Olfactory inputs activate the medial entorhinal cortex via the hippocampus

Area of Science:

• Neuroscience
• Olfactory System
• Brain Connectivity

Background:

• The entorhinal cortex, crucial for memory, has distinct lateral and medial regions with differing connectivity.
• A detailed map of olfactory projections to the entorhinal cortex is lacking, despite anatomical evidence suggesting lateral and rostral confinement.

Purpose of the Study:

• To investigate the distinct contributions of medial and lateral entorhinal areas to olfactory processing.
• To map the physiological responses to olfactory stimuli in the entorhinal cortex.

Main Methods:

• Used in vitro isolated guinea pig brain preparations.
• Stimulated the lateral olfactory tract to analyze entorhinal cortex responses.
• Employed simultaneous multisite recordings and current source density analysis with 16-channel silicon probes.

Main Results:

• A direct, monosynaptic olfactory input was found exclusively in the superficial lateral entorhinal cortex.
• Disynaptic connections to the lateral entorhinal cortex were mediated by piriform cortex associative fibers.
• Medial entorhinal cortex activation was indirect, occurring late and dependent on hippocampal (dentate gyrus, CA1) population spikes or specific low-frequency lateral olfactory tract stimulation.

Conclusions:

• The lateral entorhinal cortex receives direct olfactory input, suggesting a primary role in initial olfactory processing.
• The medial entorhinal cortex's indirect activation points to a role in integrating olfactory information with other sensory or cognitive inputs, possibly via hippocampal pathways.
• These findings highlight the differential processing of olfactory information by the medial and lateral entorhinal cortex.