Systemic amantadine diminishes inflammatory and neuropathic nociception in the rat

The role of dopamine as a possible central inhibitory mediator of pain processes has been demonstrated. The administration of L-Dopa diminishes pain perception in humans as well as the response to nociceptive stimuli in animals. Also, the intracerebral microinjection of dopamine in inflammatory and neuropathic pain models (formalin test and deafferentation, respectively) reduces nociceptive response. In this sense, the selective activation of dopamine D2 receptors and the blockade of D1 in the insular cortex and spinal cord diminish nociception. Furthermore, the microinjection of dopamine or amantadine (dopamine releaser) in the anterior cingulate cortex (ACC) also reduces chronic nociception. The efficacy of amantadine has been tested in the treatment of neuropathic pain, even when given as a single dose. There is evidence of the role of amantadine as a releaser of dopamine (DA) calcium channel (N type) dependent in the striatum as well as a low affinity non-competitive antagonist of blockade/non-blockade kinetics of the NMDA receptor. This compound has also been described as a DA agonist and an inhibitor of its reuptake. With this background, we decided to test if the effects of systemically given amantadine related to acute nociception, hyperalgesia and neuropathic nociception can be reverted by a dopaminergic blockade (using haloperidol) within the ACC. The experiments were conducted in agreement with the Ethics Committee of the International Association for the Study of Pain and the approval of the Projects Commission of the Instituto Nacional de Psiquiatría Ramón de la Fuente (INPRF). Male Wistar rats (250-300 g) were housed in the INPRF. During the observation period, the animals were maintained in transparent acrylic individual cages with light-dark cycles of 12 × 12 h, with feeding and hydration ad libitum. For all surgical procedures, rats were anaesthetised with halothane 2% mixed with O2 98%. In order to test the dopaminergic effect of amantadine within the ACC in nociception, we used the hyperalgesia model as well as a neuropathic nociception model induced by denervation. In the model of hyperalgesia, carrageenan was injected in the plantar region (50 μl at 1%), followed by a thermonociception test in which paw withdrawal latency was measured. In the neuropathic nociception model, the right sciatic nerve was denervated and chronic nociception was measured as autotomy behaviour. Moreover, in another series of experiments, haloperidol (3 mg/ 200 nl) was microinjected into the ACC before the induction of hyperalgesia and neuropathic nociception. Amantadine was then injected (90 mg/kg i.p.) and the behavioural development was observed in both models. Systemic amantadine was able to reduce both neuropathic nociception and hyperalgesia. Also, the results show, on the one hand, that haloperidol significantly decreases the antinociceptive effect of amantadine measured as paw withdrawal latency. On the other hand, amantadine can reduce nociception when administered systemically and, according to what has been published previously, when administered directly into the ACC. Our results show that amantadine is effective in diminishing hyperalgesia and nociception induced by deafferentation. This suggests that amantadine can be a therapeutic alternative for the treatment and prevention of neuropathic pain such as phantom limb pain or pain due to deafferentation, among others.