Development of synapses between chick retinal neurons in dispersed culture

Inhibition of endocytosis suppresses the nitric oxide-dependent release of Cl- in retinal amacrine cells

Our lab has previously shown that nitric oxide (NO) can alter the synaptic response properties of amacrine cells by releasing Cl- from internal acidic compartments. This alteration in the Cl- gradient brings about a positive shift in the reversal potential of the GABA-gated current, which can convert inhibitory synapses into excitatory synapses. Recently, we have shown that the cystic fibrosis transmembrane regulator (CFTR) Cl- channel is involved in the Cl- release. Here, we test the hypothesis that (acidic) synaptic vesicles are a source of NO-releasable Cl- in chick retinal amacrine cells. If SVs are a source of Cl-, then depleting synaptic vesicles should decrease the nitric oxide-dependent shift in the reversal potential of the GABA-gated current. The efficacy of four inhibitors of dynamin (dynasore, Dyngo 4a, Dynole 34-2, and MiTMAB) were evaluated. In order to deplete synaptic vesicles, voltage-steps were used to activate V-gated Ca2+ channels and stimulate the synaptic vesicle cycle either under control conditions or after treatment with the dynamin inhibitors. Voltage-ramps were used to measure the NO-dependent shift in the reversal potential of the GABA-gated currents under both conditions. Our results reveal that activating the synaptic vesicle cycle in the presence of dynasore or Dyngo 4a blocked the NO-dependent shift in EGABA. However, we also discovered that some dynamin inhibitors reduced Ca2+ signaling and L-type Ca2+ currents. Conversely, dynasore also increased neurotransmitter release at autaptic sites. To further resolve the mechanism underlying the inhibition of the NO-dependent shift in the reversal potential for the GABA-gated currents, we also tested the effects of the clathrin assembly inhibitor Pitstop 2 and found that this compound also inhibited the shift. These data provide evidence that dynamin inhibitors have multiple effects on amacrine cell synaptic transmission. These data also suggest that inhibition of endocytosis disrupts the ability of NO to elicit Cl- release from internal stores which may in part be due to depletion of synaptic vesicles.

Discrete influx events refill depleted Ca2+ stores in a chick retinal neuron

The depletion of ER Ca2+ stores, following the release of Ca2+ during intracellular signalling, triggers the Ca2+ entry across the plasma membrane known as store-operated calcium entry (SOCE). We show here that brief, local [Ca2+]i increases (motes) in the thin dendrites of cultured retinal amacrine cells derived from chick embryos represent the Ca2+ entry events of SOCE and are initiated by sphingosine-1-phosphate (S1P), a sphingolipid with multiple cellular signalling roles. Externally applied S1P elicits motes but not through a G protein-coupled membrane receptor. The endogenous precursor to S1P, sphingosine, also elicits motes but its action is suppressed by dimethylsphingosine (DMS), an inhibitor of sphingosine phosphorylation. DMS also suppresses motes induced by store depletion and retards the refilling of depleted stores. These effects are reversed by exogenously applied S1P. In these neurons formation of S1P is a step in the SOCE pathway that promotes Ca2+ entry in the form of motes.

Endocannabinoid signaling regulates spontaneous transmitter release from embryonic retinal amacrine cells

GABAergic amacrine cells, cultured from embryonic chick retina, display spontaneous mini frequencies ranging from 0-4.6 Hz as a result of the release of quanta of transmitter from both synapses and autapses. We show here that at least part of this variation originates from differences in the degree to which endocannabinoids, endogenously generated within the culture, are present at terminals presynaptic to individual cells. Though all cells examined scored positive for cannabinoid receptor type I (CB1R), only those showing a low initial rate of spontaneous minis responded to CB1R agonists with an increase in mini frequency, caused by a Gi/o-mediated reduction in [cAMP]. Cells displaying a high initial rate of spontaneous minis, on the other hand, were unaffected by CB1R agonists, but they did show a rate decrease with CB1R antagonists. Such a regulation of spontaneous transmitter release by endocannabinoids might be important in network maintenance in amacrine cells and other inhibitory interneurons.

Onset of synaptogenesis in the plexiform layers of the chick retina: A transmission electron microscopic study

The presently acknowledged onset of synaptogenesis in the chick retina from embryonic day 12 (E12) onward stands in contrast with the appearance of spontaneous electrical activity, of presynaptic proteins, or of neurotransmitters during early formation of the inner (E6-E8) and outer (E9) plexiform layers. Therefore, we investigated the chick retina from E6 to E12 at which age first synapses appear by transmission electron microscopy (TEM). The study provides evidence that synaptogenesis in the chick retina begins shortly after the plexiform layers have started to emerge. The first synapses are electrical synapses, which appear on E7, one day after the future inner plexiform layer emerged, and towards the end of E8 in the nascent outer plexiform layer. Conventional chemical synapses appear in both plexiform layers on E8, in the inner plexiform layer (stage 34) only a few hours earlier than in the outer plexiform layer (stage 35). The first synapses are formed close to the apex of the optic fissure and their frequency increases rapidly with age. The onset, the topography, and the developmental course of synaptogenesis correlate with the chronotopic course of maturation of retinal neurons and the age when spontaneous electrical activity occurs in the retina.

Calcium From Internal Stores Triggers GABA Release From Retinal Amacrine Cells

The Ca2+ that promotes transmitter release is generally thought to enter presynaptic terminals through voltage-gated Ca2+channels. Using electrophysiology and Ca2+ imaging, we show that, in amacrine cell dendrites, at least some of the Ca2+ that triggers transmitter release comes from endoplasmic reticulum Ca2+ stores. We show that both inositol 1,4,5-trisphosphate receptors (IP3Rs) and ryanodine receptors (RyRs) are present in these dendrites and both participate in the elevation of cytoplasmic [Ca2+] during the brief depolarization of a dendrite. Only the Ca2+ released through IP3Rs, however, seems to promote the release of transmitter. Antagonists for the IP3R reduced transmitter release, whereas RyR blockers had no effect. Application of an agonist for metabotropic glutamate receptor, known to liberate Ca2+ from internal stores, enhanced both spontaneous and evoked transmitter release.

L-type calcium channels mediate transmitter release in isolated, wide-field retinal amacrine cells

Transmitter release in neurons is triggered by intracellular Ca2+ increase via the opening of voltage-gated Ca2+ channels. Here we investigated the voltage-gated Ca2+ channels in wide-field amacrine cells (WFACs) isolated from the white-bass retina that are functionally coupled to transmitter release. We monitored transmitter release through the measurement of the membrane capacitance (Cm). We found that 500-ms long depolarizations of WFACs from −70 mV to 0 mV elicited about a 6% transient increase in the Cm or membrane surface area. This Cm jump could be eliminated either by intracellular perfusion with 10 mM BAPTA or by extracellular application of 4 mM cobalt. WFACs possess N-type and L-type voltage-gated Ca2+ channels. Depolarization-evoked Cm increases were unaffected by the specific N-type channel blocker ω-conotoxin GVIA, but they were markedly reduced by the L-type blocker diltiazem, suggesting a role for the L-type channel in synaptic transmission. Further supporting this notion, in WFACs the synaptic protein syntaxin always colocalized with the pore-forming subunit of the retinal specific L-type channels (CaV1.4 or α1F), but never with that of the N-type channels (CaV2.2 or α1B).

Synaptic development in semi-dissociated cultures of rat retina

Cultured neurons provide a simpler and more accessible environment to study the synaptic physiology. However, it is not clear if development of synapses in culture is similar to that in the in vivo condition. We studied the developmental sequence and morphological differentiation of chemical synapses in semi-dissociated rat retinal cultures that consisted of dissociated neurons as well as undissociated retinal aggregates. Synapses were quantified by synaptophysin immunoreactive puncta. During second week of in vitro development the average number of chemical synapses on the cell body decreased while that on the neurites increased significantly. Conventional synapses appeared both in aggregate and in dissociated neurons, with the developmental profile similar to that reported for in vivo retina. In contrast, the development of ribbon synapses was adversely affected by the in vitro microenvironment as suggested by following observations. The ribbon synapses were more frequently found in aggregate than in dissociated neurons, and were not associated with dyadic or triadic synaptic arrangement. The photoreceptor ribbons did not contact a postsynaptic process while bipolar ribbons made single (monadic) synapses. Further, photoreceptor ribbons in dissociated neurons were late to form and took more time to mature as compared to those in the aggregate cultures. Most of the rod bipolar cells, identified by their immunoreactivity to protein kinase C (PKC), had three or more neurites. Unlike in the in vivo retina, the dissociated rod bipolar cells did not show any PKC immunoreactive varicosities, suggesting that they failed to develop a well-differentiated synaptic terminal. Interestingly, we did not find any parvalbumin positive AII amacrine cells that are normally postsynaptic to rod bipolar cells. These results show that the conventional synapses of retina, which are similar to chemical synapses in other parts of the brain, develop normally both in aggregate and dissociated neurons. However, the highly specialized ribbon synapses have more stringent developmental requirements, and their normal development may require the presence of postsynaptic neurons in their close vicinity.

Blockade of cadherin-6B activity perturbs the distribution of PSD-95 family proteins in retinal neurones

BACKGROUND

Synaptic junctions have cadherin-catenin complexes, but their functions are poorly understood. Using retinal neurones, we investigated the role of this adhesion machinery in synaptic organization.

RESULTS

In cultures of chicken retinal cells, cadherin-6B (cad6B) and cadherin-7 (cad7) are expressed by distinct neurones, each being distributed in a punctate pattern along their neurites as well as in the soma. Double-immunostaining for cad6B and PSD-95/SAP90 or other PSD-95 family members, known to localize in the postsynaptic density, showed that their distributions overlapped each other. To assess the role for cad6B, we incubated retinal cells with antibodies that could specifically block cad6B-mediated adhesion. In the antibody-treated neurones, the localization pattern of PSD-95 family proteins was altered, that is, their staining signals tended to be reduced or disarranged. We then examined whether cadherins interacted molecularly with PSD-95: Cadherin immunoprecipitates from brain lysates did not contain PSD-95; nevertheless, this protein was co-precipitated with alphaN- and beta-catenins. When PSD-95 proteins were ectopically expressed in epithelial cells, some of these molecules were concentrated in cell-cell junctions, co-localizing with E-cadherin, and this junctional localization of PSD-95 was abolished by blocking of E-cadherin activity.

CONCLUSION

These results suggest that cadherins play a role in the subcellular organization of postsynaptic density components through some, perhaps indirect, molecular interactions.

Synapse formation and agrin expression in stratospheroid cultures from embryonic chick retina

Stratospheroids are three-dimensional cellular spheres which develop in vitro through the proliferation and differentiation of retinal neuroepithelial precursor cells. We investigated synapse formation in stratospheroids by analyzing the development of aggregates of synapse-associated molecules and of electron microscopically identifiable synaptic specializations. Our results show that the first aggregates of the GABA(A) receptor, the glycine receptor, and gephyrin appear in the inner plexiform layer after 8 days in culture simultaneously with the development of the first active zones and postsynaptic densities. In contrast, presynaptic molecules including synaptophysin could be detected in the inner plexiform layer before synaptogenesis, suggesting functions for these molecules in addition to neurotransmitter exocytosis at mature synapses. Similar to the retina in vivo, synapses were not found in the nuclear layers of stratospheroids. We also analyzed the isoform pattern, expression, and distribution of the extracellular matrix molecule agrin, a key regulator during formation, maintenance, and regeneration of the neuromuscular junction. In stratospheroids, several agrin isoforms were expressed as highly glycosylated proteins with an apparent molecular weight of approximately 400 kDa, similar to the molecular weight of agrin in the retina in vivo. The expression specifically of the neuronal isoforms of agrin was concurrent with the onset of synaptogenesis. Moreover, the neuronal agrin isoforms were exclusively found in the synapse-containing inner plexiform layer, whereas other agrin isoforms were associated also with the inner limiting membrane and with Müller glial cells. These results show that synapse formation is very similar in stratospheroids and in the retina in vivo, and they suggest an important role for agrin during CNS development.

Early in vitro development of voltage- and transmitter-gated currents in GABAergic amacrine cells

It has been shown in previous studies that a subpopulation of neurons in monolayer cultures prepared from immature embryonic chicken retina acquired a series of functional properties which characterized them as GABAergic amacrine cells after 1 week in vitro. In the present study, we demonstrate that immature precursors of these cells were already identifiable by morphological criteria after 2 days in vitro (DIV). Using the whole cell patch-clamp technique we have studied the time-course of the expression of voltage-dependent and of glutamate and GABA receptor-associated conductances in these identified retinal interneurons developing in vitro. Recordings after 2 DIV revealed a very homogeneous pattern of membrane conductances. In all cells tested, whole cell responses to depolarizing voltage steps consisted solely of a sustained outward potassium current and 100% of the cells responded to the glutamate receptor agonist kainic acid (KA) and to GABA. Fast activating inward sodium currents first appeared after 3 DIV, whereas a transient component of outward potassium currents was not detectable before day 4 in vitro. N-Methyl-D-aspartate (NMDA)-evoked currents were first observed at 3 DIV in the GABAergic neurons. Only 1 day later they were found in all of the GABAergic neurons. Expression of responses to quisqualic acid (QU) started at 3 DIV, but remained restricted to a subpopulation of the GABAergic cells even at later stages (59% at 4 DIV, 63% at 6-9 DIV). Antagonistic effects of QU on KA responses, however, were detectable in all cells tested, independent of the developmental stage and the presence of QU-evoked currents.(ABSTRACT TRUNCATED AT 250 WORDS)

Synapse formation in dissociated cell cultures of embryonic chick cerebral neurons

The development of synapses was confirmed in the primary cultures of dissociated cerebral cortex neurons from chick embryos. Whole-cell patch clamp recording applied to dissociated neurons from 6- to 12-day-old embryos revealed that these neurons form functional synapses. In these cultures, both excitatory and inhibitory postsynaptic responses were observed. Synaptogenesis in our cultures seemed to be in proportion to the embryonic equivalent days, which are the sum of the days in incubation and culture.

Fatty acid composition of phospholipids from chick neural retina during development

The fatty acid composition of retina phospholipids from developing chicks was investigated to determine what changes, if any, occur in the relative levels of polyunsaturated fatty acids. Embryonic chicks were killed at 3-day intervals from day 6 through hatching (day 21), and at 1 week post-hatch. Fatty acids were prepared from retina phospholipids and were analysed by capillary gas-liquid chromatography. A comparison of the composition of yolk taken on day 6 with retinas isolated on that day revealed a much greater proportion of polyunsaturated fatty acids in the latter, suggesting an ability of the embryo to metabolize selectively unsaturated fatty acids at this early stage of development. Throughout the time course studied, saturated fatty acids constituted 50% of all fatty acids, most of which was due to palmitic acid (16:0; 33-41%). Among other saturated fatty acids, myristic acid (14:0) increased to maximal levels by day 18, then declined, while stearic acid (18:0) was minimal on day 12 and then increased. Polyunsaturated fatty acids varied between 14 and 23% of total fatty acids, depending on the developmental stage. One of the most remarkable changes in polyunsaturated fatty acids occurred in the levels of 22:4 (n-6). The proportion of this single fatty acid decreased from 9.4 to 2.4% between days 15 and 18. Relative levels of 22:5 (n-6) increased significantly between day 21 and 1 week post-hatch, from 1.1 to 3.2%. In this same time period, the proportion of 22:6 (n-3), the fatty acid known to be prominent in the outer segments of rod-dominant retinas, did not change.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of functional calcium channels in cultured avian photoreceptors

Vertebrate photoreceptors are unusual neurons in that they are capable of continuous calcium-mediated release of neurotransmitter (Trifonov, 1968; Hagins et al., 1970). In this study, we have examined the development and characteristics of calcium currents in chick cone cells placed in culture on embryonic day 8. Cone cells were identified by their lectin-binding properties, rhodopsin-like immunoreactivity, and the presence of an oil droplet. Using the whole-cell patch-clamp method, we have seen calcium currents in these cells after three days in culture, slightly before the appearance of synapses (Gleason & Wilson, 1989). Because cone calcium currents are blocked by cadmium and nifedipine but are enhanced by Bay K 8644, they most closely resemble L-type current (Nowycky et al., 1985). An unexpected feature of these currents is that their gating ranges varied widely between cells so that some cells showed the foot of their activation range at -70 mV and others as positive as -25 mV. Calcium imaging of fura-2 loaded cells was used to confirm the time course of calcium current development and describe the distribution of cytosolic calcium. As expected, depolarization of young cells failed to increase cytosolic calcium but in older cells an increase of threefold to fourfold was usually observed. Both at rest and during depolarization, most cone cells showed regional differences in internal calcium concentration. In the most mature cones, depolarization strongly elevated cytosolic calcium at the terminal end of the cell while producing a lesser change around the oil droplet and the ellipsoid region, suggesting that calcium channels are localized to the terminal.

Clonal cells from embryonic retinal cell lines express qualitative electrophysiological differences

Cells from the embryonic quail retina were immortalized with the v-mil oncogene and cloned by limiting dilution. Their phenotype was examined using the whole-cell patch clamp method. Three membrane currents, IK(IR), INa and IK, were found at different frequencies within a sample of 170 cells drawn from a large clone. Nearly all combinations of these three markers were found and the frequency of combinations showed that the markers assorted independently. Examination of clones of less than 10 cells showed that heterogeneity originates with a high probability within clones, arguing that chromosomal mutation, for example, is unlikely to account for phenotypic diversity. A possible explanation is that phenotypic differences between cells might reflect the local exchange of instructive signals. If so, then the genes for the three phenotypic markers are controlled independently.