Bernard Calvino

Differential regulation of NGF receptors in primary sensory neurons by adjuvant-induced arthritis in the rat.

&NA; In the adult brain, neurotrophins play a key role in adaptive processes linked to increased neuronal activity. A growing body of evidence suggests that chronic pain results from long‐term plasticity of central pathways involved in nociception. We have investigated the involvement of nerve growth factor (NGF) in adaptive responses of primary sensory neurons during the course of a long‐lasting inflammatory pain model. The amount and distribution of the NGF receptors p75NTR and TrkA were measured in the dorsal horn and dorsal root ganglia (DRG) of animals subjected to Freunds adjuvant‐induced arthritis (AIA). We observed an increased immunoreactivity of both receptors in the central terminals of primary sensory neurons in the arthritic state. The increases were seen in the same population of afferent terminals in deep dorsal horn laminae. These changes paralleled the variations of clinical and behavioral parameters that characterize the course of the disease. They occurred in NGF‐sensitive, but not GDNF‐sensitive, nerve terminals. However, p75NTR and TrkA protein levels in the DRG (in the cell body of these neurons) showed different response patterns. An immediate rise of p75NTR was seen in parallel with the initial inflammation that developed after administration of Freunds adjuvant in hindpaws. In contrast, increases of the mature (gp140trk) form of TrkA occurred later and seemed to be linked to the development of the long‐lasting inflammatory response. The changes in receptor expression were observed exclusively at lumbar levels, L3–L5, somatotopically appropriate for the inflammation. Together, these results implicate NGF in long‐term mechanisms accompanying chronic inflammatory pain, via the up‐regulation of its high affinity receptor, and offer additional evidence for differential processes underlying short‐ versus long‐lasting inflammatory pain.

Characterisation of sensory abnormalities observed in an animal model of multiple sclerosis: a behavioural and pharmacological study.

Multiple sclerosis is a chronic inflammatory demyelinating disease, associated, in 50–80% of patients, with persistent pain. While the type of pain that affects these patients is being more documented, the mechanisms underlying this pathology are still poorly understood and animal models of such chronic pain associated with MS are required. The aim of our study was to characterize the sensory abnormalities and in particular the clinical signs linked to persistent pain in two models of Experimental Autoimmune Encephalomyelitis (EAE) in the rat. This behavioural characterization tested several sensory modalities such as mechanical and thermal (heat/cold) hyperalgesia or allodynia and explored some of these modalities on two different extremities: the hindpaws and the tail. Our study showed that while one of the model produced more robust motor impairment, animals of both models suffer from mechanical hyperalgesia and thermal allodynia to cold, both at the level of the tail and the hindpaws. While the time‐course changes of some of these modalities are shifted in the time between the two models, they represent good models of the sensory abnormalities experienced by MS patients. The second part of our study aimed at characterizing from a pharmacological point of view the most robust model (“EAE+Cyclosporine”) and showed that Gabapentin, Duloxetine and Tramadol partially relieved some of the clinical signs. Our results suggest that the model “EAE+Cyclosporine” in the rat is a good model of chronic sensory abnormalities observed in MS patients both from a behavioural and pharmacological point of view.

Cortical effect of oxaliplatin associated with sustained neuropathic pain: Exacerbation of cortical activity and down-regulation of potassium channel expression in somatosensory cortex

Summary Down‐regulation of potassium channel in somatosensory cortex is a possible origin of cortical hyperexcitability in an oxaliplatin model. Abstract Oxaliplatin is a third‐generation platinum‐based chemotherapy drug that has gained importance in the treatment of advanced metastatic colorectal cancer. Its dose‐limiting side effect is the production of chronic peripheral neuropathy. Using a modified model of oxaliplatin‐induced sensory neuropathy, we investigated plastic changes at the cortical level as possible mechanisms underlying the chronicity of pain sensation in this model. Changes in gene expression were studied using DNA microarray which revealed that when oxaliplatin‐treated animals displayed clinical neuropathic pain symptoms, including mechanical and thermal hypersensitivity, approximately 900 were down‐regulated in the somatosensory cortex. Because of the known role of potassium channels in neuronal excitability, the study further focussed on the down‐regulation of these channels as the possible molecular origin of cortical hyperexcitability. Quantification of the magnitude of neuronal extracellular signal‐regulated kinase (ERK) phosphorylation in cortical neurons as a marker of neuronal activity revealed a 10‐fold increase induced by oxaliplatin treatment, suggesting that neurons of cortical areas involved in transmission of painful stimuli undergo a chronic cortical excitability. We further demonstrated, using cortical injection of lentiviral vector shRNA against Kv2.2, that down‐regulation of this potassium channel in naive animals induced a sustained thermal and mechanical hypersensitivity. In conclusion, although the detailed mechanisms leading to this cortical excitability are still unknown, our study demonstrated that a cortical down regulation of potassium channels could underlie pain chronicity in this model of chemotherapy‐induced neuropathic pain.

Structural and Molecular Alterations of Primary Afferent Fibres in the Spinal Dorsal Horn in Vincristine-Induced Neuropathy in Rat

Vincristine is one of the most common anti-cancer drug therapies administered for the treatment of many types of cancer. Its dose-limiting side effect is the emergence of peripheral neuropathy, resulting in chronic neuropathic pain in many patients. This study sought to understand the mechanisms underlying the development of neuropathic pain by vincristine-induced neurotoxicity. We focused on signs of functional changes and revealed that deep layers of the spinal cord (III–IV) experience increased neuronal activity both in the absence of peripheral stimulation and, as a result of tactile mechanical stimulations. These laminae and superficial laminae I–II were also subject to structural changes as evidenced by an increase in immunoreactivity of Piccolo, a marker of active presynaptic elements. Further investigations performed, using DNA microarray technology, describe a large number of genes differentially expressed in dorsal root ganglions and in the spinal dorsal horn after vincristine treatment. Our study describes an important list of genes differentially regulated by vincristine treatment that will be useful for future studies and brings forward evidence for molecular and anatomical modifications of large diameter sensory neurons terminating in deep dorsal horn laminae, which could participate in the development of tactile allodynia.

Prevaccination with diluted freund adjuvant prevents the development of chronic pain and transient release of cerebrospinal fluid substance P in adjuvant-induced arthritis in rats

&NA; Parallel time courses of preclinical and behavioural pain‐related parameters and levels of substance P‐like immunoreactivity in plasma (plasma‐SPLI) and cerebrospinal fluid (CSF‐SPLI) were studied in 2 groups of rats injected with an arthritogenic solution (concentrated Freund adjuvant) over a 9‐week post‐infection (PI) period; 1 group was pretreated with saline (control) and 1 pretreated with diluted Freund adjuvant (immunized). In control rats all symptoms of adjuvant‐induced arthritis (AIA) developed while in immunized rats AIA symptoms were significantly reduced or did not appear. A significant increase in plasma‐SPLI was obvious as early as the 2nd week PI and remained at this level in both groups of animals until the end of the 9‐week PI observation period, but with a significantly higher increase in control versus immunized group at all stages. In contrast, CSF‐SPLI transiently peaked only in the control group at 3 weeks PI whereas CSF‐SPLI values did not differ from one week to another in both groups of rats. These results suggest that successive injections of diluted Freund adjuvant impairs the development of chronic inflammation and pain in AIA in rats, as well as the transient increase in SP release in CSF at 3 weeks PI, but not the long‐lasting increased SP release in plasma. Since there is a clear dissociation between our biochemical and preclinical and behavioral data, this study does not provide evidence for the role of substance P as a possible biologic marker of chronic pain either in plasma or in CSF. This study questions the use of SP measurements as a biological marker of chronic pain.

Role of NGF in Neuronal Plasticity in the Lateral Reticular Nucleus in Chronic Inflammatory Pain

In pathological states, repetitive inputs from the ascending pathways involved in the genesis and integration of nociception, leads to molecular, anatomical and electrophysiological adaptive changes of these pathways, contributing to the development of pain chronicity. In the past, neurotrophic factors have been implicated in neuronal plasticity in adult central nervous system. We have previously described plastic changes associated with the up-regulation of NGFs high affinity receptor, TrkA, in the spinoreticular pathway in a chronic inflammatory pain model of arthritis induced by complete Freunds adjuvant. The present study investigated the role of central NGF in the maintenance of inflammatory pain. Analysis of TrkA and NGF expression revealed that they are expressed in the medial thalamus and several reticular nuclei of the brain stem such as the lateral reticular nucleus (LRt) and not in pathways classically described to be involved in the sensory-discriminative aspect of pain such as the lateral thalamus. In addition NGF was over-expressed in the LRt, lateral thalamus and cortex of polyarthritic rats. Using micro-injection of an adenoviral vector synthesizing NGF (or green fluorescent protein) in the LRt of normal animals, we showed that increased NGF levels in the LRt leads to the development of mechanical hypersensitivity and increased nocifensive behavior following an inflammatory stimulus. These results suggest that, NGF acts centrally as a possible molecular inducer of synaptic plasticity in the LRt in conditions of chronic inflammatory pain.

Les modèles de douleur chez l'animal

RésuméCet article présente une revue qui se veut la plus large et la plus complète possible des différents types de modèles expérimentaux de nociception et de douleur chronique chez l’animal. Au-delà de cette revue, il tente de souligner l’importance de la prise en compte de la distance entre le modèle animal et la pathologie douloureuse chez l’Homme. Ce problème est au cœur de l’interrogation qui doit être celle de tout chercheur dans le domaine de la douleur dans la mesure où ’utilisation de modèles animaux n’est pas exempte de toute critique, interrogation éthique et déontologique, travail qu’il faut poursuivre sans excès d’autocritique ni de culpabilisation, mais avec lucidité et rigueur scientifique. De cette réflexion dépend la pertinence de ces modèles, en référence à la seule justification de leur utilisation, c’est-à-dire la compréhension de la douleur clinique chez l’Homme et de son traitement.SummaryThis paper reviews as completely as possible differents types of experimental models in animals of behavioural reactions to nociceptive stimuli and chronic pain. Beyond this review, it is attempted to emphasize the importance of analysing the distance between animal models and clinical painful pathologies in human beings. This problem must be a central questioning for scientists working in the field of pain research since the use of animals in experimental research involving painful stimuli or chronic pain is able to be criticized from an ethical point of view. But this field of research must go on without excess of autocriticism and this question must be analysed with lucidity and scientific severity. The pertinence of these models with clinical pathology is greatly dependent on efforts made in this way. Scientists must be aware of the fact that the only justification of experimenting with such animal models is to understand clinical pain mechanisms in human beings in order to better alleviate pain.