Functional analysis of identified interneurons in the mouse visual cortex

Abstract

Event Abstract Back to Event Functional analysis of identified interneurons in the mouse visual cortex GABAergic interneurons are integral to cortical circuit structure and function but the breadth of functions subserved by each of subtypes of this highly diverse cortical neuron class is less understood in an in vivo preparation. Conventional extracellular electrophysiology falls short in distinguishing interneuron subtypes. With a custom built two photon scanning microscope and in vivo calcium indicator dye imaging we are studying tuning properties of genetically identified subtypes of cortical interneuron in anesthetized mouse V1 to moving grating stimuli. Genetically identified neuronal types in cortex can be reliably labeled with fluorescent reporters which persists throughout the development of the organism. Therefore, our goal is to study a few selected interneuron subtypes at time points around developmentally important critical period for ocular dominance plasticity in V1. In our preparation, mice are anesthetized and ventilated with 0.7% isoflurane in a 20:80, O2:N2 mixture. We bulk-load calcium indicator dye, Oregon Green BAPTA-1 (OGB), using patch pipettes into layer 2/3 of the V1. Anesthetized mice were presented with moving gratings visual stimuli at 2Hz temporal frequency, 0.02 cycles per degree spatial frequency, six orientations and twelve directions. We were able to distinguish GFP (or red tdTomato) labeled cell from the OGB signal by using two different 2-photon excitation wavelengths i.e., 800nm and 950nm respectively (Sohya 2007). Preliminary analysis of somatostatin positive interneurons in 28-days (P28) old GIN (GFP labeled interneuron) mice suggest that these neurons (n = 6 cells) are sparse in layer 2/3 and show only very modest calcium dye response amplitudes compared to the other nearby cortical neurons. The weakly evoked responses were non-selective to orientation and directions of gratings compared to 10% of nearby neurons (n = 54 cells) showing responses selectively tuned to an orientation and/or direction. The lack of response in these cells could be attributed to non-optimal visual stimulation. To overcome this we used a contrast invariant Gaussian noise movie (Niell 2008) which in principle drives most visually tuned V1 neurons (though not necessarily optimally). The noise stimuli was successful in driving a large number of neurons but the calcium responses from GIN neurons were at the best weak and sparse. These results are in contrast with recent reports of synaptically driven spiking and calcium transients of Martinotti cells (GIN cells) in response to activity in neighboring pyramidal neurons in cortical slices (Kaiser 2004, Fanselow 2008) . We are duplicating our study in P21 (+2) and P56 GIN mice in order to identify developmentally relevant changes. We are also performing a comparative study with parvalbumin and GAD-positive interneurons. Support: Allen Institute founders, Paul G. Allen and Jody Patton