SHANK3 mutations identified in autism lead to modification of dendritic spine morphology via an actin-dependent mechanism

Genetic mutations of SHANK3 have been reported in patients with intellectual disability, autism spectrum disorder (ASD) and schizophrenia. At the synapse, Shank3/ProSAP2 is a scaffolding protein that connects glutamate receptors to the actin cytoskeleton via a chain of intermediary elements. Although genetic studies have repeatedly confirmed the association of SHANK3 mutations with susceptibility to psychiatric disorders, very little is known about the neuronal consequences of these mutations. Here, we report the functional effects of two de novo mutations (STOP and Q321R) and two inherited variations (R12C and R300C) identified in patients with ASD. We show that Shank3 is located at the tip of actin filaments and enhances its polymerization. Shank3 also participates in growth cone motility in developing neurons. The truncating mutation (STOP) strongly affects the development and morphology of dendritic spines, reduces synaptic transmission in mature neurons and also inhibits the effect of Shank3 on growth cone motility. The de novo mutation in the ankyrin domain (Q321R) modifies the roles of Shank3 in spine induction and morphology, and actin accumulation in spines and affects growth cone motility. Finally, the two inherited mutations (R12C and R300C) have intermediate effects on spine density and synaptic transmission. Therefore, although inherited by healthy parents, the functional effects of these mutations strongly suggest that they could represent risk factors for ASD. Altogether, these data provide new insights into the synaptic alterations caused by SHANK3 mutations in humans and provide a robust cellular readout for the development of knowledge-based therapies.

GPCR-interacting proteins (GIPs): nature and functions

The simplistic idea that seven transmembrane receptors are single monomeric proteins that interact with heterotrimeric G-proteins after agonist binding is definitively out of date. Indeed, GPCRs (G-protein-coupled receptors) are part of multiprotein networks organized around scaffolding proteins. These GIPs (GPCR-interacting proteins) are either transmembrane or cytosolic proteins. Proteomic approaches can be used to get global pictures of these 'receptosomes'. This approach allowed us to identify direct but also indirect binding partners of serotonin receptors. GIPs are involved in a wide range of functions including control of the targeting, trafficking and signalling of GPCRs. One of them, Shank, which is a secondary and tertiary partner of metabotropic and ionotropic glutamate receptors, respectively, can induce the formation of a whole functional glutamate 'receptosome' and the structure to which it is associated, the dendritic spine.

Activation of a Large-conductance Ca2+-Dependent K+ Channel by Stimulation of Glutamate Phosphoinositide-coupled Receptors in Cultured Cerebellar Granule Cells

Trans-1-amino-cyclopentyl-1,3-dicarboxylic acid (trans-ACPD), a specific agonist of the glutamate phosphoinositide-coupled receptor (Qp receptor), increased the amplitude of the outward K+ current recorded in the whole-cell configuration of the patch-clamp technique in mouse cultured cerebellar granule cells. This effect was abolished by buffering internal Ca2+ with BAPTA [1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid]. Activation of a large-conductance K+ channel was observed when trans-ACPD or quisqualic acid (QA), another Qp receptor agonist, was applied outside the cell-attached patch pipettes. No activation was observed with alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), a specific agonist of ionotropic non-N-methyl-d-aspartate (non-NMDA) receptors. The effects of trans-ACPD or QA were potentiated in the presence of external Ca2+. The channel was also directly activated by both micromolar concentrations of internal Ca2+ and membrane depolarization. Its unitary conductance was 100 - 115 pS under asymmetrical K+ and 195 - 235 pS under high symmetrical K+ conditions. In the absence of agonist, the channel was blocked by 1 mM external tetraethylammonium. This is the first description of a large conductance Ca2+-activated K+ channel in cultured cerebellar granule cells. It possesses properties similar to those of the so-called 'big K+ channel' described in other preparations. Our cell-attached experiments demonstrated an indirect coupling between Qp receptors and this channel. The most likely hypothesis is that the second messenger system inositol 1,4,5-triphosphate (IP3)-Ca2+ was involved in the coupling process. This hypothesis was further strengthened by our whole-cell experiments. On the basis of the voltage- and Ca2+-sensitivities of the studied channel, we estimated an increase of 350 to 570 nM in internal Ca2+ concentration when Qp receptors were stimulated by 100 microM trans-ACPD. Under physiological conditions, stimulation of Qp receptors by the endogenous neurotransmitter should lead to similar K+ channel activation and therefore would tend to reduce the efficacy of ionotropic glutamate synaptic receptor stimulation responsible for cell excitation.

PICK1 is required for the control of synaptic transmission by the metabotropic glutamate receptor 7

Both postsynaptic density and presynaptic active zone are structural matrix containing scaffolding proteins that are involved in the organization of the synapse. Little is known about the functional role of these proteins in the signaling of presynaptic receptors. Here we show that the interaction of the presynaptic metabotropic glutamate (mGlu) receptor subtype, mGlu7a, with the postsynaptic density-95 disc-large zona occludens 1 (PDZ) domain-containing protein, PICK1, is required for specific inhibition of P/Q-type Ca(2+) channels, in cultured cerebellar granule neurons. Furthermore, we show that activation of the presynaptic mGlu7a receptor inhibits synaptic transmission and this effect also requires the presence of PICK1. These results indicate that the scaffolding protein, PICK1, plays an essential role in the control of synaptic transmission by the mGlu7a receptor complex.

The C terminus of the metabotropic glutamate receptor subtypes 2 and 7 specifies the receptor signaling pathways

There is accumulating evidence that the specificity of the transduction cascades activated by G protein-coupled receptors cannot solely depend on the nature of the coupled G protein. To identify additional structural determinants, we studied two metabotropic glutamate (mGlu) receptors, the mGlu2 and mGlu7 receptors, that are both coupled to G(o) proteins but are known to affect different effectors in neurons. Thus, the mGlu2 receptor selectively blocks N- and L-type Ca(2+) channels via a protein kinase C-independent pathway, whereas the mGlu7 receptor selectively blocks P/Q-type Ca(2+) channels via a protein kinase C-dependent pathway, and both effects are pertussis toxin-sensitive. We examined the role of the C-terminal domain of these receptors in this coupling. Chimeras were constructed by exchanging the C terminus of these receptors and transfected into neurons. Different chimeric receptors bearing the C terminus of mGlu7 receptor blocked selectively P/Q-type Ca(2+) channels, whereas chimeras bearing the C terminus of mGlu2 receptor selectively blocked N- and L-type Ca(2+) channels. These results show that the C terminus of mGlu2 and mGlu7 receptors is a key structural determinant that allows these receptors to select a specific signaling pathway in neurons.

AMPA receptor activation induces association of G-beta protein with the alpha subunit of the sodium channel in neurons

Glutamatergic transmission is mediated by ionotropic receptors that directly gate cationic channels and metabotropic receptors that are coupled to second messenger generating systems and to ionic channels via heterotrimeric guanine-nucleotide binding- (G) proteins. This distinction cannot be made for the ionotropic receptor subclass activated by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), which has been shown to be physically associated with the alpha-subunit of Gi1 protein and activates this G-protein. Here, we report that, in addition to a Ca2+ influx, AMPA induces the mobilization of Ca2+ from the mitochondrial pool by reversing the mitochondrial Na+/Ca2+ exchanger in mouse neurons in primary culture. Both processes required the activation of tetrodotoxin-sensitive Na+ channels. AMPA receptor activation modified the gating properties of the Na+ channel, independently of the AMPA current, suggesting a G-protein-mediated process. Indeed, co-immunoprecipitation experiments indicated that AMPA receptor activation induced the association of Gbeta with the alpha-subunit of the Na+ channel. These results suggest that, in addition to its ionic channel function, the AMPA receptor is coupled to Na+ channels through G-proteins and that this novel metabotropic function is involved in the control of neuronal excitability.

Agonist-independent activation of metabotropic glutamate receptors by the intracellular protein Homer

G-protein-coupled receptors (GPCRs) transduce signals from extracellular transmitters to the inside of the cell by activating G proteins. Mutation and overexpression of these receptors have revealed that they can reach their active state even in the absence of agonist, as a result of a natural shift in the equilibrium between their inactive and active conformations. Such agonist-independent (constitutive) activity has been observed for the glutamate GPCRs (the metabotropic glutamate receptors mGluR1a and mGluR5) when they are overexpressed in heterologous cells. Here we show that in neurons, the constitutive activity of these receptors is controlled by Homer proteins, which bind directly to the receptors' carboxy-terminal intracellular domains. Disruption of this interaction by mutagenesis or antisense strategies, or expression of endogenous Homer1a (H1a), induces constitutive activity in mGluR1a or mGluR5. Our results show that these glutamate GPCRs can be directly activated by intracellular proteins as well as by agonists.

Dendritic and axonal targeting of type 5 metabotropic glutamate receptor is regulated by homer1 proteins and neuronal excitation

The physiological actions of neurotransmitter receptors are intimately linked to their proper neuronal compartment localization. Here we studied the effect of the metabotropic glutamate receptor (mGluR)-interacting proteins, Homer1a, b, and c, in the targeting of mGluR5 in neurons. We found that mGluR5 was exclusively localized in cell bodies when transfected alone in cultured cerebellar granule cells. In contrast, mGluR5 was found also in dendrites when coexpressed with Homer1b or Homer1c, and in both dendrites and axons when cotransfected with Homer1a. In dendrites, cotransfected mGluR5 and Homer1b/c formed clusters that colocalized with the synaptic marker synaptophysin. Interestingly when transfected alone, the Homer proteins were also translocated to neurites but did not form such clusters. Depolarization of the neurons with a mixture of ionotropic glutamate receptor agonists, NMDA and kainate, or potassium channel blockers, tetraethylammonium and 4-aminopyridine, induced transient expression of endogenous Homer1a and persistent neuritic localization of transfected mGluR5 even long after degradation of Homer1a. These results suggest that Homer1a/b/c proteins are involved in the targeting of mGluR5 to dendritic synaptic sites and/or axons and that this effect can be regulated by neuronal activity. Because the activity-dependent effect of endogenous Homer1a was also long-lasting, the axonal targeting of mGluR5 by this protein is likely to play an important role in synaptic plasticity.

Selective blockade of P/Q-type calcium channels by the metabotropic glutamate receptor type 7 involves a phospholipase C pathway in neurons

Although presynaptic localization of mGluR7 is well established, the mechanism by which the receptor may control Ca(2+) channels in neurons is still unknown. We show here that cultured cerebellar granule cells express native metabotropic glutamate receptor type 7 (mGluR7) in neuritic processes, whereas transfected mGluR7 was also expressed in cell bodies. This allowed us to study the effect of the transfected receptor on somatic Ca(2+) channels. In transfected neurons, mGuR7 selectively inhibited P/Q-type Ca(2+) channels. The effect was mimicked by GTPgammaS and blocked by pertussis toxin (PTX) or a selective antibody raised against the G-protein alphao subunit, indicating the involvement of a G(o)-like protein. The mGuR7 effect did not display the characteristics of a direct interaction between G-protein betagamma subunits and the alpha1A Ca(2+) channel subunit, but was abolished by quenching betagamma subunits with specific intracellular peptides. Intracellular dialysis of G-protein betagamma subunits did not mimic the action of mGluR7, suggesting that both G-protein betagamma and alphao subunits were required to mediate the effect. Inhibition of phospholipase C (PLC) blocked the inhibitory action of mGluR7, suggesting that a coincident activation of PLC by the G-protein betagamma with alphao subunits was required. The Ca(2+) chelator BAPTA, as well as inhibition of either the inositol trisphosphate (IP(3)) receptor or protein kinase C (PKC) abolished the mGluR7 effect. Moreover, activation of native mGluR7 induced a PTX-dependent IP(3) formation. These results indicated that IP(3)-mediated intracellular Ca(2+) release was required for PKC-dependent inhibition of the Ca(2+) channels. Possible control of synaptic transmission by the present mechanisms is discussed.

Imidazoline-induced neuroprotective effects result from blockade of NMDA receptor channels in neuronal cultures

Imidazolines have been shown to be neuroprotective in focal and global ischemia in the rat. However, their mechanism of action is still unclear. We have studied the neuroprotective effects of imidazolines against NMDA-induced neuronal death and hypoxic insult in cerebellar and striatal neuronal cultures. All of the imidazolines tested decreased the NMDA-mediated neurotoxicity in a non-competitive manner. Antazoline was the most effective (IC(50) of 5 microM, maximal neuroprotection reaching 90% at 100 microM). The neuroprotective effects were still present when the imidazolines were applied during the post-insult period. Antazoline, idazoxan and guanabenz also showed neuroprotective effects against hypoxia-induced neuronal death (neuroprotection reaching 95% for antazoline at 100 microM). Antazoline was still active if applied during the reoxygenation period (15% neuroprotection). To determine the mechanism of the neuroprotective effects, the possible interaction of imidazolines with NMDA receptors was studied. Imidazolines dose-dependently and non-competitively inhibited NMDA currents. As found for the neuroprotective effects, antazoline was the most effective imidazoline, with an IC(50) of 4 microM and a maximal inhibition of 90% at 100 microM. This blockade was rapid, reversible and voltage-dependent. We compared these effects to those of the classical non-competitive antagonist of NMDA channels, MK-801. In contrast to imidazolines, blockade of the NMDA current by MK-801 was voltage-independent and reversible only at positive potentials. When co-applied with MK-801, antazoline prevented the long lasting blockade of the NMDA current by MK-801. These results are consistent with the existence of overlapping binding sites for these drugs on the NMDA receptor channel. They indicate that imidazolines exert a strong neuroprotective effect against excitotoxicity and hypoxia in cerebellar and striatal primary neuronal cultures by inhibiting NMDA receptors. Since these effects were non-competitive, imidazolines appear to be interesting new drugs with therapeutic potential.

Complex interactions between mGluRs, intracellular Ca2+ stores and ion channels in neurons

Metabotropic glutamate receptors (mGluRs) can increase intracellular Ca2+ concentration via Ins(1,4,5)P3- and ryanodine-sensitive Ca2+ stores in neurons. Both types of store are coupled functionally to Ca2+-permeable channels found in the plasma membrane. The mGluR-mediated increase in intracellular Ca2+ concentration can activate Ca2+-sensitive K+ channels and Ca2+-dependent nonselective cationic channels. These mGluR-mediated effects often result from mobilization of Ca2+ from ryanodine-sensitive, rather than Ins(1,4, 5)P3-sensitive, Ca2+ stores, suggesting that close functional interactions exist between mGluRs, intracellular Ca2+ stores and Ca2+-sensitive ion channels in the membrane.

Inositol-1,4,5-trisphosphate-mediated rescue of cerebellar long-term depression in subtype 1 metabotropic glutamate receptor mutant mouse

Recent reports have outlined that cerebellar long-term depression requires the activation of subtype 1 metabotropic glutamate receptors, since long-term depression is impaired in subtype 1 metabotropic glutamate receptor (mGluR1) knockout mice. In order to better define the role of mGluR1-activated signal transduction pathways, we attempted to rescue cerebellar long-term depression in mGluR1 knockout mice by direct activation of subsequent intracellular cascades. The present results demonstrate that the inositol-1,4,5-trisphosphate signal transduction pathway remains functional in mGluR1 knockout mice, that calcium release from internal stores evoked by the combined photolytic release of inositol- 1,4,5-trisphosphate/pairing protocol is sufficient to rescue long-term depression in these mutants, and that this long-term depression is sensitive to a protein kinase C inhibitor. Therefore, our results provide compelling evidence that the impairment of long-term depression observed in mGluR1 knockout mice is not a consequence of developmental abnormalities, but is directly due to mGluR1 gene inactivation.

A simple method to transfer plasmid DNA into neuronal primary cultures: functional expression of the mGlu5 receptor in cerebellar granule cells

We describe a method to transfer cDNA into neuronal primary cultures with a commercialised cationic lipid, Transfast. Cultures were transfected at a rate of about 5% with green fluorescent protein (GFP) cDNA. Comparing Transfast to other transfection reagents, we found this compound to be the most efficient. GFP-transfected mouse cerebellar granule cells displayed normal whole-cell voltage-sensitive and unitary big K+ channel currents. We also used this transfection method with success to transfer GFP cDNA into primary cultures of striatum and colliculus. Transfast was then used to cotransfect cultured cerebellar cells with GFP cDNA, in conjunction with cDNA coding for the metabotropic glutamate receptor type 5 (mGlu5 receptor). Ninety percent of the cells expressing GFP also expressed mGlu5 receptor. Though neurones were best transfected one day after plating, they still expressed both GFP and mGlu5 receptor proteins 2 weeks after plating, i.e. after full differentiation. A functional test of the expressed mGlu5 receptor was thus performed in GFP-transfected neurones. Stimulation of mGlu5 receptor induced single big K+ channel activity, as it was the case for the native mGlu1 receptor. This indicated that the transfected mGlu5 receptor plasmid was functionally expressed and that both mGlu1 and mGlu5 receptors may share common coupling mechanisms to big K+ channels in neurones.

mGluR7-like metabotropic glutamate receptors inhibit NMDA-mediated excitotoxicity in cultured mouse cerebellar granule neurons

Glutamate-induced glutamate release may be involved in the delayed neuronal death induced by N-methyl-D-aspartate (NMDA). In order to examine a possible modulatory effect of the presynaptic group III mGluRs on glutamate excitotoxicity, the effect of L-2-amino-4-phosphonobutyrate (L-AP4) was examined on NMDA-induced delayed death of mouse cerebellar granule neurons in culture. We found that L-AP4, at high concentration (in the millimolar range), inhibited in a non-competitive manner the NMDA-induced toxicity. This effect was mimicked by high concentration of L-serine-o-phosphate (L-SOP), and was inhibited by pertussis toxin (PTX) indicating the involvement of a Gi/o protein. This suggests the involvement of mGluR7 in the L-AP4 effect, and this was consistent with the detection of both mGluR7 protein and mRNA in these cultured neurons. To examine the mechanism of the L-AP4-induced protection from excitotoxic damage, the effect of L-AP4 on glutamate release was examined. L-AP4 (> or = 1 mM) noncompetitively inhibited by more than 60% the glutamate release induced by NMDA during the insult. We also observed that the 10-min NMDA receptor stimulation resulted in a dramatic increase in the extracellular glutamate concentration reaching 6000% of the control value 24 h after the insult. This large increase was also inhibited when NMDA was applied in the presence of > or = 1 mM L-AP4. Part of the L-AP4-induced protection from excitotoxic damage of granule neurons may therefore result from the inhibition of the vicious cycle: dying cells release glutamate, glutamate induced cell death. The present results add to the hypothesis that presynaptic mGluRs, probably mGluR7, may be the targets of drugs decreasing glutamate release and then neuronal death observed in some pathological situations.

cAMP-mediated long-term modulation of voltage-dependent K+ channels in cultured colliculi neurons

Previously, we have described prolonged cAMP-induced inhibition of a K+ current in cultured colliculi neurons. The aim of the present study was to characterize the channel responsible for this cAMP-dependent effect. We detected the presence of a non-inactivating voltage-dependent 16-pS K+ channel that displayed long-lasting inhibition upon a brief application of cAMP and greater sensitivity to tetraethylammonium than to 4-aminopyridine. In addition to this channel, colliculi neurons express two other voltage-sensitive, non-inactivating K+ channels (8 and 49 pS) whose activity is facilitated by a brief application of cAMP, the effect of which is also long-lasting. These results suggest the presence of common sustained cAMP-dependent processes responsible for both up- and down-regulation of these channels in the neurons studied. They indicate that the 16-pS, but not the 8-pS or the 49-pS channels, participates in the cAMP-inhibited macroscopic K+ current.

Modulation of big K+ channel activity by ryanodine receptors and L-type Ca2+ channels in neurons

As metabotropic glutamate receptor type 1 (mGluR1) is known to couple L-type Ca2+ channels and ryanodine receptors (RyR, Chavis et al., 1996) in cerebellar granule cells, we examined if such a coupling could activate a Ca2+-sensitive K+ channel, the big K+ (BK) channel, in cultured cerebellar granule cells. We observed that (+/-)-1-amino-cyclopentane-trans-1,3-dicarboxylic acid (t-ACPD) and quisqualate (QA) stimulated the activity of BK channels. On the other hand, (2S, 3S, 4S)-alpha-carboxycyclopropyl-glycine (L-CCG-I) and L-(+)-2-amino-4-phosphonobutyrate (L-AP4) had no effect on BK channels, indicating a specific activation by group I mGluRs. Group I mGluRs stimulation of the basal BK channel activity was mimicked by caffeine and both effects were blocked by ryanodine and nifedipine. Interestingly, carbachol stimulated BK channel activity but through a pertussis toxin (PTX)-sensitive pathway that was independent of L-type Ca2+ channel activity. Our report indicates that unlike the muscarinic receptors, group I mGluRs activate BK channels by mobilizing an additional pathway involving RyR and L-type Ca2+ channels.

Synthesis and pharmacological characterization of aminocyclopentanetricarboxylic acids: new tools to discriminate between metabotropic glutamate receptor subtypes

The four stereoisomers of 1-aminocyclopentane-1,3,4-tricarboxylic acid {ACPT-I (18) and -II (19), (3R, 4R)-III [(-)-20], and (3S,4S)-III [(+)-20]} have been synthesized and evaluated for their effects at glutamate receptors subtypes. ACPTs are ACPD analogues in which a third carboxylic group has been added at position 4 in the cyclopentane ring. None of the ACPT isomers showed a significant effect on ionotropic NMDA, KA, and AMPA receptors. On the other hand, ACPT-II (19) was found to be a general competitive antagonist for metabotropic receptors (mGluRs) and exhibited a similar affinity for mGluR1a (KB = 115 +/- 2 microM), mGluR2 (KB = 88 +/- 21 microM), and mGluR4a (KB = 77 +/- 9 microM), the representative members of group I, II and III mGluRs, respectively. Two other isomers, ACPT-I (18) and (+)-(3S,4S)-ACPT-III [(+)-20], were potent agonists at the group III receptor mGluR4a (EC50 = 7.2 +/- 2.3 and 8.8 +/- 3.2 microM) and competitive antagonists with low affinity for mGluR1a and mGluR2 (KB > 300 microM). Finally, (-)-(3R,4R)-ACPT-III [(-)-20] was a competitive antagonist with poor but significant affinity for mGluR4a (KB = 220 microM). These results demonstrate that the addition of a third carboxylic group to ACPD can change its activity (from agonist to antagonist) and either increase or decrease its selectivity and/or affinity for the various mGluR subtypes.

Functional coupling between ryanodine receptors and L-type calcium channels in neurons

In skeletal muscle, L-type Ca2+ channels act as voltage sensors to control ryanodine-sensitive Ca2+ channels in the sarcoplasmic reticulum. It has recently been demonstrated that these ryanodine receptors generate a retrograde signal that modifies L-type Ca2+ -channel activity. Here we demonstrate a tight functional coupling between ryanodine receptors and L-type Ca2+ channel in neurons. In cerebellar granule cells, activation of the type-1 metabotropic glutamate receptor (mGluR1) induced a large, oscillating increase of the L-type Ba2+ current. Activation occurred independently of inositol 1,4,5-trisphosphate and classical protein kinases, but was mimicked by caffeine and blocked by ryanodine. The kinetics of this blockade were dependent on the frequency of Ba2+ current stimulation. Both mGluR1 and caffeine-induced increase in L-type Ca2+ -channel activity persisted in inside-out membrane patches. In these excised patches, ryanodine suppressed both the mGluR1- and caffeine-activated L-type Ca2+ channels. These results demonstrate a novel mechanism for Ca2+ -channel modulation in neurons.

Effects of nitric oxide on glutamate-gated channels and other ionic channels

Nitric oxide is an endogenous molecule that plays a role of second messenger in the central and peripheral nervous system. A major action of this molecule is to control ionic channel activity. Because of technical difficulties to use nitric oxide as a gaseous compound, nitric oxide donors are often utilized under controlled experimental conditions. Here we will review the advantages and limitations in using these compounds. Nitric oxide can affect ionic channels through direct interactions or through the production of cGMP. We will describe an example of direct action of nitric oxide on glutamate-gated channels. We will also review indirect actions of nitric oxide on various potassium and calcium channels. Finally, we will discuss the complex physiological consequences of the action of nitric oxide on these ionic channels.

Modulation of calcium channels by metabotropic glutamate receptors in cerebellar granule cells

We investigated the mechanisms by which metabotropic glutamate receptors (mGluRs) modulate specific Ca2+ channels in cerebellar granule cells. A large fraction of the current in granule cells is carried by L- and Q-type Ca2+ channels (about 26% each), whereas N- and P-type contribute proportionally less to the global current (9 and 15%, respectively). l-Aminocyclopentane-dicarboxylate (t-ACPD), (2S,3S,4S)-alpha-(carboxycyclopropyl)-glycine (L-CCGI) and (S)-4-carboxy-3-hydroxyphenylglycine [(S)-4C3HPG], but not L(+)-2-amino-4-phosphonobutyrate (L-AP4) reduced the Ca2+ current amplitude. The t-ACPD-induced inhibition was fully antagonized by (+/-)-methyl-4-carboxyphenylglycine [(+/-)-MCPG] and blocked by pertussis toxin (PTX). These results are consistent with inhibitory response mediated by mGluR2/R3. The use of specific Ca2+ channel blockers provided evidence that mGluR2/R3 inhibited both L- and N-type Ca2+ currents. In PTX-treated cells, Glu or t-ACPD, but not L-CCGI or L-AP4, increased the Ca2+ current. Consistent with the activation of mGluR1, the antagonists (+)-MCPG and (S)-4C3HPG prevented the facilitation of Ca2+ current produced by t-ACPD. The mGluR1-activated facilitation was completely blocked by nimodipine, indicating that L-type Ca2+ currents were selectively potentiated.

cAMP-dependent, long-lasting inhibition of a K+ current in mammalian neurons

We report the long-term modulation of K+ channels by cAMP in cultured murine colliculi neurons. A short (1-2 s) application of 8-Br-cAMP induced a long-lasting broadening of the action potential, a loss of after-hyperpolarization, and a reduction in spike accommodation. In agreement with these changes, 8-Br-cAMP produced a long-lasting (2 hr) inhibition of a K+ current. These effects were also observed after a short activation of the pituitary adenylyl cyclase-activating polypeptide, beta-adrenergic, and 5-hydroxytryptamine type 4 (5-HT4) receptors, all known to increase cAMP. A transient activation of the cAMP-dependent protein kinase and a long-lasting inhibition of phosphatases (up to 2 hr) were detected. The blockade of the K+ current resulting from a brief application of 8-Br-cAMP or 5-hydroxytryptamine was prolonged from 2 to 4 hr when protein-serine/threonine phosphatases 1 and 2A were inhibited with 10 nM okadaic acid. The critical steps following the cAMP-dependent protein kinase activation and resulting in a long-term blockade of phosphatases are discussed in this report.

Involvement of divalent ions in the nitric oxide-induced blockade of N-methyl-D-aspartate receptors in cerebellar granule cells

We have previously shown that nitric oxide blocks the N-methyl-D-aspartate (NMDA) receptor without affecting the agonist binding site. We now report that in cerebellar granule cells nitric oxide decreases the NMDA channel conductance and open probability, in voltage-dependent and -independent manners, respectively, by acting on an extracellular site different from the redox, glycine, and pH modulatory sites of the receptor-channel complex. This inhibition is not additive with those of Mg2+ and Zn2+. Moreover, removal of trace concentrations of metal ions in the external medium by means of metal ion-chelators significantly reduced the inhibitory action of nitric oxide on NMDA currents. These results indicate that divalent ions are required for the blockade of NMDA receptors by NO donors.

Action of argiotoxin636 on N-methyl-D-aspartate channels in cerebellar cells

We investigated the mode of action of argiotoxin636 on isolated N-methyl-D-aspartate (NMDA) receptor channels in cultured cerebellar granule cells. We found that the toxin blocks NMDA channels by decreasing their opening probability and by inducing a flickering activity, in a voltage-dependent manner. Our results indicate that argiotoxin636 acts as an open-channel blocker and might therefore be a useful tool for studying the structure of glutamate-gated channels.

[The role of nitric oxide and superoxides in the neurotoxicity of glutamate]

Glutamate is the major neurotransmitter of the mammalian brain. Stimulation of glutamate receptors, especially the subgroup of NMDA receptors, induces nitric oxide and arachidonic acid synthesis in neurons. These agents freely diffuse across membranes and thus can play roles of messengers in particular brain functions. The aim of our study was to identify these roles in in vitro and in vivo models from mouse and rat. Exaggerated stimulation of NMDA receptors leads to neurological disorders such as some types of epilepsy and neurodegenerative diseases. We show that superoxide ions, which probably result from metabolic degradation of arachidonic acid, would be responsible of the neurotoxic action of NMDA. On the other hand, we observed that nitric oxide inhibits NMDA receptors. This effect would protect animals against epileptic and neurodegenerative diseases mediated by over-stimulation of these receptors. This endogenous regulation may play important roles in the functioning of glutamatergic neurotransmission.

Facilitatory coupling between a glutamate metabotropic receptor and dihydropyridine-sensitive calcium channels in cultured cerebellar granule cells

The effect of metabotropic glutamate receptor activation on Ca dihydropyridine (DHP)-sensitive channels recorded in the presence of 1 microM Bay K 8644 was examined on cultured cerebellar granule cells using the patch-clamp technique in the cell-attached configuration. Bath-applied agonist (trans-ACPD, 1S,3R-, and 1R,3S-ACPD isomers, and glutamate or quisqualate in the presence of CPP and CNQX) evoked an increase in Ca channel activity with a variable latency of 8.9 +/- 8.6 sec in 40% of the recorded cells. Neither L-CCG1, L-AP3, L-AP4, nor AMPA or NMDA activated Ca channels. Two dihydropyridine-sensitive channels present in this cell type were activated by trans-ACPD: the classical 24 pS L-type channel and a smaller-conductance 7 pS channel. The effect was shown to be mediated by neither intracellular Ca2+ nor a pertussis toxin (PTX)-sensitive G protein. Interestingly treatment with BAPTA-AM increased the number of responding patches and the activity was more sustained throughout the drug application. After overnight PTX treatment, activation of the Ca channels persisted even after washout of the agonist. These results indicate that mGluR1/mGluR5 probably mediate the facilitation of dihydropyridine-sensitive Ca channels.

Inhibitory effects of dihydropyridines on macroscopic K+ currents and on the large-conductance Ca(2+)-activated K+ channel in cultured cerebellar granule cells

In cultured cerebellar granule cells, we examined the effects of dihydropyridines (DHPs) on K+ currents, using the whole-cell recording configuration of the patch-clamp technique and on Ca(2+)-activated K+ channels ("maxi K+ channels") using outside-out patches. We found that micromolar concentrations of nicardipine, nifedipine, (+) and (-) BAY K 8644, nitrendipine, nisoldipine and (-) nimodipine block 10-60% of macroscopic K+ currents. The most potent of these DHPs was nicardipine and the least potent, (-) BAY K 8644. (+) Nimodipine had no effect on this current. The inhibitory effects of nifedipine and nicardipine were not additive with those of 1 mM tetraethylammonium (TEA). Outside-out recordings of "maxi K+ channels" showed a main conductance of 200 pS (in 77% of the patches) and two subconductance states (in 23% of the patches). Neither nifedipine nor nicardipine affected the main conductance, but decreased the values of the subconductance levels. In 10% of these patches, nicardipine induced a flickering activity of the channel. These findings show that both Ca2+ and K+ channels have DHP-sensitive sites, suggesting similarity in electrostatic binding properties of these channels. Furthermore, cerebellar granule cells may express different subtypes of "maxi K+ channels" having different sensitivities to DHPs. These drugs may provide new tools for the molecular study of K+ channels.

Blockade of nitric oxide synthesis by tyrosine kinase inhibitors in neurones

In striatal neurones in culture, N-methyl-D-aspartate-(NMDA), kainate-(Kai) and K(+)-dependent cGMP production is entirely mediated via nitric oxide (NO). Low concentrations of lavendustin-A (< or = 0.3 microM), a highly specific tyrosine kinase inhibitor, reduced irreversibly and in a time-dependent manner NMDA-stimulated cGMP production. After a preincubation period of 20 min with lavendustin-A (0.3 microM), the inhibition of NMDA-induced cGMP production was equal to 56 +/- 8% (n = 6). After the same preincubation period, the IC50 of the lavendustin-A blockade was 30 +/- 15 nM. Genistein, another tyrosine kinase inhibitor also inhibited NMDA-dependent cGMP production with high potencies (< or = 3 microM). Whatever the tyrosine kinase inhibitor tested, the basal cGMP production remained unaffected. Kai-, K(+)-, and ionomycin-induced cGMP production was also inhibited by lavendustin-A, and genistein. In contrast, tyrosine kinase inhibitors were unable to block NO donor-induced cGMP production. Using patch clamp experiments, we have also found that lavendustin-A (0.3-1 microM), the most potent tyrosine kinase inhibitor used, (a) did not reduce the NMDA receptor-mediated current, (b) only slighly affected Kai receptor-mediated current (16.4 +/- 3.4% inhibition) and (c) had a marked effect on voltage-sensitive Ca2+ channel- (VSCC) mediated currents (44.4 +/- 4.9% inhibition). A reduction in VSCC activity certainly explains the inhibition of K(+)-, Kai- and possibly part of the NMDA-induced cGMP production.(ABSTRACT TRUNCATED AT 250 WORDS)

The metabotropic glutamate receptor types 2/3 inhibit L-type calcium channels via a pertussis toxin-sensitive G-protein in cultured cerebellar granule cells

Modulation of Ca2+ channels by metabotropic glutamate receptors (mGluRs) was investigated in cerebellar granule cells using the cell-attached configuration of the patch-clamp technique. Experiments were performed in the absence of external Ca2+ and Ba2+ was used as charge carrier. Bath applied glutamate or (1S,3R) trans-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R t-ACPD) inhibited Ca2+ channels activated by depolarizing pulses. These channels were sensitive to dihydropyridines and displayed a 23 pS conductance. This effect was mimicked by (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (L-CCG-I), a selective agonist of mGluR2/R3 receptors, but not by quisqualate at a concentration that stimulated inositol phosphate (InsP) synthesis, showing that mGluR1 and mGluR5 did not participate to this mechanism. The phosphodiesterase inhibitor, isobutylmethylxanthine (IBMX), did not alter the action of the mGluR agonists and biochemical measurements showed that 1S,3R t-ACPD, in the presence of IBMX, decreased cAMP formation in such a small amount that this change could not explain the almost complete inhibition of the channel activity observed under similar experimental conditions. Moreover, whole-cell recorded L-type Ca2+ currents were inhibited by L-CCG-I, in the presence of 1 mM intracellular cAMP. These observations were consistent with the hypothesis that cyclic nucleotide second messengers were not involved in this effect. Neither the protein kinase C activator phorbol-12,13-dibutyrate (PDBU) nor the phosphatase inhibitor okadaic acid affected the action of 1S,3R t-ACPD. The inhibitory action of 1S,3R t-ACPD was abolished by pertussis toxin (PTX). These results suggest that mGluR2 or mGluR3 receptors suppress the activity of L-type Ca2+ channels by a mechanism involving Gi or G(o) proteins. A likely direct effect of G-proteins on the channels is discussed.

The metabotropic glutamate receptor (MGR): pharmacology and subcellular location

A pharmacological characterization of the metabotropic glutamate receptor (MGR) was performed in striatal neurons. Among the excitatory amino acid receptor antagonists tested, only D, L-2-amino-3-phosphonopropionate (D, L-AP3) inhibited QA-induced inositol phosphate (InsP) formation in a competitive manner (mean pKi = 4.45 +/- 0.43, n = 4). However, this drug was a partial agonist of MGR since it stimulated the inositol-phosphate formation. We found that D, L-AP3 also inhibited NMDA-induced calcium increase, in a competitive manner (mean pIC50 = 4.34 +/- 0.22, n = 8, and mean pKi = 3.7 +/- 0.11 n = 5). 1 mM of the ionotropic agonists alpha-amino-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), kainate (KA) or domoate (DO) (100 microM or higher) induced a significant InsP formation in striatal neurons. The InsP responses induced by all these agonists were totally blocked by the phorbol ester phorbol-12,13-dibutyrate (PdBu), but not by atropine or prazosin. Agonist-induced increases of intracellular calcium concentrations ([Ca2+]i) were insensitive to PdBu, suggesting that all these substances were able to stimulate the MGR in striatal neurons. Trans-1-amino-cyclopentyl-1,3-dicarboxylate (trans-ACPD) evoked dose-dependent inositol phosphate formations with an EC50 of 29 microM but had no significant effect on NMDA or AMPA receptors, as measured by the patch clamp technique. In the presence of 30 microM of AMPA, trans-ACPD induced a significant release of arachidonic acid (AA) in striatal neurons. No important AA release was observed by any of these agonists alone. 56 mM K+ did not mimic AMPA in this associative ionotropic/metabotropic effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Characteristics of calcium channels responsible for voltage-activated calcium entry in rat cerebellar granule cells

The properties and characteristics of calcium channel openings in cerebellar granule cells were analysed by the cell-attached patch-clamp technique. At depolarized potentials, with 110 mM Ba2+ as the divalent charge carrier, 36% of the patches displayed activity that consisted of elementary events whose amplitude ranged from -0.3 to -1.75 pA at 0 mV, giving rise to a high threshold current. In this population of events at least four different types of channel openings were identified by their distinct biophysical and pharmacological properties. Two types of channel openings, with conductances around 24 and 7 pS, had similar characteristics in that both opened following two modes of gating characterized by brief (approximately 2 ms) and longer openings (approximately 8 ms) and both were sensitive to dihydropyridines. A further type of channel opening, with a conductance around 11 pS gated mainly with brief openings (approximately 1 ms), was shown to be insensitive to dihydropyridines but was undetectable in recordings from the cells that had been treated with omega-conotoxin. The last type of event was revealed after treatment of the cell with nicardipine or nifedipine and omega-conotoxin. The corresponding channel had a conductance of 19 pS and opened in one dominant mode characterized by brief openings (approximately 1 ms). The data obtained on single-channel activity of cerebellar granule cells are compared with the properties of the total current recorded in whole-cell conditions.

Metabotropic glutamate receptors: an original family of G protein-coupled receptors

In 1985, we discovered a new glutamate receptor which was coupled to phospholipase C via a G protein and which was later termed metabotropic glutamate receptor (mGluR). In this review, both the diversity of mGluRs and the cellular events they control are discussed, as well as their roles in physiological regulation and brain function.

Stimulation by glutamate receptors of arachidonic acid release depends on the Na+/Ca2+ exchanger in neuronal cells

In primary cultures of striatal neurons, stimulation of N-methyl-D-aspartic acid (NMDA) receptors or associative activation (but not separate activation) of (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and metabotropic glutamate receptors (mGluR) strongly increased arachidonic acid (AA) release via activation of phospholipase A2 (PLA2). Depolarizing agents, such as veratridine, were as potent as NMDA in stimulating AA release. However, increasing the intracellular Ca2+ concentration via voltage-sensitive Ca2+ channels did not result in a significant stimulation of PLA2. Substitution of sodium by lithium, a monovalent cation that does not participate in the Na+/Ca2+ exchanger activity but permeates ionotropic glutamate receptor channels, blocked AA release induced by veratridine or AMPA plus mGluR agonists. It also reduced the NMDA-induced AA release, to a lesser extent. The contribution of the Na+/Ca2+ exchanger to the activation of PLA2 after veratridine, NMDA receptor, or AMPA receptor plus mGluR stimulation was confirmed by using a selective inhibitor of the Na+/Ca2+ exchanger.

Nitric oxide-induced blockade of NMDA receptors

We studied the effects of nitric oxide (NO)-producing agents on N-methyl-D-aspartate (NMDA) receptor activation in cultured neurons. 3-Morpholino-sydnonimine (SIN-1) blocked both NMDA-induced currents and the associated increase in intracellular Ca2+. The actions of SIN-1 were reversible and suppressed by hemoglobin. A degraded SIN-1 solution that did not release NO was unable to block NMDA receptors. This showed that the SIN-1 effects were due to NO and not to another breakdown product. Similar results were obtained with 1-nitrosopyrrolidine (an NO-containing drug) and with NO released from NaNO2. Pretreatment with hemoglobin potentiated NMDA-induced effects, demonstrating that endogenous NO modulates NMDA receptors. Since NMDA receptor activation induces NO synthesis, these results suggest a feedback inhibition of NMDA receptors by NO under physiological condition.

The 5-HT4 receptor subtype inhibits K+ current in colliculi neurones via activation of a cyclic AMP-dependent protein kinase

1. The aim of the present study was to examine the effect of 5-hydroxytryptamine (5-HT) on K+ current in primary culture of mouse colliculi neurones and to identify the 5-HT receptor subtype that could be involved in this effect. 2. The voltage-activated K+ current of the neurones was partially blocked by 8-bromo adenosine 3':5'-cyclic monophosphate (8-bromo-cyclic AMP). This effect was mimicked by 5-HT and the action of 5-HT could be antagonized by H7, a non specific protein kinase inhibitor, and by PKI, the specific cyclic AMP-dependent protein kinase blocker. 3. A similar cyclic AMP-dependent blockade of the K+ current was found with renzapride (BRL 24,924) and other 5-HT4 receptor agonists such as cisapride, BIMU 8, zacopride and 5-methoxytryptamine (5-MeOT). ICS 205,930, the classical 5-HT4 receptor blocker, could not be used in this study because it inhibited the studied K+ current by itself. However, the novel 5-HT4 receptor antagonist, DAU 6285 blocked the effects of 5-HT and renzapride on the K+ current. 4. The current was insensitive to the 5-HT1 and 5-HT3 receptor agonists (8-hydroxy-2-(di-n-propylamino) tetralin, RU 24,969, carboxamidotryptamine, 2-CH3-5-HT) as well as to 5-HT1, 5-HT2 and 5-HT3 antagonists (methiothepin, ketanserin, ondansetron [GR 38,032]). Moreover, these antagonists did not affect the actions of the tested 5-HT4 receptor agonists. 5. The present results show that part of the voltage-activated K+ current in mouse colliculi neurones is cyclic AMP-sensitive and the blockade of the current by 5-HT involves the 5-HT4 receptor subtype.The putative implication of 5-HT4 receptors in neuronal plasticity, via a blockade of K+ channels, is discussed.

Sodium nitroprusside blocks NMDA receptors via formation of ferrocyanide ions

The effects of a nitric oxide (NO) donor, sodium nitroprusside (SNP), on N-methyl-D-aspartate (NMDA) receptors were assessed by optical measurements of intracellular calcium concentration ([Ca2+]i) and patch-clamp techniques in cultured central neurons. SNP selectively blocked NMDA-mediated currents and increases in [Ca2+]i. SNP inhibited the binding of [3H]-CGS 19755. The blockade of NMDA responses by SNP was prevented by CPP or APV which are selective competitive NMDA receptor antagonists. These effects were not necessarily mediated by NO, since they were mimicked by ferrocyanide ions, the NO companion photolysis product of SNP.

(trans)-1-amino-cyclopentyl-1,3-dicarboxylate stimulates quisqualate phosphoinositide-coupled receptors but not ionotropic glutamate receptors in striatal neurons and Xenopus oocytes

The effects of a novel glutamate analogue, (trans)-1-amino-cyclopentyl-1,3-dicarboxylate (ACPD), have been tested in striatal neurons in primary culture and in Xenopus oocytes injected with rat brain RNA. Both systems have been previously shown to contain well characterized metabotropic receptors coupled to phospholipase C (Qp), as well as ionotropic glutamate receptors. In striatal neurons, ACPD stimulated inositol phosphate (InsP) accumulation (EC50 = 9.7 +/- 2.5 microM; maximal effect, 184.7 +/- 11.6% of basal accumulation). This effect of ACPD was likely to be mediated by Qp receptors, because maximal ACPD and quisqualate-induced InsP formation were not additive. In contrast, the effects of ACPD and norepinephrine on InsP formation were additive. ACPD-induced InsP formation was not antagonised by antagonists of muscarinic and alpha 1-adrenergic receptors (1 microM atropine and 0.1 microM prazosin, respectively). In Xenopus oocytes, ACPD and quisqualate induced an oscillatory increase of a Ca2(+)-dependent chloride conductance, which is characteristic of the activation of phospholipase C-coupled receptors in this model. The specificity of ACPD on Qp receptors was demonstrated by testing the effect of this drug on quisqualate/kainate as well as on N-methyl-D-aspartate ionotropic receptors. In striatal neurons, the activation of quisqualate/kainate and N-methyl-D-aspartate receptors was tested by measurement of [3H]-gamma-aminobutyric acid release and by electrophysiological recordings using the patch-clamp technique. At concentrations as high as 1 mM, ACPD was inactive on these inotropic receptors, either as agonist or as antagonist. In conclusion, ACPD appeared to be a highly specific agonist of Qp receptors, with no activity on ionotropic glutamate receptors. It will be a useful drug to study the physiological properties of Qp receptors in vertebrate brains.

Neurons of the nucleus tractus solitarius, in vitro, generate bursting activities by solitary tract stimulation

Extracellular recordings of the activity of nucleus tractus solitarius (NTS) neurons were performed on rat brainstem slice preparations. Neurons localized in the medial part of the lateral NTS, which displayed a synaptic response to single pulse stimulation of the tractus solitarius (TS), generated bursting activity following repetitive TS stimulation (20-50 Hz frequency, 100-600 ms duration). According to their patterns of discharge and to the duration and frequency of their bursting activities, these neurons were classified in three groups called type A, B and C. We suggest that different cellular intrinsic properties, rather than local synaptic interactions, might be involved in the generation of these three types of bursting activities. These results are discussed in terms of the role of NTS neurons in the generation of the swallowing motor pattern.

Charybdotoxin blocks dendrotoxin-sensitive voltage-activated K+ channels

Charybdotoxin, a short scorpion venom neurotoxin, which was thought to be specific for the blockade of Ca2+-activated K+ channels also blocks a class of voltage-sensitive K+ channels that are known to be the target of other peptide neurotoxins from snake and bee venoms such as dendrotoxin and MCD peptide. Charybdotoxin also inhibits 125I-dendrotoxin and 125I-MCD peptide binding to their receptors. All these effects are observed with an IC50 of about 30 nM.

Voltage-activated calcium channels in rat Purkinje cells maintained in culture

Cell-attached patch recordings were used to study calcium channels on the dendritic membrane of rat cerebellar Purkinje cells maintained in culture. Experiments were performed with isotonic BaCl2 (110 mM) in the pipette and isotonic potassium gluconate in the bath to zero the cell membrane potential. Two distinct types of voltage-activated calcium channels were identified. The first one had a small conductance (9 pS), was activated at a low threshold (congruent to -50 mV) and could be inactivated by holding the membrane potential at -30 mV. This channel had the same characteristics as the T channel described in other neuronal preparations. The second type of Ca channel activated at a high threshold (-30 or +10 mV depending on whether BAY K 8644 was added or not to the pipette solution) and was still activatable even when the membrane was held at -40 mV. In the presence of BAY K 8644 this channel had a conductance of 21 pS with long openings. All these characteristics are similar to those of the S (L) Ca channel described in many preparations. The present study is in agreement with our previous experiments on Purkinje dendrites, where we identified low and high threshold Ca currents using the whole-cell configuration. Up to now, no channel corresponding to the N current has been observed but we cannot exclude its presence.

The action of hydrogen peroxide on paired pulse and long-term potentiation in the hippocampus

The action of a reactive oxygen intermediate, that is, hydrogen peroxide (H2O2) on modulation of synaptic transmission was examined in the hippocampal brain slice preparation. Microinjection of H2O2 into the apical dendritic region of the CA1 pyramidal cells produced no change in either the pattern or amplitude of paired pulse facilitation compared to saline injection (control). Long term potentiation (LTP), induced by high frequency stimulation of homosynaptic inputs, however, was blocked by microinjection of H2O2 into the dendritic tree. LTP was seen in only 2 out of 10 slices investigated when treated with H2O2 while LTP was seen in 4 out of 5 slices when saline injected. The results suggest that a reactive oxygen intermediate can selectively modify synaptic mechanisms in the hippocampus.

Decrease of recurrent and feed-forward inhibitions under high pressure of helium in rat hippocampal slices

The effect of high helium pressure on inhibitory synaptic transmission was studied in rat hippocampal slices with extracellular recordings. Both feed-forward and recurrent GABAergic inhibition were tested in the CA1 region with paired-pulse stimulation paradigms. The efficiency of both types of inhibition decreased under high pressure (80 atm). However, the depression of synaptic and antidromic field potentials induced by perfusion of GABA or muscimol were not significantly affected by pressure. High pressure induced hyperexcitability of CA1 pyramidal cells. This effect was reduced by the application of 2-aminophosphonovalerate or GABA. The present results suggest that: (1) high pressure reduces the efficiency of the GABAergic inhibitory transmission but does not affect the sensitivity of GABAA receptors; (2) two different processes (reduction of GABAergic inhibition and facilitation of N-methyl-D-aspartate-mediated excitation) might be a direct consequence of the change in the voltage-sensitive ion channels under high pressure and might be involved in the development of the pressure-induced hyperexcitability of CA1 pyramidal cells.

The influence of helium pressure on the reduction induced in field potentials by various amino acids and on the GABA-mediated inhibition in the CA1 region of hippocampal slices in the rat

In a previous study, it was shown that helium pressure depressed excitatory synaptic transmission mediated by the Schaffer-commissural afferents and increased the intrinsic excitability of pyramidal cells, in the CA1 region of hippocampal slices in the rat. In the present study, the neurochemical bases of these changes was investigated. Various excitatory amino acids were studied under normal and up to 80 atm of helium. At normal pressure, the amino acids tested induced a decrease in the field excitatory postsynaptic potential (EPSP) and antidromic field potential of CA1 pyramidal cells. These changes probably resulted from the well known depolarizing effect of the compounds. Quisqualate is supposed to activate the synaptic receptors of the pathway tested. Since the effect of this amino acid and other agonists were not significantly affected by helium pressure, it is suggested that the depressed hippocampal synaptic potentials under pressure did not result from reduced sensitivity of synaptic receptors. On the other hand, helium pressure enhanced the action of N-methyl-D-aspartate (NMDA) and depressed the GABA-mediated inhibition of CA1 pyramidal cells. Given that the excitability of these neurones is modulated by NMDA-related events and GABA inhibition, these results indicate that both neurochemical systems were probably involved in the helium pressure-induced hyperexcitability of the cells studied.

Helium pressure potentiates the N-methyl-D-aspartate- and D,L-homocysteate-induced decreases of field potentials in the rat hippocampal slice preparation

We examined the influence of helium pressure on the depression induced by various excitatory amino acids in CA1 hippocampal field potentials. The effects of quisqualate, L-glutamate, L-aspartate and kainate were not significantly affected by helium pressure, while those of N-methyl-D-aspartate and D,L-homocysteate were enhanced. These findings suggest that helium pressure specifically increased the sensitivity of the N-methyl-D-aspartate receptor type in the hippocampus. Other hypotheses are discussed.

A versatile chamber for microphysiologic studies with gas mixtures under high pressure

A pressure chamber, designed for microelectrode recordings in isolated tissues or organs an capable of withstanding pressures up to 200 bar, is described. The versatility of the vessel allows a wide variety of experimental configurations and several types of studies. It features a complete access, easy visibility, interchangeable tissue bath and micromanipulator modules, as continuous perfusion and temperature control of the preparation under pressure. The ability to move micropipettes in 1-micron steps allows single unit recording on a variety of in vitro preparations at normal or elevated gas pressures. Successful physiologic tests, using the hippocampal slice preparation, are described. The results testify to the reliability of the system and to the usefulness of this in vitro model to study the gas pressure effects on isolated networks of mammalian CNS.

Evoked potential changes in rat hippocampal slices under helium pressure

High pressures of helium affect the physiology of the central nervous system in animals and humans. We examined these effects in rat hippocampal slices. The in vitro preparation displayed a reversible reduction in postsynaptic and antidromic field potentials of CA1 pyramidal cells, but no significant change in the amplitude of the afferent volley. Although the subliminal synaptic response of CA1 neurons was depressed, the ability of these cells to produce population spikes was enhanced. These changes resembled those previously found in vivo in the rat hippocampus. The present results support the hypothesis of a helium pressure-induced depolarization of hippocampal neurons. Other possible mechanisms are discussed.

A study of spontaneous and evoked activity in the rat hippocampus under helium-oxygen high pressure

High pressures affect the physiology of the central nervous system. For a better understanding of this effect, we examined the hippocampal activity in the rat under high pressures (91 bars) of helium-oxygen. Effects of high pressure on hippocampal physiology are: an abnormally sustained 5-8 Hz pattern of spontaneous activity, followed, in some cases, by seizures; a marked decrease in the responses of CA1 pyramidal cells to stimulation of their commissural afferents; and a 50% decrease in the afterdischarge threshold. On the basis of the relatively well understood hippocampal physiology in normobaric conditions, our observations suggest that high pressures induce hypoexcitability of afferents and/or target cells.