UC Berkeley

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.