Dendritic Spikes Amplify the Synaptic Signal to Enhance Detection of Motion in a Simulation of the Direction-Selective Ganglion Cell

The On-Off direction-selective ganglion cell (DSGC) in mammalian retinas responds most strongly to a stimulus moving in a specific direction. The DSGC initiates spikes in its dendritic tree, which are thought to propagate to the soma with high probability. Both dendritic and somatic spikes in the DSGC display strong directional tuning, whereas somatic PSPs (postsynaptic potentials) are only weakly directional, indicating that spike generation includes marked enhancement of the directional signal. We used a realistic computational model based on anatomical and physiological measurements to determine the source of the enhancement. Our results indicate that the DSGC dendritic tree is partitioned into separate electrotonic regions, each summing its local excitatory and inhibitory synaptic inputs to initiate spikes. Within each local region the local spike threshold nonlinearly amplifies the preferred response over the null response on the basis of PSP amplitude. Using inhibitory conductances previously measured in DSGCs, the simulation results showed that inhibition is only sufficient to prevent spike initiation and cannot affect spike propagation. Therefore, inhibition will only act locally within the dendritic arbor. We identified the role of three mechanisms that generate directional selectivity (DS) in the local dendritic regions. First, a mechanism for DS intrinsic to the dendritic structure of the DSGC enhances DS on the null side of the cell's dendritic tree and weakens it on the preferred side. Second, spatially offset postsynaptic inhibition generates robust DS in the isolated dendritic tips but weak DS near the soma. Third, presynaptic DS is apparently necessary because it is more robust across the dendritic tree. The pre- and postsynaptic mechanisms together can overcome the local intrinsic DS. These local dendritic mechanisms can perform independent nonlinear computations to make a decision, and there could be analogous mechanisms within cortical circuitry

Potassium channel-based optogenetic silencing

Optogenetics enables manipulation of biological processes with light at high spatio-temporal resolution to control the behavior of cells, networks, or even whole animals. In contrast to the performance of excitatory rhodopsins, the effectiveness of inhibitory optogenetic tools is still insufficient. Here we report a two-component optical silencer system comprising photoactivated adenylyl cyclases (PACs) and the small cyclic nucleotide-gated potassium channel SthK. Activation of this ‘PAC-K’ silencer by brief pulses of low-intensity blue light causes robust and reversible silencing of cardiomyocyte excitation and neuronal firing. In vivo expression of PAC-K in mouse and zebrafish neurons is well tolerated, where blue light inhibits neuronal activity and blocks motor responses. In combination with red-light absorbing channelrhodopsins, the distinct action spectra of PACs allow independent bimodal control of neuronal activity. PAC-K represents a reliable optogenetic silencer with intrinsic amplification for sustained potassium-mediated hyperpolarization, conferring high operational light sensitivity to the cells of interest

In vitro techniques for assessing neurotoxicity using human IPSC-derived neuronal models

The central nervous system consists of a multitude of different neurons and supporting cells that form networks for transmitting neuronal signals. Proper function of the nervous system depends critically on a wide range of highly regulated processes including intracellular calcium homeostasis, neurotransmitter release, and electrical activity. Due to the diversity of cell types and complexity of signaling processes, the (central) nervous system is very vulnerable to toxic insults. Nowadays, a broad range of approaches and cell models is available to study neurotoxicity. In this chapter we show the applicability of human induced pluripotent stem cell (hiPSC)-derived neuronal co-cultures for in vitro neurotoxicity testing. We demonstrate that immunocytochemistry can be used to visualize networks of cultured cells and to differentiate between different cell types. Live cell imaging and electrophysiology techniques demonstrate that the neuronal networks develop spontaneous activity, including synchronized calcium oscillations that coincide with spontaneous changes in membrane potential as well as spontaneous electrical activity with defined (network) bursting. Importantly, as shown in this chapter, spontaneously active human iPSC-derived neuronal co-cultures are suitable for in vitro neurotoxicity assessment. Future application of live imaging and electrophysiological techniques on hiPSC from different donors and/or patients differentiated in different cell types holds great promise for personalized neurotoxicity assessment and safety screening