Retinal lesions induce differential changes in the expression of flip and flop isoforms of the glutamate receptor subunit GluR1 in the chick optic tectum

Differential Expression of AMPA Subunits Induced by NMDA Intrahippocampal Injection in Rats

Glutamate is involved in excitotoxic mechanisms by interacting with different receptors. Such interactions result in neuronal death associated with several neurodegenerative disorders of the central nervous system (CNS). The aim of this work was to study the time course of changes in the expression of GluR1 and GluR2 subunits of glutamate amino-acid-3-hydroxy-5-methyl-isoxazol-4-propionic acid (AMPA) receptors in rat hippocampus induced by NMDA intrahippocampal injection. Rats were submitted to stereotaxic surgery for NMDA or saline (control) microinjection into dorsal hippocampus and the parameters were evaluated 24 h, 1, 2, and 4 weeks after injection. The extension and efficacy of the NMDA-induced injury were evaluated by Morris water maze (MWM) behavioral test and Nissl staining. The expression of GluR1 and GluR2 receptors, glial fibrillary acidic protein (GFAP), and neuronal marker (NeuN) was analyzed by immunohistochemistry. It was observed the impairment of learning and memory functions, loss of neuronal cells, and glial proliferation in CA1 area of NMDA compared with control groups, confirming the injury efficacy. In addition, NMDA injection induced distinct changes in GluR1 and GluR2 expression over the time. In conclusion, such changes may be related to the complex mechanism triggered in response to NMDA injection resulting in a local injury and in the activation of neuronal plasticity.

Alternatively spliced three novel transcripts of gria1 in the cerebellum and cortex of mouse brain

Glutamate receptor type 1 (GluR1) subunit of α-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors plays an important role in the expression of long-term potentiation and memory formation. GluR1 is encoded by gria1 gene containing 16 exons and 15 introns in mouse. Previous studies have reported two alternatively spliced variants of this subunit. These flip and flop variants differ enormously in their properties as well as expression. In our studies, we report the presence of three new transcripts of this gene present in the cerebellum and cortex of mouse brain produced by alternative splicing at 5' end. Four new exons are reported; N1 is located in 5' untranslated region, N2 is located in the 1st intronic region while N3 and N4 are located in the 2nd intronic region. The properties of these new exons encoding N-terminal variants are highly diverse. N1, N3 and N4 are coding while N2 is a non-coding exon and results in a truncated transcript. The existence of N2 exon containing transcript is further supported by the presence of an Expressed Sequence Tag from the database. The translated amino acid sequences of these transcripts differ in the presence of signal peptide as well as in their phosphorylation and acetylation pattern. The differences in their properties might be involved in receptor modulation.

Retinal removal up-regulates cannabinoid CB(1) receptors in the chick optic tectum

The endocannabinoid system has been implicated in several neurobiological processes, including neurodegeneration and neuroprotection. The aim of this study was to evaluate the effects of unilateral retinal ablation on the expression of the cannabinoid receptor subtype 1 (CB(1)) at both protein and mRNA levels in the optic tectum of the adult chick brain. After different survival times postlesion (2-30 days), the chick brains were subjected to immunohistochemical, immunoblotting, and real-time PCR procedures to evaluate CB(1) expression. TUNEL and Fluoro-Jade B were used to verify the possible occurrence of cell death, and immunostaining for the microtubule-associated protein MAP-2 was performed to verify possible dendritic remodeling after lesions. No cell death could be observed in the deafferented tectum, at least up to 30 days postlesion, although Fluoro-Jade B could reveal degenerating axons and terminals. Retinal ablation seems to generate an increase of CB(1) protein in the optic tectum and other retinorecipient visual areas, which paralleled an increase in MAP-2 staining. On the other hand, CB(1) mRNA levels were not changed after retinal ablation. Our results reveal that CB(1) expression in visual structures of the adult chick brain may be negatively regulated by the retinal innervation. The increase of CB(1) receptor expression observed after retinal removal indicates that these receptors are not presynaptic in retinal axons projecting to the tectum and suggests a role of the cannabinoid system in plasticity processes ensuing after lesions.

Differential regulation of the neuronal isoform of nitric oxide synthase in the superior colliculus and dorsal lateral geniculate nucleus of the adult rat brain following eye enucleation

Nitric oxide has been shown to play various physiological and pathological roles in the visual system. We studied here the expression of the neuronal isoform of nitric oxide synthase in the rat superior colliculus and in the dorsal lateral geniculate nucleus after unilateral enucleation, by means of immunohistochemistry, immunoblotting, and real-time PCR. Immunohistochemistry revealed an increase of nitric oxide synthase-positive neurons in specific layers of the superior colliculus and in the dorsal lateral geniculate nucleus between 1 and 30 days post-lesion. Immunoblotting analyses confirmed that the neuronal isoform of nitric oxide synthase is upregulated in the superior colliculus and in the dorsal lateral geniculate nucleus after retinal removal. Diaminofluorescein histochemistry suggested that nitric oxide production was increased in both deafferented retinorecipient areas. Our real-time PCR results indicated that nitric oxide synthase transcript levels in the superior colliculus were not significantly altered after monocular enucleation, although an upregulation of the enzyme transcription was detected into the deafferented dorsal lateral geniculate nucleus. These findings indicated that neuronal nitric oxide synthase may undergo different forms of regulation in the adult deafferented visual system.

Ontogeny of subunits 2 and 3 of the AMPA-type glutamate receptors in Purkinje cells of the developing chick cerebellum

Several molecules, involved in cellular communication in the mature nervous system, appear to play important roles during neural development. These roles include neuronal growth, morphological changes of neurites, and neuronal survival. Such plasticity processes seem to be in part the result of activation of different receptor subtypes, which could cause Ca(2+) influx, a major candidate to be an outgrowth promoter. In this context, we performed immunohistochemical and in situ hybridization experiments to examine the following aspects of the development of chick cerebellum Purkinje cells: (i) expression of AMPA-type glutamate receptor GluR2/3 proteins; (ii) the levels of mRNAs coding for the GluR2 and GluR3 flip/flop isoforms; and (iii) expression of calbindin (CB) and parvalbumin (PV). Expression of GluR2/3 proteins, CB, PV, and the mRNAs coding for GluR2 and GluR3 splice variants all revealed a differential expression during development in chick Purkinje cells. GluR2/3 proteins and the GluR3 flop variant start to be expressed at E10, while the expression of CB, PV, the GluR3 flip isoform and the splice variants of GluR2 all started around E12-E14. All proteins showed an increasing expression from embryonic stages into the posthatching period. These results reveal a developmentally regulated expression of GluR2/3 proteins, including their splice variants, and of CB and PV in Purkinje cells. These findings may suggest a relationship between these proteins and specific cerebellar developmental processes.

The molecular pharmacology and cell biology of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors (AMPARs) are of fundamental importance in the brain. They are responsible for the majority of fast excitatory synaptic transmission, and their overactivation is potently excitotoxic. Recent findings have implicated AMPARs in synapse formation and stabilization, and regulation of functional AMPARs is the principal mechanism underlying synaptic plasticity. Changes in AMPAR activity have been described in the pathology of numerous diseases, such as Alzheimer's disease, stroke, and epilepsy. Unsurprisingly, the developmental and activity-dependent changes in the functional synaptic expression of these receptors are under tight cellular regulation. The molecular and cellular mechanisms that control the postsynaptic insertion, arrangement, and lifetime of surface-expressed AMPARs are the subject of intense and widespread investigation. For example, there has been an explosion of information about proteins that interact with AMPAR subunits, and these interactors are beginning to provide real insight into the molecular and cellular mechanisms underlying the cell biology of AMPARs. As a result, there has been considerable progress in this field, and the aim of this review is to provide an account of the current state of knowledge.

Expression of ionotropic glutamate receptors in the retina of the rdta transgenic mouse

BACKGROUND

The expression of retinal CaMKII is up-regulated in the retina of the rdta mouse in which rod photoreceptors are genetically ablated. As ionotropic glutamate receptors are known substrates of CAMKII, this study set out to determine if the protein levels of ionotropic glutamate receptors in the rdta mouse retina are also affected.

RESULTS

The NMDA receptor subunits (NR1, NR2A/B) and the GluR1; AMPA receptor subunit (GluR1) were examined in immunolabeled western blots. The results demonstrate that the amounts of NR1 and NR2A/B receptor subunits are significantly increased in crude synaptic membrane fractions isolated from retinae of the rdta mice when compared to their normal, littermate controls. The GluR1 receptor subunit and its phosphorylation are simultaneously increased in retinae of the rdta mice.

CONCLUSIONS

These data indicate that the NMDA receptors and AMPA (GluR1) receptors are altered in the retinae of rdta mice that lack rod photoreceptors. Because the rods are lost at an early stage in development, it is likely that these results are indicative of synaptic reorganization in the retina.