Molecular mechanisms underlying the enhanced analgesic effect of oxycodone compared to morphine in chemotherapy-induced neuropathic pain.

Oxycodone is a µ-opioid receptor agonist, used for the treatment of a large variety of painful disorders. Several studies have reported that oxycodone is a more potent pain reliever than morphine, and that it improves the quality of life of patients. However, the neurobiological mechanisms underlying the therapeutic action of these two opioids are only partially understood. The aim of this study was to define the molecular changes underlying the long-lasting analgesic effects of oxycodone and morphine in an animal model of peripheral neuropathy induced by a chemotherapic agent, vincristine. Using a behavioural approach, we show that oxycodone maintains an optimal analgesic effect after chronic treatment, whereas the effect of morphine dies down. In addition, using DNA microarray technology on dorsal root ganglia, we provide evidence that the long-term analgesic effect of oxycodone is due to an up-regulation in GABAB receptor expression in sensory neurons. These receptors are transported to their central terminals within the dorsal horn, and subsequently reinforce a presynaptic inhibition, since only the long-lasting (and not acute) anti-hyperalgesic effect of oxycodone was abolished by intrathecal administration of a GABAB receptor antagonist; in contrast, the morphine effect was unaffected. Our study demonstrates that the GABAB receptor is functionally required for the alleviating effect of oxycodone in neuropathic pain condition, thus providing new insight into the molecular mechanisms underlying the sustained analgesic action of oxycodone.

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.

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

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.

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.

Functional brain imaging of trigeminal neuralgia.

We used functional magnetic resonance imaging (fMRI) to analyze changes in brain activity associated with stimulation of the cutaneous trigger zone in patients with classic trigeminal neuralgia (CTN). Fifteen consecutive patients with CTN in the second or third division of the nerve, were included in this study. The fMRI paradigm consisted of light tactile stimuli of the trigger zone and the homologous contralateral area. Stimulation of the affected side induced pain in seven patients, but was not painful in eight patients on the day of the experiment. Painful stimuli were associated with significantly increased activity in the spinal trigeminal nucleus (SpV), thalamus, primary and secondary somatosensory cortices (S1, S2), anterior cingulate cortex (ACC), insula, premotor/motor cortex, prefrontal areas, putamen, hippocampus and brainstem. Nonpainful stimulation of the trigger zone activated all but three of these structures (SpV, brainstem and ACC). After a successful surgical treatment, activation induced by stimulation of the operated side was confined to S1 and S2. Our data demonstrate the pathological hyperexcitability of the trigeminal nociceptive system, including the second order trigeminal sensory neurons during evoked attacks of CTN. Such sensitization may depend on pain modulatory systems involving both the brainstem (i.e. periaqueductal gray and adjacent structures) and interconnected cortical structures (i.e. ACC). The fact that large portions of the classical 'pain neuromatrix' were also activated during nonpainful stimulation of the trigger zone, could reflect a state of maintained sensitization of the trigeminal nociceptive systems in CTN.

Serotonin 5-HT2A receptor involvement and Fos expression at the spinal level in vincristine-induced neuropathy in the rat.

We recently showed that peripheral and spinal 5-HT2A receptors (5-HT2AR) are involved in a rodent model of neuropathy induced by a nucleoside analogue reverse transcriptase inhibitor. In this paper, we show that 5-HT2AR are also involved in neuropathy induced by an anti-neoplasic drug, vincristine. Vincristine-treated rats (0.1mg/kg, daily i.p. administration for two 5-day cycles) developed thermal allodynia and mechanical hypersensitivity, which decreased in a dose-related manner after epidural injection a 5-HT2A receptor antagonist. Moreover, 5-HT2A-/- mice did not develop vincristine-induced neuropathy contrarily to their 5-HT2A+/+ littermates. In vincristine-treated rats, the number of nociceptive dorsal root ganglion cells expressing the 5-HT2AR was increased by 38%, and 5-HT2AR immunolabelling was enhanced in layers I-IV of the dorsal horn. At the EM level, a 76.3% increase in the density of 5-HT2AR immunopositive axon terminals within superficial layers of the dorsal horn was noted after vincristine treatment. Immunocytochemical study of Fos expression in vincristine-treated rats revealed a significant increase in the number of Fos-positive neurons not only in regions where nociceptive fibres terminate superficial (I-II) and deep layers (V-VI) of the spinal cord, but also in intermediate layers, suggesting that Abeta fibres could be involved in the spinal sensitization observed in this model. Double labelling experiments showed that Fos-positive neurons were endowed with 5-HT2AR immunolabelling in the dorsal horn of vincristine-treated rats. These data provide support to the idea that, in vincristine-induced neuropathy, 5-HT2AR are involved in the sensitization of peripheral nociceptors and spinal nociceptive processing.

[Neural basis of pain]. / Les bases neurales de la douleur.

Main elements concerning the physiology of pain are described, as well as the structures of the nervous system at the origin of the central control of pain: peripheral fibres (small diameter myelinated A delta and unmyelinated C fibres); spinal ascending pathways; cerebral structures relaying nociceptive information (medial and ventro-postero-lateral thalamic relays); SI and SII cortical areas; spinal segmentary and supraspinal excitatory and inhibitory controls; diffuse noxious inhibitory controls (DNIC). Chronic pain is a result of two processes: peripheral and central sensitization, in relation with inflammation and nerve injury at peripheral level and with neuroplasticity at central level. Neurotrophins, mainly NGF and BDNF and their receptors (LNTR, TrkA and TrkB) are involved in these processes. Pain is a result of an unpleasant emotional experience: its various components, mainly the emotional one, may be increased or decreased considering the different characteristics of the stimulus and of the affective state of the patient, as well as the context in which this stimulus is applied. The role of physiological systems, unconnected with those classically involved in the physiology of nociception and pain, such as the motor cortex in phantom limb pain, are described in conclusion, to focus on the extreme complexity of the control systems of pain in humans.

Central pain control.

We describe the anatomic and physiological components involved in pain physiology, with the goal of providing readers with the background information needed to understand central pain control mechanisms. These include spinal segmental controls, supraspinal excitatory and inhibitory controls, and diffuse noxious inhibitory controls (DNICs). Pain is a subjective sensation produced by an emotionally unpleasant experience considered to originate in adaptive processes taking place within neuron networks located at various levels of the central nervous system. The intensity of the components of pain is influenced by the stimulus characteristics, patient-related factors, and the setting in which the stimulus occurs. The various components of pain and the psychological and neurophysiological mechanisms that underlie the affective dimension of pain are reviewed. As a conclusion, phantom pain is used to illustrate the role for physiological systems independent from those involved in the physiology of nociception and pain, such as the motor cortex. This example highlights the extreme complexity of pain and pain control systems in humans.

Chronic pain induces a paradoxical increase in growth hormone secretion without affecting other hormones related to acute stress in the rat.

In several diseases chronic pain is associated with long-lasting pathophysiological responses which differ strongly from those observed in acute situations. When persisting, acute pain often results in physical and psychological stress which may in turn aggravate the initial pathological state. In the present work we examined the secretory patterns of pituitary hormones related to acute stress (growth hormone (GH), prolactin (PRL) and beta-endorphin (beta-END)) in rats during the phase of Freund adjuvant-induced arthritis (AIA, a model used for chronic pain studies) when chronic pain is maximum (14 and 21 days, postinoculation (PI)). Using radio-immunoassay hormones were measured in plasma samples taken every 30 min for 7 h in free-moving rats 14 and 21 days after Freund adjuvant or vehicle injection and in control animals. The total amount of GH secretion was higher at 14 and 21 days PI in AIA rats as compared to vehicle-treated and control animals, and the pulsatility of GH secretory pattern was not modified by AIA. PRL and beta-END secretion were not significantly different in arthritic rats as compared to controls. These results show that GH, PRL and beta-END responses induced by acute stress are not observed during the AIA phase when chronic pain is maximum. Thus, in our experimental conditions, beta-END and PRL do not seem to be good plasma markers of chronic pain.(ABSTRACT TRUNCATED AT 250 WORDS)

Dorsal horn (convergent) neurones in the intact anaesthetized arthritic rat. II. Heterotopic inhibitory influences.

Recordings were made from dorsal horn neurones in the spinal cord and trigeminal nucleus caudalis of intact anaesthetized rats. These rats had been rendered polyarthritic by s.c. injection of Mycobacterium butyricum suspended in oil into the base of the tail. The experiments were carried out during the acute phase of the illness (3-4 weeks post inoculation) during which hyperalgesia occurred. The disease mainly affected the hind paws and the tail and, to a lesser extent, the forepaws. The facial area of the animals was not at all affected. As described in a previous paper, recordings from lumbar dorsal horn neurones revealed that two subpopulations could be described on the basis of their electrophysiological characteristics. Namely, 'typical' units which include convergent, non-noxious and proprioceptive neurones and which have properties essentially similar to those found in healthy rats, and 'atypical' cells which have no counterpart in healthy rats and which include convergent and non-noxious neurones. All the typical convergent neurones were inhibited by noxious stimuli applied to heterotopic body areas, whereas typical non-noxious and proprioceptive neurones were not; these observations are similar to those described in healthy rats as diffuse noxious inhibitory controls (DNIC). However, it was also found that 88% of the atypical convergent and 85% of the atypical non-noxious cells were inhibited by various heterotopic stimuli. The most important observation was that gentle stimulation such as mild pressure applied to the inflamed contralateral ankle joint--a stimulus intensity which has never been found to be effective in healthy animals--was capable of triggering inhibition of both typical and atypical convergent neurones. Recordings from trigeminal nucleus caudalis neurones revealed that the entire population presented essentially the same properties as those observed in healthy animals in terms of activity evoked by natural or electrical stimulation of their excitatory receptive fields. The activity of non-noxious neurones was never modified by any heterotopically applied stimuli. By contrast, all convergent neurones were inhibited by heterotopic stimuli, noxious (52 degrees C, pinch) or non-noxious (light and mild pressure), applied to inflamed areas. While the inhibition triggered by noxious stimuli was reminiscent of that observed in healthy rats, the inhibition triggered by non-noxious mechanical stimuli was related to the inflammatory state of the part of the body stimulated, the most sensitive areas being the hind paws.(ABSTRACT TRUNCATED AT 400 WORDS)

Dorsal horn (convergent) neurones in the intact anaesthetized arthritic rat. I. Segmental excitatory influences.

Recordings were made from dorsal horn neurones in intact anaesthetized rats rendered polyarthritic by s.c. injection into the base of the tail, of Mycobacterium butyricum suspended in oil; the experiments were carried out during the acute phase of the illness (3-4 weeks post inoculation) during which hyperaesthesia occurred. The majority (60.8%) of the neurones studied had properties close to those of corresponding groups of units in healthy rats. These 'typical' neurones could be subdivided into convergent (13.2%), non-noxious (34.4%) and proprioceptive (13.2%) units. By contrast, and in agreement with a previous study in the unanaesthetized spinal arthritic rat, the segmental electrophysiological characteristics of the remaining large proportion of neurones were changed both in terms of the size and distribution of their excitatory receptive fields and their responsiveness to peripheral stimuli; these were designated as 'atypical' neurones. According to their electrophysiological properties, these neurones were differentiated as atypical convergent (27.8%) and atypical non-noxious (11.4%) units. The main qualitative difference between the typical and atypical neurones was that the atypical had an additional receptive field on the oedematous ipsilateral ankle and, in several cases, showed high levels of background activity with sometimes dramatic increases. By comparison with neurones recorded in healthy rats, quantitative data revealed other modifications: typical and atypical convergent neurones and atypical non-noxious neurones had larger classical excitatory receptive fields; while C-fibre responses evoked by transcutaneous electrical stimulation were facilitated in the case of typical convergent neurones, 47% of the atypical convergent neurones had no C-fibre responses, and when present (53%) the threshold for obtaining these C-fibre responses was higher with suprathreshold stimuli producing a minimal number of spikes; in these cells, gentle mechanical stimuli gave rise to high rates of firing which sometimes resulted in dramatic, long lasting after-discharges. The possibility that typical convergent, atypical convergent and atypical non-noxious neurones were derived from the homogeneous population of convergent neurones in the healthy rat is discussed; the atypical properties could be the result of a change in the characteristics of convergent neurones, resulting from arthritis.