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Gastrointestinal (GI) discomfort is a hallmark of most gut disorders and represents an important component of chronic visceral pain1. For the growing population afflicted by irritable bowel syndrome, GI hypersensitivity and pain persist long after tissue injury has resolved2. Irritable bowel syndrome also exhibits a strong sex bias, afflicting women three times more than men1. Here, we focus on enterochromaffin (EC) cells, which are rare excitable, serotonergic neuroendocrine cells in the gut epithelium3-5. EC cells detect and transduce noxious stimuli to nearby mucosal nerve endings3,6 but involvement of this signalling pathway in visceral pain and attendant sex differences has not been assessed. By enhancing or suppressing EC cell function in vivo, we show that these cells are sufficient to elicit hypersensitivity to gut distension and necessary for the sensitizing actions of isovalerate, a bacterial short-chain fatty acid associated with GI inflammation7,8. Remarkably, prolonged EC cell activation produced persistent visceral hypersensitivity, even in the absence of an instigating inflammatory episode. Furthermore, perturbing EC cell activity promoted anxiety-like behaviours which normalized after blockade of serotonergic signalling. Sex differences were noted across a range of paradigms, indicating that the EC cell-mucosal afferent circuit is tonically engaged in females. Our findings validate a critical role for EC cell-mucosal afferent signalling in acute and persistent GI pain, in addition to highlighting genetic models for studying visceral hypersensitivity and the sex bias of gut pain.

Chronic pelvic pain (CPP) is the most debilitating symptom of gynaecological disorders such as endometriosis. However, it remains unclear how sensory neurons from pelvic organs affected by endometriosis, such as the female reproductive tract, detect and transmit nociceptive events and how these signals are processed within the central nervous system (CNS). Using a previously characterized mouse model of endometriosis, we investigated whether the increased pain sensitivity occurring in endometriosis could be attributed to (i) changes in mechanosensory properties of sensory afferents innervating the reproductive tract, (ii) alterations in sensory input from reproductive organs to the spinal cord or (iii) neuroinflammation and sensitization of spinal neural circuits. Mechanosensitivity of vagina‐innervating primary afferents was examined using an ex vivo single‐unit extracellular recording preparation. Nociceptive signalling from the vagina to the spinal cord was quantified by phosphorylated MAP kinase ERK1/2 immunoreactivity. Immunohistochemistry was used to determine glial and neuronal circuit alterations within the spinal cord. We found that sensory afferents innervating the rostral, but not caudal portions of the mouse vagina, developed mechanical hypersensitivity in endometriosis. Nociceptive signalling from the vagina to the spinal cord was significantly enhanced in mice with endometriosis. Moreover, mice with endometriosis developed microgliosis, astrogliosis and enhanced substance P neurokinin‐1 receptor immunoreactivity within the spinal cord, suggesting the development of neuroinflammation and sensitization of spinal circuitry in endometriosis. These results demonstrate endometriosis‐induced neuroplasticity occurring at both peripheral and central sites of sensory afferent pathways. These findings may help to explain the altered sensitivity to pain in endometriosis and provide a novel platform for targeted pain relief treatments for this debilitating disorder. [ABSTRACT FROM AUTHOR]

Chronic pelvic pain (CPP) is the primary symptom of endometriosis patients, but adequate treatments are lacking. Modulation of ion channels expressed by sensory nerves innervating the viscera has shown promise for the treatment of irritable bowel syndrome and overactive bladder. However, similar approaches for endometriosis‐associated CPP remain underdeveloped. Here, we examined the role of the voltage‐gated sodium (NaV) channel NaV1.7 in (i) the sensitivity of vagina‐innervating sensory afferents and investigated whether (ii) NaV1.7 inhibition reduces nociceptive signals from the vagina and (iii) ameliorates endometriosis‐associated CPP. The mechanical responsiveness of vagina‐innervating sensory afferents was assessed with ex vivo single‐unit recording preparations. Pain evoked by vaginal distension (VD) was quantified by the visceromotor response (VMR) in vivo. In control mice, pharmacological activation of NaV1.7 with OD1 sensitised vagina‐innervating pelvic afferents to mechanical stimuli. Using a syngeneic mouse model of endometriosis, we established that endometriosis sensitised vagina‐innervating pelvic afferents to mechanical stimuli. The highly selective NaV1.7 inhibitor Tsp1a revealed that this afferent hypersensitivity occurred in a NaV1.7‐dependent manner. Moreover, in vivo intra‐vaginal treatment with Tsp1a reduced the exaggerated VMRs to VD which is characteristic of mice with endometriosis. Conversely, Tsp1a did not alter ex vivo afferent mechanosensitivity nor in vivo VMRs to VD in Sham control mice. Collectively, these findings suggest that NaV1.7 plays a crucial role in endometriosis‐induced vaginal hyperalgesia. Importantly, NaV1.7 inhibition selectively alleviated endometriosis‐associated CPP without the loss of normal sensation, suggesting that selective targeting of NaV1.7 could improve the quality of life of women with endometriosis. [ABSTRACT FROM AUTHOR]

Functional bowel disorder patients can suffer from chronic abdominal pain, likely due to visceral hypersensitivity to mechanical stimuli. As there is only a limited understanding of the basis of chronic visceral hypersensitivity (CVH), drug-based management strategies are ill defined, vary considerably, and include NSAIDs, opioids, and even anticonvulsants. We previously reported that the 1.1 subtype of the voltage-gated sodium (NaV; NaV1.1) channel family regulates the excitability of sensory nerve fibers that transmit a mechanical pain message to the spinal cord. Herein, we investigated whether this channel subtype also underlies the abdominal pain that occurs with CVH. We demonstrate that NaV1.1 is functionally upregulated under CVH conditions and that inhibiting channel function reduces mechanical pain in 3 mechanistically distinct mouse models of chronic pain. In particular, we use a small molecule to show that selective NaV1.1 inhibition (a) decreases sodium currents in colon-innervating dorsal root ganglion neurons, (b) reduces colonic nociceptor mechanical responses, and (c) normalizes the enhanced visceromotor response to distension observed in 2 mouse models of irritable bowel syndrome. These results provide support for a relationship between NaV1.1 and chronic abdominal pain associated with functional bowel disorders.

Dietary, microbial, and inflammatory factors modulate the gut-brain axis and influence physiological processes ranging from metabolism to cognition. The gut epithelium is a principal site for detecting such agents, but precisely how it communicates with neural elements is poorly understood. Serotonergic enterochromaffin (EC) cells are proposed to fulfill this role by acting as chemosensors, but understanding how these rare and unique cell types transduce chemosensory information to the nervous system has been hampered by their paucity and inaccessibility to single-cell measurements. Here, we circumvent this limitation by exploiting cultured intestinal organoids together with single-cell measurements to elucidate intrinsic biophysical, pharmacological, and genetic properties of EC cells. We show that EC cells express specific chemosensory receptors, are electrically excitable, and modulate serotonin-sensitive primary afferent nerve fibers via synaptic connections, enabling them to detect and transduce environmental, metabolic, and homeostatic information from the gut directly to the nervous system.

The distal colon and rectum (colorectum) are innervated by spinal and vagal afferent pathways. The central circuits into which vagal and spinal afferents relay colorectal nociceptive information remain to be comparatively assessed. To address this, regional colorectal retrograde tracing and colorectal distension (CRD)‐evoked neuronal activation were used to compare the circuits within the dorsal vagal complex (DVC) and dorsal horn (thoracolumbar [TL] and lumbosacral [LS] spinal levels) into which vagal and spinal colorectal afferents project. Vagal afferent projections were observed in the nucleus tractus solitarius (NTS), area postrema (AP), and dorsal motor nucleus of the vagus (DMV), labeled from the rostral colorectum. In the NTS, projections were opposed to catecholamine and pontine parabrachial nuclei (PbN)‐projecting neurons. Spinal afferent projections were labeled from rostral through to caudal aspects of the colorectum. In the dorsal horn, the number of neurons activated by CRD was linked to pressure intensity, unlike in the DVC. In the NTS, 13% ± 0.6% of CRD‐activated neurons projected to the PbN. In the dorsal horn, at the TL spinal level, afferent input was associated with PbN‐projecting neurons in lamina I (LI), with 63% ± 3.15% of CRD‐activated neurons in LI projecting to the PbN. On the other hand, at the LS spinal level, only 18% ± 0.6% of CRD‐activated neurons in LI projected to the PbN. The collective data identify differences in the central neuroanatomy that support the disparate roles of vagal and spinal afferent signaling in the facilitation and modulation of colorectal nociceptive responses. [ABSTRACT FROM AUTHOR]

Voltage-gated sodium (Na[sub.v]) channels initiate action potentials in most neurons, including primary afferent nerve fibres of the pain pathway. Local anaesthetics block pain through non-specific actions at all Na[sub.v] channels, but the discovery of selective modulators would facilitate the analysis of individual subtypes of these channels and their contributions to chemical, mechanical, or thermal pain. Here we identify and characterize spider (Heteroscodra maculata) toxins that selectively activate the Na[sub.v]1.1 subtype, the role of which in nociception and pain has not been elucidated. We use these probes to show that Na[sub.v]1.1-expressing fibres are modality-specific nociceptors: their activation elicits robust pain behaviours without neurogenic inflammation and produces profound hypersensitivity to mechanical, but not thermal, stimuli. In the gut, high-threshold mechanosensitive fibres also express Na[sub.v]1.1 and show enhanced toxin sensitivity in a mouse model of irritable bowel syndrome. Together, these findings establish an unexpected role for Na[sub.v]1.1 channels in regulating the excitability of sensory nerve fibres that mediate mechanical pain., Author(s): Jeremiah D. Osteen [1]; Volker Herzig [2]; John Gilchrist [3]; Joshua J. Emrick [1]; Chuchu Zhang [1]; Xidao Wang [4]; Joel Castro [5, 6]; Sonia Garcia-Caraballo [5, 6]; Luke [...]