Developmental expression and self-regulation of Ca2+ entry via AMPA/KA receptors in the embryonic chick retina

Calcium-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors trigger neuronal nitric-oxide synthase activation to promote nerve cell death in an Src kinase-dependent fashion

In the retina information decoding is dependent on excitatory neurotransmission and is critically modulated by AMPA glutamate receptors. The Src-tyrosine kinase has been implicated in modulating neurotransmission in CNS. Thus, our main goal was to correlate AMPA-mediated excitatory neurotransmission with the modulation of Src activity in retinal neurons. Cultured retinal cells were used to access the effects of AMPA stimulation on nitric oxide (NO) production and Src phosphorylation. 4-Amino-5-methylamino-2',7'-difluorofluorescein diacetate fluorescence mainly determined NO production, and immunocytochemistry and Western blotting evaluated Src activation. AMPA receptors activation rapidly up-regulated Src phosphorylation at tyrosine 416 (stimulatory site) and down-regulated phosphotyrosine 527 (inhibitory site) in retinal cells, an effect mainly mediated by calcium-permeable AMPA receptors. Interestingly, experiments confirmed that neuronal NOS was activated in response to calcium-permeable AMPA receptor stimulation. Moreover, data suggest NO pathway as a key regulatory signaling in AMPA-induced Src activation in neurons but not in glial cells. The NO donor SNAP (S-nitroso-N-acetyl-DL-penicillamine) and a soluble guanylyl cyclase agonist (YC-1) mimicked AMPA effect in Src Tyr-416 phosphorylation, reinforcing that Src activation is indeed modulated by the NO pathway. Gain and loss-of-function data demonstrated that ERK is a downstream target of AMPA-induced Src activation and NO signaling. Furthermore, AMPA stimulated NO production in organotypic retinal cultures and increased Src activity in the in vivo retina. Additionally, AMPA-induced apoptotic retinal cell death was regulated by both NOS and Src activity. Because Src activity is pivotal in several CNS regions, the data presented herein highlight that Src modulation is a critical step in excitatory retinal cell death.

Calcium-permeable AMPA receptors in the retina

The retina transforms light entering the eye into a sophisticated neural representation of our visual world. Specialized synapses, cells, and circuits in the retina have evolved to encode luminance, contrast, motion, and other complex visual features. Although a great deal has been learned about the cellular morphology and circuitry that underlies this image processing, many of the synapses in the retina remain incompletely understood. For example, excitatory synapses in the retina feature the full panoply of glutamate receptors, but in most cases specific roles for different receptor subtypes are unclear. In this brief review, I will discuss recent progress toward understanding how Ca²⁺-permeable AMPA receptors (CP-GluARs) contribute to synaptic transmission and newly discovered forms of synaptic plasticity in the retina.

Diltiazem-induced neuroprotection in glutamate excitotoxicity and ischemic insult of retinal neurons

PURPOSE

Cell death is often related to an abnormal increase in Ca(2+) flux. In the retina, Ca(2+) channels are mainly from the L-type that do not inactivate with time. Under excitotoxic and ischemic conditions, their continuous activation may therefore contribute significantly to the lethal Ca(2+) influx. To assess this hypothesis, the Ca(2+) channel blocker, diltiazem, was applied in excitotoxic and ischemic conditions.

METHODS

To induce excitotoxicity, retinal cell cultures from newborn rats were incubated with glutamate. The toxicity of glutamate was quantified by neuronal immunostaining with an antibody directed against the neuron specific enolase. Glutamate receptor function in vitro was assessed in pig retinal cell cultures by patch clamp recording. Retinal ischemia was induced by raising the intraocular pressure in adult rats. Retinal cell loss was quantified on retinal sections by measuring nuclear cell densities.

RESULTS

In retinal cell culture, glutamate application induced a major cell loss. This cell loss was attributed to glutamate excitotoxicity because glutamate receptor blockers like MK-801 and CNQX increased significantly neuronal survival. MK-801 and CNQX, which block NMDA and AMPA/Kainate receptors, respectively, had additive effects. Expression of AMPA/Kainate glutamate receptors in mixed adult retinal cell cultures was attested by patch clamp recording. In newborn rat retinal culture, glutamate excitotoxicity was significantly reduced by addition of the L-type Ca(2+) channel blocker, diltiazem. In in vivo experiments, the increase in ocular pressure induced a decrease in cell number in the inner nuclear and ganglion cell layers. When animals received diltiazem injections, the ischemic treatment induced a less severe reduction in retinal cells; this neuroprotection was statistically significant in the ganglion cell layer.

CONCLUSION

These results are consistent with previous studies suggesting that Ca(2+) channel activation contributes to retinal cell death following either glutamate excitotoxicity or retinal ischemia. Under both conditions, the L-type Ca(2+) channel blocker, diltiazem, can limit cell death. These results extend the potential application of diltiazem in retinal neuroprotection to retinal pathologies involving glutamate excitotoxicity and ischemia.

Control of programmed cell death by neurotransmitters and neuropeptides in the developing mammalian retina

It has long been known that a barrage of signals from neighboring and connecting cells, as well as components of the extracellular matrix, control cell survival. Given the extensive repertoire of retinal neurotransmitters, neuromodulators and neurotrophic factors, and the exhuberant interconnectivity of retinal interneurons, it is likely that various classes of released neuroactive substances may be involved in the control of sensitivity to retinal cell death. The aim of this article is to review evidence that neurotransmitters and neuropeptides control the sensitivity to programmed cell death in the developing retina. Whereas the best understood mechanism of execution of cell death is that of caspase-mediated apoptosis, current evidence shows that not only there are many parallel pathways to apoptotic cell death, but non-apoptotic programs of execution of cell death are also available, and may be triggered either in isolation or combined with apoptosis. The experimental data show that many upstream signaling pathways can modulate cell death, including those dependent on the second messengers cAMP-PKA, calcium and nitric oxide. Evidence for anterograde neurotrophic control is provided by a variety of models of the central nervous system, and the data reviewed here indicate that an early function of certain neurotransmitters, such as glutamate and dopamine, as well as neuropeptides such as pituitary adenylyl cyclase-activating polypeptide and vasoactive intestinal peptide is the trophic support of cell populations in the developing retina. This may have implications both regarding the mechanisms of retinal organogenesis, as well as pathological conditions leading to retinal dystrophies and to dysfunctional cellular behavior.

Local differences in GABA release induced by excitatory amino acids during retina development: selective activation of NMDA receptors by aspartate in the inner retina

Glutamate and GABA are the major excitatory and inhibitory neurotransmitters in the CNS. In the retina, it has been shown that glutamate and aspartate and their agonists kainate and NMDA promote the release of GABA. In the chick retina, at embryonic day 14 (E14), glutamate and kainate were able to induce the release of GABA from amacrine and horizontal cells as detected by GABA-immunoreactivity. NMDA also induced GABA release restricted to amacrine cell population and its projections to the inner plexiform layer (E14 and E18). Although aspartate reduced GABA immunoreactivity, specifically in amacrine cells of E18 retinas, it was not efficient to promote GABA release from retinas at E14. As observed in differentiated retinas, dopamine inhibited the GABA release promoted by NMDA and aspartate but not by kainate. Our data show that different retinal sites respond to distinct EAAs via different receptor systems.

The appearance and maturation of excitatory and inhibitory neurotransmitter sensitivity during retinal regeneration of the adult newt

Using living slice preparations from newt retinas at different stages of regeneration, we examined the time course of appearance and maturation of neurotransmitter-induced currents with whole-cell patch-clamp methods. Neurons from which currents were recorded were identified by Lucifer Yellow fills. All progenitor cells examined at the regenerating retinas did not express any voltage-gated Na+ currents and responsiveness to excitatory amino acid analogues (AMPA and NMDA) and inhibitory amino acids (GABA and glycine). Voltage-gated Na+ currents were first detected in premature ganglion cells with round cell body located at the most proximal level of the 'intermediate-II' regenerating retina. AMPA- GABA- and glycine-induced currents were simultaneously observed in many premature ganglion cells expressing Na+ channels, but not all, suggesting that the onset of the Na+ channels is slightly earlier than that of excitatory and inhibitory amino acid receptors in regeneration. NMDA-evoked currents were first observed in the 'intermediate-III' regenerating retina just before the synaptogenesis. Pharmacological properties and reversal potential values of the excitatory and inhibitory amino acid responses did not change substantially between regenerating ganglion cells and mature ganglion cells, while rectification properties of current-voltage relations for AMPA and NMDA responses were somewhat different between them.

Development of chick retina cells in culture: cobalt entry through AMPA receptors and expression of GluR4 AMPA receptor subunit

The functionality of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors in chick embryo retina cells during development in vitro was studied by using Co(2+) uptake, and these data were correlated with the expression of the AMPA receptor subunit GluR4. We found that at 5 h in vitro only a small number of cells took up Co(2+) upon stimulation with 100 microM kainate or other AMPA receptor agonists, in the presence of cyclothiazide (CTZ), to inhibit desensitisation. The number of cells sensitive to kainate increased from 5 h in vitro to 3 days in vitro (DIV), and remained relatively constant until 14 DIV. When the cells were stimulated with (2S,4S)-4-methylglutamic acid (30 microM), a specific kainate receptor agonist, after inhibiting desensitisation with concanavalin A, we did not observe an increase in the number of cells responding, as compared to the control. The expression of the AMPA receptor subunit GluR4 during development was detected by immunofluorescence mainly at the perinuclear region of the cells, and the number of positive cells increased from 5 h in vitro to 7 DIV, and remained relatively constant until 14 DIV. The results suggest that AMPA receptors can be functionally active at early embryonic stages (5 h in vitro) in cultured retinal neurons, although in only a few cells, before synapse formation (E12). The localisation of GluR4 was well correlated with Co(2+) entry, since the strongest GluR4 immunoreactivity was found in the regions that showed the most intense labelling with Co(2+). Finally, we found that the expression levels of GluR4 at the neurites increased between 5 h in vitro and 7 DIV, near the period of synapse formation.

Agonist-stimulated cobalt uptake provides selective visualization of neurons expressing AMPA- or kainate-type glutamate receptors in the retina

Fast-acting excitatory neurotransmission in the retina is mediated primarily by glutamate, acting at alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) -selective and kainate-selective receptors. To localize these sites of action, cat retinas were stimulated with either AMPA or kainate and processed for histochemical visualization of cobalt uptake through calcium-permeable channels. Treatment with both agonists resulted in staining of A- and B-type horizontal cells and several types of OFF cone bipolar cells; there was no evidence for staining of ON cone bipolar cells or rod bipolar cells. The subpopulations of OFF cone bipolar cells differed in their responses with two distinct types that stained heavily with cobalt after exposure to AMPA and three different types that were preferentially labeled after exposure to kainate. Although many amacrine and ganglion cells appeared to respond to both agonists, AII amacrine cells were stained after stimulation by AMPA but not by kainate. The OFF cone bipolar cells that exhibit AMPA-stimulated cobalt uptake were found to have a high level of correspondence with cells that show immunocytochemical staining for the AMPA-selective glutamate receptor subunits GluR1 and GluR2/3. Similarly, the cone bipolar cells exhibiting kainate-stimulated cobalt uptake resemble those that are immunoreactive for the kainate subunit GluR5. The results indicate that, whereas many retinal neurons express both AMPA and kainate receptors, AII amacrine cells and subpopulations of OFF cone bipolar cells are limited to the expression of either AMPA or kainate receptors. This differential expression may contribute to the unique character of transmission by these cell types.

Expression of AMPA/kainate receptors during development of chick embryo retina cells: in vitro versus in vivo studies

The activity and the subunit expression of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate ionotropic glutamate receptors were studied in retina cells developing in chick embryos and in retina cells cultured as retinospheroids, at the same stages of development. In the retinospheroids, the activity of the AMPA/kainate receptors was monitored by following the changes in the intracellular free calcium concentration ([Ca(2+)](i)), in response to AMPA, kainate or to L-glutamate, and the expression of the receptor subunits GluR1, GluR2/3, GluR4 and GluR6/7 was determined in the retinospheroids and in chick retinas by immunodetection using polyclonal antibodies. The changes in [Ca(2+)](i) in response to 400 microM kainate increased from 5h in vitro to 3 days, and remained constant until day 14, whereas the [Ca(2+)](i) in response to 500 microM L-glutamate or 400 microM AMPA increased from 5h in vitro to 3 days, and thereafter decreased slightly until day 14. The [Ca(2+)](i) responses to kainate are mainly due to AMPA receptor stimulation, since the signals were abolished by LY303070, the AMPA receptor antagonist, and were not affected by MK-801, the NMDA receptor antagonist. In retinospheroids, the levels of expression of GluR1 subunit increased from 5h in vitro until day 7, then decreased until day 14. The levels of expression of GluR2/3 and GluR4 subunits increased from 5h in vitro until day 10, and remained constant until day 14. The levels of kainate receptor subunits GluR6/7 increased from 5h in vitro until day 3, and thereafter decreased slightly until day 14. In the retinas, the expression of GluR1 and GluR6/7 subunits increased from day 8 until day 15, and then decreased until day 22 (post-natal 1). The subunits GluR2/3 and GluR4 increased from day 8 until day 18, and remained constant until day 22. The results suggest that AMPA/kainate receptors are expressed at early embryonic stages, although at low levels and before synapse formation (E12). However, the AMPA receptors are not completely functional at the first stage studied since they do not respond to the agonist AMPA. Also, the patterns of AMPA/kainate receptor subunit expression in retinospheroids of chick embryo retina cells cultured in vitro and in retina cells developing in the embryo (in vivo) were similar, indicating that the AMPA/kainate receptor subunits expression in these primary cultures mimics their expression in the developing chick retina.

Ionotropic glutamate receptors during the development of the chick retina

Glutamate is the main neurotransmitter of photoreceptors, bipolar cells, and ganglion cells of the vertebrate retina. Three main classes of ionotropic glutamate receptors comprising different subunits can be distinguished: AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxasolepropionate), KA (kainate), and NMDA (N-methyl-D-aspartate). This study was undertaken to characterize the AMPA (GluR1, GluR2/3, and GluR4), KA (GluR5/6/7), and NMDA (NR1) ionotropic glutamate receptor subunits and to determine their distribution during the development of the chick retina by Western blotting and immunohistochemistry. Western blotting analysis at 1 day after hatching indicated that the antibodies against GluR1, 2/3, 4, and 5/6/7 and NR1 recognized specifically a single band of 100-110 kDa. In turn, immunohistochemistry at P1 showed that all subunits were expressed in cells of the inner nuclear and ganglion cell layers of the chick retina, mostly amacrine and ganglion cells, and their processes in the inner plexiform layer. In addition, stained processes in the outer plexiform layer were observed with the antibodies against GluR2/3, GluR4, and GluR5/6/7. Although all subunits appeared around E5-E6 in the prospective ganglion cell layer, and later in the prospective inner nuclear layer, the distribution of cells containing these glutamate receptor subunits revealed distinct ontogenetic patterns. This multiplicity of glutamate receptors may contribute to different processes that occur in the chick retina during development.

Early activation of Ca(2+)-permeable AMPA receptors reduces neurite outgrowth in embryonic chick retinal neurons

Calcium entry through Ca(2+)-permeable AMPA/kainate receptors may activate signaling cascades controlling neuronal development. Using the fluorescent Ca(2+)-indicator Calcium Green 1-AM we showed that the application of kainate or AMPA produced an increase of intracellular [Ca(2+)] in embryonic chick retina from day 6 (E6) onwards. This Ca(2+) increase is due to entry through AMPA-preferring receptors, because it was blocked by the AMPA receptor antagonist GYKI 52466 but not by the N-methyl-D-aspartic acid (NMDA) receptor antagonist AP5, the voltage-gated Ca(2+) channel blockers diltiazem or nifedipine, or by the substitution of Na+ for choline in the extracellular solution to prevent the depolarizing action of kainate and AMPA. In dissociated E8 retinal cultures, application of glutamate, kainate, or AMPA reduced the number of neurites arising from these cells. The effect of kainate was prevented by the AMPA/kainate receptor antagonist CNQX and by GYKI 52466 but not by AP5, indicating that the reduction in neurite outgrowth resulted from the activation of AMPA receptors. Blocking Ca(2+) influx through L-type voltage-gated Ca(2+) channels with diltiazem and nifedipine prevented the effect of 10-100 microM kainate but not that of 500 microM kainate. In addition, joro spider toxin-3, a blocker of Ca(2+)-conducting AMPA receptors, prevented the effect of all doses of kainate. Neither GABA, which is depolarizing at this age in the retina, nor the activation of metabotropic glutamate receptors with tACPD mimicked the effects of AMPA receptor activation. Calcium entry via AMPA receptor channels themselves may therefore be important in the regulation of neurite outgrowth in developing chick retinal cells.

Blockade of AMPA/kainate receptors can either decrease or increase the survival of cultured neocortical cells depending on the stage of maturation

Neurotoxicity has often been associated with glutamate receptor stimulation and neuroprotection with glutamate receptor blockade. However, the relationship may be much more complex. We dissociated cells from the rat neocortical anlage at an early stage of prenatal development (embryonic day 14). The cells were exposed in vitro to agonists and antagonists of alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA)/kainate and N-methyl-D-aspartate (NMDA) receptors and the effects on differentiation and survival have been quantitatively and qualitatively evaluated. NMDA and the non-competitive antagonist (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10-imine hydrogen maleate (MK-801) had the expected effects (the agonist decreasing and the antagonist increasing neuronal survival) when applied at a relatively advanced stage of in vitro maturation, but no significant effect in either direction at earlier stages. Kainate also had an effect on cell survival only at an advanced stage (where it decreased the number of cells). However, this cannot be attributed to the absence of functional AMPA/kainate receptors at earlier stages, since: (1) cells could be loaded with cobalt; and (2) early application of kainate dramatically reduced the number of cobalt-positive cells. Furthermore, exposure at early stages to 6,7-dinitroquinoxaline-2,3-dione (DNQX), or GYKI 53655, (competitive and non-competitive AMPA receptor antagonists, respectively) strongly reduced cell survival. The effects were concentration- and time-dependent with a complex time--curve. The decrease in cell number was maximal after antagonist application from 2 to 5 days in vitro. The effects of DNQX could be cancelled by co-application of kainate. When exposed to an antagonist at later stages of development, the number of surviving cells gradually approached control values and finally became significantly higher. Our results suggest that cells of the developing neocortex (and perhaps newly generated cells in the adult brain) require at different stages of their development, an appropriate level of AMPA/kainate receptor activation.

Synaptic and non-synaptic AMPA receptors permeable to calcium

For a long time, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors permeable to calcium have been considered to be either non-existent or as "atypical". There is now ample evidence that these receptors exist in numerous regions of the nervous system and in many neuronal as well as non-neuronal cell populations. This evidence has been accumulated by several methods, including electrophysiological recording, calcium imaging and cobalt-loading. Functional AMPA receptors permeable to calcium are already expressed at very early stages of embryonic development, well before the onset of synaptogenesis. They are probably involved in the paracrine signaling necessary for construction of the nervous system before becoming involved in synaptic transmission. In immature cells, cyclothiazide strongly increases the steady-state level of responses not only to AMPA, but also to kainate. Ingestion, during pregnancy, of food or drug substances that can cross the placental barrier and act upon the embryonic receptors may constitute a risk for normal development. In the adult nervous system, synaptic as well as non-synaptic (paracrine) AMPA receptors permeable to calcium are probably widely expressed in both glial and neuronal cells. They may also participate in controlling some aspects related to adult neurogenesis, in particular the migration of newly formed neurons.

Kainate-activated cobalt uptake in the primary gustatory nucleus in goldfish: visualization of the morphology and distribution of cells expressing AMPA/kainate receptors in the vagal lobe

Gustatory afferent fibers of the vagus nerve that innervate taste buds of the oropharynx of the goldfish, Carassius auratus, project to the vagal lobe, which is a laminated gustatory nucleus in the dorsal medulla. As in the mammalian gustatory system, responses by second-order cells in the goldfish medulla are mediated by N-methyl-D-aspartate (NMDA) and non-NMDA ionotropic glutamate receptors. We utilized a cobalt uptake technique to label vagal lobe neurons that possess cobalt-permeable ionotropic glutamate receptors. Vagal lobe slices were bathed in kainate (40 microM) or glutamate (0.5 or 1 mM) in the presence of CoCl(2), which can pass into cells through the ligand-gated cation channels of non-NMDA receptors made up of certain subunit combinations. Cobalt-filled cells and dendrites were observed in slices that were activated by kainate or glutamate, but not in control slices that were bathed in CoCl(2) alone, nor in slices that were bathed with the non-NMDA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (10 microM) in addition to an agonist. Likewise, simple depolarization of the cells with KCl failed to induce cobalt loading. Cobalt-filled round unipolar cells, elongate or globular bipolar cells, and multipolar cells with elongate or polygonal perikarya were distributed throughout the cell layers in the sensory zone of the vagal lobe. Numerous labeled neurons had dendrites spanning layers IV and VI, the two principal layers of primary afferent input. Apical and basal dendrites often extended radially through neighboring laminae, but many cells also extended dendrites tangential to the lamination of the sensory zone. In the motor layer, cell bodies and proximal dendrites of small, multipolar neurons, and large motoneurons were regularly loaded with cobalt.

Regulation of brain-derived neurotrophic factor messenger RNA levels in avian hypothalamic slice cultures

Mechanisms regulating the expression of brain-derived neurotrophic factor, a member of the neurotrophin family, have been extensively studied in the rat cerebral cortex, hippocampus and cerebellum. In contrast, little is known regarding the regulation of this growth factor in the hypothalamus. Here we present an analysis of the regulation of brain-derived neurotrophic factor messenger RNA levels in chick embryo hypothalamic slice cultures following exposure to potassium chloride, glutamate agonists and sex steroids. Following a week in chemically-defined media the tissue was depolarized by exposure to 50 mM potassium chloride for 6h, resulting in a significant 4.2-fold increase in the level of brain-derived neurotrophic factor messenger RNA. This result is consistent with studies of other brain regions. Similar 6-h acute exposures of the hypothalamic cultures to 25 microM N-methyl-D-aspartic acid, 25 microM kainic acid and 25 microM alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid also significantly increased messenger RNA levels 2.5-, 2.1- and 1.4-fold, respectively. It was previously reported that brain-derived neurotrophic factor levels within the rat cerebral cortex, olfactory bulb and hippocampus are altered by exposure to 17beta-estradiol. Here we show that in hypothalamic slice cultures neither acute nor chronic treatments with 10 and 100 nM 17beta-estradiol and 10nM testosterone significantly altered the steady-state level of this growth factor. These findings show that neuronal activity, induced by glutamate agonists and potassium chloride, can regulate brain-derived neurotrophic factor messenger RNA levels within embryonic hypothalamic slice cultures. This regulation could play a critical role in the modulation of programmed cell death and synaptic maturation during development of the hypothalamus.

Guanine nucleotides block agonist-driven 45Ca2+ influx in chick embryo retinal explants

AMPA (EC50 = 1.0 x 10(-6) M) and NMDA (EC50 = 1.3 x 10(-4) M) trigger 45Ca2+ influx in 13-day chick embryonic retinal explants. This agonist-driven cationic flux is specifically inhibited by typical competitive antagonists, such as 6,7-dinitroquinoxaline-2,3-dione (DNQX) and 2-amino-7-phosphonoheptanoate (AP7), respectively. Guanine nucleotides, with different degrees of phosphorylation, namely 5'-GMP, guanosine 5'-O-(2-thiodiphosphate) (GDPbetaS), guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) and 5'-guanylyl-imidodiphosphate (GppNHp), are also efficient blockers of 45Ca2+ influx. These results confirm the antagonistic behavior of guanine nucleotides towards ionotropic glutamate receptors and suggest a convenient experimental approach for screening of novel agonists and antagonists.

Effects of xanthine derivatives on electroretinographic responsiveness

In view of the use of synthetic propentofylline (PPF) as a protective agent in brain ischemia, its possible side effects on vision capacities have been explored by electroretinography in comparative experiments with theophylline. We used eyecup preparations of small-spotted dogfish sharks and of European eels, particularly suitable for long-lasting experiments. The drug exerted profound but reversible modifications of ERG records: (1) a dose-dependent increase of the amplitude and duration of the chemically isolated late receptor potential (LRP), (2) a partial unmasking of LRP, (3) a strong potentiation of the LRP-unmasking effect of low temperature, (4) a potentiation of light adaptation effects, and (5) a strong potentiation of the post-illumination hyperexcitability. The effects were explicable as due to a strong phosphodiesterase (PDE) inhibiting, cyclic guanosine monophosphate (cGMP) promoting, action of the drug. The effects were considerably stronger, or even of opposite sign, in comparison to those of the chemically related theophylline. PPF did not seriously affect the ERG c-wave originating in the pigment epithelium. The results suggested that the effects of PPF on vision may not seriously hamper the therapeutic use of the drug. They indicated, on the other hand, that PPF was a retinoactive drug of potential usefulness in the exploration of the complex biochemical events underlying visual transduction.

Opposing effects of excitatory amino acids on chick embryo spinal cord motoneurons: excitotoxic degeneration or prevention of programmed cell death

Acute administration of a single dose of NMDA on embryonic day (E) 7 or later induces a marked excitotoxic injury in the chick spinal cord, including massive necrotic motoneuron (MN) death. When the same treatment was performed before E7, little, if any, excitotoxic response was observed. Chronic treatment with NMDA starting on E5 prevents the excitotoxic response produced by a later "acute" administration of NMDA. Additionally, chronic NMDA treatment also prevents the later excitotoxic injury induced by non-NMDA glutamate receptor agonists, such as kainate or AMPA. Chronic NMDA treatment also reduces normal MN death when treatment is maintained during the period of naturally occurring programmed cell death (PCD) of MNs and rescues MNs from PCD induced by early peripheral target deprivation. The trophic action of chronic NMDA treatment appears to involve a downregulation of glutamate receptors as shown by both a reduction in the obligatory NR1 subunit protein of the NMDA receptor and a decrease in the kainate-induced Co(2+) uptake in MNs. Both tolerance to excitotoxicity and trophic effects of chronic NMDA treatment are prevented by the NMDA receptor antagonist MK-801. Additionally, administration of MK-801 alone results in an increase in MN PCD. These data indicate for the first time that early activation of NMDA receptors in developing avian MNs in vivo has a trophic, survival-promoting effect, inhibiting PCD by a target-independent mechanism that involves NMDA receptor downregulation.

Potentiation of glutamatergic agonist-induced inositol phosphate formation by basic fibroblast growth factor is related to developmental features in hippocampal cultures: neuronal survival and glial cell proliferation

We investigated the modulation by growth factors of phospholipase C (PLC)-linked glutamate receptors during in vitro development of hippocampal cultures. In defined medium, glial cells represent between 3 and 14% of total cell number. When we added basic fibroblast growth factor (bFGF) 2 h after plating, we found: (i) a neuroprotection from naturally occurring death for up to 5 days; (ii) a proliferation of glial cells from day 3; and (iii) a potentiation of quisqualate (QA)-induced inositol phosphate (IP) formation from 1 to 10 days in vitro (DIV) and 1S, 3R-amino-cyclopentane-1,3-dicarboxylate (ACPD) response from 3 to 10 DIV. The antimitotic cytosine-beta,D-arabinofuranoside (AraC) blocked glial cell proliferation induced by bFGF, but not neuroprotection. Under these conditions, the early potentiation of the QA response (1-3 DIV) was not changed, while the ACPD and late QA response potentiations were prevented (5-10 DIV). Epidermal growth factor was not neuroprotective but it induced both glial cell proliferation and late QA or ACPD potentiation. Surprisingly, the early bFGF-potentiated QA-induced IP response was blocked by 6, 7-dinitro-quinoxaline-2,3-dione (DNQX), suggesting the participation of ionotropic (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate (KA) receptors. The delayed bFGF-potentiated ACPD-induced IP response is inhibited by (S)-alpha-methyl-4-carboxyphenylglycine (MCPG), indicating possible activation of glial metabotropic receptors. These results suggest that, in hippocampal cultures, bFGF modulates AMPA and metabotropic glutamate receptors linked to the IP cascade, possibly in relation to the regulation of neuronal survival and glial cell proliferation, respectively.

Subpopulations of GABAergic and non-GABAergic rat dorsal horn neurons express Ca2+-permeable AMPA receptors

Subpopulations of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors that are either permeable or impermeable to Ca2+ are expressed on dorsal horn neurons in culture. While both mediate synaptic transmission, the Ca2+ -permeable AMPA receptors provide a Ca2+ signal that may result in a transient change in synaptic strength [Gu, J.G., Albuquerque, C., Lee, C.J. & MacDermott, A.B. (1996) Nature, 381, 793]. To appreciate the relevance of these receptors to dorsal horn physiology, we have investigated whether they show selective expression in identified subpopulations of dorsal horn neurons. Expression of Ca2+-permeable AMPA receptors was assayed using the kainate-induced cobalt loading technique first developed by Pruss et al. [Pruss, R.M., Akeson, R.L., Racke, M.M. & Wilburn, J.L. (1991) Neuron, 7, 509]. Subpopulations of dorsal horn neurons were identified using immunocytochemistry for gamma-aminobutyric acid (GABA), glycine, substance P receptor (NK1 receptor) and the Ca2+-binding proteins, calretinin and calbindin D28K. We demonstrate that, in dorsal horn neurons in culture, kainate-induced cobalt uptake is selectively mediated by Ca2+-permeable AMPA receptors, and that a majority of GABA and NK1 receptor-expressing neurons express Ca2+-permeable AMPA receptors. GABAergic dorsal horn neurons are important in local inhibition as well as in the regulation of transmitter release from primary afferent terminals. NK1 receptor-expressing dorsal horn neurons include many of the projection neurons in the nociceptive spino-thalamic pathway. Thus, we have identified two populations of dorsal horn neurons representing important components of dorsal horn function that express Ca2+-permeable AMPA receptors. Furthermore, we show that several subpopulations of putative excitatory interneurons defined by calretinin and calbindin expression do not express Ca2+-permeable AMPA receptors.

Glutamate receptor phenotypes in the auditory brainstem and mid-brain of the developing rat

Glutamate receptors mediate most excitatory synaptic transmission in the adult vertebrate brain, but their activation in developing neurons also influences developmental processes. However, little is known about the developmental regulation of the subunits composing these receptors. Here we have studied age-dependent changes in the expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA) and N-methyl-D-aspartate (NMDA) receptor subunits in the cochlear nucleus complex (CN), the superior olivary complex (SOC), the nuclei of the lateral lemniscus, and the inferior colliculus of the developing rat. In the lateral superior olive, the medial nucleus of the trapezoid body, and the ventral nucleus of the lateral lemniscus, the distribution of AMPA receptor subunits changed drastically with age. While GluR1 and GluR2 subunits were highly expressed in the first 2 postnatal weeks, GluR4 staining was detectable only thereafter. GluR1 and GluR2 immunoreactivities rapidly decreased during the third postnatal week, with the GluR1 subunits disappearing from most neurons. In contrast, the adult pattern of the distribution of AMPA receptor subunits emerged gradually in most of the other auditory nuclei. Thus, progressive as well as regressive events characterized AMPA receptor development in some nuclei, while a monotonically maturation was seen in other regions. In contrast, the staining patterns of NMDA receptor subunits remained stable or only decreased during the same period. Although our data are not consistent with a generalized pattern of AMPA receptor development, the abundance of GluR1 subunits is a distinctive feature of early AMPA receptors. As similar AMPA receptors are present during plasticity periods throughout the brain, neurons undergoing synaptic and structural remodelling might have a particular need for these receptors.

Development of calcium-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors in cultured neocortical neurons visualized by cobalt staining

The developmental expression of calcium (Ca2+)-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate receptors in cultured neocortical neurons was evaluated by using cobalt uptake, a histochemical method that identifies cells expressing Ca2+-permeable, non-N-methyl-D-aspartate (non-NMDA) receptors. At a concentration of 500 microM, AMPA was found to stimulate cobalt uptake only late in development, resulting in staining of 2.7%+/-0.3% of the neurons maintained in culture for 12 days in vitro (DIV). When AMPA receptor desensitization was blocked with 50 microM cyclothiazide, the developmental profile of cobalt uptake mediated by 25 microM AMPA changed dramatically. The cobalt staining now appeared in young cultures (5 DIV), and the percentage of stained cells increased from 3.4%+/-0.2% at 5 DIV to 21.7%+/-1.6% at 12 DIV. The effect of 200 microM kainate was similar to that seen with 25 microM AMPA plus 50 microM cyclothiazide, resulting in 17.7%+/-0.3% stained neurons at 12 DIV. The cobalt uptake was specific to AMPA and kainate receptors because NMDA receptors and voltage-gated calcium channels were found not to mediate any cobalt staining. In addition, 10 microM 6-nitro-7-sulphamoylbenzo-[f]-quinoxaline-2,3-dione (NBQX) was able to prevent all staining at 5 and 8 DIV and most of the staining at 12 DIV, indicating that the non-NMDA ionotropic glutamate receptors are involved in cobalt uptake into the neurons. The AMPA receptor-selective antagonist GYKI 53655 was used to differentiate between cobalt influx through AMPA- or kainate-preferring receptors. After pretreatment with concanavalin A (con A), an inhibitor of kainate receptor desensitization, cobalt uptake was assessed after stimulation by 200 microM kainate in the presence of 25 microM GYKI 53655. No cobalt staining was observed under these conditions, indicating that most if not all of the cobalt influx induced by kainate was mediated through AMPA receptor channels.

Guanine nucleotides protect against kainate toxicity in an ex vivo chick retinal preparation

Ex vivo preparations of chick neural retina have been successfully used in the assessment of excitotoxicity and in the evaluation of the protective effects of glutamate antagonists. Using a variation of this approach, and measuring the acute and delayed toxic effects of kainate (KA) in terms of lactate dehydrogenase release, we have shown that guanine nucleotides behave as effective neuroprotecting agents. The anti-excitotoxic potency of guanine nucleotides (in the case of GMP and GDPbetaS it is about 100 times lower than that of DNQX, a powerful kainate antagonist) correlates well with their ability to displace KA from retinal KA receptors.

Inhibition of carbachol-induced inositol phosphate accumulation in the embryonic retina promoted by kainate and veratridine

In the present study, we report that low concentrations of the glutamate ionotropic agonist kainate decreased the turnover of [3H]-phosphoinositides ([3H]-InsPs) induced by muscarinic receptors in the chick embryonic retina. When 100 microM carbachol was used, the estimated IC50 value for kainate was 0.2 microM and the maximal inhibition of approximately 50% was obtained with 1 microM or higher concentrations of the glutamatergic agonist. Our data also show that veratridine, a neurotoxin that increases the permeability of voltage-sensitive sodium channels, had no effect on [3H]-InsPs levels of the embryonic retina. However, 50 microM veratridine, but not 50 mM KCl, inhibited approximately 65% of the retinal response to carbachol. While carbachol increased [3H]-InsPs levels from 241.2 +/- 38.0 to 2044.5 +/- 299.9 cpm/mg protein, retinal response decreased to 861.6 +/- 113.9 cpm/mg protein when tissues were incubated with carbachol plus veratridine. These results suggest that the accumulation of phosphoinositides induced by activation of muscarinic receptors can be inhibited by the influx of Na+ ions triggered by activation of kainate receptors or opening of voltage-sensitive sodium channels in the chick embryonic retina.

High sensitivity quantitative analysis of cobalt uptake in rat cerebellar granule cells with and without excitatory amino acids

We quantified the effect of the excitatory amino acids kainate and glutamate on the uptake of cobalt in primary rat cerebellar granule neurons, by using inductively coupled plasma-atomic emission spectroscopy (ICP-AES). We quantitatively demonstrated that Co2+ uptake, although enhanced by glutamate and kainate also takes place in the absence of excitatory amino acids. We also found that cobalt uptake is not significantly altered by the presence of glutamate receptor competitive or noncompetitive antagonists, indicating that cobalt uptake in granule neurons does not require glutamate receptor stimulation. Our results suggest, therefore, that Co2+ may enter the cell by passive diffusion through the plasma membrane.

Glutamate-induced Co2+ uptake in rat auditory brainstem neurons reveals developmental changes in Ca2+ permeability of glutamate receptors

Ca2+ influx through glutamate receptors (GluRs) is thought to play a crucial part in developmental processes and neuronal plasticity. Here we have examined the spatiotemporal distribution of Ca2+-permeable GluRs in auditory brainstem neurons of the rat from birth to adulthood, using the cobalt-staining technique of Pruss and collaborators. In slices of young adult rats, 1 mM glutamate evoked intense cobalt uptake in subsets of neurons in the ventral cochlear nuclei, the medial nucleus of the trapezoid body, the lateral and the medial superior olive, and the lateral lemniscal nuclei. Neurones in the central nucleus of the inferior colliculus, and thalamic auditory nuclei appear to express few, if any, Ca2+-permeable GluRs. Thus, in adults, Ca2+-permeable GluRs are present in neurons of almost all main relay stations of the auditory brainstem. During development, cobalt-stained cells first appeared at about hearing onset (at postnatal day 12 [P12]). At P16, staining levels were highest and the pattern of distribution was already adult-like. The staining intensity slightly declined during the fourth postnatal week. In contrast, Ca2+-permeable receptors were detected in the external cortex of the inferior colliculus as early as P4. Our results show that auditory neurons, characterized by a high temporal precision in neuronal activity, display Ca2+-permeable GluRs. Because Ca2+ permeability appears at about the onset of hearing and is highest during the following 2 weeks, Ca2+ influx through GluRs is likely to be implicated in remodelling processes occurring during this ontogenetic period.

Developing rat brainstem motoneurones in organotypic culture express calcium permeable AMPA-gated receptors

Recent studies suggest that calcium permeable excitatory amino acid receptors may play an important role in many developmental processes including dendritic differentiation, synaptogenesis and activity dependent plasticity or excitotoxicity related disorders. In this work, we investigate the existence of calcium permeable AMPA receptors in embryonic rat motoneurones in organotypic slice culture, which display an early sensitivity to AMPA and Kainate. We used excitatory amino acids induced cobalt uptake and show that developing motoneurones express Ca2+ permeable AMPA receptors. We demonstrate, as already described for other neuronal types in acute slices of adult animals, that the cobalt loading of motoneurones is not suppressed by blockade of synaptic activity by TTX. It is not induced by NMDA stimulation and does not result from the activation of voltage dependent calcium channels. It is specifically suppressed by the non-competitive AMPA antagonist GYKI 53784 (LY303070) and enhanced by the AMPA-receptor desensitization blocker Aniracetam. We conclude that cobalt loading results from the specific activation of AMPA receptors. We further show that, when cobalt loading is induced by threshold doses of Glutamate agonists, cobalt-sulfide deposits are found specifically in primary dendrites, dendritic spines and localized spots on the somatic and peripheral dendritic membrane. We suggest that this particular pattern of staining, different from the Golgi-like staining obtained with excitotoxic doses, may offer new information regarding the membrane density and distribution of calcium permeable AMPA receptors.

AMPA/kainate receptors modulate the survival in vitro of embryonic brainstem cells

This study aimed at analyzing the involvement of (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate (AMPA/kainate) receptors in the survival of cultured rat embryonic brainstem cells, dissociated on embryonic day 14. The cell number was estimated after pharmacological manipulation of the receptors by exposure to agonists or antagonists. The developmental stage at the moment of drug application was critical for cell survival. We observed after 8 days in vitro a much stronger decrease in the number of gamma-enolase-positive cells when the cultures were treated for 3 days with the antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) starting on the day of plating than when DNQX was added after 5 days in vitro. Conversely, exposure to the agonists (RS)-2-amino-3-(3-hydroxy-5-tri-fluoromethyl-4-isoxazolyl)-propion ic acid (T-AMPA) or kainate for 3 days significantly reduced cell survival only when the treatment was initiated after 5 days in vitro. Survival of S-100-positive cells was not affected after exposure to either agonists or antagonists. Neither agonist nor antagonist treatment modified cell proliferation, as assessed by 5-bromo-2'-deoxyuridine (BrdU) staining, suggesting that the decrease in the number of gamma-enolase-positive cells is essentially due to cell death. If some of the processes we observed in vitro correspond to analogous events in vivo, then exposure to excitatory amino acid receptor agonists or antagonists at critical stages of embryogenesis may alter the development of the central nervous system.