After prolonged stress, mice may avoid open spaces, groom less and show reduced preference for sweet water. Does one pathway drive all these changes? A study published in Translational Psychiatry on March 16, 2024 follows an orexin pathway from lateral hypothalamus to medial prefrontal cortex. Increasing or reducing its activity mainly changes grooming and sucrose-related behavior, without parallel changes in tested anxiety-like measures or immobility. Sucrose preference compares choices between sweet and plain water, while grooming describes self-care behavior. These measures make reward-related changes observable, with distinct meanings. Behaviors that appear together may involve distinct neural contributions.

01 / Chronic stress changes several behaviors

Chronic stress changes several behaviors

After weeks of unpredictable mild stress, mice spend less time in the center of an open field and on exposed maze arms, and take longer to begin feeding in a novel environment. Feeding latency in the home cage remains similar, as does total open-field distance, distinguishing exploration changes from simply moving less.

Immobility increases during swimming and suspension tests, while grooming and sucrose preference decline. Examining these measures alongside one another allows the study to ask which components an orexin pathway affects.

Original figure 1Chronic stress changes several behaviors

02 / Orexin cells change with behavioral context

Orexin cells change with behavioral context

Orexin is known for regulating wakefulness and appetite, while its cells also send information to emotion-related regions. Restraint stress increases activity markers in hypothalamus. After chronic stress, orexin-cell calcium responses are stronger as mice enter the open-field center, step onto exposed arms or begin grooming.

The increase is context-dependent: calcium responses grow at immobility-to-struggling transitions during suspension, while the corresponding swimming comparison is not significant. Activity changes with particular events, motivating closer examination of the pathway.

Original figure 2Orexin cells change with behavioral context

03 / Tracing from hypothalamus to prefrontal cortex

Tracing from hypothalamus to prefrontal cortex

Tracing backward from medial prefrontal cortex labels hypothalamic cells predominantly bearing orexin. Connectivity tracing and receptor labeling further show that, among prefrontal cells receiving hypothalamic input, orexin-receptor-bearing populations are mainly glutamatergic, with a smaller GABAergic component.

After chronic stress, tested prefrontal glutamatergic cells are more excitable: the same current evokes more spikes and less current is required to elicit the first spike. The upstream–downstream connection provides a route for subsequent functional experiments.

Original figure 3Tracing from hypothalamus to prefrontal cortex

04 / Greater excitability and rising local peptide signals

Greater excitability and rising local peptide signals

Recordings from hypothalamic orexin cells projecting to prefrontal cortex similarly show more evoked spikes and a lower firing threshold after chronic stress. Changes appear in the tested cells at both ends of the connection.

Prefrontal orexin-sensor recordings show rapid fluorescence increases during tail stimulation or restraint, followed by declines after stimulation ends. These local orexin-related changes associate with acute stress and complement the cellular-excitability findings.

Original figure 4Greater excitability and rising local peptide signals

05 / Increasing the signal reduces grooming without changing exploration

Increasing the signal reduces grooming without changing exploration

In mice without chronic stress, activating orexin terminals within prefrontal cortex reduces grooming. Open-field center time, exposed-arm exploration, novel-environment feeding latency, swimming and suspension immobility, and total distance do not change significantly.

A separate experiment activates the hypothalamic population projecting to prefrontal cortex, reducing grooming and sucrose-related drinking measures. This population manipulation complements local-terminal activation but operates at a different location and may involve other axonal branches.

Original figure 5Increasing the signal reduces grooming without changing exploration

06 / Weakening the population improves selected reward-related behaviors

Weakening the population improves selected reward-related behaviors

After chronic stress, inhibiting prefrontal-projecting orexin cells increases grooming and sucrose preference. Anxiety-like tests and immobility measures remain similar. Sucrose-drinking duration and bout number do not significantly improve, showing that the change affects selected behavioral components.

Orexin cells also project to anterior cingulate cortex, but inhibiting the population targeting that destination does not significantly change tested sucrose measures. Output destination helps distinguish contributions within the orexin system.

Original figure 6Weakening the population improves selected reward-related behaviors

07 / Receptor-level results echo the behavioral distinction

Receptor-level results echo the behavioral distinction

Separately blocking OX1R or OX2R in prefrontal cortex also increases grooming and sucrose preference after stress. Drinking bouts increase while duration remains similar, without parallel improvements in anxiety-like or immobility measures. These local pharmacological results align with the behavioral direction of population inhibition.

The study uses adult male mice and behaviors such as grooming and sucrose preference to characterize reward-related changes. Its central distinction is that one pathway affects some behaviors following chronic stress more clearly than others. Examining these components separately reveals the contributions of different neural connections.

Original figure 7Receptor-level results echo the behavioral distinction