Ca2+ influx regulates BDNF transcription by a CREB family transcription factor-dependent mechanism

Depresssion--emerging insights from neurobiology

An emerging hypothesis suggests that the pathogenesis and treatment of depression is likely to involve a plasticity of neuronal pathways. The inability of neuronal systems to exhibit appropriate, adaptive plasticity could contribute to the pathogenesis of depression. Antidepressant treatments may exert their therapeutic effects by stimulating appropriate adaptive changes in neuronal systems. Recent studies have demonstrated that chronic antidepressant administration up-regulates the cAMP signal transduction cascade resulting in an increased expression and function of the transcription factor CREB. Enhanced CREB expression leads to an up-regulation of specific target genes, including the neurotrophin BDNF. Chronic antidepressant treatments enhance BDNF expression within hippocampal and cortical neurons and can prevent the stress-induced decrease in BDNF expression. Stress has been shown to: (i) induce neuronal atrophy/death; and (ii) decrease neurogenesis of hippocampal neurons. Clinical studies indicate significant hippocampal damage in cases of major, recurrent depression. It is possible that antidepressant treatments through enhanced expression of growth and survival promoting factors like BDNF may prevent or reverse the atrophy and damage of hippocampal neurons. Indeed, studies have indicated that chronic antidepressant treatments enhance hippocampal neurogenesis, promote neuronal sprouting and prevent atrophy. The molecular mechanisms underlying the effects of antidepressant treatments including adaptations in the cAMP transduction cascade, CREB and BDNF gene expression, and structural neuronal plasticity are discussed.

Calcium/calmodulin-dependent protein kinase type IV (CaMKIV) inhibits apoptosis induced by potassium deprivation in cerebellar granule neurons

The neuroprotective mechanisms of the Ca2+/calmodulin kinase (CaMK) signaling pathway were studied in primary cerebellar neurons in vitro. When switched from depolarizing culture conditions HK (extracellular K+ 30 mM) to LK (K+ 5 mM), these neurons rapidly undergo nuclear fragmentation, a typical feature of apoptosis. We present evidence that blockade of L-type Ca2+ channels (nifedipine sensitive) but not N/P/Q-type Ca2+ channels (omega-conotoxin MVIIC sensitive) triggered apoptosis and CPP32/caspase-3-like activity. The entry into apoptosis was associated with a progressive caspase-3-dependent cleavage of CaMKIV, but not of CaMKII. CaMKIV function in neuronal apoptosis was further investigated by overexpression of CaMKIV mutants by gene transfer. A dominant-active CaMKIV mutant inhibited LK-induced apoptosis whereas a dominant-negative form induced apoptosis in HK, suggesting that CaMKIV exerts neuroprotective effects. The transcription factor CREB is a well-described nuclear target of CaMKIV in neurons. When switched to LK, the level of phosphorylation of CREB, after an initial drop, further declined progressively with kinetics comparable to those of CaMKIV degradation. This decrease was abolished by caspase-3 inhibitor. These data are compatible with a model where Ca2+ influx via L-type Ca2+ channels prevents caspase-dependent cleavage of CaMKIV and promotes neuronal survival by maintaining a constitutive level of CaMKIV/CREB-dependent gene expression.

Neurotrophins: roles in neuronal development and function

Neurotrophins regulate development, maintenance, and function of vertebrate nervous systems. Neurotrophins activate two different classes of receptors, the Trk family of receptor tyrosine kinases and p75NTR, a member of the TNF receptor superfamily. Through these, neurotrophins activate many signaling pathways, including those mediated by ras and members of the cdc-42/ras/rho G protein families, and the MAP kinase, PI-3 kinase, and Jun kinase cascades. During development, limiting amounts of neurotrophins function as survival factors to ensure a match between the number of surviving neurons and the requirement for appropriate target innervation. They also regulate cell fate decisions, axon growth, dendrite pruning, the patterning of innervation and the expression of proteins crucial for normal neuronal function, such as neurotransmitters and ion channels. These proteins also regulate many aspects of neural function. In the mature nervous system, they control synaptic function and synaptic plasticity, while continuing to modulate neuronal survival.

Regulation of GFRalpha-1 and GFRalpha-2 mRNAs in rat brain by electroconvulsive seizure

The influence of both acute and chronic electroconvulsive seizure (ECS) or antidepressant drug treatments on expression of mRNAs encoding glial cell line-derived neurotrophic factor (GDNF) and its receptors, GFRalpha-1, GFRalpha-2, and c-Ret proto-oncogene (RET) in the rat hippocampus was examined by in situ hybridization. Two hours after acute ECS, levels of GFRalpha-1 mRNA in the dentate gyrus were significantly increased. This increase peaked to nearly 3-fold at 6 h after acute ECS and returned to basal levels 24 h after treatment. Chronic (once daily for 10 days) ECS significantly increased the expression of GFRalpha-1 mRNA nearly 5-fold after the last treatment. Levels of GFRalpha-2 mRNA in the dentate gyrus were also significantly increased by acute and chronic ECS, although this effect was less than that observed for GFRalpha-1. Maximum induction of GFRalpha-2 was 30% and 70% compared to sham in response to acute or chronic ECS, respectively. Levels of GDNF and RET mRNAs were not significantly changed following either acute or chronic ECS treatment at the time points examined. Chronic (14 days) administration of different classes of antidepressant drugs, including tranylcypromine, desipramine, or fluoxetine, did not significantly affect the GDNF, GFRalpha-1, GFRalpha-2, or RET mRNA levels in CA1, CA3, and dentate gyrus areas of hippocampus. The results demonstrate that acute ECS increases the expression of GFRalpha-1 and GFRalpha-2 and that these effects are enhanced by chronic ECS. The results also imply that regulation of the binding components of GDNF receptor complex may mediate the adaptive responses of the GDNF system to acute and chronic stimulation.

Magnitude of the CREB-dependent transcriptional response is determined by the strength of the interaction between the kinase-inducible domain of CREB and the KIX domain of CREB-binding protein

The activity of the transcription factor CREB is regulated by extracellular stimuli that result in its phosphorylation at a critical serine residue, Ser133. Phosphorylation of Ser133 is believed to promote CREB-dependent transcription by allowing CREB to interact with the transcriptional coactivator CREB-binding protein (CBP). Previous studies have established that the domain encompassing Ser133 on CREB, known as the kinase-inducible domain (KID), interacts specifically with a short domain in CBP termed the KIX domain and that this interaction depends on the phosphorylation of Ser133. In this study, we adapted a recently described Escherichia coli-based two-hybrid system for the examination of phosphorylation-dependent protein-protein interactions, and we used this system to study the kinase-induced interaction between the KID and the KIX domain. We identified residues of the KID and the KIX domain that are critical for their interaction as well as two pairs of oppositely charged residues that apparently interact at the KID-KIX interface. We then isolated a mutant form of the KIX domain that interacts more tightly with wild-type and mutant forms of the KID than does the wild-type KIX domain. We show that in the context of full-length CBP, the corresponding amino acid substitution resulted in an enhanced ability of CBP to stimulate CREB-dependent transcription in mammalian cells. Conversely, an amino acid substitution in the KIX domain that weakens its interaction with the KID resulted in a decreased ability of full-length CBP to stimulate CREB-dependent transcription. These findings demonstrate that the magnitude of CREB-dependent transcription in mammalian cells depends on the strength of the KID-KIX interaction and suggest that the level of transcription induced by coactivator-dependent transcriptional activators can be specified by the strength of the activator-coactivator interaction.

BDNF, NT-3 and NGF induce distinct new Ca2+ channel synthesis in developing hippocampal neurons

Neurotrophins exert short- and long-term effects on synaptic transmission. The mechanism underlying these forms of synaptic plasticity is unknown although it is likely that intracellular Ca2+ and presynaptic Ca2+ channels play a critical role. Here we show that BDNF, NGF and NT-3 (10-100 ng/mL) exhibit a selective long-term up-regulation of voltage-gated Ca2+ current densities in developing hippocampal neurons of 6-20 days in culture. NGF and NT-3 appear more effective in up-regulating L-currents, while BDNF predominantly acts on non-L-currents (N, P/Q and R). The effects of the three neurotrophins were time- and dose-dependent. The EC50 was comparable for BDNF, NGF and NT-3 (10-16 ng/mL) while the time of half-maximal activation was significantly longer for NGF compared to BDNF (58 vs. 25 h). Despite the increased Ca2+ current density, the neurotrophins did not alter the voltage-dependence of channel activation, the kinetics parameters or the elementary properties of Ca2+ channels (single-channel conductance, probability of opening and mean open time). Neurotrophin effects were completely abolished by coincubation with the nonspecific Trk-receptor inhibitor K252a, the protein synthesis blocker anisomycin and the MAP-kinase inhibitor PD98059, while cotreatment with the PLC-gamma blocker, U73122, was without effect. Immunocytochemistry and Western blotting revealed that neurotrophins induced an increased MAP-kinase phosphorylation and its translocation to the nucleus. The present findings suggest that on a long time scale different neurotrophins can selectively up-regulate different Ca2+ channels. The action is mediated by Trk-receptors/MAP-kinase pathways and induces an increased density of newly available Ca2+ channels with unaltered gating activity.

Alteration of cyclic adenosine monophosphate response element binding protein in rat brain after hypoglycemic coma

In the current study, the temporal and regional changes of the transcription factor cyclic adenosine monophosphate response element binding protein (CREB) were investigated in rat brains subjected to 30 minutes of hypoglycemic coma followed by varied periods of recovery using Western blot and confocal microscopy. The total amount of CREB was not altered in any area examined after coma. The level of the phosphorylated form of CREB decreased during coma but rebounded after recovery. In the relatively resistant areas, such as the inner layers of the neocortex and the inner and outer blades of the dentate gyms (DG), phospho-CREB increased greater than the control level after 30 minutes of recovery and continued to increase up to 3 hours of recovery. In contrast, little or no increase of phospho-CREB was observed during the recovery period in the outer layers of the neocortex and at the tip of the DG, that is, regions that are selectively vulnerable to hypoglycemic insults. The current findings suggest that a neuroprotective signaling pathway may be more activated in the resistant regions than in the vulnerable ones after hypoglycemic coma.

Neuroplasticity and cellular resilience in mood disorders

Although mood disorders have traditionally been regarded as good prognosis diseases, a growing body of data suggests that the long-term outcome for many patients is often much less favorable than previously thought. Recent morphometric studies have been investigating potential structural brain changes in mood disorders, and there is now evidence from a variety of sources demonstrating significant reductions in regional CNS volume, as well as regional reductions in the numbers and/or sizes of glia and neurons. Furthermore, results from recent clinical and preclinical studies investigating the molecular and cellular targets of mood stabilizers and antidepressants suggest that a reconceptualization about the pathophysiology and optimal long-term treatment of recurrent mood disorders may be warranted. It is proposed that impairments of neuroplasticity and cellular resilience may underlie the pathophysiology of mood disorders, and further that optimal long-term treatment for these severe illnesses may only be achieved by the early and aggressive use of agents with neurotrophic/neuroprotective effects. It is noteworthy that lithium, valproate and antidepressants indirectly regulate a number of factors involved in cell survival pathways including CREB, BDNF, bcl-2 and MAP kinases, and may thus bring about some of their delayed long-term beneficial effects via underappreciated neurotrophic effects. The development of novel treatments which more directly target molecules involved in critical CNS cell survival and cell death pathways have the potential to enhance neuroplasticity and cellular resilience, and thereby modulate the long-term course and trajectory of these devastating illnesses.

Regulation of brain-derived neurotrophic factor messenger RNA levels in avian hypothalamic slice cultures

Mechanisms regulating the expression of brain-derived neurotrophic factor, a member of the neurotrophin family, have been extensively studied in the rat cerebral cortex, hippocampus and cerebellum. In contrast, little is known regarding the regulation of this growth factor in the hypothalamus. Here we present an analysis of the regulation of brain-derived neurotrophic factor messenger RNA levels in chick embryo hypothalamic slice cultures following exposure to potassium chloride, glutamate agonists and sex steroids. Following a week in chemically-defined media the tissue was depolarized by exposure to 50 mM potassium chloride for 6h, resulting in a significant 4.2-fold increase in the level of brain-derived neurotrophic factor messenger RNA. This result is consistent with studies of other brain regions. Similar 6-h acute exposures of the hypothalamic cultures to 25 microM N-methyl-D-aspartic acid, 25 microM kainic acid and 25 microM alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid also significantly increased messenger RNA levels 2.5-, 2.1- and 1.4-fold, respectively. It was previously reported that brain-derived neurotrophic factor levels within the rat cerebral cortex, olfactory bulb and hippocampus are altered by exposure to 17beta-estradiol. Here we show that in hypothalamic slice cultures neither acute nor chronic treatments with 10 and 100 nM 17beta-estradiol and 10nM testosterone significantly altered the steady-state level of this growth factor. These findings show that neuronal activity, induced by glutamate agonists and potassium chloride, can regulate brain-derived neurotrophic factor messenger RNA levels within embryonic hypothalamic slice cultures. This regulation could play a critical role in the modulation of programmed cell death and synaptic maturation during development of the hypothalamus.

Clinical and preclinical evidence for the neurotrophic effects of mood stabilizers: implications for the pathophysiology and treatment of manic-depressive illness

Recent neuroimaging studies have demonstrated regional central nervous system volume reductions in mood disorders, findings that are complemented by postmortem observations of cell atrophy and loss. It is thus noteworthy that lithium and valproate have recently been demonstrated to robustly increase the expression of the cytoprotective protein bcl-2 in the central nervous system. Chronic lithium not only exerts neuroprotective effects in several preclinical paradigms but also enhances hippocampal neurogenesis. Valproate robustly promotes neurite outgrowth and activates the ERK mitogen-activated protein kinase pathway, a signaling pathway utilized by many endogenous neurotrophic factors. Consistent with its preclinical neurotrophic/neuroprotective effects, chronic lithium treatment of patients with manic-depressive illness increases brain N-acetylaspartate (a putative marker of neuronal viability and function) levels, an effect that is localized almost exclusively to gray matter. To determine if lithium was producing neuropil increases, quantitative three-dimensional magnetic resonance imaging studies were undertaken, which revealed that chronic lithium significantly increases total gray matter volume in the human brain of patients with manic-depressive illness. Together, these results suggest that a reconceptualization about the optimal long-term treatment of recurrent mood disorders is warranted. Optimal long-term treatment for these severe illnesses may only be achieved by the early use of agents with neurotrophic/neuroprotective effects, irrespective of the primary, symptomatic treatment.

Neuronal plasticity and survival in mood disorders

Studies at the basic and clinical levels demonstrate that neuronal atrophy and cell death occur in response to stress and in the brains of depressed patients. Although the mechanisms have yet to be fully elucidated, progress has been made in characterizing the signal transduction cascades that control neuronal atrophy and programmed cell death and that may be involved in the action of antidepressant treatment. These pathways include the cyclic adenosine monophosphate and neurotrophic factor signal transduction cascades. It is notable that these same pathways have been demonstrated to play a pivotal role in cellular models of neural plasticity. This overlap of plasticity and cell survival pathways, together with studies demonstrating that neuronal activity enhances cell survival, suggests that neuronal atrophy and death could result from a disruption of the mechanisms underlying neural plasticity. The role of these pathways and failure of neuronal plasticity in stress-related mood disorders are discussed.

Enhancement of hippocampal neurogenesis by lithium

Increasing evidence suggests that mood disorders are associated with a reduction in regional CNS volume and neuronal and glial cell atrophy or loss. Lithium, a mainstay in the treatment of mood disorders, has recently been demonstrated to robustly increase the levels of the cytoprotective B-cell lymphoma protein-2 (bcl-2) in areas of rodent brain and in cultured cells. In view of bcl-2's antiapoptotic and neurotrophic effects, the present study was undertaken to determine if lithium affects neurogenesis in the adult rodent hippocampus. Mice were chronically treated with lithium, and 5-bromo-2-deoxyuridine (BrdU) labeling of dividing cells was conducted over 12 days. Immunohistochemical analysis was undertaken 1 day after the last injection, and three-dimensional stereological cell counting revealed that lithium produced a significant 25% increase in the BrdU-labeled cells in the dentate gyrus. Double-labeling immunofluorescence studies were undertaken to co-localize BrdU-positive cells with neuron-specific nuclear protein and showed that approximately 65% of the cells were double-labeled. These results add to the growing body of evidence suggesting that mood stabilizers and antidepressants exert neurotrophic effects and may therefore be of use in the long-term treatment of other neuropsychiatric disorders.

Capsaicin effects on brain-derived neurotrophic factor in rat dorsal root ganglia and spinal cord

The effects of capsaicin systemically administered in adult rats, with the major focus on the expression of brain-derived neurotropic factor (BDNF) and its mRNA in the dorsal root ganglion (DRG) and spinal cord, has been investigated by means of immunohistochemistry and reverse transcriptase-polymerase chain reactions. The percentage of BDNF-immunoreactive neurons in the L5 DRG was found to increase significantly 1 day after capsaicin injection. Subsequently, it decreased slowly returning to near normal levels 1 week later. Four weeks post-injection, a significant reduction to below normal levels was observed. The temporal pattern of BDNF mRNA expression in the DRG was similar to BDNF-immunoreactivity. In the spinal cord, 1 and 3 days post-injection, no changes in the expression of the BDNF-immunoreactive axonal fibers was noted. However, the expression had decreased significantly after 1 and 4 weeks. The mechanism by which capsaicin induces changes in expression of BDNF in DRG neurons and the functional significance of the rapid increase in BDNF levels in the DRG is discussed briefly.

Impaired synaptic plasticity and cAMP response element-binding protein activation in Ca2+/calmodulin-dependent protein kinase type IV/Gr-deficient mice

The Ca(2+)/calmodulin-dependent protein kinase type IV/Gr (CaMKIV/Gr) is a key effector of neuronal Ca(2+) signaling; its function was analyzed by targeted gene disruption in mice. CaMKIV/Gr-deficient mice exhibited impaired neuronal cAMP-responsive element binding protein (CREB) phosphorylation and Ca(2+)/CREB-dependent gene expression. They were also deficient in two forms of synaptic plasticity: long-term potentiation (LTP) in hippocampal CA1 neurons and a late phase of long-term depression in cerebellar Purkinje neurons. However, despite impaired LTP and CREB activation, CaMKIV/Gr-deficient mice exhibited no obvious deficits in spatial learning and memory. These results support an important role for CaMKIV/Gr in Ca(2+)-regulated neuronal gene transcription and synaptic plasticity and suggest that the contribution of other signaling pathways may spare spatial memory of CaMKIV/Gr-deficient mice.

Spatial considerations for stimulus-dependent transcription in neurons

Most neurons have elaborate dendrites as well as an axon emanating from the cell body that form synaptic connections with one or many target cells, which may be located a considerable distance from the cell body. Such complex and impressive morphologies allow some types of neurons to integrate inputs from one to many thousands of pre-synaptic partners and to rapidly propagate electrical signals, often over long distances, to post-synaptic target cells. Much slower, non-electrical signals also propagate from dendrites and distal axons to neuronal nuclei that influence survival, growth, and plasticity. The distances between distal dendrites and/or distal axons and cell bodies of neurons can be hundreds of microns to more than one meter. This long-range biochemical signal propagation from distal dendrites and distal axons to neuronal nuclei is entirely unique to neurons. This review is focused on excitatory neurotransmitter signaling from dendritic synapses to neuronal nuclei as well as on retrograde growth factor signaling from distal axons to neuronal nuclei.

The relationship between neuronal survival and regeneration

The ability of peripheral nervous system (PNS) but not central nervous system (CNS) neurons to regenerate their axons is a striking peculiarity of higher vertebrates. Much research has focused on the inhibitory signals produced by CNS glia that thwart regenerating axons. Less attention has been paid to the injury-induced loss of trophic stimuli needed to promote the survival and regeneration of axotomized neurons. Could differences in the mechanisms that control CNS and PNS neuronal survival and growth also contribute to the disparity in regenerative capacity? Here we review recent studies concerning the nature of the signals necessary to promote neuronal survival and growth, with an emphasis on their significance to regeneration after CNS injury.

Chronic ethanol exposure attenuates the anti-apoptotic effect of NMDA in cerebellar granule neurons

Ethanol, added to primary cultures of cerebellar granule neurons simultaneously with NMDA, was previously shown to inhibit the anti-apoptotic effect of NMDA. The in vitro anti-apoptotic effect of NMDA is believed to mimic in vivo protection against apoptosis afforded by innervation of developing cerebellar granule neurons by glutamatergic mossy fibers. Therefore, the results suggested that the presence of ethanol in the brain at a critical period of development would promote apoptosis. In the present studies, we examined the effect of chronic ethanol exposure on the anti-apoptotic action of NMDA in cerebellar granule neurons. The neurons were treated with ethanol in vitro for 1-3 days in the absence of NMDA. Even after ethanol was removed from the culture medium, as ascertained by gas chromatography, the protective effect of added NMDA was significantly attenuated. The decreased anti-apoptotic effect of NMDA was associated with a change in the properties of the NMDA receptor, as indicated by a decrease in ligand binding, decreased expression of NMDA receptor subunit proteins, and decreased functional responses including stimulation of increases in intracellular Ca(2+) and induction of brain-derived neurotrophic factor expression. The latter effect may directly underlie the attenuated protective effect of NMDA in these neurons. The results suggest that ethanol exposure during development can have long-lasting effects on neuronal survival. The change in the NMDA receptor caused by chronic ethanol treatment may contribute to the loss of cerebellar granule neurons that is observed in animals and humans exposed to ethanol during gestation.

Transcription-dependent neuronal plasticity: The nuclear calcium hypothesis

In neurons, calcium ions control gene transcription induced by synaptic activity. The states and histories of neuronal activity are represented by a calcium code that comprises the site of calcium entry, and the amplitude, duration and spatial properties of signal-evoked calcium transients. The calcium code is used to transform specific firing patterns into qualitatively and quantitatively distinct transcriptional responses. The following hypothesis is proposed: electrical activity causes long-lasting, transcription-dependent changes in neuronal functions when synaptically evoked calcium transients associated with the stimulation propagate to the nucleus; gene transcription activated by dendritic calcium signals only is insufficient to consolidate functional alterations long-term. Similar to enduring increases in synaptic efficacy, nuclear calcium transients are induced by high-frequency firing patterns or by weak synaptic inputs coinciding with backpropagating dendritic action potentials. Nuclear calcium stimulates CREB-mediated transcription and, through inducing the activity of the transcriptional coactivator CREB-binding protein (CBP), may modulate the expression of numerous genes including neurotransmitter receptors and scaffolding proteins. Increases in the transcription rate of target genes are predicted to be transient and in many cases small, however, they collectively contribute to the maintenance of changes in synaptic efficacy. Nuclear calcium may be the common regulator of diverse transcription-dependent forms of neuronal plasticity.

Protein phosphorylation cascades associated with methamphetamine-induced glial activation

Reactive gliosis is the most prominent response to diverse forms of central nervous system (CNS) injury. The signaling events that mediate this characteristic response to neural injury are under intense investigation. Several studies have demonstrated the activation of phosphoproteins within the mitogen-activated protein kinase (MAPK) and Janus kinase (JAK) pathways following neural insult. These signaling pathways may be involved or responsible for the glial response following injury, by virtue of their ability to phosphorylate and dynamically regulate the activity of various transcription factors. This study sought to delineate, in vivo, the relative contribution of MAPK- and JAK-signaling components to reactive gliosis as measured by induction of glial-fibrillary acidic protein (GFAP), following chemical-induced neural damage. At time points (6, 24, and 48 h) following methamphetamine (METH, 10 mg/kg x 4, s.c.) administration, female C57BL/6J mice were sacrificed by focused microwave irradiation, a technique that preserves steady-state phosphorylation. Striatal (target) and nontarget (hippocampus) homogenates were assayed for METH-induced changes in markers of dopamine (DA) neuron integrity as well as differences in the levels of activated phosphoproteins. GFAP upregulation occurred as early as 6 h, reaching a threefold induction 48 h following METH exposure. Neurotoxicant-induced reductions in striatal levels of DA and tyrosine hydroxylase (TH) paralleled the temporal profile of GFAP induction. Blots of striatal homogenates, probed with phosphorylation-state specific antibodies, demonstrated significant changes in activated forms of extracellular-regulated kinase 1/2 (ERK 1/2), c-jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK), MAPK/ERK kinase (MEK1/2), 70-kDa ribosomal S6 kinase (p70 S6), cAMP responsive element binding protein (CREB), and signal transducer and activator of transcription 3 (STAT3). MAPK-related phosphoproteins exhibited an activation profile that peaked at 6 h, remained significantly increased at 24, and fell to baseline levels 48 h following neurotoxicant treatment. The ribosomal S6 kinase was enhanced over 60% for all time points examined. Immunoreactivity profiles for the transcription factors CREB and STAT3 indicated maximal increases in phosphorylation occurring at 24 h, and measuring greater than 2- or 17-fold, respectively. Specific signaling events were found to occur with a time course suggestive of their involvement in the gliotic response. The toxicant-induced activation of these growth-associated signaling cascades suggests that these pathways could be obligatory for the triggering and/or persistence of reactive gliosis and may therefore serve as potential targets for modulation of glial response to neural damage.

Effects of blockade of voltage-sensitive Ca(2+)/Na(+) channels by a novel phenylpyrimidine derivative, NS-7, on CREB phosphorylation in focal cerebral ischemia in the rat

NS-7 is a novel blocker of voltage-sensitive Ca(2+) and Na(+) channels, and it significantly reduces infarct size after occlusion of the middle cerebral artery. Persistent activation of cyclic AMP response element binding protein (CREB), which can be induced by increase in intracellular Ca(2+) concentrations or other second messengers, has recently been found to be closely associated with neuronal survival in cerebral ischemia. The present study was therefore undertaken to evaluate the neuroprotective effects of NS-7 by analyzing changes in CREB phosphorylation in a focal cerebral ischemia model. CREB phosphorylation in the brain of rats was investigated immunohistochemically at 3.5-48-h recirculation after 1. 5-h occlusion of the middle cerebral artery. NS-7 (1 mg/kg; NS-7 group) or saline (saline group) was intravenously injected 5 min after the start of recirculation. The NS-7 group showed significantly milder activation of CREB phosphorylation in various cortical regions after 3.5 h of recirculation than the saline group. The inner border zone of ischemia in the NS-7 group subsequently exhibited a moderate, but persistent, increase in number of phosphorylated CREB-positive neurons with no apparent histological damage. By contrast, the saline group displayed a marked, but only transient, increase in number of immunopositive neurons in this border zone after 3.5 h of recirculation, and this was followed by clear suppression of CREB phosphorylation and subsequent loss of normal neurons. These findings suggest that: (1) the marked enhancement of CREB phosphorylation in the acute post-ischemic phase may be triggered largely by an influx of calcium ions as a result of activation of the voltage-sensitive Ca(2+) and Na(+) channels; and that (2) the neuroprotective effects of NS-7 may be accompanied by persistent activation of CREB phosphorylation in the inner border zone of ischemia.

Real-time analysis of preprotachykinin promoter activity in single cortical neurons

Technological limitations have hindered the study of gene elements regulating transcription within CNS neurons. In the present stuides, rat cortical brain slices endogenously expressing the preprotachykinin (PPT) gene were transfected with gene constructs encompassing green fluorescent protein (GFP) under the control of the PPT promoter. These slices were maintained in organotypic culture so that the fluorescence intensity within individual living cells could be quantified using laser scanning confocal microscopy before and after application of stimulatory agents. Combined treatment with forskolin and elevated potassium significantly increased expression of both endogenous PPT mRNA and the PPT promoter-GFP construct. The ability to follow fluorescence changes within single neurons in real time offers a powerful "within-subject" experimental approach for analysis of neural gene promoters.

Gluco- and mineralocorticoid receptor-mediated regulation of neurotrophic factor gene expression in the dorsal hippocampus and the neocortex of the rat

Gluco- and mineralocorticoid receptors (GR and MR) act via common promoter elements but may exert different effects on gene regulation in various regions of the forebrain. In order to separately analyse the role of GR and MR in the regulation of neurotrophic factor genes and their receptors, we used adrenalectomy and subsequent hormone injections in the rat as a model system. Twenty-four hours after adrenalectomy rats were injected with a single dose of corticosterone (2 and 10 mg/kg), aldosterone (0.5 mg/kg) or the synthetic glucocorticoid agonist RU 28362 (4 mg/kg). Gene expression of basic fibroblast growth factor (bFGF) and its high-affinity receptors [fibroblast growth factor receptor subtypes 1-3 (FGF-R1, FGF-R2, FGF-R3)], as well as brain-derived growth factor (BDNF) and neurotrophin-3 (NT-3) was analysed at 4 h after the hormone injection in CA1-CA4 (cornus of Ammon areas of the hippocampus) and dentate gyrus of the dorsal hippocampus and in neocortex by means of in situ hybridization. We found that bFGF is regulated in CA2, CA3 and dentate gyrus by GR and MR together, and in CA1, CA4 and neocortex by GR alone. FGF-R2 expression in the hippocampus seems to be regulated only by MR, while BDNF expression appears to depend on both receptors. FGF-R1, FGF-R3 and NT-3 were only moderately affected by the hormone activation of GR and MR acting in concert or alone in the various regions. Thus, the present findings suggest that the adrenal cortical system through GR and MR participate in the control of neurotrophic factor signalling in a highly subregion- and cellular-dependent manner.

Neural activity and survival in the developing nervous system

Recent evidence suggests that blockade of normal excitation in the immature nervous system may have profound effects on neuronal survival during the period of natural cell death. Cell loss following depression of electrical activity in the central nervous system (CNS) may explain the neuropsychiatric deficits in humans exposed to alcohol or other CNS depressants during development. Thus, understanding the role of electrical activity in the survival of young neurons is an important goal of modern basic and clinical neuroscience. Here we review the evidence from in vivo and in vitro model systems that electrical activity participates in promoting neuronal survival. We discuss the potential role of moderate elevations of intracellular calcium in promoting survival, and we address the possible ways in which activity and conventional trophic factors may interact.

Brain-derived neurotrophic factor and trkB are essential for cAMP-mediated induction of the serotonergic neuronal phenotype

Serotonergic neurons in the central nervous system are crucial in the control of autonomic functions and behavior. Mechanisms by which development and maintenance of the serotonergic transmitter phenotype is regulated include activation of protein kinase A (PKA). Using cultures established from the E14 rat raphe we show here that forskolin (10 microM) increases numbers of neurons expressing tryptophan hydroxylase (TpOH), the key enzyme of serotonin synthesis, and uptake of the false serotonergic transmitter 5, 7-dihydroxytryptamine (5,7-DHT). As shown by short-term treatments the effect is due to phenotype induction rather than survival. To begin to understand downstream or parallel signaling pathways required for the PKA-mediated induction of serotonergic markers, we have studied the putative implication of brain-derived neurotrophic factor (BDNF) and its receptor trkB. Treatment of raphe neurons with forskolin induced BDNF mRNA assayed by competitive RT-PCR. Moreover, trkB-IgG receptor bodies fully prevented the forskolin-induced numerical increase in TpOH- and 5,7-DHT-positive cells suggesting an implication of a TrkB-activated pathway. TrkC-IgG had no effect. K252b, a specific inhibitor of trk kinase activity likewise abolished the induction of serotonergic markers by forskolin. In turn, the inductive effect of BDNF on serotonergic markers was blocked by KT5720, a specific inhibitor of PKA. Taken together, these data suggest that co-activation of cAMP- and trkB-dependent signaling pathways plays a crucial role in the regulation of the serotonergic neuronal phenotype.

Traumatic brain injury alters the molecular fingerprint of TUNEL-positive cortical neurons In vivo: A single-cell analysis

The cerebral cortex is selectively vulnerable to cell death after traumatic brain injury (TBI). We hypothesized that the ratio of mRNAs encoding proteins important for cell survival and/or cell death is altered in individual damaged neurons after injury that may contribute to the cell's fate. To investigate this possibility, we used amplified antisense mRNA (aRNA) amplification to examine the relative abundance of 31 selected candidate mRNAs in individual cortical neurons with fragmented DNA at 12 or 24 hr after lateral fluid percussion brain injury in anesthetized rats. Only pyramidal neurons characterized by nuclear terminal deoxynucleotidyl transferase-mediated biotinylated dUTP nick end labeling (TUNEL) reactivity with little cytoplasmic staining were analyzed. For controls, non-TUNEL-positive neurons from the cortex of sham-injured animals were obtained and subjected to aRNA amplification. At 12 hr after injury, injured neurons exhibited a decrease in the relative abundance of specific mRNAs including those encoding for endogenous neuroprotective proteins. By 24 hr after injury, many of the mRNAs altered at 12 hr after injury had returned to baseline (sham-injured) levels except for increases in caspase-2 and bax mRNAs. These data suggest that TBI induces a temporal and selective alteration in the gene expression profiles or "molecular fingerprints" of TUNEL-positive neurons in the cerebral cortex. These patterns of gene expression may provide information about the molecular basis of cell death in this region after TBI and may suggest multiple avenues for therapeutic intervention.

Sustained activation of hippocampal Lp-type voltage-gated calcium channels by tetanic stimulation

The molecular heterogeneity of voltage-gated calcium channels is mirrored by extensive biophysical diversity. Subtype-selective antagonists have been used to place different kinds of calcium channels in functional categories. Dihydropyridine (DHP) antagonists have been used, for example, to implicate L-type calcium channels in the induction of NMDA receptor-independent forms of synaptic plasticity. DHPs, however, do not discriminate between the recently identified Lp and Ls subtypes of L-type calcium channel. The different properties of the two kinds of L-type channels suggest that they may have different functional roles. Ls channels are comparable with cardiac L-type channels, whereas Lp channels show low-threshold voltage-dependent potentiation. To clarify the potential roles of Lp and Ls channels in the induction of synaptic plasticity, we studied the responses of these channels to trains of action potentials. The frequency and duration of the trains were chosen to mimic the stimuli used to induce changes in synaptic strength. Cell-attached single-channel recordings from cultured hippocampal neurons revealed that both Lp and Ls channels responded to these trains, but only Lp channels showed persistent activation that outlasted the train. The magnitude of Lp channel activity increased with increasing action potential frequency and train duration. Stimuli that reproduced the postsynaptic response to action potential trains were also examined, and Lp channels were found to show much greater responses than were Ls channels. These results suggest that the Lp channel may play a critical role in the induction of long-lasting changes in synaptic strength.

Dynamic regulation of neuronal NO synthase transcription by calcium influx through a CREB family transcription factor-dependent mechanism

Neuronal nitric oxide (NO) synthase (nNOS) is dynamically regulated in response to a variety of physiologic and pathologic stimuli. Although the dynamic regulation of nNOS is well established, the molecular mechanisms by which such diverse stimuli regulate nNOS expression have not yet been identified. We describe experiments demonstrating that Ca(2+) entry through voltage-sensitive Ca(2+) channels regulates nNOS expression through alternate promoter usage in cortical neurons and that nNOS exon 2 contains the regulatory sequences that respond to Ca(2+). Deletion and mutational analysis of the nNOS exon 2 promoter reveals two critical cAMP/Ca(2+) response elements (CREs) that are immediately upstream of the transcription start site. CREB binds to the CREs within the nNOS gene. Mutation of the nNOS CREs as well as blockade of CREB function results in a dramatic loss of nNOS transcription. These findings suggest that nNOS is a Ca(2+)-regulated gene through the interactions of CREB on the CREs within the nNOS exon 2 promoter and that these interactions are likely to be centrally involved in the regulation of nNOS in response to neuronal injury and activity-dependent plasticity.

A relationship between behavior, neurotrophin expression, and new neuron survival

The high vocal center (HVC) controls song production in songbirds and sends a projection to the robust nucleus of the archistriatum (RA) of the descending vocal pathway. HVC receives new neurons in adulthood. Most of the new neurons project to RA and replace other neurons of the same kind. We show here that singing enhances mRNA and protein expression of brain-derived neurotrophic factor (BDNF) in the HVC of adult male canaries, Serinus canaria. The increased BDNF expression is proportional to the number of songs produced per unit time. Singing-induced BDNF expression in HVC occurs mainly in the RA-projecting neurons. Neuronal survival was compared among birds that did or did not sing during days 31-38 after BrdUrd injection. Survival of new HVC neurons is greater in the singing birds than in the nonsinging birds. A positive causal link between pathway use, neurotrophin expression, and new neuron survival may be common among systems that recruit new neurons in adulthood.

CREB: a stimulus-induced transcription factor activated by a diverse array of extracellular signals

Extracellular stimuli elicit changes in gene expression in target cells by activating intracellular protein kinase cascades that phosphorylate transcription factors within the nucleus. One of the best characterized stimulus-induced transcription factors, cyclic AMP response element (CRE)-binding protein (CREB), activates transcription of target genes in response to a diverse array of stimuli, including peptide hormones, growth factors, and neuronal activity, that activate a variety of protein kinases including protein kinase A (PKA), pp90 ribosomal S6 kinase (pp90RSK), and Ca2+/calmodulin-dependent protein kinases (CaMKs)[corrected]. These kinases all phosphorylate CREB at a particular residue, serine 133 (Ser133), and phosphorylation of Ser133 is required for CREB-mediated transcription. Despite this common feature, the mechanism by which CREB activates transcription varies depending on the stimulus. In some cases, signaling pathways target additional sites on CREB or proteins associated with CREB, permitting CREB to regulate distinct programs of gene expression under different conditions of stimulation. This review discusses the molecular mechanisms by which Ser133-phosphorylated CREB activates transcription, intracellular signaling pathways that lead to phosphorylation of CREB at Ser133, and features of each signaling pathway that impart specificity at the level of CREB activation.

Congenital deafness and sinoatrial node dysfunction in mice lacking class D L-type Ca2+ channels

Voltage-gated L-type Ca2+ channels (LTCCs) containing a pore-forming alpha1D subunit (D-LTCCs) are expressed in neurons and neuroendocrine cells. Their relative contribution to total L-type Ca2+ currents and their physiological role and significance as a drug target remain unknown. Therefore, we generated D-LTCC deficient mice (alpha1D-/-) that were viable with no major disturbances of glucose metabolism. alpha1D-/-mice were deaf due to the complete absence of L-type currents in cochlear inner hair cells and degeneration of outer and inner hair cells. In wild-type controls, D-LTCC-mediated currents showed low activation thresholds and slow inactivation kinetics. Electrocardiogram recordings revealed sinoatrial node dysfunction (bradycardia and arrhythmia) in alpha1D-/- mice. We conclude that alpha1D can form LTCCs with negative activation thresholds essential for normal auditory function and control of cardiac pacemaker activity.

BDNF-Induced potentiation of spontaneous twitching in innervated myocytes requires calcium release from intracellular stores

Brain-derived neurotrophic factor (BDNF) can potentiate synaptic release at newly developed frog neuromuscular junctions. Although this potentiation depends on extracellular Ca(2+) and reflects changes in acetylcholine release, little is known about the intracellular transduction or calcium signaling pathways. We have developed a video assay for neurotrophin-induced potentiation of myocyte twitching as a measure of potentiation of synaptic activity. We use this assay to show that BDNF-induced synaptic potentiation is not blocked by cadmium, indicating that Ca(2+) influx through voltage-gated Ca(2+) channels is not required. TrkB autophosphorylation is not blocked in Ca(2+)-free conditions, indicating that TrkB activity is not Ca(2+) dependent. Additionally, an inhibitor of phospholipase C interferes with BDNF-induced potentiation. These results suggest that activation of the TrkB receptor activates phospholipase C to initiate intracellular Ca(2+) release from stores which subsequently potentiates transmitter release.

Potassium depolarization-induced cAMP stimulates somatostatin mRNA levels in cultured diencephalic neurons

In the nervous system, signals transmitted across synapses are known to regulate gene expression in the postsynaptic cells. This process often involves membrane depolarization and subsequent elevation of intracellular Ca(2+). We have previously demonstrated in fetal cerebrocortical cells, that somatostatin (SS) mRNA levels can be induced by depolarizing agents such as high potassium concentrations and veratridine (VTD), and that these effects are calcium dependent. SS expression is regulated by cAMP, and in the cerebral cortex adenylate cyclase activity is regulated through fluctuations in intracellular Ca(2+) concentrations. The present experiments were undertaken to determine the mechanism by which calcium upregulates the levels of SS mRNA. Cerebrocortical cells from 17-day-old fetuses were exposed to the different agents for 24 h and the levels of SS mRNA were measured by Northern blot. Incubation of cells with the calcium channel antagonist nifedipine (Nf), the calcium chelating agent EGTA, calcium free KRB and the calcium calmodulin inhibitors trifluoroperazine (TFP) and the napthelene sulfonamide, W7, resulted in the inhibition of K(+)-induced SS mRNA levels. K(+)-evoked depolarization increased the intracellular concentration of cAMP and this effect was antagonized by verapamil (VPM). Forskolin (Fk) provoked a higher increment in cAMP concentration than potassium, although the induction of SS mRNA was more evident following K(+) depolarization indicating a lack of correlation between levels of cAMP and induction of SS mRNA. The role of K(+)-induced cAMP on the increment of SS mRNA that occurred upon membrane depolarization was further explored with the inhibitor of protein kinase A (PKA), Rp cAMP whose presence significantly reduced depolarization-induced SS mRNA levels. This study confirms that Ca(2+) influx is required for K(+)depolarization-induced stimulation of cAMP whereby the increment of SS mRNA is partly produced.

cAMP response element-mediated gene transcription is upregulated by chronic antidepressant treatment

Regulation of gene transcription via the cAMP-mediated second messenger pathway has been implicated in the actions of antidepressant drugs, but studies to date have not demonstrated such an effect in vivo. To directly study the regulation of cAMP response element (CRE)-mediated gene transcription by antidepressants, transgenic mice with a CRE-LacZ reporter gene construct were administered one of three different classes of antidepressants: a norepinephrine selective reuptake inhibitor (desipramine), a serotonin selective reuptake inhibitor (fluoxetine), or a monoamine oxidase inhibitor (tranylcypromine). Chronic, but not acute, administration of these antidepressants significantly increased CRE-mediated gene transcription, as well as the phosphorylation of CRE binding protein (CREB), in several limbic brain regions thought to mediate the action of antidepressants, including the cerebral cortex, hippocampus, amygdala, and hypothalamus. These results demonstrate that chronic antidepressant treatment induces CRE-mediated gene expression in a neuroanatomically differentiated pattern and further elucidate the molecular mechanisms underlying the actions of these widely used therapeutic agents.

Differential activation of brain-derived neurotrophic factor gene promoters I and III by Ca2+ signals evoked via L-type voltage-dependent and N-methyl-D-aspartate receptor Ca2+ channels

Although the brain-derived neurotrophic factor (BDNF) gene is activated by the intracellular Ca(2+) signals evoked via Ca(2+) influx into neurons, little is known about how the activation of alternative BDNF gene promoters is controlled by the Ca(2+) signals evoked via N-methyl-d-aspartate receptors (NMDA-R) and L-type voltage-dependent Ca(2+) channels (L-VDCC). There is a critical range in the membrane depolarization caused by high K(+) concentrations (25-50 mm KCl) for effective BDNF mRNA expression and transcriptional activation of BDNF gene promoters I and III (BDNF-PI and -PIII, respectively) in rat cortical culture. The increase in BDNF mRNA expression induced at high K(+) was repressed not only by nicardipine, an antagonist for L-VDCC, but also by dl-amino-5-phosphonovalerate, an antagonist for NMDA-R, which was supported by the effects of antagonists on the Ca(2+) influx. Although the promoter activations at 25 and 50 mm KCl were different, BDNF-PIII was activated by either the Ca(2+) influx through NMDA-R or L-VDCC, whereas BDNF-PI was predominantly by the Ca(2+) influx through L-VDCC. Direct stimulation of NMDA-R supported the activation of BDNF-PIII but not that of BDNF-PI. Thus, the alternative BDNF gene promoters responded differently to the intracellular Ca(2+) signals evoked via NMDA-R and L-VDCC.

Rapid and selective induction of BDNF expression in the hippocampus during contextual learning

The hippocampus is required for many forms of long-term memory in both humans and animals, and formation of long-lasting memories requires the synthesis of new proteins. Furthermore, the long-term potentiation (LTP) of hippocampal synapses, a widely studied model of memory, also depends on both de novo gene transcription and protein synthesis and results in the activation of transcription from promotors containing the cAMP response element (CRE). Expression of several genes is induced during the establishment of LTP; these include the immediate-early genes (IEGs) BDNF (brain-derived neurotrophic factor), zif268 and C/EBPbeta (CCAAT-enhancer binding protein beta), all of which contain CRE sites within their promotor regions. However, these genes induced by LTP are not known to be rapidly induced following learning in a natural setting. Here we demonstrate rapid and selective induction of BDNF expression during hippocampus-dependent contextual learning.

Developmentally regulated NMDA receptor-dependent dephosphorylation of cAMP response element-binding protein (CREB) in hippocampal neurons

Developmental changes in the signaling properties of NMDA receptors have been proposed to underlie the loss of plasticity that accompanies brain maturation. Calcium influx through postsynaptic NMDA receptors can stimulate neuronal gene expression via signaling pathways such as the Ras-MAP kinase (MAPK) pathway and the transcription factor cAMP response element-binding protein (CREB). We analyzed MAPK (Erk1/2) and CREB activation in response to NMDA receptor stimulation during the development of hippocampal neurons in culture. At all stages of development NMDA stimulation induced a rapid phosphorylation of CREB on Ser-133 (phospho-CREB). However, the time course of decline in phospho-CREB changed dramatically with neuronal maturation. At 7 d in vitro (7 DIV) phospho-CREB remained elevated 2 hr after strong NMDA stimulation, whereas at 14 DIV phospho-CREB rose only transiently and fell back to below basal levels within 30 min. Moreover, at 14 DIV, but not at 7 DIV, NMDA receptor stimulation induced a dephosphorylation of CREB that previously had been phosphorylated by KCl depolarization or forskolin, suggesting an NMDA receptor-dependent activation of a CREB phosphatase. There was no developmental change in the time course of phospho-CREB induction that followed KCl depolarization or PKA activation, nor was there a developmental change in the time course of phospho-Erk1/2 induced by NMDA receptor activation. We suggest that, during neuronal maturation, NMDA receptor activation becomes linked specifically to protein phosphatases that act on Ser-133 of CREB. Such a developmentally regulated switch in the mode of NMDA receptor coupling to intracellular signaling pathways may contribute to the changes in neural plasticity observed during brain development.

Brief electrical stimulation promotes the speed and accuracy of motor axonal regeneration

Functional recovery is often poor despite the capacity for axonal regeneration in the peripheral nervous system and advances in microsurgical technique. Regeneration of axons in mixed nerve into inappropriate pathways is a major contributing factor to this failure. In this study, we use the rat femoral nerve model of transection and surgical repair to evaluate (1) the effect of nerve transection on the speed of regeneration and the generation of motor-sensory specificity, (2) the efficacy of electrical stimulation in accelerating axonal regeneration and promoting the reinnervation of appropriate muscle pathways by femoral motor nerves, and (3) the mechanism of action of electrical stimulation. Using the retrograde neurotracers fluorogold and fluororuby to backlabel motoneurons that regenerate axons into muscle and cutaneous pathways, we found the following. (1) There is a very protracted period (10 weeks) of axonal outgrowth that adds substantially to the delay in axonal regeneration (staggered regeneration). This process of staggered regeneration is associated with preferential motor reinnervation (PMR). (2) One hour to 2 weeks of 20 Hz continuous electrical stimulation of the parent axons proximal to the repair site dramatically reduces this period (to 3 weeks) and accelerates PMR. (3) The positive effect of short-term electrical stimulation is mediated via the cell body, implicating an enhanced growth program. The effectiveness of such a short-period low-frequency electrical stimulation suggests a new therapeutic approach to accelerate nerve regeneration after injury and, in turn, improve functional recovery.

Increase of brain-derived neurotrophic factor gene expression in NG108-15 cells by the nuclear isoforms of Ca2+/ calmodulin-dependent protein kinase II

We have reported that the delta3 isoform of Ca2+/ calmodulin-dependent protein kinase II (CaM kinase II) is abundant in the nucleus in cerebellar granule cells. To examine the possibility that the nuclear isoforms of CaM kinase II are involved in the expression of brain-derived neurotrophic factor (BDNF), we transiently overexpressed the delta3 isoform in NG108-15 cells. The quantitative RT-PCR analysis revealed that rat cerebellum and NG108-15 cells expressed the exon IV-containing mRNA of BDNF (exon IV-BDNF mRNA) more than the exon III-BDNF mRNA. Treatment of NG108-15 cells with Bay K 8644 increased both exon III- and exon IV-BDNF mRNAs, and overexpression of the 83 isoform potentiated the expression of the exon IV-BDNF mRNA. The potentiation was not observed in the cells that were overexpressed with either the 61 isoform, a nonnuclear isoform, or the inactive mutant of the delta3 isoform. We constructed the luciferase reporter gene following the promoter upstream of exon IV and confirmed that overexpression of the delta3 isoform increased luciferase gene expression. Double-immunostaining of NG108-15 cells with the antibodies to CaM kinase II and BDNF clearly showed that BDNF was highly expressed in the cells that were overexpressed with the delta3 isoform or the alphaB isoform, another nuclear isoform of CaM kinase II. These results suggest that the nuclear isoforms of CaM kinase II are involved in the expression of BDNF.

Corticosterone effects on BDNF expression in the hippocampus. Implications for memory formation

The adrenal steroid corticosterone has profound effect on the structure and function of the hippocampus. Probably as a result of that, it modulates memory formation. In this review, the question is addressed if the corticosterone effects on memory processes are mediated by alterations in the expression of the neurotrophin Brain-Derived Neurotrophic Factor (BDNF) in the hippocampus. First, studies are described investigating the effect of corticosterone on BDNF expression in the rat hippocampus. It appears that corticosterone suppresses the BDNF expression at the mRNA and protein level in a subfield-specific way. Second, a model for the mechanism of action is proposed. In this model, activated mineralocorticoid and glucocorticoid receptors repress transcriptional activity of the BDNF promoter site-specifically via interaction with other transcription factors. Third, the implications for learning and memory are discussed. Studies show that during water maze training, corticosterone levels rise significantly, but the BDNF expression is not suppressed in any hippocampal subfield. Furthermore, high BDNF expression levels in specific subfields correlate with a good memory performance. Therefore, we suggest that the resistance of the hippocampal BDNF expression to suppression by corticosterone, as seen after water maze training, may contribute to an optimal memory performance.

Is CREB a key to neuronal survival?

A range of molecules control nerve-cell survival in the brain. Many of these molecules might be neuroprotective through activation of the transcription factor cAMP-response-element-binding protein (CREB). Activation of CREB, by phosphorylation of Ser133, occurs in brain-damage-resistant hippocampal dentate granule cells and is triggered by neuroprotective environmental stimulation. In addition, the Akt neuroprotective signaling pathway activates CREB, and CREB synthesis and phosphorylation promote the survival of many cells, including neurons, in vitro. Thus, CREB might be responsible for programmed nerve-cell survival. Studies investigating its role in the brain are now required to confirm these in vitro results, and the downstream survival genes, whose expression is activated by CREB in neurons, need to be identified.

Persistent CREB phosphorylation with protection of hippocampal CA1 pyramidal neurons following temporary occlusion of the middle cerebral artery in the rat

Phosphorylation of the DNA-binding transcription factor, cyclic AMP response element binding protein (CREB), was immunohistochemically examined in rat brain hippocampal CA1 in order to examine the ischemic vulnerability of this region from the viewpoint of CREB activation. The rat brain had been subjected to 90-min focal ischemia followed by various periods of recirculation. Focal ischemia was induced by occlusion of the middle cerebral artery using the intraluminal suture method. CA1 pyramidal neurons in the sham animals showed definite immunoreactivity with anti-CREB antibody, which binds to both unphosphorylated and phosphorylated CREB, while reactivity with anti-phosphorylated CREB antibody was barely detectable in these neurons. In contrast, at 3.5 h of recirculation, a significant increase in the number of phosphorylated CREB-positive neurons was noted in the CA1 on both sides, and the increase continued until 48 h of recirculation with a tendency for gradual decline. At each period, the ischemic side showed a more marked increase in the number of immunoreactive cells as compared to the nonischemic side. Cresyl violet staining revealed CA1 pyramidal neurons to be maintained intact until 14 day of recirculation, at which time CREB phosphorylation has returned to the control level. Transient global ischemia is known to induce only mild CREB phosphorylation in the CA1 followed by a frank neuronal loss in this region. These data suggest that CREB phosphorylation can be persistently activated in CA1 neurons after focal ischemia and that this phenomenon may be closely associated with protection of these neurons.

Critical dependence of cAMP response element-binding protein phosphorylation on L-type calcium channels supports a selective response to EPSPs in preference to action potentials

Activity-dependent gene expression in neurons shows a remarkable ability to differentiate between different types of stimulation: orthodromic inputs that engage synaptic transmission are much more effective than antidromic stimuli that do not. We have studied the basis of such selectivity in cultured hippocampal neurons in which nuclear cAMP response element-binding protein (CREB) phosphorylation is induced by synaptic activity but not by action potential (AP) stimulation in the absence of EPSPs, although spikes by themselves generate large elevations in intracellular Ca(2+). Previous work has shown that Ca(2+) entry through L-type Ca(2+) channels plays a dominant role in triggering calmodulin mobilization and activation of calmodulin-dependent kinases that phosphorylate CREB, raising the possibility that L-type channels contribute to the selective response to EPSPs rather than APs. Accordingly, we performed voltage-clamp experiments to compare the currents carried by L-type channels during depolarizing waveforms that approximated APs or dendritic EPSPs. The integrated current generated by L-type channels was significantly less after mock APs than with EPSP-like depolarizations. The difference was traced to two distinct factors. Compared with other channels, L-type channels activated at relatively negative potentials, favoring their opening with EPSP stimulation; they also exhibited relatively slow activation kinetics, weighing against their contribution during an AP. The relative ineffectiveness of APs as a stimulus for CREB phosphorylation could be overcome by exposure to the agonist Bay K8644, which potentiated the AP-induced influx through L-type channels by approximately 10-fold. Under normal conditions, the unique biophysical properties of L-type channels allow them to act as a kinetic filter to support spike-EPSP discrimination.

Positive modulation of AMPA receptors increases neurotrophin expression by hippocampal and cortical neurons

This study investigated whether positive modulators of AMPA-type glutamate receptors influence neurotrophin expression by forebrain neurons. Treatments with the ampakine CX614 markedly and reversibly increased brain-derived neurotrophic factor (BDNF) mRNA and protein levels in cultured rat entorhinal/hippocampal slices. Acute effects of CX614 were dose dependent over the range in which the drug increased synchronous neuronal discharges; threshold concentrations for acute responses had large effects on mRNA content when applied for 3 d. Comparable results were obtained with a second, structurally distinct ampakine CX546. Ampakine-induced upregulation was broadly suppressed by AMPA, but not NMDA, receptor antagonists and by reducing transmitter release. Antagonism of L-type voltage-sensitive calcium channels blocked induction in entorhinal cortex but not hippocampus. Prolonged infusions of suprathreshold ampakine concentrations produced peak BDNF mRNA levels at 12 hr and a return to baseline levels by 48 hr. In contrast, BDNF protein remained elevated throughout a 48 hr incubation with the drug. Nerve growth factor mRNA levels also were increased by ampakines but with a much more rapid return to control levels during chronic administration. Finally, intraperitoneal injections of CX546 increased hippocampal BDNF mRNA levels in aged rats and middle-aged mice. The present results provide evidence of regional differences in mechanisms via which activity regulates neurotrophin expression. Moreover, these data establish that changes in synaptic potency produce sufficient network level physiological effects for inducing neurotrophin genes, indicate that the response becomes refractory during prolonged ampakine exposure, and raise the possibility of using positive AMPA modulators to regulate neurotrophin levels in aged brain.

Cell death in regenerating populations of neurons in BDNF mutant mice

There are two populations of neurons which are continually renewed in the adult, the dentate gyrus granule neurons and the olfactory bulb granule and periglomerular neurons. In the dentate gyrus, a secondary proliferative zone termed the subgranular zone is established along the interface between the dentate gyrus and the hilus where granule cells are born throughout life. Olfactory bulb neurons are generated in the anterior subventricular zone of the lateral ventricle and migrate via the rostral migratory stream to the olfactory bulb. We examined animals lacking brain-derived neurotrophic factor (BDNF) in order to establish whether this neurotrophin could be involved in the generation and/or survival of these neurons in vivo. We find that cells in nestin-positive regions of both the subgranular layer of the dentate gyrus and the subventricular zone of the olfactory bulb undergo apoptosis starting 2 weeks after birth in the absence of BDNF. However, increased apoptosis was not limited to precursors, as apoptotic cells were also found in the granule cell layer of the dentate gyrus and in the granule and periglomerular layers of the olfactory bulb. The excessive cell death was limited to these populations of neurons as no excessive cell death was detected in other forebrain areas. We conclude that BDNF is essential for the survival of neurons specifically in populations which are continuously being regenerated in the brain.

Mediation by a CREB family transcription factor of NGF-dependent survival of sympathetic neurons

Nerve growth factor (NGF) and other neurotrophins support survival of neurons through processes that are incompletely understood. The transcription factor CREB is a critical mediator of NGF-dependent gene expression, but whether CREB family transcription factors regulate expression of genes that contribute to NGF-dependent survival of sympathetic neurons is unknown. CREB-mediated gene expression was both necessary for NGF-dependent survival and sufficient on its own to promote survival of sympathetic neurons. Moreover, expression of Bcl-2 was activated by NGF and other neurotrophins by a CREB-dependent transcriptional mechanism. Overexpression of Bcl-2 reduced the death-promoting effects of CREB inhibition. Together, these data support a model in which neurotrophins promote survival of neurons, in part through a mechanism involving CREB family transcription factor-dependent expression of genes encoding prosurvival factors.