Animal Studies

The dorsal root ganglia is a key structure in nociception and chronic pain disorders. Several gene expression studies of dorsal root ganglia in pre-clinical pain models have been performed, but it is unclear if consistent gene changes are identifiable. We, therefore, compared several recent RNA-Seq datasets on the whole dorsal root ganglia in rodent models of nerve injury. Contrary to previous findings, we show hundreds of common differentially expressed genes and high positive correlation between studies, despite model and species differences. We also find, in contrast to prior studies, that 60% of the common rodent gene response after injury is likely to occur in nociceptors of the dorsal root ganglia. Substantial expression changes are observed at a one-week time-point, with smaller changes in the same genes at a later three to four-week time-point. However, a subset of genes shows a similar magnitude of changes at both early and late time-points, suggesting their potential involvement in the maintenance of chronic pain. These genes are centred around suppression of endogenous opioid signalling. Reversal of this suppression could allow endogenous and exogenous opioids to exert their analgesic functions and may be an important strategy for treating chronic pain disorders. Currently used drugs, such as amitriptyline and duloxetine, do not appear to appropriately modulate many of the critical pain genes and indeed may transcriptionally suppress endogenous opioid signalling further.

The dorsal root (DRG) and trigeminal (TG) ganglia contain cell bodies of sensory neurons of spinal and trigeminal systems, respectively. They are homologs of each other; however, differences in how the two systems respond to injury exist. Trigeminal nerve injuries rarely result in chronic neuropathic pain (NP). To date, no genes involved in the differential response to nerve injury between the two systems have been identified. We examined transcriptional changes involved in the development of trigeminal and spinal NP.

Although tolerance serves as a major limitation in the long-term clinical use of opioids in patients with chronic severe pain, mechanisms of opioid tolerance are poorly understood. In this study, a morphine tolerance model was established by subcutaneously injecting male rats with morphine (10 mg/kg) twice a day for 10 consecutive days. In addition, a subset of morphine-tolerant rats underwent testosterone replacement therapy. The levels of mu-opioid receptor (MOR) mRNA and protein in the trigeminal ganglia (TGs) of morphine-tolerant versus control rats and of morphine-tolerant rats with vs. without testosterone replacement therapy were measured. We found that testosterone levels were significantly lower in morphine-tolerant rats than in the controls (1.248 ± 0.231 ng/ml vs. 2.223 ± 0.153 ng/ ml, respectively; p =  0.008). Furthermore, chronic morphine exposure led to a downregulation in the levels of MOR mRNA to 79.3%, and of MOR protein to 68.9%. Testosterone replacement therapy restored MOR mRNA and protein levels specifically in rats who had developed a tolerance to morphine, thereby suggesting a potential role of testosterone in the opioid-receptor response to chronic morphine exposure. In summary, our study provides evidence for the involvement of testosterone in the proper regulation of the peripheral MOR system in rats following prolonged morphine exposure. We also suggest that analgesic therapeutic measures should take into account the testosterone levels of patients who have built up a tolerance to morphine.

The mesopontine tegmental anesthesia area (MPTA) is a small brainstem nucleus that, when exposed to minute quantities of GABA receptor agonists, induces a state of general anesthesia. In addition to immobility and analgesia this state is accompanied by widespread suppression of neural activity in the cerebral cortex and high delta-band power in the electroencephalogram. Collectively, MPTA neurons are known to project to a variety of forebrain targets which are known to relay to the cortex in a highly distributed manner. Here we ask whether ascending projections of individual MPTA neurons collateralize to several of these cortical relay nuclei, or access only one. Using rats, contrasting retrograde tracers were microinjected pairwise on one side into three ascending relays: the basal forebrain, the zona incerta-lateral hypothalamus and the intralaminar thalamic nuclear group. In addition, in separate animals, each target was microinjected bilaterally. MPTA neurons were then identified as being single-or double-labeled, indicating projection to one target nucleus or collateralization to both. Results indicated that double-labeling was rare, occurring on average in only 1.3% of the neurons sampled. The overwhelming majority of individual MPTA neurons showed specific connectivity, contributing to only one of the major ascending pathways, either ipsilaterally or contralaterally, but not bilaterally. This architecture would permit particular functional aspects of anesthetic loss-of-consciousness to be driven by specific subpopulations of MPTA neurons.

Central neuropathic pain is a common untreated symptom in progressive multiple sclerosis (PMS) and is associated with poor quality of life and interference with patients' daily activities. The neuroinflammation process and mitochondrial dysfunction in the PMS lesions generate reactive species. The transient potential receptor ankyrin 1 (TRPA1) has been identified as one of the major mechanisms that contribute to neuropathic pain signaling and can be activated by reactive compounds. Thus, the goal of our study was to evaluate the role of spinal TRPA1 in the central neuropathic pain observed in a PMS model in mice. We used C57BL/6 female mice (20-30 g), and the PMS model was induced by the experimental autoimmune encephalomyelitis (EAE) using mouse myelin oligodendrocyte glycoprotein (MOG) antigen and CFA (complete Freund's adjuvant). Mice developed progressive clinical score, with motor impairment observed after 15 days of induction. This model induced mechanical and cold allodynia and heat hyperalgesia which were measured up to 14 days after induction. The hypersensitivity observed was reduced by the administration of selective TRPA1 antagonists (HC-030031 and A-967079, via intrathecal and intragastric), antioxidants (α-lipoic acid and apocynin, via intrathecal and intragastric), and TRPA1 antisense oligonucleotide (via intrathecal). We also observed an increase in TRPA1 mRNA levels, NADPH oxidase activity, and 4-hydroxinonenal (a TRPA1 agonist) levels in spinal cord samples of PMS-EAE induced animals. In conclusion, these results support the hypothesis of the TRPA1 receptor involvement in nociception observed in a PMS-EAE model in mice.

Behavioral alterations, like mechanical and thermal hyperalgesia, and modulation of biomarkers in the peripheral and central nervous systems (CNS) are markers of chronic pain. Transcranial direct current stimulation (tDCS) with exercise is a promising therapy for pain due to its neuromodulatory capacity.

Chronic pain patients often complain of their poor memory. The mechanisms underlying chronic pain-related memory impairment remain elusive, and there are few clinical therapeutic strategies available for this condition.