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Stimulation of the Parasubiculum Modulates Entorhinal Cortex Responses to Piriform Cortex Inputs In Vivo

Center for Studies in Behavioral Neurobiology, Department of Psychology, Concordia University, Montreal, Québec H4B 1R6, Canada

Submitted 12 January 2004; accepted in final form 16 March 2004

Although a major output of the hippocampal formation is from the subiculum to the deep layers of the entorhinal cortex, the parasubiculum projects to the superficial layers of the entorhinal cortex and may therefore modulate how the entorhinal cortex responds to sensory inputs from other cortical regions. Recordings at multiple depths in the entorhinal cortex were first used to characterize field potentials evoked by stimulation of the parasubiculum in urethan-anesthetized rats. Current source density analysis showed that a prominent surface-negative field potential component is generated by synaptic activation in layer II. The surface-negative field potential was also observed in rats with chronically implanted electrodes. The response was maintained during short stimulation trains of 125 Hz, suggesting that it is generated by activation of monosynaptic inputs to the entorhinal cortex. The piriform cortex also projects to layer II of the entorhinal cortex, and interactions between parasubicular and piriform cortex inputs were investigated using double-site stimulation tests. Simultaneous activation of parasubicular and piriform cortex inputs with high-intensity pulses resulted in smaller synaptic potentials than were expected on the basis of summing the individual responses, consistent with the termination of both pathways onto a common population of neurons. Paired-pulse tests were then used to assess the effect of parasubicular stimulation on responses to piriform cortex stimulation. Responses of the entorhinal cortex to piriform cortex inputs were inhibited when the parasubiculum was stimulated 5 ms earlier and were enhanced when the parasubiculum was stimulated 20–150 ms earlier. These results indicate that excitatory inputs to the entorhinal cortex from the parasubiculum may enhance the propagation of neuronal activation patterns into the hippocampal circuit by increasing the responsiveness of the entorhinal cortex to appropriately timed inputs.

This article has been cited by other articles:

				S. D. Glasgow and C. A. Chapman Conductances Mediating Intrinsic Theta-Frequency Membrane Potential Oscillations in Layer II Parasubicular Neurons J Neurophysiol, November 1, 2008; 100(5): 2746 - 2756. [Abstract] [Full Text] [PDF]

				S. D. Glasgow and C. A. Chapman Local Generation of Theta-Frequency EEG Activity in the Parasubiculum J Neurophysiol, June 1, 2007; 97(6): 3868 - 3879. [Abstract] [Full Text] [PDF]

				D. A. Caruana, R. E. Sorge, J. Stewart, and C. A. Chapman Dopamine Has Bidirectional Effects on Synaptic Responses to Cortical Inputs in Layer II of the Lateral Entorhinal Cortex J Neurophysiol, December 1, 2006; 96(6): 3006 - 3015. [Abstract] [Full Text] [PDF]