Muscarinic control of long-range GABAergic inhibition within the rhinal cortices.

The perirhinal cortex plays a critical role in memory formation, in part because it forms reciprocal connections with the neocortex and entorhinal cortex and is thus in a position to integrate and transfer higher-order information to and from the hippocampus. However, for reasons that remain unclear, perirhinal transfer of neocortical inputs to the entorhinal cortex occurs with a low probability. Using patch recordings in vitro and tract-tracing combined with GAD-67 immunohistochemistry, we show that the perirhinal cortex contains GABAergic neurons with long-range projections to superficial entorhinal cells. This finding challenges the traditional model of cortical inhibition in which all trans-areal inhibition is thought to be disynaptic because the axons of GABAergic interneurons are assumed to be confined within the area in which their somata are located. Moreover, consistent with recent studies indicating that the formation of perirhinal-dependent memories requires activation of muscarinic receptors, long-range IPSPs were presynaptically inhibited by M2 receptor activation. Overall, these results suggest that long-range feedforward inhibition regulates perirhinal transfer of neocortical inputs to the entorhinal cortex, but that cholinergic inputs can presynaptically adjust the impact of this control mechanism as a function of environmental contingencies.