Sex differences in pain along the neuraxis.

Despite the overwhelming female-predominance in chronic pain disorders, preclinical pain studies have historically excluded females as research subjects. Male-biased explanations of pathological pain mechanisms may not fully translate to pain processes in females, necessitating the exploration of pain processing and modulation in both sexes at the preclinical and clinical levels. This review highlights historical trends in the study of sex differences within the pain field and examines the current literature regarding new techniques for the mechanistic analysis of pain modulation in males and females. A large body of evidence suggests that sex differences exist at the molecular, cellular, and systems levels of pain processing, likely influenced by a combination of genetic, hormonal, and neuroimmune factors that may differ at distinct levels of the neuraxis.

Evaluation of Urea-Based Inhibitors of the Dopamine Transporter Using the Experimental Autoimmune Encephalomyelitis Model of Multiple Sclerosis.

The dopaminergic system is involved in the regulation of immune responses in various homeostatic and disease conditions. For conditions such as Parkinson's disease and multiple sclerosis (MS), pharmacological modulation of dopamine (DA) system activity is thought to have therapeutic relevance, providing the basis for using dopaminergic agents as a treatment of relevant states. In particular, it was proposed that restoration of DA levels may inhibit neuroinflammation. We have recently reported a new class of dopamine transporter (DAT) inhibitors with high selectivity to the DAT over other G-protein coupled receptors tested. Here, we continue their evaluation as monoamine transporter inhibitors. Furthermore, we show that the urea-like DAT inhibitor (compound 5) has statistically significant anti-inflammatory effects and attenuates motor deficits and pain behaviors in the experimental autoimmune encephalomyelitis model mimicking clinical signs of MS. To the best of our knowledge, this is the first study reporting the beneficial effects of DAT inhibitor-based treatment in animals with induced autoimmune encephalomyelitis, and the observed results provide additional support to the model of DA-related neuroinflammation.

Fear Extinction-Based Inter-Individual and Sex Differences in Pain-Related Vocalizations and Anxiety-like Behaviors but Not Nocifensive Reflexes.

Inter-individual and sex differences in pain responses are recognized but their mechanisms are not well understood. This study was intended to provide the behavioral framework for analyses of pain mechanisms using fear extinction learning as a predictor of phenotypic and sex differences in sensory (mechanical withdrawal thresholds) and emotional-affective aspects (open field tests for anxiety-like behaviors and audible and ultrasonic components of vocalizations) of acute and chronic pain. In acute arthritis and chronic neuropathic pain models, greater increases in vocalizations were found in females than males and in females with poor fear extinction abilities than females with strong fear extinction, particularly in the neuropathic pain model. Female rats showed higher anxiety-like behavior than males under baseline conditions but no inter-individual or sex differences were seen in the pain models. No inter-individual and sex differences in mechanosensitivity were observed. The data suggest that vocalizations are uniquely suited to detect inter-individual and sex differences in pain models, particularly in chronic neuropathic pain, whereas no such differences were found for mechanosensitivity, and baseline differences in anxiety-like behaviors disappeared in the pain models.