Retinal waves

Last updated December 03, 2023

Retinal waves are spontaneous bursts of action potentials that propagate in a wave-like fashion across the developing retina. These waves occur before rod and cone maturation and before vision can occur. The signals from retinal waves drive the activity in the dorsal lateral geniculate nucleus (dLGN) and the primary visual cortex. The waves are thought to propagate across neighboring cells in random directions determined by periods of refractoriness that follow the initial depolarization. Retinal waves are thought to have properties that define early connectivity of circuits and synapses between cells in the retina. There is still much debate about the exact role of retinal waves. Some contend that the waves are instructional in the formation of retinogeniculate pathways, while others argue that the activity is necessary but not instructional in the formation of retinogeniculate pathways.

Discovery

One of the first scientists to theorize the existence of spontaneous cascades of electrical activity during retinal development was computational neurobiologist David J. Willshaw. He proposed that adjacent cells generate electrical activity in a wave-like formation through layers of interconnected pre-synaptic and postsynaptic cells. Activity propagating through a close span of pre- and postsynaptic cells is thought to result in strong electrical activity in comparison to pre- and postsynaptic cells that are farther apart, which results in weaker activity. Willshaw thought this difference in the firing strength and the location of cells was responsible for determining the activities' boundaries. The lateral movement of firing from neighboring cell to neighboring cell, starting in one random area of cells and moving throughout both the pre- and postsynaptic layers, is thought to be responsible for the formation of the retinotopic map. To simulate the cascade of electrical activity, Willshaw wrote a computer program to demonstrate the movement of electrical activity between pre- and postsynaptic cell layers. What Willshaw called "spontaneous patterned electrical activity" is today referred to as "retinal waves."^[1]

From this purely theoretical concept, Italian scientists Lucia Galli and Lamberto Maffei used animal models to observe electrical activity in ganglion cells of the retina. Before Galli and Maffei, retinal ganglion cell activity had never been recorded during prenatal development. To study ganglion activity, Galli and Maffei used premature rat retinas, between embryonic days 17 and 21, to record electrical activity. Several isolated, single cells were used for this study. The recordings showed cell activity was catalyzed from ganglion cells. Galli and Maffei speculated that the electrical activity seen in the retinal ganglion cells may be responsible for the formation of retinal synaptic connections and for the projections of retinal ganglion cells to the superior colliculus and lateral geniculate nucleus (LGN).^[2]

As the idea of retinal waves became established, neurobiologist Carla Shatz used calcium imaging and microelectrode recording to visualize the movement of action potentials in a wave-like formation. For more information on calcium imaging and microelectrode recording, see section below. The calcium imaging showed ganglion cells initiating the formation of retinal waves, along with adjacent amacrine cells, which take part in the movement of the electrical activity. Microelectrode recordings were also thought to show LGN neurons being driven by the wave-like formation of electrical activity across neighboring retinal ganglion cells. From these results, it was suggested that the waves of electrical activity were responsible for driving the pattern of spatiotemporal activity and also playing a role in the formation of the visual system during prenatal development.^[3]

Rachel Wong is another researcher involved in the study of retinal waves. Wong speculated that electrical activity, within the retina, is involved in the organization of retinal projections during prenatal development. More specifically, the electrical activity may be responsible for the segregation and organization of the dLGN. Wong also speculated that specific parts of the visual system, such as the ocular dominance columns, require some form of electrical activity in order to develop completely. She also believed being able to figure out the signals encoded by retinal waves, may allow scientists to better understand how retinal waves play a role in retinal development.^[4]

Some of the most recent research being conducted is attempting to better understand the encoded signals of retinal waves during development. According to research conducted by Evelyne Sernagor, it is thought that retinal waves are not just necessary for their spontaneous electrical activity but are also responsible for encoding information to be used in the formation of spatiotemporal patterns allowing retinal pathways to become more refined. Using turtles to test this concept, Sernagor used calcium imaging to look at the change in retinal waves during various stages of retinal development. From the study, at the very first stages of development, retinal waves fire quickly and repeatedly, causing what is thought to be a large wave of action potentials across the retina. However, as the turtle nears completion of development, the retinal waves gradually stop spreading and instead become immobile clumps of retinal ganglion cells. This is thought to be a result of GABA changing from excitatory to inhibitory during continual retinal development. Whether the change in retinal wave formation during development is unique to turtles, is still largely unknown.^[5]

Observation of waves in other systems

Spontaneous generation and propagation of waves is seen elsewhere in developing circuits. Similar synchronized spontaneous activity early in development has been seen in neurons of the hippocampus, spinal cord, and auditory nuclei.^[6] Patterned activity shaping neuronal connections and control of synaptic efficiency in multiple systems including the retina are important for understanding interaction between presynaptic and postsynaptic cells that create precise connections essential to the function of the nervous system.^[4]

Development

During development, communication via synapse is important between amacrine cells and other retinal interneurons as well as ganglion cells, which act as a substrate for retinal waves.^[4] There are three stages of development that characterize retinal wave activity in mammals. Before birth, the waves are mediated by non-synaptic currents, waves during the period from birth until 10 days after birth are mediated by the neurotransmitter acetylcholine acting on nicotinic acetylcholine receptors, and waves during the third period, from 10 days after birth to 2 weeks, are mediated by ionotropic glutamate receptors.^[7]

Chemical synapses during the cholinergic wave period involve the starburst amacrine cells (SACs) releasing acetylcholine onto other SACs, which then propagate waves. During this period, cholinergic wave production exceeds wave production via gap junctions, of which the signals are quite reduced. This signaling happens before bipolar cells form connections in the inner plexiform layer. SACs are thought to be the source of retinal waves because spontaneous depolarizations have been observed without synaptic excitation.^[7]

Cholinergic wave activity eventually dies out, and the release of glutamate in bipolar cells generates waves.^[7] Bipolar cells differentiate later than amacrine and ganglion cells, which could be the cause for this change in wave behavior.^[4] The change from cholinergic mediation to glutamatergic mediation occurs when bipolar cells make their first synaptic connections with ganglion cells.^[7] Glutamate, the neurotransmitter contained in bipolar cells, generates spontaneous activity in ganglion cells. Waves are still present after bipolar cells establish synaptic connection with amacrine and ganglion cells.^[4]

Additional activity involved in retinal waves includes the following. In certain species, GABA appears to play a role in the frequency and duration of the bursts in ganglion cells. The interactions in cells vary in different test subjects and at different maturity levels, especially the complex interactions mediated by amacrine cells. Activity propagated via gap junctions has not been observed in all test subjects; for example, research has shown that ferret retina ganglion cells are not coupled. Other studies have shown that extracellular excitatory agents such as potassium could be instrumental in wave propagation. Research suggests that synaptic networks of amacrine and ganglion cells are necessary for the production of waves. Broadly put, waves are produced and continue over a relatively long developmental period, during which new cellular components of the retina and synapses are added. Variation in the mechanisms of retinal waves account for diversity in the connections between cells and the maturation of processes in the retina.^[4]

Activity pattern of waves

Waves are generated at random but limited spatially due to a refractory period in cells after bursts of action potentials have been produced. After a wave has been propagated in one place, it cannot be propagated in the same place again. Wave-induced refractory areas last about 40 to 60 seconds. Research suggests that every region of the retina has an equal probability of generating and propagating a wave. The refractory period also determines the velocity (distance between wave fronts per unit of time) and periodicity (average time interval between wave-induced calcium transients or depolarizations recorded in a particular neuron in the ganglion cell layer). The density of refractory cells corresponds to how fast retinal waves propagate; for instance, if there is a low number or density of refractory cells, the velocity of propagation will be high.^[8]

Experimental procedures

Visualization of waves

Two primary methods of visualizing retinal waves are the use of calcium imaging and multielectrode array. Calcium imaging allows analysis of wave pattern over a large area of the retina (more than with multielectrode recording). Imaging as such has allowed researchers to investigate spatiotemporal properties or waves as well as wave mechanism and function in development.

Disrupting waves

There are three main techniques currently used to disrupt retinal waves: intraocular injection of pharmacological substances that alter wave patterns, use of immunotoxins that eliminate certain classes of amacrine cells, or use of knockout mouse lines that have altered spontaneous firing patterns.^[9] There are several pharmacological agents that can be used to disrupt retinal activity. Tetrodotoxin (TTX) can be injected near the optic tract to block incoming retinal activity in addition to the outgoing activity of lateral geniculate neurons.^[10] Intraocular injections of epibatidine, a cholinergic agonist, can be used to block spontaneous firing in half of all retinal ganglion cells and cause uncorrelated firing in the remaining half.^[9] Effects of the pharmacological agents on retinal ganglion cell activity are observed using either MEA or calcium imaging. Immunotoxins can be used to target starburst amacrine cells. Starburst amacrine cells are retinal interneurons responsible for cholinergic retinal waves.^[9] The third method is to use knockout mice with altered spontaneous firing patterns. The most common line of mouse for this method is the neuronal nicotinic acetylcholine receptor beta-2 subunit knockout (β2-nAChR-KO). β2-nAChR-KO mice have been observed to have reduced eye-specific retinotopic refinement similar to epitbatidine injection as well as no correlated waves, as observed with calcium imaging and MEA recording.^[9]

Controversial role in neuronal development

There is currently still much controversy about whether retinal waves play an 'instructive' or a 'permissive' role in the formation of eye-specific projections in the retinogeniculate pathway. Injections of pharmacological agents prevents the formation of eye-specific retinogeniculate inputs, which indicates that retinal waves play some role in the formation. β2-nAChR-KO mice have been found to have altered patterns of spontaneous firing. It is important to note that while experiments done in knock-out lines to date have helped to explain some things about retinal waves, only experiments done in vivo at normal body temperature and in a normal chemical environment can truly determine what the true pattern of firing is in the knock-out animals.^[9]^[10]

Instructive argument

Retinal wave activity has been found to coincide with the period in which eye-specific retinogeniculate projections are formed. This temporal overlap would be necessary for a causal relationship. TTX injections in fetal cats prevented the formation of eye-specific retinogeniculate projections, which indicates that neuronal activity is necessary for the formation of eye-specific layers.^[10] After treatment with epibatidine, the lack of correlated firing in the remaining half of retinal ganglion cells despite the robust firing as well as the lack of eye-specific layer formation can be indicated as proof that the waves play an instructional role.^[9] Calcium imaging observation following immunotoxin use showed that some correlated firing still remained where coupled voltage clamp recording showed significant reduction in correlated firing.^[9] The remaining correlated firing could explain the formation of eye-specific retinogeniculate projections that was found. Using calcium imaging and MEA recording these cells have shown to have no correlated firing. Instead, reduced firing rates have been observed, and depolarization in one cell seemed to inhibit surrounding cells.^[9] The altered firing pattern of the β2-nAChR-KO mice is also controversial as there has been some evidence that correlated firing still occurs in the knock-out mice, as detailed in the next section.

Permissive argument

Retinal waves have been found while eye-specific retinogeniculate pathways are formed; however, it is important to note that in all species studied to date retinal waves begin prior to and continue after these eye-specific pathways are formed. It also is noted that some species in which retinal waves have been documented to have projections that are crossed. This suggests that retinal waves can be present and not play an instructive role in eye-specific inputs.^[10] There are several issues to be considered when looking at data from use of pharmacological substance to block retinal activity. First, the long-term effects of treatment with TTX are unknown, as it is not yet possible to monitor the retinal activity for a long duration in an intact animal.^[10] The finding that long-term injection of TTX did not inhibit and instead merely delayed eye-specific layer formation could be explained then by the reduced effects of TTX on retinal activity at a longer duration. This supports the argument that blocking all retinal activity prevents eye-specific projection formation remains to be determined.^[10] Furthermore, since immunotoxin treatment to kill starburst amacrine cells shows no difference in the formation of eye-specific retinogeniculate projections while treatment with epibatidine does, it could suggest that some sort of retinal activity is essential for the eye-specific layer formation, but not retinal waves.^[10] One study showed that β2-nAChR-KO mice did still have robust retinal wave activity, unlike previously reported; however, they found that the retinal waves were propagated using gap junctions in the knock-out line, instead of cholinergic transmission wild-type mice display.^[10]

Related Research Articles

The retina is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then processes that image within the retina and sends nerve impulses along the optic nerve to the visual cortex to create visual perception. The retina serves a function which is in many ways analogous to that of the film or image sensor in a camera.

The development of the nervous system, or neural development (neurodevelopment), refers to the processes that generate, shape, and reshape the nervous system of animals, from the earliest stages of embryonic development to adulthood. The field of neural development draws on both neuroscience and developmental biology to describe and provide insight into the cellular and molecular mechanisms by which complex nervous systems develop, from nematodes and fruit flies to mammals.

A photoreceptor cell is a specialized type of neuroepithelial cell found in the retina that is capable of visual phototransduction. The great biological importance of photoreceptors is that they convert light into signals that can stimulate biological processes. To be more specific, photoreceptor proteins in the cell absorb photons, triggering a change in the cell's membrane potential.

<span class="mw-page-title-main">Retinal ganglion cell</span> Type of cell within the eye

A retinal ganglion cell (RGC) is a type of neuron located near the inner surface of the retina of the eye. It receives visual information from photoreceptors via two intermediate neuron types: bipolar cells and retina amacrine cells. Retina amacrine cells, particularly narrow field cells, are important for creating functional subunits within the ganglion cell layer and making it so that ganglion cells can observe a small dot moving a small distance. Retinal ganglion cells collectively transmit image-forming and non-image forming visual information from the retina in the form of action potential to several regions in the thalamus, hypothalamus, and mesencephalon, or midbrain.

The receptive field, or sensory space, is a delimited medium where some physiological stimuli can evoke a sensory neuronal response in specific organisms.

<span class="mw-page-title-main">Retina bipolar cell</span> Type of neuron

As a part of the retina, bipolar cells exist between photoreceptors and ganglion cells. They act, directly or indirectly, to transmit signals from the photoreceptors to the ganglion cells.

Motion perception is the process of inferring the speed and direction of elements in a scene based on visual, vestibular and proprioceptive inputs. Although this process appears straightforward to most observers, it has proven to be a difficult problem from a computational perspective, and difficult to explain in terms of neural processing.

<span class="mw-page-title-main">Electroretinography</span>

Electroretinography measures the electrical responses of various cell types in the retina, including the photoreceptors, inner retinal cells, and the ganglion cells. Electrodes are placed on the surface of the cornea or on the skin beneath the eye to measure retinal responses. Retinal pigment epithelium (RPE) responses are measured with an EOG test with skin-contact electrodes placed near the canthi. During a recording, the patient's eyes are exposed to standardized stimuli and the resulting signal is displayed showing the time course of the signal's amplitude (voltage). Signals are very small, and typically are measured in microvolts or nanovolts. The ERG is composed of electrical potentials contributed by different cell types within the retina, and the stimulus conditions can elicit stronger response from certain components.

<span class="mw-page-title-main">Amacrine cell</span> Interneuron cells in the retina of the eye

In the anatomy of the eye, amacrine cells are interneurons in the retina. They are named from Greek a– 'non', makr– 'long', and in– 'fiber', because of their short neuronal processes. Amacrine cells are inhibitory neurons, and they project their dendritic arbors onto the inner plexiform layer (IPL), they interact with retinal ganglion cells, and bipolar cells or both of these.

<span class="mw-page-title-main">Retina horizontal cell</span>

Horizontal cells are the laterally interconnecting neurons having cell bodies in the inner nuclear layer of the retina of vertebrate eyes. They help integrate and regulate the input from multiple photoreceptor cells. Among their functions, horizontal cells are believed to be responsible for increasing contrast via lateral inhibition and adapting both to bright and dim light conditions. Horizontal cells provide inhibitory feedback to rod and cone photoreceptors. They are thought to be important for the antagonistic center-surround property of the receptive fields of many types of retinal ganglion cells.

Intrinsically photosensitive retinal ganglion cells (ipRGCs), also called photosensitive retinal ganglion cells (pRGC), or melanopsin-containing retinal ganglion cells (mRGCs), are a type of neuron in the retina of the mammalian eye. The presence of ipRGCs was first suspected in 1927 when rodless, coneless mice still responded to a light stimulus through pupil constriction, This implied that rods and cones are not the only light-sensitive neurons in the retina. Yet research on these cells did not advance until the 1980s. Recent research has shown that these retinal ganglion cells, unlike other retinal ganglion cells, are intrinsically photosensitive due to the presence of melanopsin, a light-sensitive protein. Therefore, they constitute a third class of photoreceptors, in addition to rod and cone cells.

Retinotopy is the mapping of visual input from the retina to neurons, particularly those neurons within the visual stream. For clarity, 'retinotopy' can be replaced with 'retinal mapping', and 'retinotopic' with 'retinally mapped'.

Ocular dominance columns are stripes of neurons in the visual cortex of certain mammals that respond preferentially to input from one eye or the other. The columns span multiple cortical layers, and are laid out in a striped pattern across the surface of the striate cortex (V1). The stripes lie perpendicular to the orientation columns.

In the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another neuron or to the target effector cell.

Leo M. Chalupa is a Ukrainian-American Neuropsychologist who was Vice President for Research and is Professor of Pharmacology and Physiology at George Washington University. He was previously a Distinguished Professor of Ophthalmology and Neurobiology at the University of California, Davis and Chairman of the Department of Neurobiology, Physiology and Behavior where he also served as the Director of the UC Davis Center for Neuroscience and Interim Dean of the College of Biological Sciences.

Non-spiking neurons are neurons that are located in the central and peripheral nervous systems and function as intermediary relays for sensory-motor neurons. They do not exhibit the characteristic spiking behavior of action potential generating neurons.

Retinal precursor cells are biological cells that differentiate into the various cell types of the retina during development. In the vertebrate, these retinal cells differentiate into seven cell types, including retinal ganglion cells, amacrine cells, bipolar cells, horizontal cells, rod photoreceptors, cone photoreceptors, and Müller glia cells. During embryogenesis, retinal cells originate from the anterior portion of the neural plate termed the eye field. Eye field cells with a retinal fate express several transcription factor markers including Rx1, Pax6, and Lhx2. The eye field gives rise to the optic vesicle and then to the optic cup. The retina is generated from the precursor cells within the inner layer of the optic cup, as opposed to the retinal pigment epithelium that originate from the outer layer of the optic cup. In general, the developing retina is organized so that the least-committed precursor cells are located in the periphery of the retina, while the committed cells are located in the center of the retina. The differentiation of retinal precursor cells into the mature cell types found in the retina is coordinated in time and space by factors within the cell as well as factors in the environment of the cell. One example of an intrinsic regulator of this process is the transcription factor Ath5. Ath5 expression in retinal progenitor cells biases their differentiation into a retinal ganglion cell fate. An example of an environmental factor is the morphogen sonic hedge hog (Shh). Shh has been shown to repress the differentiation of precursor cells into retinal ganglion cells.

Tripartite synapse refers to the functional integration and physical proximity of the presynaptic membrane, postsynaptic membrane, and their intimate association with surrounding glia as well as the combined contributions of these three synaptic components to the production of activity at the chemical synapse. Tripartite synapses occur at a number of locations in the central nervous system with astrocytes and may also exist with Muller glia of retinal ganglion cells and Schwann cells at the neuromuscular junction. The term was first introduced in the late 1990s to account for a growing body of evidence that glia are not merely passive neuronal support cells but, instead, play an active role in the integration of synaptic information through bidirectional communication with the neuronal components of the synapse as mediated by neurotransmitters and gliotransmitters.

Douglas G. McMahon is a professor of Biological Sciences and Pharmacology at Vanderbilt University. McMahon has contributed several important discoveries to the field of chronobiology and vision. His research focuses on connecting the anatomical location in the brain to specific behaviors. As a graduate student under Gene Block, McMahon identified that the basal retinal neurons (BRNs) of the molluscan eye exhibited circadian rhythms in spike frequency and membrane potential, indicating they are the clock neurons. He became the 1986 winner of the Society for Neuroscience's Donald B. Lindsley Prize in Behavioral Neuroscience for his work. Later, he moved on to investigate visual, circadian, and serotonergic mechanisms of neuroplasticity. In addition, he helped find that constant light can desynchronize the circadian cells in the suprachiasmatic nucleus (SCN). He has always been interested in the underlying causes of behavior and examining the long term changes in behavior and physiology in the neurological modular system. McMahon helped identifying a retrograde neurotransmission system in the retina involving the melanopsin containing ganglion cells and the retinal dopaminergic amacrine neurons.

Frank Werblin is Professor of the Graduate School, Division of Neurobiology at the University of California, Berkeley.

References

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