There is growing interest in studying the useful effects of functional plant extracts to control diseases and improve broiler performance. G. glabra (licorice) is one of the most commonly used medicinal plants, and its extract is currently used in the food and pharmaceutical industries (Herrera et al., 2009).
The present study was designed for the first time to investigate the possible involvement of glycyrrhizin in the central regulation of food intake in broilers injected with LPS. The results of experiment 1 (Fig. 3) showed that IP injection of glycyrrhizin significantly increases cumulative food intake in chickens. In addition, it is claimed that the use of glycyrrhizic acid in broilers revealed that these chickens had higher body weights and better FCR than the control group. Previous study reported that dietary glycyrrhizin has been recommended as an alternative to growth promoters administered with antibiotics in broilers (Ibrahim et al., 2020). In a study using glycyrrhizic acid (GA) (60 µg/ml in water), broilers had higher final body weight, better FCR, and a lower mortality rate compared to control (without using Gly) (Ocampo et al., 2016). In the second experiment, intracerebroventricular (ICV) injection of LPS (12.5 and 25 ng) reduced food intake compared with the control group (Fig. 4). Our results were in agreement with previous studies in rats (Langhans et al., 1989), mammals (Inui, 2002), and chickens (Johnson et al., 1993). LPS injection is known to induce inflammation in broilers, which can lead to decreased appetite and body weight (Xie et al., 2000; Langhans, 2007). At 12, 24, and 48 hours after injection, the rate of weight change in LPS-treated birds was lower than that control group. This is probably due to reduced food intake and diarrhea early in treatment (Xie et al., 2000). In another study was reported that peripheral injection of LPS in chickens leads to reduced appetite (Sell et al., 2003).
Similar results showed that injection of LPS into the body induced an acute inflammatory response (Tan et al., 2014). IL-1β and IL-6 are pro-inflammatory cytokines that trigger acute-phase reactions in the liver and alter brain-controlled functions such as fever and anorexia (Zhang et al., 2013). Also in broilers, LPS disrupts the intestinal protective function. This is associated with the inhibition of growth function and increased activity of lactate and diamine oxidase (DAO), two biomarkers that reflect intestinal permeability (Wu et al., 2013). In our study, it was found that the ICV injection of LPS reduces cumulative food intake in broiler chickens at 30, 60, 90, 120, and 180 min after injection. Studies have shown that LPS injection in broilers can lead to reduced food intake, impaired growth potential, and increased oxidative stress (Wu et al., 2013; Kooshki et al., 2019).
We detected inflammation by measuring body temperature in the hours following injection. Fever induction by endotoxin is known in many species, including chickens (JONES et al., 1983). Aligned with our study, Boever et al. investigated the effect of LPS on body temperature in broiler chickens. After the injection of an effective dose of LPS, the body temperature initially fell below normal but then increased (De Boever et al., 2008). In another study conducted by Xie et al., the effects of intravenous injection of LPS on three-week-old broiler chickens were investigated. This study confirms our results that LPS increased cloacal temperature 3 h after injection (Xie et al., 2000).
LPS injection promotes an acute pro-inflammatory cytokine response, resulting in a variety of acute-phase responses such as anorexia. Thus, immune system signaling pathways in LPS-induced anorexia converge on central nervous system signaling to control food intake and energy balance for individual health. Inflammation triggers host defense responses, including physiological and behavioral changes, containing anorexia (Lennie, 1998; Langhans, 2007). According to the results of this study, we showed that LPS injection triggers an inflammation response, leading to anorexia.
In experiment 3, IP injections of Glycyrrhizin reduced the effect of LPS on food intake (Fig. 5). These results suggest that there is an interaction between Glycyrrhizin and anti-inflammatory mechanisms. Various studies confirm our results. The effects of glycyrrhizin in neurological diseases are mainly attributed to the inhibition of HMGB1 expression and the reduction of neuronal damage through translocation and downregulation of inflammatory cytokine expression (Paudel et al., 2020). Wang et al. confirmed that Gly. Supplementation increased the expression of appetite-stimulating factor genes, digestive enzyme activity, and antioxidant capacity, while Gly. supplementation reduced inflammation (Wang et al., 2022). Aminian et al. investigated the anti-inflammatory effect of the glycyrrhizin component of licorice plant on inflamed nerve cells, The inhibitory activity of glycyrrhizin in these cells may have important therapeutic effects for the treatment of neuroinflammatory diseases (Aminian Akram, Hamidi Farshid, Moheb Alian, 2020). Our results confirmed a previous study that Gly. injection significantly reverses reduced food intake caused by inflammation in broilers, highlighting its potential role in modulating food intake in broilers. In addition, Song et al. 2013 investigated the effect of glycyrrhizin on neuroinflammation and memory loss in mice and showed that it is possible to use glycyrrhizin as a drug for neurodegenerative diseases (Song et al., 2013).
Glycyrrhizin directly prevents the formation of prostaglandin E2 (PGE2) and the activity of cyclooxygenase (COX) and indirectly inhibits the accumulation of platelets and inflammatory factors (Bahmani et al., 2014). Bhattacharjee et al. reported that Gly. successfully suppressed inflammation during Leishmania donovani infection by inhibiting COX-2-mediated prostaglandin E2 (PGE2) release in L. donovani-infected macrophages (Bhattacharjee et al., 2012). Moreover, Gly. could improve the ability of chicken macrophages to kill Salmonella (Wang et al., 2018). Also, Gly. could reduce LPS-induced acute lung injury in mice (Wang et al., 2020). Gly. could effectively inhibit the proliferation and adhesion of Mycoplasma gallisepticum (MG). Furthermore, Gly. reduced the morbidity of CRD (Chronic respiratory disease) and mortality in MG-infected broilers. Gly. showed a significant therapeutic effect on MG infection and had no adverse effects on broilers (Hu et al., 2022). Our results suggest that glycyrrhizin effectively mitigates the adverse effects of lipopolysaccharide (LPS) on food intake.