Abstract
Dendritic ion channels play a critical role in shaping synaptic input and are fundamentally important for synaptic integration and plasticity. In the hippocampal region CA1, somato-dendritic gradients of AMPA receptors and the hyperpolarization-activated cation conductance (Ih) counteract the effects of dendritic filtering on the amplitude, time-course, and temporal integration of distal Schaffer collateral (SC) synaptic inputs within stratum radiatum (SR). While ion channel gradients in CA1 distal apical trunk dendrites within SR have been well characterized, little is known about the patterns of ion channel expression in the distal apical tuft dendrites within stratum lacunosum moleculare (SLM) that receive distinct input from the entorhinal cortex via perforant path (PP) axons. Here, we measured local ion channels densities within these distal apical tuft dendrites to determine if the somato-dendritic gradients of Ih and AMPA receptors extend into distal tuft dendrites. We also determined the densities of voltage-gated sodium channels and NMDA receptors. We found that the densities of AMPA receptors, Ih, and voltage-gated sodium channels are similar in tuft dendrites in SLM when compared with distal apical dendrites in SR, while the ratio of NMDA receptors to AMPA receptors increases in tuft dendrites relative to distal apical dendrites within SR. These data indicate that the somato-dendritic gradients of Ih and AMPA receptors in apical dendrites do not extend into the distal tuft, and the relative densities of voltage-gated sodium channels and NMDA receptors are poised to support nonlinear integration of correlated SC and PP input.
Highlights
Pyramidal neurons in hippocampal area CA1 receive spatial information from the entorhinal cortex both directly from layer III via perforant path (PP) axons and indirectly from layer II via Schaffer collateral (SC) axons
To determine whether the scaling of synaptic AMPA-type glutamate receptors (AMPARs) density previously measured within the distal stratum radiatum (SR) apical dendritic trunk continues into distal tuft dendrites of CA1 pyramidal neurons, we measured the amplitudes of miniature EPSCs using whole cell voltage-clamp of distal apical dendrites within SR (250–350 mm from the soma) or tuft dendrites within stratum lacunosum-moleculare (SLM) (50– 150 mm from the start of the perforant path axons) in response to a local pressure application of hyperosmotic sucrose solution
We measured AMPAR miniature EPSCs (mEPSCs), Ih currents, voltagegated Na+ channel currents, and NMDA-type glutamate receptors (NMDARs)/AMPAR ratios to determine the ionic mechanisms of synaptic integration within the distal apical tuft dendrites of CA1 pyramidal neurons
Summary
Pyramidal neurons in hippocampal area CA1 receive spatial information from the entorhinal cortex both directly from layer III via perforant path (PP) axons and indirectly from layer II via Schaffer collateral (SC) axons. While somato-dendritic gradients of AMPARs and Ih current have been demonstrated for the SR segment of the apical trunk innervated by SC axons [7,8,9,10], it is unknown whether these gradients continue into the distal tuft dendrites This is relevant because the apical tuft receives functionally distinct input from the EC (PP) that appears to be integrated in a distinctly nonlinear mode through the generation of long duration dendritic plateau potentials that possess sodium (Na+), calcium (Ca2+) and NMDA dependent components [11,12,13,14]. We observed that Na+ channel density is comparable between SR and SLM, but that the relative density of synaptic NMDARs with respect to AMPARs increases from SR to SLM
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