Messenger In Long-term Potentiation Research Articles

Retrograde adaptive resonance theory based on the role of nitric oxide in long-term potentiation

Adaptive resonance theory (ART) demonstrates how the brain learns to recognize and categorize vast amounts of information by using top-down expectations and attentional focusing. ART 3, one member of the ART family, embeds the computational properties of the chemical synapse in its search process, but it converges slowly and is lack of stability when being applied in pattern recognition and analysis. To overcome these problems, Nitric Oxide (NO), which serves as a newly discovered retrograde messenger in Long-Term Potentiation (LTP), is introduced in retrograde adaptive resonance theory (ReART) model presented in this paper. In the presented model a novel search hypothesis is proposed to incorporate angle and amplitude information of an external input vector to decide whether the input matches the long-term memory (LTM) weights of an active node or not, and the embedded NO retrograde mechanism makes the search procedure a closed loop, which improves the stability and convergence speed of the transmitter releasing mechanism in a synapse. To make the model more adaptive and practical, a forgetting mechanism is built to improve the weights updating process. Experimental results indicate that the proposed ReART model achieves low error rate, fast convergence and self-organizing weights regulation.

NO enhances presynaptic currents during cerebellar mossy fiber-granule cell LTP.

Nitric oxide (NO) is a candidate retrograde messenger in long-term potentiation (LTP). The NO metabolic pathway is expressed in the cerebellar granule cell layer but its physiological role remained unknown. In this paper we have investigated the role of NO in cerebellar mossy fiber-granule cell LTP, which has postsynaptic N-methyl-d-aspartate (NMDA) receptor-dependent induction. Pre- and postsynaptic current changes were simultaneously measured by using extracellular focal recordings, and NO release was monitored with an electrochemical probe in P21 rat cerebellar slices. High-frequency mossy fiber stimulation induced LTP and caused a significant NO release (6.2 +/- 2.8 nM; n = 5) in the granular layer that was dependent on NMDA receptor as well as on nitric oxide synthase (NOS) activation. Preventing NO production by perfusing the NOS inhibitor 100 microM NG-nitro-l-arginine (L-NNA), blocking extracellular NO diffusion by 10 microM MbO2, or inhibiting the NO target guanylyl cyclase (sGC) with 10 microM 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-dione (ODQ) prevented LTP. Moreover, the NO donor 10 microM 2-(N,N-diethylamino)-diazenolate-2-oxide.Na (DEA-NO) induced LTP, which was mutually occlusive with LTP generated by high-frequency stimulation, prevented by ODQ, and insensitive to NMDA channel blockade (50 microM APV + 25 microM 7-Cl-kyn) or interruption of mossy fiber stimulation. Thus NO is critical for LTP induction at the cerebellar mossy fiber-granule cell relay. Interestingly, LTP manipulations were accompanied by consensual changes in the presynaptic current, suggesting that NO acts as a retrograde signal-enhancing presynaptic terminal excitability.

Synaptic secretion of BDNF after high-frequency stimulation of glutamatergic synapses.

The protein brain-derived neurotrophic factor (BDNF) has been postulated to be a retrograde or paracrine synaptic messenger in long-term potentiation and other forms of activity-dependent synaptic plasticity. Although crucial for this concept, direct evidence for the activity-dependent synaptic release of BDNF is lacking. Here we investigate secretion of BDNF labelled with green fluorescent protein (BDNF-GFP) by monitoring the changes in fluorescence intensity of dendritic BDNF-GFP vesicles at glutamatergic synaptic junctions of living hippocampal neurons. We show that high-frequency activation of glutamatergic synapses triggers the release of BDNF-GFP from synaptically localized secretory granules. This release depends on activation of postsynaptic ionotropic glutamate receptors and on postsynaptic Ca(2+) influx. Release of BDNF-GFP is also observed from extrasynaptic dendritic vesicle clusters, suggesting that a possible spatial restriction of BDNF release to specific synaptic sites can only occur if the postsynaptic depolarization remains local. These results support the concept of BDNF being a synaptic messenger of activity-dependent synaptic plasticity, which is released from postsynaptic neurons.

NEURODEGENERATION AND PLASTICITY OF THE BRAIN

Aging involves neural circuitry rearrangements that are broadly referred to as synaptic plasticity. This is the result of several events, mainly of synaptic activity. Long-term potentiation (LTP), learning and memory promote plasticity of synapses, notably in the hippocampus. There are also pathological conditions that trigger plasticity. For example, in ischemic stroke, damaged neurons (penumbra), sprout and reestablish connections, partially or wholly recovering function. In epileptogenesis, aberrant synaptic plasticity involve a network of intra- and extracellular messengers that modulate receptors, ion channels, and gene expression. Intracellular pathways of signal transduction modulate neuronal survival. Often, upregulation of certain steps of these pathways leads to neuronal damage or death. An early event is phospholipase A2 activation and the release of arachidonic acid (and subsequent synthesis of prostaglandins and other bioactive derivatives), as well as PAF (platelet-activating factor). PAF is a retrograde messenger in LTP and also, when over-produced, becomes an endogenous neurotoxin by enhancing glutamate release and activating the transcription of the inducible cyclooxygenase (COX)-2 (gene linked to synaptic plasticity and also to the inflammatory response). Molecular events involved in neuronal plasticity to be described involve messenger-activated stress sensitive protein quinces, the subsequent stimulation of transcription factors, and how these DNA-binding proteins modulate gene expression by interacting with consensus sequences in gene promoters. NFK-B (an inflammation-related transcription factor) may be critical in turning on the COX-2 gene in human neocortical samples from Alzheimer's patients. This highlights the potential importance of inflammatory signaling and plasticity in neurodegenerative disease. The identification of the cellular and molecular mechanisms of plasticity of neural circuitry is opening up avenues for novel interventions in aging and in neurodegenerative diseases. (NIH NS23002)

Chapter 20 The neuromessenger platelet-activating factor in plasticity and neurodegeneration

Synaptic activation leads to the formation of arachidonic acid, platelet-activating factor (PAF, 1-O-alkyl-2-acyl-sn-3-phosphocholine) and other lipid messengers. PAF is a potent bioactive phospholipid in synaptic plasticity. PAF enhances presynaptic glutamate release, is a retrograde messenger in long-term potentiation and enhances memory formation. PAF also couples synaptic events with gene expression by stimulating a FOS/JUN/AP-1 transcriptional signaling system, as well as transcription of COX-2 (inducible prostaglandin synthase). Since the COX-2 gene is also involved in synaptic plasticity, the PAF-COX-2 pathway may have physiological significance. Seizures, ischemia and other forms of brain injury promote phospholipase A2 (PLA2) overactivation, resulting in the accumulation of bioactive lipids at the synapse. PAF, under these pathological conditions, behaves as a neuronal injury messenger by at least two mechanisms: (a) enhancing glutamate release; and, (b) by sustained augmentation of COX-2 transcription. These events link PAF with neurodegeneration. The upstream intracellular pathways of signal transduction involved in neuronal or photoreceptor cell apoptosis are not well understood and involve stress sensitive kinases. PAF is a transcriptional activator of the COX-2 gene. BN 50730, a potent intracellular PAF antagonist, blocks COX-2 induction. COX-2 transcription and protein expression are upregulated in the hippocampus in kainic acid induced epileptogenesis. There is a selectively elevated induction of COX-2 (72-fold) by kainic acid preceding neuronal cell death. BN 50730 administered by i.c.v. injection blocks seizure-induced COX-2 induction. Overall, PAF is a dual modulator of neural function and becomes an endogenous neurotoxin when over produced.

Arachidonic acid inhibits uptake of amino acids and potentiates PKC effects on glutamate, but not GABA, exocytosis in isolated hippocampal nerve terminals

Arachidonic acid (AA), the putative retrograde messenger in long-term potentiation, enhanced extracellular aspartate, glutamate, and GABA levels in rat hippocampus synaptosomes. Whether this effect was determined by stimulating the release and/or inhibiting the uptake of amino acids was further investigated using different experimental conditions. To approach physiological conditions, a static incubation assay was used where both release and uptake occur. Under these conditions, AA dose-dependently (10–25 μM) enhanced basal extracellular amino acid levels in a completely Ca 2+-independent way. AA still exerted this effect in the presence of inhibitors of PKC or of AA metabolism. When using the superfusion release assay, in which amino acid uptake cannot occur, no potentiating effect of AA on superfusate amino acid levels was observed. Therefore, AA possibly enhances the extracellular levels of aspartate, glutamate and GABA by inhibiting the uptake of these amino acids and not their efflux. Indeed, AA reduced the Na +-dependent uptake of endogenously released amino acids, which were labelled with traces of tritiated d-aspartate and GABA. When stimulating hippocampus synaptosomes with 4-aminopyridine, AA (2 μM) potentiated the Ca 2+-dependent release of glutamate, but not of GABA, synergistically with PKC activation by 4β-phorbol-12,13-dibutyric acid. In rat hippocampus, AA exerts different presynaptic effects to regulate extracellular amino acid levels, by inhibiting carrier-mediated uptake and, for glutamate, by stimulating exocytosis.

Bioactive lipids in excitatory neurotransmission and neuronal plasticity

Long-term potentiation (LTP), a model of activity-dependent synaptic plasticity and of certain forms of memory, comprises the persistent enhancement of excitatory neurotransmission that results from high-frequency activation. A presynaptic component of LTP is thought to be modulated by a retrograde messenger generated by the postsynaptic neuron. Arachidonic acid, nitrix oxide, carbon monoxide and PAF have each been proposed as retrograde messengers in LTP, but arachidonic acid, unlike PAF, requires NMDA receptor activation. A PAF antagonist (BN 52021) that provides neuroprotection in ischemia-reperfusion displaces [ 3H] PAF bound to presynaptic membranes, blocks PAF-induced glutamate exocytosis and inhibits LTP. An antagonist selective for the intracellular PAF binding site (BN 50730) did not affect LTP, nor did BN 52021 modify NMDA currents. LTP was induced with weak synaptic stimulation coupled with postsynaptically administered enzyme resistant mcPAF. Theta-burst stimulation (10 min) after bath applications of mcPAF (1 μM) induced APV-independent LTP that was blocked by 5 μM BN 52021. When this antagonist was infused into the hippocampus before or immediately after training, it impaired memory of inhibitory avoidance training in the rat. Memory was not altered if the antagonist is infused 30 or 60 min after training. Moreover, mcPAF enhances memory on retention test performance of step-down inhibitory avoidance habituation and learning in rats. Also, memory was studied using a caudate nucleus-dependent cued water maze task. Rats received an 8 trial (30 s intertrial interval) training session in which a visible cued escape platform was located in a different quadrant of the maze of each trial. Following trial 8, the rats received a unilateral post-training intra-caudate injection of mcPAF (1 μg/0.5 μl), BN 52021 (0.5 μg/0.5 μl) or vehicle. On a retention test session 24 h later, latency to mount the escape platform was used as a measure of memory. The retention test escape latencies of rats given mcPAF were significantly lower than those of the vehicle-injected controls, indicating a memory enhancing effect of mcPAF. Injection of mcPAF did not affect retention when administered 2 h post-training indicating a time-dependent effect of mcPAF on memory. The latencies for animals injected with BN 52021 were significantly higher than those of the controls, indicating that antagonism of endogenous PAF impairs memory. The findings show that PAF plays a role in memory formation in a caudate-mediated cued discrimination task. Administration of BN 52021 2 h post-training had no effect on retention, indicating a time-dependent effect of endogenous PAF on memory formation. PAF, the most potent bioactive lipid known, modulates excitatory synaptic transmission, neuronal plasticity and memory. When PAF production is overstimulated as in seizures or ischemia, it becomes neurotoxic.

Platelet activating factor (PAF) in memory formation: role as a retrograde messenger in long-term potentiation

Long-term potentiation (LTP) is a neurophysiological process that has been implicated in memory formation. The elevation of intracellular Ca 2+ levels in postsynaptic neurons, an essential step in the induction of LTP in the hippocampus, can lead to activation of the enzyme acetyl-CoA:lyso-PAF acetyltransferase that is required for PAF synthesis in neurons. Thus, during the induction of LTP, stimulation of Ca 2+ influx by glutamate receptors would lead to a postsynaptic increase in PAF biosynthesis. A main target for PAF action in neurons is the stimulation of neurotransmitter release via Ca 2+-dependent vesicular exocytosis, a process that occurs presynaptically. In this article we describe the evidence obtained to-date for the pre- and postsynaptic events outlined above, and demonstrate for the first time that during the induction of LTP by high-frequency stimulation (HFS) a 9-fold increase in PAF release to the extracellular environment occurs within 60 min following HFS. This finding provides the evidence that PAF can diffuse from postsynaptic sites of synthesis to presynaptic sites of action, and thus function as a retrograde messenger in the induction of LTP. Based on these data, we present a scheme in which postsynaptic glutamate receptors cooperate with presynaptic PAF receptors in a reverberating cycle that can amplify the transmission in a Hebbian synapse.

Long-term potentiation and N-methyl- d-aspartate receptors: Foundations of memory and neurologic disease?

Understanding the physiology of learning and memory is one of the great challenges of neuroscience. The discovery in recent years of long-term potentiation (LTP) of synaptic transmission and the elaboration of the mechanisms involved, in particular the NMDA receptor, offers the prospect not only of improving our understanding of normal memory storage and retrieval, but may also yield insights about various neurological and psychiatric clinical disorders. In this review, we begin by examining the different forms, properties, and methods of inducing LTP, followed by a description of molecular mechanisms thought to underlie the phenomenon. Molecular structure of the receptor is discussed, along with the roles of Ca 2+ second messenger systems, synaptic morphology changes, and retrograde messengers in LTP. Finally, implications of the NMDA receptor and LTP in learning, memory, and certain clinical conditions such as epilepsy, Alzheimer's disease, and schizophrenia are discussed.

Mediators of injury in neurotrauma: intracellular signal transduction and gene expression.

Membrane lipid-derived second messengers are generated by phospholipase A2 (PLA2) during synaptic activity. Overstimulation of this enzyme during neurotrauma results in the accumulation of bioactive metabolites such as arachidonic acid, oxygenated derivatives of arachidonic acid, and platelet-activating factor (PAF). Several of these bioactive lipids participate in cell damage, cell death, or repair-regenerative neural plasticity. Neurotransmitters may activate PLA2 directly when linked to receptors coupled to G proteins and/or indirectly as calcium influx or mobilization from intracellular stores is stimulated. The release of arachidonic acid and its subsequent metabolism to prostaglandins are early responses linked to neuronal signal transduction. Free arachidonic acid may interact with membrane proteins, i.e., receptors, ion channels, and enzymes, modifying their activity. It can also be acted upon by prostaglandin synthase isoenzymes (the constitutive prostaglandin synthase PGS-1 or the inducible PGS-2) and by lipoxygenases, with the resulting formation of different prostaglandins and leukotrienes. Glutamatergic synaptic activity and activation of postsynaptic NMDA receptors are examples of neuronal activity, linked to memory and learning processes, which activate PLA2 with the consequent release of arachidonic acid and platelet-activating factor (PAF), another lipid mediator. Both mediators may exert presynaptic and postsynaptic effects contributing to long-lasting changes in glutamate synaptic efficacy or long-term potentiation (LTP), PAF, a potential retrograde messenger in LTP, stimulates glutamate release. The PAF antagonist BN 52021 competes for receptors in presynaptic membranes and blocks this effect. PAF may also be involved in plasticity responses because PAF leads to the expression of early response genes and subsequent gene cascades. The PAF antagonist BN 50730, selective for PAF intracellular binding, blocks PAF-mediated induction of gene expression. A consequence of neural injury induced by ischemia, trauma, or seizures is an increased release of neurotransmitters, that in turn generates an overproduction of second messengers. Glutamate, a key player in excitotoxic neuronal damage, triggers increased permeation of calcium mediated by NMDA receptors and activation of PLA2 in postsynaptic neurons. NMDA receptor antagonists reduce the accumulation of free fatty acids and elicit neuroprotection in ischemic damage. Increased production of free arachidonic acid and PAF converges to exacerbate glutamate-mediated neurotransmission. These neurotoxic actions may be brought about by arachidonic acid-induced potentiation of NMDA receptor activity and decreased glutamate reuptake. On the other hand, PAF stimulates the further release of glutamate at presynaptic endings. The neuroprotective effects of the PAF antagonist BN 52021 in ischemia-reperfusion are due, at least in part, to an inhibition of presynaptic glutamate release. PAF also induces expression of the inducible prostaglandin synthase gene, and PAF antagonists selective for the intracellular sites inhibit this effect. The PAF antagonist also inhibits the enhanced abundance, due to vasogenic cerebral edema and ischemia-reperfusion damage, of inducible prostaglandin synthase mRNA in vivo. Therefore, PAF, an injury-generated mediator, may favor the formation of other cell injury and inflammation mediators by turning on the expression of the gene that encodes prostaglandin synthase.

Memory enhancement by intrahippocampal, intraamygdala, or intraentorhinal infusion of platelet-activating factor measured in an inhibitory avoidance task.

Platelet-activating factor (PAF; 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine), which is thought to be a retrograde messenger in long-term potentiation (LTP), enhances glutamate release and LTP through an action on presynaptic nerve endings. The PAF antagonist BN 52021 blocks CA1 LTP in hippocampal slices, and, when infused into rat dorsal hippocampus pre- or posttraining, blocks retention of inhibitory avoidance. Here we report that memory is affected by pre- or posttraining infusion of the PAF analog 1-O-hexadecyl-2-N-methylcarbamoyl-sn-glycerol-3-phosphocholine (mc-PAF) into either rat dorsal hippocampus, amygdala, or entorhinal cortex. Male Wistar rats were implanted bilaterally with cannulae in these brain regions. After recovery from surgery, the animals were trained in step-down inhibitory avoidance or in a spatial habituation task and tested for retention 24 h later. mc-PAF (1.0 microgram per side) enhanced retention test performance of the two tasks when infused into the hippocampus before training without altering training session performance. In addition, mc-PAF enhanced retention test performance of the avoidance task when infused into (i) the hippocampus 0 but not 60 min after training; (ii) the amygdala immediately after training; and (iii) the entorhinal cortex 100 but not 0 or 300 min after training. In confirmation of previous findings, BN 52021 (0.5 microgram per side) was found to be amnestic for the avoidance task when infused into the hippocampus or the amygdala immediately but not 30 or more minutes after training or into the entorhinal cortex 100 but not 0 or 300 min after training. These findings support the hypothesis that memory involves PAF-regulated events, possibly LTP, generated at the time of training in hippocampus and amygdala and 100 min later in the entorhinal cortex.

Nitric oxide synthase expression in single hippocampal neurons

The presence of nitric oxide synthase (NOS) in CA1 pyramidal cells of the rat hippocampus was demonstrated by single-cell PCR. NOS-specific primers were used to amplify mRNA isolated from single hippocampal neurons. The sequence of the major amplification-product obtained was identical to that of the constitutively expressed brain-isoform of NOS. These results confirm immunocytochemical data that NOS is present in CA1, and, therefore, nitric oxide could function as a retrograde messenger in long-term potentiation.

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

Several lines of evidence suggest that cyclic GMP might be involved in long-term potentiation (LTP) in the hippocampus. Arachidonic acid, nitric oxide and carbon monoxide, three molecules that have been proposed to act as retrograde messengers in LTP, all activate soluble guanylyl cyclase. We report here that an inhibitor of guanylyl cyclase blocks the induction of LTP in the CA1 region of hippocampal slices. Conversely, cGMP analogues produce long-lasting enhancement of the excitatory postsynaptic potential if they are applied at the same time as weak tetanic stimulation of the presynaptic fibres. The enhancement is spatially restricted, is not blocked by valeric acid (APV), nifedipine, or picrotoxin, and partially occludes LTP. This synaptic enhancement may be mediated by the cGMP-dependent protein kinase (PKG). Inhibitors of PKG block the induction of LTP, and activators of PKG produce activity-dependent long-lasting enhancement. These results suggest that guanylyl cyclase and PKG contribute to LTP, possibly as activity-dependent presynaptic effectors of retrograde messengers.

Reduction of depolarization-induced glutamate release by heme oxygenase inhibitor: Possible role of carbon monoxide in synaptic transmission

The effects of carbon monoxide on the release of glutamate were studied in synaptoneurosomal preparations using a fluorometric system. Zinc protoporphyrin IX, an inhibitor of the heme oxygenase that produces carbon monoxide, reduced depolarization-induced glutamate release in a calcium-dependent manner, but had no effect on calcium-independent glutamate release. On the basis of this finding, we suggest that carbon monoxide plays a pivotal role in glutamate release in synapses, and may be a retrograde messenger in long-term potentiation.

NADPH-diaphorase (NOS) is induced in pyramidal neurones of hippocampal slices.

We found NADPH-diaphorase (presumably identical with nitric oxide synthase) in pyramidal neurones of the hippocampus in slices that stayed in a chamber for 30 min or longer. In some instances parallel slices showed normal membrane properties when studied electrophysiologically. In freshly made slices the pyramidal neurones were not stained. Thus, after induction of the enzyme, the hippocampal pyramidal neurones can synthesize nitric oxide which may serve as a retrograde messenger in long-term potentiation. The enzyme may also play a role in cell loss seen in slices which stayed in a chamber for 9-22 h before fixation.

The search for retrograde messengers in long-term potentiation

The possibility that retrograde messengers play an essential role in the induction and maintenance of synaptic plasticity, first suggested as long ago as 1966, gained plausibility with the demonstration that the induction of long-term potentiation (LTP) was triggered by postsynaptic events while its maintenance was at least in part presynaptic. We discuss the criteria which a retrograde messenger needs to satisfy and describe experiments to assess three types of candidate retrograde messenger: soluble proteins, arachidonic acid and nitrix oxide.

Effect of PAF antagonists in retinal diseases

Long-term potentiation (LTP), a model of activity-dependent synaptic plasticity and of certain forms of memory, comprises the persistent enhancement of excitatory neurotransmission that results from high-frequency activation. A presynaptic component of LTP is thought to be modulated by a retrograde messenger generated by the postsynaptic neuron. Arachidonic acid, nitrix oxide, carbon monoxide and PAF have each been proposed as retrograde messengers in LTP, but arachidonic acid, unlike PAF, requires NMDA receptor activation. A PAF antagonist (BN 52021) that provides neuroprotection in ischemia-reperfusion displaces [3H] PAF bound to presynaptic membranes, blocks PAF-induced glutamate exocytosis and inhibits LTP. An antagonist selective for the intracellular PAF binding site (BN 50730) did not affect LTP, nor did BN 52021 modify NMDA currents. LTP was induced with weak synaptic stimulation coupled with postsynaptically administered enzyme resistant mcPAF. Theta-burst stimulation (10 min) after bath applications of mcPAF (1 μM) induced APV-independent LTP that was blocked by 5 μM BN 52021. When this antagonist was infused into the hippocampus before or immediately after training, it impaired memory of inhibitory avoidance training in the rat. Memory was not altered if the antagonist is infused 30 or 60 min after training. Moreover, mcPAF enhances memory on retention test performance of step-down inhibitory avoidance habituation and learning in rats. Also, memory was studied using a caudate nucleus-dependent cued water maze task. Rats received an 8 trial (30 s intertrial interval) training session in which a visible cued escape platform was located in a different quadrant of the maze of each trial. Following trial 8, the rats received a unilateral post-training intra-caudate injection of mcPAF (1 μg/0.5 μl), BN 52021 (0.5 μg/0.5 μl) or vehicle. On a retention test session 24 h later, latency to mount the escape platform was used as a measure of memory. The retention test escape latencies of rats given mcPAF were significantly lower than those of the vehicle-injected controls, indicating a memory enhancing effect of mcPAF. Injection of mcPAF did not affect retention when administered 2 h post-training indicating a time-dependent effect of mcPAF on memory. The latencies for animals injected with BN 52021 were significantly higher than those of the controls, indicating that antagonism of endogenous PAF impairs memory. The findings show that PAF plays a role in memory formation in a caudate-mediated cued discrimination task. Administration of BN 52021 2 h post-training had no effect on retention, indicating a time-dependent effect of endogenous PAF on memory formation. PAF, the most potent bioactive lipid known, modulates excitatory synaptic transmission, neuronal plasticity and memory. When PAF production is overstimulated as in seizures or ischemia, it becomes neurotoxic.

Tests of the roles of two diffusible substances in long-term potentiation: evidence for nitric oxide as a possible early retrograde messenger.

Although long-term potentiation (LTP) in the CA1 region of the hippocampus is initiated postsynaptically by the influx of Ca2+ through N-methyl-D-aspartate receptor channels, the maintenance of LTP seems to be at least in part presynaptic. This suggests that the postsynaptic cell releases a retrograde messenger to activate the presynaptic terminals. It is likely that this messenger is membrane-permeant and reaches the presynaptic neuron by diffusion. We therefore have investigated two major membrane-permeant candidate retrograde messengers, arachidonic acid and nitric oxide (NO). Consistent with arachidonic acid or a lipoxygenase metabolite being a retrograde messenger, the phospholipase A2 and lipoxygenase inhibitor nordihydroguaiaretic acid blocked LTP in the guinea pig CA1 region in vitro. However, arachidonic acid (up to 100 microM) did not reliably produce activity-independent LTP, and activity-dependent potentiation by arachidonic acid was blocked by DL-aminophosphonovaleric acid. Since nordihydroguaiaretic acid also interferes with signal transduction involving NO, we next examined whether inhibitors of NO synthase block LTP. NG-Nitro-L-arginine blocked LTP when given in the bath, and this inhibition was partially overcome by high concentrations of L-arginine, suggesting that the inhibitor is specific to NO synthase. NG-Nitro-L-arginine and NG-methyl-L-arginine (but not NG-methyl-D-arginine) also blocked LTP when injected intracellularly, indicating that NO synthase is located in the postsynaptic cell. The NO, in turn, seems to be released into the extracellular space, since bathing the slice with hemoglobin, a protein that binds NO and is not taken up by cells, also blocked LTP. Moreover, NO enhances spontaneous presynaptic release of transmitter from hippocampal neurons in dissociated cell culture. These data favor the idea that NO might be a retrograde messenger in LTP.

Is arachidonic acid a retrograde messenger in long-term potentiation?

Is arachidonic acid a retrograde messenger in long-term potentiation?

Arachidonic Acid Increases Inositol Phospholipid Metabolism and Glutamate Release in Synaptosomes Prepared from Hippocampal Tissue

We have been interested in the possibility that arachidonic acid or one of its 12-lipoxygenase metabolites may function as a retrograde messenger in long-term potentiation (LTP) in the dentate gyrus of the hippocampus. One criterion required of a retrograde messenger is that it stimulates presynaptic changes. Here, two possible presynaptic actions of arachidonic acid and its 12-lipoxygenase metabolites, 12-hydroxyeicosatetraenoic acid (HETE) and 12-hydroperoxyeicosatetraenoic acid (HPETE), are examined. We report that arachidonic acid, HETE, and HPETE significantly increase both K(+)-stimulated release of [3H]glutamate and [3H]inositol labelling of inositol phosphates in synaptosomes, whereas other biologically important fatty acids (oleic, palmitic, and stearic) failed to induce a similar response. The findings of these experiments are consistent with the hypothesis that arachidonic acid, HETE, or HPETE may play the role of a retrograde messenger in LTP.