Gamma-aminobutyric acid type A receptors modulate cAMP-mediated long-term potentiation and long-term depression at monosynaptic CA3-CA1 synapses

cAMP induces a protein-synthesis-dependent late phase of long-term potentiation (LTP) at CA3-CA1 synapses in acute hippocampal slices. Herein we report cAMP-mediated LTP and long-term depression (LTD) at monosynaptic CA3-CA1 cell pairs in organotypic hippocampal slice cultures. After bath application of the membrane-permeable cAMP analog adenosine 3',5'-cyclic monophosphorothioate, Sp isomer (Sp-cAMPS), synaptic transmission was enhanced for at least 2 h. Consistent with previous findings, the late phase of LTP requires activation of cAMP-dependent protein kinase A and protein synthesis. There is also an early phase of LTP induced by cAMP; the early phase depends on protein kinase A but, in contrast to the later phase, does not require protein synthesis. In addition, the cAMP-induced LTP is associated with a reduction of paired-pulse facilitation, suggesting that presynaptic modification may be involved. Furthermore, we found that Sp-cAMPS induced LTD in slices pretreated with picrotoxin, a gamma-aminobutyric acid type A (GABA(A)) receptor antagonist. This form of LTD depends on protein synthesis and protein phosphatase(s) and is accompanied by an increased ratio of failed synaptic transmission. These results suggest that GABA(A) receptors can modulate the effect of cAMP on synaptic transmission and thus determine the direction of synaptic plasticity.     

Caveolin-1 knockout mice exhibit impaired induction of mGluR-dependent long-term depression at CA3-CA1 synapses

Group I metabotropic glutamate receptors (mGluR1/5) are important to synaptic circuitry formation during development and to forms of activity-dependent synaptic plasticity. Dysregulation of mGluR1/5 signaling is implicated in some disorders of neurodevelopment, including fragile X syndrome, the most common inherited form of intellectual disabilities and leading cause of autism. Site(s) in the intracellular loops of mGluR1/5 directly bind caveolin-1, an adaptor protein that associates with membrane rafts. Caveolin-1 is the main coat component of caveolae and organizes macromolecular signaling complexes with effector proteins and membrane receptors. We report that long-term depression (LTD) elicited by a single application of the group I mGluR selective agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) was markedly attenuated at Schaffer collateral-CA1 synapses of mice lacking caveolin-1 (Cav1(-/-)), as assessed by field recording. In contrast, multiple applications of DHPG produced LTD comparable to that in WT mice. Passive membrane properties, basal glutamatergic transmission and NMDA receptor (NMDAR)-dependent LTD were unaltered. The remaining LTD was reduced by anisomycin, an inhibitor of protein synthesis, by U0126, an inhibitor of MEK1/2 kinases, and by rapamycin, an inhibitor of mammalian target of rapamycin (mTOR), suggesting mediation by the same mechanisms as in WT. mGluR1/5-dependent activation (phosphorylation) of MEK and extracellular signal-regulated kinase (ERK1/2) was altered in Cav1(-/-) mice; basal phosphorylation was increased, but a single application of DHPG had no further effect, and after DHPG, phosphorylation was similar in WT and Cav1(-/-) mice. Taken together, our findings suggest that caveolin-1 is required for normal coupling of mGluR1/5 to downstream signaling cascades and induction of mGluR-LTD.     

The synaptic glycoprotein neuroplastin is involved in long-term potentiation at hippocampal CA1 synapses

Neuroplastin-65 and -55 (previously known as gp65 and gp55) are glycoproteins of the Ig superfamily that are enriched in rat forebrain synaptic membrane preparations. Whereas the two-Ig domain isoform neuroplastin-55 is expressed in many tissues, the three-Ig domain isoform neuroplastin-65 is brain-specific and enriched in postsynaptic density (PSD) protein preparations. Here, we have assessed the function of neuroplastin in long-term synaptic plasticity. Immunocytochemical studies with neuroplastin-65-specific antibodies differentially stain distinct synaptic neuropil regions of the rat hippocampus with most prominent immunoreactivity in the CA1 region and the proximal molecular layer of the dentate gyrus. Kainate-induced seizures cause a significant enhancement of neuroplastin-65 association with PSDs. Similarly, long-term potentiation (LTP) of CA1 synapses in hippocampal slices enhanced the association of neuroplastin-65 with a detergent-insoluble PSD-enriched protein fraction. Several antibodies against the neuroplastins, including one specific for neuroplastin-65, inhibited the maintenance of LTP. A similar effect was observed when recombinant fusion protein containing the three extracellular Ig domains of neuroplastin-65 was applied to hippocampal slices before LTP induction. Microsphere binding experiments using neuroplastin-F(c) chimeric proteins show that constructs containing Ig1-3 or Ig1 domains, but not Ig2-3 domains mediate homophilic adhesion. These data suggest that neuroplastin plays an essential role in implementing long-term changes in synaptic activity, possibly by means of a homophilic adhesion mechanism.     

Synaptic inhibition regulates associative interactions between afferents during the induction of long-term potentiation and depression.

The induction of long-term potentiation and depression depends upon associative interactions between synapses that converge on individual dendrites. The distance over which these associative interactions occur is limited. The present study evaluates whether this limitation is regulated by synaptic inhibition. We evaluated the associative interactions between two inputs that terminate on different proximo-distal locations along the dendrites of dentate granule cells in the presence of the gamma-aminobutyric acid (GABA) antagonist bicuculline methiodide. Local blockade of GABAergic inhibition enhanced associative interactions between nonoverlapping inputs, compared to within-animal control sites, where inhibitory transmission was intact. The results suggest that synaptic inhibition limits interactions between excitatory synapses by creating current shunts that limit the spread of depolarization within the dendritic tree.     

Postsynaptic protein kinase C essential to induction and maintenance of long-term potentiation in the hippocampal CA1 region.

Previous studies on the effects of protein kinase C (PKC) inhibitors intracellularly introduced into the postsynaptic neuron on long-term potentiation (LTP) in the hippocampal CA1 region showed that given before the tetanic stimulation they only blocked the development of the maintenance phase of LTP and that given after the tetanus they did not affect the continued maintenance of established LTP. We now report different results in such experiments obtained by looking into the dose-effect relationship of the inhibitors given to the postsynaptic neuron and making use of a synergistic effect of two inhibitors given together. We used the following three PKC inhibitors: polymyxin B (PMB), PKC-(19-31), and H7. With the intracellular delivery of the inhibitor(s) beginning 30 min before the tetanus, PMB in adequate dosage or a combination of PMB and PKC-(19-31), each at a low dosage, could block the development of LTP completely including its initial induction phase. With the delivery beginning at the time of the tetanus, PKC-(19-31) or H7 slowly caused the established LTP to decline to the baseline; this decline was greatly accelerated when PMB and PKC-(19-31) or PMB and H7 were given together. PMB and PKC-(19-31) given together 75-90 min or even 3 h after the tetanus caused a decline of the maintained LTP similar to the decline observed when both inhibitors were given at the time of the tetanus. These results show that postsynaptic PKC is essentially involved in both the initial induction and the subsequent maintenance of LTP, contrary to current views on the subject.     

Synaptic disinhibition during maintenance of long-term potentiation in the CA1 hippocampal subfield.

Long-term potentiation (LTP) in the CA1 region of the hippocampus is widely believed to occur through a strengthening of efficacy of excitatory synapses between afferent fibers and pyramidal cells. An alternative mechanism of LTP, reduction of efficacy of synaptic inhibition, was examined in the present report. The present study demonstrates that the maintenance of LTP in the CA1 hippocampal subfield of guinea pigs is accompanied by impairment of type A gamma-aminobutyric acid (GABA) receptor function, particularly at apical dendritic sites of CA1 pyramidal cells. Enhanced excitability of GABAergic interneurons during LTP represents a strengthening of inhibitory efficacy. The net effect of opposite modifications of synaptic inhibition during LTP of CA1 pyramidal cells is an overall impairment of the strength of GABAergic inhibition, and disinhibition could contribute importantly to CA1 pyramidal cell LTP.     

Age-related deterioration of long-term potentiation in the CA3 and CA1 regions of hippocampal slices from the senescence-accelerated mouse

Long-term potentiation (LTP) is one candidate for the mechanism underlying memory storage. In the present study, we carried out electrophysiological studies on hippocampal slices prepared from the senescence-accelerated mouse (SAM-P/8), a strain which shows accelerated senescence and failure of certain types of learning in behavioral tests. The findings were compared with those noted in the SAM-R/1 substrain without severe symptoms of senescence. No significant differences were found between SAM-R/1 and SAM-P/8 of the same ages in responses in the absence of tetanic stimulation, and in LTP after tetanic stimulation. However, there were marked decreases in the degree of potentiation with aging in both strains.     

Cooperative interactions among afferents govern the induction of homosynaptic long-term depression in the hippocampus.

Prolonged periods of low-frequency stimulation have been shown to produce a robust, long-term synaptic depression (LTD) in both hippocampus and visual cortex. In the present study we have examined the extent to which interactions among afferents govern the induction of homosynaptic LTD in young-adult rats in hippocampal region CA1 in vitro. Field excitatory postsynaptic potentials were assessed before and after conditioning stimulation consisting of two 10-min trains of low-frequency stimulation (LFS; 1 Hz) of the Schaffer collateral/commissural pathway. LFS at an intensity producing a 0.5-mV response did not produce significant synaptic depression. However, LFS administered at a higher intensity resulted in significant input-specific LTD of a 0.5-mV test response. Picrotoxin, which also facilitates depolarization of CA1 neurons, significantly enhanced the magnitude of LTD after LFS at 0.5 mV. In addition, LFS at 0.5 mV in normal perfusion medium (no picrotoxin) produced only small changes in synaptic efficacy when either of two converging pathways was conditioned separately but produced a robust LTD when both pathways were conditioned simultaneously. This cooperative LTD was reversibly blocked by prior administration of 100 microM DL-aminophosphonovaleric acid but not by 20 microM nimodipine. Taken together, these results suggest that cooperative interactions among afferents contribute to voltage-dependent processes underlying the induction of homosynaptic LTD.     

N-methyl-D-aspartate receptor blockade during development lowers long-term potentiation threshold without affecting dynamic range of CA3-CA1 synapses

During development, excitatory synapses in the CA1 region of the hippocampus undergo activity-dependent and N-methyl-D-aspartate (NMDA) receptor-dependent long-lasting changes in synaptic efficacy. These bidirectional changes occur between limits that determine the dynamic range within which synapses operate. It is unknown whether the dynamic range itself is also activity-dependent and NMDA receptor-dependent. Here, we show that chronic blockade of NMDA receptors in hippocampal slice cultures during early postnatal development does not affect the dynamic range but results in a lower threshold for the induction of long-term potentiation. Thus, the dynamic range of CA3-CA1 synapses, unlike long-term potentiation threshold, is NMDA receptor-independent, thereby providing functional stability to the hippocampal network during development.     

Transient protein kinase C activation primes long-term depression and suppresses long-term potentiation of synaptic transmission in hippocampus.

Activity-dependent long-lasting plasticity in hippocampus and neocortex includes long-term potentiation (LTP) and long-term depression (LTD) of synaptic strength. Recent studies have confirmed theoretical predictions that the sensitivity of LTP- and LTD-inducing mechanisms is dynamically regulated by previous synaptic history. In particular, prior induction of either repeated short-term potentiations or LTP lowers the threshold for induction of LTD and raises the threshold for LTP. In the current study, transient activation of protein kinase C with phorbol 12,13-diacetate was able to substitute for synaptic activity in priming synapses to exhibit enhanced homosynaptic LTD and to suppress the induction of LTP at Schaffer collateral synapses in area CA1 of hippocampal slices. This priming lasted 30 min, but not 3 hr, following phorbol 12,13-diacetate bath application. These data suggest that a protein kinase C-sensitive phosphorylation site may be an activity-sensitive target mediating the rapid expression of LTP and LTD.     

Virus-mediated gene transfer into hippocampal CA1 region restores long-term potentiation in brain-derived neurotrophic factor mutant mice.

Long-term potentiation (LTP) has been shown to be impaired in mice deficient in the brain-derived neurotrophic factor (BDNF) gene, as well as in a number of other knockout animals. Despite its power the gene-targeting approach is always fraught with the danger of looking at the cumulative direct and indirect effects of the absence of a particular gene rather than its immediate function. The re-expression of a specific gene at a selective time point and at a specific site in gene-defective mutants presents a potent procedure to overcome this limitation and to evaluate the causal relationship between the absence of a particular gene and the impairment of a function in gene-defective animals. Here we demonstrate that the re-expression of the BDNF gene in the CA1 region almost completely restores the severely impaired LTP in hippocampal slices of BDNF-deficient mice. The results therefore provide strong evidence for the direct involvement of BDNF in the process of LTP.     

Short-term synaptic enhancement and long-term potentiation in neocortex.

Repetitive stimuli reliably induce long-term potentiation (LTP) of synapses in the upper layers of the granular somatosensory cortex but not the agranular motor cortex of rats. Herein we examine, in these same cortical areas, short-term changes in synaptic strength that occur during the LTP induction period. theta-Burst stimulation produced a strong short-term enhancement of synapses in the granular area but only weak enhancement in the agranular area. The magnitude of enhancement during stimulation was strongly correlated with the magnitude of LTP subsequently expressed. Short-term enhancement was abolished by an antagonist of N-methyl-D-aspartate (NMDA) receptors but remained in the presence of a non-NMDA receptor antagonist. Inhibitory postsynaptic potentials of the granular and agranular areas displayed similar frequency sensitivity, but the frequency sensitivity of NMDA receptor-dependent excitatory postsynaptic potentials differed significantly between areas. We propose that pathway-specific differences in short-term enhancement are due to variations in the frequency dependence of NMDA currents; different capacities for short-term enhancement may explain why repetitive stimulation more readily induces LTP in the somatosensory cortex than in the motor cortex.     

Frequency-dependent associative long-term potentiation at the hippocampal mossy fiber-CA3 synapse.

The mossy fiber-CA3 synapse displays an N-methyl-D-aspartate-receptor-independent mu-opioid-receptor-dependent form of long-term potentiation (LTP) that is thought not to display cooperativity or associativity with coactive afferents. However, because mossy fiber LTP requires repetitive synaptic activity for its induction, we reevaluated cooperativity and associativity at this synapse by using trains of mossy fiber stimulation. Moderate-, but not low-, intensity trains induced mossy fiber LTP, indicating cooperativity. Low-intensity mossy fiber trains that were normally ineffective in inducing LTP could induce mossy fiber LTP when delivered in conjunction with trains delivered to commissural-CA3 afferents. Associative mossy fiber LTP also could be induced with single mossy fiber pulses when delivered with commissural trains in the presence of a mu-opioid-receptor agonist. Our findings suggest a frequency-dependent variation of Hebbian associative LTP induction that is regulated by the release of endogenous opioid peptides.     

Evidence that associative interactions between synapses during the induction of long-term potentiation occur within local dendritic domains.

The present study evaluates whether the associative interactions between synapses that lead to long-term potentiation and depression (LTP and LTD) can occur between spatially segregated synapses of the medial and lateral temporodentate pathway of the rat. Coconditioning of crossed and ipsilateral pathways resulted in LTP of the crossed system only when the current sinks of the two conditioned pathways overlapped sufficiently. Likewise, conditioning of an ipsilateral pathway alone resulted in LTD of the crossed pathway only when those current sinks overlapped sufficiently. These observations support the idea that associative events that lead to LTP or LTD can be restricted to a local dendritic domain. The postsynaptic cell can therefore serve as more than one unit of integration for synaptic modification.     

Asynchronous pre- and postsynaptic activity induces associative long-term depression in area CA1 of the rat hippocampus in vitro.

Associative long-term depression (LTD) was induced in hippocampal slice cultures with repeated low-frequency (0.3 Hz) stimulation of the Schaffer collateral pathway, only when such stimuli were preceded by intracellular injection of brief depolarizing current pulses in the postsynaptic CA1 pyramidal cell. The decrease in excitatory postsynaptic potential amplitude lasted > 30 min, could be reversed by induction of potentiation, could be induced at previously potentiated inputs, was input-specific, and did not require activation or potentiation of other inputs. The magnitude of the depression depended upon the time interval between depolarization and stimulation and upon the duration of the depolarization pulse. LTD was not observed in neurons impaled with electrodes containing a Ca2+ chelator. LTD could not be induced in the presence of an N-methyl-D-aspartate receptor antagonist, suggesting that voltage-dependent Ca2+ influx is necessary but not sufficient for LTD induction. We conclude that associative LTD results when synaptic activity follows postsynaptic depolarization within a circumscribed time window.     

Long-term synaptic transformation of hippocampal CA1 gamma-aminobutyric acid synapses and the effect of anandamide.

Evidence is presented for a distinctive type of hippocampal synaptic modification [previously described for a molluscan gamma-aminobutyric acid (GABA) synapse after paired pre- and postsynaptic excitation]: transformation of GABA-mediated synaptic inhibition into synaptic excitation. This transformation persists with no further paired stimulation for 60 min or longer and is termed long-term transformation. Long-term transformation is shown to contribute to pairing-induced long-term potentiation but not to long-term potentiation induced by presynaptic stimulation alone. Further support for such mechanistic divergence is provided by pharmacologic effects on long-term transformation as well as these two forms of long-term potentiation by Cl- channel blockers, glutamate and GABA antagonists, as well as the endogenous cannabinoid ligand anandamide.     

Norepinephrine induces pathway-specific long-lasting potentiation and depression in the hippocampal dentate gyrus.

The study presented here indicates that norepinephrine (NE) selectively induces long-lasting modifications of synaptically mediated responses in the dentate gyrus of the rat hippocampal slice. A low concentration of NE (1.0 microM; in the presence of 50 microM phentolamine, an alpha-adrenergic antagonist) or a 1.0 microM concentration of the specific beta-adrenergic agonist isoproterenol induced long-lasting pathway-specific alterations of granule cell electrophysiological responses. Excitatory postsynaptic potentials and population spikes evoked by stimulation of the medial perforant pathway (PP) were potentiated for more than 45 min. In contrast, responses to lateral PP stimulation were depressed for the same period. Both potentiation and depression were blocked by the beta-adrenergic antagonist propranolol (1.0 microM). These results indicate that NE can act differentially on projections to the dentate gyrus arising in the entorhinal cortex. Such selective persistent modifications of cortical circuits may be involved in processes in the mammalian brain underlying attention, learning, and memory.     

A hebbian form of long-term potentiation dependent on mGluR1a in hippocampal inhibitory interneurons.

Hippocampal inhibitory interneurons play important roles in controlling the excitability and synchronization of pyramidal cells, but whether they express long-term synaptic plasticity that contributes to hippocampal network function remains uncertain. We found that pairing postsynaptic depolarization with theta-burst stimulation induced long-term potentiation (LTP) of putative single-fiber excitatory postsynaptic currents in interneurons. Either postsynaptic depolarization or theta-burst stimulation alone failed to induce LTP. LTP was expressed as a decrease in failure rates and an increase in excitatory postsynaptic current amplitude, independent of N-methyl-d-aspartate receptors, and dependent on metabotropic glutamate receptors subtype 1a. LTP was induced specifically in interneurons in stratum oriens and not in interneurons of stratum radiatum/lacunosum-moleculare. Thus, excitatory synapses onto specific subtypes of inhibitory interneurons express a new form of hebbian LTP that will contribute to hippocampal network plasticity.     

Time-restricted role for dendritic activation of the mTOR-p70S6K pathway in the induction of late-phase long-term potentiation in the CA1

Mammalian target of rapamycin (mTOR) is a key regulator of translational capacity. The mTOR inhibitor rapamycin can prevent forms of protein synthesis-dependent synaptic plasticity such as long-term facilitation in Aplysia and late-phase long-term potentiation (L-LTP) in the hippocampal CA1 region of rodents. In the latter model, two issues remain to be addressed: defining the L-LTP phase sensitive to rapamycin and identifying the site of rapamycin-sensitive protein synthesis. Here, we show that L-LTP is sensitive to application of rapamycin only during the induction paradigm, whereas rapamycin application after the establishment of L-LTP was ineffective. Second, we observed that Thr-389-phosphorylated p70 S6 kinase (p70S6K), the main active phosphoform of the mTOR effector p70S6K, was induced in an N-methyl-D-aspartate and phosphatidylinositol 3-kinase-dependent manner throughout the dendrites but not in the cell bodies of CA1 neurons in hippocampal slices after L-LTP induction. A similar dendrite-wide activation of p70S6K was induced in primary hippocampal neurons by depolarization with KCL or glutamate. In primary hippocampal neurons, the sites of dendritic activation of p70S6K appeared as discrete compartments along dendritic shafts like the hotspots for fast dendritic translation. Conversely, only a subset of dendritic spines also displayed activated p70S6K. Taken together, the present data suggest that the N-methyl-d-aspartate-, phosphatidylinositol 3-kinase-dependent dendritic activation of the mTOR-p70S6K pathway is necessary for the induction phase of protein synthesis-dependent synaptic plasticity. Newly synthesized proteins in dendritic shafts could be targeted selectively to activity-tagged synapses. Thus, coordinated activation of dendrite-wide translation and synaptic-specific activation is likely to be necessary for long-term synaptic plasticity.     

Homosynaptic long-term depression in area CA1 of hippocampus and effects of N-methyl-D-aspartate receptor blockade.

We tested a theoretical prediction that patterns of excitatory input activity that consistently fail to activate target neurons sufficiently to induce synaptic potentiation will instead cause a specific synaptic depression. To realize this situation experimentally, the Schaffer collateral projection to area CA1 in rat hippocampal slices was stimulated electrically at frequencies ranging from 0.5 to 50 Hz. Nine hundred pulses at 1-3 Hz consistently yielded a depression of the CA1 population excitatory postsynaptic potential that persisted without signs of recovery for greater than 1 hr after cessation of the conditioning stimulation. This long-term depression was specific to the conditioned input, ruling out generalized changes in postsynaptic responsiveness or excitability. Three lines of evidence suggest that this effect is accounted for by a modification of synaptic effectiveness rather than damage to or fatigue of the stimulated inputs. First, the effect was dependent on the stimulation frequency; 900 pulses at 10 Hz caused no lasting change, and at 50 Hz a synaptic potentiation was usually observed. Second, the depressed synapses continued to support long-term potentiation in response to a high-frequency tetanus. Third, the effects of conditioning stimulation could be prevented by application of NMDA receptor antagonists. Thus, our data suggest that synaptic depression can be triggered by prolonged NMDA receptor activation that is below the threshold for inducing synaptic potentiation. We propose that this mechanism is important for the modifications of hippocampal response properties that underlie some forms of learning and memory.