Tomomi Ichinose

Zinc induces a Src family kinase-mediated up-regulation of NMDA receptor activity and excitotoxicity

Zinc is coreleased with glutamate from excitatory nerve terminals throughout the central nervous system and acutely inhibits N-methyl-d-aspartate (NMDA) receptor activation. Here we report that cultured murine cortical neurons briefly exposed to sublethal concentrations of zinc developed increased intracellular free Na+, phosphorylation of Src kinase at tyrosine 220, and tyrosine phosphorylation of NMDA receptor 2A/2B subunits, in a fashion sensitive to the Src family kinase inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine, PP2. Functionally, this zinc exposure produced a delayed increase in NMDA receptor current in perforated patch but not conventional whole-cell recordings, as well as an increase in NMDA receptor-mediated cell death. These observations suggest that the effect of synaptically released zinc on neuronal NMDA receptors may be biphasic: acute block, followed by Src family kinase-mediated up-regulation of NMDA receptor activity and cytotoxicity.

Sodium Channels in Transient Retinal Bipolar Cells Enhance Visual Responses in Ganglion Cells

Retinal bipolar cells are slow potential neurons that respond to photoreceptor inputs with graded potentials and do not fire action potentials. We found that transient ON bipolar cells recorded in retinal slices possess voltage-gated sodium channels located on either their dendrites or somas. The sodium currents in these neurons did not generate spikes but enhanced voltage responses evoked by visual stimulation, which selectively boosted transmission to transient ganglion cells. In contrast, sodium currents were not found in sustained ON bipolar cells, and light responses in sustained bipolar cells and ganglion cells were not affected by TTX. The presence of sodium channels in transient ON bipolar cells contributed to the separation of transient and sustained signals by selectively enhancing the responses of ON transient ganglion cells to light. Our results suggest that bipolar cell sodium channels augment transient signals and contribute to the temporal segregation of visual information.

Inner and outer retinal pathways both contribute to surround inhibition of salamander ganglion cells

Illumination of the receptive‐field surround reduces the sensitivity of a retinal ganglion cell to centre illumination. The steady, antagonistic receptive‐field surround of retinal ganglion cells is classically attributed to the signalling of horizontal cells in the outer plexiform layer (OPL). However, amacrine cell signalling in the inner plexiform layer (IPL) also contributes to the steady receptive‐field surround of the ganglion cell. We examined the contributions of these two forms of presynaptic lateral inhibition to ganglion cell light sensitivity by measuring the effects of surround illumination on EPSCs evoked by centre illumination. GABAC receptor antagonists reduced inhibition attributed to dim surround illumination, suggesting that this inhibition was mediated by signalling to bipolar cell axon terminals. Brighter surround illumination further reduced the light sensitivity of the ganglion cell. The bright surround effects on the EPSCs were insensitive to GABA receptor blockers. Perturbing outer retinal signalling with either carbenoxolone or cobalt blocked the effects of the bright surround illumination, but not the effects of dim surround illumination. We found that the light sensitivities of presynaptic, inhibitory pathways in the IPL and OPL were different. GABAC receptor blockers reduced dim surround inhibition, suggesting it was mediated in the IPL. By contrast, carbenoxolone and cobalt reduced bright surround, suggesting it was mediated by horizontal cells in the OPL. Direct amacrine cell input to ganglion cells, mediated by GABAA receptors, comprised another surround pathway that was most effectively activated by bright illumination. Our results suggest that surround activation of lateral pathways in the IPL and OPL differently modulate the sensitivity of the ganglion cell to centre illumination.

Ambient Light Regulates Sodium Channel Activity to Dynamically Control Retinal Signaling

The retinal network increases its sensitivity in low-light conditions to detect small visual inputs and decreases its sensitivity in bright-light conditions to prevent saturation. However, the cellular mechanisms that adjust visual signaling in the retinal network are not known. Here, we show that voltage-gated sodium channels in bipolar cells dynamically control retinal light sensitivity. In dim conditions, sodium channels amplified light-evoked synaptic responses mediated by cone pathways. Conversely, in bright conditions, sodium channels were inactivated by dopamine released from amacrine cells, and they did not amplify synaptic inputs, minimizing signal saturation. Our findings demonstrate that bipolar cell sodium channels mediate light adaptation by controlling retinal signaling gain.

Roles of ON cone bipolar cell subtypes in temporal coding in the mouse retina

In the visual system, diverse image processing starts with bipolar cells, which are the second-order neurons of the retina. Thirteen subtypes of bipolar cells have been identified, which are thought to encode different features of image signaling and to initiate distinct signal-processing streams. Although morphologically identified, the functional roles of each bipolar cell subtype in visual signal encoding are not fully understood. Here, we investigated how ON cone bipolar cells of the mouse retina encode diverse temporal image signaling. We recorded bipolar cell voltage changes in response to two different input functions: sinusoidal light and step light stimuli. Temporal tuning in ON cone bipolar cells was diverse and occurred in a subtype-dependent manner. Subtypes 5s and 8 exhibited low-pass filtering property in response to a sinusoidal light stimulus, and responded with sustained fashion to step-light stimulation. Conversely, subtypes 5f, 6, 7, and XBC exhibited bandpass filtering property in response to sinusoidal light stimuli, and responded transiently to step-light stimuli. In particular, subtypes 7 and XBC were high-temporal tuning cells. We recorded responses in different ways to further examine the underlying mechanisms of temporal tuning. Current injection evoked low-pass filtering, whereas light responses in voltage-clamp mode produced bandpass filtering in all ON bipolar cells. These findings suggest that cone photoreceptor inputs shape bandpass filtering in bipolar cells, whereas intrinsic properties of bipolar cells shape low-pass filtering. Together, our results demonstrate that ON bipolar cells encode diverse temporal image signaling in a subtype-dependent manner to initiate temporal visual information-processing pathways.

The Mode of Retinal Presynaptic Inhibition Switches with Light Intensity

Excitatory amino acid transporters (EAATs) terminate signaling in the CNS by clearing released glutamate. Glutamate also evokes an EAAT-mediated Cl− current, but its role in CNS signaling is poorly understood. We show in mouse retina that EAAT-mediated Cl− currents that were evoked by light inhibit rod pathway signaling. EAATs reside on rod bipolar cell axon terminals where GABA and glycine receptors also mediate light-evoked inhibition. We found that the mode of inhibition depended on light intensity. Dim light evoked GABAergic and glycinergic inhibition with rapid kinetics and a large spatial extent. Bright light evoked predominantly EAAT-mediated inhibition with slow kinetics and a small spatial extent. The switch to EAAT-mediated signaling in bright light supplements receptor-mediated signaling to expand the dynamic range of inhibition and contributes to the transition from rod to cone signaling by suppressing rod pathway signaling in bright light conditions.

Ca2+‐independent, but voltage‐ and activity‐dependent regulation of the NMDA receptor outward K+ current in mouse cortical neurons

To test the novel hypothesis that the K+ efflux mediated by NMDA receptors might be regulated differently than the influx of Ca2+ and Na+ through the same receptor channels, NMDA receptor whole‐cell currents carried concurrently or individually by Ca2+, Na+ and K+ were analysed in cultured mouse cortical neurons. In contrast to the NMDA inward current carried by Ca2+ and Na+, the NMDA receptor outward K+ current or NMDA‐K current, recorded either in the presence or absence of extracellular Ca2+ and Na+, and at different or the same membrane potentials, showed much less sensitivity to alterations in intracellular Ca2+ concentration and underwent little rundown. In line with a selective regulation of the NMDA receptor K+ permeability, the ratio of the NMDA inward/outward currents decreased, and the reversal potential of composite NMDA currents recorded in physiological solutions shifted by −8.5 mV after repeated activation of NMDA receptors. Moreover, a depolarizing pre‐pulse of a few seconds or a burst of brief depolarizing pulses selectively augmented the subsequent NMDA‐K current, but not the NMDA inward current. On the other hand, a hyperpolarizing pre‐pulse showed the opposite effect of reducing the NMDA‐K current. The voltage‐ and activity‐dependent regulation of the NMDA‐K current did not require the existence of extracellular Ca2+ or Ca2+ influx; it was, however, affected by the duration of the pre‐pulse and was subject to a time‐dependent decay. The burst of excitatory activity revealed a lasting upregulation of the NMDA‐K current even 5 s after termination of the pre‐pulses. Our data reveal a selective regulation of the NMDA receptor K+ permeability and represent a novel model of voltage‐ and excitatory activity‐dependent plasticity at the receptor level.

Differential effects of TrkC isoforms on sensory axon outgrowth

Neurotrophins are powerful regulators of neuronal morphology. Several lines of evidence are consistent with the idea that characteristic axonal and dendritic morphologies throughout the nervous system may be determined by local patterns of neurotrophin and neurotrophin receptor expression. Neurotrophin receptor tryosine kinases (Trks) exist in both tyrosine‐containing (TK+) and tyrosine‐lacking (TK−) isoforms, both of which are expressed in many neuronal populations. However, ratios of TK+ to TK− isoforms may vary at different stages of development and may be differentially distributed to cellular compartments. To test whether these isoforms have different functions related to axon outgrowth, full‐length or tyrosine kinase‐lacking TrkC receptors were overexpressed in embryonic dorsal root ganglion neurons maintained in explant cultures in neurotrophin‐3 (NT‐3)‐containing media. Neurons were transfected with plasmid DNA encoding enhanced yellow fluorescent protein (EYFP) and TrkC receptor isoforms by particle‐mediated gene transfer. Control neurons possessed 3.7 ± 1.3 primary processes and 113.8 ± 46 branch points. About 80% of the branches were located along the distal part of the axon. Transfection with the trkC TK+ increased the number of primary processes (6.5 ± 2.8), whereas transfection with trkC TK− reduced the formation of primary processes (3.0 ± 1.3). Surprisingly, the distribution of branch points was shifted to the proximal region of axons in neurons transfected with trkC TK−. These observations are consistent with the idea that differential expression of Trk isoforms during development may sculpt axonal morphology. J. Neurosci. Res. 59:365–371, 2000

Differential signalling and glutamate receptor compositions in the OFF bipolar cell types in the mouse retina

Using whole‐cell clamp methods, we characterized the temporal coding in each type of OFF bipolar cell. We found that type 2 and 3a cells are transient, type 1 and 4 cells are sustained, and type 3b cells are intermediate. The light‐evoked excitatory postsynaptic potentials in some types were rectified, suggesting that they provide inputs to the non‐linear ganglion cells. Visual signalling from the photoreceptors was mediated exclusively through the kainate receptors in the transient OFF bipolar cells, whereas both kainate and AMPA receptors contributed in the other cells. This study demonstrates, for the first time, that parallel visual encoding starts at the OFF bipolar cells in a type‐specific manner.

Dopamine D1 receptor expression is bipolar cell type-specific in the mouse retina.

In the retina, dopamine is a key molecule for daytime vision. Dopamine is released by retinal dopaminergic amacrine cells and transmits signaling either by conventional synaptic or by volume transmission. By means of volume transmission, dopamine modulates all layers of retinal neurons; however, it is not well understood how dopamine modulates visual signaling pathways in bipolar cells. Here we analyzed Drd1a‐tdTomato BAC transgenic mice and found that the dopamine D1 receptor (D1R) is expressed in retinal bipolar cells in a type‐dependent manner. Strong tdTomato fluorescence was detected in the inner nuclear layer and localized to type 1, 3b, and 4 OFF bipolar cells and type 5‐2, XBC, 6, and 7 ON bipolar cells. In contrast, type 2, 3a, 5‐1, 9, and rod bipolar cells did not express Drd1a‐tdTomato. Other interneurons were also found to express tdTomato including horizontal cells and a subset (25%) of AII amacrine cells. Diverse visual processing pathways, such as color or motion‐coded pathways, are thought to be initiated in retinal bipolar cells. Our results indicate that dopamine sculpts bipolar cell performance in a type‐dependent manner to facilitate daytime vision. J. Comp. Neurol. 524:2059–2079, 2016.