Features of proximal and distal excitatory synaptic inputs to layer V neurons of the rat medial entorhinal cortex

Authors


  • V. Medinilla and O. Johnson contributed equally to this work.

S. Gasparini: Neuroscience Center, Louisiana State University Health Sciences Center, 2020 Gravier Street, New Orleans, LA 70112, USA. Email: sgaspa1@lsuhsc.edu

Key points

  • • Layer V principal neurons of the entorhinal cortex receive the hippocampal output on their proximal and basal dendrites and send their axons to cortical areas, playing a fundamental role in memory processing.
  • • The apical dendrites of these neurons are rich in spines and extend to the entorhinal superficial layers, in the proximity of axons from cortical neurons, which could make synapses onto these spines.
  • • We stimulated afferent fibres in the superficial layers and recorded depolarizing responses in entorhinal layer V neurons, indicating that they receive excitatory inputs onto their distal dendrites.
  • • The responses were completely blocked by glutamatergic receptor antagonists; stimulation of distal afferents could initiate dendritic spikes, which propagated to the soma to generate an action potential.
  • • These results show that the distal dendrites of entorhinal layer V neurons have access to information that could affect the integration of the input from the hippocampus.

Abstract  The entorhinal cortex (EC) has a fundamental function in transferring information between the hippocampus and the neocortex. EC layer V principal neurons are the main recipients of the hippocampal output and send processed information to the neocortex, likely playing an important role in memory processing and consolidation. Most of these neurons have apical dendrites that extend to the superficial layers and are rich in spines, which could be the targets of excitatory inputs from fibres innervating that region. We have used electrical stimulation of afferent fibres coupled with whole-cell patch-clamp somatic recordings to study the features of distal excitatory inputs and compare them with those of proximal ones. The amplitude of putative unitary excitatory responses was ∼1.5 times larger for distal compared with proximal inputs. The responses were purely glutamatergic, as they were abolished by a combination of AMPA and NMDA glutamatergic receptor antagonists. Blockade of Ih by 4-ethylphenylamino-1,2-dimethyl-6-methylaminopyrimidinium chloride (ZD7288) increased temporal summation; the increase was comparable for proximal and distal inputs. Proximal inputs initiated a somatic spike more reliably than distal ones; in some instances, somatic action potentials triggered by distal stimulation were preceded by dendritic spikes that fully propagated to the soma. Altogether, our results show that medial layer V entorhinal neurons receive excitatory synapses at distal dendritic locations, which gives them access to information encoded by inputs to the superficial layers as well as the deep layers. These findings are fundamentally relevant to understanding the role of the EC in the formation and consolidation of episodic memory.

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