Phorbol ester-induced synaptic facilitation is different than long-term potentiation

Cognitive and hippocampal synaptic profiles in monosodium glutamate-induced obese mice

Obesity is a growing worldwide public health issue and is associated with a range of comorbidities, including cognitive deficits. The present study investigated synaptic changes in the hippocampus during the development of obesity. The treatment of newborn mice with monosodium-L-glutamate (MSG, 2 mg/g) induced obesity and recognition memory deficits in the novel object recognition (NOR) test at 16-17 weeks, but not at 8-9 weeks. Hippocampal synaptic plasticity, including long-term potentiation (LTP) and long-term depression (LTD), and excitatory synaptic transmission at Schaffer collateral-CA1 (SC-CA1) synapses were compared between MSG-treated mice and age-matched control mice. LTP and fiber volley amplitudes were enhanced in MSG-treated mice at 16-17 weeks, but not at 8-9 weeks. Furthermore, the strength of paired-pulse facilitation (PPF) changed in MSG-treated mice at 16-17 weeks, but not at 8-9 weeks. These results suggest that enhanced LTP in the SC-CA1 synapses of MSG-induced obese mice involves presynaptic rather than postsynaptic mechanisms.

Rapid activation of dormant presynaptic terminals by phorbol esters

Presynaptic stimulation stochastically recruits transmission according to the release probability (P(r)) of synapses. The majority of central synapses have relatively low P(r), which includes synapses that are completely quiescent presynaptically. The presence of presynaptically dormant versus active terminals presumably increases synaptic malleability when conditions demand synaptic strengthening or weakening, perhaps by triggering second messenger signals. However, whether modulator-mediated potentiation involves recruitment of transmission from dormant terminals remains unclear. Here, by combining electrophysiological and fluorescence imaging approaches, we uncovered rapid presynaptic awakening by select synaptic modulators. A phorbol ester phorbol 12,13-dibutyrate (PDBu) (a diacylglycerol analog), but not forskolin (an adenylyl cyclase activator) or elevated extracellular calcium, recruited neurotransmission from presynaptically dormant synapses. This effect was not dependent on protein kinase C activation. After PDBu-induced awakening, these previously dormant terminals had a synaptic P(r) spectrum similar to basally active synapses naive to PDBu treatment. Dormant terminals did not seem to have properties of nascent or immature synapses, judged by NR2B NMDAR (NMDA receptor) receptor subunit contribution after PDBu-stimulated awakening. Strikingly, synapses rendered inactive by prolonged depolarization, unlike basally dormant synapses, were not awakened by PDBu. These results suggest that the initial release competence of synapses can dictate the acute response to second messenger modulation, and the results suggest multiple pathways to presynaptic dormancy and awakening.

SNAP-25 Ser187 does not mediate phorbol ester enhancement of hippocampal synaptic transmission

Phorbol esters, activators of protein kinase C (PKC), have been shown to enhance synaptic transmission. One potential downstream target of PKC in the presynaptic terminal is the soluble N-ethylmaleimide sensitive factor (NSF) attachment protein receptor (SNARE) SNAP-25, which has a PKC phosphorylation site in its C-terminal coil centered at serine 187 (S187/Ser187). We examined the role of S187 in hippocampal synaptic transmission. After proteolytic cleavage of native SNAP-25 by botulinum neurotoxin E (BoNT/E), synaptic transmission was restored in a subset of transfected CA3 pyramidal cells with a toxin-resistant form of SNAP-25 containing unaltered S187 (Swt), S187 mutated to alanine (SA) or S187 mutated to glutamate (SE). We observed that phorbol-12,13-diacetate (PDAc, 10 microM) induced potentiation of neurotransmission to a similar degree for both Swt and SA (2.4-fold and 3.1-fold increase, respectively). Furthermore, basal levels of transmission mediated by SE were reduced relative to that of Swt (failure rates of 72% and 41%, respectively). Together, these data suggest that phosphorylation of SNAP-25 S187 does not mediate the observed enhancement of neurotransmission by phorbol esters at hippocampal synapses.

The Relative Contribution of NMDA Receptor Channels in the Expression of Long-term Potentiation in the Hippocampal CA1 Region

Long-term potentiation (LTP) was studied in the hippocampal CA1 region of guinea-pigs using a solution containing 0.1 mM magnesium and 10 microM of the non-N-methyl-d-aspartate (non-NMDA) antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), leaving an NMDA-mediated field excitatory postsynaptic potential (EPSP). Brief high-frequency afferent tetanization induced a substantial synapse-specific potentiation of the NMDA EPSP with a time course closely resembling that previously described for LTP of the non-NMDA-mediated EPSP. This NMDA EPSP potentiation was occluded by prior induction of LTP in normal solution. Using a solution containing 0.1 mM magnesium and 1 microM CNQX, the EPSP was composed of both a non-NMDA- and an NMDA-mediated component which could be measured separately and in parallel. Manipulations that cause increased transmitter release, such as phorbol ester application and changes in stimulation frequency, enhanced the two measures nearly equally. Afferent tetanization induced an increase of both EPSP components, with a similar time course, the NMDA component showing a relative increase of about one-third of that of the non-NMDA one. These results suggest that, to the extent that LTP is based on an increased release of transmitter, the mechanism exhibits features distinct from those underlying other forms of enhanced release.

Activation of a K-252b-Sensitive Protein Kinase is Necessary for a Post-Synaptic Phase of Long-Term Potentiation in Area CA1 of Rat Hippocampus

K-252b, a potent inhibitor of protein kinases blocked a late phase of long-term potentiation (LTP) in area CA1 of rat hippocampal slices, resulting in decremental LTP. It also prevented the slowly developing increase in sensitivity of CA1 neurons to iontophoretically administered alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) which was seen in control slices that exhibit nondecremental LTP. However, K-252b applied 60 - 180 min after the induction of LTP had no effect on the potentiated synaptic and AMPA-induced responses. A K-252b-sensitive protein kinase may therefore be involved in a slowly developing postsynaptic component of LTP.

Long-term potentiation alters the modulator pharmacology of AMPA-type glutamate receptors

Changes in the biophysical properties of AMPA-type glutamate receptors have been proposed to mediate the expression of long-term potentiation (LTP). The present study tested if, as predicted from this hypothesis, AMPA receptor modulators differentially affect potentiated versus control synaptic currents. Whole cell recordings were collected from CA1 pyramidal neurons in hippocampal slices from adult rats. Within-neuron comparisons were made of the excitatory postsynaptic currents (EPSCs) elicited by two separate groups of Schaffer-collateral/commissural synapses. LTP was induced by theta burst stimulation in one set of inputs; cyclothiazide (CTZ), a drug that acts on the desensitization kinetics of AMPA receptors, was infused 30 min later. The decay time constants of the potentiated EPSCs prior to drug infusion were slightly, but significantly, shorter than those of control EPSCs. CTZ slowed the decay of the EPSCs, as reported in prior studies, and did so to a significantly greater degree in the potentiated synapses. Additionally, infusion of CTZ resulted in significantly greater effects on amplitude in potentiated pathways as compared with control pathways. The interaction between LTP and CTZ was also obtained in a separate set of experiments in which GABA receptor antagonists were used to block inhibitory postsynaptic currents. Additionally, there was no significant change in paired-pulse facilitation in the presence of CTZ, indicating that presynaptic effects of the drug were negligible. These findings provide new evidence that LTP modifies AMPA receptor kinetics. Candidates for the changes responsible for the observed effects of LTP were evaluated using a model of AMPA receptor kinetics; a simple increase in the channel opening rate provided the most satisfactory match with the LTP data.

Use-dependent decline of paired-pulse facilitation at Aplysia sensory neuron synapses suggests a distinct vesicle pool or release mechanism

We have characterized paired-pulse facilitation at Aplysia sensory neuron-to-motoneuron synapses. This simple form of very short-term synaptic plasticity displayed an unusual feature: it decreased dramatically with repeated testing. Synaptic depression at these synapses and this use-dependent decrease in paired-pulse facilitation occurred independently of each other. Paired-pulse facilitation was inversely correlated with the size of the initial synaptic connection and was absent at stronger synapses. The use-dependent decrease in paired-pulse facilitation occurred at the same rate at large synapses as at small synapses, although the initial paired-pulse facilitation at large synapses was substantially smaller. Rates of synaptic depression were also independent of initial synaptic strength. Paired-pulse facilitation was blocked by presynaptic EGTA injection, but not by postsynaptic EGTA or BAPTA injection. These results indicate that presynaptic Ca2+ influx plays a critical role in paired-pulse facilitation. However, the persistence of the decrease in paired-pulse facilitation for longer than 15 min suggests that Ca2+ from the first paired action potential produces facilitation via a modulatory mechanism rather than by summating with Ca2+ influx during the second paired action potential in activating the Ca2+ binding sites that initiate exocytosis. This modulatory mechanism may not involve protein phosphorylation because paired-pulse facilitation was unaffected by the protein kinase inhibitors H7 and KN-62. These findings further suggest that release by the second paired action potential occurs at sites distinct from those that mediate release by the first action potential.

Use-dependent decline of paired-pulse facilitation at Aplysia sensory neuron synapses suggests a distinct vesicle pool or release mechanism

We have characterized paired-pulse facilitation at Aplysia sensory neuron-to-motoneuron synapses. This simple form of very short-term synaptic plasticity displayed an unusual feature: it decreased dramatically with repeated testing. Synaptic depression at these synapses and this use-dependent decrease in paired-pulse facilitation occurred independently of each other. Paired-pulse facilitation was inversely correlated with the size of the initial synaptic connection and was absent at stronger synapses. The use-dependent decrease in paired-pulse facilitation occurred at the same rate at large synapses as at small synapses, although the initial paired-pulse facilitation at large synapses was substantially smaller. Rates of synaptic depression were also independent of initial synaptic strength. Paired-pulse facilitation was blocked by presynaptic EGTA injection, but not by postsynaptic EGTA or BAPTA injection. These results indicate that presynaptic Ca2+ influx plays a critical role in paired-pulse facilitation. However, the persistence of the decrease in paired-pulse facilitation for longer than 15 min suggests that Ca2+ from the first paired action potential produces facilitation via a modulatory mechanism rather than by summating with Ca2+ influx during the second paired action potential in activating the Ca2+ binding sites that initiate exocytosis. This modulatory mechanism may not involve protein phosphorylation because paired-pulse facilitation was unaffected by the protein kinase inhibitors H7 and KN-62. These findings further suggest that release by the second paired action potential occurs at sites distinct from those that mediate release by the first action potential.

Effects of the Aconitum alkaloid songorine on synaptic transmission and paired-pulse facilitation of CA1 pyramidal cells in rat hippocampal slices

1. The present study investigated the electrophysiological effects of songorine (1 100 microM), an alkaloid occurring in plants of the Aconitum genus, in rat hippocampal slices. 2. Songorine (10-100 microM) evoked a concentration-dependent increase in the amplitude of the orthodromic population spike and in the slope of the field e.p.s.p. The enhancement was long-lasting and was not reversed by up to 90 min of washout. Songorine failed to affect size and shape of the presynaptic fiber spike which represents the compound action potential of the Schaffer collaterals. This indicates that enhancement of the synaptic response is no consequence of an increased afferent excitability. 3. The antidromically evoked population spike was not affected by songorine at concentrations up to 100 microM suggesting that the enhancement of the orthodromic population spike and of the field e.p.s.p. was not due to an increase in pyramidal cell excitability. 4 The input-output curve for the postsynaptic population spike was shifted to the left implying that a presynaptic fiber spike of the same size elicited a larger postsynaptic response, indicating a decrease in threshold for generation of the population spike. 5. The songorine-evoked increase in excitability was not affected by the NMDA receptor antagonist, D-AP5. However, the effect of songorine was completely abolished by the selective dopamine D2 receptor antagonist sulpiride (0.1 microM) as well as by haloperidol (10 microM) and was mimicked by application of the dopamine releaser, amantadine (100 mM). In contrast, the selective D1 receptor antagonist, SCH23390, did not block the action of songorine. 6. The results indicate that the plant alkaloid songorine enhances excitatory synaptic transmission which may be due to an agonistic action at D2 receptors.

GABAa receptor-mediated field potentials are enhanced in area CA1 following prenatal cocaine exposure

Prenatal cocaine exposure results in several documented changes in neurotransmitter receptor number and structure. Increases have been reported for cortical catecholamine and indoleamine receptor number and binding affinity, in the subunit expression of glutamatergic NMDA and AMPA receptors in the striatum, and in GABA immunoreactivity in the anterior cingulate cortex. We sought information on the functional consequences of cocaine-induced alterations in receptor structure/number. Since hippocampal amino acid neurotransmitters are of critical importance and have been shown to be affected by cocaine, we studied field potentials produced by synaptic activation of isolated glutamatergic NMDA and AMPA receptors and GABAa and GABAb responsive receptors in area CA1 of rabbit hippocampal slices. We found the GABAa receptor population produced significantly larger field potentials in cocaine-exposed offspring compared to controls, while other receptors produced responses similar to controls.

Memory and the brain: unexpected chemistries and a new pharmacology

Efforts to characterize long-term potentiation (LTP) and to identify its substrates have led to the discovery of novel synaptic chemistries, computational algorithms, and, most recently, pharmacologies. Progress has also been made in using LTP to develop a "standard model" of how unusual, but physiologically plausible, levels of afferent activity create lasting changes in the operating characteristics of synapses in the cortical telencephalon. Hypotheses of this type typically distinguish induction, expression, and consolidation stages in the formation of LTP. Induction involves a sequence consisting of theta-type rhythmic activity, suppression of inhibitory currents, intense synaptic depolarization, NMDA receptor activation, and calcium influx into dendritic spines. Calcium-dependent lipases, kinases, and proteases have been implicated in LTP induction. Regarding the last group, it has been recently reported that theta pattern stimulation activates calpain and that translational suppression of the protease blocks potentiation. It is thus likely that proteolysis is readily driven by synaptic activity and contributes to structural reorganization. LTP does not interact with treatments that affect transmitter release, has a markedly differential effect on the currents mediated by colocalized AMPA vs NMDA synaptic receptors, changes the waveform of the synaptic current, modifies the effects of drugs that modulate AMPA receptors, and is sensitive to the subunit composition of those receptors. These results indicate that LTP is expressed by changes in AMPA receptor operations. LTP is accompanied by modifications in the anatomy of synapses and spines, something which accounts for its extreme duration (weeks). As with various types of memory, LTP requires about 30 min to consolidate (become resistant to disruption). Consolidation involves adhesion chemistries and, in particular, activation of integrins, a class of transmembrane receptors that control morphology in numerous cell types. Platelet activating factor and adenosine may contribute to consolidation by regulating the engagement of latent integrins. How consolidation stabilizes LTP expression is a topic of intense investigation but probably involves modifications to one or more of the following: membrane environment of AMPA receptors; access of regulatory proteins (e.g., kinases, proteases) to the receptors; receptor clustering; and space available for receptor insertion. Attempts to enhance LTP have focused on the induction phase and resulted in a class of centrally active drugs ("ampakines") that positively modulate AMPA receptors. These compounds promote LTP in vivo and improve the encoding of variety of memory types in animals. Positive results have also been obtained in preliminary studies with humans.

Postsynaptic calcineurin activity downregulates synaptic transmission by weakening intracellular Ca2+ signaling mechanisms in hippocampal CA1 neurons

Protein phosphorylation and dephosphorylation are believed to functionally couple neuronal activity and synaptic plasticity. Our previous results indicated that postsynaptic Ca2+/calmodulin (CaM) signaling pathways play an important role in setting synaptic strength, and calcineurin (CaN) activity limits synaptic responses during basal synaptic transmission and long-term potentiation expression. The inhibition of postsynaptic CaN activity by FK-506 or an autoinhibitory peptide induced synaptic potentiation in hippocampal slices, which occludes tetanus-induced LTP. FK-506-induced synaptic potentiation was expressed in adult but not young rats. To elucidate mechanisms underlying CaN-inhibited synaptic potentiation, we co-injected certain agents affecting Ca2+ signaling pathways with CaN inhibitors into CA1 neurons. Synaptic potentiation induced by FK-506 was significantly attenuated by co-injecting BAPTA, heparin/dantrolene (inhibitors of intracellular Ca2+ release), a CaM-binding peptide, or CaM-KII/PKC pseudosubstrate peptides. These results indicate that postsynaptic CaN activity can downregulate evoked synaptic transmission by weakening intracellular Ca2+ signals and downstream protein kinase activities.

Postsynaptic calcineurin activity downregulates synaptic transmission by weakening intracellular Ca2+ signaling mechanisms in hippocampal CA1 neurons

Protein phosphorylation and dephosphorylation are believed to functionally couple neuronal activity and synaptic plasticity. Our previous results indicated that postsynaptic Ca2+/calmodulin (CaM) signaling pathways play an important role in setting synaptic strength, and calcineurin (CaN) activity limits synaptic responses during basal synaptic transmission and long-term potentiation expression. The inhibition of postsynaptic CaN activity by FK-506 or an autoinhibitory peptide induced synaptic potentiation in hippocampal slices, which occludes tetanus-induced LTP. FK-506-induced synaptic potentiation was expressed in adult but not young rats. To elucidate mechanisms underlying CaN-inhibited synaptic potentiation, we co-injected certain agents affecting Ca2+ signaling pathways with CaN inhibitors into CA1 neurons. Synaptic potentiation induced by FK-506 was significantly attenuated by co-injecting BAPTA, heparin/dantrolene (inhibitors of intracellular Ca2+ release), a CaM-binding peptide, or CaM-KII/PKC pseudosubstrate peptides. These results indicate that postsynaptic CaN activity can downregulate evoked synaptic transmission by weakening intracellular Ca2+ signals and downstream protein kinase activities.

Lithium enhances synaptic transmission in neonatal rat hippocampus

The effects of lithium on excitatory synaptic transmission were studied in the CA1 region of hippocampal slices taken from 14- to 30-day-old rats using extracellular recording techniques. Lithium (2-18 mM) reversibly increased the field excitatory postsynaptic potentials in a concentration-dependent manner. Application of lithium for 6-15 min had no effect on the synaptic input-output function, while application of lithium for 20-35 min shifted this curve to the left. Lithium reversibly increased the amplitude of the presynaptic fibre volley in a concentration- and calcium-dependent manner. Lithium decreased paired-pulse facilitation measured at 50-ms interstimulus intervals. The results indicate that lithium enhances excitatory synaptic transmission in CA1 pyramidal cells by at least two different actions.

Implication of protein kinase C in mechanisms of potassium-induced long-term potentiation in rat hippocampal slices

The involvement of Ca2+/phospholipid-dependent (alpha, beta, gamma, PKCs) and Ca(2+)-independent PKC (epsilon and zeta isoforms) in mechanisms of long-term potentiation was investigated in CA1 hippocampal slices, using a brief high potassium pulse (50 mM, 40 s) to induce long-term potentiation (K+/LTP). The K+ pulse induced first, in 15 s a translocation of PKC activity to the membrane. This was rapidly followed, from 1 to 60 min after the pulse, by a selective activation of PKC in the cytosol. This activation, which could be blocked by the NMDA (N-methyl-D-aspartate) receptor antagonist 2-amino-5-phosphonovalerate (APV), was associated with a significant increase n immunoreactivity for gamma PKC in he cytosol, and also to a less degree for beta PKC. In contrast, application of the phorbol ester PMA (phorbol 12-mirystate 13 acetate) to other slices induced a rapid and persistent translocation to the membrane of alpha, beta, epsilon and zeta PKCs. A major role for the activation role for the activation of cytosolic gamma PKC in the maintenance of LTP is discussed.

Ca2+/calmodulin-dependent protein kinase II phosphorylation of the presynaptic protein synapsin I is persistently increased during long-term potentiation

Long-term potentiation (LTP) is an increase in synaptic responsiveness thought to be involved in mammalian learning and memory. The localization (presynaptic and/or postsynaptic) of changes underlying LTP has been difficult to resolve with current electrophysiological techniques. Using a biochemical approach, we have addressed this issue and attempted to identify specific molecular mechanisms that may underlie LTP. We utilized a novel multiple-electrode stimulator to produce LTP in a substantial portion of the synapses in a hippocampal CA1 minislice and tested the effects of such stimulation on the presynaptic protein synapsin I. LTP-inducing stimulation produced a long-lasting 6-fold increase in the phosphorylation of synapsin I at its Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) sites without affecting synapsin I levels. This effect was fully blocked by either the N-methyl-D-aspartate receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (APV) or the CaM kinase II inhibitor KN-62. Our results indicate that LTP expression is accompanied by persistent changes in presynaptic phosphorylation, and specifically that presynaptic CaM kinase II activity and synapsin I phosphorylation may be involved in LTP expression.

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.

Presynaptic receptors and the control of glutamate exocytosis

When a typical glutamate-containing neurone fires, an action potential is propagated down the branching axon through more than a thousand varicosities. At each of these release sites the probability that a synaptic vesicle will be exocytosed into the synaptic cleft is individually controlled by means of presynaptic receptors: autoreceptors responding by positive or negative feedback to previously released transmitter, or heteroreceptors under the influence of other neurotransmitters or modulators. The simplest system in which to investigate presynaptic modulation is the isolated nerve terminal or synaptosome; studies with this preparation have revealed a complex interplay of signal-transduction pathways.

A critical period of protein kinase activity after tetanic stimulation is required for the induction of long-term potentiation

A critical period of protein kinase activity required for the induction of long-term potentiation (LTP) was determined in area CA1 or hippocampal slices using the broad-range and potent protein kinase inhibitors K-252a and staurosporine. As reported previously, K-252a and staurosporine blocked LTP induction when applied before, during, and after high-frequency stimulation (HFS). In contrast, K-252a did not block LTP when applied only before and during HFS and washed out immediately after HFS. K-252a and staurosporine both attenuated LTP magnitude when applied immediately after or as late as 5 min after HFS. However, K-252a applications beginning 30-45 min after HFS did not affect LTP expression significantly. K-252a had no detectable effect on isolated N-methyl-D-aspartate (NMDA) receptor-mediated EPSPs but significantly inhibited the in situ phosphorylation of specific hippocampal proteins (synapsin I, MARCKS, and B-50). In addition, K-252a attenuated 4 beta-phorbol-12,13-dibutyrate (PDBu)-enhanced synaptic transmission. Our results indicate that there is a critical period of protein kinase activity required for LTP induction that extends for approximately 20 min after HFS. In addition, our results suggest that protein kinase activity during and immediately after HFS is not sufficient for LTP induction. These results provide new information about the mechanisms that underlie LTP induction and expression and provide evidence for persistent and/or Ca(2+)-independent protein kinase activity involvement in LTP.

Potentiated transmission and prevention of further LTP by increased CaMKII activity in postsynaptic hippocampal slice neurons

Calcium-calmodulin-dependent protein kinase II (CaMKII) is a necessary component of the cellular machinery underlying learning and memory. Here, a constitutively active form of this enzyme, CaMKII(1-290), was introduced into neurons of hippocampal slices with a recombinant vaccinia virus to test the hypothesis that increased postsynaptic activity of this enzyme is sufficient to produce long-term synaptic potentiation (LTP), a prominent cellular model of learning and memory. Postsynaptic expression of CaMKII(1-290) increased CaMKII activity, enhanced synaptic transmission, and prevented more potentiation by an LTP-inducing protocol. These results, together with previous studies, suggest that postsynaptic CaMKII activity is necessary and sufficient to generate LTP.

Ionomycin-induced acetylcholine release and its inhibition by adenosine at frog motor nerve endings

1. Acetylcholine (ACh) evoked secretion by the calcium ionophore, ionomycin, was studied at frog motor nerve endings. 2. Bath application of ionomycin stimulated an irreversible increase in the rate of spontaneous, quantal ACh release in the presence of extracellular Ca2+. In contrast, local application of ionomycin stimulated a rapid, reversible acceleration of spontaneous ACh release. 3. The magnitude of the secretory response to ionomycin was dependent both upon the concentration of ionophore and the concentration of extracellular Ca2+. 4. Adenosine or 2-chloroadenosine inhibited ionomycin-stimulated ACh release with the same potency and efficacy observed previously for these adenosine analogues as inhibitors of ACh secretion evoked by nerve impulses. 5. These results support the conclusion that adenosine receptor activation inhibits quantal ACh secretion at a site distal to that of Ca2+ entry at frog motor nerve endings.

Persistent activation of the zeta isoform of protein kinase C in the maintenance of long-term potentiation

Long-term potentiation in the CA1 region of the hippocampus, a model for memory formation in the brain, is divided into two phases. A transient process (induction) is initiated, which then generates a persistent mechanism (maintenance) for enhancing synaptic strength. Protein kinase C (PKC), a gene family of multiple isozymes, may play a role in both induction and maintenance. In region CA1 from rat hippocampal slices, most of the isozymes of PKC translocated to the particulate fraction 15 sec after a tetanus. The increase of PKC in the particulate fraction did not persist into the maintenance phase of long-term potentiation. In contrast, a constitutively active kinase, PKM, a form specific to a single isozyme (zeta), increased in the cytosol during the maintenance phase. The transition from translocation of PKC to formation of PKM may help to explain the molecular mechanisms of induction and maintenance of long-term potentiation.

A synaptic model of memory: long-term potentiation in the hippocampus

Long-term potentiation of synaptic transmission in the hippocampus is the primary experimental model for investigating the synaptic basis of learning and memory in vertebrates. The best understood form of long-term potentiation is induced by the activation of the N-methyl-D-aspartate receptor complex. This subtype of glutamate receptor endows long-term potentiation with Hebbian characteristics, and allows electrical events at the postsynaptic membrane to be transduced into chemical signals which, in turn, are thought to activate both pre- and postsynaptic mechanisms to generate a persistent increase in synaptic strength.

Protein kinase C-mediated enhancement of NMDA currents by metabotropic glutamate receptors in Xenopus oocytes

1. N-Methyl-D-aspartate (NMDA) receptors were expressed in Xenopus oocytes injected with rat brain RNA. The modulation of NMDA-induced currents was examined by activating protein kinase C (PKC) either directly (using phorbol esters) or indirectly (via metabotropic glutamate agonists). 2. Bath application of the PKC activator, 4-beta-phorbol-12,13-dibutyrate (PDBu) resulted in a two-fold increase in the NMDA-evoked current at all holding potentials examined (-80 to 0 mV). The inactive (alpha) stereoisomer of phorbol ester was ineffective. 3. The increase was observed under conditions that eliminate the oocyte's endogenous calcium-dependent chloride current, which often contributes to the NMDA response in oocytes. 4. The PDBu effect was specific to the NMDA subclass of glutamate receptors in that no increase was observed in the responses to two other glutamate agonists, kainate and AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid). 5. Stimulation of PKC by activation of metabotropic receptors via either quisqualate or trans-ACPD (trans-1-aminocyclopentane-1,3-dicarboxylic acid) also led to an increase in NMDA currents. 6. Both methods of enhancement induced transient effects. PDBu effects lasted 10-45 min, depending upon both dose and length of application. Quisqualate and trans-ACPD effects were shorter, lasting less than 10 min under these conditions of application. 7. Both methods of enhancement were blocked by the PKC inhibitor, staurosporine. In addition, the phorbol ester-induced enhancement of NMDA responses occluded further enhancement by quisqualate. 8. The results suggest a role for metabotropic glutamate receptors in modulation of NMDA-mediated processes.

Activators of protein kinase C increase the phosphorylation of the synapsins at sites phosphorylated by cAMP-dependent and Ca2+/calmodulin-dependent protein kinase in the rat hippocampal slice

Previous studies have shown that activators of protein kinase C (C kinase) produce synaptic potentiation in the hippocampus. For example, the C kinase activator phorbol dibutyrate has been shown to increase transmitter release in the hippocampus. In addition, a role for C kinase in long-term potentiation has been proposed. A common assumption in such studies has been that substrates for C kinase were responsible for producing these forms of synaptic potentiation. However, we have recently shown that phorbol dibutyrate increased the phosphorylated of synapsin II (formerly protein III, Browning et al., 1987) in chromaffin cells (Haycock et al., 1988). Synapsin II is a synaptic vesicle-associated phosphoprotein that is a very poor substrate for C kinase but an excellent substrate for cAMP-dependent and Ca2+/calmodulin-dependent protein kinase. We felt, therefore, that activation of C kinase might lead to activation of a kinase cascade. Thus effects of C kinase activation might be produced via the phosphorylation of proteins that are not substrates for C kinase. In this report we test the hypothesis that activators of C kinase increase the phosphorylation of synapsin II and an homologous protein synapsin I. Our data indicate that PdBu produced dose-dependent increases in the phosphorylation of synapsin I and synapsin II. We also performed phospho-site analysis of synapsin I using limited proteolysis. These studies indicated that PdBu increased the phosphorylation of multiple sites on synapsin I. These sites have previously been shown to be phosphorylated by both cAMP-dependent protein kinase and the multifunctional Ca2+/calmodulin-dependent protein kinase II.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term potentiation, protein kinase C, and glutamate receptors

Among the various molecular events that have been proposed to contribute to the mechanisms of long-term potentiation (LTP), one of the most cited possibilities has been the activation of protein kinase C (PKC). Here we review various aspects of the cellular actions of PKC activation and inhibition, with special emphasis on the effects of the kinase on synaptic transmission and the N-methyl-D-aspartate (NMDA) and non-NMDA receptor-mediated components of synaptic responses. We discuss the implications of these effects for interpretations of the role of PKC in the mechanisms of LTP induction and maintenance.

Pharmacology of long-term potentiation. A model for learning reviewed

Long-term potentiation is widely used as a model for memory formation. Recently, much information concerning this topic like the involvement of protein kinase C, arachidonic acid and N-methyl-D-aspartate receptors has been reported. In this review recent discoveries concerning long-term potentiation and the pharmacological implications for the development of cognition-enhancing drugs are discussed.

Comparison of two forms of long-term potentiation in single hippocampal neurons

In invertebrate nervous systems, some long-lasting increases in synaptic efficacy result from changes in the presynaptic cell. In the vertebrate nervous system, the best understood long-lasting change in synaptic strength is long-term potentiation (LTP) in the CA1 region of the hippocampus. Here the process is initiated postsynaptically, but the site of the persistent change is unresolved. Single CA3 hippocampal pyramidal cells receive excitatory inputs from associational-commissural fibers and from the mossy fibers of dentate granule cells and both pathways exhibit LTP. Although the induction of associational-commissural LTP requires in the postsynaptic cell N-methyl-D-aspartate (NMDA) receptor activation, membrane depolarization, and a rise in calcium, mossy fiber LTP does not. Paired-pulse facilitation, which is an index of increased transmitter release, is unaltered during associational-commissural LTP but is reduced during mossy fiber LTP. Thus, both the induction and the persistent change may be presynaptic in mossy fiber LTP but not in associational-commissural LTP.

Protein kinase C activity is not responsible for the expression of long-term potentiation in hippocampus

Long-term potentiation (LTP) in hippocampus has been proposed to result from a tonic activation of protein kinase C. This hypothesis predicts that stimulation of the kinase would produce a smaller change in response size on potentiated versus control pathways and, conversely, that inhibition of the kinase would reduce potentiated inputs to a greater degree than control responses. We tested these predictions using phorbol esters to activate and using the antagonist H-7 to inhibit protein kinase C; we found that the actions of these drugs on synaptic transmission were not affected by prior induction of LTP. Both compounds, however, significantly decreased the contribution of N-methyl-D-aspartate receptors to synaptic potentials, a result that accounts for the suppressive effects of these compounds on LTP formation. Thus protein kinase C is probably not involved in the expression of LTP but may play a role in the receptor-mediated events participating in its induction.

The nature and causes of hippocampal long-term potentiation

One of the most fascinating features of the hippocampus is its capacity for plasticity. Long-term potentiation (LTP), a stable facilitation of synaptic potentials after high-frequency synaptic activity, is very prominent in hippocampus and is a leading candidate memory storage mechanism. Here, we discuss the nature and causes of LTP and relate them to endogenous rhythmic neuronal activity patterns and their potential roles in memory. Anatomical studies indicate that LTP is accompanied by postsynaptic structural modifications while pharmacological studies strongly suggest that LTP is not due to an increase in presynaptic transmitter release. In field CA1, LTP induction appears to be triggered by a postsynaptic influx of calcium through NMDA receptor-linked channels. Possible roles of several calcium-sensitive enzyme systems in LTP are discussed and it is argued that activation of a calcium-dependent protease (calpain) could produce the structural changes linked to LTP. Rhythmic bursting activity is highly effective in inducing LTP and it is argued that the endogenous hippocampal theta rhythm plays a role in LTP induction in vivo. Finally, studies indicate that LTP and certain types of memory share a common pharmacology and the use of electrical brain stimulation as a sensory cue suggests that LTP develops when the significance of that cue is learned.

The protein kinase C inhibitor 1-(5-isoquinolinesulphonyl)-2-methylpiperazine (H-7) disinhibits CA1 pyramidal cells in rat hippocampal slices

1. The effects of the protein kinase C (PKC) inhibitor 1-(5-isoquinolinesulphonyl)-2-methylpiperazine (H-7) on evoked synaptic potentials were investigated in the CA1 region of rat hippocampal slices by use of extracellular and intracellular recording techniques. 2. Extracellular recordings showed that superfusion with H-7 (10-100 microM) increased the amplitude of the population spike and the initial slope of the dendritic field e.p.s.p. H-7 also produced the appearance of multiple population spikes in the somatic region and in the dendritic field e.p.s.p. 3. H-7 (30 microM) induced the disappearance of intracellularly recorded inhibitory potentials elicited by orthodromic stimulation of CA1 pyramidal cells. At this concentration H-7 had no effect on resting membrane potential, input membrane resistance, and spike threshold. In voltage-clamped neurones H-7 blocked the antidromically evoked inhibitory currents and the spontaneous miniature inhibitory currents. 4. The hyperpolarizing effect of bath applied gamma-aminobutyric acid (GABA, 500 microM) or isoguvacine (30 microM) was not affected by 30 microM H-7. 5. Neither the PKC activity regulator sphingosine (10-40 microM) nor the H-7 analogue N-(2-guanidinoethyl)-5-isoquinolinesulphonamide (HA-1004, 20-50 microM) which is devoid of activity on PKC at these concentrations, affected the extracellularly recorded dendritic field e.p.s.p. or population spike. 6. It is concluded that the disinhibitory effect produced by H-7 is due to the block of a H-7-sensitive PKC which is involved in the spontaneous and evoked release of GABA.

Inhibition of postsynaptic PKC or CaMKII blocks induction but not expression of LTP

Long-term potentiation (LTP) of synaptic transmission is a widely studied cellular example of synaptic plasticity. However, the identity, localization, and interplay among the biochemical signals underlying LTP remain unclear. Intracellular microelectrodes have been used to record synaptic potentials and deliver protein kinase inhibitors to postsynaptic CA1 pyramidal cells. Induction of LTP is blocked by intracellular delivery of H-7, a general protein kinase inhibitor, or PKC(19-31), a selective protein kinase C (PKC) inhibitor, or CaMKII(273-302), a selective inhibitor of the multifunctional Ca2+-calmodulin-dependent protein kinase (CaMKII). After its establishment, LTP appears unresponsive to postsynaptic H-7, although it remains sensitive to externally applied H-7. Thus both postsynaptic PKC and CaMKII are required for the induction of LTP and a presynaptic protein kinase appears to be necessary for the expression of LTP.