Obesogenic diet induces circuit-specific memory deficits in mice

Abstract

Obesity is associated with neurocognitive dysfunction, including memory deficits. This is particularly worrisome when obesity occurs during adolescence, a maturational period for brain structures critical for cognition. In rodent models, we recently reported that memory impairments induced by obesogenic high-fat diet (HFD) intake during the periadolescent period can be reversed by chemogenetic manipulation of the ventral hippocampus (vHPC). Here, we used an intersectional viral approach in HFD-fed male mice to chemogenetically inactivate specific vHPC efferent pathways to nucleus accumbens (NAc) or medial prefrontal cortex (mPFC) during memory tasks. We first demonstrated that HFD enhanced activation of both pathways after training and that our chemogenetic approach was effective in normalizing this activation. Inactivation of the vHPC-NAc pathway rescued HFD-induced deficits in recognition but not location memory. Conversely, inactivation of the vHPC-mPFC pathway restored location but not recognition memory impairments produced by HFD. Either pathway manipulation did not affect exploration or anxiety-like behaviour. These findings suggest that HFD intake throughout adolescence impairs different types of memory through overactivation of specific hippocampal efferent pathways and that targeting these overactive pathways has therapeutic potential.

Keywords: adolescence; connectivity; high-fat diet; hippocampus; mouse; neuroscience; obesity; pathways.

Figure 1.. Characterization of ventral hippocampus (vHPC) projections to nucleus accumbens (NAc) and medial prefrontal cortex (mPFC).

Representative images illustrating expression of TdTomato in fibres in the NAc (A) and the mPFC (B) after AAV-CaMKII-Cre injection in the vHPC of Ai14(RCL-tdT)-D mice. Schematics adapted from Figures 21 and 23 (A) and Figures 14 and 16 (B) from Paxinos and Franklin, 2004, indicating the levels of fibres labelling. (C, D) Schema of intersectional chemogenetic approach. An AAV-hSyn1-dlox-hM4D(Gi)-mCherry vector was injected into the vHPC, while a retrograde CAV2-Cre vector was injected in the NAc (C) or the mPFC (D). (E, F) Expression of mCherry is depicted for each condition after amplification using immunohistochemistry. Schematics adapted from Figures 51, 55, 59 and 61 from Paxinos and Franklin, 2004, indicating the largest (light red) or the smallest (dark red) viral infection. Representative images illustrating mCherry expression after CAV2-Cre injection in (E) the NAc or (F) the mPFC. Scale bar is set to 100 µm, 500 µm, or 1 mm.

Figure 1—figure supplement 1.. Labelling in the ventral hippocampus (vHPC).

Parasagittal (A) and frontal (B) schema illustrating AAV-CaMKII-Cre injection in the vHPC of Ai14(RCL-tdT)-D mice and representative image illustrating expression of TdTomato in vHPC neurons (C). Representative example of mCherry labelling in the vHPC after injections of AAV-hSyn1-dloxhM4D(Gi)-mCherry in the vHPC and retrograde CAV2-Cre vector in the nucleus accumbens (NAc, D) or the medial prefrontal cortex (mPFC, E).

Figure 2.. Effects of high-fat diet and chemogenetic silencing of ventral hippocampus (vHPC)–nucleus accumbens (NAc) or vHPC–medial prefrontal cortex (mPFC) pathway on c-Fos expression in ventral CA1/subiculum.

(A) Schema of exposure to a novel arena containing two identical and novel objects. (B) Representative images of c-Fos-positive (Fos+) cells in the ventral CA1/subiculum for each group. (C) Quantification of c-Fos+ cells in the ventral CA1/subiculum of the different groups. Data are shown as the number of c-Fos+ cells per mm². The area corresponding to ventral CA1/subiculum area is delineated. Scale bar is set at 500 nm. (D) Representative example of mCherry and c-Fos labelling in the ventral CA1/subiculum. (E) Percentage of mCherry+ and c-Fos+ cells over total number of mCherry+ in the different groups. Scale bar is set to 250 nm and data are shown as a percentage of cells. Each point represents a single animal value. Diet effect: *p < 0.05, **p < 0.01; DREADD effect: ^##p < 0.01 (two-way analysis of variance [ANOVA]).

Figure 2—figure supplement 1.. Effect of high-fat diet and chemogenetic manipulation on c-Fos expression in hippocampus, nucleus accumbens (NAc) and medial prefrontal cortex (mPFC).

Quantification of c-Fos-positive cells in the ventral CA3 (A), the NAc (B) and the mPFC (C) of the different groups. Data are shown as the number of c-Fos-positive cells per mm². Diet effect: *p < 0.05, **p < 0.01, ***p < 0.001; DREADD effect: ^##p < 0.01.

Figure 3.. Impacts of high-fat diet and chemogenetic silencing of ventral hippocampus (vHPC)–nucleus accumbens (NAc) or vHPC–medial prefrontal cortex (mPFC) pathway on object-based memory.

(A) Schema of object recognition memory (ORM) task (top) and ORM performance expressed as percentage of exploration time of novel (empty bars) or familiar (striped bars) object over both objects (bottom). (B) Schema of object location memory (OLM) task (top) and OLM performance expressed as percentage of exploration time of novel (empty bars) or familiar (striped bars) location over both objects (bottom). Each point represents a single animal value. Diet effect: **p < 0.01 (two-way analysis of variance [ANOVA], diet × pathway). Difference between groups: *p < 0.05, **p < 0.01 (two-way ANOVA, diet × DREADD for each pathway, significant interaction followed by post hoc test). Different from 50%: ^#p < 0.05 (one-sample t-test).

Figure 3—figure supplement 1.. Effect of high-fat diet and chemogenetic manipulation on anxiety-like behaviours.

Schema of elevated plus-maze task (A). Results are presented as percentage of open arm time (calculated as open arm time over total time spent in both open and closed arms × 100; B) and percentage of open arm entries (calculated as open arm entries with respect to the total number of entries in both open and closed arms × 100; C).