Physiological evidence for an excitatory pathway from entorhinal cortex to amygdala in the rat.

We studied the responses of amygdala neurons to entorhinal cortex stimulation in anaesthetized rats. Intracellular and extracellular data were obtained in a total of 16 cells located throughout the amygdaloid complex and two cells in adjacent piriform cortex. In addition, antidromic responses to amygdala stimulation were obtained in 7 cells of the entorhinal or perirhinal cortex. All recordings in the amygdala showed orthodromic excitatory responses (spikes or EPSPs), with a mean latency of 8 ms. These were succeeded by IPSPs with a mean latency of 15 ms. Two cells in piriform cortex responded to entorhinal stimulation with inhibition alone. A cell in the region of the basomedial nucleus showed characteristics of an inhibitory interneuron. Cells in entorhinal and perirhinal cortex responding antidromically to amygdala stimulation were found primarily in layers III-V. Axons of one such cell, which was injected with HRP, were seen to course rostrally to the region of the amygdala within the fiber tract of the external capsule. Three entorhinal cells (layer III) responded antidromically to both amygdala and hippocampal formation stimulation. A neuronal circuit diagram accounting for our findings is presented.