Preclinical Neuropathic Pain Assessment; the Importance of Translatability and Bidirectional Research.

For patients suffering with chronic neuropathic pain the need for suitable novel therapies is imperative. Over recent years a contributing factor for the lack of development of new analgesics for neuropathic pain has been the mismatch of primary neuropathic pain assessment endpoints in preclinical vs. clinical trials. Despite continuous forward translation failures across diverse mechanisms, reflexive quantitative sensory testing remains the primary assessment endpoint for neuropathic pain and analgesia in animals. Restricting preclinical evaluation of pain and analgesia to exclusively reflexive outcomes is over simplified and can be argued not clinically relevant due to the continued lack of forward translation and failures in the clinic. The key to developing new analgesic treatments for neuropathic pain therefore lies in the development of clinically relevant endpoints that can translate preclinical animal results to human clinical trials. In this review we discuss this mismatch of primary neuropathic pain assessment endpoints, together with clinical and preclinical evidence that supports how bidirectional research is helping to validate new clinically relevant neuropathic pain assessment endpoints. Ethological behavioral endpoints such as burrowing and facial grimacing and objective measures such as electroencephalography provide improved translatability potential together with currently used quantitative sensory testing endpoints. By tailoring objective and subjective measures of neuropathic pain the translatability of new medicines for patients suffering with neuropathic pain will hopefully be improved.

Local translation in primary afferent fibers regulates nociception.

Recent studies have demonstrated the importance of local protein synthesis for neuronal plasticity. In particular, local mRNA translation through the mammalian target of rapamycin (mTOR) has been shown to play a key role in regulating dendrite excitability and modulating long-term synaptic plasticity associated with learning and memory. There is also increased evidence to suggest that intact adult mammalian axons have a functional requirement for local protein synthesis in vivo. Here we show that the translational machinery is present in some myelinated sensory fibers and that active mTOR-dependent pathways participate in maintaining the sensitivity of a subpopulation of fast-conducting nociceptors in vivo. Phosphorylated mTOR together with other downstream components of the translational machinery were localized to a subset of myelinated sensory fibers in rat cutaneous tissue. We then showed with electromyographic studies that the mTOR inhibitor rapamycin reduced the sensitivity of a population of myelinated nociceptors known to be important for the increased mechanical sensitivity that follows injury. Behavioural studies confirmed that local treatment with rapamycin significantly attenuated persistent pain that follows tissue injury, but not acute pain. Specifically, we found that rapamycin blunted the heightened response to mechanical stimulation that develops around a site of injury and reduced the long-term mechanical hypersensitivity that follows partial peripheral nerve damage--a widely used model of chronic pain. Our results show that the sensitivity of a subset of sensory fibers is maintained by ongoing mTOR-mediated local protein synthesis and uncover a novel target for the control of long-term pain states.

Disruption of noradrenergic transmission and the behavioural response to a novel environment in NK1R-/- mice.

The behaviour of neurokinin-1-receptor gene knockout (NK1R-/-) mice, which lack functional, substance P-preferring receptors, resembles that of NK1R+/+ mice treated with an antidepressant. Because all antidepressants increase central monoamine transmission, we have investigated whether noradrenergic transmission is increased in NK1R-/- mice and, if so, whether this could influence their behaviour. In anaesthetized subjects, the concentration of extracellular noradrenaline in NK1R-/- mice was two-fourfold greater than in NK1R+/+ mice. Systemic administration of the alpha2-adrenoceptor antagonist, 2-(2,3-dihydro-2-methoxy-1,4-benzodioxan-2-yl)-4,5-dihydro-1H-imidazoline (RX 821002), in anaesthetized or freely moving animals increased extracellular noradrenaline in NK1R+/+ mice only. This suggests that the function of alpha2a-autoreceptors, which modulate noradrenergic transmission, is impaired in NK1R-/- mice. Consistent with this, [35S]GTPgammaS binding to activated alpha2a-adrenoceptors was lower (-70%) in the locus coeruleus, but not the frontal cortex, of NK1R-/- mice compared with their NK1R+/+ counterparts. RX 821002-pretreatment, followed by retrodialysis of the noradrenaline reuptake inhibitor, desipramine, into the frontal cortex of anaesthetized mice increased extracellular noradrenaline to the same extent in the two genotypes. Western blots confirmed that there was no difference in the amount of noradrenaline transporter protein in NK1R-/- and NK1R+/+ mice. Finally, the effects of RX 821002 on certain behaviours in a light/dark exploration box were blunted in NK1R-/- mice, but there was no consistent effect on anxiety-like behaviour in the two genotypes. It is concluded that the greater basal efflux of noradrenaline in NK1R-/- mice is explained by increased transmitter release, coupled with desensitization of somatodendritic alpha2a-adrenoceptors. These changes could contribute to the difference in the behavioural phenotypes.

Increased neurogenesis and brain-derived neurotrophic factor in neurokinin-1 receptor gene knockout mice.

It has previously been shown that chronic treatment with antidepressant drugs increases neurogenesis and levels of brain-derived neurotrophic factor in the hippocampus. These changes have been correlated with changes in learning and long-term potentiation and may contribute to the therapeutic efficacy of antidepressant drug treatment. Recently, antagonists at the neurokinin-1 receptor, the preferred receptor for the neuropeptide substance P, have been shown to have antidepressant activity. Mice with disruption of the neurokinin-1 receptor gene are remarkably similar both behaviourally and neurochemically to mice maintained chronically on antidepressant drugs. We demonstrate here that there is a significant elevation of neurogenesis but not cell survival in the hippocampus of neurokinin-1 receptor knockout mice. Neurogenesis can be increased in wild-type but not neurokinin-1 receptor knockout mice by chronic treatment with antidepressant drugs which preferentially target noradrenergic and serotonergic pathways. Hippocampal levels of brain-derived neurotrophic factor are also two-fold higher in neurokinin-1 receptor knockout mice, whereas cortical levels are similar. Finally, we examined hippocampus-dependent learning and memory but found no clear enhancement in neurokinin-1 receptor knockout mice. These data argue against a simple correlation between increased levels of neurogenesis or brain-derived neurotrophic factor and mnemonic processes in the absence of increased cell survival. They support the hypothesis that increased neurogenesis, perhaps accompanied by higher levels of brain-derived neurotrophic factor, may contribute to the efficacy of antidepressant drug therapy.