From Retinal Waves to Activity-Dependent Retinogeniculate Map Development

A major topic of investigation in the field of developmental neuroscience is to understand how newly formed excitatory synapses receive and transmit information. It is now widely accepted that neural activity plays an important role in a number of developmental processes, including the refinement and establishment of orderly connections (Grubb and Thompson 2004). However, it remains unclear how many immature excitatory synapses (i.e., ones that utilize glutamate as a neurotransmitter) participate in the relay of activity. Contributing to this problem is the finding that immature glutamatergic synapses are comprised largely of N-methyl-D-aspartate (NMDA) receptors and lack AMPA receptors that mediate fast excitatory transmission. Such synapses are said to be “silent,” incapable of causing postsynaptic action potentials from resting levels because in addition to glutamate, the NMDA receptor requires membrane depolarization to relieve a voltage-dependent Mg 2 block. How then do silent synapses “speak” loudly enough to cause postsynaptic firing and thereby successfully relay information? In a recent article, Liu and Chen (2008) provide some answers to this question. They examine the synaptic responses and intrinsic properties of developing neurons by making use of an in vitro thalamic slice preparation that maintains the excitatory synaptic connections between retinal ganglion cells and relay cells of the dorsal lateral geniculate nucleus (LGN). In an elegant series of whole cell recording experiments, they show how a constellation of properties, including ligand-gated channel kinetics, receptor subunit composition, extended presence of neurotransmitter

The spontaneous activity in the developing retina is necessary for the maturation of the neuronal circuitry in visual-associated brain areas. While previous studies have shown that the activation of intrinsically photosensitive retinal ganglion cells (ipRGCs) contributes to visual development, its effects and mechanisms remain largely unclear. Here, using microelectrode array recordings from both male and female mice, we demonstrated that prolong light exposure reduces the interwave interval and coupling distance of Stage 2 retinal waves, which can persist for at least 1 h after light exposure. Notably, these light-induced effects on cholinergic waves were impaired in melanopsin knock-out mice. Additionally, the light-induced retinal wave modulation is mediated by the dopaminergic pathway, primarily through D4 receptors. Using single-molecule fluorescence in situ hybridization, we identified high expression of D2-like receptors, particularly D4R, in the ganglion cell layer and ON-type starburst amacrine cells (SACs) during early postnatal development. Furthermore, we found that gap junction coupling in SACs was increased after light exposure, which can be blocked by D2-like receptor antagonists. Overall, our study reveals that the key properties of spontaneous Stage 2 retinal waves can be regulated by environmental light through ipRGCs. This regulation involves dopaminergic signaling, highlighting the critical role of ipRGC in retinal wave modulation and the convergence of experience-dependent and independent circuitry refinement processes.

Timing and Specificity of Feed-Forward Inhibition within the LGN

Spontaneous retinal waves during development are thought to provide an instructive signal for precise retinotopic mapping by correlating the activity of neighboring retinal ganglion cells. In mutant mice (beta2-/-) that lack correlated waves, retinocollicular map refinement is impaired. In vivo recordings reveal that neurons in the superior colliculus of beta2-/- mice have large receptive fields and low peak visual responses, resulting in a conservation of total integrated response. We find that this "response homeostasis" is maintained on a cell-by-cell basis, and argue that it does not depend on regulation from the visual cortex during adulthood. Instead, in vitro recordings show that homeostasis arises from the conservation of total synaptic input from the retina, and that it is maintained via different mechanisms over development. In the absence of correlated retinal waves, beta2-/- neurons sample a larger number of weaker retinal inputs relative to controls after the first postnatal week. Once retinal waves are restored, developmental learning rules and homeostasis drive refinement so that fewer, stronger synapses are retained, as in wild-type mice, but from a larger retinal area. Homeostasis in neurons has been shown previously to regulate the gain of synaptic transmission in response to perturbations of activity. Our results suggest that during the development of sensory maps, a unique consequence of homeostatic mechanisms is the precise shaping of neuronal receptive fields in conjunction with activity-dependent competition.

Properties of cat retinal ganglion cells: a comparison of W-cells with X- and Y-cells.

Spontaneous retinal waves are critical for the development of the mammalian visual system. However, their role in transcriptional regulation in the retina across the diverse retinal ganglion cell (RGC) types that underpin the detection and transmission of visual features is unclear. Using single-cell RNA sequencing, we analyzed RGC transcriptome from wild-type mice and mice with disrupted retinal waves. We identified several genes that show RGC-type-specific regulation in their expression, including multiple neuropeptides and ion channels. However, wave-dependent changes in the transcriptome were more subtle than developmental changes, indicating that spontaneous activity-dependent molecular changes in retinal ganglion cells are not primarily manifested at the transcriptomic level.

Visual areas I and II of cerebral cortex of rabbit.

Sleep spindles are brief cortical oscillations at 10–15 Hz that occur predominantly during non-REM (quiet) sleep in adult mammals and are thought to contribute to learning and memory. Spindle bursts are phenomenologically similar to sleep spindles, but they occur predominantly in early infancy and are triggered by peripheral sensory activity (e.g., by retinal waves); accordingly, spindle bursts are thought to organize neural networks in the developing brain and establish functional links with the sensory periphery. Whereas the spontaneous retinal waves that trigger spindle bursts in visual cortex are a transient feature of early development, the myoclonic twitches that drive spindle bursts in sensorimotor cortex persist into adulthood. Moreover, twitches—and their associated spindle bursts—occur exclusively during REM (active) sleep. Curiously, despite the persistence of twitching into adulthood, twitch-related spindle bursts have not been reported in adult sensorimotor cortex. This raises the question of whether such spindle burst activity does not occur in adulthood or, alternatively, occurs but has yet to be discovered. If twitch-related spindle bursts do occur in adults, they could contribute to the calibration, maintenance, and repair of sensorimotor systems.

PurposeA lesion to primary visual cortex (V1) in primates can produce retrograde transneuronal degeneration in the dorsal lateral geniculate nucleus (LGN) and retina. We investigated the effect of age at time of lesion on LGN volume and retinal ganglion cell (RGC) density in marmoset monkeys.MethodsRetinas and LGNs were obtained about 2 years after a unilateral left-sided V1 lesion as infants (n = 7) or young adult (n = 1). Antibodies against RBPMS were used to label all RGCs, and antibodies against CaMKII or GABAA receptors were used to label nonmidget RGCs. Cell densities were compared in the left and right hemiretina of each eye. The LGNs were stained with the nuclear marker NeuN or for Nissl substance.ResultsIn three animals lesioned within the first 2 postnatal weeks, the proportion of RGCs lost within 5 mm of the fovea was ∼twofold higher than after lesions at 4 or 6 weeks. There was negligible loss in the animal lesioned at 2 years of age. A positive correlation between RGC loss and LGN volume reduction was evident. No loss of CaMKII-positive or GABAA receptor-positive RGCs was apparent within 2 mm of the fovea in any of the retinas investigated.ConclusionsSusceptibility of marmoset RGCs to transneuronal degeneration is high at birth and declines over the first 6 postnatal weeks. High survival rates of CaMKII and GABAA receptor-positive RGCs implies that widefield and parasol cells are less affected by neonatal cortical lesions than are midget-pathway cells.

Development of visual system circuitry requires the formation of precise synaptic connections between neurons in the retina and brain. For example, axons from retinal ganglion cells (RGCs) form synapses onto...

Ambient GABA modulates firing patterns in adult neural circuits by tonically activating extrasynaptic GABA(A) receptors. Here, we demonstrate that during a developmental period when activation of GABA(A) receptors causes membrane depolarization, tonic activation of GABA(A) receptors blocks all spontaneous activity recorded in retinal ganglion cells (RGCs) and starburst amacrine cells (SACs). Bath application of the GABA(A) receptor agonist muscimol blocked spontaneous correlated increases in intracellular calcium concentration and compound postsynaptic currents in RGCs associated with retinal waves. In addition, GABA(A) receptor agonists activated a tonic current in RGCs that significantly reduced their excitability. Using a transgenic mouse in which green fluorescent protein is expressed under the metabotropic glutamate receptor subtype 2 promoter to target recordings from SACs, we found that GABA(A) receptor agonists blocked compound postsynaptic currents and also activated a tonic current. GABA(A) receptor antagonists reduced the holding current in SACs but not RGCs, indicating that ambient levels of GABA tonically activate GABA(A) receptors in SACs. GABA(A) receptor antagonists did not block retinal waves but did alter the frequency and correlation structure of spontaneous RGC firing. Interestingly, the drug aminophylline, a general adenosine receptor antagonist used to block retinal waves, induced a tonic GABA(A) receptor antagonist-sensitive current in outside-out patches excised from RGCs, indicating that aminophylline exerts its action on retinal waves by direct activation of GABA(A) receptors. These findings have implications for how various neuroactive drugs and neurohormones known to modulate extrasynaptic GABA(A) receptors may influence spontaneous firing patterns that are critical for the establishment of adult neural circuits.

Control of thalamic transmission by corticofugal and ascending reticular pathways in the visual system.

In the developing vertebrate retina, ganglion cells fire spontaneous bursts of action potentials long before the eye becomes exposed to sensory experience at birth. These early bursts are synchronised between neighbouring retinal ganglion cells (RGCs), yielding unique spatiotemporal patterns: 'waves' of activity sweep across large retinal areas every few minutes. Both at retinal and extraretinal levels, these embryonic retinal waves are believed to guide the wiring of the visual system using hebbian mechanisms of synaptic strengthening. In the first part of this review, we recapitulate the evidence for a role of these embryonic spontaneous bursts of activity in shaping developing complex receptive field properties of RGCs in the turtle embryonic retina. We also discuss the role of visual experience in establishing RGC visual functions, and how spontaneous activity and visual experience interact to bring developing receptive fields to maturation. We have hypothesised that the physiological changes associated with development reflect modifications in the dendritic arbours of RGCs, the anatomical substrate of their receptive fields. We demonstrate that there is a temporal correlation between the period of receptive field expansion and that of dendritic growth. Moreover, the immature spontaneous activity contributes to dendritic growth in developing RGCs. Intracellular staining of RGCs reveals, however, that immature receptive fields only rarely show direct correlation with the layout of the corresponding dendritic tree. To investigate the possibility that not only the presence of the spontaneous activity, but even the precise spatiotemporal patterns encoded in retinal waves might contribute to the refinement of retinal neural circuitry, first we must clarify the mechanisms mediating the generation and propagation of these waves across development. In the second part of this review, we present evidence that turtle retinal waves, visualised using calcium imaging, exhibit profound changes in their spatiotemporal patterns during development. From fast waves sweeping across large retinal areas and recruiting many cells on their trajectory at early stages, waves become slower and eventually stop propagating towards hatching, when they become stationary patches of neighbouring coactive RGCs. A developmental switch from excitatory to inhibitory GABAA responses appears to mediate the modification in spontaneous activity patterns while the retina develops. Future chronic studies using specific spatiotemporal alterations of the waves will shed a new light on how the wave dynamics help in sculpting retinal receptive fields.

Partial decussation of sensory pathways allows neural inputs from both sides of the body to project to the same target region where these signals will be integrated. Here, in order to better understand mechanisms of eye-specific targeting, we studied how retinal ganglion cell (RGC) axons terminate in their thalamic target, the dorsal lateral geniculate nucleus (dLGN), when crossing at the optic chiasm midline is altered. In models with gain- and loss-of-function of EphB1, the receptor that directs the ipsilateral projection at the optic chiasm, misrouted RGCs target the appropriate retinotopic zone in the opposite dLGN. However, in EphB1-/- mice, the misrouted axons do not intermingle with normally projecting RGC axons and segregate instead into a distinct patch. We also revisited the role of retinal activity on eye-specific targeting by blocking correlated waves of activity with epibatidine into both eyes. We show that in wild-type mice, retinal waves are necessary during the first postnatal week for both proper distribution and eye-specific segregation of ipsilateral axons in the mature dLGN. Moreover, in EphB1-/- mice, refinement of ipsilateral axons is perturbed in control conditions and is further impaired after epibatidine treatment. Finally, retinal waves are required for the formation of the segregated patch of misrouted axons in EphB1-/- mice. These findings implicate molecular determinants for targeting of eye-specific zones that are independent of midline guidance cues and that function in concert with correlated retinal activity to sculpt retinogeniculate projections.

The ability to process visual stimuli rich with motion represents an essential skill for animal survival and is largely already present at the onset of vision. Although the exact mechanisms underlying its maturation remain elusive, spontaneous activity patterns in the retina, known as retinal waves, have been shown to contribute to this developmental process. Retinal waves exhibit complex spatio-temporal statistics and contribute to the establishment of circuit connectivity and function in the visual system, including the formation of retinotopic maps and the refinement of receptive fields in downstream areas such as the thalamus and visual cortex. Recent work in mice has shown that retinal waves have statistical features matching those of natural visual stimuli, such as optic flow, suggesting that they could prime the visual system for motion processing upon vision onset. Motivated by these findings, we examined whether artificial neural network (ANN) models trained on natural movies show improved performance if pre-trained with retinal waves. We employed the spatio-temporally complex task of next-frame prediction, in which the ANN was trained to predict the next frame based on preceding input frames of a movie. We found that pre-training ANNs with retinal waves enhances the processing of real-world visual stimuli and accelerates learning. Strikingly, when we merely replaced the initial training epochs on naturalistic stimuli with retinal waves, keeping the total training time the same, we still found that an ANN trained on retinal waves temporarily outperforms one trained solely on natural movies. Similar to observations made in biological systems, we also found that pre-training with spontaneous activity refines the receptive field of ANN neurons. Overall, our work sheds light on the functional role of spatio-temporally patterned spontaneous activity in the processing of motion in natural scenes, suggesting it acts as a training signal to prepare the developing visual system for adult visual processing.