Dorsal hippocampus and medial prefrontal cortex each contribute to the retrieval of a recent spatial memory in rats

Systems-level consolidation models propose that recent memories are initially hippocampus-dependent. When remote, they are partially or completely dependent upon the medial prefrontal cortex (mPFC). An implication of the mPFC in recent memory, however, is still debated. Different amounts of muscimol (MSCI 0, 30, 50, 80 and 250 ng in 1 µL PBS) were used to assess the impact of inactivation of the dorsal hippocampus (dHip) or the mPFC (targeting the prelimbic cortex) on a 24-h delayed retrieval of a platform location that rats had learned drug-free in a water maze. The two smallest amounts of MSCI (30 and 50 ng) did not affect recall, whatever the region. 80 ng MSCI infused into the dHip disrupted spatial memory retrieval, as did the larger amount. Infusion of MSCI into the mPFC did not alter performance in the 0-80 ng range. At 250 ng, it induced an as dramatic memory impairment as after efficient dHip inactivation. Stereological quantifications showed that 80 ng MSCI in the dHip and 250 ng MSCI in the mPFC induced a more than 80% reduction of c-Fos expression, suggesting that, beyond the amounts infused, it is the magnitude of the neuronal activity decrease which is determinant as to the functional outcome of the inactivation. Because, based on the literature, even 250 ng MSCI is a small amount, our results point to a contribution of the mPFC to the recall of a recently acquired spatial memory and thereby extend our knowledge about the functions of this major actor of cognition.