Homeostatic control of Drosophila neuromuscular junction function

The ability to adapt to changing internal and external conditions is a key feature of biological systems. Homeostasis refers to a regulatory process that stabilizes dynamic systems to counteract perturbations. In the nervous system, homeostatic mechanisms control neuronal excitability, neurotransmitter release, neurotransmitter receptors, and neural circuit function. The neuromuscular junction (NMJ) of Drosophila melanogaster has provided a wealth of molecular information about how synapses implement homeostatic forms of synaptic plasticity, with a focus on the transsynaptic, homeostatic modulation of neurotransmitter release. This review examines some of the recent findings from the Drosophila NMJ and highlights questions the field will ponder in coming years.     

Cocaine withdrawal reduces GABABR transmission at entopeduncular nucleus – lateral habenula synapses

Lateral habenula (LHb) hyperactivity plays a pivotal role in the emergence of negative emotional states, including those occurring during withdrawal from addictive drugs. We have previously implicated cocaine‐driven adaptations at synapses from the entopeduncular nucleus (EPN) to the LHb in this process. Specifically, ionotropic GABA_A receptor (R)‐mediated neurotransmission at EPN‐to‐LHb synapses is reduced during cocaine withdrawal, due to impaired vesicle filling. Recent studies have shown that metabotropic GABA_BR signaling also controls LHb activity, although its role at EPN‐to‐LHb synapses during drug withdrawal is unknown. Here, we predicted that cocaine treatment would reduce GABA_BR‐mediated neurotransmission at EPN‐to‐LHb synapses. We chronically treated mice with saline or cocaine, prepared brain slices after two days of withdrawal and performed voltage‐clamp recordings from LHb neurons whilst optogenetically stimulating EPN terminals. Compared with controls, mice in cocaine withdrawal exhibited reduced GABA_AR‐mediated input to LHb neurons, and a reduced occurrence of GABA_BR‐signaling at EPN‐to‐LHb synapses. We then assessed the underlying mechanism of this decrease. Application of GABA_BR agonist baclofen evoked similar postsynaptic responses in EPN‐innervated LHb neurons in saline‐ and cocaine‐treated mice. Release probability at EPN‐to‐LHb GABAergic synapses was also comparable between groups. However, incubating brain slices in glutamine to facilitate GABA vesicle filling, normalized GABA_BR‐currents at EPN‐to‐LHb synapses in cocaine‐treated mice. Overall, we show that during cocaine withdrawal, together with reduced GABA_AR transmission, also GABA_BR‐mediated inhibitory signaling is diminished at EPN‐to‐LHb synapses, likely via the same presynaptic deficit. In concert, these alterations are predicted to contribute to the emergence of drug withdrawal symptoms, facilitating drug relapse.     

Drebrin a knockout eliminates the rapid form of homeostatic synaptic plasticity at excitatory synapses of intact adult cerebral cortex

Homeostatic synaptic plasticity (HSP) is important for maintaining neurons' excitability within the dynamic range and for protecting neurons from unconstrained long‐term potentiation that can cause breakdown of synapse specificity (Turrigiano [2008] Cell 135:422–435). Knowledge of the molecular mechanism underlying this phenomenon remains incomplete, especially for the rapid form of HSP. To test whether HSP in adulthood depends on an F‐actin binding protein, drebrin A, mice deleted of the adult isoform of drebrin (DAKO) but retaining the embryonic isoform (drebrin E) were generated. HSP was assayed by determining whether the NR2A subunit of N‐methyl‐D‐aspartate receptors (NMDARs) can rise rapidly within spines following the application of an NMDAR antagonist, D ‐APV, onto the cortical surface. Electron microscopic immunocytochemistry revealed that, as expected, the D ‐APV treatment of wild‐type (WT) mouse cortex increased the proportion of NR2A‐immunolabeled spines within 30 minutes relative to basal levels in hemispheres treated with an inactive enantiomer, L ‐APV. This difference was significant at the postsynaptic membrane and postsynaptic density (i.e., synaptic junction) as well as at nonsynaptic sites within spines and was not accompanied by spine size changes. In contrast, the D ‐APV treatment of DAKO brains did not augment NR2A labeling within the spine cytoplasm or at the synaptic junction, even though basal levels of NR2A were not significantly different from those of WT cortices. These findings indicate that drebrin A is required for the rapid (<30 minutes) form of HSP at excitatory synapses of adult cortices, whereas drebrin E is sufficient for maintaining basal NR2A levels within spines. J. Comp. Neurol. 517:105–121, 2009. © 2009 Wiley‐Liss, Inc.     

Effects of intravesicular loading of a Ca2+ chelator and depolymerization of actin fibers on neurotransmitter release in frog motor nerve terminals

Ca²⁺‐induced Ca²⁺ release (CICR) via type‐3 ryanodine receptor enhances neurotransmitter release in frog motor nerve terminals. To test a possible role of synaptic vesicle in CICR, we examined the effects of loading of EGTA, a Ca²⁺ chelator, into synaptic vesicles and depolymerization of actin fibers. Intravesicular EGTA loading via endocytosis inhibited the ryanodine sensitive enhancement of transmitter release induced by tetanic stimulation and the associated rises in intracellular‐free Ca²⁺ ([Ca²⁺]_i: Ca²⁺ transients). Latrunculin A, a depolymerizer of actin fibers, enhanced both spontaneous and stimulation‐induced transmitter release, but inhibited the enhancement of transmitter release elicited by successive tetanic stimulation. The results suggest a possibility that the activation of CICR from mobilized synaptic vesicles caused the enhancement of neurotransmitter release.     

Cellular and subcellular distribution of spinophilin, a PP1 regulatory protein that bundles F-actin in dendritic spines

Spinophilin is an actin binding protein that positions protein phosphatase 1 next to its substrates in dendritic spines. It contains a single PDZ domain and has the biochemical characteristics of a cytoskeletal scaffolding protein. Previous studies suggest that spinophilin is present in most spines, but the concentration of spinophilin varies from brain region to region in a manner that does not simply reflect differences in spine density. Here, we show that spinophilin is enriched in the great majority of dendritic spines in cerebral cortex, caudatoputamen, hippocampal formation, and cerebellum, irrespective of regional differences in spinophilin concentration. In addition, spinophilin is present postsynaptic to asymmetrical contacts on interneuronal dendritic shafts. We further show that, in hippocampus and ventral pallidum, spinophilin is occasionally present in dendritic shafts adjacent to gamma-aminobutyric acid-containing contacts. Thus, the functional role of spinophilin may not be exclusively restricted to excitatory synapses and may be significant at a small fraction of inhibitory contacts. These data also suggest that the concentration of spinophilin per spine is variable and is likely regulated by local physiological factors and/or regional influences.     

Isoproterenol increases the phosphorylation of the synapsins and increases synaptic transmission in dentate gyrus, but not in area CA1, of the hippocampus.

Previous studies have shown that either norepinephrine (NE) or isoproterenol (ISO) enhances the slope of the field excitatory postsynaptic potential (EPSP) in the dentate gyrus of the rat hippocampal formation. In contrast, NE and ISO cause no increase in excitatory transmission in area CA1 of the hippocampus. The molecular mechanism underlying this brain region-specific increase in synaptic transmission is not known. The phosphorylation of synapsin I and synapsin II, two homologous presynaptic vesicle-associated proteins, is thought to promote neurotransmitter release. The authors have observed previously NE- and ISO-enhanced phosphorylation of synapsins I and II in the dentate gyrus. The purpose of this study was to determine whether ISO-stimulated phosphorylation also occurs in the CA1, where ISO has no effect on excitatory neurotransmission. These studies were correlated with electrophysiological studies in in vitro hippocampal slices. Superfusion of slices with ISO resulted in an increase in EPSP slope in the dentate but not in area CA1. The enhanced dentate EPSP returned to baseline levels within 30 minutes of washout of the drug. Isoproterenol produced corresponding increases in the phosphorylation of the synapsins in dentate slices but had no effect on these proteins in CA1 slices. Moreover, in dentate slices exposed to a 30-minute wash following incubation with ISO, phosphorylation of the synapsins returned to control levels. This close temporal and brain regional correlation between ISO stimulation of both synapsin phosphorylation and synaptic transmission suggests that the synapsin proteins may play a role in the synaptic potentiation produced by ISO in the dentate.     

Frequency-dependent requirement for calcium store-operated mechanisms in induction of homosynaptic long-term depression at hippocampus CA1 synapses

For induction of long-term depression (LTD), mechanisms dependent on N-methyl-D-aspartate receptors (NMDARs) and on intracellular calcium stores have been separately known. How these two mechanisms coexist at the same synapses is not clear. Here, induction of LTD at hippocampal Schaffer collateral-to-CA1 pyramidal cell synapses was shown to depend on NMDARs throughout the theoretically predicted activation range for LTD induction. With stimulation at 1 Hz, the largest LTD was induced in a store-independent manner. With stimulation at 0.5 and 2.0 Hz the induced LTD was much smaller, and dependence on calcium stores appeared. Under caffeine application, an enlarged LTD was induced with 0.5 Hz stimulation. Postsynaptic blockade of ryanodine receptors prevented this caffeine-induced enhancement of LTD. It is therefore suggested that calcium release from calcium stores facilitated by caffeine contributed to the LTD enhancement, and that the caffeine effect was exerted on the postsynaptic side. Induction of this enhanced LTD was resistant to NMDAR blockade. We thus propose that the store-dependent mechanism for LTD induction is dormant at the centre of the theoretically predicted activation range for LTD induction, but operates at the fringes of this activation range, with its contribution more emphasized when ample calcium release occurs.     

Effects of synaptic antagonists on perforant path paired-pulse plasticity: differentiation of pre- and postsynaptic antagonism

The effects of different synaptic antagonists on paired-pulse plasticity of medial perforant path responses were studied in rat hippocampal slices. Baclofen reduces the response to activation of the perforant path, but does not have the same net effect on the first and second responses to paired stimulation: baclofen lessens the percent paired-pulse depression of medial perforant path responses. Furthermore, at doses that reduced the control medial perforant path response by half, paired-pulse plasticity changed from paired-pulse depression to paired-pulse potentiation. A similar effect on medial perforant path paired-pulse plasticity is produced by decreasing extracellular calcium concentration. Kynurenic acid reduces the first and second responses to paired stimulation proportionately the same, and, therefore, has no effect on the percent paired-pulse depression. These results suggest that baclofen acts presynaptically to reduce the synaptic response, whereas kynurenate acts postsynaptically. Adenosine was also found to be a potent antagonist of medial perforant path responses, with effects on paired-pulse plasticity similar to baclofen: a new synaptic antagonist, N-p-chlorobenzoyl-piperazine-2,3-dicarboxylate, was found to have effects like kynurenate, suggesting that it is also a postsynaptic receptor blocker.     

Ethanol inhibits long‐term potentiation in hippocampal CA1 neurons,irrespective of lamina and stimulus strength,through neurosteroidogenesis

Ethanol inhibits memory encoding and the induction of long‐term potentiation (LTP) in CA1 neurons of the hippocampus. Hippocampal LTP at Schaffer collateral synapses onto CA1 pyramidal neurons has been widely studied as a cellular model of learning and memory, but there is striking heterogeneity in the underlying molecular mechanisms in distinct regions and in response to distinct stimuli. Basal and apical dendrites differ in terms of innervation, input specificity, and molecular mechanisms of LTP induction and maintenance, and different stimuli determine distinct molecular pathways of potentiation. However, lamina or stimulus‐dependent effects of ethanol on LTP have not been investigated. Here, we tested the effect of acute application of 60 mM ethanol on LTP induction in distinct dendritic compartments (apical versus basal) of CA1 neurons, and in response to distinct stimulation paradigms (single versus repeated, spaced high frequency stimulation). We found that ethanol completely blocks LTP in apical dendrites, whereas it reduces the magnitude of LTP in basal dendrites. Acute ethanol treatment for just 15 min altered pre‐ and post‐synaptic protein expression. Interestingly, ethanol increases the neurosteroid allopregnanolone, which causes ethanol‐dependent inhibition of LTP, more prominently in apical dendrites, where ethanol has greater effects on LTP. This suggests that ethanol has general effects on fundamental properties of synaptic plasticity, but the magnitude of its effect on LTP differs depending on hippocampal sub‐region and stimulus strength. © 2014 Wiley Periodicals, Inc.     

Synaptogenesis in the mushroom body calyx during metamorphosis in the honeybee Apis mellifera: an electron microscopic study

The goals of this study are to determine relationships between synaptogenesis and morphogenesis within the mushroom body calyx of the honeybee Apis mellifera and to find out how the microglomerular structure characteristic for the mature calyx is established during metamorphosis. We show that synaptogenesis in the mushroom body calycal neuropile starts in early metamorphosis (stages P1-P3), before the microglomerular structure of the neuropile is established. The initial step of synaptogenesis is characterized by the rare occurrence of distinct synaptic contacts. A massive synaptogenesis starts at stage P5, which coincides with the formation of microglomeruli, structural units of the calyx that are composed of centrally located presynaptic boutons surrounded by spiny postsynaptic endings. Microglomeruli are assembled either via accumulation of fine postsynaptic processes around preexisting presynaptic boutons or via ingrowth of thin neurites of presynaptic neurons into premicroglomeruli, tightly packed groups of spiny endings. During late pupal stages (P8-P9), addition of new synapses and microglomeruli is likely to continue. Most of the synaptic appositions formed there are made by boutons (putative extrinsic mushroom body neurons) into small postsynaptic profiles that do not exhibit presynaptic specializations (putative intrinsic mushroom body neurons). Synapses between presynaptic boutons characteristic of the adult calyx first appear at stage P8 but remain rare toward the end of metamorphosis. Our observations are consistent with the hypothesis that most of the synapses established during metamorphosis provide the structural basis for afferent information flow to calyces, whereas maturation of local synaptic circuitry is likely to occur after adult emergence.     

Transsynaptic modulation of the synaptic vesicle cycle by cell-adhesion molecules

Delicate control of the synaptic vesicle cycle is required to meet the demands imposed on synaptic transmission by the brain's complex information processing. In addition to intensively analyzed intrinsic regulation, extrinsic modulation of the vesicle cycle by the postsynaptic target neuron has become evident. Recent studies have demonstrated that several families of synaptic cell-adhesion molecules play a significant role in transsynaptic retrograde signaling. Different adhesion systems appear to specifically target distinct steps of the synaptic vesicle cycle. Signaling via classical cadherins regulates the recruitment of synaptic vesicles to the active zone. The neurexin/neuroligin system has been shown to modulate presynaptic release probability. In addition, reverse signaling via the EphB/ephrinB system plays an important role in the activity-dependent induction of long-term potentiation of presynaptic transmitter release. Moreover, the first hints of involvement of cell-adhesion molecules in vesicle endocytosis have been published. A general hypothesis is that specific adhesion systems might use different but parallel transsynaptic signaling pathways able to selectively modulate each step of the synaptic vesicle cycle in a tightly coordinated manner.     

Altered dendritic spine density in animal models of depression and in response to antidepressant treatment.

Olfactory bulbectomy, neonatal clomipramine administration, and maternal deprivation have been employed as animal models of depression. Each model is unique with respect to the experimental manipulations required to produce "depressive" signs, expression and duration of these signs, and response to antidepressant treatments. Dendritic spines represent a possible anatomical substrate for the enduring changes seen with depression and we have previously shown that chronic antidepressant drug exposure alters the density of hippocampal dendritic spines in an enduring fashion. The purpose of the present study was to determine whether persistent alteration of hippocampal spine density is a common element in each of these different models of depression and whether such alterations could be reversed with chronic antidepressant treatment. The results show that olfactory bulbectomy reduced spine density in CA1, CA3, and dentate gyrus compared to sham-operated controls. Chronic treatment with amitriptyline, a tricyclic antidepressant, reversed the bulbectomy- induced reduction in dendritic spine density in CA1, CA3, and dentate gyrus, whereas treatment with mianserin, an atypical antidepressant, reversed this reduction only in dentate gyrus. On the other hand, neither neonatal clomipramine administration nor maternal deprivation affected hippocampal dendritic spine density. Repeated neonatal handling, however, as a control or as part of the maternal deprivation procedure, elevated spine density in dentate gyrus. These data suggest that long-lasting alterations in hippocampal dendritic spine density contribute to the neural mechanism underlying the olfactory bulbectomy model of depression, but not the neonatal clomipramine or maternal deprivation models.     

Developmental changes in the excitatory short‐term plasticity at input synapses in the rat inferior colliculus

The inferior colliculus (IC) is the primal center of convergence and integration in the auditory pathway. Although extensive functional changes are known to occur at the relay synapses in the auditory brainstem during development, the changes in the IC remain to be investigated. Here, we have measured excitatory postsynaptic currents (EPSCs) of the neurons in the central nucleus of the IC in response to stimulation of either the lateral lemniscus or the commissure of the inferior colliculus. Before hearing onset, the lemniscus inputs exhibited short‐term depression, whereas commissural inputs showed facilitation. After hearing onset, the N‐methyl‐d ‐aspartate‐EPSCs exhibited faster decay for both pathways, whereas the decay of the AMPA‐EPSCs were unaltered. Furthermore, the EPSCs showed more constant responses during repetitive stimulation in both pathways. These developmental changes ensure faster and more reliable signal transmission to the inferior colliculus after onset of hearing.     

Preserved induction of long-term potentiation in the stratum radiatum in the CA1 field of hippocampal slices from transgenic mice overexpressing ornithine decarboxylase and overproducing putrescine

The role of putrescine in synaptic neurotransmission and plasticity was studied using transgenic mice overexpressing ornithine decarboxylase (ODC), a polyamine-synthesizing enzyme. Transgenic mice were produced using the standard microinjection technique leading to elevated levels of putrescine in the periphery and in the brain. The experiments investigated whether or not ODC mice with elevated levels of putrescine show alterations in synaptic transmission and induction of long-term potentiation in the CA1 field of the hippocampus in vitro. Our results indicated that (1) putrescine levels in brain slices of the transgenic mice were more than ten times higher than those in fresh slices of control mice, although the absolute levels of putrescine and spermine decreased (by 15 and 40%, respectively) after 3–6 h incubation in vitro, while the levels of spermidine slightly increased (by 10%), (2) the excitatory synaptic response waveforms were wider (an increased half-width), and paired-pulse facilitation was somewhat reduced in ODC mice as compared to controls, and (3) potentiation of excitatory synaptic responses (measured 30–45 min after theta burst stimulation) did not differ between ODC and control mice. These results indicate that synaptic transmission is affected, but synaptic plasticity in the field CA1 assessed in vitro is not changed by elevated levels of intracellular putrescine. Synapse 28:288–293, 1998. © 1998 Wiley-Liss, Inc.     

Actin in the nervous system

Since synaptic plasticity is an important property of the brain, it is timely to try to understand the possible mechanisms underlying this phenomenon. The role of the cytoplasm for neuronal functions has until now been largely overlooked, the main emphases being on the plasma membrane for fast electrical events and on cytoplasmic organelles for the slower metabolic processes. However, recent studies on the cytoplasm of non-muscle cells have stressed the importance of contractile proteins, like actin, on maintaining the cell shape and a number of vital cellular functions which may be related to the phase transitions in the cytoplasm. The necessary versatility is conferred on the actin networks by actin-associated proteins and by the free cytosolic calcium. In the nervous system, in addition to actin and myosin, a number of actin regulatory proteins was recently isolated, and they were shown to have properties similar to those of other non-muscle cells. Consequently, actin networks in neurons like those in non-muscle cells may be capable of contraction and phase transitions. The phase transitions have a rapid onset, and they may be quickly terminated or they may last over extended periods of time. In this way actin networks may gain control over the state of the cytoplasm and hence over the function of the neuron. Actin may be therefore uniquely suited to regulate various plastic reactions. The cytoplasm of growth cones and dendritic spines contains solely actin networks and is devoid of microtubules and neurofilaments. Since both these structures contain myosin and since growth cones are endowed with a considerable motility, dendritic spines also may have a likewise property. The necessary regulation of the levels of free cytosolic calcium may be provided by the spine apparatus in addition to calcium pumps in the plasma membrane and calcium regulatory proteins in the spine cytoplasm. Various types of stimulation which change the level of free cytosolic calcium may induce contraction of the spine actin network which may be responsible for the morphometric changes observed following different experimental interventions and pathological conditions. Although most of the conclusions in this review are rather speculative they may provide directions for future research in the spine and synaptic plasticity.     

Unique changes in synaptic morphology following tetanization under pharmacological blockade

Long-term potentiation (LTP) in the hippocampus has been associated with changes in synaptic morphology. Whether these changes are LTP-dependent or simply a result of electrophysiological stimulation has not yet been fully determined. This study involved an examination of synaptic morphology in the rat dentate gyrus 24 h after electrophysiological stimulation sufficient to induce LTP. In one group, ketamine, a competitive NMDA antagonist, was injected prior to stimulation to block the formation of LTP. Synaptic morphological quantification included estimating the total number of synapses per neuron, determining synaptic curvature and the presence of synaptic perforations, and measuring the maximal PSD profile length of the synapses. The results indicated that most of the changes observed following the induction of LTP (increases in the proportion of concave-shaped synapses, increases in perforated concave synapses, and a decrease in the length of nonperforated concave synapses) are not observed under ketamine blockade, suggesting that they are LTP-specific and not simply the result of tetanic stimulation. Ketamine was associated, however, with several novel structural changes including a decrease in the length of the perforations in the concave perforated synapses, a reduction in the number of convex perforated synapses, and a nonlayer-specific increase in synaptic length compared to controls. Based on previous research, this combination of morphological characteristics is potentially less efficacious, which suggests that synapses that are tetanized but not potentiated, due to pharmacological blockade, appear to undergo opposing, compensatory, or homeostatic changes. These results support the suggestion that synaptic morphology changes are both stimulation- and area-specific, are highly complex, and depend on the specific local physiology.     

Differences in paired-pulse facilitation and long-term potentiation between dorsal and ventral CA1 regions in anesthetized rats.

To clarify hippocampal regional differences in synaptic plasticity, paired-pulse facilitation (PPF, a form of short-term plasticity), long-term potentiation (LTP, a form of long-term plasticity), and their interactions were studied in the dorsal and ventral hippocampal CA1 regions of anesthetized rats. Baseline PPF and post-LTP PPF experiments were conducted at interstimulus intervals (ISIs) of 20-320 ms. A general protocol (100 Hz, 1 s) and a stronger protocol (250-Hz pulse series) were applied for LTP induction. PPF were observed in both regions; however, the degree was lower and the range of ISIs was narrower in the ventral region compared with the dorsal region. The degree of ventral LTP was lower than that of the dorsal LTP. The interaction between PPF and LTP was observed in both regions (PPF change correlated inversely with degree of baseline PPF). However, this was also different in each region. Dorsal PPF increased or decreased; in contrast, ventral PPF of short ISIs after LTP only decreased. These regional differences in short-term and long-term synaptic plasticity may explain a consequence of different afferent inputs and information processing.     

Imaging LTP of presynaptic release of FM1-43 from the rapidly recycling vesicle pool of Schaffer collateral-CA1 synapses in rat hippocampal slices

Recent studies using the styryl dye FM1-43 and two-photon microscopy to directly visualize transmitter release at CA3-CA1 excitatory synapses in the hippocampus have demonstrated that activity-dependent long-term potentiation (LTP) and long-term depression are associated with alterations in vesicular release. It is not known whether particular vesicle pools preferentially express these alterations or what second messenger cascades are involved. To address these questions, we selectively loaded FM1-43 into the rapidly recycling pool (RRP) of vesicles by use of a brief hypertonic shock to release and load the RRP. We demonstrate here that the induction of LTP can lead to a selective long-lasting enhancement in presynaptic release from the RRP, while reserve pool kinetics remain unchanged. LTP of RRP release was N-methyl-d-aspartate receptor-dependent and also required production of the intercellular messenger NO and activation of receptor tyrosine kinase. Measurement of FM1-43 stimulus-evoked uptake rates following induction of LTP confirmed that LTP produces more rapid recycling of vesicles released by electrical stimulation, consistent with an enhanced release probability from the RRP.     

Prior short-term synaptic disinhibition facilitates long-term potentiation and suppresses long-term depression at CA1 hippocampal synapses

Long-term potentiation (LTP) and long-term depression (LTD) are two main forms of activity-dependent synaptic plasticity that have been extensively studied as the putative mechanisms underlying learning and memory. Current studies have demonstrated that prior synaptic activity can influence the subsequent induction of LTP and LTD at Schaffer collateral-CA1 synapses. Here, we show that prior short-term synaptic disinhibition induced by type A gamma-aminobutyric acid (GABA) receptor antagonist picrotoxin exhibited a facilitation of LTP induction and an inhibition of LTD induction. This effect lasted between 10 and 30 min after washout of picrotoxin and was specifically inhibited by the L-type voltage-operated Ca2+ channel (VOCC) blocker nimodipine, but not by the N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphopentanoic acid (D-APV). Moreover, this picrotoxin-induced priming effect was mimicked by forskolin, an activator of cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA), and was blocked by the adenylyl cyclase inhibitor 9-(tetrahydro-2-furanyl)-9H-purin-6-amine (SQ 22536) and the PKA inhibitor Rp-adenosine 3',5'-cyclic monophosphothioate (Rp-cAMPS). It was also found that following picrotoxin application, CA1 neurons have a higher probability of synchronous discharge in response to a population of excitatory postsynaptic potential (EPSP) of fixed slope (EPSP/spike potentiation). However, picrotoxin treatment did not significantly affect paired-pulse facilitation (PPF). These findings suggest that a brief of GABAergic disinhibition can act as a priming stimulus for the subsequent induction of LTP and LTD at Schaffer collateral-CA1 synapses. The increase in Ca2+ influx through L-type VOCCs in turn triggering a cAMP/PKA signalling pathway is a possible molecular mechanism underlying this priming effect.