Reorganization of inhibitory synapses and increased PSD length of perforated excitatory synapses in hippocampal area CA1 of dystrophin-deficient mdx mice.

Dystrophin is a cytoskeletal membrane-bound protein expressed in both muscle and brain. Brain dystrophin is thought to be involved in the stabilization of gamma-aminobutyric acid (GABA)(A)-receptor (GABA(A)-R)clusters in postsynaptic densities (PSDs) at inhibitory synapses onto pyramidal cells, and its loss has been linked to cognitive impairments in Duchenne muscular dystrophy. Dystrophin-deficient mdx mice have learning deficits and altered synaptic plasticity in cornu ammonis (CA1) hippocampus, but the possibility that altered synapse morphology or distribution may underlie these alterations has not been examined. Here we used in vivo magnetic resonance imaging and histological analyses to assess brain volumetric and cytoarchitectonic abnormalities and quantitative electron microscopy to evaluate the density and ultrastructure of CA1 hippocampal synapses in mdx mice. We found that mdx mice have increased density of axodendritic symmetric inhibitory synapses and larger PSDs in perforated asymmetric excitatory synapses in the proximal, but not distal, CA1 apical dendrites that normally express dystrophin, in the absence of gross brain malformations. Data are discussed in light of the known molecular and neurophysiological alterations in mdx mice. We suggest that increased inhibitory synapse density reflects tenuous compensation of altered clustering of alpha2 subunit-containing GABA(A)-Rs in CA1 dendrites, whereas increased PSD length in perforated synapses suggests secondary alterations in excitatory synapse organization associated with enhanced synaptic excitation.

Structure and expression of three Emx genes in the dogfish Scyliorhinus canicula: functional and evolutionary implications.

We report the characterization of three Emx genes in a chondrichthyan, the dogfish Scyliorhinus canicula. Comparisons of these genes with their osteichthyan counterparts indicate that the gnathostome Emx genes belong to three distinct orthology classes, each containing one of the dogfish genes and either the tetrapod Emx1 genes (Emx1 class), the osteichthyan Emx2 genes (Emx2 class) or the zebrafish Emx1 gene (Emx3 class). While the three classes could be retrieved from the pufferfish genome data, no indication of an Emx3-related gene in tetrapods could be found in the databases, suggesting that this class may have been lost in this taxon. Expression pattern comparisons of the three dogfish Emx genes and their osteichthyan counterparts indicate that not only telencephalic, but also diencephalic Emx expression territories are highly conserved among gnathostomes. In particular, all gnathostomes share an early, dynamic phase of Emx expression, spanning presumptive dorsal diencephalic territories, which involves Emx3 in the dogfish, but another orthology class, Emx2, in tetrapods. In addition, the dogfish Emx2 gene shows a highly specific expression domain in the cephalic paraxial mesoderm from the end of gastrulation and throughout neurulation, which suggests a role in the segmentation of the cephalic mesoderm.

Otx1 gene-controlled morphogenesis of the horizontal semicircular canal and the origin of the gnathostome characteristics.

The horizontal semicircular canal of the inner ear is a unique feature of gnathostomes and is predated by the two vertical semicircular canals, which are already present in lampreys and some fossil, armored jawless vertebrates regarded as close relatives of gnathostomes. Inactivation in mice of the orthodenticle-related gene Otx1 results in the absence of this structure. In bony fishes and tetrapods (osteichthyans), this gene belongs to a small multigene family comprising at least two orthology classes, Otx1 and Otx2. We report that, as in the mouse, xenopus and zebrafish, Otx1- and Otx2-related genes are present in a chondrichthyan, the dogfish Scyliorhinus canicula, with an Otx1 expression domain in the otocyst very similar to those observed in osteichthyans. A strong correlation is thus observed in extant vertebrates between the distribution of the horizontal semicircular canal and the presence of an Otx1 ortholog expressed in the inner ear, which supports the hypothesis that the absence of this characteristic in Otx1-/- mice may correspond to an atavism. The same conclusion applies to two other gnathostome-specific characteristics also deleted in Otx1-/- mice, the utriculosaccular duct and the ciliary process. Together with functional analyses of Otx1 and Otx2 genes in mice and comparative analyses of the Otx gene families characterized in chordates, these discoveries lead to the hypothesis that some of the anatomic characteristics of gnathostomes have appeared quite suddenly and almost simultaneously in vertebrate evolution, possibly as a consequence of gene functional diversifications following duplications of an ancestral chordate gene.

Presynaptic and postsynaptic effects of nitric oxide donors at synapses between parallel fibres and Purkinje cells: involvement in cerebellar long-term depression.

The involvement of nitric oxide in cerebellar long-term depression is widely accepted. Nevertheless, its site of action has remained unclear. Using the coefficient of variation method applied to the parallel fibre-mediated excitatory postsynaptic currents recorded in voltage-clamped Purkinje cells. this study shows that nitric oxide donors exert their effects at both presynaptic and postsynaptic sites. The presynaptic depression fades away with washout of nitric oxide donors and is mediated through the potentiation of A1 adenosine receptors. Part of this effect may be due to non-nitric oxide products. In contrast, long-term depression induced by nitric oxide donors is expressed at a postsynaptic site, and is independent of the ADP ribosylation. Long-term depression induced by pairing is also expressed mainly at a postsynaptic level. These results establish that long-term depression at the parallel fibre Purkinje cell synapse induced by pairing of nitric oxide donors is mostly expressed at a postsynaptic site.

Incomplete regression of multiple climbing fibre innervation of cerebellar Purkinje cells in mGLuR1 mutant mice.

Recent reports have suggested the existence of a causal relationship between impaired regression of multiple climbing fibre innervation and impaired motor coordination in protein kinase C gamma subunit (PKC gamma) mutant mice. In the present patch-clamp study, performed in thin cerebellar slices prepared from adult mutant mice deficient in metabotropic glutamate receptors of the mGluR1 subtype, only 15% of Purkinje cells remained multiply innervated by climbing fibres, but motor coordination was largely impaired in these animals. The present results do not preclude the existence of a causal relationship between impairement of regression of multiple innervation during development and improper motor coordination in the adult.

Receptors and second messengers involved in long-term depression in rat cerebellar slices in vitro: a reappraisal.

In patch-clamped Purkinje cells (PCs), bath application of the ionotropic glutamate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) prevents induction of long-term depression (LTD) of parallel fibre (PF)-mediated EPSPs by a pairing protocol between Ca2+ spike firing and PF stimulation whereas bath application of (RS)-alpha-methyl-4-carboxyphenylglycine (MCPG), a metabotropic glutamate (mGLU) receptor antagonist, does not. On the other hand, LTD can be also induced by pairing direct depolarization of PCs with activation of mGLU receptors by 1S,3R-aminocyclopentyl-dicarboxylate (1S,3R-ACPD), even in the presence of CNQX. In this case, LTD induction is not consistently blocked by bath application of the nitric oxide synthase inhibitor, NG-methyl-L-arginine (L-NMMA), whereas it is strongly blocked when the protein kinase C inhibitor peptide 19-36 is dialysed into PCs. These results are at variance with LTD induced by a pairing protocol between Ca2+ spikes and PF-mediated EPSPs which depends to the same extent on both cascades. Finally, thapsigargin, which depletes most intracellular Ca2+ pools, does not block induction of LTD by a pairing protocol between Ca2+ spikes and PF-mediated EPSPs whereas it prevents the induction of LTD depending on strong mGLU receptor activation.

Cellular locus of the nitric oxide-synthase involved in cerebellar long-term depression induced by high external potassium concentration.

The cellular location of the NO-synthase involved in long-term depression (LTD) of parallel fiber (PF)-mediated EPSCs induced by raising the external potassium (K) concentration has been investigated by using both whole-cell patch-clamp recordings (WCR) of Purkinje cells (PCs) in thin slices in vitro, and reverse transcription followed by polymerase chain reaction (PCR) applied to mRNAs harvested from these single PCs during WCR. In all tested cells in the control group, a large LTD of PF-mediated EPSCs was induced by perfusing the slices for 3 min with a high (30 mM) K perfusing medium. In a second group of cells for which the protein kinase C (PKC) inhibitor peptide 19-36 was added to the intrapipette solution at a concentration of 10 microM, the LTD following complete wash out of the high K solution was significantly less prominent than in the control group. Very similar results were also obtained when 30 microM NG-methyl-L-arginine (L-NMMA) was added to the perfusing medium. In contrast, when both the PKC inhibitor peptide 19-36 and L-NMMA were added to the intrapipette solution at a concentration of 10 and 30 microM respectively, no LTD was revealed following wash out of the high K solution. Finally, the PCR amplification of mRNAs harvested from these single PCs during WCR, as well as from granule cells from the same slices, confirms that mRNAs encoding the NO-synthase are expressed by granule cells, whereas they are not detected in PCs.

Properties of glutamate receptors are modified during long-term depression in rat cerebellar Purkinje cells.

Long-term depression (LTD) of synaptic transmission at parallel fiber (PF)-Purkinje cell (PC) synapses occurs when these synapses are activated in conjunction with direct activation of voltage-gated calcium (Ca2+) channels of PCs. In the present study, we have used Aniracetam to test whether the expression of LTD at PF-PC synapses is due to a genuine modification of properties of alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) receptors of these neurons. Whole-cell recordings of PF-mediated EPSCs were performed in thin slices taken from 16-22-day-old rats. In all tested cells, bath application of Aniracetam potentiated PF-mediated EPSCs and prolonged their decay without notably changing their rising phase. On the other hand, Aniracetam prevented the induction of LTD by a pairing protocol with Ca2+ spikes and, conversely, the nootropic compound had a larger potentiating effect on PF-mediated EPSCs during expression of LTD than normally, when this change in synaptic efficacy had been induced prior to Aniracetam application. These data strongly suggest that LTD involves a desensitization of postsynaptic AMPA receptors at PF-PC synapses, or, at least, a change in their functional characteristics.

Long-term depression requires nitric oxide and guanosine 3':5' cyclic monophosphate production in rat cerebellar Purkinje cells.

In patch-clamped Purkinje cells, bath application of the nitric oxide synthase inhibitor NG-methyl-L-arginine consistently prevents the induction of long-term depression (LTD) of parallel fibre-mediated excitatory postsynaptic potentials (EPSPs) induced by their pairing with calcium spikes. On the other hand, bath application of nitric oxide donors and of 8-bromoguanosine 3':5' cyclic monophosphate is able to reproduce an LTD-like phenomenon. LTD of parallel fibre-mediated EPSPs also occurs when nitric oxide donors or guanosine 3':5' cyclic monophosphate are directly dialysed into Purkinje cells, and this effect partially occludes LTD induced by pairing protocols. These results show that nitric oxide does play a role in LTD induction, and demonstrate for the first time that its site of action is probably the soluble guanylate cyclase of Purkinje cells.

Coactivation of metabotropic glutamate receptors and of voltage-gated calcium channels induces long-term depression in cerebellar Purkinje cells in vitro.

Using an in vitro slice preparation, we studied the effects, on parallel fiber (PF)-mediated EPSPs, of coactivation of metabotropic-glutamate receptors and of voltage-gated calcium (Ca) channels of Purkinje cells (PCs) by bath application of 50 microM trans-1-amino-cyclopentyl-1,3-dicarboxylate (trans-ACPD) and by direct depolarization of the cells, respectively. These effects were compared with changes in synaptic efficacy obtained when alpha-amino-3hydroxy-5-methylisoxalone-4-propionate (AMPA) receptors of PCs were also activated through stimulation of PFs during the pairing protocol, as well as when similar experiments were performed without trans-ACPD in the bath. In a control medium, pairing for 1 min of PF-mediated EPSPs evoked at 1 Hz with Ca spikes evoked by steady depolarization of PCs (n = 13) led to LTD of synaptic transmission in 9 cases whereas for the others EPSPs were not affected. No LTD occurred in 9 out of 10 other cells tested when PF stimulation was omitted during the 1 min period of Ca spike firing of PCs. Bath application of 50 microM trans-ACPD, in conjunction with the same pairing protocol as before (n = 8), led to a significantly larger LTD of PF-mediated EPSPs after washing out of this drug. Moreover, a clear-cut LTD of PF-mediated EPSPs was also observed in 5 of the 8 other cells, when PF stimulation was omitted during Ca spike firing in the presence of trans-ACPD.(ABSTRACT TRUNCATED AT 250 WORDS)

Pairing of pre- and postsynaptic activities in cerebellar Purkinje cells induces long-term changes in synaptic efficacy in vitro.

1. An in vitro slice preparation of rat cerebellar cortex was used to analyse long-lasting modifications of synaptic transmission at parallel fibre (PF)-Purkinje cell (PC) synapses. These use-dependent changes were induced by pairing PF-mediated EPSPs evoked at low frequency (1 Hz) with different levels of membrane polarization (or bioelectrical activities) of PCs for 15 min. 2. Experiments were performed on forty-eight PCs recorded intracellularly in a conventional perfused chamber, and in fifty other cells maintained in a static chamber either in the presence (n = 21) or in the absence (n = 29) of 400 nM-phorbol 12,13-dibutyrate (PDBu). 3. In these three experimental conditions, PF-mediated EPSPs were always measured on PCs maintained at a holding potential of -75 mV, and further hyperpolarized by constant hyperpolarizing pulses. This allowed us both to test the input resistance of PCs and to avoid their firing during PF-mediated EPSPs. 4. In all cells retained for the present study, latencies of PF-mediated EPSPs evoked at 0.2 Hz were stable during the pre-pairing period, and the same was true for their amplitude and time course. 5. In the perfused chamber, pairing of PF-mediated EPSPs with the same hyperpolarization of PCs as that used for measurements of synaptic responses had no effect on these EPSPs in 30% of PCs. It induced long-term depression (LTD) and long-term potentiation (LTP) in 23 and 47% of the tested cells respectively (n = 17). 6. In the perfused chamber, pairing of PF-mediated EPSPs with moderate depolarization of PCs (n = 19) giving rise to a sustained firing of sodium spikes significantly favoured the appearance of LTP as compared to the previous pairing protocol. However, there were still 27 and 15% of cells which showed no modification and LTD respectively. 7. In contrast, pairing of PF-mediated EPSPs with calcium (Ca2+) spikes evoked by strong depolarization of PCs (n = 12) led to LTD of synaptic transmission in nearly half of the tested cells, whereas LTP was now observed in less than 20% of them. 8. In the static chamber and in the absence of PDBu, LTD of PF-mediated EPSPs was observed in most cells, whatever the pairing protocol with sodium or Ca2+ spikes. 9. This shift towards LTD was significantly reversed by PDBu in the pairing protocol using firing of sodium spikes, but not in the case of pairings with Ca2+ spikes.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of ACPD and AP3 on parallel-fibre-mediated EPSPs of Purkinje cells in cerebellar slices in vitro.

The effects of trans-1-amino-cyclopentyl-1,3-dicarboxylate (trans-ACPD) and of DL-2-amino-3-phosphonopropionic acid (AP3), i.e. selective agonist and antagonist of metabotropic quisqualate receptors respectively, on parallel fibre (PF)-mediated EPSPs of Purkinje cells (PCs) were studied in an in vitro slice preparation. Bath application of 500 microM trans-ACPD in conjunction with PF stimulation at 0.2 or 1 Hz depending on the cell always induced a marked depression of PF-mediated EPSPs, which was fully reversible in most cases after wash-out of this compound. Trans-ACPD also often induced a transient depolarization of PCs which induced calcium spike firing in these cells and which again no longer persisted after wash-out of trans-ACPD. Even in cells which were depolarized by trans-ACPD, the decrease in amplitude of PF-mediated EPSPs started before the appearance of calcium spikes, lasted longer than the transient depolarizing effect of trans-ACPD, and was accompanied by no variation in input resistance of the cells when they were manually clamped at their initial resting potential. Bath application of 600 microM DL-AP3 had no effect on PF-mediated EPSPs or the bioelectrical activities of PCs. Moreover, it did not prevent the effects of trans-ACPD mentioned before. The present results are not consistent with the view that coactivation of ionotropic and metabotropic quisqualate receptors of PCs is sufficient to induce a long-term depression of PF-mediated EPSPs.

Protein kinases, nitric oxide and long-term depression of synapses in the cerebellum.

We have analysed the effects of polymyxin B, a potent inhibitor of calcium-dependent protein kinases, of L-N-monomethylarginine, an inhibitor of nitric oxide synthesis, and of methylene blue which prevents activation of soluble guanylate cyclase, on long-term depression of parallel fibre-mediated EPSPs of rat cerebellar Purkinje cells in slices maintained in-vitro. In control conditions, a long-term depression of parallel fibre-mediated EPSPs was consistently induced by their pairing with calcium spikes directly elicited in the postsynaptic cells. This long-term change in synaptic strength was not observed in the presence of polymyxin B, of L-N-monomethylarginine, or of methylene blue, suggesting that calcium-dependent protein kinases and nitric oxide are both involved.