In recent years, multielectrode arrays and large silicon probes have been developed to record simultaneously between hundreds and thousands of electrodes packed with a high density. However, they require novel methods to extract the spiking activity of large ensembles of neurons. Here, we developed a new toolbox to sort spikes from these large-scale extracellular data. To validate our method, we performed simultaneous extracellular and loose patch recordings in rodents to obtain ‘ground truth’ data, where the solution to this sorting problem is known for one cell. The performance of our algorithm was always close to the best expected performance, over a broad range of signal-to-noise ratios, in vitro and in vivo. The algorithm is entirely parallelized and has been successfully tested on recordings with up to 4225 electrodes. Our toolbox thus offers a generic solution to sort accurately spikes for up to thousands of electrodes.

In the mammalian retina, excitatory and inhibitory circuitries enable retinal ganglion cells (RGCs) to signal the occurrence of visual features to higher brain areas. This functionality disappears in certain diseases of retinal degeneration because of the progressive loss of photoreceptors. Recent work in a mouse model of retinal degeneration (rd1) found that, although some intraretinal circuitry is preserved and RGCs maintain characteristic physiological properties, they exhibit increased and aberrant rhythmic activity. Here, extracellular recordings were made to assess the degree of aberrant activity in adult rd1 retinas and to investigate the mechanism underlying such behavior. A multi-transistor array with thousands of densely packed sensors allowed for simultaneous recordings of spiking activity in populations of RGCs and of local field potentials (LFPs). The majority of identified RGCs displayed rhythmic (7-10 Hz) but asynchronous activity. The spiking activity correlated with the LFPs, whichreflectanaveragesynchronizedexcitatoryinputtotheRGCs.LFPsinitiatedfromrandompositionsandpropagatedacrosstheretina.They disappeared when ionotrophic glutamate receptors or electrical synapses were blocked. They persisted in the presence of other pharmacological blockers, including TTX and inhibitory receptor antagonists. Our results suggest that excitation-transmitted laterally through a network of electrically coupled interneurons-leads to large-scale retinal network oscillations, reflected in the rhythmic spiking of most rd1 RGCs. This result may explain forms of photopsias reported by blind patients, while the mechanism involved should be considered in future treatment strategies targeting the disease of retinitis pigmentosa.

A hybrid circuit of a semiconductor chip and synaptically connected neurons was implemented and characterized. Individual nerve cells from the snail Lymnaea stagnalis were immobilized on a silicon chip by microscopic picket fences of polyimide. The cells formed a network with electrical synapses after outgrowth in brain conditioned medium. Pairs of neurons were electronically interfaced for noninvasive stimulation and recording. Voltage pulses were applied to a capacitive stimulator on the chip to excite the attached neuron. Signals were transmitted in the neuronal net and elicited an action potential in a second neuron. The postsynaptic excitation modulated the current of a transistor on the chip. The implementation of the silicon-neuron-neuron-silicon circuit constitutes a proof-of-principle experiment for the development of neuroelectronic systems to be used in studies on neuronal signal processing, neurocomputation, and neuroprosthetics.

Eickenscheidt M, Jenkner M, Thewes R, Fromherz P, Zeck G. Electrical stimulation of retinal neurons in epiretinal and subretinal configuration using a multicapacitor array. J Neurophysiol 107: 2742-2755, 2012. First published February 22, 2012 doi:10.1152/jn.00909.2011.-Electrical stimulation of retinal neurons offers the possibility of partial restoration of visual function. Challenges in neuroprosthetic applications are the long-term stability of the metal-based devices and the physiological activation of retinal circuitry. In this study, we demonstrate electrical stimulation of different classes of retinal neurons with a multicapacitor array. The array-insulated by an inert oxideallows for safe stimulation with monophasic anodal or cathodal current pulses of low amplitude. Ex vivo rabbit retinas were interfaced in either epiretinal or subretinal configuration to the multicapacitor array. The evoked activity was recorded from ganglion cells that respond to light increments by an extracellular tungsten electrode. First, a monophasic epiretinal cathodal or a subretinal anodal current pulse evokes a complex burst of action potentials in ganglion cells. The first action potential occurs within 1 ms and is attributed to direct stimulation. Within the next milliseconds additional spikes are evoked through bipolar cell or photoreceptor depolarization, as confirmed by pharmacological blockers. Second, monophasic epiretinal anodal or subretinal cathodal currents elicit spikes in ganglion cells by hyperpolarization of photoreceptor terminals. These stimuli mimic the photoreceptor response to light increments. Third, the stimulation symmetry between current polarities (anodal/cathodal) and retina-array configuration (epi/sub) is confirmed in an experiment in which stimuli presented at different positions reveal the center-surround organization of the ganglion cell. A simple biophysical model that relies on voltage changes of cell terminals in the transretinal electric field above the stimulation capacitor explains our results. This study provides a comprehensive guide for efficient stimulation of different retinal neuronal classes with low-amplitude capacitive currents.

Human daylight vision depends on cone photoreceptors and their degeneration results in visual impairment and blindness as observed in several eye diseases including age-related macular degeneration, cone-rod dystrophies, or late stage retinitis pigmentosa, with no cure available. Preclinical cell replacement approaches in mouse retina have been focusing on rod dystrophies, due to the availability of sufficient donor material from the rod-dominated mouse retina, leaving the development of treatment options for cone degenerations not well studied. Thus, an abundant and traceable source for donor cone-like photoreceptors was generated by crossing neural retina leucine zipper-deficient (Nrl 2/2 ) mice with an ubiquitous green fluorescent protein (GFP) reporter line resulting in double transgenic tg(Nrl 2/2 ; aGFP) mice. In Nrl 2/2 retinas, all rods are converted into cone-like photoreceptors that express CD73 allowing their enrichment by CD73-based magnetic activated cell sorting prior transplantation into the subretinal space of adult wild-type, cone-only (Nrl 2/2 ), or cone photoreceptor function loss 1 (Cpfl1) mice. Donor cells correctly integrated into host retinas, acquired mature photoreceptor morphology, expressed cone-specific markers, and survived for up to 6 months, with significantly increased integration rates in the cone-only Nrl 2/2 retina. Individual retinal ganglion cell recordings demonstrated the restoration of photopic responses in cone degeneration mice following transplantation suggesting, for the first time, the feasibility of daylight vision repair by cell replacement in the adult mammalian retina. STEM CELLS 2015;33:79-90