Dissociated GABAergic retinal interneurons exhibit spontaneous increases in intracellular calcium

Somatostatin receptor-mediated suppression of gabaergic synaptic transmission in cultured rat retinal amacrine cells

Somatostatin (SRIF) modulates neurotransmitter release by activating the specific receptors (sst1-sst5). Our previous study showed that sst5 receptors are expressed in rat retinal GABAergic amacrine cells. Here, we investigated modulation of GABA release by SRIF in cultured amacrine cells, using patch-clamp techniques. The frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) in the amacrine cells was significantly reduced by SRIF, which was partially reversed by BIM 23056, an sst5 receptor antagonist, and was further rescued by addition of CYN-154806, an sst2 receptor antagonist. Both nimodipine, an L-type Ca2+ channel blocker, and ω-conotoxin GVIA, an N-type Ca2+ channel blocker, suppressed the sIPSC frequency, and in the presence of nimodipine and ω-conotoxin GVIA, SRIF failed to further suppress the sIPSC frequency. Extracellular application of forskolin, an activator of adenylate cyclase, increased the sIPSC frequency, while the membrane permeable protein kinase A (PKA) inhibitor Rp-cAMP reduced it, and in the presence of Rp-cAMP, SRIF did not change sIPSCs. However, SRIF persisted to suppress the sIPSCs in the presence of KT5823, a protein kinase G (PKG) inhibitor. Moreover, pre-incubation with Bis IV, a protein kinase C (PKC) inhibitor, or pre-application of xestospongin C, an inositol 1,4,5-trisphosphate receptor (IP3R) inhibitor, SRIF still suppressed the sIPSC frequency. All these results suggest that SRIF suppresses GABA release from the amacrine cells by inhibiting presynaptic Ca2+ channels, in part through activating sst5/sst2 receptors, a process that is mediated by the intracellular cAMP-PKA signaling pathway.

Dissection, culture, and analysis of Xenopus laevis embryonic retinal tissue

The process by which the anterior region of the neural plate gives rise to the vertebrate retina continues to be a major focus of both clinical and basic research. In addition to the obvious medical relevance for understanding and treating retinal disease, the development of the vertebrate retina continues to serve as an important and elegant model system for understanding neuronal cell type determination and differentiation(1-16). The neural retina consists of six discrete cell types (ganglion, amacrine, horizontal, photoreceptors, bipolar cells, and Müller glial cells) arranged in stereotypical layers, a pattern that is largely conserved among all vertebrates (12,14-18). While studying the retina in the intact developing embryo is clearly required for understanding how this complex organ develops from a protrusion of the forebrain into a layered structure, there are many questions that benefit from employing approaches using primary cell culture of presumptive retinal cells (7,19-23). For example, analyzing cells from tissues removed and dissociated at different stages allows one to discern the state of specification of individual cells at different developmental stages, that is, the fate of the cells in the absence of interactions with neighboring tissues (8,19-22,24-33). Primary cell culture also allows the investigator to treat the culture with specific reagents and analyze the results on a single cell level (5,8,21,24,27-30,33-39). Xenopus laevis, a classic model system for the study of early neural development (19,27,29,31-32,40-42), serves as a particularly suitable system for retinal primary cell culture (10,38,43-45). Presumptive retinal tissue is accessible from the earliest stages of development, immediately following neural induction (25,38,43). In addition, given that each cell in the embryo contains a supply of yolk, retinal cells can be cultured in a very simple defined media consisting of a buffered salt solution, thus removing the confounding effects of incubation or other sera-based products (10,24,44-45). However, the isolation of the retinal tissue from surrounding tissues and the subsequent processing is challenging. Here, we present a method for the dissection and dissociation of retinal cells in Xenopus laevis that will be used to prepare primary cell cultures that will, in turn, be analyzed for calcium activity and gene expression at the resolution of single cells. While the topic presented in this paper is the analysis of spontaneous calcium transients, the technique is broadly applicable to a wide array of research questions and approaches (Figure 1).

Retinal remodeling

Retinal photoreceptor degeneration takes many forms. Mutations in rhodopsin genes or disorders of the retinal pigment epithelium, defects in the adenosine triphosphate binding cassette transporter, ABCR gene defects, receptor tyrosine kinase defects, ciliopathies and transport defects, defects in both transducin and arrestin, defects in rod cyclic guanosine 3',5'-monophosphate phosphodiesterase, peripherin defects, defects in metabotropic glutamate receptors, synthetic enzymatic defects, defects in genes associated with signaling, and many more can all result in retinal degenerative disease like retinitis pigmentosa (RP) or RP-like disorders. Age-related macular degeneration (AMD) and AMD-like disorders are possibly due to a constellation of potential gene targets and gene/gene interactions, while other defects result in diabetic retinopathy or glaucoma. However, all of these insults as well as traumatic insults to the retina result in retinal remodeling. Retinal remodeling is a universal finding subsequent to retinal degenerative disease that results in deafferentation of the neural retina from photoreceptor input as downstream neuronal elements respond to loss of input with negative plasticity. This negative plasticity is not passive in the face of photoreceptor degeneration, with a phased revision of retinal structure and function found at the molecular, synaptic, cell, and tissue levels involving all cell classes in the retina, including neurons and glia. Retinal remodeling has direct implications for the rescue of vision loss through bionic or biological approaches, as circuit revision in the retina corrupts any potential surrogate photoreceptor input to a remnant neural retina. However, there are a number of potential opportunities for intervention that are revealed through the study of retinal remodeling, including therapies that are designed to slow down photoreceptor loss, interventions that are designed to limit or arrest remodeling events, and optogenetic approaches that target appropriate classes of neurons in the remnant neural retina.

Cellular mechanisms underlying spatiotemporal features of cholinergic retinal waves

Before vision, a transient network of recurrently connected cholinergic interneurons, called starburst amacrine cells (SACs), generates spontaneous retinal waves. Despite an absence of robust inhibition, cholinergic retinal waves initiate infrequently and propagate within finite boundaries. Here, we combine a variety of electrophysiological and imaging techniques and computational modeling to elucidate the mechanisms underlying these spatial and temporal properties of waves in developing mouse retina. Waves initiate via rare spontaneous depolarizations of SACs. Waves propagate through recurrent cholinergic connections between SACs and volume release of ACh as demonstrated using paired recordings and a cell-based ACh optical sensor. Perforated-patch recordings and two-photon calcium imaging reveal that individual SACs have slow afterhyperpolarizations that induce SACs to have variable depolarizations during sequential waves. Using a computational model in which the properties of SACs are based on these physiological measurements, we reproduce the slow frequency, speed, and finite size of recorded waves. This study represents a detailed description of the circuit that mediates cholinergic retinal waves and indicates that variability of the interneurons that generate this network activity may be critical for the robustness of waves across different species and stages of development.

Network deficiency exacerbates impairment in a mouse model of retinal degeneration

Neural oscillations play an important role in normal brain activity, but also manifest during Parkinson’s disease, epilepsy, and other pathological conditions. The contribution of these aberrant oscillations to the function of the surviving brain remains unclear. In recording from retina in a mouse model of retinal degeneration (RD), we found that the incidence of oscillatory activity varied across different cell classes, evidence that some retinal networks are more affected by functional changes than others. This aberrant activity was driven by an independent inhibitory amacrine cell oscillator. By stimulating the surviving circuitry at different stages of the neurodegenerative process, we found that this dystrophic oscillator further compromises the function of the retina. These data reveal that retinal remodeling can exacerbate the visual deficit, and that aberrant synaptic activity could be targeted for RD treatment.

Histamine elevates free intracellular calcium in mouse retinal dopaminergic cells via H1-receptors

PURPOSE

Previously, retinopetal axons containing histamine and dopaminergic neurons expressing histamine H(1)-receptor had been localized in mouse retinas using anatomic techniques. The goal of these experiments was to demonstrate that these receptors are functional.

METHODS

Dopaminergic cells were acutely isolated from retinas of transgenic mice expressing red fluorescent protein under control of the tyrosine hydroxylase promoter and loaded with the calcium indicator Fura-2.

RESULTS

Under control conditions, there were spontaneous oscillations in the levels of free intracellular calcium in dopaminergic cells. These oscillations were abolished in nominally calcium-free extracellular medium and in 1 μM tetrodotoxin, findings suggesting that the oscillations were mediated by calcium entry across the plasma membrane in response to sodium-dependent action potentials. Histamine increased the mean free intracellular calcium in the dopaminergic cells by increasing the frequency and/or amplitude of the calcium oscillations. The effects of histamine were dose-dependent and reached maximum at 5 μM. With this dose, there was a 65% increase in the mean free intracellular calcium concentration. The histamine H(1)-receptor antagonist, pyrilamine, blocked the effects of 5 μM histamine when applied at 50 μM. The selective histamine H(1)-receptor agonists, 2-(3-trifluoromethylphenyl) histamine and methylhistaprodifen significantly increased mean free intracellular calcium when applied at 5 μM.

CONCLUSIONS

Histamine released from retinopetal axons in the mouse retina can elevate intracellular calcium levels in the perikarya of dopaminergic cells via the activation of histamine H(1)-receptors.

Non-cell-autonomous factor induces the transition from excitatory to inhibitory GABA signaling in retina independent of activity

During development, the effect of activating GABA(A) receptors switches from depolarizing to hyperpolarizing. Several environmental factors have been implicated in the timing of this GABA switch, including neural activity, although these observations remain controversial. By using acutely isolated retinas from KO mice and pharmacological manipulations in retinal explants, we demonstrate that the timing of the GABA switch in retinal ganglion cells (RGCs) is unaffected by blockade of specific neurotransmitter receptors or global activity. In contrast to RGCs in the intact retina, purified RGCs remain depolarized by GABA, indicating that the GABA switch is not cell-autonomous. Indeed, purified RGCs cocultured with dissociated cells from the superior colliculus or cultured in media conditioned by superior collicular cells undergo a normal switch. Thus, a diffusible signal that acts independent of local circuit activity regulates the maturation of GABAergic inhibition in mouse RGCs.

Spontaneous activity in the developing mouse midbrain driven by an external pacemaker

Central nervous system (CNS) development depends upon spontaneous activity (SA) to establish networks. We have discovered that the mouse midbrain has SA expressed most robustly at embryonic day (E) 12.5. SA propagation in the midbrain originates in midline serotonergic cell bodies contained within the adjacent hindbrain and then passes through the isthmus along ventral midline serotonergic axons. Once within the midbrain, the wave bifurcates laterally along the isthmic border and then propagates rostrally. Along this trajectory, it is carried by a combination of GABAergic and cholinergic neurons. Removing the hindbrain eliminates SA in the midbrain. Thus, SA in the embryonic midbrain arises from a single identified pacemaker in a separate brain structure, which drives SA waves across both regions of the developing CNS. The midbrain can self-initiate activity upon removal of the hindbrain, but only with pharmacological manipulations that increase excitability. Under these conditions, new initiation foci within the midbrain become active. Anatomical analysis of the development of the serotonergic axons that carry SA from the hindbrain to the midbrain indicates that their increasing elongation during development may control the onset of SA in the midbrain.

Neural reprogramming in retinal degeneration

PURPOSE

Early visual defects in degenerative diseases such as retinitis pigmentosa (RP) may arise from phased remodeling of the neural retina. The authors sought to explore the functional expression of ionotropic (iGluR) and group 3, type 6 metabotropic (mGluR6) glutamate receptors in late-stage photoreceptor degeneration.

METHODS

Excitation mapping with organic cations and computational molecular phenotyping were used to determine whether retinal neurons displayed functional glutamate receptor signaling in rodent models of retinal degeneration and a sample of human RP.

RESULTS

After photoreceptor loss in rodent models of RP, bipolar cells lose mGluR6 and iGluR glutamate-activated currents, whereas amacrine and ganglion cells retain iGluR-mediated responsivity. Paradoxically, amacrine and ganglion cells show spontaneous iGluR signals in vivo even though bipolar cells lack glutamate-coupled depolarization mechanisms. Cone survival can rescue iGluR expression by OFF bipolar cells. In a case of human RP with cone sparing, iGluR signaling appeared intact, but the number of bipolar cells expressing functional iGluRs was double that of normal retina.

CONCLUSIONS

RP triggers permanent loss of bipolar cell glutamate receptor expression, though spontaneous iGluR-mediated signaling by amacrine and ganglion cells implies that such truncated bipolar cells still release glutamate in response to some nonglutamatergic depolarization. Focal cone-sparing can preserve iGluR display by nearby bipolar cells, which may facilitate late RP photoreceptor transplantation attempts. An instance of human RP provides evidence that rod bipolar cell dendrite switching likely triggers new gene expression patterns and may impair cone pathway function.