In an attempt to improve behavioral memory, we devised a strategy to amplify the signal-to-noise ratio of the cAMP pathway, which plays a central role in hippocampal synaptic plasticity and behavioral memory. Multiple highfrequency trains of electrical stimulation induce long-lasting long-term potentiation, a form of synaptic strengthening in hippocampus that is greater in both magnitude and persistence than the short-lasting long-term potentiation generated by a single tetanic train. Studies using pharmacological inhibitors and genetic manipulations have shown that this difference in response depends on the activity of cAMPdependent protein kinase A. Genetic studies have also indicated that protein kinase A and one of its target transcription factors, cAMP response element binding protein, are important in memory in vivo. These findings suggested that amplification of signals through the cAMP pathway might lower the threshold for generating long-lasting long-term potentiation and increase behavioral memory. We therefore examined the biochemical, physiological, and behavioral effects in mice of partial inhibition of a hippocampal cAMP phosphodiesterase. Concentrations of a type IV-specific phosphodiesterase inhibitor, rolipram, which had no significant effect on basal cAMP concentration, increased the cAMP response of hippocampal slices to stimulation with forskolin and induced persistent long-term potentiation in CA1 after a single tetanic train. In both young and aged mice, rolipram treatment before training increased long-but not short-term retention in freezing to context, a hippocampus-dependent memory task.The second messenger, cAMP, and cAMP-dependent protein kinase A (PKA) have been implicated in short-and longlasting synaptic plasticity in Aplysia and in short-and longlasting behavioral learning in Aplysia and Drosophila (1, 2). Recently, convergent pharmacological and genetic evidence has also implicated the cAMP system in short-lasting longterm potentiation (LTP) at the mossy fiber-CA3 synapse of rodent hippocampus (3-6), and, strikingly, in the stronger longer-lasting intermediate and late phases of long-lasting LTP (L-LTP) that follow three to four trains of tetanic stimulation in all three hippocampal pathways: the perforant, the mossy fiber, and the Schaeffer collateral (CA3-CA1) (3,(7)(8)(9)(10)(11)(12)(13)(14). LTP is a well studied example of synaptic plasticity in mammals, thought to be a candidate cellular mechanism for mediating some forms of explicit hippocampus-dependent memory (15, 16). L-LTP has been of particular interest in regard to this behavioral correlation, because it is much more persistent than the short-lasting long-term potentiation that follows a single tetanic train (7,8,9). L-LTP persists as long as it has been observed, up to 29 hr in vitro, and depends at later time points not only on PKA activity but also on transcription and translation (3,6,8,9), much like behavioral long-term memory.The dependence of L-LTP, in hippocampal slices and behavioral memory, on PKA a...

MAP kinase (ERK) translates cell surface signals into alterations in transcription. We have found that ERK also regulates hippocampal neuronal excitability during 5 Hz stimulation and thereby regulates forms of long-term potentiation (LTP) that do not require macromolecular synthesis. Moreover, ERK-mediated changes in excitability are selectively required for some forms of LTP but not others. ERK is required for the early phase of LTP elicited by brief 5 Hz stimulation, as well as for LTP elicited by more prolonged 5 Hz stimulation when paired with beta1-adrenergic receptor activation. By contrast, ERK plays no role in LTP elicited by a single 1 s 100 Hz train. Consistent with these results, we find that ERK is activated by beta-adrenergic receptors in CA1 pyramidal cell somas and dendrites.

REVIEWSConsider a busy Manhattan street. Cars and people moving from store to store, apartment to place of business, leaving behind their tyre tracks, coffee cups, oil spills and so on. This rubbish is then steadily cleaned away by street sweepers and janitors. A common school of thought in neuroscience has been much in the same vein -kinases moving from protein to protein within the dendritic spine or presynaptic terminal, phosphorylating these proteins and changing their function, while phosphatases work behind the scenes in a mundane, constitutive custodial role. Increasingly, however, this bias is being challenged, as data now show that phosphatase activity in neurons is dynamically regulated, and that specific phosphatases have important roles in neuronal function. Perhaps in no field is this more apparent than that of hippocampal synaptic plasticity. In the hippocampus, NMDA (N-methyl-D-aspartate)-receptor-dependent longterm potentiation (LTP) and long-term depression (LTD) are robust forms of persistent modifications of synaptic transmission in response to transient stimuli, and are leading cellular candidates for the mediation of aspects of learning and memory. Although it has been clear for some time that protein phosphorylation has a key role in hippocampus-based synaptic plasticity and learning, it has only more recently been appreciated that phosphatases might have an important, active role in governing these processes.In this review, we discuss the roles of phosphatases in synaptic plasticity, focusing on the CA1 region of the hippocampus. First we discuss the enzymes and their regulation, emphasizing their propensity to be regulated by neuronal activity. Then we examine the evidence for a role of phosphatases in synaptic plasticity in area CA1 of the hippocampus, focusing on two broad areas: the effects of manipulating phosphatase activity on synaptic plasticity, and the measurement of phosphatase activation or inactivation in association with synaptic plasticity. We present a discussion of the phosphoproteins and processes on which phosphatases might act in achieving their physiological effects, as well as recent data indicating the behavioural relevance of the phosphatase regulation of plasticity. Finally, we provide a summary model of the roles of phosphatases in the hippocampus, and highlight what we feel to be some important future directions in this rapidly developing area.The regulation of glutamate-mediated excitatory neurotransmission has a critical role in many aspects of behaviour. Great effort has gone into understanding the signal transduction cascades and effectors recruited in these processes, and protein phosphorylation has been identified as an important element. Although initial research in the field focused on the activity-dependent activation of kinases and the kinase dependence of various forms of synaptic plasticity, it has become increasingly clear that phosphatases have an equally dynamic and critical role in the activity-dependent alterations of synaptic transmission. Here, ...

To investigate the role of phosphatases in synaptic plasticity using genetic approaches, we generated transgenic mice that overexpress a truncated form of calcineurin under the control of the CaMKIIalpha promoter. Mice expressing this transgene show increased calcium-dependent phosphatase activity in the hippocampus. Physiological studies of these mice and parallel pharmacological experiments in wild-type mice reveal a novel, intermediate phase of LTP (I-LTP) in the CA1 region of the hippocampus. This intermediate phase differs from E-LTP by requiring multiple trains for induction and in being dependent on PKA. It differs from L-LTP in not requiring new protein synthesis. These data suggest that calcineurin acts as an inhibitory constraint on I-LTP that is relieved by PKA. This inhibitory constraint acts as a gate to regulate the synaptic induction of L-LTP.

The threshold for hippocampal-dependent synaptic plasticity and memory storage is thought to be determined by the balance between protein phosphorylation and dephosphorylation mediated by the kinase PKA and the phosphatase calcineurin. To establish whether endogenous calcineurin acts as an inhibitory constraint in this balance, we examined the effect of genetically inhibiting calcineurin on plasticity and memory. Using the doxycycline-dependent rtTA system to express a calcineurin inhibitor reversibly in the mouse brain, we find that the transient reduction of calcineurin activity facilitates LTP in vitro and in vivo. This facilitation is PKA dependent and persists over several days in vivo. It is accompanied by enhanced learning and strengthened short- and long-term memory in several hippocampal-dependent spatial and nonspatial tasks. The LTP and memory improvements are reversed fully by suppression of transgene expression. These results demonstrate that endogenous calcineurin constrains LTP and memory.

Stress is a major risk factor for numerous neuropsychiatric diseases. However, susceptibility to stress and the qualitative nature of stress effects on behavior differ markedly among individuals. This is partly because of the moderating influence of genetic factors. Inbred mouse strains provide a relatively stable and restricted range of genetic and environmental variability that is valuable for disentangling gene-stress interactions. Here, we screened a panel of inbred strains for anxiety-and depression-related phenotypes at baseline (trait) and after exposure to repeated restraint. Two strains, DBA/2J and C57BL/6J, differed in trait and restraint-induced anxiety-related behavior (dark/light exploration, elevated plus maze). Gene expression analysis of amygdala, medial prefrontal cortex, and hippocampus revealed divergent expression in DBA/2J and C57BL/6J both at baseline and after repeated restraint. Restraint produced strain-dependent expression alterations in various genes including glutamate receptors (e.g., Grin1, Grik1). To elucidate neuronal correlates of these strain differences, we performed ex vivo analysis of glutamate excitatory neurotransmission in amygdala principal neurons. Repeated restraint augmented amygdala excitatory postsynaptic signaling and altered metaplasticity (temporal summation of NMDA receptor currents) in DBA/2J but not C57BL/6J. Furthermore, we found that the C57BL/6J-like changes in anxiety-related behavior after restraint were absent in null mutants lacking the modulatory NMDA receptor subunit Grin2a, but not the AMPA receptor subunit Gria1. Grin2a null mutants exhibited significant (ϳ30%) loss of dendritic spines on amygdala principal neurons under nonrestraint conditions. Collectively, our data support a model in which genetic variation in glutamatergic neuroplasticity in corticolimbic circuitry underlies phenotypic variation in responsivity to stress.

Learning-induced synaptic plasticity commonly involves the interaction between cAMP and p42/44MAPK. To investigate the role of Rap1 as a potential signaling molecule coupling cAMP and p42/44MAPK, we expressed an interfering Rap1 mutant (iRap1) in the mouse forebrain. This expression selectively decreased basal phosphorylation of a membrane-associated pool of p42/44MAPK, impaired cAMP-dependent LTP in the hippocampal Schaffer collateral pathway induced by either forskolin or theta frequency stimulation, decreased complex spike firing, and reduced the p42/44MAPK-mediated phosphorylation of the A-type potassium channel Kv4.2. These changes correlated with impaired spatial memory and context discrimination. These results indicate that Rap1 couples cAMP signaling to a selective membrane-associated pool of p42/44MAPK to control excitability of pyramidal cells, the early and late phases of LTP, and the storage of spatial memory.

Anxiety and addiction disorders are two of the most common mental disorders in the United States, and are typically chronic, disabling, and comorbid. Emerging evidence suggests the bed nucleus of the stria terminalis (BNST) mediates both anxiety and addiction through connections with other brain regions, including the amygdala and nucleus accumbens. Although BNST structural connections have been identified in rodents and a limited number of structural connections have been verified in non-human primates, BNST connections have yet to be described in humans. Neuroimaging is a powerful tool for identifying structural and functional circuits in vivo. In this study, we examined BNST structural and functional connectivity in a large sample of humans. The BNST has structural and functional connections with multiple subcortical regions, including limbic, thalamic, and basal ganglia structures, confirming structural findings in rodents. We describe two novel connections in the human brain that have not been previously reported in rodents or non-human primates, including structural connections with the temporal pole, and functional connections with the paracingulate gyrus. The findings of this study provide a map of the BNST’s structural and functional connectivity across brain in healthy humans. In large part, the BNST neurocircuitry in humans is similar to findings from rodents and non-human primates; however, several connections are unique to humans. Future explorations of BNST neurocircuitry in anxiety and addiction disorders have the potential to reveal novel mechanisms underlying these disabling psychiatric illnesses.