Spinal neuron-glia-immune interaction in cross-organ sensitization

Abstract

Inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS), historically considered as regional gastrointestinal disorders with heightened colonic sensitivity, are increasingly recognized to have concurrent dysfunction of other visceral and somatic organs, such as urinary bladder hyperactivity, leg pain, and skin hypersensitivity. The interorgan sensory cross talk is, at large, termed "cross-organ sensitization." These organs, anatomically distant from one another, physiologically interlock through projecting their sensory information into dorsal root ganglia (DRG) and then the spinal cord for integrative processing. The fundamental question of how sensitization of colonic afferent neurons conveys nociceptive information to activate primary afferents that innervate distant organs remains ambiguous. In DRG, primary afferent neurons are surrounded by satellite glial cells (SGCs) and macrophage accumulation in response to signals of injury to form a neuron-glia-macrophage triad. Astrocytes and microglia are major resident nonneuronal cells in the spinal cord to interact, physically and chemically, with sensory synapses. Cumulative evidence gathered so far indicate the indispensable roles of paracrine/autocrine interactions among neurons, glial cells, and immune cells in sensory cross-activation. Dichotomizing afferents, sensory convergency in the spinal cord, spinal nerve comingling, and extensive sprouting of central axons of primary afferents each has significant roles in the process of cross-organ sensitization; however, more results are required to explain their functional contributions. DRG that are located outside the blood-brain barrier and reside upstream in the cascade of sensory flow from one organ to the other in cross-organ sensitization could be safer therapeutic targets to produce less central adverse effects.

Keywords: cross-organ sensitization; intercell interaction; neuroinflammation; sensory neurons; visceral hypersensitivity.

Graphical abstract

Fig. 1.

Acute noxious colorectal distension-induced inhibition-activation switch in bladder activity. A total of five male adult mice (25 g) were implanted with intravesical catheters according to our previous publication (88). Cystometry was performed under continuous saline infusion at 1 mL/h. After surgery, the animals were recovered for at least 2 h. The animals were then inserted with intracolonic miniballoons under light anesthesia (0.5%–1% isoflurane) and placed into a recording cage for 30 min before recording. Each animal was first recorded for baseline with mini-balloon uninflated (control). Noxious colorectal distension (CRD, 80 mmHg for 10-s with 5-s intervals and repeated for five times) was performed at the beginning of a filling phase and with saline infusion paused. Cystometry was immediately resumed after CRD. After at least four micturition cycles were obtained (average 10–20 min), cystometry was restarted on the hour for an average of 10–20 min. Among the five animals, one was only recorded for baseline and 3 h and another one was recorded for baseline and the first period without stopping saline infusion during CRD. Results were consistent in all animals that CRD-induced immediate bladder inhibition was shown as longer micturition intervals (MI) by 1.3-fold (B). Bladder hyperactivity was developed at 3 h post-CRD that reduced MI from baseline by 32% (B). One-way ANOVA and Newman–Keuls multiple comparison test was used for data analysis, with the baseline normalized as 1 (B, ***P < 0.001). The noxious stimulation of colon excites colonic afferent neurons and conveys information to the urinary bladder through primary afferents (C). The mechanisms underlying the development of bladder inhibition and the switch from inhibition to activation following mechanical or chemical irritation of the colon are unknown. CRD, colorectal distension.

Fig. 2.

Macrophage accumulation around DRG neurons following colitis. We generated mice by the Csf1r-Cre-based expression of GFP to label CSF1R expressing macrophages and microglia. We induced colonic inflammation by TNBS in adult female mice according to publications (25, 47). Control animals received EtOH vehicle treatment. Three days later, animals were perfused by transcardiac Krebs buffer followed by 4% paraformaldehyde. T13 to L2 DRGs were extracted and sectioned for 10-µm thickness for visualization of GFP-labeled macrophages. Macrophages in control animals were low (A). Macrophages clustered around DRG neurons following TNBS treatment (B and C, indicated by the red arrows; bar = 20 µm). Macrophages and SGCs surround DRG neurons to form a network to regulate DRG neuron intraganglionic interaction (D). SGCs are coupled by gap junctions (GJs). Mediators released by colonic afferent neurons following colonic inflammation can act on SGCs and macrophages; likewise, gliotransmitters and inflammatory factors released by SGCs and/or macrophages can also act on DRG neurons (D). DRG, dorsal root ganglia; GFP, green fluorescent protein; SGCs, satellite glial cells; TNBS, 2,4,6-trinitrobenzene sulfonic acid.

Fig. 3.

Possible mechanisms of cross-organ sensitization involve intraganglionic signal cross talk and spinal central sensitization. Upon noxious stimulation of the distal colon such as TNBS-induced colonic inflammation (1), sensory mediators released by colonic afferent neurons act on adjacent macrophages and SGCs to convey sensory information to nearby neurons to mediate sensory neuron cross-activation within DRG (2). Sensory mediators released into the spinal dorsal horn induce microgliosis and astrogliosis (3). Activation of nonneuronal cells (macrophages, SGCs, microglia, and astrocytes) produces inflammatory factors, causing neuroinflammation. The formation of gap junctions between astrocytes also conveys information to a distant area in the spinal cord. The close contact of spinal factors to the central terminals of primary afferent neurons leads to retrograde sensitization of these neurons (4). CGRP fiber sprouting transsegmentally and volume transmission are also critical contributors in spinal central sensitization and cross-sensitization of organs when their primary afferent neurons do not locate in the same DRG (e.g., colon and lower extremity). CGRP, calcitonin gene-related peptide; DRG, dorsal root ganglia; SGCs, satellite glial cells; TNBS, 2,4,6-trinitrobenzene sulfonic acid.