Increased hypothalamic serotonin turnover in inflammation-induced anorexia

Abstract

Background: Anorexia can occur as a serious complication of disease. Increasing evidence suggests that inflammation plays a major role, along with a hypothalamic dysregulation characterized by locally elevated serotonin levels. The present study was undertaken to further explore the connections between peripheral inflammation, anorexia and hypothalamic serotonin metabolism and signaling pathways. First, we investigated the response of two hypothalamic neuronal cell lines to TNFα, IL-6 and LPS. Next, we studied transcriptomic changes and serotonergic activity in the hypothalamus of mice after intraperitoneal injection with TNFα, IL-6 or a combination of TNFα and IL-6.

Results: In vitro, we showed that hypothalamic neurons responded to inflammatory mediators by releasing cytokines. This inflammatory response was associated with an increased serotonin release. Mice injected with TNFα and IL-6 showed decreased food intake, associated with altered expression of inflammation-related genes in the hypothalamus. In addition, hypothalamic serotonin turnover showed to be elevated in treated mice.

Conclusions: Overall, our results underline that peripheral inflammation reaches the hypothalamus where it affects hypothalamic serotoninergic metabolism. These hypothalamic changes in serotonin pathways are associated with decreased food intake, providing evidence for a role of serotonin in inflammation-induced anorexia.

Fig. 1

Serotonergic transmission in neurons. Conversion of tryptophan (TRP) to serotonin (5HT) is catalysed by TPH and DDC. Serotonin is then, by transportation via VMAT, stored into vesicles before it can be released into the synaptic cleft. There, serotonin signalling resulting from binding to serotonin receptors (5HTR) can be terminated by the reuptake of serotonin via SERT transporter. Once serotonin is taken up into the presynaptic neuron it is degraded by MAO and ALDH to 5-HIAA, which is considered a marker for serotonergic activity and measurable for a longer period compared to levels of secreted serotonin. TRP tryptophan, TPH tryptophan hydroxylase, 5-HT serotonin, 5HTR serotonin receptor, 5-HIAA 5 hydroxyindolacetic acid, DDC dopadecarboxylase, VMAT vesicular monoamine transporter, SERT serotonin reuptake transporter, MAO monoamine oxidase, ALDH aldehyde dehydrogenase

Fig. 2

Effect of TNFα, IL-6 and LPS on 5-HIAA, 5HT, IL-6 and MCP-1 in HypoE-46 and mHypoA-2/12 cells. Murine derived hypothalamic cell lines were 24 h exposed to various concentrations TNFα (100 pg/ml), IL-6 (100 pg/ml) and LPS (1 μg/ml). KCL was used to depolarize cells (positive control for 5HT). a, b Intracellular 5-HIAA and secretion of 5HT in HypoE-46 (a) and HypoA-2/12 cells (b). c, d Production of IL-6 and MCP-1 in HypoE-46 (c) and HypoA-2/12 cells (d). All treatments were significantly different from untreated controls. Data are expressed as mean ± SEM (n = 3)

Fig. 3

Effect of injection with TNFα, IL-6 or both on food intake and plasma cytokines. a Time course of food intake after injection with TNFα, IL-6 or both. b, c IL-6 and TNFα plasma levels 5 h after injection. d Level of serum amyloid 1 (SAA) in liver homogenates 5 h after injection. ^aSignificantly different from Control group (P < 0.05). Data is expressed as mean ± SEM (n = 12)

Fig. 4

Levels of tryptophan, serotonin and dopamine and their metabolites in the hypothalamus. Effect of ip injection with TNFα, IL-6 or both on a 5-hydroxyindoleacetic acid (5HIAA), b Serotonin (5HT), c 3,4-dihydroxyphenylacetic acid (DOPAC), d Dopamine (DA) and e tryptophan (TRP). ^aSignificantly different from Control group (P < 0.05), ^bsignificantly different from TNF, IL-6 Low and IL-6 High group (P < 0.05). Data is expressed as mean ± SEM (n = 6)

Fig. 5

Gene expression changes in hypothalamus after ip injection with TNFα, IL-6 or both. Top upregulated genes and top downregulated genes in treated groups compared to control group. Each row represents a gene and each column represents a group of animals. Magenta colour indicates genes that were higher expressed as control and green colour indicates genes that were lower expressed as the control. Black indicates genes whose expression was similar to compared to control. ID: Entrez ID

Fig. 6

Gene expression changes in hypothalamus after ip injection with TNFα, IL-6 or both. Top upregulated genes and top downregulated genes in treated groups compared to control group. Each row represents a gene and each column represents a group of animals. Magenta colour indicates genes that were higher expressed as control and green colour indicates genes that were lower expressed as the control. Black indicates genes whose expression was similar to compared to control. ID: Entrez ID

Fig. 7

Gene expression changes in hypothalamus after ip injection with TNFα, IL-6 or both. a, b Relative gene expression compared to control group of NPY and AgRP. ^aSignificantly different from Control group (P < 0.05), AgRP agouti related protein, NPY neuropeptide Y (n = 6)