FEAR MEMORY UNDER ACUTE PHASE SHIFT: POTENTIAL MODULATION OF PREFRONTAL-HIPPOCAMPAL CIRCUIT BY OREXIN

Circadian disturbances compromise cognitive function and disrupt the information flow from prefrontal cortex (PFC) to hippocampus (HIP) in humans and in rodent models. An acute ‘jet-lag’ in mice using a six-hour phase delay (PD) of the normal light-dark cycle impaired contextual fear memory. Immunohistochemistry analysis showed that neuronal activation, indicated by cFos, was increased in orexin expressing neurons, cells in the dentate gyrus (DG) and in the supramammillary nucleus (SuM), an important relay station for PFC-to-HIP information flow. Chemogenetic activation of either SuM or DG neurons reproduced such a freezing reduction, suggesting that over‑activation of this circuit is sufficient to disrupt fear conditioning.

To investigates the role of orexin, a wake-promoting neuropeptide in the PFC-SUM-DG pathway, mRNA levels for orexin receptors were measured in laser microdissected neurons of the SuM, tagged by anterograde or retrograde viral tracers. SUM-to-DG projection neurons expressed low levels of the orexin receptor type 2 but not type 1 (OXR1). OXR1-positive PFC-to-DG relay cells in the SuM were confirmed by a double tracing approach paired with immunohistochemistry. In the DG, OXR1 was located in mossy cells (MC), as demonstrated by RNAScope. Intranasal application of orexin evoked a bimodal effect on contextual fear memory retrieval during PD and normal light conditions. The freezing levels were negatively correlated with the activation of orexin-expressing neurons in the lateral hypothalamus, MCs in the hilus, and cells in the SuM, measured by cFos immunohistochemistry. Thus, orexinergic projections are well suited to modulate fear memory expression acting via the PFC-SuM-DG network.