Functional reciprocal connections of the rat entorhinal cortex and subicular complex with the medial frontal cortex: an in vivo intracellular study.

We used in vivo intracellular recording techniques in the rat in order to determine the properties of projections from the medial frontal cortex to the entorhinal cortex and subicular complex. Three main results were obtained. (1) A high proportion (65%) of neurons within the medial frontal cortex were antidromically activated at short latency (0.4-1.9 ms) by electrical stimulation of the entorhinal cortex or subicular complex. This provided physiological evidence for fast direct projections from the medial frontal cortex to the entorhinal cortex and subicular complex. (2) Clear excitatory postsynaptic potentials (EPSPs) were evoked in 8% of the cells within the entorhinal cortex, subicular complex, or adjacent cortices after electrical stimulation of the medial frontal cortex. (3) The most salient synaptic response was inhibition, as shown by the presence of inhibitory postsynaptic potentials (IPSPs) in 50% of the cells sampled. Similar results were obtained for the reciprocal pathway: 56% of the sampled cells in the entorhinal cortex or subicular complex responded with antidromic spikes to stimulation of the medial cortex; 4% of medial frontal neurons responded to stimulation of the entorhinal cortex or subicular complex with clear EPSPs, and 48% with IPSPs. The latencies of most synaptic responses, 15-25 ms, were inconsistent with monosynaptic activation. This suggests that oligosynaptic relays amplified the signal within or en route to their targets, and/or that cells with more slowly propagating axons were also present but not sampled by the intracellular electrodes. Finally, responsive fast-spiking cells (candidate inhibitory neurons) were encountered within target structures. The results provide evidence that these distant cortical regions are functionally connected in a reciprocal manner, and that both principal and inhibitory neurons are excited by this projection system.