cAMP and arachidonic acid simulate long-term structural and functional changes produced by neurotransmitters in Aplysia sensory neurons

Evidence of Active-Forgetting Mechanisms? Blocking Arachidonic Acid Release May Slow Forgetting of Sensitization in Aplysia

Long-term sensitization in Aplysia is accompanied by a persistent up-regulation of mRNA encoding the peptide neurotransmitter Phe-Met-Arg-Phe-amide (FMRFa), a neuromodulator that opposes the expression of sensitization through activation of the arachidonic acid second-messenger pathway. We completed a preregistered test of the hypothesis that FMRFa plays a critical role in the forgetting of sensitization. Aplysia received long-term sensitization training and were then given whole-body injections of vehicle (N = 27), FMRFa (N = 26), or 4-bromophenacylbromide (4-BPB; N = 31), a phospholipase inhibitor that prevents the release of arachidonic acid. FMRFa produced no changes in forgetting. 4-BPB decreased forgetting measured 6 d after training [d s = 0.55 95% CI(0.01, 1.09)], though the estimated effect size is uncertain. Our results provide preliminary evidence that forgetting of sensitization may be a regulated, active process in Aplysia, but could also indicate a role for arachidonic acid in stabilizing the induction of sensitization.

Long-term habituation of the C-start escape response in zebrafish larvae

The cellular and molecular basis of long-term memory in vertebrates remains poorly understood. Knowledge regarding long-term memory has been impeded by the enormous complexity of the vertebrate brain, particularly the mammalian brain, as well as by the relative complexity of the behavioral alterations examined in most studies of long-term memory in vertebrates. Here, we demonstrate a long-term form of nonassociative learning-specifically, long-term habituation (LTH)-of a simple reflexive escape response, the C-start, in zebrafish larvae. The C-start is triggered by the activation of one of a pair of giant neurons in the zebrafish's hindbrain, the Mauthner cells. We show that LTH of the C-start requires the activity of NMDA receptors and involves macromolecular synthesis. We further show that the long-term habituated reflex can by rapidly dishabituated by a brief tactile stimulus. Our results set the stage for rigorous, mechanistic investigations of the long-term memory for habituation of a reflexive behavioral response, one that is mediated by a relatively simple, neurobiologically tractable, neural circuit. Moreover, the demonstration of NMDAR and transcriptionally dependent LTH in a translucent vertebrate organism should facilitate the use of optical recording, and optogenetic manipulation, of neuronal activity to elucidate the cellular basis of a long-term vertebrate memory.

Measuring the dynamics of cyclic adenosine monophosphate level in living cells induced by low-level laser irradiation using bioluminescence resonance energy transfer

Several studies demonstrated that the cyclic adenosine monophosphate (cAMP), an important second messenger, is involved in the mechanism of low-level laser irradiation (LLLI) treatment. However, most of these studies obtained the cAMP level in cell culture extracts or supernatant. In this study, the cAMP level in living cells was measured with bioluminescence resonance energy transfer (BRET). The effect of LLLI on cAMP level in living cells with adenosine receptors blocked was explored to identify the role of adenosine receptors in LLLI. The results showed that LLLI increased the cAMP level. Moreover, the rise of cAMP level was light dose dependent but wavelength independent for 658-, 785-, and 830-nm laser light. The results also exhibited that the adenosine receptors, a class of G protein-coupled receptor (GPCR), modulated the increase of cAMP level induced by LLLI. The cAMP level increased more significantly when the A3 adenosine receptors (A3R) were blocked by A3R antagonist compared with A1 adenosine receptor or A2a adenosine receptor blocked in HEK293T cells after LLLI, which was in good agreement with the adenosine receptors’ expressions. All these results suggested that measuring the cAMP level with BRET could be a useful technique to study the role of GPCRs in living cells under LLLI.

Dendritic spine dynamics leading to spine elimination after repeated inductions of LTD

Memory is fixed solidly by repetition. However, the cellular mechanism underlying this repetition-dependent memory consolidation/reconsolidation remains unclear. In our previous study using stable slice cultures of the rodent hippocampus, we found long-lasting synaptic enhancement/suppression coupled with synapse formation/elimination after repeated inductions of chemical LTP/LTD, respectively. We proposed these phenomena as useful model systems for analyzing repetition-dependent memory consolidation. Recently, we analyzed the dynamics of dendritic spines during development of the enhancement, and found that the spines increased in number following characteristic stochastic processes. The current study investigates spine dynamics during the development of the suppression. We found that the rate of spine retraction increased immediately leaving that of spine generation unaltered. Spine elimination occurred independent of the pre-existing spine density on the dendritic segment. In terms of elimination, mushroom-type spines were not necessarily more stable than stubby-type and thin-type spines.

Involvement of TrkB- and p75(NTR)-signaling pathways in two contrasting forms of long-lasting synaptic plasticity

The repetition of experience is often necessary to establish long-lasting memory. However, the cellular mechanisms underlying this repetition-dependent consolidation of memory remain unclear. We previously observed in organotypic slice cultures of the rodent hippocampus that repeated inductions of long-term potentiation (LTP) led to a slowly developing long-lasting synaptic enhancement coupled with synaptogenesis. We also reported that repeated inductions of long-term depression (LTD) produced a long-lasting synaptic suppression coupled with synapse elimination. We proposed these phenomena as useful in vitro models for analyzing repetition-dependent consolidation. Here, we hypothesized that the enhancement and suppression are mediated by the brain-derived neurotrophic factor (BDNF)-TrkB signaling pathway and the proBDNF-p75(NTR) pathway, respectively. When we masked the respective pathways, reversals of the enhancement and suppression resulted. These results suggest the alternative activation of the p75(NTR) pathway by BDNF under TrkB-masking conditions and of the TrkB pathway by proBDNF under p75(NTR)-masking conditions, thus supporting the aforementioned hypothesis.

Protein kinase C regulates local synthesis and secretion of a neuropeptide required for activity-dependent long-term synaptic plasticity

Long-term facilitation (LTF) of sensory neuron synapses in Aplysia is produced by either nonassociative or associative stimuli. Nonassociative LTF can be produced by five spaced applications of serotonin (5-HT) and requires a phosphoinosotide 3-kinase (PI3K)-dependent and rapamycin-sensitive increase in the local synthesis of the sensory neuron neuropeptide sensorin and a protein kinase A (PKA)-dependent increase in the secretion of the newly synthesized sensorin. We report here that associative LTF produced by a single pairing of a brief tetanus with one application of 5-HT requires a rapid protein kinase C (PKC)-dependent and rapamycin-sensitive increase in local sensorin synthesis. This rapid increase in sensorin synthesis does not require PI3K activity or the presence of the sensory neuron cell body but does require the presence of the motor neuron. The secretion of newly synthesized sensorin by 2 h after stimulation requires both PKA and PKC activities to produce associative LTF because incubation with exogenous anti-sensorin antibody or the kinase inhibitors after tetanus plus 5-HT blocked LTF. The secreted sensorin leads to phosphorylation and translocation of p42/44 mitogen-activated protein kinase (MAPK) into the nuclei of the sensory neurons. Thus, different stimuli activating different signaling pathways converge by regulating the synthesis and release of a neuropeptide to produce long-term synaptic plasticity.

Two signaling pathways regulate the expression and secretion of a neuropeptide required for long-term facilitation in Aplysia

Activation of several signaling pathways contributes to long-term synaptic plasticity, but how brief stimuli produce coordinated activation of these pathways is not understood. In Aplysia, the long-term facilitation (LTF) of sensory neuron synapses by 5-hydroxytryptamine (serotonin; 5-HT) requires the activation of several kinases, including mitogen-activated protein kinase (MAPK). The 5-HT-enhanced secretion of the sensory neuron-specific neuropeptide sensorin mediates the activation of MAPK. We find that stimulus-induced activation of two signaling pathways, phosphoinositide 3-kinase (PI3K) and type II protein kinase A (PKA), regulate sensorin secretion and responses. Treatment with 5-HT produces a rapid increase in sensorin synthesis, especially at varicosities, which precedes the secretion of sensorin. PI3K inhibitor and rapamycin block LTF and the rapid synthesis of sensorin at varicosities even in the absence of sensory neuron cell bodies. Secretion of the newly synthesized sensorin from the varicosities and activation of the autocrine responses of sensorin to produce LTF require type II PKA interaction with AKAPs (A-kinase anchoring proteins). Thus, long-term synaptic plasticity is produced when multiple signaling pathways that are important for regulating distinct cellular functions are activated in a specific sequence and recruit the secretion of a neuropeptide to activate additional critical pathways.

Synapse to nucleus signaling during long-term synaptic plasticity; a role for the classical active nuclear import pathway

The requirement for transcription during long-lasting plasticity indicates that signals generated at the synapse must be transported to the nucleus. We have investigated whether the classical active nuclear import pathway mediates intracellular retrograde signal transport in Aplysia sensory neurons and rodent hippocampal neurons. We found that importins localize to distal neuronal processes, including synaptic compartments, where they are well positioned to mediate synapse to nucleus signaling. In Aplysia, stimuli known to produce long-lasting but not short-lasting facilitation triggered importin nuclear translocation. In hippocampal neurons, NMDA receptor activation but not depolarization induced importin nuclear translocation. We further showed that LTP-inducing stimuli recruited active nuclear import in hippocampal slices. Together with our finding that long-term facilitation of Aplysia sensory-motor synapses required active nuclear import, our results indicate that regulation of the active nuclear import pathway plays a critical role in transporting synaptically generated signals into the nucleus during learning-related forms of plasticity.

Serotonin regulates the secretion and autocrine action of a neuropeptide to activate MAPK required for long-term facilitation in Aplysia

In Aplysia, long-term facilitation (LTF) of sensory neuron synapses requires activation of both protein kinase A (PKA) and mitogen-activated protein kinase (MAPK). We find that 5-HT through activation of PKA regulates secretion of the sensory neuron-specific neuropeptide sensorin, which binds autoreceptors to activate MAPK. Anti-sensorin antibody blocked LTF and MAPK activation produced by 5-HT and LTF produced by medium containing sensorin that was secreted from sensory neurons after 5-HT treatment. A single application of 5-HT followed by a 2 hr incubation with sensorin produced protein synthesis-dependent LTF, growth of new presynaptic varicosities, and activation of MAPK and its translocation into sensory neuron nuclei. Inhibiting PKA during 5-HT applications and inhibiting receptor tyrosine kinase or MAPK during sensorin application blocked both LTF and MAPK activation and translocation. Thus, long-term synaptic plasticity is produced when stimuli activate kinases in a specific sequence by regulating the secretion and autocrine action of a neuropeptide.

The two regulatory subunits of aplysia cAMP-dependent protein kinase mediate distinct functions in producing synaptic plasticity

Activation of the cAMP-dependent protein kinase (PKA) is critical for both short- and long-term facilitation in Aplysia sensory neurons. There are two types of the kinase, I and II, differing in their regulatory (R) subunits. We cloned Aplysia RII; RI was cloned previously. Type I PKA is mostly soluble in the cell body whereas type II is enriched at nerve endings where it is bound to two prominent A kinase-anchoring-proteins (AKAPs). Disruption of the binding of RII to AKAPs by Ht31, an inhibitory peptide derived from a human thyroid AKAP, prevents both the short- and the long-term facilitation produced by serotonin (5-HT). During long-term facilitation, RII is transcriptionally upregulated; in contrast, the amount of RI subunits decreases, and previous studies have indicated that the decrease is through ubiquitin-proteosome-mediated proteolysis. Experiments with antisense oligonucleotides injected into the sensory neuron cell body show that the increase in RII protein is essential for the production of long-term facilitation. Using synaptosomes, we found that 5-HT treatment causes RII protein to increase at nerve endings. In addition, using reverse transcription-PCR, we found that RII mRNA is transported from the cell body to nerve terminals. Our results suggest that type I operates in the nucleus to maintain cAMP response element-binding protein-dependent gene expression, and type II PKA acts at sensory neuron synapses phosphorylating proteins to enhance release of neurotransmitter. Thus, the two types of the kinase have distinct but complementary functions in the production of facilitation at synapses of an identified neuron.

Acute and long-term synaptic modulation by neurotrophins

While it has now been well accepted that neurotrophins play an important role in synapse development and plasticity, the specific effects of each neurotrophin on different populations of neurons at different developmental stages have just begun to be worked out. Moreover, the cellular and molecular mechanisms underlying the synaptic function of neurotrophins remain poorly understood. In general, synaptic effects of neurotrophins could be divided into two categories: acute effect on synaptic transmission and plasticity occurring within seconds or minutes after cells are exposed to a neurotrophin, and long-term effect on synaptic structures and function that takes days to accomplish. In this review I have considered the previous findings on neurotrophic regulation of synapses in view of these two categories. Acute and long-term effects of neurotrophins are reexamined in detail in three model systems: the neuromuscular junction, the hippocampus and the visual cortex. Potential molecular mechanisms that mediate the acute or long-term neurotrophic regulation are discussed. Efforts are made to understand the mechanistic differences between the two effects and their relationships. Further study of these mechanisms will help us better understand how neurotrophins can achieve diverse and synapse-specific modulation.

Regulation of neuritogenesis and synaptic transmission by msec7-1, a guanine nucleotide exchange factor, in cultured Aplysia neurons

msec7-1, a mammalian homologue of yeast sec7p, is known as a GDP/GTP exchange factor (GEF) for the ADP ribosylation factor (ARF) family of small GTPases. Here, we report that msec7-1 overexpression in cultured Aplysia neurons leads to an extensive neuritogenesis in a GEF activity-dependent manner through the modulation of the actin cytoskeleton. Similarly, the overexpression of ARNO, another mammalin GEF, produces extensive neuritogenesis in Aplysia neurons. In addition, msec7-1 overexpression increases the number of varicosities with an altered size and shape in a GEF activity-dependent manner. The overexpression of msec7-1 in pre-synaptic sensory neurons co-cultured with post-synaptic target motor neurons leads to an increase in the amplitude of the excitatory post-synaptic potential through its GEF activity. Our results demonstrate that msec7-1 regulates neuritogenesis and synaptic transmission.

Redistribution of syntaxin mRNA in neuronal cell bodies regulates protein expression and transport during synapse formation and long-term synaptic plasticity

Syntaxin has an important role in regulating vesicle docking and fusion essential for neurotransmitter release. Here, we demonstrate that the distribution of syntaxin mRNA in cell bodies of sensory neurons (SNs) of Aplysia maintained in cell culture is affected by synapse formation, synapse stabilization, and long-term facilitation (LTF) produced by 5-HT. The distribution of the mRNA in turn regulates expression and axonal transport of the protein. Syntaxin mRNA and protein accumulated at the axon hillock of SNs during the initial phase of synapse formation. Significant numbers of granules containing syntaxin were detected in the SN axon. When synaptic strength was stable, both mRNA and protein were targeted away from the axon hillock, and the number of syntaxin granules in the SN axon was reduced. Dramatic increases in mRNA and protein accumulation at the axon hillock and number of syntaxin granules in the SN axon were produced when cultures with stable connections were treated with 5-HT that evoked LTF. Anisomycin (protein synthesis inhibitor) or KT5720 (protein kinase A inhibitor) blocked LTF, accumulation of syntaxin mRNA and protein at the axon hillock, and the increase in syntaxin granules in SN axons. The results indicate that without significant effects on overall mRNA expression, both target interaction and 5-HT via activation of protein kinase A pathway regulate expression of syntaxin and its packaging for transport into axons by influencing the distribution of its mRNA in the SN cell body.

Repetitive activation of protein kinase A induces slow and persistent potentiation associated with synaptogenesis in cultured hippocampus

Mammalian brain memory is hypothesized to be established through two phases; short-term plasticity, as exemplified by long-term potentiation (LTP) where pre-existing synapses change transmission efficiency, and long-lasting plasticity where new synapses are formed. This hypothesis, however, has not been verified experimentally. Using cultured hippocampal slices, we show that the repeated induction of late-phase LTP by brief applications of forskolin (FK) led to a slowly-developing long-lasting synaptogenesis, as judged from electrophysiological, cytological and ultrastructural indices. These indices include (1) field postsynaptic potential standardized by field action potential, which should represent the number of synapses per neuron; (2) the amounts of synaptic marker proteins; (3) the number of synaptophysin-immunopositive puncta; (4) the number of dendritic spines per length; (5) the density of synaptic ultrastructures; (6) ultrastructures similar to synapse perforation. Increment in these indices occurred approximately 10 days after FK-application and outlasted the following weeks. The increment depended on the times and intervals of FK-application. A biologically inert FK analogue failed to produce the similar effect. An inhibitor for cyclic AMP-dependent protein kinase (PKA) blocked the synaptogenesis. The cultured brain slice repeatedly exposed to FK should serve as a good model system for the analysis of persistent synaptogenesis possibly related to long-term memory in mammalian CNS.

Synaptic precedence during synapse formation between reciprocally connected neurons involves transmitter-receptor interactions and AA metabolites

The cellular mechanisms that determine specificity of synaptic connections between mutually connected neurons in the nervous system have not yet been fully examined in vertebrate and invertebrate species. Here we report on a novel form of synaptic interaction during early stages of synapse formation between reciprocally connected Lymnaea neurons. Specifically, using soma-soma synapses between an identified dopaminergic neuron (also known as the giant dopamine cell), right pedal dorsal 1 (RPeD1), and a FMRFamidergic neuron, visceral dorsal 4 (VD4), we demonstrate that although reciprocal inhibitory synapses re-form between the somata after 24-36 h of pairing, VD4 is, however, the first cell to establish synaptic contacts with RPeD1 (within 12-18 h). We show that VD4 "captures" RPeD1 first as a postsynaptic cell by suppressing its transmitter secretory machinery during early stages of cell-cell pairing. The VD4-induced suppression of transmitter release from RPeD1 was transient, and it required transcription and de novo protein synthesis dependent step in VD4 but not in RPeD1. The VD4-induced effects on RPeD1 were mimicked by a FMRFamide-like peptide. Perturbation of FMRFamide-activated metabolites of the arachidonic acid pathway in RPeD1 not only prevented FMRFamide-induced suppression of transmitter release from the giant dopamine cell but also shifted the synaptic balance in favor of RPeD1, thus making it the first cell to begin synaptic transmission with VD4 within 12-18 h. A single RPeD1 that had developed dopamine secretory capabilities overnight and was subsequently paired with VD4 for 12-18 h was, however, immune to VD4-induced suppression of transmitter release. Under these experimental conditions, both cells developed mutual inhibitory synapses concurrently. Taken together, our data provide evidence for novel synaptic interaction between reciprocally connected neurons and underscore the importance of transmitter-receptor interplay in regulating the timing of synapse formation in the nervous system.

Localized neuronal outgrowth induced by long-term sensitization training in aplysia

Biophysical, biochemical, and morphological studies have implicated sensory neurons as key sites of plasticity in the formation and retention of the memory of long-term sensitization in Aplysia californica. This study examined the effects of different sensitization training protocols on the structure of sensory neurons mediating the tail-siphon withdrawal reflex. A 4 d training period produced a robust localized outgrowth in these sensory neurons observed 24 hr after the end of training. These changes are consistent with previous results in siphon sensory neurons (Bailey and Chen, 1988a). In contrast, 1 d of sensitization training, which has been shown to effectively induce long-term behavioral sensitization and synaptic facilitation (Frost et al., 1985; Cleary et al., 1998), is not associated with morphological changes in tail sensory neurons at either 24 hr or 4 d after training. Similarly, a single treatment with the growth factor TGF-beta, which also induced facilitation, did not alter sensory neuron morphology. The different effectiveness of the two protocols was not simply a reflection of the number of stimuli presented, because a 1 d massed training protocol did not produce sensitization 24 hr after training, nor did it induce neuronal outgrowth. These results suggest that extensive sensitization training is required to induce neuronal outgrowth in tail sensory neurons, indicating that the memory of long-term sensitization induced by 1 d of training is mechanistically different from that induced by 4 d of training. Moreover, the induction of a form of long-term sensitization associated with neuronal outgrowth does not appear to be a function of the amount of stimulation but does appear to be dependent on the temporal spacing of the stimulation over multiple days.

Signal transduction mechanisms and behavioral sensitization to stimulant drugs: an overview of cAMP and PLA2

Behavioral sensitization refers to the progressive increase of behavioral responses to psychomotor stimulants, which provides a model for the intensification of drug craving and relapse alleged to underlie addiction in humans. Mechanisms related to sensitization may also contribute to schizophrenia and bipolar disorder. While the phenomenon has been observed for years, only recently have molecular or intracellular mechanisms associated with behavioral sensitization been studied. An overview of cAMP and PLA2 (intracellular, signal transduction mechanisms) relevant to behavioral sensitization will be presented.

Evidence for the involvement of cyclooxygenase activity in the development of cocaine sensitization

Phospholipase A2 (PLA(2)) activation generates the release of arachidonic acid (AA) and platelet-activating factor (PAF), two compounds which may be involved in neuroplasticity. In previous studies, we found that PLA(2) activation is involved in the development of stimulant sensitization. In the present study, we have examined the roles of AA and PAF in the development of stimulant sensitization using agonists and antagonists selective for PAF receptors or the induction of various AA cascade-mediated eicosanoids. Sprague-Dawley rats were treated for 5 days with cocaine (30 mg/kg) or D-amphetamine (1 mg/kg) preceded 15 min earlier by various antagonists, and then tested following a 10-day withdrawal period for cocaine (15 mg/kg) or D-amphetamine (0.5 mg/kg)-induced locomotion. Consistent with our earlier work, pretreatment with the PLA(2) inhibitor quinacrine (25 mg/kg) blocked the development of cocaine and amphetamine sensitization. The lipoxygenase (LOX) inhibitors nordihydroguaiaretic acid (NDGA) (5-10 mg/kg) and MK-886 (1 mg/kg) had no effect on cocaine sensitization. The PAF receptor antagonist WEB 2086 (5-10 mg/kg) reduced the development of cocaine sensitization. The cyclooxygenase (COX) inhibitors indomethacin (1-2 mg/kg), piroxicam (0.5-1 mg/kg), 6-methoxy-2-napthylacetic acid (6-MNA; 0.5-1 mg/kg), and NS-398 (0.5-1 mg/kg) blocked the development of cocaine sensitization. The COX inhibitors indomethacin (2 mg/kg) and 6-MNA (1 mg/kg) also reduced the development of amphetamine sensitization. Rats were administered bilateral intraventral tegmental area (VTA) injections of D-amphetamine (5 microg/side) or saline coadministered with indomethacin (0.5 microg/side) or vehicle three times over 5 days and were then tested after a 10-day withdrawal for D-amphetamine (0.5 mg/kg ip)-induced locomotion. Intra-VTA amphetamine induced a robust form of amphetamine sensitization, which was blocked by coadministration of indomethacin. Unilateral intra-VTA injections of PAF (1 microg) did not significantly alter cocaine (15 mg/kg ip)-induced locomotion when tested after a 3-day withdrawal. These findings suggest that COX, and possibly PAF, activity is involved in the development of stimulant sensitization. Neuroanatomical studies demonstrate that this may occur at the level of the VTA.

Arachidonic acid both inhibits and enhances whole cell calcium currents in rat sympathetic neurons

We recently reported that arachidonic acid (AA) inhibits L- and N-type Ca(2+) currents at positive test potentials in the presence of the dihydropyridine L-type Ca(2+) channel agonist (+)-202-791 in dissociated neonatal rat superior cervical ganglion neurons [Liu L and Rittenhouse AR. J Physiol (Lond) 525: 291-404, 2000]. In this first of two companion papers, we characterized the mechanism of inhibition by AA at the whole cell level. In the presence of either omega-conotoxin GVIA or nimodipine, AA decreased current amplitude, confirming that L- and N-type currents, respectively, were inhibited. AA-induced inhibition was concentration dependent and reversible with an albumin-containing wash solution, but appears independent of AA metabolism and G protein activity. In characterizing inhibition, an AA-induced enhancement of current amplitude was revealed that occurred primarily at negative test potentials. Cell dialysis with albumin minimized inhibition but had little effect on enhancement, suggesting that AA has distinct sites of action. We examined AA's actions on current kinetics and found that AA increased holding potential-dependent inactivation. AA also enhanced the rate of N-type current activation. These findings indicate that AA causes multiple changes in sympathetic Ca(2+) currents.

Rapid bidirectional modulation of mRNA expression and export accompany long-term facilitation and depression of Aplysia synapses

Serotonin (5-HT) and the neuropeptide Phe-Met-Arg-Phe-amide (FMRFa) modulate synaptic efficacy of sensory neurons (SNs) of Aplysia in opposite directions and for long duration. Both long-term responses require changes in mRNA and protein synthesis. The SN-specific neuropeptide, sensorin A, is a gene product that appears to be increased by 5-HT and decreased by FMRFa. We examined whether changes in sensorin A mRNA levels in the cell body and neurites of SNs accompany long-term facilitation and depression. Both 5-HT and FMRFa evoked rapid changes in sensorin A mRNA levels in the SN cell bodies: an increase with 5-HT and a decrease with FMRFa. Parallel changes in sensorin A mRNA levels in SN neurites were detected 2 h and 4 h later. These rapid changes in mRNA expression and net export required the presence of the appropriate target motor cell L7. The neuromodulators failed to produce changes in mRNA expression or export when SNs were cultured alone or with the inappropriate target cell L11. The changes in mRNA expression were transient because mRNA levels returned to control values 24 h after treatment, while synaptic efficacy remained altered by the respective treatments. These results indicate that two neuromodulators produce distinct, but transient, target-dependent effects on expression and export of a cell-specific mRNA that correlate with changes in synaptic plasticity.

Cell-specific changes in expression of mRNAs encoding splice variants of aplysia cell adhesion molecule accompany long-term synaptic plasticity

Aplysia neurons express several splice variants of apCAM, a member of the Ig superfamily of cell adhesion molecules. The major transmembrane isoform is endocytosed in sensory neurons (SNs) during the early phases of long-term facilitation (LTF) of SN synapses evoked by serotonin (5-HT) or in the motor neuron L7 during the early phases of long-term depression (LTD) of SN synapses evoked by Phe-Met-Arg-Phe-amide (FMRFa). We used single cell RT-PCR to evaluate whether expression of mRNAs encoding for different apCAM isoforms in SNs and L7 is regulated during LTF produced by 5-HT, and LTD produced by FMRFa. Single SNs and L7s express mRNAs encoding for all major isoforms, but the proportion of each isoform expressed differs for the two cells. SN expresses more mRNA encoding for GPI-linked isoforms, while L7 expresses more mRNA encoding for the major transmembrane isoform. The neuromodulators produced significant changes in the proportional levels of mRNAs encoding for specific apCAM isoforms during the first 4 h after treatments without affecting overall levels of apCAM mRNA. 5-HT evoked changes that exaggerated cell-specific differences in isoform expression. FMRFa evoked changes that reduced cell-specific differences in isoform expression. The effects of the neuromodulators on apCAM mRNA expression were not detected when cells were cultured alone or when SNs were cocultured with another motor cell that failed to induce synapse formation (L11). The results suggest that rapid cell-specific regulation of splice variant expression may contribute to different forms of long-term synaptic plasticity.

Localization of a short-term memory in Drosophila

Memories are thought to be due to lasting synaptic modifications in the brain. The search for memory traces has relied predominantly on determining regions that are necessary for the process. However, a more informative approach is to define the smallest sufficient set of brain structures. The rutabaga adenylyl cyclase, an enzyme that is ubiquitously expressed in the Drosophila brain and that mediates synaptic plasticity, is needed exclusively in the Kenyon cells of the mushroom bodies for a component of olfactory short-term memory. This demonstrates that synaptic plasticity in a small brain region can be sufficient for memory formation.

Tissue-specific expression of a type I adenylyl cyclase rescues the rutabaga mutant memory defect: in search of the engram

Most attempts to localize physical correlates of memory in the central nervous system (CNS) rely on ablation techniques. This approach has the limitation of defining just one of an unknown number of structures necessary for memory formation. We have used the Drosophila rutabaga type I Ca(2+)/CaM-dependent adenylyl cyclase (AC) gene to determine in which CNS region AC expression is sufficient for memory formation. Using pan-neural and restricted CNS expression with the GAL4 binary transcription activation system, we have rescued the memory defect of the rutabaga mutant in a fast robust spatial learning paradigm. The ventral ganglion, antennal lobes, and median bundle are likely the CNS structures sufficient for rutabaga AC- dependent spatial learning.

Arcadlin is a neural activity-regulated cadherin involved in long term potentiation

Neural activity results in long term changes that underlie synaptic plasticity. To examine the molecular basis of activity-dependent plasticity, we have used differential cloning techniques to identify genes that are rapidly induced in brain neurons by synaptic activity. Here, we identify a novel cadherin molecule Arcadlin (activity-regulated cadherin-like protein). arcadlin mRNA is rapidly and transiently induced in hippocampal granule cells by seizures and by N-methyl-D-aspartate-dependent synaptic activity in long term potentiation. The extracellular domain of Arcadlin is most homologous to protocadherin-8; however, the cytoplasmic region is distinct from that of any cadherin family member. Arcadlin protein is expressed at the synapses and shows a homophilic binding activity in a Ca2+-dependent manner. Furthermore, application of Arcadlin antibody reduces excitatory postsynaptic potential amplitude and blocks long term potentiation in hippocampal slices. Its close homology with cadherins, its rapid inducibility by neural activity, and its involvement in synaptic transmission suggest that Arcadlin may play an important role in activity-induced synaptic reorganization underlying long term memory.

Drosophila larval neuromuscular junction's responses to reduction of cAMP in the nervous system

We investigated the effects of chronically lowered cyclic adenosine monophosphate (cAMP) on the morphology and physiology of the Drosophila larval neuromuscular junction, using two fly lines in which cAMP was significantly lower than normal in the nervous system: (a) transgenic flies in which the dunce (dnc) gene product was overexpressed in the nervous system, and (b) flies mutant for the rutabaga gene (rut1) which have reduced adenylyl cyclase activity. In comparison with controls, larvae with reduced cAMP exhibited a smaller number of synaptic varicosities. This effect was more pronounced in transgenic larvae, in which the reduction of neural cAMP was more pronounced. Synaptic transmission was also reduced in both cases, as evidenced by smaller excitatory junctional potentials (EJPs). Synaptic currents recorded from individual synaptic varicosities of the neuromuscular junction indicated almost normal transmitter release properties in transgenic larvae and a modest impairment in rut1 larvae. Thus, reduction in EJP amplitude in transgenic larvae is primarily due to reduced innervation, while in rut1 larvae it is attributable to the combined effects of reduced innervation and a mild impairment of transmitter release. We conclude that the major effect of chronically lowered cAMP is reduction of innervation rather than impairment of transmitter release properties.

Activation of protein kinase A contributes to the expression but not the induction of long-term hyperexcitability caused by axotomy of Aplysia sensory neurons

Nociceptive sensory neurons (SNs) in Aplysia provide useful models to study both memory and adaptive responses to nerve injury. Induction of long-term memory in many species, including Aplysia, is thought to depend on activation of cAMP-dependent protein kinase (PKA). Because Aplysia SNs display similar alterations in models of memory and after nerve injury, a plausible hypothesis is that axotomy triggers memory-like modifications by activating PKA in damaged axons. The present study disproves this hypothesis. SN axotomy was produced by (1) dissociation of somata from the ganglion [which is shown to induce long-term hyperexcitability (LTH)], (2) transection of neurites of dissociated SNs growing in vitro, or (3) peripheral nerve crush. Application of the competitive PKA inhibitor Rp-8-CPT-cAMPS at the time of axotomy failed to alter the induction of LTH by each form of axotomy, although the inhibitor antagonized hyperexcitability produced by 5-HT application. Strong activation of PKA in the nerve by coapplication of a membrane-permeant analog of cAMP and a phosphodiesterase inhibitor was not sufficient to induce LTH of either the SN somata or axons. Furthermore, nerve crush failed to activate axonal PKA or stimulate its retrograde transport. Therefore, PKA activation plays little if any role in the induction of LTH by axotomy. However, the expression of LTH was reduced by intracellular injection of the highly specific PKA inhibitor PKI several days after nerve crush. This suggests that long-lasting activation of PKA in or near the soma contributes to the maintenance of long-term modifications produced by nerve injury.

Sequestration of cAMP response element-binding proteins by transcription factor decoys causes collateral elaboration of regenerating Aplysia motor neuron axons

Axonal injury increases intracellular Ca2+ and cAMP and has been shown to induce gene expression, which is thought to be a key event for regeneration. Increases in intracellular Ca2+ and/or cAMP can alter gene expression via activation of a family of transcription factors that bind to and modulate the expression of CRE (Ca2+/cAMP response element) sequence-containing genes. We have used Aplysia motor neurons to examine the role of CRE-binding proteins in axonal regeneration after injury. We report that axonal injury increases the binding of proteins to a CRE sequence-containing probe. In addition, Western blot analysis revealed that the level of ApCREB2, a CRE sequence-binding repressor, was enhanced as a result of axonal injury. The sequestration of CRE-binding proteins by microinjection of CRE sequence-containing plasmids enhanced axon collateral formation (both number and length) as compared with control plasmid injections. These findings show that Ca2+/cAMP-mediated gene expression via CRE-binding transcription factors participates in the regeneration of motor neuron axons.

Binding of serotonin to receptors at multiple sites is required for structural plasticity accompanying long-term facilitation of Aplysia sensorimotor synapses

Long-term changes in the efficacy of Aplysia sensorimotor synapses accompany nonassociative and associative forms of behavioral plasticity. This synapse expresses long-term facilitation either with repeated applications of 5-hydroxytryptamine (5-HT) or with a single pairing of tetanus in the sensory neuron (SN) and bath application of 5-HT. We examined whether structural changes in the SN accompany all forms of long-term synaptic enhancement and the locations at which 5-HT must bind receptors to evoke long-term functional and/or structural changes. Pairing tetanus with one application of 5-HT evoked both functional and structural changes after 24 hr only when 5-HT application was temporally paired with the tetanus and activated receptors on both the SN cell body and terminal region. Repeated application of 5-HT to the terminal region alone failed to evoke any long-term change. Repeated applications of 5-HT to the SN cell body alone evoked a change in synaptic efficacy at 24 hr but failed to increase SN varicosities. Repeated applications of 5-HT to both the SN cell body and the terminal region evoked increases in both synaptic efficacy and the number of SN varicosities at 24 hr. The results indicate that different external stimuli can evoke equivalent forms of long-term synaptic facilitation with or without structural changes in the SNs. Changes in the number of SN varicosities can accompany different forms of long-term facilitation and require the activation of 5-HT receptors at multiple sites.

Comparison of plasticity in vivo and in vitro in the developing visual cortex of normal and protein kinase A RIbeta-deficient mice

Developing sensory systems are sculpted by an activity-dependent strengthening and weakening of connections. Long-term potentiation (LTP) and depression (LTD) in vitro have been proposed to model this experience-dependent circuit refinement. We directly compared LTP and LTD induction in vitro with plasticity in vivo in the developing visual cortex of a mouse mutant of protein kinase A (PKA), a key enzyme implicated in the plasticity of a diverse array of systems. In mice lacking the RIbeta regulatory subunit of PKA, we observed three abnormalities of synaptic plasticity in layer II/III of visual cortex in vitro. These included an absence of (1) extracellularly recorded LTP, (2) depotentiation or LTD, and (3) paired-pulse facilitation. Potentiation was induced, however, by pairing low-frequency stimulation with direct depolarization of individual mutant pyramidal cells. Together these findings suggest that the LTP defect in slices lacking PKA RIbeta lies in the transmission of sufficient net excitation through the cortical circuit. Nonetheless, functional development and plasticity of visual cortical responses in vivo after monocular deprivation did not differ from normal. Moreover, the loss of all responsiveness to stimulation of the originally deprived eye in most cortical cells could be restored by reverse suture of eyelids during the critical period in both wild-type and mutant mice. Such an activity-dependent increase in response would seem to require a mechanism like potentiation in vivo. Thus, the RIbeta isoform of PKA is not essential for ocular dominance plasticity, which can proceed despite defects in several common in vitro models of neural plasticity.

Long-term effects of axotomy on excitability and growth of isolated Aplysia sensory neurons in cell culture: potential role of cAMP

Crushing nerves, which contain the axons of central sensory neurons, in Aplysia causes the neurons to become hyperexcitable and to sprout new processes. Previous experiments that examined the effects of axonal injury on Aplysia sensory neurons have been performed in the intact animal or in the semi-intact CNS of Aplysia. It therefore has been unclear to what extent the long-term neuronal consequences of injury are due to intrinsic or extrinsic cellular signals. To determine whether injury-induced changes in Aplysia sensory neurons are due to intrinsic or extrinsic signals, we have developed an in vitro model of axonal injury. Isolated central sensory neurons grown for 2 days in cell culture were axotomized. Approximately 24 h after axotomy, sensory neurons exhibited a greater excitability-reflected, in part, as a significant reduction in spike accommodation-and greater neuritic outgrowth than did control (unaxotomized) neurons. Rp diastereoisomer of the cyclic adenosine 3',5'-monophosphorothiate (Rp-cAMPS), an inhibitor of protein kinase A, blocked both the reduction in accommodation and increased neuritic outgrowth induced by axotomy. Rp-cAMPS also blocked similar, albeit smaller, alterations observed in control sensory neurons during the 24-h period of our experiments. These results indicate that axonal injury elevates cAMP levels within Aplysia sensory neurons, and that this elevation is directly responsible, in part, for the previously described long-term electrophysiological and morphological changes induced in Aplysia sensory neurons by nerve crush. In addition, the results indicate that control sensory neurons in culture are also undergoing injury-related electrophysiological and structural changes, probably due to cellular processes triggered when the neurons are axotomized during cell culturing. Finally, the results provide support for the idea that the cellular processes activated within Aplysia sensory neurons by injury, and those activated during long-term behavioral sensitization, overlap significantly.

A cyclic nucleotide-gated channel inhibits sensory axon outgrowth in larval and adult Caenorhabditis elegans: a distinct pathway for maintenance of sensory axon structure

The tax-2 and tax-4 genes of C. elegans encode two subunits of a cyclic nucleotide-gated channel that is required for chemosensation, thermosensation and normal axon outgrowth of some sensory neurons. Here we show that, in tax-2 and tax-4 mutants, young larvae have superficially normal axons, but axon outgrowth resumes in inappropriate regions in late larval stages. Using a temperature-sensitive mutation in tax-2, we find that tax-2 activity is required during the adult stage to preserve normal axon morphology. These results indicate that tax-2 and tax-4 are required for the maintenance of correct axon structure, and reveal an unexpected plasticity that allows C. elegans axons to be remodeled long after their initial connections have been established. TAX-2 and TAX-4 have been proposed to form a transduction channel for chemosensation and thermosensation, and tax-2 activity is required in the adult stage for normal chemotaxis to NaCl and odorants. Animals mutant for the daf-11 gene have axon phenotypes that are similar to those of tax-2 and tax 4 mutants; this axon phenotype also has a late time of action. daf-11 regulates a developmental process called dauer larva formation that is controlled by sensory stimuli, and tax-2 and tax-4 can either stimulate or inhibit dauer larva formation in different contexts.

Pathway-specific synaptic plasticity: activity-dependent enhancement and suppression of long-term heterosynaptic facilitation at converging inputs on a single target

To explore mechanisms of long-term, pathway-specific synaptic plasticity, we examined consequences of differential stimulation of Aplysia sensorimotor connections in culture where two sensory neuron (SN) inputs converge on a single target motor cell L7. A single pairing of tetanus in one SN with bath application of 5-HT evoked long-term (24 hr) increase in efficacy of the SN connection given paired stimulation that was comparable in magnitude to the increase in synaptic efficacy evoked with repeated applications of 5-HT. Repeated pairing of tetanus in one SN with applications of 5-HT evoked a significant increase in efficacy of the SN connection given paired stimuli, and significant reduction in facilitation that is normally evoked by repeated applications of 5-HT in the unpaired SN connection. Hyperpolarization of L7 or incubation with APV interfered with both enhancement of facilitation with paired stimulation and suppression of facilitation with unpaired stimulation, but without interfering with long-term facilitation evoked either by repeated applications of 5-HT or by a single pairing. The results suggest that a single connection can undergo at least two forms of activity-dependent, pathway-specific facilitation lasting more than 24 hr. One form, evoked with a single pairing, is initiated and maintained primarily by activity in the presynaptic neuron. The other form, evoked with repeated paired stimuli, requires target-dependent activity that differentially modulates long-term heterosynaptic facilitation at the converging inputs.

Modulation of a cAMP/protein kinase A cascade by protein kinase C in sensory neurons of Aplysia

The synaptic connections between the sensory neurons of Aplysia and their follower neurons have been used as a model system for examining the cellular mechanisms contributing to neuronal and synaptic plasticity. Recent studies suggest that at least two protein kinases, protein kinase A (PKA) and protein kinase C (PKC), contribute to serotonin (5-HT)-induced short-term facilitation. The interaction between these two kinase cascades has not been examined, however. Using electrophysiological and biochemical approaches, we examined possible interactions between PKA and PKC cascades. The results indicated that prolonged activation of PKC by preincubation with phorbol esters attenuated PKA-mediated actions of 5-HT, including increases in sensory neuron excitability and spike broadening in the presence of tetraethylammonium (TEA) and nifedipine. Although phorbol esters also attenuated increases in excitability by an analog of cAMP and small cardioactive peptide B (SCPB), the degree of attenuation was smaller. In addition, phorbol esters did not attenuate broadening of TEA spikes by the cAMP analog and SCPB. Thus, phorbol esters appeared specifically to attenuate aspects of the 5-HT activation of the cAMP/PKA cascade. Measurements of cAMP levels with radioimmunoassays revealed that phorbol esters did not attenuate 5-HT-induced cAMP synthesis, however. Finally, the results indicated that phorbol esters themselves induced a small but significant increase in excitability as well as an increase in the level of cAMP. Our results suggest that there is crosstalk between the PKC and PKA cascades. The mechanisms by which phorbol esters specifically attenuate 5-HT-induced activation of the cAMP/PKA cascade are not known, however.

Cyclooxygenases and the central nervous system

Prostaglandins (PGs) were first described in the brain by Samuelsson over 30 years ago (Samuelsson, 1964). Since then a large number of studies have shown that PGs are formed in regions of the brain and spinal cord in response to a variety of stimuli. The recent identification of two forms of cyclooxygenase (COX; Kujubu et al., 1991; Xie et al., 1991; Smith and DeWitt, 1996), both of which are expressed in the brain, along with superior tools for mapping COX distribution, has spurred a resurgence of interest in the role of PGs in the central nervous system (CNS). In this review we will describe new data in this area, focusing on the distribution and potential role of the COX isoforms in brain function and disease.

Site-specific and sensory neuron-dependent increases in postsynaptic glutamate sensitivity accompany serotonin-induced long-term facilitation at Aplysia sensorimotor synapses

Long-term changes in the efficacy of Aplysia sensory neuron (SN) connections accompany behavioral training or applications with 5-HT. The changes evoked by training or 5-HT include formation of new SN varicosities and transmitter release sites. Because new synapse formation requires proper alignment of presynaptic structures with postsynaptic zones containing a high density of transmitter receptors, we examined whether changes in postsynaptic sensitivity to the presumed SN transmitter (glutamate) were correlated with formation and distribution of new SN varicosities in contact with motor cell L7 in cell culture. The formation of stable SN connections after 4 d in culture did not significantly change overall responses to focal applications of glutamate. However, specific sites along L7's axon apposed to SN varicosities expressed larger responses to glutamate compared with adjacent sites with few SN varicosities. After treatments with 5-HT that evoked long-term changes in both the structure and the function of SN-L7 synaptic interaction, glutamate responses increased selectively at sites along the surface of L7's axon with preexisting or new SN varicosities. Increases in postsynaptic response to glutamate 24 hr after 5-HT treatment required interaction with an SN. These results suggest that new synapse formation between neurons, either with regeneration or after external stimuli that evoke increases in synaptic efficacy, involves site-specific changes in expression of functional neurotransmitter receptors on the postsynaptic cell that is regulated by interaction with the presynaptic neuron.

Prostaglandin F2alpha is required for NMDA receptor-mediated induction of c-fos mRNA in dentate gyrus neurons

Activation of NMDA receptors has been linked to a diversity of lasting physiological and pathological changes in the mammalian nervous system. The cellular and molecular mechanisms underlying permanent modifications of nervous system structure and function after brief episodes of neuronal activity are unknown. Immediate-early genes (IEGs) have been implicated in the conversion of short-term stimuli to long-term changes in cellular phenotype by regulation of gene expression. The intracellular signaling pathways coupling activation of receptors at the cell surface with induction of IEGs in the nucleus are incompletely understood. NMDA produces a striking increase in the IEG c-fos in dentate gyrus (DG) neurons in vitro; this induction is dependent, in part, on the arachidonic acid cascade. Here we show that NMDA receptor activation triggers the synthesis of the prostaglandins PGF2alpha and PGE2, but not PGD2, in rat cerebral cortical neurons in vitro. We further demonstrate that PGF2alpha, but not PGE2 or PGD2, is necessary but not sufficient for NMDA induction of c-fos mRNA in DG neurons. These findings provide insight into the molecular events coupling activation of the NMDA receptor with regulation of the IEG c-fos and identify the diffusable messenger PGF2alpha as obligatory for NMDA receptor-mediated transcription of a nuclear IEG.

Rheb interacts with Raf-1 kinase and may function to integrate growth factor- and protein kinase A-dependent signals

Rheb is a recently described member of the Ras family that was originally identified as an immediate-early gene in brain but is also widely expressed in other tissues. Here we demonstrate that Rheb interacts with and appears to regulate Raf-1 kinase, an essential component of the H-Ras signaling pathway. In direct contrast to H-Ras, however, the interaction of Rheb with Raf-1 is potentiated by growth factors in combination with agents that increase cyclic AMP (cAMP) levels. Protein kinase A-dependent phosphorylation of serine 43 within the regulatory domain of Raf-1 reciprocally potentiates its interaction with Rheb and decreases its interaction with H-Ras. A single amino acid in the G2 effector domain is critical for the differential properties of Rheb. Since Rheb is an immediate-early gene, our studies suggest that Rheb functions in concert with H-Ras to dynamically integrate cAMP and growth factor signaling.

Differential distribution of functional receptors for neuromodulators evoking short-term heterosynaptic plasticity in Aplysia sensory neurons

Synaptic transmission and excitability in Aplysia sensory neurons (SNs) are bidirectionally modulated by 5-HT and FMRFamide. To explore the regional distribution of different functional receptors that modulate SN properties, we examined changes in synaptic efficacy and excitability with brief focal applications of the neuromodulators to different regions of SNs that have established connections with motor cell L7 in culture. Short-term changes in synaptic efficacy were evoked only when 5-HT or FMRFamide was applied to regions with SN varicosities along the surface of L7 axons. Applications to adjacent SN neurites with few varicosities in contact with L7 axons failed to evoke a significant change in synaptic efficacy. The distribution of functional receptors mediating changes in excitability differed for 5-HT and FMRFamide. Whereas excitability increases were evoked only when 5-HT was applied to SN cell bodies, excitability decreases in SNs were evoked only when FMRFamide was applied to regions along the L7 axon with SN varicosities. Without the target cell, cell bodies of SNs expressed both 5-HT and FMRFamide receptors that modulate excitability. These results indicate that functional G-protein-coupled receptors for two neuromodulators are distributed differentially along the surface of a presynaptic neuron that forms chemical connections in vitro. This differential distribution of receptors on the presynaptic neuron is regulated by a target and does not require the physical presence of neurons that release the neuromodulators.

A putative cyclic nucleotide-gated channel is required for sensory development and function in C. elegans

In vertebrate visual and olfactory systems, a cyclic nucleotide-gated channel couples receptor activation to electrical activity of the sensory neurons. The Caenorhabditis elegans tax-2 gene is required for some forms of olfaction, for chemosensation of salts, and for thermosensation. We show here that tax-2 encodes a predicted subunit of a cyclic nucleotide-gated channel that is expressed in olfactory, gustatory, and thermosensory neurons, implicating this channel in multiple sensory modalities. Some sensory neurons display axon outgrowth defects in tax-2 mutants. Thus, the channel has an unexpected role in sensory neuron development in addition to its role in sensation. Consistent with this proposed dual function, a Tax-2::GFP fusion protein is present both in sensory cilia and in sensory axons.

A protein synthesis-dependent late phase of cerebellar long-term depression

Synthesis of new protein has been shown to be required for establishment of a late phase in hippocampal long-term potentiation. Whether a similar requirement is needed to produce the late phase of long-term depression (LTD) remains to be determined. Application of transcription inhibitors or of the translation inhibitor anisomycin, immediately, but not 30 min after glutamate/depolarization conjunction, attenuated a late phase of cerebellar LTD in culture. LTD, produced in a perforated outside-out macropatch of Purkinje neuron dendrite, which lacks nuclear material, returned to baseline values with a time course paralleling that observed with protein synthesis inhibitors in intact cultured Purkinje neurons. These findings suggest that there is a distinct late phase of cerebellar LTD that is dependent upon postsynaptic protein synthesis.

Altered habituation of an identified escape circuit in Drosophila memory mutants

Genetic approaches in Drosophila have advanced our understanding of the molecular mechanisms of different forms of learning, including habituation, but relevant neural components have not been explored. We show that a well defined neural circuit that underlies an escape response can be habituated, providing for the first time excellent opportunities for studying physiological parameters of learning in a functional circuit in the fly. Compared with other forms of conditioning, relatively little is known of the physiological mechanisms of habituation. The giant fiber pathway mediates a jump-and-flight escape response to visual stimuli. The jump may also be triggered electrically at multiple sites in the tethered fly. This response shows parameters of habituation, including frequency-dependent decline in responsiveness, spontaneous recovery, and dishabituation by a novel stimulus, attributable to plasticity in the brain. Mutations of rutabaga that diminish cAMP synthesis reduced the rate of habituation, whereas dunce mutations that increase cAMP levels led to a detectable but moderate increase in habituation rates. Surprisingly, habituation was extremely rapid in dunce rutabaga double mutants. This corresponds to the extreme defects seen in double mutants in other learning tasks, and demonstrates that defects of the rutabaga and dunce products interact synergistically in ways that could not have been predicted on the basis of simple counterbalancing biochemical effects. Although habituation is localized to afferents to the giant fiber, cAMP mutations also affected performance of thoracic portions of the pathway on a millisecond time scale that did not account for behavioral plasticity. More significantly, spontaneous recovery and dishabituation were not as clearly affected as habituation in mutants, indicating that these processes may not overlap entirely in terms of cAMP-regulating mechanisms. The analysis of habituation of the giant fiber response in available learning and memory mutants could be a crucial step toward realizing the promise of memory mutations to elucidate mechanisms in neural circuits that underlie behavioral plasticity.

Tetanic stimulation and cyclic adenosine monophosphate regulate segregation of presynaptic inputs on a common postsynaptic target neuron in vitro

Previous studies indicated that Aplysia sensory neurons (SNs) compete when reestablishing synapses with a motor cell target (L7) in vitro. The competition is characterized by a cell number-dependent decrease in the efficacy of each connection, an increase in the elimination of SN varicosities, a reduction in the formation of new SN varicosities, and the segregation of varicosities of each SN to restricted portions of the target axons. The changes do not require spike activity, since both the SNs and L7 do not fire spontaneously. Here, we examined whether adding activity to SNs during the early stages of synapse formation with stimuli known to evoke facilitatory responses in stable SN-L7 connections--tetanic stimulation or increase in intracellular cyclic adenosine monophosphate (cAMP)--would modulate the intrinsic segregatory process. Tetanic stimulation to one SN increased synapse efficacy and the number of varicosities of the stimulated SNs while reducing the functional changes by the nonstimulated SNs in the same cultures. An increase in the stability of preexisting varicosities contributed to the overall increase in varicosities evoked by tetanus. The functional changes evoked by tetanus were not expressed when the same tetanic stimulation was also given to the other SN, or when L7 was hyperpolarized during the tetanus to the SN. Raising cAMP levels in one SN increased synapse efficacy and the rate of new varicosity formation by the injected SNs without affecting the development of the connections formed by the noninjected SNs. These results suggest that different forms of presynaptic and postsynaptic activities in neurons can regulate specific aspects of the competitive process associated with the fine-tuning of connections formed by converging presynaptic inputs.

Arc, a growth factor and activity-regulated gene, encodes a novel cytoskeleton-associated protein that is enriched in neuronal dendrites

Neuronal activity is an essential stimulus for induction of plasticity and normal development of the CNS. We have used differential cloning techniques to identify a novel immediate-early gene (IEG) cDNA that is rapidly induced in neurons by activity in models of adult and developmental plasticity. Both the mRNA and the encoded protein are enriched in neuronal dendrites. Analysis of the deduced amino acid sequence indicates a region of homology with alpha-spectrin, and the full-length protein, prepared by in vitro transcription/translation, coprecipitates with F-actin. Confocal microscopy of the native protein in hippocampal neurons demonstrates that the IEG-encoded protein is enriched in the subplasmalemmal cortex of the cell body and dendrites and thus colocalizes with the actin cytoskeletal matrix. Accordingly, we have termed the gene and encoded protein Arc (activity-regulated cytoskeleton-associated protein). Our observations suggest that Arc may play a role in activity-dependent plasticity of dendrites.

Arachidonic acid: toxic and trophic effects on cultured hippocampal neurons

Arachidonic acid (20:4) is a component of membrane lipids that has been implicated as a messenger both in physiological and pathophysiological processes, including ischemic injury and synaptic plasticity. In order to clarify direct trophic or toxic effects of arachidonic acid on central neurons, primary cultures of rat hippocampal neurons were exposed to arachidonic acid under chemically-defined conditions. Arachidonic acid present in the culture medium at concentrations over 5 x 10(-6) M showed profound toxicity, whereas at lower concentrations (10(-6) M) it significantly supported the survival of hippocampal neurons. These effects were not mimicked by oleic acid (18:1) or palmitic acid (16:0). The toxic action of 10(-5) M arachidonic acid was markedly and significantly prevented by a lipoxygenase inhibitor nordihydroguaiaretic acid (10(-6) M). AA861 and baicalein (each at 10(-6) M), a selective inhibitor for 5- and 12-lipoxygenase, respectively, also showed a significant protective effect, whereas cyclooxygenase inhibitor indomethacin (10(-5) M) had no effect. The toxic action was also prevented by an antioxidant alpha-tocopherol (10(-6) M), but not by superoxide dismutase (100 U/ml) or catalase (200 U/ml). The trophic effect of 10(-6) M arachidonic acid was not suppressed by the treatments listed above. At lower concentrations (10(-7)-10(-6) M), arachidonic acid promoted neurite elongation, which was not inhibited by nordihydroguaiaretic acid or indomethacin. Overall, arachidonic acid has both trophic and toxic actions on cultured hippocampal neurons, part of which involves its metabolism by lipoxygenases. The mechanisms and the physiological significance of these effects are discussed.

Postsynaptic regulation of the development and long-term plasticity of Aplysia sensorimotor synapses in cell culture

The monosynaptic component of the neuronal circuit that mediates the withdrawal reflex of Aplysia californica can be reconstituted in dissociated cell culture. Study of these in vitro monosynaptic connections has yielded insights into the basic cellular mechanisms of synaptogenesis and long-term synaptic plasticity. One such insight has been that the development of the presynaptic sensory neurons is strongly regulated by the postsynaptic motor neuron. Sensory neurons which have been cocultured with a target motor neuron have more elaborate structures--characterized by neurites with more branches and varicosities--than do sensory neurons grown alone in culture or sensory neurons that have been cocultured with an inappropriate target cell. Another way in which the motor neuron regulates the development of sensory neurons is apparent when sensorimotor cocultures with two presynaptic cells are examined. In such cocultures the outgrowth from the different presynaptic cells is obviously segregated on the processes of the postsynaptic cell. By contrast, when two sensory neurons are placed into cell culture without a motor neuron, their processes readily grow together. In addition to regulating the in vitro development of sensory neurons, the motor neuron also regulates learning-related changes in the structure of sensory neurons. Application of the endogenous facilitatory transmitter serotonin (5-HT) causes long-term facilitation of in vitro sensorimotor synapses due in part to growth of new presynaptic varicosities. But 5-HT applied to sensory neurons alone in culture does not produce structural changes in these cells. More recently it has been found that sensorimotor synapses in cell culture can exhibit long-term potentiation (LTP). Like LTP of some hippocampal synapses, LTP of in vitro Aplysia synapses is regulated by the voltage of the postsynaptic cell. Pairing high-frequency stimulation of sensory neurons with strong hyperpolarization of the motor neuron blocks the induction of LTP. Moreover, LTP of sensorimotor synapses can be induced in Hebbian fashion by pairing weak presynaptic stimulation with strong postsynaptic depolarization. These findings implicate a Hebbian mechanism in classical conditioning in Aplysia. They also indicate that Hebbian LTP is a phylogenetically ancient form of synaptic plasticity.