Effects of nitric oxide on proprioceptive signaling

We have analysed the effects of the neuromodulator nitric oxide (NO) on proprioceptive information processing by ascending intersegmental interneurons that form part of the local circuits within the terminal abdominal ganglion of the crayfish. NO modulates the synaptic inputs to ascending interneurons, enhancing the amplitude of class I interneurons and reducing the amplitude of class II interneurons. Repetitive proprioceptive stimulation leads to rapid depression in a specific set of identified interneurons but not in others. Bath application of a nitric oxide scavenger, PTIO, causes a significant decrease in the rate of depression of the interneurons showing a rapid depression, independent of interneuron class, but has no effect on the dynamic responses of the interneurons that show little initial depression. These results indicate that NO exerts multiple effects at the very first stage of synaptic integration in local circuits.     

Distribution of NADPH-diaphorase-positive ascending interneurones in the crayfish terminal abdominal ganglion

Previous neuropharmacological studies have described the presence of a nitric oxide-cGMP signalling pathway in the crayfish abdominal nervous system. In this study we have analysed the distribution of putative nitric oxide synthase (NOS)-containing ascending interneurones in the crayfish terminal abdominal ganglion using NADPH-diaphorase (NADPHd) histochemistry. Ascending intersegmental interneurones were stained intracellularly using the fluorescent dye Lucifer yellow and the ganglia containing the stained interneurones subsequently processed for NADPHd activity. Fluorescence persisted throughout histochemical processing. These double-labelling experiments showed that 12 of 18 identified ascending interneurones were NADPHd positive. Thus many ascending interneurones that process mechanosensory signals in the terminal ganglion may contain NOS, and are themselves likely sources of NO which is known to modulate their synaptic inputs. Three clear relationships emerged from our analysis between the effects of NO on the synaptic inputs of interneurones, their output properties and their staining for NADPH-diaphorase. First were class 1 interneurones with no local outputs in the terminal ganglion, the NE type interneurones, which had sensory inputs that were enhanced by NO and were NADPHd positive. Second were class 1 interneurones with local and intersegmental output effects that had sensory inputs that were also enhanced by NO but were NADPHd negative. Third were class 2 interneurones with local and intersegmental outputs that had synaptic inputs that were depressed by the action of NO but were NADPHd positive. These results suggest that NO could selectively enhance specific synaptic connections and sensory processing pathways in local circuits.     

Frontal cortical afferents facilitate striatal nitric oxide transmission in vivo via a NMDA receptor and neuronal NOS-dependent mechanism

Striatal nitric oxide (NO) signaling plays a critical role in modulating neural processing and motor behavior. Nitrergic interneurons receive synaptic inputs from corticostriatal neurons and are activated via ionotropic glutamate receptor stimulation. However, the afferent regulation of NO signaling is poorly characterized. The role of frontal cortical afferents in regulating NO transmission was assessed in anesthetized rats using amperometric microsensor measurements of NO efflux and local field potential recordings. Low frequency (3 Hz) electrical stimulation of the ipsilateral cortex did not consistently evoke detectable changes in striatal NO efflux. In contrast, train stimulation (30 Hz) of frontal cortical afferents facilitated NO efflux in a stimulus intensity-dependent manner. Nitric oxide efflux evoked by train stimulation was transient, reproducible over time, and attenuated by systemic administration of either the NMDA receptor antagonist MK-801 or the neuronal NO synthase inhibitors 7-nitroindazole and NG-propyl-L-arginine. The interaction between NO efflux evoked via train stimulation and local striatal neuron activity was assessed using dual microsensor and local field potential recordings carried out concurrently in the contralateral and ipsilateral striatum, respectively. Systemic administration of the non-specific NO synthase inhibitor methylene blue attenuated both evoked NO efflux and the peak oscillation frequency (within the delta band) of local field potentials recorded immediately after train stimulation. Taken together, these observations indicate that feed-forward activation of neuronal NO signaling by phasic activation of frontal cortical afferents facilitates the synchronization of glutamate driven oscillations in striatal neurons. Thus, NO signaling may act to amplify coherent corticostriatal transmission and synchronize striatal output.     

Nitric oxide modulates local reflexes of the tailfan of the crayfish

Electrical stimulation of sensory neurons that innervate receptors on the tailfan of crayfish evokes a reflex response of motor neurons that produce movements of the blades of the tailfan, the uropods. We analyzed the modulatory effects of nitric oxide (NO) on the spike frequency of the reflex response. Bath application of L-arginine and SNAP, which elevate endogenous and exogenous NO levels, increased the frequency of the evoked response, whereas the application of L-NAME and PTIO, which reduce NO levels, decreased the frequency of the response. To determine through what pathway and target NO exerted these effects we bath applied ODQ, an inhibitor of soluble guanylyl cyclase (sGC), which decreased the frequency of response, and 8-br-cGMP, which increased the spike frequency of response. To provide further evidence that NO acts via sGC, we elevated NO levels with L-arginine while simultaneously inhibiting sGC with ODQ. This application reduced the response to control levels, indicating that NO in the terminal ganglion of crayfish acts via sGC to modulate cGMP levels, which in turn regulate the responses of the uropod motor neurons.     

Regulation of cyclic GMP elevation in the developing antennal lobe of the sphinx moth,Manduca sexta

In the moth, Manduca sexta, 3′,5′‐guanosine monophosphate (cGMP) is transiently elevated during adult development in about 100 neurons of the antennal lobe. We demonstrate that nearly all of these neurons are local interneurons of the lateral cluster I, that their capacity to show a strong cGMP response during development is regulated by the steroid hormone 20‐hydroxyecdysone, and that in a subpopulation of these neurons cGMP elevation seems to be controlled directly by the gaseous messenger molecule nitric oxide (NO). Treatment with the acetylcholine esterase inhibitor eserine, antennal nerve transection, and electrical stimulation of the antennae suggest that NO/cGMP signaling during development is an activity‐dependent process. Besides input from the antennae, input from the central brain and the ventral ganglia is involved in upregulating cGMP in the antennal‐lobe neurons. Possible sources are centrifugal aminergic neurons, since application of serotonin and histamine enhances the GMP signal in local interneurons. Comparing the time course of cGMP elevation with events occurring during development leads us to the hypothesis that the NO/cGMP signaling pathway might be involved in synapse formation of a subset of antennal‐lobe neurons. © 1999 John Wiley & Sons, Inc. J Neurobiol 41: 359–375, 1999     

Regulation of cyclic GMP elevation in the developing antennal lobe of the Sphinx moth, Manduca sexta.

In the moth, Manduca sexta, 3',5'-guanosine monophosphate (cGMP) is transiently elevated during adult development in about 100 neurons of the antennal lobe. We demonstrate that nearly all of these neurons are local interneurons of the lateral cluster I, that their capacity to show a strong cGMP response during development is regulated by the steroid hormone 20-hydroxyecdysone, and that in a subpopulation of these neurons cGMP elevation seems to be controlled directly by the gaseous messenger molecule nitric oxide (NO). Treatment with the acetylcholine esterase inhibitor eserine, antennal nerve transection, and electrical stimulation of the antennae suggest that NO/cGMP signaling during development is an activity-dependent process. Besides input from the antennae, input from the central brain and the ventral ganglia is involved in upregulating cGMP in the antennal-lobe neurons. Possible sources are centrifugal aminergic neurons, since application of serotonin and histamine enhances the GMP signal in local interneurons. Comparing the time course of cGMP elevation with events occurring during development leads us to the hypothesis that the NO/cGMP signaling pathway might be involved in synapse formation of a subset of antennal-lobe neurons.     

GABAergic and glutamatergic inhibition of nonspiking local interneurons in the terminal abdominal ganglion of the crayfish

Nonspiking local interneurons in the terminal abdominal ganglion of the crayfish Procambarus clarkii receive inhibitory inputs from mainly glutamatergic spiking local interneurons and GABAergic nonspiking interneurons. In this study, the inhibitory responses of nonspiking interneurons to local application of glutamate and GABA into the neuropil were compared. Glutamate and GABA injection mediated the hyperpolarization of the nonspiking interneurons with an increase in membrane conductance. The glutamate-mediated membrane hyperpolarization was reversed by injection of 1 or 2 nA hyperpolarizing current. By contrast, more than 3 nA hyperpolarizing current was frequently necessary to reverse the GABA-mediated hyperpolarization. Bath application of a chloride channel blocker, 50 microM picrotoxin (PTX), reduced the glutamate-mediated hyperpolarization, but had no effect on the GABA-mediated hyperpolarization. The GABA-mediated hyperpolarization was not consistently affected by bath application of low chloride solution. These results suggest that the glutamate-mediated inhibition was related to the gating of a Cl(-) conductance, while the GABA-mediated inhibition was not. Electrical stimulation of sensory afferents innervating the exopodite elicited ipsps in uropod opener motor neurons. These sensory-evoked ipsps were also PTX-insensitive, suggesting GABAergic nonspiking interneurons could be the predominant premotor elements in organizing the uropod motor control system.     

Nitric oxide inhibits nociceptive transmission by differentially regulating glutamate and glycine release to spinal dorsal horn neurons

Nitric oxide (NO) is involved in many physiological functions, but its role in pain signaling remains uncertain. Surprisingly, little is known about how endogenous NO affects excitatory and inhibitory synaptic transmission at the spinal level. Here we determined how NO affects excitatory and inhibitory synaptic inputs to dorsal horn neurons using whole-cell recordings in rat spinal cord slices. The NO precursor L-arginine or the NO donor SNAP significantly increased the frequency of glycinergic spontaneous and miniature inhibitory postsynaptic currents (IPSCs) of lamina II neurons. However, neither L-arginine nor SNAP had any effect on GABAergic IPSCs. L-arginine and SNAP significantly reduced the amplitude of monosynaptic excitatory postsynaptic currents (EPSCs) evoked from the dorsal root with an increase in paired-pulse ratio. Inhibition of the soluble guanylyl cyclase abolished the effect of L-arginine on glycinergic IPSCs but not on evoked monosynaptic EPSCs. Also, inhibition of protein kinase G blocked the increase in glycinergic sIPSCs by the cGMP analog 8-bromo-cGMP. The inhibitory effects of L-arginine on evoked EPSCs and high voltage-activated Ca(2+) channels expressed in HEK293 cells and dorsal root ganglion neurons were abolished by blocking the S-nitrosylation reaction with N-ethylmaleimide. Intrathecal injection of L-arginine and SNAP significantly increased mechanical nociceptive thresholds. Our findings suggest that spinal endogenous NO enhances inhibitory glycinergic input to dorsal horn neurons through sGC-cGMP-protein kinase G. Furthermore, NO reduces glutamate release from primary afferent terminals through S-nitrosylation of voltage-activated Ca(2+) channels. Both of these actions probably contribute to inhibition of nociceptive transmission by NO at the spinal level.     

Nitric oxide and cGMP signal transduction positively regulates the motility of human neuronal precursor (NT2) cells

Developmental studies in both vertebrates and invertebrates implicate an involvement of nitric oxide (NO) signaling in cell proliferation, neuronal motility, and synaptic maturation. However, it is unknown whether NO plays a role in the development of the human nervous system. We used a model of human neuronal precursor cells from a well-characterized teratocarcinoma cell line (NT2). The precursor cells proliferate during retinoic acid treatment as spherical aggregate culture that stains for nestin and βIII-tubulin. Cells migrate out of the aggregates to acquire fully differentiated neuronal phenotypes. The cells express neuronal nitric oxide synthase and soluble guanylyl cyclase (sGC), an enzyme that synthesizes cGMP upon activation by NO. The migration of the neuronal precursor cell is blocked by the use of nNOS, sGC, and protein kinase G (PKG) inhibitors. Inhibition of sGC can be rescued by a membrane permeable analog of cGMP. In gain of function experiments the application of a NO donor and cGMP analog facilitate cell migration. Our results from the differentiating NT2 model neurons point towards a vital role of the NO/cGMP/PKG signaling cascade as positive regulator of cell migration in the developing human brain.     

NO-cGMP Signaling and Regenerative Medicine Involving Stem Cells

Nitric oxide (NO) is a short lived diatomic free radical species synthesized by nitric oxide synthases (NOS). The physiological roles of NO depend on its local concentrations as well as availability and the nature of downstream target molecules. At low nanomolar concentrations, activation of soluble guanylyl cyclase (sGC) is the major event initiated by NO. The resulting elevation in the intracellular cyclic GMP (cGMP) levels serves as signals for regulating diverse cellular and physiological processes. The participation of NO and cGMP in diverse physiological processes is made possible through cell type specific spatio-temporal regulation of NO and cGMP synthesis and signal diversity downstream of cGMP achieved through specific target selection. Thus cyclic GMP directly regulates the activities of its downstream effectors such as Protein Kinase G (PKG), Cyclic Nucleotide Gated channels (CNG) and Cyclic nucleotide phosphodiesterases, which in turn regulate the activities of a number of proteins that are involved in regulating diverse cellular and physiological processes. Localization and activity of the NO-cGMP signaling pathway components are regulated by G-protein coupled receptors, receptor and non receptor tyrosine kinases, phosphatases and other signaling molecules. NO also serves as a powerful paracrine factor. At micromolar concentrations, NO reacts with superoxide anion to form reactive peroxinitrite, thereby leading to the oxidation of important cellular proteins. Extensive research efforts over the past two decades have shown that NO is an important modulator of axon outgrowth and guidance, synaptic plasticity, neural precursor proliferation as well as neuronal survival. Excessive NO production as that evoked by inflammatory signals has been identified as one of the major causative reasons for the pathogenesis of a number of neurodegenerative diseases such as ALS, Alzheimers and Parkinson diseases. Regenerative therapies involving transplantation of embryonic stem cells (ES cells) and ES cell derived lineage committed neural precursor cells have recently shown promising results in animal models of Parkinson disease (PD). Recent studies from our laboratory have shown that a functional NO-cGMP signaling system is operative early during the differentiation of embryonic stem cells. The cell type specific, spatio-temporally regulated NO-cGMP signaling pathways are well suited for inductive signals to use them for important cell fate decision making and lineage commitment processes. We believe that manipulating the NO-cGMP signaling system will be an important tool for large scale generation of lineage committed precursor cells to be used for regenerative therapies. Special issue dedicated to John P. Blass.     

Metabotropic glutamate receptors as targets of neuromodulatory influence of nitric oxide

A possible effect of nitric oxide (NO) on metabotropic glutamate receptor (mGluR) function in the amino acid afferent synapse was investigated in the isolated labyrinth of the frog Rana temporaria. The modification of the amplitude of responses of metabotropic glutamate receptor agonist trans-ACPD was analyzed during bath applied NO donor S-nitroso-N-acetyl-DL-penicillamine SNAP (0.1–100 μM) or nitric oxide synthase inhibitor L-NAME. It was shown that NO donor SNAP (1 μM) inhibited mGluR induced responses, and the inhibitor of NO-synthase L-NAME (100 μM) increased the amplitude of trans-ACPD evoked answers. The results suggest that NO can depress mGluR function due to modulation of functions of the endoplasmic reticulum channels.     

Differing afferent connections of spiking and nonspiking wind-sensitive local interneurons in the terminal abdominal ganglion of the cricket Gryllus bimaculatus

Fifteen local spiking interneurons (LSIs) and twentyone local non-spiking interneurons (LNIs) were identified in the terminal abdominal ganglion (TAG) of the cricket Gryllus bimaculatus on the basis of intracellular recording and staining (Figs. 1, 5, 6). Although the majority of LNIs showed sharp directionalities (Fig. 7) the LSIs did not (Fig. 3). The directionality of LNIs varied with the recording sites within a single cell (Fig. 8). Electrical stimulations of the cereal sensory nerve suggested that the LNIs are connected monosynaptically with the sensory afferents of both the cerci, and that LSIs may possess a variety of bilateral combinations of polysynaptic connections with the sensory afferents. We found that the spiking and the non-spiking local interneurons in the cereal sensory system differ not only in their membrane properties, but also in their afferent connections, and concluded that their differing connectivity to the sensory afferents will associate them with different roles in signal processing.Abbreviations TAG terminal abdominal ganglion - LSI local spiking interneuron - LNI local non-spiking interneurons - CNS central nervous system - PSP post synaptic potential - GI giant interneuron     

Gaseous Oxides and Olfactory Computation

The gaseous neurotransmitters nitric oxide (NO) and carbon monoxide(CO) are prominent and universal components of the array ofneurotransmitters found in olfactory information processingsystems. These highly mobile communication compounds have effectson both second messenger signaling and directly on ion channelgating in olfactory receptors and central synaptic processingof receptor input. Olfactory systems are notable for the plasticityof their synaptic connections, revealed both in higher-orderassociative learning mechanisms using odor cues and developmentalplasticity operating to maintain function during addition ofnew olfactory receptors and new central olfactory interneurons.We use the macrosmatic terrestrial mollusk Limax maximus toinvestigate the role of NO and CO in the dynamics of centralodor processing and odor learning. The major central site ofodor processing in the Limax CNS is the procerebral (PC) lobeof the cerebral ganglion, which displays oscillatory dynamicsof its local field potential and periodic activity waves modulatedby odor input. The bursting neurons in the PC lobe are dependenton local NO synthesis for maintenance of bursting activity andwave propagation. New data show that these bursting PC interneuronsare also stimulated by carbon monoxide. The synthesizing enzymefor carbon monoxide, heme oxygenase 2, is present in the neuropilof the PC lobe. Since the PC lobe exhibits two forms of synapticplasticity related to both associative odor learning and continualconnection of new receptors and interneurons, the use of multiplegaseous neurotransmitters may be required to enable these multipleforms of synaptic plasticity.     

Nitric oxide and cyclic GMP induce vesicle release at Drosophila neuromuscular junction.

Nitric oxide (NO) diffuses as short-lived messenger through the plasma membrane and serves, among many other functions, as an activator of the cGMP synthesizing enzyme soluble guanylyl cyclase (sGC). In view of recent genetic investigations that postulated a retrograde signal from the larval muscle fibers to the presynaptic terminals, we looked for the presence of an NO/cGMP signaling system at the neuromuscular junction (NMJ) of Drosophila melanogaster larvae. Application of NO donors induced cGMP immunoreactivity in the presynaptic terminals but not the postsynaptic muscle fibers at an identified NMJ. The NO-induced cGMP immunoreactivity was sensitive to a specific inhibitor (ODQ) of the sGC. Since presynaptic terminals which were surgically isolated from the central nervous system are capable of synthesizing cGMP, we suggest that an NO-sensitive guanylyl cyclase is present in the terminal arborizations. Using a fluorescent dye that is known to stain recycling synaptic vesicles, we demonstrate that NO donors and membrane permeant cGMP analogues cause vesicle release at the NMJ. Moreover, the NO-induced release could be blocked by the specific inhibitor of the sGC. A destaining of synaptic terminals after NO exposure in Ca2+-free solution in the presence of cobalt chloride as a channel blocker suggested that NO stimulates Ca2+-independent vesicle release at the NMJ. The combined immunocytochemical and exocytosis imaging experiments imply the involvement of cGMP and NO in the regulation of vesicle release at the NMJ of Drosophila larvae.     

Nitric oxide-evoked cGMP production in Purkinje cells in rat cerebellum: an immunocytochemical and pharmacological study

The cerebellar cells that account for glutamate-dependent cyclic GMP (cGMP) production, involving activation of the ionotropic glutamate receptors/nitric oxide synthase/soluble guanylyl cyclase pathway, are not fully established. In the present paper we have searched for the localisation of the cGMP response to the nitric oxide (NO) donor S-nitroso-penicillamine (SNAP 1muM), expected to generate local NO concentrations in the low nanomolar physiological range and evoking a cGMP response dependent on glutamate release and on the consequent activation of ionotropic glutamate NMDA/non-NMDA receptors, in cerebellar slices from adult rat. We have found that low concentration of exogenous NO evoked cGMP accumulation in Purkinje cells in an ionotropic glutamate receptor-dependent and tetrodotoxin-sensitive manner. Such immunocytochemical localisation appears consistent with functional evidence for physiologically relevant glutamate-dependent cGMP production in Purkinje cells in rat cerebellar cortex.     

Nitric oxide and cyclic GMP induce vesicle release at Drosophila neuromuscular junction

Nitric oxide (NO) diffuses as short‐lived messenger through the plasma membrane and serves, among many other functions, as an activator of the cGMP synthesizing enzyme soluble guanylyl cyclase (sGC). In view of recent genetic investigations that postulated a retrograde signal from the larval muscle fibers to the presynaptic terminals, we looked for the presence of an NO/cGMP signaling system at the neuromuscular junction (NMJ) of Drosophila melanogaster larvae. Application of NO donors induced cGMP immunoreactivity in the presynaptic terminals but not the postsynaptic muscle fibers at an identified NMJ. The NO‐induced cGMP immunoreactivity was sensitive to a specific inhibitor (ODQ) of the sGC. Since presynaptic terminals which were surgically isolated from the central nervous system are capable of synthesizing cGMP, we suggest that an NO‐sensitive guanylyl cyclase is present in the terminal arborizations. Using a fluorescent dye that is known to stain recycling synaptic vesicles, we demonstrate that NO donors and membrane permeant cGMP analogues cause vesicle release at the NMJ. Moreover, the NO‐induced release could be blocked by the specific inhibitor of the sGC. A destaining of synaptic terminals after NO exposure in Ca²⁺‐free solution in the presence of cobalt chloride as a channel blocker suggested that NO stimulates Ca²⁺‐independent vesicle release at the NMJ. The combined immunocytochemical and exocytosis imaging experiments imply the involvement of cGMP and NO in the regulation of vesicle release at the NMJ of Drosophila larvae. © 1999 John Wiley & Sons, Inc. J Neurobiol 39: 337–346, 1999     

Nitric oxide-cGMP signaling regulates axonal elongation during optic nerve regeneration in the goldfish in vitro and in vivo

Nitric oxide (NO) signaling results in both neurotoxic and neuroprotective effects in CNS and PNS neurons, respectively, after nerve lesioning. We investigated the role of NO signaling on optic nerve regeneration in the goldfish ( Carassius auratus ). NADPH diaphorase staining revealed that nitric oxide synthase (NOS) activity was up-regulated primarily in the retinal ganglion cells (RGCs) 5–40 days after axotomy. Levels of neuronal NOS (nNOS) mRNA and protein also increased in the RGCs alone during this period. This period (5–40 days) overlapped with the process of axonal elongation during regeneration of the goldfish optic nerve. Therefore, we evaluated the effect of NO signaling molecules upon neurite outgrowth from adult goldfish axotomized RGCs in culture. NO donors and dibutyryl cGMP increased neurite outgrowth dose-dependently. In contrast, a nNOS inhibitor and small interfering RNA, specific for the nNOS gene, suppressed neurite outgrowth from the injured RGCs. Intra-ocular dibutyryl cGMP promoted the axonal regeneration from injured RGCs in vivo . None of these molecules had an effect on cell death/survival in this culture system. This is the first report showing that NO-cGMP signaling pathway through nNOS activation is involved in neuroregeneration in fish CNS neurons after nerve lesioning.     

Absence of implication of L-arginine/nitric oxide pathway on neuronal cell injury induced by L-glutamate or hypoxia.

L-glutamate, N-methyl-D-aspartate (NMDA), kainate, quisqualate and sodium nitroprusside increased cyclic GMP (cGMP) level on rat whole brain cell culture. The accumulation of cGMP evoked by L-glutamate was inhibited by a NMDA antagonist MK-801, an inhibitor of guanylate cyclase methylene blue and two nitric oxide (NO) synthase inhibitors NG-monomethyl-L-arginine (L-NMMA) and L-NG-nitroarginine (NO2Arg). The inhibition of L-NMMA on cGMP level was reversed partially by addition of L-arginine. Although MK-801 was able to protect cells from neuronal injury induced by L-glutamate or by 5 h hypoxia, L-NMMA and NO2Arg were ineffective. The present study suggests that cGMP elevation mediated by NO following activation by L-glutamate is not involved in neuronal cell injury.     

Synaptic interactions between nonspiking local interneurones in the terminal abdominal ganglion of the crayfish

Nonspiking local interneurones are the important premotor elements in arthropod motor control systems. We have analyzed the synaptic interactions between nonspiking interneurones in the crayfish terminal (6th) abdominal ganglion using simultaneous intracellular recordings. Only 15% of nonspiking interneurones formed bi-directional excitatory connections. In 77% of connections, however, the nonspiking interneurones showed a one-way inhibitory interaction. In these cases, the presynaptic nonspiking interneurones received excitatory synaptic inputs from the sensory afferents innervating hairs on the surface of the uropods and the postsynaptic nonspiking interneurones received inhibitory synaptic inputs that were partly mediated by the inputs to the presynaptic nonspiking interneurones. The membrane hyperpolarization of the postsynaptic nonspiking interneurones mediated by the presynaptic nonspiking interneurones was reduced in amplitude when the hyperpolarizing current was injected into the postsynaptic interneurones, or when the external bathing solution was replaced with one containing low calcium and high magnesium concentrations. The role of these interactions in the circuits controlling the movements of the terminal appendages is discussed.Abbreviations AL antero-lateral - epsp excitatory postsynaptic potential - ipsp inhibitory postsynaptic potential - PL postero-lateral     

The role of nitric oxide in hippocampal long-term potentiation.

Long-term potentiation is a long-lasting, use-dependent increase in the strength of synaptic connections. We investigated the role of nitric oxide (NO) in determining the duration of potentiation induced by high frequency stimulation of afferents in the CA1 region of the rat hippocampus. The calcium/calmodulin-dependent production of NO can be initiated by activation of excitatory amino acid receptors and results in increased levels of cGMP in target cells. Here we report that only a relatively short-term potentiation can be induced in the presence of nitro-L-arginine methyl ester (L-NAME), an NO synthase inhibitor. The effects of L-NAME on the duration of potentiation are partially reversed by coadministration of L-arginine, a precursor of neuronal NO, and by dibutyryl cGMP. Hemoglobin, which binds extracellular NO, also shortens the duration of stimulus-induced potentiation. The results suggest a role for NO in the maintenance of activity-dependent synaptic enhancements, possibly via the generation of cGMP.