Background & aims: Tachykinins are involved in physiological and pathophysiological mechanisms in the gastrointestinal tract. The major sources of tachykinins in the gut are intrinsic enteric neurons in the enteric nervous system and extrinsic nerve fibers from the dorsal root and vagal ganglia. Although tachykinins are important mediators in the enteric nervous system, how they contribute to neuroinflammation through effects on neurons and glia is not fully understood. Here, we tested the hypothesis that tachykinins contribute to enteric neuroinflammation through mechanisms that involve intercellular neuron-glia signaling.
Methods: We used immunohistochemistry and quantitative real-time polymerase chain reaction, and studied cellular activity using transient-receptor potential vanilloid-1 (TRPV1)tm1(cre)Bbm/J::Polr2atm1(CAG-GCaMP5g,-tdTomato)Tvrd and Sox10CreERT2::Polr2atm1(CAG-GCaMP5g,-tdTomato)Tvrd mice or Fluo-4. We used the 2,4-di-nitrobenzene sulfonic acid (DNBS) model of colitis to study neuroinflammation, glial reactivity, and neurogenic contractility. We used Sox10::CreERT2+/-/Rpl22tm1.1Psam/J mice to selectively study glial transcriptional changes.
Results: Tachykinins are expressed predominantly by intrinsic neuronal varicosities whereas neurokinin-2 receptors (NK2Rs) are expressed predominantly by enteric neurons and TRPV1-positive neuronal varicosities. Stimulation of NK2Rs drives responses in neuronal varicosities that are propagated to enteric glia and neurons. Antagonizing NK2R signaling enhanced recovery from colitis and prevented the development of reactive gliosis, neuroinflammation, and enhanced neuronal contractions. Inflammation drove changes in enteric glial gene expression and function, and antagonizing NK2R signaling mitigated these changes. Neurokinin A-induced neurodegeneration requires glial connexin-43 hemichannel activity.
Conclusions: Our results show that tachykinins drive enteric neuroinflammation through a multicellular cascade involving enteric neurons, TRPV1-positive neuronal varicosities, and enteric glia. Therapies targeting components of this pathway could broadly benefit the treatment of dysmotility and pain after acute inflammation in the intestine.
Keywords: BzATP, 2’(3’)-O-(4-benzoylbenzoyl)adenosine 5’-triphosphate triethylammonium salt; Ca2+, calcium; Colitis; Cx43, connexin-43; DMEM, Dulbecco's modified Eagle medium; DNBS, dinitrobenzene sulfonic acid; EFS, electrical field stimulation; ENS, enteric nervous system; Enteric Nervous System; FGID, functional gastrointestinal disorder; GFAP, glial fibrillary acidic protein; GI, gastrointestinal; Glia; HA, hemagglutinin; IPAN, intrinsic primarily afferent neuron; LMMP, longitudinal muscle–myenteric plexus; MSU, Michigan State University; NK1R, neurokinin-1 receptor; NK2R, neurokinin-2 receptor; NKA, neurokinin A; Neurokinins; SP, substance P; TRPV1, transient receptor potential vanilloid-1; mRNA, messenger RNA.