind limb of rats and mice is frequently used in neuropathic pain models and the different types of lesion may afford difference in the spread and quality of the pain provoked. The most frequently used models are presented, with special focus on the spared nerve injury (SNI) and the spinal nerve ligation/transection (SNL/SNT) models, which are extensively used and validated in rats and mice. Measures of mechanical and thermal hypersensitivity with von Frey filaments and Hargreaves test, respectively, are described and shown in figures. Conclusions A number of animal models have been developed and described for neuropathic pain showing predictive value in parallel for both humans and animals. On the other hand, there are still large knowledge gaps in the pathophysiologic mechanisms for the development, maintenance and progression of the neuropathic pain syndrome Implications Better understanding of pathogenic mechanisms of neuropathic pain in animal models may support the search for new treatment paradigms in patients with complex neuropathic pain conditions.

Predictive validity of pharmacologic interventions in animal models of neuropathic pain.

Introduction The pathophysiologic and neurochemical characteristics of neuropathic pain must be considered in the search for new treatment targets. Breakthroughs in the understanding of the structural and biochemical changes in neuropathy have opened up possibilities to explore new treatment paradigms. However, long term sequels from the damage are still difficult to treat. Aim of the study To examine the validity of pharmacological treatments in humans and animals for neuropathic pain. Method An overview from the literature and own experiences of pharmacological treatments employed to interfere in pain behavior in different animal models was performed. Results The treatment principles tested in animal models of neuropathic pain may have predictive validity for treatment of human neuropathies. Opioids, neurotransmitter blockers, drugs interfering with the prostaglandin syntheses as well as voltage gated sodium channel blockers and calcium channel blockers are treatment principles having efficacy and similar potency in humans and in animals. Alternative targets have been identified and have shown promising results in the validated animal models. Modulators of the glutamate system with an increased expression of glutamate re-uptake transporters, inhibition of pain promoters as nitric oxide and prostaglandins need further exploration. Modulation of cytokines and neurotrophins in neuropathic pain implies new targets for study. Further, a combination of different analgesic treatments may as well improve management of neuropathic pain, changing the benefit/risk ratio. Implications Not surprisingly most pharmacologic principles that are tested in animal models of neuropathic pain are also found to be active in humans. Whereas many candidate drugs that were promising in animal models of neuropathic pain turned out not to be effective or too toxic in humans, animal models for neuropathic pain are still the best tools available to learn more about mechanisms of neuropathic pain. Better understanding of pathogenesis is the most hopeful approach to improve treatment of neuropathic pain.

Low dose of donepezil improves gabapentin analgesia in the rat spared nerve injury model of neuropathic pain: single and multiple dosing studies.

The use of cholinergic drugs, either alone or in combination with other drugs, has been suggested as an approach to improve treatment outcome for patients suffering from neuropathic pain. The present study was undertaken in the rat spared nerve injury model of neuropathic pain to evaluate the effect of the cholinesterase inhibitor donepezil when administered (1) alone and (2) as low-dose in combination with the first-line recommendation gabapentin. The co-administration studies were performed following single and multiple dosing. Single, parenteral dosing of donepezil (1, 1.5 and 3 mg/kg s.c.) produced a dose-dependent reversal of the neuropathic pain behaviour. Co-administration of a sub-effective dose of donepezil (0.5 mg/kg s.c.) and low doses of gabapentin (10 and 30 mg/kg s.c.) resulted in a three- to fourfold increase of the analgesic effect, in comparison with gabapentin administered alone. Following multiple, oral dosing, gabapentin (25 mg/kg p.o.) was administered once daily over 20 days. Addition of donepezil (1.5 mg/kg p.o.) from day 11 to day 20 resulted in improved analgesia during the period of combination therapy, in comparison with the gabapentin monotherapy period. Furthermore, the treatment effects were stable in both the mono- and the combination therapy period, indicating that tolerance development does not occur within the studied time frame. In conclusion, the results from this preclinical study support the use of donepezil as adjunctive to gabapentin to improve the therapeutic outcome in the management of neuropathic pain.

Cytomegalovirus infection in lung transplant patients: the role of prophylaxis and recipient-donor serotype matching.

Cytomegalovirus (CMV) remains an important cause of morbidity and mortality in lung transplant recipients. We investigated the incidence of CMV infection in relation to CMV prophylaxis, and recipient-donor CMV serotype, in a cohort of 250 consecutive lung transplant recipients. All patients received 3 months CMV prophylaxis with acyclovir (n = 67) or gancyclovir (n = 183). Recipient-donor CMV serotype matching was performed in patients receiving acyclovir: R+/D+(n = 38), R+/D-(n = 10), R-/D+(n = 1), R- /D-(n = 16), unknown (n = 2). Recipient-donor CMV serotype matching was not performed in patients receiving gancyclovir: R+/D+(n = 71), R+/D-(n = 42), R-/D+(n = 38), R-/D-(n = 31), unknown (n = 1). The overall incidence of CMV infection was 51% (n = 34) in the acyclovir group, and 42% (n = 77) in the gancyclovir group (p = 0.14). During the first 9 months after transplantation, the rate of CMV infection was higher in the acyclovir group (42%) compared with the gancyclovir group (30%) (p = 0.005). Multivariate analysis demonstrated the incidence of CMV infection during the first 9 months was higher for acyclovir prophylaxis (p<0.001) and R-/D+ serostatus (p<0.001) and lower with R-/D- serostatus (p = 0.02). In conclusion, gancyclovir significantly delays the onset of first CMV infection among lung transplant patients. CMV surveillance and choice of prophylaxis may be modified according to donor-recipient CMV serotype.