Neuropathic pain can be defined as pain caused or initiated by a primary lesion of the peripheral or central nervous system, manifesting with sensory symptoms and signs [1]. It is associated with allodynia, hyperalgesia and spontaneous pain with varying intensity, in which pain could be so severe that daily activities like wearing clothes might be difficult. Neuropathic pain can also decrease quality of life, cause patients to withdraw socially and emotionally, and cause problems with mood and sleep. Moreover, unrelieved pain inhibits respiration, decreases mobility, and decreases patient functional status, which may lead to iatrogenic events, including pneumonia, constipation, and deep vein thrombosis [2].

Medications primarily used for neuropathic pain generally fall into the categories of anticonvulsants, antidepressants, opioids, and topical agents. The current treatment options, however, are not effective enough to make the pain endurable and the response rate has been estimated at a 30% to 50% reduction in pain in up to 50% of patients [3].

Recently, accumulating evidence has demonstrated that both microglia and astrocytes release a variety of proinflammatory cytokines including interleukin (IL)-1β, IL-6, and tumor necrosis factor (TNF-α), which play a major role in mediating and/or maintaining hyperalgesia and allodynia in inflammatory, as well as neuropathic pain [4-6].

Thalidomide, a derivative of glutamic acid, was developed initially as a sedative and used for morning sickness until 1961 when its use was banned due to its association with major birth defects. It reappeared in clinical practice because of its immunomodulatory properties. Furthermore, investigations into the mechanisms underlying thalidomide's pharmacotherapeutic action have increased [7,8], with its effects on the production and release of TNF-α being of notable importance. Thalidomide has been used in several cutaneous inflammatory disorders (erythema nodosum leprosum in lepromatous leprosy, cutaneous lupus erythematosus, severe aphtosis), cancers (relapsed/refractory multiple myeloma) and inflammatory conditions [9]. Previous studies have shown that thalidomide inhibits the inflammatory hyperalgesia in rats and the writhing nociceptive response in mice [10,11]. Thalidomide was found to be effective in animal models of neuropathic pain, such as chronic sciatic nerve injury (CCI) and Lumbar (L) 5 ventral root transection (VRT) neuropathy [12,13]; however, several human case reports and series have demonstrated mixed results, with some patients having dramatic responses, especially in chronic intractable conditions such as complex regional pain syndrome [14,15].

Opioids are among the most powerful analgesics in clinical use for the treatment of nociceptive pain, but data concerning their effectiveness in neuropathic pain has been controversial [16,17]. However, evidence now suggests that opioids are effective in relieving some pain symptoms in patients with neuropathic pain of primarily peripheral origin [18,19].

In the present study, we investigated the antiallodynic effects of morphine and thalidomide in a spinal nerve ligation rat model.

Neuropathic pain encompasses a myriad of painful disease states that are often hard to treat, especially with one single medication. There is no "gold standard" in regard to one medication for neuropathic pain. Some new medications have emerged during the past few years that help to augment the armamentarium of medications used in neuropathic pain [3]. Recently, increasing evidence indicates that glial cells, one of non-neuronal cells, also play an important role in the pathogenesis of neuropathic pain and the mechanism of neuropathic pain by nerve injury. Neuropathic pain is newly regarded as complex and integral in the interaction of neuronal and non-neuronal cells [4-6]. Not only neurotransmitters from neurons at the synapses, but also many cytokines released from glial cells are involved in the development of hyperalgesia and allodynia in inflammatory as well as neuropathic pain. In addition, much research indicates that proinflammatory cytokines such as TNF-α, IL-1β and IL-6 are major elements contributing to development and maintenance of the pathological state of pain [6,23]; consequently, they have been proposed as therapeutic targets [24].

Thalidomide has profound immunomodulatory actions in addition to its originally intended pharmacological actions. Thalidomide is currently approved by the US Food and Drug Administration in conjunction with dexamethasone in newly diagnosed multiple myeloma, and in the acute treatment of cutaneous manifestations of erythema nodosum leprosum, as well as in maintenance therapy for its prevention and suppression.

In the present study, intraperitoneal thalidomide significantly increased PWT compared to control and produced dose-responsiveness. The lower dose of thalidomide (6.25 mg/kg) produced no significant reversal of tactile allodynia, but the higher dose of thalidomide (12.5, 25 and 50 mg/kg, respectively) effectively increased mechanical withdrawal threshold compared to vehicle.

Although thalidomide and its analogues have broad-spectrum properties, their major mode of action is the inhibition of TNF-α synthesis. Thalidomide reduces synthesis of TNF-α in cultures of lipopolysaccharide-stimulated human microglial cells [25], and also modulates TNF-α production by several mechanisms, for example, by enhancing TNF-α mRNA degradation [26] or by binding to a1-acid glycoprotein [27]. Thalidomide was also found to be effective in animal models by inhibiting TNF-alpha production [10-13]. Previous studies have shown that thalidomide inhibits inflammatory hyperalgesia in rats and the writhing nociceptive response in mice as a possible consequence of inhibition of TNF-α production [10,11]. It has also been reported that thalidomide reversed and prevented the hyperalgesia by inhibiting TNF-α production in chronic sciatic nerve injury and L5 VRT neuropathy, respectively [12,13]. However, in other studies [13,28], thalidomide was only effective when given preemptively, i.e., before manifestation of the pain related symptom. However, in our study intraperitoneal injection of thalidomide effectively attenuated mechanical allodynia when it was applied at day 10 after surgery. This discrepancy may be related to different animal models and experimental methods. Also, it has also been reported that when applied 7 days after surgery thalidomide was still capable of inhibiting upregulation of TNF-α in L5 VRT neuropathy [13]. Thus, it is most likely that the antinociceptive effect of thalidomide in this present study comes from the interruption of TNF-α production. The pharmacological properties of thalidomide aside from their immunomodulatory capacity could also explain the effects observed in our study. The sedative-hypnotic effect of thalidomide was suggested as mediated via interruption or modulation of serotonin's actions within the CNS. However, rats injected with thalidomide did not show any sedative state at the dose of maximum analgesic effect during the behavioral test.

Various studies have reported reduced antinociceptive efficacy of morphine in animal models of peripheral or central nerve injury. However, conflicting results regarding the efficacy of opioids on neuropathic pain in different neuropathic animal models have been reported. While systemic administration of morphine attenuates allodynia and hyperalgesia in chronic constriction injury and spinal nerve ligation (SNL) models [29,30], intrathecal morphine is apparently ineffective [29,31]. In contrast, intrathecal morphine dose-dependently reversed mechanical allodynia in a rat model of central pain, whereas systemic morphine had little effect on this measure [32]. Recently, these findings have been challenged by Zhao et al. [33] who reported antiallodynic effects of both systemic and intrathecally administered morphine in the SNI, SNL and SCI animal models of neuropathic pain. These results indicate the efficacy of opioids in neuropathic pain is variable and seems to depend on several factors (e.g., the kind of nerve injury and the route of drug administration). In this study, the 3 and 10 mg/kg doses of systemic morphine significantly increased PWT compared to vehicle. Our study indicates that systemic morphine produces antiallodynic effects in SNL rats, which is consistent with previous reports [30,33]. Our results further support the efficacy of opioid receptor agonists in alleviating signs of neuropathic pain in animal models of peripheral nerve injury.

In this present study, intraperitoneal thalidomide and morphine significantly increased PWT compared to control and produced dose-responsiveness. However, similar studies on various animals are required to obtain a reliable oversight of the effect of thalidomide on peripheral and central nerve injury-induced neuropathic pain in a clinical setting.

In conclusion, systemic morphine and thalidomide have antiallodynic effects on neuropathic pain induced by SNL in rats. These results suggest that morphine and thalidomide may be an alternative therapeutic approach for neuropathic pain.