Action Potential Generation at an Axon Initial Segment-Like Process in the Axonless Retinal AII Amacrine Cell

Abstract

In axon-bearing neurons, action potentials conventionally initiate at the axon initial segment (AIS) and are important for neuron excitability and cell-to-cell communication. However in axonless neurons, spike origin has remained unclear. Here we report in the axonless spiking AII amacrine cell of the mouse retina a dendritic process sharing organizational and functional similarities with the AIS. This process was revealed through viral-mediated expression of channelrhodopsin-2-GFP (ChR2-GFP) with the AIS-targeting motif of sodium channels (NavII-III). The AII-processes showed clustering of voltage-gated Na+ channel 1.1 (Nav1.1) as well as AIS markers ankyrin-G and neurofascin. Furthermore, NavII-III targeting disrupted Nav1.1 clustering in the AII-process which drastically decreased Na+ current and abolished the ability of the AII amacrine cell to generate spiking. Our findings indicate that despite lacking an axon, spiking in the axonless neuron can originate at a specialized AIS-like process.

Electrical synapses between AII amacrine cells in the retina: Function and modulation

“…The presence of I Na has been verified in several later studies, as has the involvement of this current in the ability of AII amacrine cells to generate low-amplitude spikes (Boos et al, 1993;Cembrowski et al, 2012;Tamalu and Watanabe, 2007;Veruki and Hartveit, 2002a, b). Immunocytochemistry and in situ hybridization studies suggest that the channels are of the Na v 1.1 type, with an intriguing localization at a specific lobular appendage (Cembrowski et al, 2012;Kaneko and Watanabe, 2007;Wu et al, 2011), seemingly corresponding to the "morphologically distinct processes" identified in an earlier study by van Wart et al (2005). Physiological evidence for expression of voltage-gated I Ca was provided by Habermann et al (2003) and imaging experiments suggested a preferential location in the region of the cell body and lobular appendages, consistent with a role in mediating transmitter release in the chemical synapses with OFF-cone bipolar cells and ganglion cells.…”

AII amacrine cells: quantitative reconstruction and morphometric analysis of electrophysiologically identified cells in live rat retinal slices imaged with multi-photon excitation microscopy

“…4). This lobular dendrite is likely to correspond to the process identified as displaying a cluster of voltage-gated Na + channels (Wu et al 2011; Cembrowski et al 2012) and having a characteristic ultrastructure in electron microscopic investigations (Tsukamoto and Omi 2013). It is also possible to observe that lobular dendrites can extend into the inner nuclear layer (Fig.…”

“…Despite morphological variability, these cells display a set of common characteristics that together contribute to defining them as a cell type. AII amacrine cells have been characterized as axon-less, narrow-field, bistratified retinal interneurons and their general morphological characteristics have been identified in a variety of different mammalian species at the light microscopic level, including cat (Kolb and Famiglietti 1974; Famiglietti and Kolb 1975; Kolb et al 1981; Vaney 1985), dog (Cajal 1892; Famiglietti and Kolb 1975), mouse (Wu et al 2011; Cembrowski et al 2012), primate (Polyak 1941; Boycott and Dowling 1969; Famiglietti and Kolb 1975; Kolb et al 1992; Wässle et al 1995), rabbit (Dacheux and Raviola 1986; Mills and Massey 1991; Vaney et al 1991), and rat (Perry and Walker 1980; Boos et al 1993; Wässle et al 1993; Mørkve et al 2002). The number of studies of AII amacrine cells at the electron microscopic level is smaller, but includes cat (Kolb and Famiglietti 1974; Famiglietti and Kolb 1975; Kolb 1979), mouse (Tsukamoto and Omi 2013), primate (Wässle et al 1995), rabbit (Dacheux and Raviola 1986; Strettoi et al 1992; Marc et al 2014), and rat (Chun et al 1993).…”

Synaptic Vesicle Exocytosis at the Dendritic Lobules of an Inhibitory Interneuron in the Mammalian Retina

“…Light flashes evoked depolarizing responses during the current clamp segment of the protocol as shown in Figure 8B. Note that small action potentials (spikelets) are present in the AII-AC light response and these are likely generated by the axon-initial-segment-like process of AII-AC (Wu et al, 2011; Cembrowski et al, 2012). As shown in Figure 8C, the light stimuli produced ΔC m jumps that were correlated with light intensity.…”