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

No prior work had resolved the specific connectivity patterns between the entorhinal cortex and the amygdala. It was already known that these brain regions participate in memory and emotional regulation. However, the exact nature of their synaptic communication remained poorly defined in rodent models. This uncertainty drove researchers to investigate the physiological responses of amygdala neurons to cortical stimulation. Prior research has shown that the amygdala receives diverse inputs from various cortical structures. That gap motivated a detailed examination of the excitatory and inhibitory signaling dynamics. The current study addresses how these regions interact at the single-cell level. Understanding this pathway provides insight into how sensory information reaches the emotional brain.

The aim of this study was to characterize the physiological responses of amygdala neurons to stimulation of the entorhinal cortex. Researchers sought to determine if a direct excitatory pathway links these two brain regions. This investigation addressed the lack of clarity regarding the functional connectivity between the cortex and the amygdala. The study focused on identifying the nature of synaptic inputs received by amygdala cells. By recording from multiple neurons, the team intended to map the flow of information within this circuit. They also aimed to identify the specific cortical layers that send projections to the amygdala. This work was motivated by the need to understand how sensory information reaches emotional processing centers. The results provide a foundation for modeling the neural architecture of this pathway.

The review approach involved analyzing intracellular and extracellular recordings from neurons in anaesthetized rats. Researchers applied electrical stimulation to the entorhinal cortex to evoke responses in the amygdala. They monitored 16 cells within the amygdaloid complex and two cells in the piriform cortex. The team also assessed antidromic activation in seven neurons located in the entorhinal or perirhinal regions. One specific neuron underwent Horseradish Peroxidase injection to map its axonal trajectory. This staining technique enabled the visualization of fibers traveling through the external capsule. The investigators compared responses from different cortical layers to identify the origin of these projections. Finally, they constructed a circuit diagram to integrate the observed physiological data.

Key findings from the literature indicate that all amygdala cells exhibited orthodromic excitatory responses upon stimulation. These excitatory spikes or excitatory postsynaptic potentials occurred with a mean latency of 8 ms. Inhibitory postsynaptic potentials followed these events with a mean latency of 15 ms. Two piriform cortex cells displayed only inhibitory responses during the stimulation process. Antidromic activation revealed that neurons in layers III-V of the entorhinal and perirhinal cortex project to the amygdala. One labeled cell showed axons extending rostrally toward the amygdala via the external capsule. Three entorhinal layer III neurons responded antidromically to both amygdala and hippocampal formation stimulation. The data support a model of direct excitatory input from the cortex to the amygdala.

The authors propose a direct excitatory connection exists between the entorhinal cortex and the amygdala. This pathway likely facilitates rapid communication between these two critical brain structures. The observed excitatory responses suggest a primary role for this circuit in sensory processing. Subsequent inhibitory signals indicate a complex regulatory mechanism within the amygdala. The researchers suggest that these connections are organized within specific cortical layers. Their findings support the existence of a feedback loop involving the hippocampal formation. The proposed circuit diagram synthesizes these physiological observations into a functional model. This work clarifies the anatomical and physiological basis for cortical-amygdala interactions.