Role of guanylyl cyclase and cGMP-dependent protein kinase in long-term potentiation

Role of nitric oxide and cyclic GMP in the dizocilpine-induced impairment of spontaneous alternation behavior in mice

The activation of N-methyl-D-aspartate receptors induces the synthesis of nitric oxide, which activates soluble guanylate cyclase and leads to the formation of cyclic GMP in the brain. The inhibition of nitric oxide production, as well as the blockade of N-methyl-D-aspartate receptors, has been reported to prevent the induction of hippocampal long-term potentiation and learning and memory formation in vivo, although the effects of inhibitors of nitric oxide synthase are still controversial. We investigated the putative role of nitric oxide and cyclic GMP in dizocilpine-induced memory impairment in mice. The nitric oxide synthase inhibitors, NG-nitro-L-arginine methyl ester and 7-nitro indazole, as well as dizocilpine, a non-competitive N-methyl-D-aspartate receptor antagonist, dose-dependently impaired spatial working memory in mice, assessed by their spontaneous alternation behavior in a Y-maze. The inhibitory effects of both NG-nitro-L-arginine methyl ester and dizocilpine on their behavior were completely reversed by 8-bromo-cyclic GMP. Cyclic GMP levels in the cerebellum were reduced by treatment with dizocilpine. NG-Nitro-L-arginine methyl ester and 7-nitro indazole reduced cyclic GMP levels in the cerebral cortex/hippocampus and cerebellum, and the suppressive effect of NG-nitro-L-arginine methyl ester on cyclic GMP levels in the cerebral cortex/hippocampus was reversed by co-treatment with L-arginine. Cyclic AMP levels in the brain were not affected by treatment with either dizocilpine, NG-nitro-L-arginine methyl ester, or 7-nitro indazole. Neither NG-nitro-L-arginine methyl ester nor L-arginine had any effect on monoamine and acetylcholine metabolism in the brain. These results suggest that the reduction in nitric oxide/cyclic GMP production in the brain may be responsible for dizocilpine-induced impairment of spontaneous alternation behavior in a Y-maze.

Opposite actions of nitric oxide on cholinergic synapses: which pathways?

Nitric oxide (NO) produced opposite effects on acetylcholine (ACh) release in identified neuroneuronal Aplysia synapses depending on the excitatory or the inhibitory nature of the synapse. Extracellular application of the NO donor, SIN-1, depressed the inhibitory postsynaptic currents (IPSCs) and enhanced the excitatory postsynaptic currents (EPSCs) evoked by presynaptic action potentials (1/60 Hz). Application of a membrane-permeant cGMP analog mimicked the effect of SIN-1 suggesting the participation of guanylate cyclase in the NO pathway. The guanylate cyclase inhibitor, methylene blue, blocked the NO-induced enhancement of EPSCs but only reduced the inhibition of IPSCs indicating that an additional mechanism participates to the depression of synaptic transmission by NO. Using nicotinamide, an inhibitor of ADP-ribosylation, we found that the NO-induced depression of ACh release on the inhibitory synapse also involves ADP-ribosylation mechanism(s). Furthermore, application of SIN-1 paired with cGMP-dependent protein kinase (cGMP-PK) inhibitors showed that cGMP-PK could play a role in the potentiating but not in the depressing effect of NO on ACh release. Increasing the frequency of stimulation of the presynaptic neuron from 1/60 Hz to 0.25 or 1 Hz potentiated the EPSCs and reduced the IPSCs. In these conditions, the potentiating effect of NO on the excitatory synapse was reduced, whereas its depressing effect on the inhibitory synapse was unaffected. Moreover the frequency-dependent enhancement of ACh release in the excitatory synapse was greatly reduced by the inhibition of NO synthase. Our results indicate that NO may be involved in different ways of modulation of synaptic transmission depending on the type of the synapse including synaptic plasticity.

Examination of the role of cGMP in long-term potentiation in the CA1 region of the hippocampus

The mechanisms underlying the generation of NMDA receptor-dependent LTP in the CA1 region of the hippocampus continue to receive a great deal of attention because of the postulated importance of LTP as a synaptic mechanism for learning and memory. It is well accepted that the initial induction of LTP occurs in the postsynaptic cell, but the site of expression remains controversial. One prominent hypothesis is that LTP involves the release of one or more retrograde messengers that act on the presynaptic terminal to enhance transmitter release. Recently, evidence has been presented that retrograde messengers function to activate presynaptic guanylyl cyclase and that the resulting rise in presynaptic cGMP levels, when accompanied by presynaptic activity, is responsible for generating an early component of LTP. We have tested this hypothesis by examining whether synaptic strength is increased by coupling tetanic stimulation with application of a membrane-permeable analog of cGMP. The experiments were done in the presence of an NMDA receptor antagonist to block postsynaptic induction mechanisms. Under a variety of experimental conditions, this manipulation failed to generate LTP, suggesting that an increase in cGMP levels accompanied by presynaptic activity is not sufficient to generate LTP in the CA1 region of the hippocampus.

A biochemist's view of long-term potentiation

This review surveys the molecular mechanisms of long-term potentiation (LTP) from the point of view of a biochemist. On the basis of available data, LTP in area CA1 of the hippocampus is divided into three phases--initial, early, and late--and the mechanisms contributing to the induction and expression of each phase are examined. We focus on evidence for the involvement of various second messengers and their effectors as well as the biochemical strategies employed in each phase to convert a transient signal into a lasting change in the neuron. We also consider, from a biochemical perspective, the implications of a multiphase model for LTP.

Ultrastructural distribution of NADPH-diaphorase in the normal hippocampus and after long-term potentiation

The distribution of the enzyme nitric oxide synthase (NOS) was investigated at the ultrastructural level in synaptic structures of the hippocampal formation in relation to long-term potentiation (LTP), based on the histochemical NADPH-diaphorase (NADPH-d) staining with the tetrazolium salt BSPT. BSPT-formazan, the osmiophilic reaction product, was found to be selectively distributed and predominantly attached to membranes of the endoplasmic reticulum. In synaptic regions mainly the presynaptic sides showed labeling. Although several groups have demonstrated a principal involvement of NO in the LTP-mechanism, we found only a low, statistically insignificant increase in NADPH-d stained presynaptic areas of the dentate gyrus, where LTP was evoked. Postsynaptic elements also did not show any noticeable differences. Based on the present results, the predominantly presynaptic localization of NOS should be preferably considered in models describing a functional role of NO in LTP formation, despite the fact that we failed to reveal any indications for an LTP-related change in synaptically located NADPH-d.

cGMP-dependent protein kinase in dorsal root ganglion: relationship with nitric oxide synthase and nociceptive neurons

Nitric oxide and cGMP influence plasticity of nociceptive processing in spinal cord. However, effectors for cGMP have not been identified in sensory pathways. We now demonstrate that cGMP-dependent protein kinase I (cGKl) occurs in the DRGs at levels comparable to that in cerebellum, the richest source of cGKl in the body. Immunohistochemical studies reveal that cGKl is concentrated in a subpopulation of small- and medium-diameter DRG neurons that partially overlap with substance P and calcitonin gene-related polypeptide containing cells. During development, cGKl expression throughout the embryo is essentially restricted to sensory neurons and to the spinal floor and roof plates. Neuronal nitric oxide synthase (nNOS) is coexpressed with cGKl in sensory neurons during embryonic development and after peripheral nerve axotomy. The primary target for cGKl in cerebellum, G-substrate, is not present in developing, mature, or regenerating sensory neurons, indicating that other proteins serve as effectors for cGKl in sensory processing. These data establish sensory neurons as a primary locus for cGMP actions during development and suggest a role for cGKl in plasticity of nociception.

NO decreases evoked quantal ACh release at a synapse of Aplysia by a mechanism independent of Ca2+ influx and protein kinase G

1. The exogenous nitric oxide (NO) donor, SIN-1, decreased the postsynaptic response evoked by a presynaptic spike at an identified cholinergic neuro-neuronal synapse in the buccal ganglion of Aplysia californica. 2. The statistical analysis of long duration postsynaptic responses evoked by square depolarizations of the voltage-clamped presynaptic neurone showed that the number of evoked acetylcholine (ACh) quanta released was decreased by SIN-1, pointing to a presynaptic action of the drug. 3. Vitamin E, a scavenger of free radicals, prevented the effects of SIN-1 on ACh release. SIN-1 still decreased ACh release in the presence of superoxide dismutase, whereas haemoglobin suppressed the effects of SIN-1. These results showed that NO is the active compound. 4. 8-Bromoguanosine 3', 5' cyclic monophosphate (8-Br-cGMP) mimicked the inhibitory effect of NO on ACh release suggesting the involvement of a NO-sensitive guanylate cyclase. This was reinforced by the reversibility of the effects of SIN-1 by inhibitors of guanylate cyclase, Methylene Blue, cystamine or LY83583. Methylene Blue partially reduced the inhibitory effect of NO. In addition, in the presence of superoxide dismutase, Methylene Blue blocked and cystamine significantly reduced the NO-induced inhibition of ACh release. 5. In the presence of KT5823 or R-p-8-pCPT-cGMPS, two inhibitors of protein kinase G, the reduction of ACh release by SIN-1 still took place indicating that the effects of NO most probably did not involve protein kinase G-dependent phosphorylation. 6. Presynaptic voltage-dependent Ca2+ (L-, N- and P-types) and K+ (IA and late outward rectifier) currents were unmodified by SIN-1. 7. The modulation of ACh release in opposite ways by L-arginine and N omega-nitro-L-arginine points to the involvement of an endogenous NO synthase-dependent regulation of transmitter release.

Effects of the 3',5'-phosphodiesterase inhibitors isobutylmethylxanthine and zaprinast on NO-mediated cGMP accumulation in the hippocampus slice preparation: an immunocytochemical study

The effect of inhibition of 3',5'-phosphodiesterase (PDE) activity on the cGMP accumulation was studied in control and nitric oxide (NO) stimulated hippocampal slices incubated in vitro using immunohistochemical visualisation of cGMP. Isobutylmethylxanthine (IBMX) was used as a non-selective PDE inhibitor and zaprinast was used as a selective inhibitor of cGMP-specific PDE activity. In the absence of PDE inhibitors cGMP-immunoreactivity (cGMP-IR) was found in blood vessel walls only. After incubation with the NO-donor sodium nitroprusside (SNP) cGMP-IR was found in a few isolated varicose fibres which were distributed throughout the slice. Incubation in the presence of either 1 mM IBMX or 10 microM zaprinast resulted in cGMP-IR in small numbers of varicose fibres distributed throughout the hippocampal slice. SNP in combination with IBMX resulted in cGMP-IR in small numbers multitude of varicose fibres throughout the slice; occasionally cell somata were observed. After incubation with SNP and zaprinast cGMP-IR was found in varicose fibres, although with a more restricted distribution and less numerous than in the presence of IBMX. In the latter combination, varicose fibres were observed predominantly in the CA2/CA3 region and in the stratum lacunosum molecular of the hippocampus, and cell somata were occasionally observed throughout the hippocampus. The differential distribution of cGMP-IR in the presence of different PDE inhibitors is consistent with the notion that there are regional differences in the localization of cGMP hydrolyzing enzymes in the hippocampus.

Localization of natriuretic peptide-activated guanylate cyclase mRNAs in the rat brain

Physiological actions of atrial natriuretic peptide (ANP) and C-type natriuretic peptide (CNP) are elaborated by membrane-bound natriuretic peptide receptors (NPRs). These receptors possess intracellular guanylate cyclase domains that mobilize cyclic guanosine monophosphate upon binding of peptide. Two distinct NPR subtypes have been described in brain: the NPR-A selectively binds ANP, whereas NPR-B exhibits high affinity for CNP. To define further the potential domains of ANP and CNP action in brain, the present study used in situ hybridization histochemistry to map NPR-A and NPR-B mRNA-expressing cell populations. Significant levels of neuronal NPR-A mRNA expression were observed only in the mitral cell layer of the olfactory bulb, medial habenula, subfornical organ, and area postrema. Expression of NPR-A mRNA was observed in forebrain white matter tracts, suggesting synthesis in glial cells. In contrast, NPR-B mRNA was widely expressed throughout the neuraxis. In the telencephalon, signal was abundant throughout limbic cortex and neocortex, olfactory bulb, hippocampus, and amygdala. Intense NPR-B mRNA hybridization was observed in preoptic-hypothalamic neuroendocrine circuits and in motor nuclei of cranial nerves. Intermediate expression of NPR-B mRNA was observed in brainstem nuclei controlling autonomic function. Labeling for NPR-B but not NPR-A mRNA was observed in pituicytes in the neural lobe of the pituitary and in scattered cells of the anterior pituitary. These results suggest that CNP is the primary biologically active natriuretic peptide in brain. In contrast with NPR-B, NPR-A appears to be expressed largely in restricted cell populations containing high levels of ANP and in circumventricular organs. These data implicate the NPR-A in autoregulation of ANP neurons and central registration of cardiac ANP release.

CaM kinase II in long-term potentiation

The observation that autophosphorylation converts CaM kinase II from the Ca(2+)-dependent form to the Ca(2+)-independent form has led to speculation that the formation of the Ca(2+)-independent form of the enzyme could encode frequency of synaptic usage and serve as a molecular explanation of "memory". In cultured rat hippocampal neurons, glutamate elevated the Ca(2+)-independent activity of CaM kinase II through autophosphorylation, and this response was blocked by an NMDA receptor antagonist, D-2-amino-5-phosphonopentanoate (AP5). In addition, we confirmed that high, but not low frequency stimulation, applied to two groups of CA1 afferents in the rat hippocampus, resulted in LTP induction with concomitant long-lasting increases in Ca(2+)-independent and total activities of CaM kinase II. In experiments with 32P-labeled hippocampal slices, the LTP induction in the CA1 region was associated with increases in autophosphorylation of both alpha and beta subunits of CaM kinase II 1 h after LTP induction. Significant increases in phosphorylation of endogenous CaM kinase II substrates, synapsin I and microtubule-associated protein 2 (MAP2), which are originally located in presynaptic and postsynaptic regions, respectively, were also observed in the same slice. All these changes were prevented when high frequency stimulation was applied in the presence of AP5 or a calmodulin antagonist, calmidazolium. Furthermore, in vitro phosphorylation of the AMPA receptor by CaM kinase II was reported in the postsynaptic density and infusion of the constitutively active CaM kinase II into the hippocampal neurons enhanced kainate-induced response. These results support the idea that CaM kinase II contributes to the induction of hippocampal LTP in both postsynaptic and presynaptic regions through phosphorylation of target proteins such as the AMPA receptor, MAP2 and synapsin I.

Regulation of gene expression by cGMP-dependent protein kinase. Transactivation of the c-fos promoter

The cAMP/cAMP-dependent protein kinase (A-kinase) and Ca2+/calmodulin-dependent protein kinase (Cam-kinase) signal transduction pathways are well known to regulate gene transcription, but this has not been demonstrated directly for the cGMP/cGMP-dependent protein kinase (G-kinase) signal transduction pathway. Here we report that transfection of G-kinase into G-kinase-deficient cells causes activation of the human c-fos promoter in a strictly cGMP-dependent manner. The effect of G-kinase appeared to be mediated by several sequence elements, most notably the serum response element (SRE), the AP-1 binding site (FAP), and the cAMP response element (CRE). The magnitude of G-kinase transactivation of the fos promoter was similar to that of A-kinase, but there were significant differences between G-kinase and A-kinase activation of single enhancer elements and of a chimeric Gal4-CREB transcription factor. Our results indicate that G-kinase transduces signals to the nucleus independently of A-kinase or Ca2+, although it may target some of the same transcription factors as A-kinase and Cam-kinase.

Apoptosis induced in neuronal cultures by either the phosphatase inhibitor okadaic acid or the kinase inhibitor staurosporine is attenuated by isoquinolinesulfonamides H-7, H-8, and H-9

Protein phosphorylation is kept in balance by an orchestrated action of kinases and phosphatases; when this balance is lost, neuronal apoptosis may occur. Okadaic acid (OKA), a marine toxin that inhibits specifically protein phosphatases 1 and 2A (EC 3.1.3.16), and staurosporine, an inhibitor of protein kinase C (PKC; EC 2.7.1.37), induced apoptosis in primary cultures of rat cerebellar granule neurons. We assayed apoptosis by the DNA gel electrophoresis, by the in situ TUNEL assay, and by morphological appearance following propidium iodide staining. Cell viability was assessed by the Trypan blue assay. Both OKA- and staurosporine-induced neuronal apoptosis were prevented by a macromolecular synthesis inhibitor actinomycin D and by a group of isoquinolinesulfonamide kinase inhibitors (H-7, 1-[5-isoquinolinesulfonyl]-2-methylpiperazine; H-8, N-¿2-[methylamino]ethyl¿-5-isoquinolinesulfonamide; H-9, N-(2-aminoethyl)-5-isoquinolinesulfonamide, but not by inhibitors of PKC, cyclic-GMP- and cyclic-AMP-dependent kinases, calcium/calmodulin-dependent kinases, tyrosine kinases, or by antioxidants. We postulate that a common mechanism, possibly an increased protein phosphorylation, is responsible for apoptosis triggered by an inhibition of phosphatases 1 and 2A and PKC. Elucidating the isoquinolinesulfonamide-sensitive mechanism may help us find new therapies for neurodegenerative diseases that involve apoptosis.

Astrocytes and Bergmann glia as an important site of nitric oxide synthase I

In the central nervous system nitric oxide appears to be critically involved in a number of physiological and pathological processes. Although there is convincing evidence for expression of nitric oxide synthase in cultured glial cells, demonstration of this enzyme in glial cells in situ remained largely unsatisfactory. In the present study we applied immunostaining to freeze-dried sections of snap-frozen hippocampi and cerebella of rats and to sections of freeze-dried brain tissue in order to minimize diffusion artefacts and thus to obtain more precise information about the true in situ localization of nitric oxide synthase. Here we show that astrocytes and Bergmann glia react strongly with antibodies raised against cerebellar nitric oxide synthase and against a type I nitric oxide synthase-specific C-terminal peptide, respectively. This finding was further substantiated by histochemical localization of NADPH-diaphorase activity in astrocytes and Bergmann glia as well as by immunoreactivity of both types of glia cells with antibodies to the NADPH-delivering enzyme glucose-6-phosphate dehydrogenase. We conclude, that astrocytes are important sites of nitric oxide synthase I in brain, suggesting that these cells might use nitric oxide as gaseous messenger molecule for various aspects of glia-neuron signalling.

Cyclic Nucleotide-Gated Channels, Nitric Oxide, and Neural Function

Cyclic nucleotide-gated (CNG) channels are nonselective cation channels that are activated by direct interaction of the second messengers cAMP and cGMP. Once activated, they provide for membrane depolarization and Ca2+ influx into cells. The functions of CNG channels are tightly coupled to the mechanisms by which cAMP and cGMP are regulated in neurons, namely through activation of G-protein–coupled membrane receptors and through the nitric oxide/guanylyl cyclase signaling system. These functions are best understood in sensory neurons of the vertebrate visual and olfactory system, where CNG channels are critical components of the signal transduction apparatus. The family of known CNG channels is expanding, and there is now increasing evidence that these channels are also present in many other areas of the nervous system. Based on their role in sensory neurons, a functional framework for the role of CNG channels in the CNS is provided.

Only 'de novo' long-term depression (LTD) in the rat hippocampus in vitro is blocked by the same low concentration of FK506 that blocks LTD in the visual cortex

It has been proposed that the long-term depression (LTD) seen following low frequency stimulation (LFS) in the rat hippocampus involves calcineurin. We have tested this by examining the effect of FK506, a macrolide which blocks calcineurin at nanomolar concentrations, on synaptic transmission in the rat hippocampal slice at a concentration of 1 microM which has been shown to block LTD in the visual cortex. The effect of FK506 on long-term potentiation (LTP) and spontaneous transmitter release was also studied. The magnitude of LTD induced by LFS was 16.7 +/- 2.4% in control which was not significantly different from the 22.3 +/- 3.0% seen in the same preparations after exposure to FK506 for 25-30 min. In contrast the magnitude of LTD induced 'de novo' in preparations exposed to FK506 was significantly reduced. FK506 had no significant effect on LTP, miniature EPSP frequency, miniature EPSP amplitude, resting membrane potential or input resistance. These results, therefore, support the hypothesis that calcineurin is involved in 'de novo' LTD but it appears that an event is triggered by LFS whereby FK506-insensitive LTD can subsequently be activated by a second episode of LFS.

Nitric oxide-dependent long-term potentiation is blocked by a specific inhibitor of soluble guanylyl cyclase

The diffusible second messenger, nitric oxide, is synthesised in central neurons in response to activation of glutamate receptors or other stimuli that increase cytosolic Ca2+ concentrations. Among the many roles suggested for nitric oxide in the central nervous system is that of mediating synaptic plasticity. For example, long-term potentiation in the CA1 region of the rat hippocampus was reported to be blocked by inhibitors of nitric oxide synthase and exogenous nitric oxide has been claimed to induce an enduring enhancement of synaptic strength under certain conditions. These findings, however, are controversial and even when a participation of nitric oxide is evident, the transduction mechanism is unclear. A well-known action of nitric oxide is to stimulate the soluble form of guanylyl cyclase, thereby evoking an accumulation of cyclic GMP in target cells but several other mechanisms have been proposed, including stimulation of ADP ribosyltransferase or cyclooxygenase, and nitrosylation of protein thiol residues. The identification of a selective inhibitor of soluble guanylyl cyclase, the oxadiazoloquinoxaline derivative, ODQ, provides, for the first time, the means to investigate the importance of the cyclic GMP pathway in nitric oxide signal transduction. We find that ODQ and the nitric oxide synthase inhibitor, nitroarginine, reduce hippocampal long-term potentiation in an equal and mutually exclusive manner, suggesting that the actions of nitric oxide in this phenomenon are entirely mediated through cyclic GMP. The experiments also show that there is a component of long-term potentiation that involves neither nitric oxide nor cyclic GMP.

Postsynaptic cAMP pathway gates early LTP in hippocampal CA1 region

The role of the cAMP pathway in LTP was studied in the CA1 region of hippocampus. Widely spaced trains of high frequency stimulation generated cAMP postsynaptically via NMDA receptors and calmodulin, consistent with the Ca2+/calmodulin-mediated stimulation of postsynaptic adenylyl cyclase. The early phase of LTP produced by the same pattern of high frequency stimulation was dependent on postsynaptic cAMP. However, synaptic transmission was not increased by postsynaptic application of cAMP. Early LTP became cAMP-independent when protein phosphatase inhibitors were injected postsynaptically. These observations indicate that in early LTP the cAMP signaling pathway, instead of transmitting signals for the generation of LTP, gates LTP through postsynaptic protein phosphatases.

Hippocampal long-term potentiation is normal in heme oxygenase-2 mutant mice

We have generated mice deficient in HO-2, the major cerebral isoform of heme oxygenase, in order to assess the potential role of carbon monoxide as a retrograde messenger in hippocampal LTP. Cerebral HO catalytic activity was markedly reduced in the HO-2 mutant mice, yet no differences were found between wild types and mutants in gross neuroanatomical structure, in basal hippocampal synaptic transmission, or in the amount of potentiation produced by various LTP induction protocols. Furthermore, zinc protoporphyrin IX, an inhibitor of HO, had nearly identical inhibitory effects on LTP in wild-type and HO-2 mutant hippocampal slices. Our data indicate that carbon monoxide produced endogenously by HO is unlikely to be a neuromodulator required for LTP in the hippocampus.

Effects of methylene blue and LY83583 on neuronal nitric oxide synthase and NADPH-diaphorase

Methylene blue and 6-anilino-5,8-quinolinedione (LY83583) have often been used as 'selective' inhibitors of soluble guanylyl cyclase. We report that in in vitro assays, both these compounds were potent inhibitors of rat cerebellar nitric oxide synthase activity. Methylene blue had an apparent Ki of 2.7 microM, while for LY83583 the Ki was 15.8 microM. Furthermore, methylene blue, but not LY83583, inhibited the NADPH-diaphorase histochemical reaction associated with nitric oxide synthase. Our results indicate that many of the effects of these drugs which have been attributed to inhibition of guanylyl cyclase, may derive from their direct inhibition of nitric oxide synthase activity instead.

Postsynaptic injection of CA2+/CaM induces synaptic potentiation requiring CaMKII and PKC activity

CA2+-regulated protein kinases play critical roles in long-term potentiation (LTP). To understand the role of Ca2+/calmodulin (CaM) signaling pathways in synaptic transmission better, Ca2+/CaM was injected into hippocampal CA1 neurons. Ca2+/CaM induced significant potentiation of excitatory synaptic responses, which was blocked by coinjection of a CaM-binding peptide and was not induced by injections of Ca2+ or CaM alone. Reciprocal experiments demonstrated that Ca2+/CaM-induced synaptic potentiation and tetanus-induced LTP occluded one another. Pseudosubstrate inhibitors or high-affinity substrates of CaMKII or PKC blocked Ca2/CaM-induced potentiation, indicating the requirement of CaMKII and PKC activities in synaptic potentiation. We suggest that postsynaptic levels of free Ca2+/CaM is a rate limiting factor and that functional cross-talk between Ca2+/CaM and PKC pathways occurs during the induction of LTP.

Presynaptic metabotropic glutamate receptors modulate omega-conotoxin-GVIA-insensitive calcium channels in the rat medulla

We have previously demonstrated that the metabotropic glutamate receptor (mGluR) agonist (1S,3R)-1 aminocyclopentane-1,3-dicarboxylate (ACPD) presynaptically inhibits evoked glutamatergic EPSCs and GABAergic IPSCs in patch clamped rat nucleus tractus solitarius (NTS) neurons recorded in this slices. The present study investigated the pharmacology of the presynaptic mGluRs, the the voltage dependent Ca2+ channel (VDCC) subtypes supporting neurotransmitter release, and possible interactions between the two. Monosynaptic EPSCs or IPSCs were evoked by electrical stimulation in the region of the tractus solitarius (TS). The effects of the mGluR agonists ACPD, (2S,3S,4S)-alpha-(carboxycyclopropyl)glycine (L-CCG-I) and L-2-amino-4-phosphonobutyrate (AP4) were examined upon EPSCs. The effects of the above compounds and quisqualate (QUIS) were examined upon IPSCs. L-CCG-I proved the most potent inhibitor of EPSCs and IPSCs. The VDCC blockers omega-AGA-IVA (AGA), omega-conotoxin GVIA (GVIA), omega-conotoxin MVIIC (MVIIC) and nimodipine (NIM) were assessed for their ability to inhibit monosynaptic EPSCs and IPSCs. EPSCs were inhibited by GVIA >> AGA > or = MVIIC. IPSCs were inhibited by AGA > or = MVIIC >> GVIA. NIM was without effect on the EPSC or IPSC. The potency of mGluR inhibition of evoked synaptic transmission was assessed in the absence and following treatment with VDCC blockers. mGluR agonists blocked a greater percentage of the EPSC or IPSC following treatment with GVIA, but not the other VDCC antagonists, than under control conditions. We have previously demonstrated that the postsynaptic inhibitory effects of mGluR activation upon GABAA mediated currents can be mimicked by cyclic guanosine monophosphate (cGMP) analogs. The cGMP-dependent protein kinase (PKG) inhibitors H8 and Rp-8-4-chlorophenylthio-guanosine-3',5'-cyclic monophosphorothioate (Rp-cG) blocked mGluR inhibition of GABAA mediated currents without blocking the ability of mGluR agonists to inhibit the IPSC. The effect of L-CCGI was enhanced following treatment with GVIA in the presence of Rp-cG, confirming a presynaptic locus of mGluR mediated inhibition of the IPSC. In contrast, cGMP analogues potentiate postsynaptic responses to glutamate agonists but depress the EPSC. As with the mGluR agonists, the inhibition of the EPSC by cGMP was potentiated following treatment with GVIA. These results suggest that presynaptic mGluR reduce both glutamate release from afferent fibers and GABA release from inhibitory interneurons following electrical stimulation in the region of the TS. Although different VDCCs support the majority of glutamate and GABA release and mGluR effects on release appear to utilize differing intracellular pathways, presynaptic GVIA-insensitive VDCCs are favorably targeted for inhibition by mGluR agonists.

Synaptic plasticity: stairway to memory

Since the idea that memory is associated with alterations in synaptic strength was accepted, studies on the cellular and molecular mechanisms responsible for the plastic changes in neurons have attracted wide interest in the scientific community. Recent studies on memory processes have also pointed out some unifying themes emerging from a wide range of nervous systems, suggesting that regardless of the species or brain regions, a common denominator for memory may exist. Thus, the present review attempted to create a hypothetical and universal synaptic model valid for a variety of nervous systems, ranging from molluscs to mammals. The cellular and molecular events leading to short- and long-term modifications of memory have been described in a sequential order, from the triggering signals to the gene expression, synthesis of new proteins and neuronal growth. These events are thought to represent the late phases of memory consolidation leading to persistent modifications in synaptic plasticity, thereby facilitating the permanent storage of acquired information throughout the individual's life.

Activity-dependent long-term enhancement of transmitter release by presynaptic 3',5'-cyclic GMP in cultured hippocampal neurons

Long-term potentiation (LTP) in hippocampus is a type of synaptic plasticity that is thought to be involved in learning and memory. Several lines of evidence suggest that LTP involves 3',5'-cyclic GMP (cGMP), perhaps as an activity-dependent presynaptic effector of one or more retrograde messengers (refs 2-12, but see ref. 13). However, previous results are also consistent with postsynaptic effects of cGMP. This is difficult to test in hippocampal slices, but more rigorous tests are possible in dissociated cell culture. We have therefore developed a reliable method for producing N-methyl-D-aspartate (NMDA) receptor-dependent LTP at synapses between individual hippocampal pyramidal neurons in culture. We report that inhibitors of guanylyl cyclase or of cGMP-dependent protein kinase block potentiation by either tetanic stimulation or low-frequency stimulation paired with postsynaptic depolarization. Conversely, application of 8-Br-cGMP to the bath or injection of cGMP into the presynaptic neuron produces activity-dependent long-lasting potentiation. The potentiation by cGMP involves an increase in transmitter release that is in part independent of changes in the presynaptic action potential. These results support a presynaptic role for cGMP in LTP.

Long-term depression: a learning-related type of synaptic plasticity in the mammalian central nervous system

Studies of various forms of synaptic plasticity in the central nervous system have provided insights into the cellular and molecular mechanisms for certain types of learning and memory. Activity-induced decreases and increases in synaptic efficacy can be elicited in mammalian neurons. Long-term depression (LTD) and long-term potentiation (LTP) are two major forms of activity-dependent synaptic plasticity in the brain. LTD of excitatory synaptic transmission in the cerebellum in the most well studied form of synaptic depression. The induction of cerebellar LTD requires conjunctive activation of alpha-amino-3-hydroxy-5-methyl-4-isoxalepropionate (AMPA) receptors, metabotropic glutamate receptors (mGluRs) and L-type voltage-dependent Ca2+ channels. Several intracellular second messengers and protein kinases are critical for cerebellar LTD, including cGMP, cGMP-dependent protein kinase and protein kinase C (PKC). A novel intercellular messenger, nitric oxide (NO), is found in the cerebellum, is released durinng synaptic stimulation, and may contribute to cerebellar LTD. The expression of cerebellar LTD is mediated by postsynaptic desensitization of AMPA receptors. Recently, a form of homosynaptic LTD has been described in the CA1 region of the hippocampus. The induction of hippocampal LTD is postsynaptic. N-Methyl-D-aspartate receptors and mGluRs are important for induction of hippocampal LTD. Other intracellular and intercellular messengers, such as NO, cGMP and cAMP, might act downstream from glutamate receptors during hippocampal LTD. The expression of hippocampal LTD is likely to be in part presynaptic. While cerebellar LTD may be important for motor learning, the behavioral role of hippocampal LTD remains to be explored.

Role of nitric oxide in learning and memory and in monoamine metabolism in the rat brain

1. We investigated the effects of NG-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide (NO) synthase, on the performance of rats in a radial arm maze and in habituation tasks, and on monoamine metabolism in the brain. 2. Daily administration of L-NAME (10-60 mg kg-1) resulted in a dose-dependent impairment of performance during the acquisition of the radial arm maze task, while it failed to affect performance in those rats that had previously acquired the task. 3. The rate of decrease in locomotor activity in the habituation task in the L-NAME-treated rats was significantly less than that in control rats. 4. NG-nitro-D-arginine methyl ester (D-NAME, a less active inhibitor of NO synthase) showed no effects in the above behavioural tasks. 5. NO synthase activity was significantly decreased in both the L-NAME and D-NAME-treated rats, with the magnitude of inhibition being greater in the L-NAME-treated animals. 6. The content of 5-hydroxyindoleacetic acid (5-HIAA) in the hippocampus and the 5-HIAA/5-hydroxytryptamine ratio in the hippocampus and cortex were significantly decreased in the L-NAME (60 mg kg-1)-treated rats compared with these values in the controls. 7. Striatal 3,4-dihydroxyphenylacetic acid (DOPAC) content was significantly increased in the L-NAME (60 mg kg-1)-treated rats compared with the values in the controls, while the DOPAC/dopamine ratio was not changed. 8. These results suggest that: (i) NO may play an important role in performance during the acquisition,but not retention, of the radial arm maze task, and (ii) that endogenous NO may be involved in the regulation of monoamine metabolism.

Synaptic plasticity: hippocampal LTP

One of the most intensively studied forms of synaptic plasticity is long-term potentiation (LTP). The past year has seen further evidence advanced on both sides of the presynaptic/postsynaptic locus of expression debate, without an obvious path to reconcile the two views. Real progress has been made, however, in clarifying the possible role of nitric oxide as a retrograde messenger and the cellular location of its synthetic enzyme. Intriguing glimpses of the complex involvement of metabotropic glutamate receptors in the induction of LTP have also appeared.

Protein phosphorylation in neuronal plasticity and gene expression

Protein kinases and phosphatases are intimately involved in several forms of synaptic plasticity. They play a critical role in the initiation of long-term potentiation and long-term depression, as well as in the induction of genes that permit long-term expression of altered synaptic states. Recent findings demonstrate a central role for the cAMP signaling pathway in the persistent phase of long-term potentiation. Genetic approaches have established that the transcription factor CREB is essential for long-term memory.

Molecular characterization of two Drosophila guanylate cyclases expressed in the nervous system

We have isolated, by interspecies hybridization, two classes of Drosophila cDNA each encoding a different guanylate cyclase (GC). One of them encodes an alpha subunit homolog of soluble GC, designated DGC alpha 1, and the other encodes a receptor-type GC, designated DrGC. The dgc alpha 1 cDNA encodes a protein of 676 amino acids and maps to 99B. In situ hybridization to adult tissue sections showed that dgc alpha 1 mRNA is found mainly in the cell bodies of the optic lobe, central brain, and thoracic ganglia. The DGC alpha 1 protein was also localized primarily to the nervous system by immunocytochemical staining, consistent with results of in situ hybridization. However, no detectable expression of this protein was found in the retina. The other class of cDNA, drgc, maps to 76C and encodes a 1525-amino acid protein displaying structural features similar to other known receptor-type guanylate cyclases. However, it has a C-terminal 430 amino acid region that has no homology to any known proteins. drgc RNA is expressed at low levels throughout development and in adult heads and bodies. In situ hybridizations to adult tissue sections showed that drgc mRNA is expressed in a wide range of tissues, including the optic lobe, central brain, thoracic ganglia, digestive tract, and the oocyte.

Complex involvement of nitric oxide and cGMP at N-methyl-D-aspartic acid receptors regulating gamma-[3H]aminobutyric acid release from striatal slices

Whilst the depolarization of postsynaptic N-methyl-D-aspartic acid (NMDA) receptors leads to an influx of Ca2+ and subsequent synthesis of nitric oxide (NO), we examined roles for NO at striatal NMDA receptors regulating transmitter release. In superfused rat striatal slices, NMDA-evoked release of gamma-[3H]aminobutyric acid ([3H]GABA) was investigated in the presence of nitrergic drugs. NMDA-induced release of [3H]GABA was attenuated by D-2-aminophosphonopentanoate, tetrodotoxin and omission of Ca2+. L-Arginine enhanced NMDA-evoked release of [3H]GABA, but exogenous NO donors were ineffective. Inhibitors of NO synthase (NG-nitro- and NG-amino-L-arginine) and guanylate cyclase (LY83583) elevated release. Since NMDA-evoked release of [3H]GABA was partially tetrodotoxin-sensitive, nitrergic-linked NMDA receptors regulating the release are both pre- and extrasynaptic. Thus not only does NO arise from multiple sites, and involve NMDA receptors with their redox site insensitive to exogenous NO donors, but the NMDA receptors are under the influence of nitrergic and cGMP-linked negative feedback mechanisms.

The cellular Na+ pump as a site of action for carbon monoxide and glutamate: a mechanism for long-term modulation of cellular activity

Carbon monoxide (CO) induces a long-lasting alteration in cerebellar alpha 3-Na,K-ATPase independent of [Na+] but linked to cGMP synthesis and localized to Purkinje neurons. The action of CO is absent in Purkinje neuron-deficient mice, mimicked by 8-Br-cGMP, and blocked by inhibition of PKG. Glutamate (Glu) and metabotropic agonists mimic the action of CO, an effect that requires PKC and is associated with CO synthesis. These data suggest that CO regulates Na,K-ATPase through cGMP and PKG, and that Glu regulates CO through mGluRs. This system is also modulated by NMDA agonists and nitric oxide, possibly via Glu release, as well as by free radicals. These findings offer a mechanism by which CO, Glu, and free radicals can exert specific effects on synaptic transmission (relevant to long-term changes in cell excitability), as well as more general actions on energy metabolism (relevant to the pathophysiology of excitotoxicity).

Extracellular phosphorylation of membrane protein modifies theta burst-induced long-term potentiation in CA1 neurons of guinea-pig hippocampal slices

The involvement of ecto-protein kinase activity in activity-dependent long-term potentiation (LTP) was studied in CA1 neurons of guinea-pig hippocampal slices. Application of 5 microM K-252b, an ecto-protein kinase inhibitor, blocked LTP induced by a theta-burst stimulation (3 bursts composed of 5 pulses at 100 Hz with inter-burst intervals of 200 ms). On the other hand, under 10 microM RK682, an ecto-phosphatase inhibitor, a robust LTP was induced by a weak theta-burst stimulation (3 bursts composed of 3 pulses) which was just at the threshold for the induction of LTP in the control perfusate. These findings suggest that ATP released from presynaptic terminals during the burst stimulation plays an important role in the induction of LTP through phosphorylation of extracellular domains of synaptic membrane proteins, as the substrate for ecto-protein kinase.

An ADP-ribosyltransferase as a potential target for nitric oxide action in hippocampal long-term potentiation

Recent studies of long-term potentiation (LTP) in the CA1 region of the hippocampus have demonstrated that nitric oxide (NO) may be involved in some forms of LTP and have suggested that postsynaptically generated NO is a candidate to act as a retrograde messenger. However, the molecular target(s) of NO in LTP remain to be elucidated. The present study examined whether either of two potential NO targets, a soluble guanylyl cyclase or an ADP-ribosyltransferase (ADPRT; EC 2.4.2.31) plays a role in LTP. The application of membrane-permeant analogs of cGMP did not produce any long-lasting alterations in synaptic strength. In addition, application of a cGMP-dependent protein kinase inhibitor did not prevent LTP. We found that the CA1 tissue from hippocampus possesses an ADPRT activity that is dramatically stimulated by NO and attenuated by two different inhibitors of mono-ADPRT activity, phylloquinone and nicotinamide. The extracellular application of these same inhibitors prevented LTP. Postsynaptic injection of nicotinamide failed to attenuate LTP, suggesting that the critical site of ADPRT activity resides at a nonpostsynaptic locus. These results suggest that ADP-ribosylation plays a role in LTP and are consistent with the idea that an ADPRT may be a target of NO action.

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)

Diverse roles for nitric oxide in synaptic signalling after activation of NMDA release-regulating receptors

NMDA receptors regulating transmitter release were studied in three model systems to investigate whether their activation involves the NO transduction system. In superfused slices of rat brain, the release of [3H]D-aspartate, [3H]noradrenaline and [3H]GABA evoked by NMDA could be modulated by nitrergic drugs. Tetrodotoxin (0.1 microM) exerted differential effects in the three systems indicative of the NMDA receptors (and hence sites of NO generation) being pre- or extra-synaptic, or a combination of both types of localization. L-Arginine (100 microM) enhanced NMDA-evoked release of [3H]GABA (110%), [3H]NA (120%) and [3H]D-ASP (700%). Exogenous NO donors could increase NMDA-induced release of [3H]NA and [3H]D-ASP from hippocampal slices, although differential effects were noted, whilst inhibitors of NO synthase (NG-nitro- and NG-amino-L-arginine, both 100 microM) attenuated (60-85%) the release. NMDA-evoked release of [3H]GABA from striatal slices were insensitive to exogenous NO donors, but NG-nitro- and NG-amino-L-arginine produced 100% increases. In all cases, the NMDA receptors regulating release are linked to a NO system, although the link to the receptors modulating release of [3H]GABA appeared different. The actions of the nitrergic drugs may depend upon the redox state and/or cellular milieu of the individual NMDA receptors involved.

Molecular mechanisms of inhibition of porcine brain nitric oxide synthase by the antinociceptive drug 7-nitro-indazole

7-Nitro-indazole (7-NI) has been described as novel nitric oxide synthase (NOS) inhibitor with in vivo selectivity for the neuronal isozyme [Moore et al. Br. J. Phaarmac. 110, 219-224 (1993)]. In the present study we have used purified porcine brain NOS to investigate the molecular mechanisms of enzyme inhibition by 7-NI. The drug was competitive with L-arginine, exhibited a kinetic KI of 2.8 microM, and additionally induced a slight reduction in Vmax. As a cytochrome P-450, NOS catalyzes a heme-mediated reduction of molecular oxygen, resulting in the formation of H2O2 in the absence of L-arginine. 7-NI turned out as a potent inhibitor of H2O2 formation (IC50 = 0.28 +/- 0.096 microM) but did not affect flavin-mediated electron transfer. Thus, 7-NI resembled imidazole, a known heme-site inhibitor of NOS. We found that imidazole was a purely competitive inhibitor of L-citrulline formation (KI = 263 microM) and blocked H2O2 formation at similar concentrations (IC50 = 280 +/- 38 microM). In accordance with their L-arginine-competitive effects in the citrulline assay, both drugs antagonized binding of radiolabeled NG-nitro-L-arginine (L-NNA), a high affinity probe for reversible labelling of the substrate site of NOS [Klatt et al., J. Biol. Chem. 269, 14781-14787 (1994)]. The calculated KI values for 7-NI and imidazole were 0.09 +/- 0.024 microM and 200 +/- 63 microM, respectively. Finally, binding of radiolabelled tetrahydrobiopterin, a NOS cofactor with unknown function, was also antagonized by 7-NI with a KI of 0.12 +/- 0.023 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

In the hippocampus in vivo, nitric oxide does not appear to function as an endogenous antiepileptic agent

Using a reverberatory epilepiform discharge of hippocampal-parahippocampal circuits termed "maximal dentate activation", this study investigated whether the local release of nitric oxide within these circuits functions as an antiepileptic agent. Two nitric oxide synthase inhibitors (L-nitro-arginine methyl ester and 7-nitro-indazole) and a guanylate cyclase inhibitor (methylene blue) were tested, and none had a significant effect on the time to onset or duration of maximal dentate activation. A membrane-permeable analogue of cyclic guanosine monophosphate (cGMP), 8-bromo-cGMP, caused an increase in the time to onset and a decrease in the duration of maximal dentate activation. The number of neurons expressing NADPH diaphorase activity (a marker for nitric oxide synthase) was also examined after repeated elicitation of maximal dentate activation. After 18 seizures there was a significant, but transient, decrease in the number of hilar/subgranular neurons that were NADPH diaphorase-positive. The decrease was only seen at 1 h after the last seizure. There was no induction of NADPH diaphorase activity. These results are not consistent with the hypothesis that, in hippocampal-parahippocampal circuits in vivo, nitric oxide is released in response to neuronal activity and then acts to terminate the neuronal activity.