Dendritic spines, characterized by a small head (volume~0.01-0.1μm3) and narrow neck (diameter~0.1μm, length~1μm), are the primary site of excitatory synaptic input in the mammalian brain. Synaptic inputs made onto spines first integrate onto dendrites, and subsequently propagate towards the soma and axon initial segment, where they further integrate with other inputs to determine overall action potential output. Elucidating the electrical properties of spines is thus paramount for understanding the first steps along this signal processing chain. Yet, their micron/sub-micron size has rendered conventional whole-cell intracellular electrophysiology infeasible.
In the first part of this talk, the speaker describes his work using quantum-dot labeled quartz nanopipettes (15-30 nm diameters) under two-photon visualization for targeted intracellular recordings from spines and small pre-synaptic terminals. He shows through detailed experiments that (i) spines receive large EPSPs (25-30mV), and (ii) estimated neck resistances are large enough to influence electrical isolation (mean ~420 MΩ), and filter synaptic input as it invades the dendrite. He briefly describes the theoretical implications of these properties
In the second part of this talk, he describes a new method in which he combines the flexible property of these nanopipettes with microprisms to enable simultaneous two-photon calcium imaging and targeted intracellular electrophysiology across (a) cortical depth; (b) different cell types; (c) somatic and dendritic segments; and (d) anaesthetized and awake head-fixed locomoting mice. As a prototypical application of the method, he describes targeted intracellular recordings from PV+ interneurons, while simultaneously imaging epileptic seizure spread across cortical layers. Dr. Jayant gives a succinct overview of recent work on biomimetic nanopipettes, custom CMOS intracellular amplifiers, and the fabrication of two-photon compatible electrode arrays. Finally, he concludes by describing recent endeavors from his lab at Purdue on using two-photon holography to stimulate spines in 3D.
Dr. Krishna Jayant
Assistant Professor of Biomedical Engineering, Purdue University, USA