Toll-Like Receptor 4 in Paraventricular Nucleus Mediates Visceral Hypersensitivity Induced by Maternal Separation

Abstract

Neonatal maternal separation (MS) is a major early life stress that increases the risk of emotional disorders, visceral pain perception and other brain dysfunction. Elevation of toll-like receptor 4 (TLR4) signaling in the paraventricular nucleus (PVN) precipitates early life colorectal distension (CRD)-induced visceral hypersensitivity and pain in adulthood. The present study aimed to investigate the role of TLR4 signaling in the pathogenesis of postnatal MS-induced visceral hypersensitivity and pain during adulthood. The TLR4 gene was selectively knocked out in C57BL/10ScSn mice (Tlr4-/-). MS was developed by housing the offspring alone for 6 h daily from postnatal day 2 to day 15. Visceral hypersensitivity and pain were assessed in adulthood. Tlr4+/+, but not Tlr4-/-, mice that had experienced neonatal MS showed chronic visceral hypersensitivity and pain. TLR4 immunoreactivity was observed predominately in microglia in the PVN, and MS was associated with an increase in the expression of protein and/or mRNA levels of TLR4, corticotropin-releasing factor (CRF), CRF receptor 1 (CRFR1), tumor necrosis factor-α, and interleukin-1β in Tlr4+/+ mice. These alterations were not observed in Tlr4-/- mice. Local administration of lipopolysaccharide, a TLR4 agonist, into the lateral cerebral ventricle elicited visceral hypersensitivity and TLR4 mRNA expression in the PVN, which could be prevented by NBI-35965, an antagonist to CRFR1. The present results indicate that neonatal MS induces a sensitization and upregulation of microglial TLR4 signaling activity, which facilitates the neighboring CRF neuronal activity and, eventually, precipitates visceral hypersensitivity in adulthood.Highlights Neonatal MS does not induce chronic visceral hypersensitivity and pain in Tlr4-/- mice.Neonatal MS increases the expression of TLR4 mRNA, CRF protein and mRNA, CRFR1 protein, TNF-α protein, and IL-1β protein in Tlr4+/+ mice.TLR4 agonist LPS (i.c.v.) elicits visceral hypersensitivity and TLR4 mRNA expression in the PVN.

Predisposition of Neonatal Maternal Separation to Visceral Hypersensitivity via Downregulation of Small-Conductance Calcium-Activated Potassium Channel Subtype 2 (SK2) in Mice

“…As per their findings, separation for 3 h daily can develop somatic and visceral hyperalgesia as well [ 49 – 52 ]. Nevertheless, we have recognized that exposure to MS 3 h daily could not develop visceral hypersensitivity in adulthood, whereas separation for 6 h daily could successfully induce visceral hypersensitivity, which was contrary to the abovementioned results [ 51 , 52 ] and consistent with our previous reports [ 53 ]. We attributed this discrepancy to the distinction in time course of separation and animal species.…”

Early-life stress and inflammation: A systematic review of a key experimental approach in rodents

“…Most studies in which animals underwent a second stress have reported an increase in TNF-α in RMS animals, although generally in non-blood tissues rather than blood. Regarding just the brain, an increase in TNF-α expression in RMS animals was reported in the hippocampus (Amini-Khoei et al, 2017;Han et al, 2019;Pinheiro et al, 2015;Zhu et al, 2017), although in Han et al (2019), this difference was not tested statistically; the PFC (Ganguly et al, 2019;Pinheiro et al, 2015), although only in males but not females in Ganguly et al (2019); the PVN (Tang et al, 2017); the striatum (Dallé et al, 2017); and the nucleus accumbens in males but not females (Ganguly et al, 2019). Several studies, however, did not find any effect of RMS in the hippocampus (Viola et al, 2019;Wang et al, 2017) and PFC (Amini-Khoei et al, 2019;Viola et al, 2019).…”

Maternal separation in rodents: a journey from gut to brain and nutritional perspectives

“…Indeed, increased circulating levels of IL-1β (71) and IL-6 (179) have been reported in MS animals. In addition, MS offspring display neuroinflammatory marks such as increased Tnfa , Il-1b and Tlr4 expression or increased reactive oxygen species levels and decreased Il-10 expression in the hippocampus (71,75,87,114) , PFC (71) and PVN (180) . Recent studies have shown a decrease in the levels of the astrocytic marker GFAP (glial fibrillary acidic protein) in the PFC of MS animals (91) and the opposite effect in the locus coeruleus of MS females only (181).…”