Neural dendrites continually prove to harness more computational complexity than previously thought. The voltage-gated ion channels distributed throughout a dendritic tree are key determinants of dendritic excitability and computation. However, little is known about the specific functional impacts of voltage-gated excitability in discrete dendritic regions. Recently, an optical revolution in neuroscience has yielded a vast array of optical tools for functional interrogation of neurons and neural circuits. One such tool, Quarternary-ammonium Azobenzene Quarternary-ammonium (QAQ), is an optically-controllable small-molecule drug that affects voltage-gated ion channels. In its trans conformation, which is photo-inducible with green light, QAQ directly blocks all voltage-gated ion channels tested, but rapidly un-blocks those channels when converted to its cis form with near-ultraviolet light (Mourot et al. 2012). It does not photo-bleach, and can be robustly photoswitched back and fourth to either block or unblock channels in a matter of milliseconds. QAQ is a promising tool to control voltage-gated excitability in neural dendrites with the spatiotemporal precision of light.;In this thesis we use QAQ to rapidly, reversibly, and locally control voltage-gated ion channel activity in neural dendrites using targeted light. A wealth of experimental evidence using traditional pharmacology is already available about specific voltage-gated ion channels in CA1 pyramidal cells, so we first apply QAQ via a patch-pipette to CA1 pyramidal cells and confirm that it works as expected in a whole-cell. We find that trans-QAQ blocks somatic action potentials, blocks dendritic calcium activity, and enhances EPSP summation. These are all processes driven by QAQ-sensitive voltage-gated ion channel types that either boost (sodium and calcium channels) or dampen (potassium channels) intrinsic excitability.;We then investigate the level of spatial control we can achieve with QAQ using dendritic calcium imaging. Indeed, for up to three seconds after photo-switching the molecule, control is extremely precise. With this knowledge, we use local block of voltage-gated ion channels and calcium imaging to confirm and extend previous findings that voltage-gated calcium channel activity is relatively uniform throughout the apical dendritic tree of CA1 pyramidal cells. Finally, we specifically photo-control voltage-gated ion channels in the apical dendrites of CA1 neurons to experimentally probe whether dendrite-specific voltage-gated excitability affects the degree of action potential back-propagation. We find that dendritic voltage-gated ion channels determine whether a CA1 pyramidal neuron will undergo strong or weak back-propagation, a notion that has only previously been modeled.
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