Neural stimulation and recording with high spatial and temporal precision is desirable for studying the real-time dynamics of neural networks, as well as developing possible therapeutic treatments for neurological diseases. Electrophysiological techniques are commonly used to monitor the activity of many neurons simultaneously with high temporal resolution. Especially, multi-electrode recording arrays (MEAs) allow to simultaneously record the spiking activity of tens of neurons and/or local field potentials at tens of recoding sites within a neural network.
On the other hand, recent developments in "optogenetics", genetic modification of brain cells to render them light sensitized by the presence of light sensitive membrane ion channel proteins such as channelrhodopsin (ChR2) or halorhodopsin (NpHR), now allow the precise spatiotemporal optical manipulation of the neuronal activity with cell specificity. With optogenetics the activity of the targeted neurons can be stimulated or suppressed by light with millisecond temporal resolution, contrary to conventional electrical/chemical stimulation techniques which are limited in their spatial targeting and/or speed of actuation.
(a) Hybrid microelectrode array with geometrically matched tapered optical waveguide
To utilize the advantages of both MEA recordings and optogenetics for studying brain function, we developed a novel dual-modality hybrid device, which consists of a tapered coaxial optical waveguide (‘optrode’) integrated into a 100 element intra-cortical MEA (Figure 1). This device offers the prospect of enabling local delivery of optical stimulation via a single optrode while the simultaneous multi-site recoding of the neural activity by electrophysiological means, both in the vicinity of and distant to the stimulation site. A concept sketch outlining the hybrid microelectrode array (Figure 1a) shows the Si-based Pt-tipped MEA (Figure 1c) but now with any specific electrode(s) replaced by a geometrically commensurate tapered optical waveguide for optical access and information retrieval. The implementation of this device (Figure 1c) is successfully used in rodents, in vivo and in vitro as explained in specific examples below.
(b) Single optrode as a tapered optical waveguide and an electrode
The optical waveguide in the hybrid MEA, the single optrode (Figure 2), is designed to simultaneously (i) deliver light for targeted neural stimulation/inhibition, and (ii) record the local neural activity electrophysiologically. The optrode is constructed from a tapered fiber coated with gold everywhere except the very tip leaving an aperture of few microns in diameter for local light delivery (Figure 2b). For allowing electrophysiological recordings via the optrode, the gold coating is insulated with a UV-curable epoxy except the final 50 µm of the tapered tip. This configuration results electrode impedances of 100-800 kΩ which is suitable for spike and local field potential recordings. The dual functionality of the optrode, both as a single element and integrated with a hybrid microelectrode array, is demonstrated by the examples below.
(c) Testing of dual functionality of the optrode in mouse retina
We have tested the dual optical delivery and electrical recording capability of a single optrode in in vitro preparations of mouse retina (Figure 3a). Either a wide spread white light (not through the optrode) or 532 nm laser light through the optrode was used as photo-stimulus while the activity of ganglion cells were recorded by the optrode (Figure 3b and c). The recordings show that the particular cell responded the OFF phase of light stimulus which is a characteristics of an OFF-transient ganglion cell.
(d) Dual modality hybrid MEA device in ChR2 expressing mouse cortical slices
Subsequentlly, a dual-modality full array device was then used in ChR2 transfected mouse brain slices. Specifically, epileptiform events have been reliably optically triggered by the optrode and their spatiotemporal patterns were simultaneously recordedby the multi-electrode array (Figures 4). In his experiments we demonstrate that local delivery of a stimulus, “write-in”, in a neural circuit can create an extended network response, in this case a propagating epileptic wave. Thus the extension of the experimental strategy to the related circumstances of spatio-temporal mapping of information storage and dynamics in the working memory should be feasible – at unprecedented detail.