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That visceral sensory afferents are functionally distinct from their somatic analogues has been known for a long time but the detailed knowledge of their synaptic connections and neurotransmitters at the first relay nucleus in the spinal cord has been limited. To provide information on these topics, we investigated the synapses and neurotransmitters of identified afferents from the urinary bladder to the superficial laminae of the rat spinal dorsal horn (DH) and the spinal parasympathetic nucleus (SPN) by tracing with horseradish peroxidase, quantitative electron microscopical analysis, and immunogold staining for GABA and glycine. In the DH, most bladder afferent boutons formed synapses with 1-2 postsynaptic dendrites, whereas in the SPN, close to a half of them formed synapses with 3-8 postsynaptic dendrites. The number of postsynaptic dendrites and dendritic spines per bladder afferent bouton, both measures of synaptic divergence and of potential for synaptic plasticity at a single bouton level, were significantly higher in the SPN than in the DH. Bladder afferent boutons frequently received inhibitory axoaxonic synapses from presynaptic endings in the DH but rarely in the SPN. The presynaptic endings were GABA- and/or glycine-immunopositive. The bouton volume, mitochondrial volume, and active zone area, all determinants of synaptic strength, of the bladder afferent boutons were positively correlated with the number of postsynaptic dendrites. These findings suggest that visceral sensory information conveyed via the urinary bladder afferents is processed differently in the DH than in the SPN, and differently from the way somatosensory information is processed in the spinal cord.

Recent studies implicate glutamate release from satellite glial cells (SGCs) surrounding the primary sensory neurons in the mechanisms of pathologic pain. However, little is known about the population of SGCs in the trigeminal ganglion that is involved in glutamate signaling associated with craniofacial neuropathic pain. To address this issue, we used quantitative analysis of electron microscopic immunogold labeling to investigate the changes in glutamate levels in trigeminal neurons and their enveloping SGCs in a rat model of craniofacial neuropathic pain, chronic constriction injury of inferior alveolar nerve (CCI-ION). The density of immunogold, a measure for glutamate concentration, in the neuronal cell bodies of all sizes, and in the SGCs surrounding them, was significantly higher in rats with CCI-ION than in sham-operated rats. This effect was more pronounced for the small neurons (2.2 times higher) and their SGCs (1.8 times higher) than for the medium and the large neurons and their SGCs, respectively. These findings suggest that each populations of SGCs and their surrounding trigeminal neurons of different type are involved in the glutamate signaling associated with neuropathic pain at a different level.

Memories are stored in synapses Memory formation is thought to change the strength of synaptic connections between neurons. However, direct measurements between neurons that participate in a learning process are difficult to obtain. Choi et al. developed the “dual-eGRASP” technique to identify synaptic connections between hippocampal CA3 and CA1 pyramidal cells. This method could label two different sets of synapses so that their convergence on the same dendrites would be quantified. After contextual fear conditioning in mice, the number and size of spines were increased on CA1 engram cells receiving input from CA3 engram cells. Science , this issue p. 430

Neur1 and Neur2, mouse homologs of the Drosophila neur gene, consist of two neuralized homology repeat domains and a RING domain. Both Neur1 and Neur2 are expressed in the whole adult brain and encode E3 ubiquitin ligases, which play a crucial role in the Notch signaling pathways. A previous study reported that overexpression of Neur1 enhances hippocampus-dependent memory, whereas the role of Neur2 remains largely unknown. Here, we aimed to elucidate the respective roles of Neur1 and Neur2 in hippocampus-dependent memory using three lines of genetically modified mice: Neur1 knock-out, Neur2 knock-out, and Neur1 and Neur2 double knock-out (D-KO). Our results showed that spatial memory was impaired when both Neur1 and Neur2 were deleted, but not in the individual knock-out of either Neur1 or Neur2. In addition, basal synaptic properties estimated by input-output relationships and paired-pulse facilitation did not change, but a form of long-term potentiation that requires protein synthesis was specifically impaired in the D-KO mice. These results collectively suggest that Neur1 and Neur2 are crucially involved in hippocampus-dependent spatial memory and synaptic plasticity.

Currently, compared to jaw-closing (JC) α-motoneurons, the information on the distribution and morphology of glutamatergic synapses on the jaw-closing (JC) γ-motoneurons, which may help elucidate the mechanism of isometric contraction of the JC muscle, is very limited. This study investigated the distribution and ultrastructural features of vesicular glutamate transporter 1 (VGLUT1)- and VGLUT2-immunopositive (+) axon terminals (boutons) on JC γ-motoneurons by retrograde tracing with horseradish peroxidase, electron microscopic immunocytochemistry, and quantitative analysis. About 35% of the boutons on identified JC γ-motoneurons were VGLUT+, and of those, 99% were VGLUT2+. The fraction of VGLUT1+ boutons of all boutons and the percentage of membrane of JC γ-motoneurons covered by these boutons were significantly lower than those for the JC α-motoneurons, revealed in our previous work. The bouton volume, mitochondrial volume, and active zone area of the VGLUT2+ boutons on the JC γ-motoneurons were uniformly small. These findings suggest that the JC γ-motoneurons, in contrast to the JC α-motoneurons, receive generally weak glutamatergic synaptic input almost exclusively from VGLUT2+ premotoneurons that form direct synapse with motoneurons.

Recently, protein kinase M ζ (PKMζ) has emerged as an important player for maintaining memory. It has been reported that PKMζ regulates the trafficking of GluA2 in postsynaptic membranes to maintain memory. However, there has been no study on PKMζ outside the synaptic region regarding memory maintenance. Here, we found that PKMζ is transported to the nucleus in a neural activity-dependent manner. Moreover, we found that PKMζ phosphorylates CREB-binding protein (CBP) at serine residues and that PKMζ inhibition reduces the acetylation of histone H2B and H3. Finally, we showed that the amnesic effect of PKMζ inhibition can be rescued by enhancing histone acetylation level. These results suggest the possibility that nuclear PKMζ has a crucial role in memory maintenance.

Nociception is one of the most complex senses that is affected not only by external stimulation but also internal conditions. Previous studies have suggested that circadian rhythm is important in modulating nociception. REV-ERBα knock-out (KO) mice have disrupted circadian rhythm and altered mood-related phenotypes. In this study, we examined the role of REV-ERBα in inflammatory nociception. We found that the nociceptive sensitivity of KO mice was partially enhanced in mechanical nociception. However, this partial alteration was independent of the circadian rhythm. Taken together, deletion of REV-ERBα induced a mild change in mechanical nociceptive sensitivity but this alteration was not dependent on the circadian rhythm.

Protein kinase M ζ is well known for its role in maintaining memory and pain. Previously, we revealed that the activation of protein kinase M ζ in the anterior cingulate cortex plays a role in sustaining neuropathic pain. However, the mechanism by which protein kinase M ζ is expressed in the anterior cingulate cortex by peripheral nerve injury, and whether blocking of protein kinase M ζ using its inhibitor, zeta inhibitory peptide, produces analgesic effects in neuropathic pain maintained chronically after injury, have not previously been resolved. In this study, we show that protein kinase M ζ expression in the anterior cingulate cortex is enhanced by peripheral nerve injury in a transcription-independent manner. We also reveal that the inhibition of protein kinase M ζ through zeta inhibitory peptide treatment is enough to reduce mechanical allodynia responses in mice with one-month-old nerve injuries. However, the zeta inhibitory peptide treatment was only effective for a limited time.

MicroRNAs (miRNAs) are small, noncoding RNAs that posttranscriptionally regulate gene expression in many tissues. Although a number of brain-enriched miRNAs have been identified, only a few specific miRNAs have been revealed as critical regulators of synaptic plasticity, learning, and memory. miR-9-5p/3p are brain-enriched miRNAs known to regulate development and their changes have been implicated in several neurological disorders, yet their role in mature neurons in mice is largely unknown. Here, we report that inhibition of miR-9-3p, but not miR-9-5p, impaired hippocampal long-term potentiation (LTP) without affecting basal synaptic transmission. Moreover, inhibition of miR-9-3p in the hippocampus resulted in learning and memory deficits. Furthermore, miR-9-3p inhibition increased the expression of the LTP-related genes Dmd and SAP97, the expression levels of which are negatively correlated with LTP. These results suggest that miR-9-3p-mediated gene regulation plays important roles in synaptic plasticity and hippocampus-dependent memory. SIGNIFICANCE STATEMENT Despite the abundant expression of the brain-specific microRNA miR-9-5p/3p in both proliferating and postmitotic neurons, most functional studies have focused on their role in neuronal development. Here, we examined the role of miR-9-5p/3p in adult brain and found that miR-9-3p, but not miR-9-5p, has a critical role in hippocampal synaptic plasticity and memory. Moreover, we identified in vivo binding targets of miR-9-3p that are involved in the regulation of long-term potentiation. Our study provides the very first evidence for the critical role of miR-9-3p in synaptic plasticity and memory in the adult mouse.

Autism spectrum disorders (ASDs) are a group of developmental disorders that cause variable and heterogeneous phenotypes across three behavioral domains such as atypical social behavior, disrupted communications, and highly restricted and repetitive behaviors. In addition to these core symptoms, other neurological abnormalities are associated with ASD, including intellectual disability (ID). However, the molecular etiology underlying these behavioral heterogeneities in ASD is unclear. Mutations in SHANK2 genes are associated with ASD and ID. Interestingly, two lines of Shank2 knockout mice (e6-7 KO and e7 KO) showed shared and distinct phenotypes. Here, we found that the expression levels of Gabra2, as well as of GABA receptor-mediated inhibitory neurotransmission, are reduced in Shank2 e6-7, but not in e7 KO mice compared with their own wild type littermates. Furthermore, treatment of Shank2 e6-7 KO mice with an allosteric modulator for the GABAA receptor reverses spatial memory deficits, indicating that reduced inhibitory neurotransmission may cause memory deficits in Shank2 e6-7 KO mice. This article is part of the Special Issue entitled 'Ionotropic glutamate receptors'.

Long-term depression (LTD) is a key form of synaptic plasticity important in learning and information storage in the brain. It has been studied in various cortical regions, including the anterior cingulate cortex (ACC). ACC is a crucial cortical region involved in such emotion-related physiological and pathological conditions as fear memory and chronic pain. In the present study, we used a multielectrode array system to map cingulate LTD in a spatiotemporal manner within the ACC. We found that low-frequency stimulation (1 Hz, 15 min) applied onto deep layer V induced LTD in layers II/III and layers V/VI. Cingulate LTD requires activation of metabotropic glutamate receptors (mGluRs), while L-type voltage-gated calcium channels and NMDA receptors also contribute to its induction. Peripheral amputation of the distal tail impaired ACC LTD, an effect that persisted for at least 2 weeks. The loss of LTD was rescued by priming ACC slices with activation of mGluR1 receptors by coapplying (RS)-3,5-dihydroxyphenylglycine and MPEP, a form of metaplasticity that involved the activation of protein kinase C. Our results providein vitroevidence of the spatiotemporal properties of ACC LTD in adult mice. We demonstrate that tail amputation causes LTD impairment within the ACC circuit and that this can be rescued by activation of mGluR1.

Phosphatidylinositol 3-kinase (PI3K) has been implicated in synaptic plasticity and other neural functions in the brain. However, the role of individual PI3K isoforms in the brain is unclear. We investigated the role of PI3Kγ in hippocampal-dependent synaptic plasticity and cognitive functions. We found that PI3Kγ has a crucial and specific role in NMDA receptor (NMDAR)-mediated synaptic plasticity at mouse Schaffer collateral-commissural synapses. Both genetic deletion and pharmacological inhibition of PI3Kγ disrupted NMDAR long-term depression (LTD) while leaving other forms of synaptic plasticity intact. Accompanying this physiological deficit, the impairment of NMDAR LTD by PI3Kγ blockade was specifically correlated with deficits in behavioral flexibility. These findings suggest that a specific PI3K isoform, PI3Kγ, is critical for NMDAR LTD and some forms of cognitive function. Thus, individual isoforms of PI3Ks may have distinct roles in different types of synaptic plasticity and may therefore influence various kinds of behavior.

APPswe+PS1/ΔE9 transgenic (Tg) mice with Aβ plaque formation in neocortex and hippocampus were evaluated in tests measuring exploratory activity, anxiety, and memory ability using open field test (OFT), Y-maze, contextual fear conditioning (CFC), and Morris water maze (MWM). Wild type (WT) and Tg mice over eight months old showed same locomotion activity and anxiety level in novel stimulation, open field, and Y-maze contexts. In other experiments that measured associative memory and spatial memory in Tg mice and their littermates, the subjects also presented similar deficiencies in memory acquisition. These two aged groups showed abnormal freezing level variance especially in CFC test. In comparison to that in non-transgenic 8-week-old mice group, the acquisition of spatial memory in MWM task was impaired in aged WT and bigenic Tg mice. Taken together, aged wild-type littermates and Tg mice present similar deficits in associative learning and spatial memory independent of amyloid plaques.

Consistent evidence from pharmacological and genetic studies shows that cAMP is a critical modulator of synaptic plasticity and memory formation. However, the potential of the cAMP signaling pathway as a target for memory enhancement remains unclear because of contradictory findings from pharmacological and genetic approaches. To address these issues, we have developed a novel conditional genetic system in mice based on the heterologous expression of an Aplysia octopamine receptor, a G-protein-coupled receptor whose activation by its natural ligand octopamine leads to rapid and transient increases in cAMP. We find that activation of this receptor transgenically expressed in mouse forebrain neurons induces a rapid elevation of hippocampal cAMP levels, facilitates hippocampus synaptic plasticity, and enhances the consolidation and retrieval of fear memory. Our findings clearly demonstrate that acute increases in cAMP levels selectively in neurons facilitate synaptic plasticity and memory, and illustrate the potential of this heterologous system to study cAMP-mediated processes in mammalian systems.

Autism spectrum disorder is a debilitating mental illness and social issue. Autism spectrum disorder patients suffer from social isolation, cognitive deficits, compulsive behavior, and sensory deficits, including hyposensitivity to pain. However, recent studies argued that autism spectrum disorder patients show physiological pain response and, in some cases, even extremely intense pain response to harmless stimulation. Recently, Shank gene family was reported as one of the genetic risk factors of autism spectrum disorder. Thus, in this study, we used Shank2(-) (/) (-) (Shank2 knock-out, KO) mice to investigate the controversial pain sensitivity issue and found that Shank2 KO mice showed reduced tactile perception and analgesia to chronic pain.

SummaryRepeated pulses of serotonin (5-HT) induce long-term facilitation (LTF) of the synapses between sensory and motor neurons of the gill-withdrawal reflex in Aplysia. To explore how apCAM downregulation at the plasma membrane and CREB-mediated transcription in the nucleus, both of which are required for the formation of LTF, might relate to each other, we cloned an apCAM-associated protein (CAMAP) by yeast two-hybrid screening. We found that 5-HT signaling at the synapse activates PKA which in turn phosphorylates CAMAP to induce the dissociation of CAMAP from apCAM and the subsequent translocation of CAMAP into the nucleus of sensory neurons. In the nucleus, CAMAP acts as a transcriptional coactivator for CREB1 and is essential for the activation of ApC/EBP required for the initiation of LTF. Combined, our data suggest that CAMAP is a retrograde signaling component that translocates from activated synapses to the nucleus during synapse-specific LTF.

Long-term memory requires transcriptional regulation by a combination of positive and negative transcription factors. Aplysia activating factor (ApAF) is known to be a positive transcription factor that forms heterodimers with ApC/EBP and ApCREB2. How these heterodimers are regulated and how they participate in the consolidation of long-term facilitation (LTF) has not, however, been characterized. We found that the functional activation of ApAF required phosphorylation of ApAF by PKA on Ser-266. In addition, ApAF lowered the threshold of LTF by forming a heterodimer with ApCREB2. Moreover, once activated by PKA, the ApAF–ApC/EBP heterodimer transactivates enhancer response element–containing genes and can induce LTF in the absence of CRE- and CREB-mediated gene expression. Collectively, these results suggest that PKA-activated ApAF–ApC/EBP heterodimer is a core downstream effector of ApCREB in the consolidation of LTF.

A consolidated memory can be transiently destabilized by memory retrieval, after which memories are reconsolidated within a few hours; however, the molecular substrates underlying this destabilization process remain essentially unknown. Here we show that at lateral amygdala synapses, fear memory consolidation correlates with increased surface expression of calcium-impermeable AMPA receptors (CI-AMPARs), which are known to be more stable at the synapse, whereas memory retrieval induces an abrupt exchange of CI-AMPARs to calcium-permeable AMPARs (CP-AMPARs), which are known to be less stable at the synapse. We found that blockade of either CI-AMPAR endocytosis or NMDA receptor activity during memory retrieval, both of which blocked the exchange to CP-AMPARs, prevented memory destabilization, indicating that this transient exchange of AMPARs may underlie the transformation of a stable memory into an unstable memory. These newly inserted CP-AMPARs gradually exchanged back to CI-AMPARs within hours, which coincided with the course of reconsolidation. Furthermore, blocking the activity of these newly inserted CP-AMPARs after retrieval impaired reconsolidation, suggesting that they serve as synaptic “tags” that support synapse-specific reconsolidation. Taken together, our results reveal unexpected physiological roles of CI-AMPARs and CP-AMPARs in transforming a consolidated memory into an unstable memory and subsequently guiding reconsolidation.

Recently, pluripotency induction or cellular reprogramming by introducing critical transcription factors has been extensively studied, but has been demonstrated only in vitro. Based on re- ports that Oct4 is critically involved in transforming neural stem cells into pluripotent cells, we used the lentiviral vector to in- troduce the Oct4 gene into the hippocampal dentate gyrus (DG) of adult mice. We examined whether this manipulation led to cellular or behavioral changes, possibly through processes in- volving the transformation of NS cells into pluripotent cells. The Oct4 lentivirus-infused group and the green fluorescent protein lentivirus-infused group showed a similar thickness of the DG and a comparable level of synaptophysin expression in the DG. Furthermore, our behavioral analyses did not show any differ- ences between the groups concerning exploratory activity, anxi- ety, or memory abilities. This first trial for pluripotency in- duction in vivo, despite negative results, provides implications and information for future studies on in vivo cellular reprogra- mming. (BMB reports 2011; 44(12): 793-798)

Synaptic plasticity in the spinal cord and the cortex is believed to be important for the amplification of painful information in chronic pain conditions. The investigation of molecular mechanism responsible for maintaining injury-related plastic changes, such as through the study of long-term potentiation in these structures, provides potential novel targets for designing new medicine for chronic pain. Recent studies using integrative neurobiological approaches demonstrate that protein kinase M zeta (PKMζ) maintains pain-induced persistent changes in the anterior cingulate cortex (ACC), and inhibiting PKMζ by ζ-pseudosubstrate inhibitory peptide produces analgesic effects in animal models of chronic pain. We propose that targeting PKMζ, or its up- or downstream signaling proteins, in the ACC may provide novel clinical treatment for chronic pain.

The cAMP cascade and vascular endothelial growth factor (VEGF) are critical modulators of depression. Here we have tested whether the antidepressive effect of the cAMP cascade is mediated by VEGF in the adult hippocampus. We used a conditional genetic system in which the Aplysia octopamine receptor (Ap oa(1)), a G(s)-coupled receptor, is transgenically expressed in the forebrain neurons of mice. Chronic activation of the heterologous Ap oa(1) by its natural ligand evoked antidepressant-like behaviors, accompanied by enhanced phosphorylation of cAMP response element-binding protein and transcription of VEGF in hippocampal dentate gyrus (DG) neurons. Selective knockdown of VEGF in these cells during the period of cAMP elevation inhibited the antidepressant-like behaviors. These findings reveal a molecular interaction between the cAMP cascade and VEGF expression, and the pronounced behavioral consequences of this interaction shed light on the mechanism underlying neuronal VEGF functions in antidepression.

Newborn neurons in the subgranular zone (SGZ) of the hippocampus incorporate into the dentate gyrus and mature. Numerous studies have focused on hippocampal neurogenesis because of its importance in learning and memory. However, it is largely unknown whether hippocampal neurogenesis is involved in memory extinction per se. Here, we sought to examine the possibility that hippocampal neurogenesis may play a critical role in the formation and extinction of hippocampus-dependent contextual fear memory. By methylazoxymethanol acetate (MAM) or gamma-ray irradiation, hippocampal neurogenesis was impaired in adult mice. Under our experimental conditions, only a severe impairment of hippocampal neurogenesis inhibited the formation of contextual fear memory. However, the extinction of contextual fear memory was not affected. These results suggest that although adult newborn neurons contribute to contextual fear memory, they may not be involved in the extinction or erasure of hippocampus-dependent contextual fear memory.

SummaryChronic pain can lead to anxiety and anxiety can enhance the sensation of pain. Unfortunately, little is known about the synaptic mechanisms that mediate these re-enforcing interactions. Here we characterized two forms of long-term potentiation (LTP) in the anterior cingulate cortex (ACC); a presynaptic form (pre-LTP) that requires kainate receptors and a postsynaptic form (post-LTP) that requires N-methyl-D-aspartate receptors. Pre-LTP also involves adenylyl cyclase and protein kinase A and is expressed via a mechanism involving hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. Interestingly, chronic pain and anxiety both result in selective occlusion of pre-LTP. Significantly, microinjection of the HCN blocker ZD7288 into the ACC in vivo produces both anxiolytic and analgesic effects. Our results provide a mechanism by which two forms of LTP in the ACC may converge to mediate the interaction between anxiety and chronic pain.