Acquisition of trace fear conditioning without functional nucleus reuniens did not require dorsal or ventral hippocampus NMDA receptor activation in male Long-Evans rats.

Importance: The nucleus reuniens (RE) is a crucial component that interconnects the medial prefrontal cortex (mPFC) and hippocampus (HPC), completing the HPC-dependent circuit underlying the regulation of trace fear. We previously demonstrated that RE inactivation during acquisition impaired the encoding of trace fear, while RE inactivation during both the acquisition and retrieval led to heightened trace fear throughout the test session, raising questions about the involvement of HPC-independent circuit in trace fear acquisition without functional RE.

Objective: To investigate whether rats without functional RE throughout the entire behavioral sessions can acquire trace fear using an HPC-independent circuit.

Design: A balanced factorial design was used to assess the role of the dorsal hippocampus (DH) or ventral hippocampus (VH) in trace fear acquisition with or without functional RE.

Setting: The study was conducted in a controlled laboratory environment.

Participants: Adult male Long-Evans rats were used as experimental subjects.

Interventions: Consecutive intracranial micro-infusions of either GABAA receptor agonist "Muscimol" or vehicle targeted the RE during both trace fear acquisition and retrieval. Micro-infusions of the N-methyl-D-aspartate (NMDA) receptor antagonist "DL-2-amino-5-phosphonovaleric acid" or saline targeted the DH or VH during trace fear acquisition.

Main outcomes: Fear level of respective groups was measured.

Measures: Freezing was quantified as immobility during baseline and conditioned stimulus during trace fear acquisition and retrieval.

Results: Control animals required proper DH or VH NMDA receptor activation for the acquisition of trace fear. Rats without functional RE still acquired trace fear, but independent of DH or VH NMDA receptor activation, suggesting the reliance of an HPC-independent circuit during fear encoding.

Conclusions and relevance: These findings highlighted potential alternative neural pathways that may support and compensate trace fear acquisition under pathological conditions. Significance Statement Impaired fear regulation resulted in psychiatric disorders like panic disorder and anxiety. Pavlovian trace fear conditioning using male Long-Evans rats as the subjects models human emotional learning. Although hippocampus (HPC)-medial prefrontal cortex (mPFC) interactions facilitate trace fear acquisition, the role of thalamic nucleus reuniens (RE), which connects these regions, is unclear. In this study, we found that in terms of within-session fear expression during acquisition, dorsal hippocampus (DH) N-methyl-D-aspartate (NMDA) receptor blockade impaired fear response only in RE-intact rats, whereas ventral hippocampus (VH) blockade impaired the response regardless of the RE functionality. Critically, rats without functional RE still acquired trace fear memory independent of DH or VH NMDA receptor activation, indicating the recruitment of alternative HPC-independent pathways. These findings highlight possible compensatory pathways engaged following RE dysfunction, providing insight into how the brain adapts under pathological conditions.