Fatty Acid Lingual Application Activates Gustatory and Reward Brain Circuits in the Mouse

Abstract

The origin of spontaneous preference for dietary lipids in humans and rodents is debated, though recent compelling evidence has shown the existence of fat taste that might be considered a sixth taste quality. We investigated the implication of gustatory and reward brain circuits, triggered by linoleic acid (LA), a long-chain fatty acid. The LA was applied onto the circumvallate papillae for 30 min in conscious C57BL/6J mice, and neuronal activation was assessed using c-Fos immunohistochemistry. By using real-time reverse transcription polymerase chain reaction (RT-qPCR), we also studied the expression of mRNA encoding brain-derived neurotrophic factor (BDNF), Zif-268, and Glut-1 in some brain areas of these animals. LA induced a significant increase in c-Fos expression in the nucleus of solitary tract (NST), parabrachial nucleus (PBN), and ventroposterior medialis parvocellularis (VPMPC) of the thalamus, which are the regions known to be activated by gustatory signals. LA also triggered c-Fos expression in the central amygdala and ventral tegmental area (VTA), involved in food reward, in conjunction with emotional traits. Interestingly, we noticed a high expression of BDNF, Zif-268, and Glut-1 mRNA in the arcuate nucleus (Arc) and hippocampus (Hipp), where neuronal activation leads to memory formation. Our study demonstrates that oral lipid taste perception might trigger the activation of canonical gustatory and reward pathways.

Keywords: BDNF; Glut-1; Zif-268; c-Fos; fat taste; gustation; hedonic; linoleic acid.

Figure 1

Linoleic acid deposition on the tongue induces c-Fos expression in the major cerebral structures of the canonical gustatory pathway. (A) Typical photomicrographs of the NST, Arc, and Hipp, showing c-Fos immunoreactivity in mice subjected to oral stimulation with linoleic acid or xanthan gum (XG, 0.3%, w/v) to mimic the texture of lipids. The dotted lines circumscribe the regions of interest. Arrowheads point to representative c-Fos immunopositive nuclei. The boxes show higher magnification (×4) of representative c-Fos immunopositive nuclei. Scale bar, 100 µm. The square windows indicate the area shown in the photomicrographs. (B) Bar graph representation of the density of c-Fos immunopositive cells (number of c-Fos positive cells/mm²) in mice subjected to oral stimulation with linoleic acid (LA) or xanthan gum (XG). Values are means ± standard error of the mean (SEM); n = 6; for each structure studied, treatment effects on c-Fos expression were assessed using unpaired one-sided t-tests. *, p < 0.05; **, p < 0.01. AI, agranular insular cortex; Arc, arcuate nucleus; Hipp, hippocampus; NST, nucleus of the solitary tract; PBN, parabrachial nucleus; VPM, ventral posteromedial thalamic nucleus; VPMPC, ventroposterior medialis parvocellularis.

Figure 2

Linoleic acid deposition on the tongue activates c-Fos expression in the major cerebral structures related to emotional and reward traits. Bar graph representation of the density of c-Fos immunopositive cells (number of c-Fos positive cells/mm²) in mice subjected to oral stimulation with linoleic acid (LA) or xanthan gum as control solution. Values are means ± SEM; n = 6; for each structure studied, treatment effects on c-Fos expression were assessed using unpaired one-sided t-tests. * p < 0.05; ** p < 0.01. Acb, accumbens nucleus; Arc, arcuate nucleus; BLA, basolateral amygdaloid nucleus; CeA, central amygdaloid nucleus; GI/DI, granular/dysgranular insular cortex; Hbn, habenula; Hipp, hippocampus; mPFC, medial prefrontal cortex; PSTN/CbN, parasubthalamic nucleus/calbindin nucleus; VTA, ventral tegmental area.

Figure 3

Lingual application of LA modulates brain-derived neurotrophic factor (BDNF), Zif-268, and Glut-1 mRNA expression in the mouse brain. Bar graphs represent the relative increase in mRNA expression (Zif-268 in (A), Glut-1 in (B), BDNF in (C)) in mice subjected to oral stimulation with linoleic acid (LA) or xanthan gum as control solution. Values are means ± SEM; n = 6; for each structure studied, treatment effects on Zif-268 and Glut-1 mRNA expression were assessed using unpaired one-sided t-tests. NST, nucleus of solitary tract; Arc, arcuate nucleus; Hipp, hippocampus.

Figure 4

Schematic representation of the gustatory pathway, depicting the major central synaptic relays and their connections with structures involved in metabolic, reward and learning, and memory processes. The lingual application of a long-chain fatty acid will trigger signaling events via CD36, localized in the circumvallate papillae. The gustatory information on dietary lipids will be conveyed to the NST via cranial nerves VII and IX. The NST that serves as the relay structure of the peripheral information will send the gustatory information to different brain areas, as mentioned in the Discussion section. Acb, accumbens nucleus; AI, agranular insular cortex; Arc, arcuate nucleus; BLA, basolateral amygdaloid nucleus; CeA, central amygdaloid nucleus; GI/DI, granular/dysgranular insular cortex; Hbn, habenula; Hipp, hippocampus; mPFC, medial prefrontal cortex; NST, nucleus of the solitary tract; PBN, parabrachial nucleus; PSTN/CbN, parasubthalamic nucleus/calbindin nucleus; VPM, ventral posteromedial thalamic nucleus; VPMPC, ventroposterior medialis parvocellularis; VTA, ventral tegmental area (adapted from Reference [27]).