The hypothalamic orexin-medial prefrontal cortex pathway modulates susceptibility to depression in the learned helplessness paradigm in male mice.

Depression is a prevalent psychiatric disorder primarily characterized by behavioral despair and helplessness. The orexinergic system plays a crucial role in stress-induced depression; however, the precise mechanisms remain unclear. The medial prefrontal cortex (mPFC), a major target of orexinergic projections, is essential for emotional regulation. This study suggests that impaired orexinergic input to the mPFC underlies depression-like behaviors in murine models. In the learned helplessness (LH) paradigm, the majority of male mice subjected to inescapable foot shocks developed depression-like behaviors (classified as LH-susceptible), whereas a subset displayed resilience (classified as LH-resilient). LH-susceptible mice exhibited a reduced number of orexin+ neuronal somata in the hypothalamus, along with decreased expression of prepro-orexin mRNA Correspondingly, the mPFC of these mice displayed decreased density of orexinergic terminals and lower Orexin A levels. In contrast, LH-resilient mice maintained stable hypothalamic orexin neuron counts, exhibited increased prepro-orexin mRNA expression, and preserved orexinergic terminal density in the mPFC. Chemogenetic activation of the hypothalamic orexin-mPFC pathway alleviated depression-like behaviors in LH-susceptible mice through Orexin A signaling. Conversely, inhibition of this pathway induced depression-like behaviors in LH-resilient mice. Additionally, LH-susceptible mice showed decreased excitatory synaptic transmission in layer V pyramidal neurons of the mPFC, which was improved by activation of hypothalamic orexinergic inputs; this improvement was attenuated by administration of an orexin receptor 1 (OX1R) antagonist. Collectively, these findings implicate the hypothalamic orexin-mPFC pathway as a pivotal regulator of excitatory synaptic transmission and suggest its involvement in modulating individual variability in susceptibility and resilience to depression.Significance statement Depression is a highly prevalent emotional disorder characterized by behavioral despair and helplessness under stress. This study demonstrates that the orexinergic system is critical for modulating resilience to aversive stimuli. Using the learned helplessness (LH) mouse model, we found that LH-susceptible mice, but not LH-resilient mice, show compromised hypothalamic orexinergic projections to the medial prefrontal cortex (mPFC), together with decreased excitatory synaptic transmission in layer V pyramidal neurons. Activation of the hypothalamic orexin-mPFC pathway alleviates depression-like behaviors in susceptible mice through Orexin A, while inhibition of this pathway induces depression-like phenotypes in resilient mice. These findings identify orexinergic signaling as a promising therapeutic target for depression.