Neurophysiology of limbic system pathways in the rat: projections from the amygdala to the entorhinal cortex.

We studied the responses of rat entorhinal neurons to electrical stimulation of the amygdala. Four main results were obtained: (1) excitatory postsynaptic potentials were recorded in entorhinal neurons in response to electrical stimulation of the amygdala. Cells in layers II, III and V of the entorhinal cortex were responsive. (2) Excitatory responses were followed by inhibitory postsynaptic potentials. (3) Frequency potentiation of both excitatory and inhibitory responses was observed when 10/s stimulation was used. (4) Three amygdala neurons were antidromically activated by entorhinal stimulation; and two layer II entorhinal cells that were excited by amygdala stimulation were also antidromically activated by dentate gyrus stimulation. These results provide evidence for a monosynaptic, excitatory projection from the amygdala to the entorhinal cortex. In addition, the data indicate that amygdala neurons are only one synapse removed from the excitation of dentate gyrus granule cells.