Calcium spike-mediated digital signaling increases glutamate output at the visual threshold of retinal bipolar cells

Graded spikes differentially signal neurotransmitter input in cerebrospinal fluid contacting neurons of the mouse spinal cord

The action potential and its all-or-none nature is fundamental to neural communication. Canonically, the action potential is initiated once voltage-activated Na⁺ channels are activated, and their rapid kinetics of activation and inactivation give rise to the action potential's all-or-none nature. Here we demonstrate that cerebrospinal fluid contacting neurons (CSFcNs) surrounding the central canal of the mouse spinal cord employ a different strategy. Rather than using voltage-activated Na⁺ channels to generate binary spikes, CSFcNs use two different types of voltage-activated Ca²⁺ channel, enabling spikes of different amplitude. T-type Ca²⁺ channels generate small amplitude spikes, whereas larger amplitude spikes require high voltage-activated Cd²⁺-sensitive Ca²⁺ channels. We demonstrate that these different amplitude spikes can signal input from different transmitter systems; purinergic inputs evoke smaller T-type dependent spikes whereas cholinergic inputs evoke larger spikes that do not rely on T-type channels. Different synaptic inputs to CSFcNs can therefore be signaled by the spike amplitude.

Calcium- and Voltage-Dependent Dual Gating ANO1 is an Intrinsic Determinant of Repolarization in Rod Bipolar Cells of the Mouse Retina

TMEM16A/anoctamin1 (ANO1), a calcium (Ca²⁺)-activated chloride (Cl^-) channel, has many functions in various excitable cells and modulates excitability in both Ca²⁺- and voltage-gating modes. However, its gating characteristics and role in primary neural cells remain unclear. Here, we characterized its Ca²⁺- and voltage-dependent components in rod bipolar cells using dissociated and slice preparations of the mouse retina. The I-V curves of Ca²⁺-dependent ANO1 tail current and voltage-gated Ca²⁺ channel (VGCC) are similar; as ANO1 is blocked by VGCC inhibitors, ANO1 may be gated by Ca²⁺ influx through VGCC. The voltage-dependent component of ANO1 has outward rectifying and sustained characteristics and is clearly isolated by the inhibitory effect of Cl^- reduction and T16Ainh-A01, a selective ANO1 inhibitor, in high EGTA, a Ca²⁺ chelator. The voltage-dependent component disappears due to VGCC inhibition, suggesting that Ca²⁺ is the essential trigger for ANO1. In perforated current-clamping method, the application of T16Ainh-A01 and reduction of Cl^- extended excitation periods in rod bipolar cells, revealing that ANO1 induces repolarization during excitation. Overall, ANO1 opens by VGCC activation during physiological excitation of the rod bipolar cell and has a voltage-dependent component. These two gating-modes concurrently provide the intrinsic characteristics of the membrane potential in rod bipolar cells.

Quantifying the effect of light activated outer and inner retinal inhibitory pathways on glutamate release from mixed bipolar cells

Inhibition mediated by horizontal and amacrine cells in the outer and inner retina, respectively, are fundamental components of visual processing. Here, our purpose was to determine how these different inhibitory processes affect glutamate release from ON bipolar cells when the retina is stimulated with full-field light of various intensities. Light-evoked membrane potential changes (ΔV_m ) were recorded directly from axon terminals of intact bipolar cells receiving mixed rod and cone inputs (Mbs) in slices of dark-adapted goldfish retina. Inner and outer retinal inhibition to Mbs was blocked with bath applied picrotoxin (PTX) and NBQX, respectively. Then, control and pharmacologically modified light responses were injected into axotomized Mb terminals as command potentials to induce voltage-gated Ca²⁺ influx (Q^Ca ) and consequent glutamate release. Stimulus-evoked glutamate release was quantified by the increase in membrane capacitance (ΔC_m ). Increasing depolarization of Mb terminals upon removal of inner and outer retinal inhibition enhanced the ΔV_m /Q^Ca ratio equally at a given light intensity and inhibition did not alter the overall relation between Q^Ca and ΔC_m . However, relative to control, light responses recorded in the presence of PTX and PTX + NBQX increased ΔC_m unevenly across different stimulus intensities: at dim stimulus intensities predominantly the inner retinal GABAergic inhibition controlled release from Mbs, whereas the inner and outer retinal inhibition affected release equally in response to bright stimuli. Furthermore, our results suggest that non-linear relationship between Q^Ca and glutamate release can influence the efficacy of inner and outer retinal inhibitory pathways to mediate Mb output at different light intensities.

Postsynaptic Plasticity Triggered by Ca²⁺-Permeable AMPA Receptor Activation in Retinal Amacrine Cells

Amacrine cells are thought to be a major locus for mechanisms of light adaptation and contrast enhancement in the retina. However, the potential for plasticity in their AMPA receptor currents remains largely unknown. Using paired patch-clamp recordings between bipolar cell terminals and amacrine cells, we have simultaneously measured presynaptic membrane capacitance changes and EPSCs. Repetitive bipolar cell depolarizations, designed to maintain the same amount of exocytosis, nevertheless significantly potentiated evoked EPSCs in a subpopulation of amacrine cells. Likewise, repetitive iontophoresis (or puffs) of glutamate (or AMPA) onto the dendrites of amacrine cells also significantly potentiated evoked currents and [Ca(2+)]i rises. However, strong postsynaptic Ca(2+) buffering with BAPTA abolished the potentiation and selective antagonists of Ca(2+)-permeable AMPA receptors also blocked the potentiation of AMPA-mediated currents. Together these results suggest that Ca(2+) influx via Ca(2+)-permeable AMPA receptors can elicit a rapid form of postsynaptic plasticity in a subgroup of amacrine cell dendrites.

Inhibitory input to the direction-selective ganglion cell is saturated at low contrast

Direction-selective ganglion cells (DSGCs) respond selectively to motion toward a "preferred" direction, but much less to motion toward the opposite "null" direction. Directional signals in the DSGC depend on GABAergic inhibition and are observed over a wide range of speeds, which precludes motion detection based on a fixed temporal correlation. A voltage-clamp analysis, using narrow bar stimuli similar in width to the receptive field center, demonstrated that inhibition to DSGCs saturates rapidly above a threshold contrast. However, for wide bar stimuli that activate both the center and surround, inhibition depends more linearly on contrast. Excitation for both wide and narrow bars was also more linear. We propose that positive feedback, likely within the starburst amacrine cell or its network, produces steep saturation of inhibition at relatively low contrast. This mechanism renders GABA release essentially contrast and speed invariant, which enhances directional signals for small objects and thereby increases the signal-to-noise ratio for direction-selective signals in the spike train over a wide range of stimulus conditions. The steep saturation of inhibition confers to a neuron immunity to noise in its spike train, because when inhibition is strong no spikes are initiated.