Peripheral GABAA receptor signaling contributes to visceral hypersensitivity in a mouse model of colitis

Evaluation of the beneficial effects of a GABA-based product containing Melissa officinalis on post-inflammatory irritable bowel syndrome: a preclinical study

Introduction

Visceral pain represents the most common digestive issue, frequently resulting from long-term inflammation, such as inflammatory bowel diseases. The lack of effective drugs prompted search of new therapeutic approaches. In this regard, gamma-aminobutyric acid (GABA) and Melissa officinalis (Mo) appear as excellent candidates as they were recognized to have several positive effects on the digestive system. The aim of this research was to evaluate the effects of a compound containing GABA and Mo (GABA-Mo 5:1) in inflammation-induced intestinal damage and visceral pain.

Methods

Colitis was induced in rats by intrarectal 2,4-dinitrobenzenesulfonic acid (DNBS) administration. DNBS-treated animals received GABA-Mo (80 mg/kg BID), starting 3 days before DNBS administration, until 14 days after colitis induction (preventive protocol), or starting 7 days after DNBS until day 21 (curative protocol). Visceral pain was assessed by measuring the viscero-motor response (VMR) and the abdominal withdrawal reflex (AWR) to colorectal distension on day 7, 14 (both protocols) and 21 (curative protocol) after DNBS administration.

Results

In the preventive protocol, GABA-Mo reduced AWR at day 14 but had no effect on VMR. In the spinal cord, treatment with GABA-Mo significantly prevented microglia reactivity (Iba-1 positive cells). In the colon, the supplement significantly decreased malondialdehyde (MDA, index of oxidative stress) and IL-1β levels and counteracted the decreased expression of claudin-1. Moreover, GABA-Mo normalized the increased levels of plasma lipopolysaccharide binding protein (LBP, index of altered intestinal permeability). In the curative protocol, GABA-Mo significantly counteracted visceral hypersensitivity persistence in DNBS-treated animals (day 14 and 21). In the spinal cord, GABA-Mo significantly reduced GFAP positive cell density (astrocytes). Histological evaluations highlighted a mild but significant effect of GABA-Mo in promoting healing from DNBS-induced colon damage. Colonic MDA and myeloperoxidase (index of leukocyte infiltration) levels were reduced, while the decreased colonic claudin-1 expression was normalized. In addition, the increased levels of plasma LBP were normalized by GABA-Mo administration.

Discussion

In conclusion GABA-Mo, particularly in the curative protocol, was able to reduce visceral pain and intestinal inflammation, likely through a reinforcement of intestinal barrier integrity, thus representing a suitable approach for the management of abdominal pain, especially in the remission stages of colitis.

Experimental colitis-induced visceral hypersensitivity is attenuated by GABA treatment in mice

Ulcerative colitis (UC) is linked with inflammation of the large intestine due to an overactive response of the colon-immune system. UC is associated with weight loss, rectal bleeding, diarrhea, and abdominal pain. Given that γ-amino butyric acid (GABA) suppresses immune cell activity and the excitability of colonic afferents, and that there is a decrease in colonic GABA during UC, we hypothesized that UC pain is due to a decrease in the inhibition of colonic afferents. Thus, restoring GABA in the colon will attenuate inflammatory hypersensitivity. We tested this hypothesis in a mouse model of colitis. Colon inflammation was induced with seven days of dextran sodium sulfate (DSS, 3%) in the drinking water. GABA (40 mg/kg) was administered orally for the same period as DSS, and body weight, colon length, colon permeability, clinical progression of colitis (disease activity index or DAI), and colon histological score (HS) were assessed to determine the effects of GABA on colitis. A day after the end of GABA treatment, visceral sensitivity was assessed with balloon distention (of the colon)-evoked visceromotor response and colon samples were collected for the measurement of GABA and cytokines. Treatment with GABA reduced the DSS-induced increase in the colon permeability, DAI, HS, and decrease in body weight and colon length. Furthermore, GABA inhibited the DSS-induced increase in the proinflammatory cytokines tumor necrosis factor-α (TNF-α), interferon-γ (IFN-γ), interleukin-12 (IL-12), and increased the expression of the anti-inflammatory cytokine IL-10 in the colon tissue. Importantly, GABA reduced DSS-induced visceral hypersensitivity. These data suggest that increasing gastrointestinal levels of GABA may be useful for the treatment of colitis.NEW & NOTEWORTHY GABA treatment reduces the severity of colitis and inflammation and produces inhibition of visceral hypersensitivity in colon-inflamed mice. These results raise the promising possibility that GABA treatment may be an effective therapeutic strategy for the management of symptoms associated with colitis. However, clinical studies are required to corroborate whether this mouse-model data translates to human colon.

Fundamental Neurochemistry Review: The role of enteroendocrine cells in visceral pain

While visceral pain is commonly associated with disorders of the gut-brain axis, underlying mechanisms are not fully understood. Dorsal root ganglion (DRG) neurons innervate visceral structures and undergo hypersensitization in inflammatory models. The characterization of peripheral DRG neuron terminals is an active area of research, but recent work suggests that they communicate with enteroendocrine cells (EECs) in the gut. EECs sense stimuli in the intestinal lumen and communicate information to the brain through hormonal and electrical signaling. In that context, EECs are a target for developing therapeutics to treat visceral pain. Linaclotide is an FDA-approved treatment for chronic constipation that activates the intestinal membrane receptor guanylyl cyclase C (GUCY2C). Clinical trials revealed that linaclotide relieves both constipation and visceral pain. We recently demonstrated that the analgesic effect of linaclotide reflects the overexpression of GUCY2C on neuropod cells, a specialized subtype of EECs. While this brings some clarity to the relationship between linaclotide and visceral analgesia, questions remain about the intracellular signaling mechanisms and neurotransmitters mediating this communication. In this Fundamental Neurochemistry Review, we discuss what is currently known about visceral nociceptors, enteroendocrine cells, and the gut-brain axis, and ongoing areas of research regarding that axis and visceral pain.

Intestinal neuropod cell GUCY2C regulates visceral pain

Visceral pain (VP) is a global problem with complex etiologies and limited therapeutic options. Guanylyl cyclase C (GUCY2C), an intestinal receptor producing cyclic GMP(cGMP), which regulates luminal fluid secretion, has emerged as a therapeutic target for VP. Indeed, FDA-approved GUCY2C agonists ameliorate VP in patients with chronic constipation syndromes, although analgesic mechanisms remain obscure. Here, we revealed that intestinal GUCY2C was selectively enriched in neuropod cells, a type of enteroendocrine cell that synapses with submucosal neurons in mice and humans. GUCY2Chi neuropod cells associated with cocultured dorsal root ganglia neurons and induced hyperexcitability, reducing the rheobase and increasing the resulting number of evoked action potentials. Conversely, the GUCY2C agonist linaclotide eliminated neuronal hyperexcitability produced by GUCY2C-sufficient - but not GUCY2C-deficient - neuropod cells, an effect independent of bulk epithelial cells or extracellular cGMP. Genetic elimination of intestinal GUCY2C amplified nociceptive signaling in VP that was comparable with chemically induced VP but refractory to linaclotide. Importantly, eliminating GUCY2C selectively in neuropod cells also increased nociceptive signaling and VP that was refractory to linaclotide. In the context of loss of GUCY2C hormones in patients with VP, these observations suggest a specific role for neuropod GUCY2C signaling in the pathophysiology and treatment of these pain syndromes.