Date of Completion

8-8-2014

Embargo Period

8-1-2015

Major Advisor

Leslie Loew

Associate Advisor

Srdjan Antic

Associate Advisor

Ji Yu

Associate Advisor

John Carson

Field of Study

Biomedical Science

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Excitatory postsynaptic potentials (EPSPs) occur when the neurotransmitter glutamate binds to postsynaptic receptors located on specialized/pleomorphic structures called dendritic spines, which are attached to dendrites through spine necks. These potentials constitute the input signals that neurons must process in order to maintain proper brain function, but due to the small size of spines, direct measurement of these potentials at their site of origin has remained elusive. In this study, we combine voltage-sensitive dye recording with glutamate-uncaging to directly measure the amplitude and duration of unitary EPSPs in single spines in cortical layer 5 pyramidal neurons from mouse brain slices. Our findings indicate that EPSPs in the spines that resemble miniature EPSPs at the soma are always less than 20 mV, and last on average 15 ms. In the same spines, we also determined the diffusional coupling of the spine with the parent dendrite to estimate the spine neck resistance (Rneck). We show that large Rneckenhances the EPSP amplitude within the spine through passive mechanisms, but at the same time, increases the amount of attenuation of the synaptic input at the soma. These findings are recapitulated with a morphologically realistic computational model of a L5 pyramidal neuron from which we can also predict the unitary synaptic conductances associated with these EPSPs. Moreover, since the propagation of the EPSP from the spine to the soma is also determined by the dendritic morphology, we performed detailed simulations to assess how the spine neck and the dendritic resistance combined can facilitate or prevent the generation of sodium dendritic spikes from the synchronous activation of synaptic input. We find from these simulations that distal spines with small Rneck are more likely to generate sodium spikes if the synaptic conductance is large enough, and the distribution of voltage-gated sodium channels (VGSCs) is homogeneous throughout the dendrite. These sodium spikes backpropagate into the spines, and are largely dependent on the density of VGSCs within the parent dendrite, but not within the spine head. The implications of these observations for dendritic input integration and future experiments to test these predictions are discussed.

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