Brain-derived neurotrophic factor expression after long-term potentiation

The Induction of Long-Term Potentiation by Medial Septum Activation under Urethane Anesthesia Can Alter Gene Expression in the Hippocampus

We studied changes in the expression of early genes in hippocampal cells in response to stimulation of the dorsal medial septal area (dMSA), leading to long-term potentiation in the hippocampus. Rats under urethane anesthesia were implanted with stimulating electrodes in the ventral hippocampal commissure and dMSA and a recording electrode in the CA1 area of the hippocampus. We found that high-frequency stimulation (HFS) of the dMSA led to the induction of long-term potentiation in the synapses formed by the ventral hippocampal commissure on the hippocampal CA1 neurons. One hour after dMSA HFS, we collected the dorsal and ventral hippocampi on both the ipsilateral (damaged by the implanted electrode) and contralateral (intact) sides and analyzed the expression of genes by qPCR. The dMSA HFS led to an increase in the expression of bdnf and cyr61 in the ipsilateral hippocampi and egr1 in the ventral contralateral hippocampus. Thus, dMSA HFS under the conditions of degeneration of the cholinergic neurons in the medial septal area prevented the described increase in gene expression. The changes in cyr61 expression appeared to be dependent on the muscarinic M1 receptors. Our data suggest that the induction of long-term potentiation by dMSA activation enhances the expression of select early genes in the hippocampus.

The Serum Brain-Derived Neurotrophic Factor Increases in Serotonin Reuptake Inhibitor Responders Patients with First-Episode, Drug-Naïve Major Depression

Brain-derived neurotrophic factor (BDNF) is a growth factor synthesized in the cell bodies of neurons and glia, which affects neuronal maturation, the survival of nervous system, and synaptic plasticity. BDNF play an important role in the pathophysiology of major depression (MD). The serum BDNF levels changed over time, or with the improvement in depressive symptoms. However, the change of serum BDNF during pharmacotherapy remains obscure in MDD. In particular, the changes in serum BDNF associated with pharmacotherapy have not yet been fully elucidated. The present study aimed to compare the changes in serum BDNF concentrations in first-episode, drug-naive patients with MD treated with antidepressants between treatment-response and treatment-nonresponse groups. The study included 35 inpatients and outpatients composed of 15 males and 20 females aged 36.7 ± 6.8 years at the Department of Psychiatry of our University Hospital. All patients met the DSM-5 diagnostic criteria for MD. The antidepressants administered included paroxetine, duloxetine, and escitalopram. Severity of depressive state was assessed using the 17-item HAMD before and 8 weeks after drug administration. Responders were defined as those whose total HAMD scores at 8 weeks had decreased by 50% or more compared to those before drug administration, while non-responders were those whose total HAMD scores had decreased by less than 50%. Here we showed that serum BDNF levels were not significantly different at any point between the two groups. The responder group, but not the non-responder group, showed statistically significant changes in serum BDNF 0 and serum BDNF 8. The results suggest that the changes of serum BDNF might differ between the two groups. The measurement of serum BDNF has the potential to be a useful predictor of pharmacotherapy in patients with first-episode, drug-naïve MD.

Fasting for 20 h does not affect exercise-induced increases in circulating BDNF in humans

Intermittent fasting and exercise provide neuroprotection from age-related cognitive decline. A link between these two seemingly distinct stressors is their capability to steer the brain away from exclusively glucose metabolism. This cerebral substrate switch has been implicated in upregulating brain-derived neurotrophic factor (BDNF), a protein involved in neuroplasticity, learning and memory, and may underlie some of these neuroprotective effects. We examined the isolated and interactive effects of (1) 20-h fasting, (2) 90-min light exercise, and (3) high-intensity exercise on peripheral venous BDNF in 12 human volunteers. A follow-up study isolated the influence of cerebrovascular shear stress on circulating BDNF. Fasting for 20 h decreased glucose and increased ketones (P ≤ 0.0157) but had no effect on BDNF (P ≥ 0.4637). Light cycling at 25% of peak oxygen uptake ( V ̇ O 2 peak ${\dot V_{{{\rm{O}}_{\rm{2}}}{\rm{peak}}}}$ ) increased serum BDNF by 6 ± 8% (independent of being fed or fasted) and was mediated by a 7 ± 6% increase in platelets (P < 0.0001). Plasma BDNF was increased from 336 pg l^-1 [46,626] to 390 pg l^-1 [127,653] by 90-min of light cycling (P = 0.0128). Six 40-s intervals at 100% of V ̇ O 2 peak ${\dot V_{{{\rm{O}}_{\rm{2}}}{\rm{peak}}}}$ increased plasma and serum BDNF, as well as the BDNF-per-platelet ratio 4- to 5-fold more than light exercise did (P ≤ 0.0044). Plasma BDNF was correlated with circulating lactate during the high-intensity intervals (r = 0.47, P = 0.0057), but not during light exercise (P = 0.7407). Changes in cerebral shear stress - whether occurring naturally during exercise or induced experimentally with inspired CO₂ - did not correspond with changes in BDNF (P ≥ 0.2730). BDNF responses to low-intensity exercise are mediated by increased circulating platelets, and increasing either exercise duration or particularly intensity is required to liberate free BDNF. KEY POINTS: Intermittent fasting and exercise both have potent neuroprotective effects and an acute upregulation of brain-derived neurotrophic factor (BDNF) appears to be a common mechanistic link. Switching the brain's fuel source from glucose to either ketone bodies or lactate, i.e. a cerebral substrate switch, has been shown to promote BDNF production in the rodent brain. Fasting for 20 h caused a 9-fold increase in ketone body delivery to the brain but had no effect on any metric of BDNF in peripheral circulation at rest. Prolonged (90 min) light cycling exercise increased plasma- and serum-derived BDNF irrespective of being fed or fasted and seemed to be independent of changes in cerebral shear stress. Six minutes of high-intensity cycling intervals increased every metric of circulating BDNF by 4 to 5 times more than prolonged low-intensity cycling; the increase in plasma-derived BDNF was correlated with a 6-fold increase in circulating lactate irrespective of feeding or fasting. Compared to 1 day of fasting with or without prolonged light exercise, high-intensity exercise is a much more efficient means to increase BDNF in circulation.

Postnatal Sox6 Regulates Synaptic Function of Cortical Parvalbumin-Expressing Neurons

Cortical parvalbumin-expressing (Pvalb⁺) neurons provide robust inhibition to neighboring pyramidal neurons, crucial for the proper functioning of cortical networks. This class of inhibitory neurons undergoes extensive synaptic formation and maturation during the first weeks after birth and continue to dynamically maintain their synaptic output throughout adulthood. While several transcription factors, such as Nkx2-1, Lhx6, and Sox6, are known to be necessary for the differentiation of progenitors into Pvalb⁺ neurons, which transcriptional programs underlie the postnatal maturation and maintenance of Pvalb⁺ neurons' innervation and synaptic function remains largely unknown. Because Sox6 is continuously expressed in Pvalb⁺ neurons until adulthood, we used conditional knock-out strategies to investigate its putative role in the postnatal maturation and synaptic function of cortical Pvalb⁺ neurons in mice of both sexes. We found that early postnatal loss of Sox6 in Pvalb⁺ neurons leads to failure of synaptic bouton growth, whereas later removal in mature Pvalb⁺ neurons in the adult causes shrinkage of already established synaptic boutons. Paired recordings between Pvalb⁺ neurons and pyramidal neurons revealed reduced release probability and increased failure rate of Pvalb⁺ neurons' synaptic output. Furthermore, Pvalb⁺ neurons lacking Sox6 display reduced expression of full-length tropomyosin-receptor kinase B (TrkB), a key modulator of GABAergic transmission. Once re-expressed in neurons lacking Sox6, TrkB was sufficient to rescue the morphologic synaptic phenotype. Finally, we showed that Sox6 mRNA levels were increased by motor training. Our data thus suggest a constitutive role for Sox6 in the maintenance of synaptic output from Pvalb⁺ neurons into adulthood.

SIGNIFICANCE STATEMENT Cortical parvalbumin-expressing (Pvalb⁺) inhibitory neurons provide robust inhibition to neighboring pyramidal neurons, crucial for the proper functioning of cortical networks. These inhibitory neurons undergo extensive synaptic formation and maturation during the first weeks after birth and continue to dynamically maintain their synaptic output throughout adulthood. However, it remains largely unknown which transcriptional programs underlie the postnatal maturation and maintenance of Pvalb⁺ neurons. Here, we show that the transcription factor Sox6 cell-autonomously regulates the synaptic maintenance and output of Pvalb⁺ neurons until adulthood, leaving unaffected other maturational features of this neuronal population.

Bilateral histone deacetylase 1 and 2 activity and enrichment at unique genes following induction of long-term potentiation in vivo

Long-term potentiation (LTP) is a synaptic plasticity mechanism critical to long-term memory. LTP induced in vivo is characterized by altered transcriptional activity, including a period of upregulation of gene expression which is followed by a later dominant downregulation. This temporal shift to downregulated gene expression is predicted to be partly mediated by epigenetic inhibitors of gene expression, such as histone deacetylases (HDACs). Further, pharmacological inhibitors of HDAC activity have previously been shown to enhance LTP persistence in vitro. To explore the contribution of HDACs to the persistence of LTP in vivo, we examined HDAC1 and HDAC2 activity over a 24 hr period following unilateral LTP induction in the dentate gyrus of freely moving rats. Surprisingly, we found significant changes in HDAC1 and HDAC2 activity in both the stimulated as well as the unstimulated hemispheres, with the largest increase in activity occurring bilaterally, 20 min after LTP stimulation. During this time point of heightened activity, chromatin immunoprecipitation assays showed that both HDAC1 and HDAC2 were enriched at distinct sets of genes within each hemispheres. Further, the HDAC inhibitor Trichostatin A enhanced an intermediate phase of LTP lasting days, which has not previously been associated with altered transcription. The inhibitor had no effect on the persistence of LTP lasting weeks. Together, these data suggest that HDAC activity early after the induction of LTP may negatively regulate plasticity-related gene expression that is involved in the initial stabilization of LTP, but not its long-term maintenance.

Acute Stress Induces Cognitive Improvement in the Novel Object Recognition Task by Transiently Modulating Bdnf in the Prefrontal Cortex of Male Rats

Stress response involves several mechanisms and mediators that allow individuals to adapt to a changing environment. The effects of stress may be adaptive or maladaptive, based on the timing and intensity of exposure as well as on the individual vulnerability. In particular, exposure to mild and brief stressors provides beneficial advantages in a short-term period, by activating protective functions to react with the external demands. On these bases, the purpose of our study was to establish the time-dependent effects of acute stress exposure on neuroplastic mechanisms in adult male rats. Moreover, we aim at establishing the consequences of the acute challenge on memory processes by testing rats in the Novel Object Recognition (NOR) test. We found that acute restraint stress up-regulated total Bdnf expression 1 h post stress specifically in rat prefrontal cortex, an effect that was sustained by the increase of Bdnf isoform IV as well as by the pool of Bdnf transcripts with long 3'UTR. Furthermore, in the same brain region, the acute stress modulated in a time-specific manner the expression of different activity-dependent genes, namely Arc, Gadd45β and Nr4a1. At behavioral level, the challenge was able to improve the performance in the NOR test specifically 1 h post stress, an effect that positively correlated with the expression of the neurotrophic factors. Taken together, our results suggest that a single session of acute stress enhances memory and learning functions with a specific temporal profile, by improving neuroplastic mechanisms within the prefrontal cortex.

Hippocampal BDNF regulates a shift from flexible, goal-directed to habit memory system function following cocaine abstinence

The transition from recreational drug use to addiction involves pathological learning processes that support a persistent shift from flexible, goal‐directed to habit behavioral control. Here, we examined the molecular mechanisms supporting altered function in hippocampal (HPC) and dorsolateral striatum (DLS) memory systems following abstinence from repeated cocaine. After 3 weeks of cocaine abstinence (experimenter‐ or self‐administered), we tested new behavioral learning in male rats using a dual‐solution maze task, which provides an unbiased approach to assess HPC‐ versus DLS‐dependent learning strategies. Dorsal hippocampus (dHPC) and DLS brain tissues were collected after memory testing to identify transcriptional adaptations associated with cocaine‐induced shifts in behavioral learning. Our results demonstrate that following prolonged cocaine abstinence rats show a bias toward the use of an inflexible, habit memory system (DLS) in lieu of a more flexible, easily updated memory system involving the HPC. This memory system bias was associated with upregulation and downregulation of brain‐derived neurotrophic factor (BDNF) gene expression and transcriptionally permissive histone acetylation (acetylated histone H3, AcH3) in the DLS and dHPC, respectively. Using viral‐mediated gene transfer, we overexpressed BDNF in the dHPC during cocaine abstinence and new maze learning. This manipulation restored HPC‐dependent behavioral control. These findings provide a system‐level understanding of altered plasticity and behavioral learning following cocaine abstinence and inform mechanisms mediating the organization of learning and memory more broadly.

Different doses of methamphetamine alter long-term potentiation, level of BDNF and neuronal apoptosis in the hippocampus of reinstated rats

Methamphetamine (METH) is a psychostimulant. The precise mechanisms of its effects remain unknown and current relapse treatments have low efficacy. However, brain-derived neurotrophic factor (BDNF) and neuronal plasticity are essential contributors, despite paradoxical reports and a lack of comprehensive studies. Therefore, we investigated the effects of different doses of METH on long-term potentiation (LTP), BDNF expression and neuronal apoptosis in the hippocampus of reinstated rats. Rats were injected intraperitoneally with METH (1, 5, or 10 mg/kg) or saline, and trained in a conditioned place preference paradigm. Following implementation of the reinstatement model, electrophysiology, western blotting and TUNEL assay were performed to assess behavior, LTP components, BDNF expression, and neuronal apoptosis, respectively. The results demonstrated that the preference scores, population spike amplitude and BDNF expression markedly decreased in the METH (10 mg/kg) group compared with the other groups. In contrast, METH (5 mg/kg) significantly increased these factors more than the control group. There was no change in variables between METH (1 mg/kg) and the control group. Also, apoptosis of the hippocampus was increased in the METH (10 mg/kg) group compared with the METH (5 mg/kg) group. These results suggest that alterations in synaptic plasticity, expression of BDNF and neuronal apoptosis in the hippocampus has a vital role in the context-induced reinstatement of METH seeking.

Juvenile cerebral ischemia reveals age-dependent BDNF-TrkB signaling changes: Novel mechanism of recovery and therapeutic intervention

Global ischemia in childhood often leads to poor neurologic outcomes, including learning and memory deficits. Using our novel model of childhood cardiac arrest/cardiopulmonary resuscitation (CA/CPR), we investigate the mechanism of ischemia-induced cognitive deficits and recovery. Memory is impaired seven days after juvenile CA/CPR and completely recovers by 30 days. Consistent with this remarkable recovery not observed in adults, hippocampal long-term potentiation (LTP) is impaired 7-14 days after CA/CPR, recovering by 30 days. This recovery is not due to the replacement of dead neurons (neurogenesis), but rather correlates with brain-derived neurotrophic factor (BDNF) expression, implicating BDNF as the molecular mechanism underlying impairment and recovery. Importantly, delayed activation of TrkB receptor signaling reverses CA/CPR-induced LTP deficits and memory impairments. These data provide two new insights (1) endogenous recovery of memory and LTP through development may contribute to improved neurological outcome in children compared to adults and (2) BDNF-enhancing drugs speed recovery from pediatric cardiac arrest during the critical school ages.

Antidepressant-like effects of ginsenoside Rg5 in mice: Involving of hippocampus BDNF signaling pathway

Ginsenoside Rg5 is one of the major bioactive ingredients of Panax ginseng with little toxicity and has been shown to have pharmacological effects on the central nervous system. In this study, we investigated the antidepressant effects of Rg5 in mice models of depression. The effects of Rg5 were first assessed in the forced swimming test (FST) and tail suspension test (TST), and then investigated in the chronic social defeat stress (CSDS) model of depression. The changes in hippocampal brain-derived neurotrophic factor (BDNF) signaling pathway after CSDS and Rg5 treatment were also examined. The tryptophan hydroxylase inhibitor and tyrosine kinase B inhibitor were used to explore the antidepressant mechanisms of Rg5. It was found that Rg5 exhibited antidepressant-like activities in the FST and TST without affecting locomotor activity. Rg5 was also effective in the CSDS model of depression, and restored the CSDS-induced decrease in hippocampal BDNF signaling cascade. Moreover, the usage of the tyrosine kinase B inhibitor blocked the antidepressant effects of Rg5, while the tryptophan hydroxylase inhibitor did not. Collectively, ginsenoside Rg5 has antidepressant activities via the activation of hippocampal BDNF system.

Review : Stress, Antidepressant Treatments, and Neurotrophic Factors: Molecular and Cellular Mechanisms

Repeated stress or an excess of glucocorticoids can exacerbate neuronal damage in response to insults and, in severe cases, can lead to neuronal atrophy and death. These effects are thought to be related to the actions of stress and glucocorticoids on glutamate function, neuronal metabolism, and the generation of cytotoxic free radicals. Recent studies demonstrate that the regulation of neurotrophic factors may contribute to the actions of stress on neuronal function. Acute or chronic stress decreases the expression of brain derived neurotrophic factor, the most abundant neurotrophin in the brain, in specific regions of the hippocampus, and other forebrain regions. In addition, chronic stress increases the expression of neurotrophin-3 in certain regions of the hippocampus and may, thereby, help to protect these regions from the neurotoxic effects of chronic stress. The deleterious effects of stress may contribute to psy chiatric illnesses, such as depression, that can be precipitated or worsened by stress and that are often characterized by hypercortisolism. Electroconvulsive seizure therapy, as well as antidepressant drugs, increase the expression of brain derived neurotrophic factor and its receptor, trkB, in the brain, demon strating that neurotrophins are a target of antidepressant treatments. These findings outline a role of neurotrophic factors in the etiology and treatment of certain psychiatric illnesses. The Neuroscientist 1:351-360, 1995

Extracellular and intracellular cleavages of proBDNF required at two distinct stages of late-phase LTP

Although late-phase long-term potentiation (L-LTP) is implicated in long-term memory, its molecular mechanisms are largely unknown. Here we provide evidence that L-LTP can be divided into two stages: an induction stage (I) and a maintenance stage (II). Both stages require mature brain-derived neurotrophic factor (mBDNF), but involve distinct underlying mechanisms. Stage I requires secretion of existing proBDNF followed by extracellular cleavage by tPA/plasmin. Stage II depends on newly synthesized BDNF. Surprisingly, mBDNF at stage II is derived from intracellular cleavage of proBDNF by furin/PC1. Moreover, stage I involves BDNF-TrkB signaling mainly through MAP kinase, whereas all three signaling pathways (phospholipase C-γ, PI3 kinase, and MAP kinase) are required for the maintenance of L-LTP at stage II. These results reveal the molecular basis for two temporally distinct stages in L-LTP, and provide insights on how BDNF modulates this long-lasting synaptic alternation at two critical time windows.

Social Isolation During Adolescence Strengthens Retention of Fear Memories and Facilitates Induction of Late-Phase Long-Term Potentiation

Social isolation during the vulnerable period of adolescence produces emotional dysregulation that often manifests as abnormal behavior in adulthood. The enduring consequence of isolation might be caused by a weakened ability to forget unpleasant memories. However, it remains unclear whether isolation affects unpleasant memories. To address this, we used a model of associative learning to induce the fear memories and evaluated the influence of isolation mice during adolescence on the subsequent retention of fear memories and its underlying cellular mechanisms. Following adolescent social isolation, we found that mice decreased their social interaction time and had an increase in anxiety-related behavior. Interestingly, when we assessed memory retention, we found that isolated mice were unable to forget aversive memories when tested 4 weeks after the original event. Consistent with this, we observed that a single train of high-frequency stimulation (HFS) enabled a late-phase long-term potentiation (L-LTP) in the hippocampal CA1 region of isolated mice, whereas only an early-phase LTP was observed with the same stimulation in the control mice. Social isolation during adolescence also increased brain-derived neurotrophic factor (BDNF) expression in the hippocampus, and application of a tropomyosin-related kinase B (TrkB) receptor inhibitor ameliorated the facilitated L-LTP seen after isolation. Together, our results suggest that adolescent isolation may result in mental disorders during adulthood and that this may stem from an inability to forget the unpleasant memories via BDNF-mediated synaptic plasticity. These findings may give us a new strategy to prevent mental disorders caused by persistent unpleasant memories.

BDNF and synaptic plasticity, cognitive function, and dysfunction

Among all neurotrophins, brain-derived neurotrophic factor (BDNF) stands out for its high level of expression in the brain and its potent effects on synapses. It is now widely accepted that the main function of BDNF in the adult brain is to regulate synapses, with structural and functional effects ranging from short-term to long-lasting, on excitatory or inhibitory synapses, in many brain regions. The diverse effects of BDNF on brain synapses stem from its complex downstream signaling cascades, as well as the diametrically opposing effects of the pro- and mature form through distinct receptors, TrkB and p75(NTR). Many aspects of BDNF cell biology are regulated by neuronal activity. The synergistic interactions between neuronal activity and synaptic plasticity by BDNF make it an ideal and essential regulator of cellular processes that underlie cognition and other complex behaviors. Indeed, numerous studies firmly established that BDNF plays a critical role in hippocampal long-term potentiation (LTP), a long-term enhancement of synaptic efficacy thought to underlie learning and memory. Converging evidence now strongly suggest that deficits in BDNF signaling contribute to the pathogenesis of several major diseases and disorders such as Huntington's disease, Alzheimer's disease, and depression. Thus, manipulating BDNF pathways represents a viable treatment approach to a variety of neurological and psychiatric disorders.

Ketamine: synaptogenesis, immunomodulation and glycogen synthase kinase-3 as underlying mechanisms of its antidepressant properties

Major depressive disorder is an extremely debilitating condition affecting millions of people worldwide. Nevertheless, currently available antidepressant medications still have important limitations, such as a low response rate and a time lag for treatment response that represent a significant problem when dealing with individuals who are vulnerable and prone to self-harm. Recent clinical trials have shown that the N-methyl-D-aspartate receptor antagonist, ketamine, can induce an antidepressant response within hours, which lasts up to 2 weeks, and is effective even in treatment-resistant patients. Nonetheless, its use is limited due to its psychotomimetic and addictive properties. Understanding the molecular pathways through which ketamine exerts its antidepressant effects would help in the developing of novel antidepressant agents that do not evoke the same negative side effects of this drug. This review focuses specifically on the effects of ketamine on three molecular mechanisms that are relevant to depression: synaptogenesis, immunomodulation and regulation of glycogen synthase kinase-3 activity.

Chronic nicotine treatment reverses hypothyroidism-induced impairment of L-LTP induction phase: critical role of CREB

We have previously shown that adult onset hypothyroidism impairs late-phase long-term potentiation (L-LTP) and reduces basal protein levels of cyclic-AMP response element binding protein (CREB), mutagen-activated protein kinase (MAPKp42/44), and calcium calmodulin kinase IV (CaMKIV) in area Cornu Ammonis 1 (CA1) of the hippocampus. These changes were reversed by chronic nicotine treatment. In the present study, levels of signaling molecules important for L-LTP were determined in CA1 area of the hippocampus during the induction phase. Standard multiple high-frequency stimulation (MHFS) was used to evoke L-LTP in the CA1 area of the hippocampus of hypothyroid, nicotine-treated hypothyroid, nicotine, and sham control anaesthetized adult rats. Chronic nicotine treatment reversed hypothyroidism-induced impairment of L-LTP at the induction phase. Five minutes after MHFS, Western blotting showed an increase in the levels of P-CREB, and P-MAPKp42/44 in sham-operated control, nicotine, and nicotine-treated hypothyroid animals, but not in hypothyroid animals. The protein levels of total CREB, total MAPK p42/44, BDNF, and CaMKIV were not altered in all groups 5 min after MHFS. Therefore, normalized phosphorylation of essential kinases such as P-CREB and P-MAPK p42/44 in the CA1 area of nicotine-treated hypothyroid animals plays a crucial role in nicotine-induced rescue of L-LTP induction during hypothyroidism.

Levothyroxin replacement therapy restores hypothyroidism induced impairment of L-LTP induction: critical role of CREB

Cyclic-AMP response element binding protein (CREB) is a transcription factor crucial for late phase long-term potentiation (L-LTP) induction and maintenance. Upon multiple high frequency stimulation (MHFS), large Ca(2+) influx activates adenylyl cyclase. This, in turn, activates PKA, which by itself or through MAPK p42/p44 can activate (phosphorylate) CREB. Upon phosphorylation, P-CREB activates multiple genes essential for L-LTP generation. Calcium calmodulin kinase IV (CaMKIV) is also activated by calcium and can directly activate CREB. We have shown previously that hypothyroidism impairs L-LTP and reduces the basal protein levels of CREB, MAPK p42/p44, and CaMKIV in area CA1 of the hippocampus. In the present study, levels of these signaling molecules were determined in area CA1 during the induction and maintenance phases of L-LTP. Standard MHFS was used to evoke L-LTP in the CA1 area of hypothyroid, levothyroxin treated hypothyroid and sham control anesthetized adult rats. Chronic levothyroxin treatment reversed hypothyroidism-induced L-LTP impairment. Five minutes after MHFS, western blotting showed an increase in the levels of P-CREB, and P-MAPK p42/p44 in sham-operated control, and levothyroxin treated hypothyroid animals, but not in hypothyroid animals. The protein levels of total CREB, total MAPK p42/p44, BDNF and CaMKIV were not altered in all groups five minutes after MHFS. Four hours after MHFS, the levels of P-CREB, and P-MAPK p42/p44 remained unchanged in hypothyroid animals, while they were elevated in sham-operated control, and levothyroxin treated hypothyroid animals. We conclude that respective normalized phosphorylation of essential kinases such as P-CREB and P-MAPK p42/p44 is correlated with restoration of normal L-LTP induction and maintenance in the CA1 area of levothyroxin-treated hypothyroid animals.

Mechanism based approaches for rescuing and enhancing cognition

Progress toward pharmacological means for enhancing memory and cognition has been retarded by the widely discussed failure of behavioral studies in animals to predict human outcomes. As a result, a number of groups have targeted cognition-related neurobiological mechanisms in animal models, with the assumption that these basic processes are highly conserved across mammals. Here we survey one such approach that begins with a form of synaptic plasticity intimately related to memory encoding in animals and likely operative in humans. An initial section will describe a detailed hypothesis concerning the signaling and structural events (a “substrate map”) that convert learning associated patterns of afferent activity into extremely stable increases in fast, excitatory transmission. We next describe results suggesting that all instances of intellectual impairment so far tested in rodent models involve a common endpoint failure in the substrate map. This will be followed by a clinically plausible proposal for obviating the ultimate defect in these models. We then take up the question of whether it is reasonable to expect, from either general principles or a very limited set of experimental results, that enhancing memory will expand the cognitive capabilities of high functioning brains. The final section makes several suggestions about how to improve translation of behavioral results from animals to humans. Collectively, the material covered here points to the following: (1) enhancement, in the sense of rescue, is not an unrealistic possibility for a broad array of neuropsychiatric disorders; (2) serendipity aside, developing means for improving memory in normals will likely require integration of information about mechanisms with new behavioral testing strategies; (3) a shift in emphasis from synapses to networks is a next, logical step in the evolution of the cognition enhancement field.

Cognitive stimulation during lifetime and in the aged phase improved spatial memory, and altered neuroplasticity and cholinergic markers of mice

In the central nervous system, the degree of decline in memory retrieval along the aging process depends on the quantity and quality of the stimuli received during lifetime. The cholinergic system modulates long-term potentiation and, therefore, memory processing. This study evaluated the spatial memory, the synaptic plasticity and the density of cholinergic markers in the hippocampi of mice submitted to cognitive stimulation during lifetime or during their aged phase. Male C57Bl/6 mice (2 months old) were exposed to enriched environment during 15 months (EE-15). An age-matched group was left in standard cages during the same period (SC-15). Spatial memory was evaluated using the Barnes maze at 2, 5, 11 and 17 months of age. At the 17-month-old time point, EE-15 mice showed better performance in the spatial memory task (P<0.05), when compared to C-15 mice. Other two groups of mice were left in regular cages until the age of 15 months, and then one of the groups was transferred to an enriched environment for two months (EE-2). The other group was kept in regular cages (C-2). After two months of stimulation, EE-2 showed a significant increase in spatial memory (P<0.01). At the end, brains were extracted and kept at -80°C. Slices were obtained from one hemisphere in a cryostat (20 μm, -18°C) and thaw-mounted on gelatin coated slides. Synaptic densities, cellular bodies, BDNF densities and α4β2 nicotinic cholinergic receptors (nAChR) were evaluated by immunohistochemistry. Autoradiography for α7 nAChR was conducted using [(125)I]-α-bungarotoxin. The other half of the brains was used for Western blotting analysis of choline acetyltransferase (ChAT) density. There was no difference in synaptophysin or MAP-2 densities, but BDNF was increased in some hippocampal areas of EE-15 and EE-2, in comparison to control groups. In the same way, increases in ChAT and α7 densities, but not in α4β2, were observed. Both cognitive stimuli during lifetime or during the aged phase improved spatial memory of mice. No difference in structural plasticity was observed, but the maintenance of memory can be due to improvement in long-term potentiation functionality in the hippocampus, modulated, at least, by BDNF and the cholinergic system.

The neurotrophic compound J147 reverses cognitive impairment in aged Alzheimer's disease mice

Introduction

Despite years of research, there are no disease-modifying drugs for Alzheimer's disease (AD), a fatal, age-related neurodegenerative disorder. Screening for potential therapeutics in rodent models of AD has generally relied on testing compounds before pathology is present, thereby modeling disease prevention rather than disease modification. Furthermore, this approach to screening does not reflect the clinical presentation of AD patients which could explain the failure to translate compounds identified as beneficial in animal models to disease modifying compounds in clinical trials. Clearly a better approach to pre-clinical drug screening for AD is required.

Methods

To more accurately reflect the clinical setting, we used an alternative screening strategy involving the treatment of AD mice at a stage in the disease when pathology is already advanced. Aged (20-month-old) transgenic AD mice (APP/swePS1ΔE9) were fed an exceptionally potent, orally active, memory enhancing and neurotrophic molecule called J147. Cognitive behavioral assays, histology, ELISA and Western blotting were used to assay the effect of J147 on memory, amyloid metabolism and neuroprotective pathways. J147 was also investigated in a scopolamine-induced model of memory impairment in C57Bl/6J mice and compared to donepezil. Details on the pharmacology and safety of J147 are also included.

Results

Data presented here demonstrate that J147 has the ability to rescue cognitive deficits when administered at a late stage in the disease. The ability of J147 to improve memory in aged AD mice is correlated with its induction of the neurotrophic factors NGF (nerve growth factor) and BDNF (brain derived neurotrophic factor) as well as several BDNF-responsive proteins which are important for learning and memory. The comparison between J147 and donepezil in the scopolamine model showed that while both compounds were comparable at rescuing short term memory, J147 was superior at rescuing spatial memory and a combination of the two worked best for contextual and cued memory.

Conclusion

J147 is an exciting new compound that is extremely potent, safe in animal studies and orally active. J147 is a potential AD therapeutic due to its ability to provide immediate cognition benefits, and it also has the potential to halt and perhaps reverse disease progression in symptomatic animals as demonstrated in these studies.

Neurobiological consequences of sleep deprivation

Although the physiological function of sleep is not completely understood, it is well documented that it contributes significantly to the process of learning and memory. Ample evidence suggests that adequate sleep is essential for fostering connections among neuronal networks for memory consolidation in the hippocampus. Sleep deprivation studies are extremely valuable in understanding why we sleep and what are the consequences of sleep loss. Experimental sleep deprivation in animals allows us to gain insight into the mechanism of sleep at levels not possible to study in human subjects. Many useful approaches have been utilized to evaluate the effect of sleep loss on cognitive function, each with relative advantages and disadvantages. In this review we discuss sleep and the detrimental effects of sleep deprivation mostly in experimental animals. The negative effects of sleep deprivation on various aspects of brain function including learning and memory, synaptic plasticity and the state of cognition-related signaling molecules are discussed.

The neurotrophins and their role in Alzheimer's disease

Besides being essential for correct development of the vertebrate nervous system the neurotrophins also play a vital role in adult neuron survival, maintenance and regeneration. In addition they are implicated in the pathogenesis of certain neurodegenerative diseases, and may even provide a therapeutic solution for some. In particular there have been a number of studies on the involvement of nerve growth factor (NGF) and brain derived neurotrophic factor (BDNF) in the development of Alzheimer’s disease. This disease is of growing concern as longevity increases worldwide, with little treatment available at the moment to alleviate the condition. Memory loss is one of the earliest symptoms associated with Alzheimer’s disease. The brain regions first affected by pathology include the hippocampus, and also the entorhinal cortex and basal cholinergic nuclei which project to the hippocampus; importantly, all these areas are required for memory formation. Both NGF and BDNF are affected early in the disease and this is thought to initiate a cascade of events which exacerbates pathology and leads to the symptoms of dementia. This review briefly describes the pathology, symptoms and molecular processes associated with Alzheimer’s disease; it discusses the involvement of the neurotrophins, particularly NGF and BDNF, and their receptors, with changes in BDNF considered particularly in the light of its importance in synaptic plasticity. In addition, the possibilities of neurotrophin-based therapeutics are evaluated.

Novelty exposure overcomes foot shock-induced spatial-memory impairment by processes of synaptic-tagging in rats

Novelty processing can transform short-term into long-term memory. We propose that this memory-reinforcing effect of novelty could be explained by mechanisms outlined in the "synaptic tagging hypothesis." Initial short-term memory is sustained by a transient plasticity change at activated synapses and sets synaptic tags. These tags are later able to capture and process the plasticity-related proteins (PRPs), which are required to transform a short-term synaptic change into a long-term one. Novelty is involved in inducing the synthesis of PRPs [Moncada D, et al. (2011) Proc Natl Acad Sci USA 108:12937-12936], which are then captured by the tagged synapses, consolidating memory. In contrast to novelty, stress can impair learning, memory, and synaptic plasticity. Here, we address questions as to whether novelty-induced PRPs are able to prevent the loss of memory caused by stress and if the latter would not interact with the tag-setting process. We used water-maze (WM) training as a spatial learning paradigm to test our hypothesis. Stress was induced by a strong foot shock (FS; 5 × 1 mA, 2 s) applied 5 min after WM training. Our data show that FS reduced long-term but not short-term memory in the WM paradigm. This negative effect on memory consolidation was time- and training-dependent. Interestingly, novelty exposure prevented the stress-induced memory loss of the spatial task and increased BDNF and Arc expression. This rescuing effect was blocked by anisomycin, suggesting that WM-tagged synapses were not reset by FS and were thus able to capture the novelty-induced PRPs, re-establishing FS-impaired long-term memory.

Hippocampal long term potentiation in rats under different regimens of vitamin D: an in vivo study

Evidence indicates that vitamin D involves in development of brain as well as its function. This study assesses occurrence of long term potentiation (LTP), as an experimental form of synaptic plasticity, in adult rats under the normal regimen (CON), and the regimens without vitamin D (CON-D) or with a supplement of 1,25(OH)2D3 (CON+D). Stimulating the Schaffer collaterals pre- and post-tetanus excitatory postsynaptic potentials (EPSPs) were recorded in the CA1 area of hippocampus in anesthetized animals. Amplitude change of the EPSPs was considered for comparisons. Our results indicated that the basic EPSPs were similar in the three groups. Tetanization elicited a considerable LTP in both the CON and CON+D rats but a moderate potentiation in the CON-D group. We concluded that optimal level of vitamin D is required for induction of LTP.

BDNF facilitates L-LTP maintenance in the absence of protein synthesis through PKMζ

Late-phase long term potentiation (L-LTP) is thought to be the cellular basis for long-term memory (LTM). While LTM as well as L-LTP is known to depend on transcription and translation, it is unclear why brain-derived neurotrophic factor (BDNF) could sustain L-LTP when protein synthesis is inhibited. The persistently active protein kinase ζ (PKMζ) is the only molecule implicated in perpetuating L-LTP maintenance. Here, in mouse acute brain slices, we show that inhibition of PKMζ reversed BDNF-dependent form of L-LTP. While BDNF did not alter the steady-state level of PKMζ, BDNF together with the L-LTP inducing theta-burst stimulation (TBS) increased PKMζ level even without protein synthesis. Finally, in the absence of de novo protein synthesis, BDNF maintained TBS-induced PKMζ at a sufficient level. These results suggest that BDNF sustains L-LTP through PKMζ in a protein synthesis-independent manner, revealing an unexpected link between BDNF and PKMζ.

New insights into "GPR40-CREB interaction in adult neurogenesis" specific for primates

Polyunsaturated fatty acids (PUFA), such as docosahexaenoic (DHA) and arachidonic acids (ARA) are known to be closely related to the brain development and also have beneficial effects on adult neurogenesis, learning, and mental disorders. Although PUFA were demonstrated as ligands for G protein-coupled receptor 40 (GPR40), their signaling mechanism in the brain, especially in the neurogenic niche, remains unknown. Using a monkey model of ischemia-enhanced hippocampal neurogenesis, we studied the spatial correlation between GPR40 and the phosphorylated cAMP response element-binding protein (pCREB), a transcription factor involved in adult neurogenesis, learning and memory. Furthermore, the brain-derived neurotrophic factor (BDNF) and its receptor tropomyosin receptor kinase B (TrkB), both being downstream gene transcripts of pCREB, were studied. Similar to the dynamic change of GPR40 as the authors reported previously, pCREB was up-regulated significantly after transient global brain ischemia on Western blots, and this was associated with an enhanced hippocampal neurogenesis. Immunofluorescence microscopic analysis showed that GPR40 and pCREB expression patterns were completely identical, and they were coexpressed in both mature and newborn neurons as well as in the astrocytes residing in the subgranular zone (SGZ). GPR40/pCREB double-positive cells significantly increased in the SGZ on day 15 after ischemia. The mature form of BDNF (mBDNF) and TrkB receptor showed no remarkable changes on Western blots, although proBDNF (precursor of mBDNF) was maximal on day 9. Immunofluorescence microscopy showed that the newborn neurons expressed BDNF, but not TrkB. These results altogether suggest that PUFA, GPR40, pCREB, and BDNF may be engaged in the same signaling pathway to promote neurogenesis in the adult primate hippocampus.

Postsynaptic BDNF-TrkB signaling in synapse maturation, plasticity, and disease

Brain-derived neurotrophic factor (BDNF) is a prototypic neurotrophin that regulates diverse developmental events from the selection of neural progenitors to the terminal dendritic differentiation and connectivity of neurons. We focus here on activity-dependent synaptic regulation by BDNF and its receptor, full length TrkB. BDNF-TrkB signaling is involved in transcription, translation, and trafficking of proteins during various phases of synaptic development and has been implicated in several forms of synaptic plasticity. These functions are carried out by a combination of the three signaling cascades triggered when BDNF binds TrkB: The mitogen-activated protein kinase (MAPK), the phospholipase Cgamma (PLC PLCgamma), and the phosphatidylinositol 3-kinase (PI3K) pathways. MAPK and PI3K play crucial roles in both translation and/or trafficking of proteins induced by synaptic activity, whereas PLCgamma regulates intracellular Ca(2+) that can drive transcription via cyclic AMP and a protein kinase C. Conversely, the abnormal regulation of BDNF is implicated in various developmental and neurodegenerative diseases that perturb neural development and function. We will discuss the current state of understanding BDNF signaling in the context of synaptic development and plasticity with a focus on the postsynaptic cell and close with the evidence that basic mechanisms of BDNF function still need to be understood to effectively treat genetic disruptions of these pathways that cause devastating neurodevelopmental diseases.

A simple role for BDNF in learning and memory?

Since its discovery almost three decades ago, the secreted neurotrophin brain-derived neurotrophic factor (BDNF) has been firmly implicated in the differentiation and survival of neurons of the CNS. More recently, BDNF has also emerged as an important regulator of synaptogenesis and synaptic plasticity mechanisms underlying learning and memory in the adult CNS. In this review we will discuss our knowledge about the multiple intracellular signalling pathways activated by BDNF, and the role of this neurotrophin in long-term synaptic plasticity and memory formation as well as in synaptogenesis. We will show that maturation of BDNF, its cellular localization and its ability to regulate both excitatory and inhibitory synapses in the CNS may result in conflicting alterations in synaptic plasticity and memory formation. Lack of a precise knowledge about the mechanisms by which BDNF influences higher cognitive functions and complex behaviours may constitute a severe limitation in the possibility to devise BDNF-based therapeutics for human disorders of the CNS.

Antagonism of glutamate receptors in the CA1 to perirhinal cortex projection prevents long-term potentiation and attenuates levels of brain-derived neurotrophic factor

The CA1 to perirhinal cortex projection is one of multiple hippocampal-neocortical projections considered to be involved in memory consolidation. This projection has been shown to sustain long-term potentiation (LTP) following stimulation of CA1. Here we examined the pharmacological properties underpinning the plasticity observed in this projection. A stimulating electrode was inserted into the area CA1 and a recording electrode was inserted into the perirhinal cortex of urethane-anaesthetised Wistar rats. Rats (n=6 in each drug group) were administered with either saline (0.09%), MK-801 (NMDA antagonist; 0.1 mg/kg) or CNQX (AMPA/kainate antagonist; 1.5 mg/kg). Baseline recordings were made for 10 min by stimulating area CA1 (0.05 Hz stimulation protocol). High-frequency stimulation (HFS; 250 Hz) was performed and post-HFS fEPSP recordings were made for 1 h (0.05 Hz, as above). Baseline and post-HFS paired-pulse facilitation (PPF) recordings were performed across six different interpulse intervals. CA1 and perirhinal cortex tissue samples were taken from the stimulated and unstimulated hemispheres of each rat brain and analysed using a brain-derived neurotrophic factor (BDNF) ELISA. Results indicate that LTP was induced in the saline and MK-801 groups but not in the CNQX group; fEPSPs in the latter group rapidly returned to baseline levels following a short period of post-tetanic potentiation. Drug treatment and HFS had no effect on PPF levels. Drug treatment significantly reduced concentrations of both CA1 and perirhinal BDNF and prevented stimulation-induced increases in BDNF in CA1. This molecular and electrophysiological data suggests that LTP in the CA1-perirhinal cortex projection may require activation of postsynaptic AMPA/kainate receptors in order to sustain LTP.

Activity-dependent expression of brain-derived neurotrophic factor in dendrites: facts and open questions

Long-lasting synaptic changes in transmission and morphology at the basis of memory storage, require delivery of newly synthesized proteins to affected synapses. Although many of these proteins are generated in the cell body, several key molecules for plasticity can be delivered in the form of silent mRNAs at synapses in extra somatic compartments where they are locally translated. One of such mRNAs encodes brain-derived neurotrophic factor (BDNF), a key molecule in neuronal development, learning and memory. A single BDNF protein is produced from several splice variants having a different 5' untranslated region. These mRNA variants have a different subcellular localization (soma, proximal or distal dendritic compartment) and may represent a spatial code for a local control of BDNF availability. This review will highlight current knowledge on the mechanisms of spatial and temporal regulation of activity-dependent BDNF mRNA localization in dendrites in relation with synaptic plasticity.

Effect of prenatal protein malnutrition on long-term potentiation and BDNF protein expression in the rat entorhinal cortex after neocortical and hippocampal tetanization

Reduction of the protein content from 25 to 8% casein in the diet of pregnant rats results in impaired neocortical long-term potentiation (LTP) of the offspring together with lower visuospatial memory performance. The present study was aimed to investigate whether this type of maternal malnutrition could result in modification of plastic capabilities of the entorhinal cortex (EC) in the adult progeny. Unlike normal eutrophic controls, 55-60-day-old prenatally malnourished rats were unable to develop LTP in the medial EC to tetanizing stimulation delivered to either the ipsilateral occipital cortex or the CA1 hippocampal region. Tetanizing stimulation of CA1 also failed to increase the concentration of brain-derived neurotrophic factor (BDNF) in the EC of malnourished rats. Impaired capacity of the EC of prenatally malnourished rats to develop LTP and to increase BDNF levels during adulthood may be an important factor contributing to deficits in learning performance having adult prenatally malnourished animals.

BDNF: a key regulator for protein synthesis-dependent LTP and long-term memory?

It is generally believed that late-phase long-term potentiation (L-LTP) and long-term memory (LTM) require new protein synthesis. Although the full complement of proteins mediating the long-lasting changes in synaptic efficacy have yet to be identified, several lines of evidence point to a crucial role for activity-induced brain-derived neurotrophic factor (BDNF) expression in generating sustained structural and functional changes at hippocampal synapses thought to underlie some forms of LTM. In particular, BDNF is sufficient to induce the transformation of early to late-phase LTP in the presence of protein synthesis inhibitors, and inhibition of BDNF signaling impairs LTM. Despite solid evidence for a critical role of BDNF in L-LTP and LTM, many issues are not resolved. Given that BDNF needs to be processed in Golgi outposts localized at the branch point of one or few dendrites, a conceptually challenging problem is how locally synthesized BDNF in dendrites could ensure synapse-specific modulation of L-LTP. An interesting alternative is that BDNF-TrkB signaling is involved in synaptic tagging, a prominent hypothesis that explains how soma-derived protein could selectively modulate the tetanized (tagged) synapse. Finally, specific roles of BDNF in the acquisition, retention or extinction of LTM remain to be established.

Tyrosine receptor kinase B (TrkB) protein expression in the human endometrium

Tyrosine kinase receptor B (TrkB) gene expression, a neurotrophic factor receptor expressed in the brain and ovary, has recently been identified in deep infiltrating endometriosis by gene array. TrkB is thought to be important in resistance to anchorage independent apoptosis (ANOIKIS) and thus could be important in the pathogenesis of endometriosis. However, TrkB protein expression in the eutopic endometrium of women with and without endometriosis is unknown. Therefore, we examined TrKB protein expression in the endometrium by Western blot (n = 50) and immunohistochemistry (n = 17). Immunoblots of endometrial biopsies were prepared from women with endometriosis (n = 21) vs. healthy controls (n = 29) undergoing benign gynecological surgery at McMaster University Medical Centre. TrkB protein expression was significantly higher in immunoblots from women with endometriosis compared to women without endometriosis. In samples of archived paraffin-embedded endometrial tissue TrkB was localized to the cytoplasm of epithelial cells of the eutopic endometrium from women with endometriosis (n = 7) and without endometriosis (n = 10). We conclude that TrkB protein is expressed in human endometrium. Our results also suggest that TrkB expression may be greater in women with endometriosis compared to women without endometriosis.

Nicotine prevents disruption of the late phase LTP-related molecular cascade in adult-onset hypothyroidism

We have shown previously that chronic nicotine treatment reverses adult-onset hypothyroidism-induced impairment of late-phase long-term potentiation (L-LTP) in area CA1 of the hippocampus. In the present study, basal and stimulated levels of signaling molecules essential for the expression of L-LTP were determined in area CA1. Immunoblots analysis showed that chronic nicotine treatment of hypothyroid rats prevented the reduction in the basal protein levels of adenylyl cyclase I (ACI), mitogen-activated protein kinases [MAPKp44/42 (ERK1/2)], calcium-calmodulin-dependent protein kinase IV (CaMKIV), and cyclic-AMP response element binding protein [CREB; phosphorylated (P-) and total]. A significant increase in the levels of P-CREB, P-MAPKp44, P-MAPKp42 and brain derived neurotrophic factor (BDNF) was seen 4 h after multiple train high frequency stimulation (MHFS) in nicotine-treated hypothyroid and control animals, but not in hypothyroid animals. The levels of total CREB, total MAPKp44, total MAPKp42, and CaMKIV were elevated in all groups 4 h after MHFS. These findings suggest that prevention of the reduced basal level of CaMKIV, MAPKp44/42, and CREB by nicotine along with the regained ability of MHFS to induce MAPKp44/42 and CREB phosphorylation in nicotine treated hypothyroid animals may be responsible for the reversal of L-LTP impairment by chronic nicotine treatment in this disease model.

Chronic psychosocial stress-induced impairment of hippocampal LTP: possible role of BDNF

Electrophysiological recording reveals that chronic nicotine treatment prevents stress-induced impairment of long-term potentiation (LTP) in the CA1 region of the hippocampus of anesthetized rats. We investigated the molecular mechanism of this action of nicotine in the CA1 region. Immunoblot analysis showed that chronic nicotine treatment (1 mg/kg, 2 times/day) normalized the stress-induced decrease in the basal levels of BDNF, CaMKII (total and phosphorylated; P-CaMKII), and calmodulin. Additionally, nicotine reversed the stress-induced increase in calcineurin basal levels. Chronic nicotine treatment also markedly increased the basal levels of BDNF in naïve rats. Furthermore, high-frequency stimulation (HFS), which increased the levels of P-CaMKII in control as well as nicotine-treated stressed rats, failed to increase P-CaMKII levels in untreated stressed rats. Compared to unstimulated control, the levels of both total CaMKII and calcineurin were increased after HFS in all groups including the stressed, but no changes were detected after HFS in the levels of BDNF and calmodulin. These results indicate that normalization by nicotine of the stress-induced changes in the levels of signaling molecules including BDNF may contribute to the recovery of LTP.

Activity-dependent modulation of the BDNF receptor TrkB: mechanisms and implications

Although brain-derived neurotrophic factor (BDNF) has emerged as a key regulator of activity-dependent synaptic plasticity, a conceptually challenging question is how this diffusible molecule achieves local and synapse-specific modulation. One hypothesis is that neuronal activity enhances BDNF signaling by selectively modulating TrkB receptors at active neurons or synapses without affecting receptors on neighboring, less-active ones. Growing evidence suggests that neuronal activity facilitates cell-surface expression of TrkB. BDNF secreted from active synapses and neurons recruits TrkB from extrasynaptic sites into lipid rafts, microdomains of membrane that are enriched at synapses. Postsynaptic rises in cAMP concentrations facilitate translocation of TrkB into the postsynaptic density. Finally, neuronal activity promotes BDNF-induced TrkB endocytosis, a signaling event important for many long-term BDNF functions. These mechanisms could collectively underlie synapse-specific regulation by BDNF.

Regulation of late-phase LTP and long-term memory in normal and aging hippocampus: role of secreted proteins tPA and BDNF

Long-lasting forms of memory are generally believed to be mediated by protein synthesis-dependent, late-phase long-term potentiation (L-LTP). L-LTP exhibits at least two distinctive characteristics compared with early phase LTP (E-LTP): synaptic growth and requirement of gene transcription and new protein synthesis. In this review, we discuss the cellular and molecular mechanisms underlying the structural and functional changes of hippocampal synapses during L-LTP, in the context of long-term memory. We describe experiments that reveal the critical role of cAMP/protein kinase A and MAP kinase pathways, and the downstream transcription factor CREB. Because transcription-dependent long-term changes are input specific, we also discuss the role of "local protein synthesis" and "synaptic tagging" mechanisms that may confer synapse specificity. We then focus on brain-derived neurotrophic factor (BDNF) and tissue plasminogen activator (tPA), two secreted proteins that have been repeatedly implicated in L-LTP. Biochemical and molecular biology experiments indicate that the expression and secretion of both factors are enhanced by strong tetanic stimulation that induces L-LTP as well as by training in hippocampal-dependent memory tasks. Inhibition of either tPA or BDNF by gene knockout and specific inhibitors results in a significant impairments in L-LTP and long-term memory. Further work will be required to address the relationship between BDNF and tPA in various forms of synaptic plasticity, and the mechanisms by which BDNF/tPA achieves synapse-specific modulation. Finally, we discuss how the aging process affects L-LTP and long-term memory.

Altered synapse formation in the adult somatosensory cortex of brain-derived neurotrophic factor heterozygote mice

Increased sensory stimulation in the adult whisker-to-barrel pathway induces the expression of BDNF as well as synapse formation in cortical layer IV. Here, we investigated whether BDNF plays a role in the alterations of connectivity between neurons by analyzing the ultrastructure of the BDNF heterozygote mouse, characterized by a reduced level of BDNF expression. Using serial section electron microscopy, we measured synapse density, spine morphology, and synaptic vesicle distribution to show that mice with a reduced level of BDNF have a barrel neuropil that is indistinguishable from wild-type controls. After 24 hr of whisker stimulation, however, there is no indication of synapse formation in the heterozygous mouse. Whereas the balance between excitatory and inhibitory synapses is modified in the controls, it remains constant in the heterozygotes. The distribution of synaptic vesicles in excitatory synapses is the same in heterozygous and wild-type mice and is not influenced by the stimulation paradigm. Spine volume, however, is unchanged by stimulation in the wild-type animals, but does increase significantly in the heterozygous animal. These results provide evidence that, in vivo, BDNF plays an important role in the structural rearrangement of adult cortical circuitry as a consequence of an increased sensory input.

Long-term potentiation and memory

One of the most significant challenges in neuroscience is to identify the cellular and molecular processes that underlie learning and memory formation. The past decade has seen remarkable progress in understanding changes that accompany certain forms of acquisition and recall, particularly those forms which require activation of afferent pathways in the hippocampus. This progress can be attributed to a number of factors including well-characterized animal models, well-defined probes for analysis of cell signaling events and changes in gene transcription, and technology which has allowed gene knockout and overexpression in cells and animals. Of the several animal models used in identifying the changes which accompany plasticity in synaptic connections, long-term potentiation (LTP) has received most attention, and although it is not yet clear whether the changes that underlie maintenance of LTP also underlie memory consolidation, significant advances have been made in understanding cell signaling events that contribute to this form of synaptic plasticity. In this review, emphasis is focused on analysis of changes that occur after learning, especially spatial learning, and LTP and the value of assessing these changes in parallel is discussed. The effect of different stressors on spatial learning/memory and LTP is emphasized, and the review concludes with a brief analysis of the contribution of studies, in which transgenic animals were used, to the literature on memory/learning and LTP.

Acute and long-term synaptic modulation by neurotrophins

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

Rapid induction of BDNF expression in the hippocampus during immobilization stress challenge in adult rats

Brain-derived neurotrophic factor (BDNF) is strongly expressed in the hippocampus, where it has been associated with memory processes. In the central nervous system, some learning processes, as well as brain insults, including stress, induce modifications in BDNF mRNA expression. Because stress and memory appear to share some neuronal pathways, we studied BDNF mRNA and BDNF peptide variations in response to short times of immobilization stress. Using an RNase protection assay, we demonstrated that short-time stress application induced a significant increase (at 60 min) in BDNF mRNA levels in the whole rat hippocampus. Changes in BDNF mRNA content appear to reflect increased expression of BDNF transcripts containing exons I, II, and III, that were also significantly modified at this time. The time course of stress-induced changes in BDNF transcript levels revealed that mRNA containing exon III was the first increased, significantly elevated by 15 min, attaining maximal levels at 60 min, as BDNF transcripts containing exons I and II. However, at longer times of stress (180 min), BDNF mRNA levels were decreased as well as mRNA containing exon IV. In situ hybridization analysis of discrete hippocampal layers demonstrated that BDNF mRNA expression increased as early as 15 min in most hippocampal regions, with no modification in the number of labeled cells. The same signal pattern, although less pronounced, was determined at 60 min, but at this time a significant increase in BDNF-positive cells was visualized in the CA3 layer. The peptide, measured by immunoassay, was significantly augmented after 180 min of stress exposure whereas at 300 min, levels were similar to those measured in control animals. These data suggest that rapid changes in BDNF expression may be part of a compensatory response to preserve hippocampal homeostasis or a form of neuronal plasticity to cope with new stimuli.

Neurotrophins and cortical development

The mammalian cerebral cortex requires the proper formation of exquisitely precise circuits to function correctly. These neuronal circuits are assembled during development by the formation of synaptic connections between hundreds of thousands of differentiating neurons. Although the development of the cerebral cortex has been well described anatomically, the cellular and molecular mechanisms that guide neuronal differentiation and formation of connections are just beginning to be understood. Moreover, despite evidence that coordinated patterns of activity underlie reorganization of brain circuits during critical periods of development, the molecular signals that translate activity into structural and functional changes in connections remain unknown. Recently, the neurotrophins have emerged as attractive candidates not only for regulating neuronal differentiation in the developing brain, but also for mediating activity-dependent synaptic plasticity. The neurotrophins meet many of the criteria required for molecular signals involved in neuronal differentiation and plasticity. They are present in the cerebral cortex during development and their expression is regulated by synaptic activity. In turn, the neurotrophins themselves strongly influence both short-term synaptic plasticity and long-term potentiation and depression. In addition to their functional effects, the neurotrophins also profoundly regulate the structural changes that underlie axonal and dendritic differentiation. Finally, the neurotrophins have been implicated in mediating synaptic competition required for activity-dependent plasticity during the critical period. This chapter presents and discusses the rapidly accumulating evidence that the neurotrophins are critical for neuronal differentiation and that they may be involved in activity-dependent synaptic refinement in the developing cerebral cortex.

Overactive bladder--experimental aspects

Supra-pontine lesions resulting from neurological disorders such as vascular disease, Parkinson's disease, or Alzheimer type senile dementia lead to an increase in bladder activity. This is due in part to the removal at the cortical inhibitory control of the micturition center in the brain stem - i.e. the pontine micturition center (PMC) - and in part to facilitation of excitatory control. These inhibitory or excitatory controls consist of several neurotransmitter systems, including glutamate, dopamine, gamma-aminobutyric acid (GABA), and acetylcholine. Bladder overactivity caused by cerebral infarction is mediated by upregulation of N-methyl-D-aspartate (NMDA) glutamatergic and D2 dopaminergic excitatory mechanisms, and by downregulation of NMDA glutamatergic and Ml muscarinic inhibitory mechanisms in the brain. Bladder overactivity associated with Parkinson's disease is reportedly induced by a loss of input to the D1 dopaminergic receptor. Furthermore, bladder overactivity caused by Alzheimer type dementia is thought to be mediated by downregulation of M1 muscarinic inhibitory mechanisms. Development of bladder overactivity following cerebral infarction is mediated by activation of the NMDA receptor and accompanied by an increase in c-fos, zif268 and COX-2 mRNA expression in the dorsal pontine tegmentum.

Neurotrophin secretion: current facts and future prospects

The proteins of the mammalian neurotrophin family (nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4/5 (NT-4/5)) were originally identified as neuronal survival factors. During the last decade, evidence has accumulated implicating them (especially BDNF) in addition in the regulation of synaptic transmission and synaptogenesis in the CNS. However, a detailed understanding of the secretion of neurotrophins from neurons is required to delineate their role in regulating synaptic function. Some crucial questions that need to be addressed include the sites of neurotrophin secretion (i.e. axonal versus dendritic; synaptic versus extrasynaptic) and the neuronal and synaptic activity patterns that trigger the release of neurotrophins. In this article, we review the current knowledge in the field of neurotrophin secretion, focussing on activity-dependent synaptic release of BDNF. The modality and the site of neurotrophin secretion are dependent on the processing and subsequent targeting of the neurotrophin precursor molecules. Therefore, the available data regarding formation and trafficking of neurotrophins in the secreting neurons are critically reviewed. In addition, we discuss existing evidence that the characteristics of neurotrophin secretion are similar (but not identical) to those of other neuropeptides. Finally, since BDNF has been proposed to play a critical role as an intercellular synaptic messenger in long-term potentiation (LTP) in the hippocampus, we try to reconcile this possible role of BDNF in LTP with the recently described features of synaptic BDNF secretion.

BDNF and activity-dependent synaptic modulation

It is widely accepted that neuronal activity plays a pivotal role in synaptic plasticity. Neurotrophins have emerged recently as potent factors for synaptic modulation. The relationship between the activity and neurotrophic regulation of synapse development and plasticity, however, remains unclear. A prevailing hypothesis is that activity-dependent synaptic modulation is mediated by neurotrophins. An important but unresolved issue is how diffusible molecules such as neurotrophins achieve local and synapse-specific modulation. In this review, I discuss several potential mechanisms with which neuronal activity could control the synapse-specificity of neurotrophin regulation, with particular emphasis on BDNF. Data accumulated in recent years suggest that neuronal activity regulates the transcription of BDNF gene, the transport of BDNF mRNA and protein into dendrites, and the secretion of BDNF protein. There is also evidence for activity-dependent regulation of the trafficking of the BDNF receptor, TrkB, including its cell surface expression and ligand-induced endocytosis. Further study of these mechanisms will help us better understand how neurotrophins could mediate activity-dependent plasticity in a local and synapse-specific manner.

Cellular mechanisms regulating activity-dependent release of native brain-derived neurotrophic factor from hippocampal neurons

Brain-derived neurotrophic factor (BDNF) plays a critical role in activity-dependent modifications of neuronal connectivity and synaptic strength, including establishment of hippocampal long-term potentiation (LTP). To shed light on mechanisms underlying BDNF-dependent synaptic plasticity, the present study was undertaken to characterize release of native BDNF from newborn rat hippocampal neurons in response to physiologically relevant patterns of electrical field stimulation in culture, including tonic stimulation at 5 Hz, bursting stimulation at 25 and 100 Hz, and theta-burst stimulation (TBS). Release was measured using the ELISA in situ technique, developed in our laboratory to quantify secretion of native BDNF without the need to first overexpress the protein to nonphysiological levels. Each stimulation protocol resulted in a significant increase in BDNF release that was tetrodotoxin sensitive and occurred in the absence of glutamate receptor activation. However, 100 Hz tetanus and TBS, stimulus patterns that are most effective in inducing hippocampal LTP, were significantly more effective in releasing native BDNF than lower-frequency stimulation. For all stimulation protocols tested, removal of extracellular calcium, or blockade of N-type calcium channels, prevented BDNF release. Similarly, depletion of intracellular calcium stores with thapsigargin and treatment with dantrolene, an inhibitor of calcium release from caffeine-ryanodine-sensitive stores, markedly inhibited activity-dependent BDNF release. Our results indicate that BDNF release can encode temporal features of hippocampal neuronal activity. The dual requirement for calcium influx through N-type calcium channels and calcium mobilization from intracellular stores strongly implicates a role for calcium-induced calcium release in activity-dependent BDNF secretion.

BDNF mRNA expression during postnatal development, maturation and aging of the human prefrontal cortex

Brain derived neurotrophic factor (BDNF) is widely distributed in the central nervous system (CNS) and has survival-promoting actions on a variety of CNS neurons. We have examined changes in the level of BDNF mRNA expression in the dorsolateral prefrontal cortex (DLPFC) of the postnatal human brain using both RNAse protection assay and in situ hybridization. Expression of BDNF mRNA in the DLPFC was compared to that in the occipital cortex. BDNF mRNA levels vary between layers, with layer VI consistently higher than other layers in both the DLPFC and occipital regions. BDNF mRNA levels increase approximately one-third from infancy to adulthood, i.e. they are relatively low during infancy and adolescence, peak during young adulthood, and are maintained at a constant level throughout adulthood and aging. The significant increase in BDNF mRNA levels in the DLPFC during the young adult period coincides with the time when the frontal cortex matures both structurally and functionally. The increase in BDNF at this critical time in human development may have important implications for the etiology and treatment of the severe mental disorders that tend to present during this time.