Sex differences in the ascending nociceptive pathways: focus on inflammatory pain and diabetic neuropathy

Noxious stimuli are encoded by peripheral nociceptors, whose fibers innervate the skin and whose cell bodies reside in the dorsal root ganglia. These neurons transmit signals to the superficial dorsal horn (SDH) that integrates and modulates nociceptive input. Inhibitory interneurons within the SDH are essential for regulating nociception and pain, and their dysfunction is linked to the development of chronic pain. While sex differences in pain perception and prevalence are increasingly recognized, their impact on spinal nociceptive modulation remains insufficiently explored. My Ph.D. research focused on the SDH’s role in nociception, examining inhibitory interneurons and primary afferent fibers, with emphasis on sex-specific differences under physiological and pathological conditions. I adopted two murine pain models: (1) inflammatory pain, induced via intraplantar zymosan injection; and (2) diabetic neuropathy (DN), induced by intraperitoneal streptozotocin injection. In vivo analysis included behavioral assays (von Frey, adhesive tape removal) and physiological measurements (weight, blood glucose, estrous cycle). Functional and morphological changes in peripheral and central neurons were assessed by confocal microscopy. Correlative investigations in human spinal tissue and optogenetic studies were performed in collaboration with Laval University (Quebec, Canada). My data confirm constitutive sex differences in SDH nociceptive circuits. Females exhibited higher inhibitory neuron density, confirmed also in human tissue, and showed greater recruitment of these neurons following inflammation. These differences were not detectable under strong peripheral drive, i.e., zymosan, but emerged when circuits were selectively challenged using optogenetic approaches. This suggests the presence of regulatory mechanisms that prevent circuit saturation in a sex-specific manner. In DN, females showed increased propensity to develop pathological pain and altered touch sensitivity. This impairment is rooted in a selective vulnerability of sensory afferents, particularly mechanosensitive fibers projecting to SDH, which destabilizes the functional balance between innocuous and nociceptive processing pathways in a sex-specific manner. Together, these findings demonstrate that nociception relies on sex-specific mechanisms that can be differentially compromised under pathological conditions. By explicitly incorporating biological sex into experimental design and interpretation, alongside baseline configurations, this thesis highlights the importance of considering sex-related differences in circuit organization when investigating nociceptive mechanisms.