How does food sight promote feeding and contextual memory in mice?

Food sight, not odor, drives the feeding response

The study asks which food-related cue actually changes feeding behavior and how the brain converts that cue into eating. In untrained, overnight-fasted mice, the authors compared a compartment containing only food odor with one containing only food sight. Mice spent similar time and made similar numbers of investigations in both compartments, so odor and sight attracted comparable interest during energy deficit. Each cue alone drew mice more than an empty compartment, and sight plus odor produced an additive effect.

The decisive test removed sight rather than adding it. Under red illumination that blocks visual input, fasted mice took longer to begin eating and ate less within the first 60 minutes of the test. In a conditioned place preference (CPP) test, where mice learn to associate a compartment with food, fasted mice with normal sight spent more time in the food zone than in non-food zones, while fed mice did not. When fasted mice were conditioned without food sight, their time in the food zone dropped relative to fasted mice with normal sight. The same manipulation did not change preference for an object-associated compartment, which the authors use to argue that energy deficit specifically strengthens food-related contextual memory.

Food sight rapidly silences VMH neurons

To locate the brain region responding to food sight, the authors fasted mice overnight and then either refed them or kept them fasting for two hours under normal or red illumination, and measured expression of the immediate early gene c-Fos. Among the hypothalamic regions examined, only the VMH showed a difference: c-Fos increased after refeeding under red illumination compared with normal illumination, consistent with food sight suppressing VMH activity. The other regions showed no such change, and fasted mice under the two lighting conditions did not differ.

Fiber photometry in Sf1-Cre mice expressing the calcium sensor GCaMP6s in VMH neurons gave a real-time view. Presenting a chow diet promptly decreased VMH SF1 neuron activity in fasted mice relative to fed mice, and activity stayed low during feeding. When food was placed in a closed chamber with holes and sight was blocked by red light, so that mice could smell but not see it, no fluorescence change appeared in either fed or fasted mice. When food was visible but inaccessible, fasted mice showed a rapid drop in VMH signal that quickly returned to baseline. The authors interpret the transient drop as the response to seeing food, with sustained low activity depending on actually eating.

GABAergic SCN input links sight to VMH inhibition

The paper identifies the source of inhibition as GABAergic neurons in the SCN, the brain's master circadian pacemaker. Retrograde tracing from the VMH in Vgat-ires-Cre mice labeled mCherry-positive cells in the SCN, and anterograde labeling showed SCN neuron terminals in the VMH, supporting a direct projection. Fiber photometry in fasted mice exposed to food sight alone showed that SCN GABAergic neuron calcium signals rose at the start of foraging or investigation, and the change differed between fed and fasted mice. A non-food object did not produce the same response.

Chemogenetic inhibition of SCN GABAergic neurons projecting to the VMH increased food intake within one hour of CNO administration, and the same manipulation altered performance in the CPP test. The authors also report that activating this SCN-to-VMH pathway increases food intake while inhibiting it decreases intake. Because VMH neurons are generally considered anorexigenic, the authors propose that the SCN GABA input facilitates feeding by suppressing VMH activity, coordinated with other energy-state-dependent circuits; they explicitly note that how this model operates requires future demonstration.

RUVBL2-expressing VMH neurons gate feeding and memory

RUVBL2 is an ATPase with roles in chromatin remodeling, glucose transport, and transcription, and it is widely expressed in the brain, including hypothalamic regions. The authors use RUVBL2 expression to define a VMH neuron population. Knockout of RUVBL2 in the VMH produced hyperphagia and subsequent obesity, and chemogenetic inhibition of VMH RUVBL2 neurons rapidly increased food intake in both male and female mice across light and dark cycles. Chemogenetic activation produced the opposite direction of effect on feeding and increased energy expenditure, while knockout or inhibition reduced energy expenditure.

The same VMH RUVBL2 neurons also carry the memory effect. Inhibiting them increased time and investigation in the food zone in a real-time place preference test and raised the discrimination index in CPP. The authors connect this to synaptic plasticity: RUVBL2 has previously been reported to regulate feeding through synaptic plasticity in the paraventricular hypothalamus, and they suggest that the feeding and memory effects here may arise from impaired synaptic transmission in the downstream circuit. That mechanism is presented as a likely interpretation rather than a directly demonstrated step.

The SuM-to-DG path carries the memory signal

Tracing the output of VMH RUVBL2 neurons, the authors found sequential projections to the SuM and then the DG. Retrograde polysynaptic tracing with PRV injected into the DG labeled cells in the SuM, VMH, and SCN, and about 30% of DG-projecting SuM neurons were co-labeled with the VMH projection marker, supporting a VMH-to-SuM-to-DG route. The VMH-projected SuM neurons were primarily excitatory: roughly 76% were glutamate positive, and none of the EGFP-expressing SuM neurons were Vgat positive.

Functionally, chemogenetic inhibition of VMH-projected glutamatergic SuM neurons significantly increased food intake. Inhibiting the SuM-to-DG circuit promoted both feeding and food-associated contextual memory, which the authors describe as ventral hippocampal memory. Optogenetic inhibition of GABAergic SCN neurons produced a robust increase in DG neuron calcium signals, consistent with the SCN acting through disinhibition along the chain. The identity of the specific DG neurons involved remains to be defined, and the authors note that more work is needed to establish how food-associated contextual memory feeds back into feeding and body weight regulation.

Energy state decides whether food sight helps or hinders

The paper's own framing places energy state at the center of the interpretation. Food sight promoted feeding and food-associated contextual memory during energy deficit, and the authors propose that the same cue could inhibit feeding and memory encoding during energy sufficiency. This is offered as a way to reconcile their results with prior work showing that food-related visual and odor cues inhibited food intake and related contextual memory through activation of hD2R neurons. They further suggest that the VMH RUVBL2-to-SuM-to-DG neurons studied here could be a different population from those hD2R neurons.

That reconciliation is a hypothesis, not a tested comparison in this paper. The experiments establish the fasted-state effects and the circuit components; the fed-state suppression of feeding by food sight is not demonstrated here. Readers should therefore treat the fasted condition as the scope in which the described circuit was shown to promote feeding and memory, and treat the proposed switch across energy states as the authors' explanatory model awaiting direct test.

A survival circuit with a modern mismatch

The study's contribution is a specific, ordered pathway: SCN GABAergic neurons, RUVBL2-expressing VMH neurons, glutamatergic SuM neurons, and DG neurons, with food sight as the trigger that sets the chain in motion during energy deficit. The authors argue that enhanced food-related memory could further stimulate feeding as a positive feedback loop, and they frame the circuit as useful for increasing feeding efficiency during food shortage.

The same logic carries a cost in an environment of abundant food cues. The authors note that the circuit could contribute to overfeeding and obesity driven at least partly by growing food-related visual cues such as pictures and videos in advertisements. The most decision-relevant distinction in the paper is therefore not simply that food sight promotes eating, but that the promotion runs through a memory-forming hippocampal path and is gated by energy state. That combination, rather than the sight of food alone, is what the evidence supports as the target for understanding cue-driven overeating.

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