Reliability and frequency response of excitatory signals transmitted to different types of retinal ganglion cell

Convergence and Divergence of CRH Amacrine Cells in Mouse Retinal Circuitry

Inhibitory interneurons sculpt the outputs of excitatory circuits to expand the dynamic range of information processing. In mammalian retina, >30 types of amacrine cells provide lateral inhibition to vertical, excitatory bipolar cell circuits, but functional roles for only a few amacrine cells are well established. Here, we elucidate the function of corticotropin-releasing hormone (CRH)-expressing amacrine cells labeled in Cre-transgenic mice of either sex. CRH cells costratify with the ON alpha ganglion cell, a neuron highly sensitive to positive contrast. Electrophysiological and optogenetic analyses demonstrate that two CRH types (CRH-1 and CRH-3) make GABAergic synapses with ON alpha cells. CRH-1 cells signal via graded membrane potential changes, whereas CRH-3 cells fire action potentials. Both types show sustained ON-type responses to positive contrast over a range of stimulus conditions. Optogenetic control of transmission at CRH-1 synapses demonstrates that these synapses are tuned to low temporal frequencies, maintaining GABA release during fast hyperpolarizations during brief periods of negative contrast. CRH amacrine cell output is suppressed by prolonged negative contrast, when ON alpha ganglion cells continue to receive inhibitory input from converging OFF-pathway amacrine cells; the converging ON- and OFF-pathway inhibition balances tonic excitatory drive to ON alpha cells. Previously, it was demonstrated that CRH-1 cells inhibit firing by suppressed-by-contrast (SbC) ganglion cells during positive contrast. Therefore, divergent outputs of CRH-1 cells inhibit two ganglion cell types with opposite responses to positive contrast. The opposing responses of ON alpha and SbC ganglion cells are explained by differing excitation/inhibition balance in the two circuits.SIGNIFICANCE STATEMENT A goal of neuroscience research is to explain the function of neural circuits at the level of specific cell types. Here, we studied the function of specific types of inhibitory interneurons, corticotropin-releasing hormone (CRH) amacrine cells, in the mouse retina. Genetic tools were used to identify and manipulate CRH cells, which make GABAergic synapses with a well studied ganglion cell type, the ON alpha cell. CRH cells converge with other types of amacrine cells to tonically inhibit ON alpha cells and balance their high level of excitation. CRH cells diverge to different types of ganglion cell, the unique properties of which depend on their balance of excitation and inhibition.

Asymmetry between ON and OFF α ganglion cells of mouse retina: integration of signal and noise from synaptic inputs

KEY POINTS

Bipolar and amacrine cells presynaptic to the ON sustained α cell of mouse retina provide currents with a higher signal-to-noise power ratio (SNR) than those presynaptic to the OFF sustained α cell. Yet the ON cell loses proportionately more SNR from synaptic inputs to spike output than the OFF cell does. The higher SNR of ON bipolar cells at the beginning of the ON pathway compensates for losses incurred by the ON ganglion cell, and improves the processing of positive contrasts.

ABSTRACT

ON and OFF pathways in the retina include functional pairs of neurons that, at first glance, appear to have symmetrically similar responses to brightening and darkening, respectively. Upon careful examination, however, functional pairs exhibit asymmetries in receptive field size and response kinetics. Until now, descriptions of how light-adapted retinal circuitry maintains a preponderance of signal over the noise have not distinguished between ON and OFF pathways. Here I present evidence of marked asymmetries between members of a functional pair of sustained α ganglion cells in the mouse retina. The ON cell exhibited a proportionately greater loss of signal-to-noise power ratio (SNR) from its presynaptic arrays to its postsynaptic currents. Thus the ON cell combines signal and noise from its presynaptic arrays of bipolar and amacrine cells less efficiently than the OFF cell does. Yet the inefficiency of the ON cell is compensated by its presynaptic arrays providing a higher SNR than the arrays presynaptic to the OFF cell, apparently to improve visual processing of positive contrasts. Dynamic clamp experiments were performed that introduced synaptic conductances into ON and OFF cells. When the amacrine-modulated conductance was removed, the ON cell's spike train exhibited an increase in SNR. The OFF cell, however, showed the opposite effect of removing amacrine input, which was a decrease in SNR. Thus ON and OFF cells have different modes of synaptic integration with direct effects on the SNR of the spike output.

Selective synaptic connections in the retinal pathway for night vision

The mammalian retina encodes visual information in dim light using rod photoreceptors and a specialized circuit: rods→rod bipolar cells→AII amacrine cell. The AII amacrine cell uses sign-conserving electrical synapses to modulate ON cone bipolar cell terminals and sign-inverting chemical (glycinergic) synapses to modulate OFF cone cell bipolar terminals; these ON and OFF cone bipolar terminals then drive the output neurons, retinal ganglion cells (RGCs), following light increments and decrements, respectively. The AII amacrine cell also makes direct glycinergic synapses with certain RGCs, but it is not well established how many types receive this direct AII input. Here, we investigated functional AII amacrine→RGC synaptic connections in the retina of the guinea pig (Cavia porcellus) by recording inhibitory currents from RGCs in the presence of ionotropic glutamate receptor (iGluR) antagonists. This condition isolates a specific pathway through the AII amacrine cell that does not require iGluRs: cone→ON cone bipolar cell→AII amacrine cell→RGC. These recordings show that AII amacrine cells make direct synapses with OFF Alpha, OFF Delta and a smaller OFF transient RGC type that co-stratifies with OFF Alpha cells. However, AII amacrine cells avoid making synapses with numerous RGC types that co-stratify with the connected RGCs. Selective AII connections ensure that a privileged minority of RGC types receives direct input from the night-vision pathway, independent from OFF bipolar cell activity. Furthermore, these results illustrate the specificity of retinal connections, which cannot be predicted solely by co-stratification of dendrites and axons within the inner plexiform layer.

A Mammalian Retinal Ganglion Cell Implements a Neuronal Computation That Maximizes the SNR of Its Postsynaptic Currents

Neurons perform computations by integrating excitatory and inhibitory synaptic inputs. Yet, it is rarely understood what computation is being performed, or how much excitation or inhibition this computation requires. Here we present evidence for a neuronal computation that maximizes the signal-to-noise power ratio (SNR). We recorded from OFF delta retinal ganglion cells in the guinea pig retina and monitored synaptic currents that were evoked by visual stimulation (flashing dark spots). These synaptic currents were mediated by a decrease in an outward current from inhibitory synapses (disinhibition) combined with an increase in an inward current from excitatory synapses. We found that the SNR of combined excitatory and disinhibitory currents was voltage sensitive, peaking at membrane potentials near resting potential. At the membrane potential for maximal SNR, the amplitude of each current, either excitatory or disinhibitory, was proportional to its SNR. Such proportionate scaling is the theoretically best strategy for combining excitatory and disinhibitory currents to maximize the SNR of their combined current. Moreover, as spot size or contrast changed, the amplitudes of excitatory and disinhibitory currents also changed but remained in proportion to their SNRs, indicating a dynamic rebalancing of excitatory and inhibitory currents to maximize SNR.SIGNIFICANCE STATEMENT We present evidence that the balance of excitatory and disinhibitory inputs to a type of retinal ganglion cell maximizes the signal-to-noise ratio power ratio (SNR) of its postsynaptic currents. This is significant because chemical synapses on a retinal ganglion cell require the probabilistic release of transmitter. Consequently, when the same visual stimulus is presented repeatedly, postsynaptic currents vary in amplitude. Thus, maximizing SNR may be a strategy for producing the most reliable signal possible given the inherent unreliability of synaptic transmission.

Synaptic noise is an information bottleneck in the inner retina during dynamic visual stimulation

In daylight, noise generated by cones determines the fidelity with which visual signals are initially encoded. Subsequent stages of visual processing require synapses from bipolar cells to ganglion cells, but whether these synapses generate a significant amount of noise was unknown. To characterize noise generated by these synapses, we recorded excitatory postsynaptic currents from mammalian retinal ganglion cells and subjected them to a computational noise analysis. The release of transmitter quanta at bipolar cell synapses contributed substantially to the noise variance found in the ganglion cell, causing a significant loss of fidelity from bipolar cell array to postsynaptic ganglion cell. Virtually all the remaining noise variance originated in the presynaptic circuit. Circuit noise had a frequency content similar to noise shared by ganglion cells but a very different frequency content from noise from bipolar cell synapses, indicating that these synapses constitute a source of independent noise not shared by ganglion cells. These findings contribute a picture of daylight retinal circuits where noise from cones and noise generated by synaptic transmission of cone signals significantly limit visual fidelity.

Cross inhibition from ON to OFF pathway improves the efficiency of contrast encoding in the mammalian retina

The retina is divided into parallel and mostly independent ON and OFF pathways, but the ON pathway "cross" inhibits the OFF pathway. Cross inhibition was thought to improve signal processing by the OFF pathway, but its effect on contrast encoding had not been tested experimentally. To quantify the effect of cross inhibition on the encoding of contrast, we presented a dark flash to an in vitro preparation of the mammalian retina. We then recorded excitatory currents, inhibitory currents, membrane voltages, and spikes from OFF α-ganglion cells. The recordings were subjected to an ideal observer analysis that used Bayesian methods to determine how accurately the recordings detected the dark flash. We found that cross inhibition increases the detection accuracy of currents and membrane voltages. Yet these improvements in encoding do not fully reach the spike train, because cross inhibition also hyperpolarizes the OFF α-cell below spike threshold, preventing small signals in the membrane voltages at low contrast from reaching the spike train. The ultimate effect of cross inhibition is to increase the accuracy with which the spike train detects moderate contrast, but reduce the accuracy with which it detects low contrast. In apparent compensation for the loss of accuracy at low contrast, cross inhibition, by hyperpolarizing the OFF α-cell, reduces the number of spikes required to detect the dark flash and thereby increases encoding efficiency.

Differential expression of Brn3 transcription factors in intrinsically photosensitive retinal ganglion cells in mouse

Several subtypes of melanopsin-expressing, intrinsically photosensitive retinal ganglion cells (ipRGCs) have been reported. The M1 type of ipRGCs exhibit distinct properties compared with the remaining (non-M1) cells. They differ not only in their soma size and dendritic arbor, but also in their physiological properties, projection patterns, and functions. However, it is not known how these differences arise. We tested the hypothesis that M1 and non-M1 cells express Brn3 transcription factors differentially. The Brn3 family of class IV POU-domain transcription factors (Brn3a, Brn3b, and Brn3c) is involved in the regulation of differentiation, dendritic stratification, and axonal projection of retinal ganglion cells during development. By using double immunofluorescence for Brn3 transcription factors and melanopsin, and with elaborate morphometric analyses, we show in mouse retina that neither Brn3a nor Brn3c are expressed in ipRGCs. However, Brn3b is expressed in a subset of ipRGCs, particularly those with larger somas and lower melanopsin levels, suggesting that Brn3b is expressed preferentially in the non-M1 cells. By using dendritic stratification to distinguish M1 from non-M1 cells, we found that whereas nearly all non-M1 cells expressed Brn3b, a vast majority of the M1 cells were negative for Brn3b. Interestingly, in the small proportion of the M1 cells that did express Brn3b, the expression level of Brn3b was significantly lower than in the non-M1 cells. These results provide insights about how expression of specific molecules in a ganglion cell could be linked to its role in visual function.

Astrocytes modulate neural network activity by Ca²+-dependent uptake of extracellular K+

Astrocytes are electrically nonexcitable cells that display increases in cytosolic calcium ion (Ca²+) in response to various neurotransmitters and neuromodulators. However, the physiological role of astrocytic Ca²+ signaling remains controversial. We show here that astrocytic Ca²+ signaling ex vivo and in vivo stimulated the Na+,K+-ATPase (Na+- and K+-dependent adenosine triphosphatase), leading to a transient decrease in the extracellular potassium ion (K+) concentration. This in turn led to neuronal hyperpolarization and suppressed baseline excitatory synaptic activity, detected as a reduced frequency of excitatory postsynaptic currents. Synaptic failures decreased in parallel, leading to an increase in synaptic fidelity. The net result was that astrocytes, through active uptake of K+, improved the signal-to-noise ratio of synaptic transmission. Active control of the extracellular K+ concentration thus provides astrocytes with a simple yet powerful mechanism to rapidly modulate network activity.

Why do axons differ in caliber?

CNS axons differ in diameter (d) by nearly 100-fold (∼0.1-10 μm); therefore, they differ in cross-sectional area (d(2)) and volume by nearly 10,000-fold. If, as found for optic nerve, mitochondrial volume fraction is constant with axon diameter, energy capacity would rise with axon volume, also as d(2). We asked, given constraints on space and energy, what functional requirements set an axon's diameter? Surveying 16 fiber groups spanning nearly the full range of diameters in five species (guinea pig, rat, monkey, locust, octopus), we found the following: (1) thin axons are most numerous; (2) mean firing frequencies, estimated for nine of the identified axon classes, are low for thin fibers and high for thick ones, ranging from ∼1 to >100 Hz; (3) a tract's distribution of fiber diameters, whether narrow or broad, and whether symmetric or skewed, reflects heterogeneity of information rates conveyed by its individual fibers; and (4) mitochondrial volume/axon length rises ≥d(2). To explain the pressure toward thin diameters, we note an established law of diminishing returns: an axon, to double its information rate, must more than double its firing rate. Since diameter is apparently linear with firing rate, doubling information rate would more than quadruple an axon's volume and energy use. Thicker axons may be needed to encode features that cannot be efficiently decoded if their information is spread over several low-rate channels. Thus, information rate may be the main variable that sets axon caliber, with axons constrained to deliver information at the lowest acceptable rate.