Stabilization of dendritic mRNAs by nitric oxide allows localized, activity-dependent enhancement of hippocampal protein synthesis

Differential regulation of MAP2 and αCamKII expression in hippocampal neurones by forskolin and calcium ionophore treatment

The genes encoding microtubule-associated protein 2 (MAP2), and the alpha subunit of calcium/calmodulin-dependent protein kinase II (alphaCaMKII), are members of a small number of genes whose expression is increased in hippocampal neurones during the intermediate phase of long-term potentiation (LTP)-a phase dependent on mRNA translation but not on gene transcription. However, the intracellular signalling pathways which mediate these increases in expression are largely unknown. Organotypic slice cultures of rat hippocampus were exposed to either forskolin (to elevate cAMP levels), A23187 (to increase intracellular Ca(2+) levels) or the corresponding vehicle. The levels of immunoreactive (ir-) MAP2 were increased 4 h after forskolin treatment, but were unaffected by A23187 treatment. Conversely, the levels of ir-alphaCaMKII were increased 4 h after A23187 treatment, but were unaffected by forskolin. The regulation of the expression of these proteins was the same in the CA3 region as in the CA1 and dentate gyrus of the hippocampus. While rapamycin reduced the basal levels of ir-MAP2, it did not affect the ability of either forskolin or A23187 to enhance ir-MAP2 or ir-alphaCaMKII levels. These results suggest that cAMP and Ca(2+) differentially modulate the expression of these two plasticity-related genes, and that translational enhancement via the mammalian target of rapamycin kinase is not involved in these effects.

Distribution of Rho family GTPases in the adult rat hippocampus and cerebellum

Small GTPases are monomeric guanine nucleotide binding proteins of 20-25 kDa mass. Rho GTPases belong to the Ras superfamily of small GTPases. The small GTPases of the Rho family have been shown to participate in the organisation of the actin cytoskeleton and signal transduction pathways leading to gene transcription. Recent evidence suggests that Rho family GTPases may play an important role in synaptic communication in the brain, and particularly in synatic plasticity. In this study the distribution of RhoA, RhoB, RhoG, Cdc42, and Rac1 was investigated in hippocampal and cerebellar tissue of adult rat brain using immunohistochemical techniques. Previous studies suggest that distribution of Rho family mRNA is uniform throughout these structures. Here we provide evidence for differences in expression of these proteins between different regions of the hippocampus, and between the molecular and granular layers in the cerebellum. These differences may prove important with regard to the physiological functions of Rho family GTPases.

Activation of p44/p42 MAP kinase in striatal neurons via kainate receptors and PI3 kinase

The members of the mitogen-activated protein (MAP) kinase family -- p44/p42 MAP kinase (ERK), c-jun N-terminal kinase (JNK) and p38 MAP kinase (p38) are known to be important mediators of the physiological plasticity or neurotoxicity induced in the striatum by activation of ionotropic glutamate receptors. However, our knowledge of the class of glutamate receptor and the intracellular pathways involved derives totally from studies on embryonic neurons, where the mechanisms are likely to be totally different from those operating in mature neurons. In superfused striatal slices from adult rats, NMDA and kainate, but not AMPA, were found to activate ERK. No activation of p38 or JNK was detected following treatment with any ionotropic glutamate receptor agonist. The activation of ERK by kainate was blocked by the ERK kinase (MEK) inhibitor PD98059, and the PI3 kinase inhibitor wortmannin, but not by the p38 MAP kinase inhibitor SB203580. This provides evidence for a novel pathway linking striatal kainate receptors to ERK activation via PI3 kinase and MEK.

Nitrergic neurons make synapses on dual-input dendritic spines of neurons in the cerebral cortex and the striatum of the rat: implication for a postsynaptic action of nitric oxide

Pre-embedding electron microscopic immunocytochemistry was used to examine the ultrastructure of neurons containing nitric oxide synthase and to evaluate their synaptic relationships with target neurons in the striatum and sensorimotor cerebral cortex. Intense nitric oxide synthase immunoreactivity was found by light and electron microscopy in a type of aspiny neuron scattered in these two regions. The intensity of the labeling was uniform in the soma, dendrites and axon terminals of these neurons. In both forebrain regions, nitric oxide synthase-immunoreactive neurons received synaptic contacts from unlabeled terminals, which were mostly apposed to small-caliber dendrites. The unlabeled symmetric contacts were generally about four times as abundant as the unlabeled asymmetric contacts on the nitric oxide synthase-immunoreactive neurons. Terminals labeled for nitric oxide synthase were filled with synaptic vesicles and were observed to contact unlabeled neurons. Only 54% (in the cerebral cortex) and 44.3% (in the striatum) of the nitric oxide synthase-immunoreactive terminals making apposition with the target structures were observed to form synaptic membrane specializations within the plane of the randomly sampled sections. The most common targets of nitric oxide synthase-immunoreactive terminals were thin dendritic shafts (54% of the immunoreactive terminals in the cortex and 75.7% of the immunoreactive terminals in the striatum), while dendritic spines were a common secondary target (42% of the immunoreactive terminals in the cortex and 20.6% of the immunoreactive terminals in the striatum). The spines contacted by nitric oxide synthase-immunoreactive terminals typically also received an asymmetric synaptic contact from an unlabeled axon terminal. These findings suggest that: (i) nitric oxide synthase-immunoreactive neurons in the cortex and striatum preponderantly receive inhibitory input; (ii) nitric oxide synthase-containing terminals commonly make synaptic contact with target structures in the cortex and striatum; (iii) spines targeted by nitric oxide synthase-containing terminals in the cortex and striatum commonly receive an asymmetric contact as well, which may provide a basis for a synaptic interaction of nitric oxide with excitatory input to individual spines.

Expression of the neuronal calcium sensor protein family in the rat brain

The neuronal calcium sensor proteins are members of the calcium-binding protein superfamily. They control localized calcium signalling on membranes and may make G-protein cascades sensitive to cytosolic calcium. The family members are recoverin (visinin, S-modulin), neuronal calcium sensor-1 (frequenin), hippocalcin, neuronal visinin-like protein-1 (visinin-like protein, neurocalcin-alpha), neuronal visinin-like protein-2 and neuronal visinin-like protein-3. Recoverin is expressed only in the retina and pineal gland. Using in situ hybridization, we mapped the expression of the other neuronal calcium sensor protein genes in the adult rat brain. Neuronal visinin-like protein-1 messenger RNA has a widespread distribution and is abundant in all brain areas except the caudate-putamen. Neuronal calcium sensor-1 gene expression is pan-neuronal. Neuronal calcium sensor-1 messenger RNA is present in the dendrites of hippocampal pyramidal and granule cells, suggesting a specific role in dendritic function. Hippocalcin and neuronal visinin-like protein-2 are mainly expressed in the forebrain and have similar expression patterns (neocortex, hippocampus and caudate-putamen). Neuronal visinin-like protein-3 has the most restricted expression; its highest expression level is in the cerebellum (Purkinje and granule cells). However, the neuronal visinin-like protein-3 gene is also expressed in many ventral nuclei throughout the fore- and midbrain, in the medial habenulae, and in the superior and inferior colliculi. The neuronal calcium sensor proteins are a relatively unexplored family of Ca(2+)-binding proteins. They are likely to be involved in many diverse areas of neuronal signalling. In this paper, we describe their expression in the rat brain as determined by in situ hybridization. As all five neuronal calcium sensor protein genes have distinctive expression patterns, they probably perform specific functions.

Brain-derived neurotrophic factor (BDNF) induces dendritic targeting of BDNF and tyrosine kinase B mRNAs in hippocampal neurons through a phosphatidylinositol-3 kinase-dependent pathway

This study aims to understand the mechanisms of dendritic targeting of brain-derived neurotrophic factor (BDNF) and tyrosine kinase B (TrkB) mRNAs. We show that brief depolarizations are sufficient to induce accumulation of BDNF and TrkB mRNAs in dendrites of hippocampal neurons. Endogenous BDNF, secreted during the KCl stimulation, contributes significantly to the dendritic accumulation of BDNF-TrkB mRNAs. In the absence of depolarization, 1 min pulses of exogenous BDNF are sufficient to induce dendritic accumulation of BDNF-TrkB mRNAs. After binding to TrkB, BDNF exerts this action by activating a PI-3 kinase-dependent pathway. The accumulation of dendritic mRNA by BDNF is not mediated by BDNF-induced neurotransmitter release. Because most hippocampal neurons coexpress BDNF and TrkB receptors, these results show that the subcellular distribution of BDNF-TrkB mRNAs is under the control of an autocrine-paracrine BDNF-TrkB-dependent loop.

Regulation of neuronal cell adhesion molecule expression by NF-kappa B

The neuronal cell adhesion molecule (NCAM) is a key mediator of structural plasticity in the central nervous system, but the mechanisms that control its expression are unknown. Equally, although the transcription factor NF-kappaB is present in the brain, few NF-kappaB-regulated genes relevant for central nervous system function have been identified. We have previously demonstrated that NF-kappaB is activated in neuronal cultures treated with kainic acid or nitric oxide. We show here that kainic acid or nitric oxide also increase the levels of NCAM mRNA and protein in neurons and that this induction of NCAM expression is sensitive to dexamethasone and to antisense, but not missense, oligonucleotides designed to suppress NF-kappaB synthesis. Nitric oxide also stimulates protein binding to an NF-kappaB site in the promoter of the NCAM gene. This indicates that NF-kappaB, which has recently been implicated in synaptic plasticity and also in the etiology of neurodegenerative disease, plays a crucial role in the activity-dependent regulation of NCAM gene expression. In addition, since both NCAM and NF-kappaB are present in the post-synaptic density, this represents a route allowing direct communication between the synapse and the nucleus.

Kainate receptors activate NF-kappaB via MAP kinase in striatal neurones

The transcription factor NF-kappaB has been implicated in the synaptic plasticity and neurotoxicity mediated by ionotropic glutamate receptors in the striatum. However, the class of glutamate receptor and the intracellular pathways involved have not been determined. Kainate, but not AMPA or NMDA, was found to activate NF-kappaB in superfused slices of rat striatum. A similar activation was produced by the calcium ionophore A23187. The NF-kappaB activation by kainate was not observed in the absence of extracellular calcium, and was blocked by the p44/p42 MAP kinase inhibitor PD98059, but not by the p38 MAP kinase inhibitor SB203580. This demonstrates that striatal kainate receptors are coupled to NF-kappaB activation via calcium influx and p44/p42 MAP kinase activation.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Characterization of transcript processing of the gene encoding precerebellin-1

Precerebellin-1 (Cbln1) is a cerebellum-specific protein that shares significant sequence identity with the globular domains of the complement components C1qA, B and C, suggesting some common aspects of function and/or structure. As the C1q complex is composed of heterotrimers of C1qA, B and C it was hypothesized that multiple precerebellins may exist in a ternary complex. Northern blotting for cbln1 revealed multiple bands that could represent further family members or alternatively spliced variants. To discriminate these alternatives, probes derived from different regions of the cbln1 gene were used to identify and clone the transcripts detected on Northern blots. Four independent transcripts were repeatedly cloned from an adult mouse cerebellum cDNA library. Upon sequencing, all of these clones were found to be derived from the cbln1 gene and no additional precerebellin-related genes were isolated. Moreover, these clones accounted for the four cbln1-hybridizing bands (1.9, 2. 2, 3.2 and 5.5 kb) detected on Northern blots of adult cerebellum RNA. With one possible exception, these clones were all derived through alterations in the 3'-untranslated region (3'-UTR) of cbln1 that did not affect the coding sequence. This was achieved by the use of two polyadenylation sites and alternative (non-canonical) splicing in the 3'-UTR. Some additional variation in mRNA structure is provided by the use of alternative transcription start sites in cbln1. The possible significance of this level of diversity in the 3'-UTR is discussed.

Dendritic localization of mRNAs

The dendritic localization of mRNAs has been proposed to underlie the structural and functional polarity of neurons, as well as certain aspects of synaptic plasticity. Even though there is no conclusive evidence that such a localization is a physiological requirement, studies of mRNA localization in relation to function in other cell types and recent experiments on synaptic plasticity suggest that this proposal may be correct.

Increased expression of dendritic mRNA following the induction of long-term potentiation

A small number of mRNAs, including Ca2+/calmodulin-dependent protein kinase II alpha-subunit (CamKIIalpha) mRNA and microtubule-associated protein 2 (MAP2) mRNA, are present in the dendrites of neurones as well as in the cell bodies. We show here that the induction of long-term potentiation (LTP) in the hippocampal perforant path/granule cell synapses in anaesthetised rats is associated with increased levels of CamKIIalpha mRNA and MAP2 mRNA in the granule cell dendrites after 2 h. Similarly, induction of LTP in the Schaffer collateral/CA1 pyramidal cell synapses in hippocampal slices maintained in vitro also results in elevated dendritic levels of CamKIIalpha mRNA and MAP2 mRNA 2 h later. In both models, the levels of various other mRNA species restricted to the cell body region were unaffected by the induction of LTP. Increased expression of dendritic CamKIIalpha mRNA and MAP2 mRNA appears to be a general feature of hippocampal plasticity, since it occurs following LTP induction in both the dentate gyrus and the CA1 region. The elevation of mRNA levels in a restricted region close to the afferent synapses would allow a highly-localised enhancement of the synthesis of the corresponding proteins, providing an elegant mechanism for protein-synthesis-dependent synaptic plasticity to maintain a high degree of anatomical specificity.