Nucleus reuniens is organised as parallel circuits rather than a prefrontal-hippocampal relay

Interactions between the prefrontal cortex (PFC) and hippocampus (HPC) are crucial for flexible and memory guided behaviour. Because there are no direct projections from PFC to HPC, it is widely presumed that information is shared through a disynaptic projection via the nucleus reuniens (nRE), a ventral midline thalamic nucleus. Yet this model relies on the implicit assumption that has not been directly tested: that prefrontal afferents monosynaptically recruit hippocampal-projecting nRE neurons. Here we combine whole-brain anatomical mapping, projection-specific circuit tracing and electrophysiology to test this model. We find that nRE contains largely distinct populations of prefrontal- and hippocampal-projecting neurons, with minimal overlap between them. Unexpectedly, prefrontal afferent input to nRE provides little anatomical or functional input to hippocampal-projecting neurons, instead preferentially targeting neurons projecting back to PFC. These findings challenge the prevailing view that nRE acts as a simple relay between PFC and HPC and instead reveal parallel projection-defined circuits likely to make distinct contributions to hippocampal-prefrontal communication.