Hippocampal BDNF mRNA shows a diurnal regulation, primarily in the exon III transcript

Circadian rhythm of brain-derived neurotrophic factor in serum and plasma

The neurotrophic growth factor brain-derived neurotrophic factor (BDNF) plays a crucial role in various neurodegenerative and psychiatric diseases, such as Alzheimer's disease, schizophrenia and depression. BDNF has been proposed as a potential biomarker for diagnosis, prognosis and monitoring therapy. Understanding the factors influencing BDNF levels and whether they follow a circadian rhythm is essential for interpreting fluctuations in BDNF measurements. We aimed to investigate the circadian rhythm of BDNF by collecting multiple peripheral venous blood samples from young, healthy male participants at 12 different time points over 24 h. In addition, vital parameters, cortisol and insulin like growth factor 1 (IGF1) were measured to explore potential regulatory mechanisms, interfering variables and their correlations with BDNF concentration. The findings revealed that plasma BDNF did not exhibit any significant fluctuations over 24 h, suggesting the absence of a circadian rhythm. However, serum BDNF levels decreased during sleep. Furthermore, serum BDNF showed a positive correlation with heart rate but a negative correlation with IGF1. No significant correlation was observed between cortisol and BDNF or IGF1. Although plasma BDNF suggests steady-state conditions, the decline of serum BDNF during the nocturnal period could be attributed to physical inactivity and associated with reduced haemodynamic blood flow (heart rate reduction during sleep). The type of sample collection (peripheral venous cannula vs. blood sampling using a butterfly system) does not significantly affect the measured BDNF levels. The sample collection during the day did not significantly affect BDNF analysis, emphasizing the importance of considering activity levels rather than timing when designing standardized protocols for BDNF assessments.

Diverse Functions of Multiple Bdnf Transcripts Driven by Distinct Bdnf Promoters

The gene encoding brain-derived neurotrophic factor (Bdnf) consists of nine non-coding exons driven by unique promoters, leading to the expression of nine Bdnf transcripts that play different roles in various brain regions and physiological stages. In this manuscript, we present a comprehensive overview of the molecular regulation and structural characteristics of the multiple Bdnf promoters, along with a summary of the current knowledge on the cellular and physiological functions of the distinct Bdnf transcripts produced by these promoters. Specifically, we summarized the role of Bdnf transcripts in psychiatric disorders, including schizophrenia and anxiety, as well as the cognitive functions associated with specific Bdnf promoters. Moreover, we examine the involvement of different Bdnf promoters in various aspects of metabolism. Finally, we propose future research directions that will enhance our understanding of the complex functions of Bdnf and its diverse promoters.

Heat shock factor HSF1 regulates BDNF gene promoters upon acute stress in the hippocampus, together with pCREB

Heat shock factor 1 (HSF1) is a master stress-responsive transcriptional factor, protecting cells from death. However, its gene regulation in vivo in the brain in response to neuronal stimuli remains elusive. Here, we investigated its direct regulation of the brain-derived neurotrophic factor (BDNF) gene (Bdnf) in response to acute neuronal stress stimuli in the brain. The results of immunohistochemistry and chromatin immunoprecipitation quantitative PCR (ChIP-qPCR) showed that administration of kainic acid (a glutamate receptor agonist inducing excitotoxity) to young adult mice induced HSF1 nuclear translocation and its binding to multiple Bdnf promoters in the hippocampus. Footshock, a physical stressor used for learning, also induced HSF1 binding to selected Bdnf promoters I and IV. This is, to our knowledge, the first demonstration of HSF1 gene regulation in response to neuronal stimuli in the hippocampus in vivo. HSF1 binding sites (HSEs) in Bdnf promoters I and IV were also detected when immunoprecipitated by an antibody of phosphorylated (p)CREB (cAMP-responsive element-binding protein), suggesting their possible interplay in acute stress-induced Bdnf transcription. Interestingly, their promoter binding patterns differed by KA and footshock, suggesting that HSF1 and pCREB orchestrate to render fine-tuned promoter control depending on the types of stress. Further, HSF1 overexpression increased Bdnf promoter activity in a luciferase assay, while virus infection of constitutively active-form HSF1 increased levels of BDNF mRNA and protein in vitro in primary cultured neurons. These results indicated that HSF1 activation of Bdnf promoter was sufficient to induce BDNF expression. Taken together, these results suggest that HSF1 promoter-specific control of Bdnf gene regulation plays an important role in neuronal protection and plasticity in the hippocampus in response to acute stress, possibly interplaying with pCREB.

Effects of photoperiod and diet on BDNF daily rhythms in diurnal sand rats

Brain-derived neurotrophic factor (BDNF), its receptors and epigenetic modulators, are implicated in the pathophysiology of affective disorders, T2DM and the circadian system function. We used diurnal sand rats, which develop type 2 diabetes (T2DM), anxiety and depressive-like behavior under laboratory conditions. The development of these disorders is accelerated when animals are maintained under short photoperiod (5:19L:D, SP) compared to neutral photoperiod (12:12L:D, NP). We compared rhythms in plasma BDNF as well as BDNF and PER2 expression in the frontal cortex and suprachiasmatic nucleus (SCN) of sand rats acclimated to SP and NP. Acclimation to SP resulted in higher insulin levels, significantly higher glucose levels in the glucose tolerance test, and significantly higher anxiety- and depression-like behaviors compared with animals acclimated to NP. NP Animals exhibited a significant daily rhythm in plasma BDNF levels with higher levels during the night, and in BDNF expression levels in the frontal cortex and SCN. No significant BDNF rhythm was found in the plasma, frontal cortex or SCN of SP acclimated animals. We propose that in sand rats, BDNF may, at least in part, mediate the effects of circadian disruption on the development of anxiety and depressive-like behavior and T2DM.

Molecular, physiological and behavioral characterization of the heterozygous Df[h15q13]/+ mouse model associated with the human 15q13.3 microdeletion syndrome

The human 15q13.3 microdeletion syndrome (DS) is caused by a heterozygous microdeletion (MD) affecting six genes: FAN1; MTMR10; TRPM1; KLF13; OTUD7A; and CHRNA7. Carriers are at risk for intellectual disability, epilepsy, autism spectrum disorder, and schizophrenia. Here we used the Df[h15q13]/+ mouse model with an orthologous deletion to further characterize molecular, neurophysiological, and behavioral parameters that are relevant to the 15q13.3 DS. First, we verified the expression and distribution of the α7 nicotinic acetylcholine receptor (nAChR), a gene product of the CHRNA7, in cortical and subcortical areas. Results revealed similar mRNA distribution pattern in wildtype (WT) and heterozygous (Het) mice, with about half the number of α7 nAChR binding sites in mutants. Hippocampal recordings showed similar input/output responses of field excitatory post-synaptic potentials and theta-burst induced long-term potentiation in WT and Het mice. Het males exhibited impaired spatial learning acquisition in the Barnes Maze. Indicative of increased seizure susceptibility, Het mice developed secondary seizures after 6-Hz corneal stimulation, and had significantly increased sensitivity to the chemoconvulsant pentylenetetrazol resulting in increased spiking in hippocampal EEG recordings. Basal mRNA expression of brain derived neurotrophic factor and activity regulated immediate early genes (c-fos, Arc, Erg-1 and Npas4) during adolescence, a critical period of brain maturation, was unaffected by genotype. Thus, the MD did not show gross neuroanatomical, molecular, and neurophysiological abnormalities despite deficits in spatial learning and increased susceptibility to seizures. Altogether, our results verify the phenotypic profile of the heterozygous Df[h15q13]/+ mouse model and underscore its translational relevance for human 15q13.3 DS.

Memory and the circadian system: Identifying candidate mechanisms by which local clocks in the brain may regulate synaptic plasticity

The circadian system is an endogenous biological network responsible for coordinating near-24-h cycles in behavior and physiology with daily timing cues from the external environment. In this review, we explore how the circadian system regulates memory formation, retention, and recall. Circadian rhythms in these memory processes may arise through several endogenous pathways, and recent work highlights the importance of genetic timekeepers found locally within tissues, called local clocks. We evaluate the circadian memory literature for evidence of local clock involvement in memory, identifying potential nodes for direct interactions between local clock components and mechanisms of synaptic plasticity. Our discussion illustrates how local clocks may pervasively modulate neuronal plastic capacity, a phenomenon that we designate here as circadian metaplasticity. We suggest that this function of local clocks supports the temporal optimization of memory processes, illuminating the potential for circadian therapeutic strategies in the prevention and treatment of memory impairment.

Chronotype and cellular circadian rhythms predict the clinical response to lithium maintenance treatment in patients with bipolar disorder

Bipolar disorder (BD) is a serious mood disorder associated with circadian rhythm abnormalities. Risk for BD is genetically encoded and overlaps with systems that maintain circadian rhythms. Lithium is an effective mood stabilizer treatment for BD, but only a minority of patients fully respond to monotherapy. Presently, we hypothesized that lithium-responsive BD patients (Li-R) would show characteristic differences in chronotype and cellular circadian rhythms compared to lithium non-responders (Li-NR). Selecting patients from a prospective, multi-center, clinical trial of lithium monotherapy, we examined morning vs. evening preference (chronotype) as a dimension of circadian rhythm function in 193 Li-R and Li-NR BD patients. From a subset of 59 patient donors, we measured circadian rhythms in skin fibroblasts longitudinally over 5 days using a bioluminescent reporter (Per2-luc). We then estimated circadian rhythm parameters (amplitude, period, phase) and the pharmacological effects of lithium on rhythms in cells from Li-R and Li-NR donors. Compared to Li-NRs, Li-Rs showed a difference in chronotype, with higher levels of morningness. Evening chronotype was associated with increased mood symptoms at baseline, including depression, mania, and insomnia. Cells from Li-Rs were more likely to exhibit a short circadian period, a linear relationship between period and phase, and period shortening effects of lithium. Common genetic variation in the IP3 signaling pathway may account for some of the individual differences in the effects of lithium on cellular rhythms. We conclude that circadian rhythms may influence response to lithium in maintenance treatment of BD.

Aerobic exercise and a BDNF-mimetic therapy rescue learning and memory in a mouse model of Down syndrome

Down syndrome (DS) is caused by the triplication of human chromosome 21 and represents the most frequent genetic cause of intellectual disability. The trisomic Ts65Dn mouse model of DS shows synaptic deficits and reproduces the essential cognitive disabilities of the human syndrome. Aerobic exercise improved various neurophysiological dysfunctions in Ts65Dn mice, including hippocampal synaptic deficits, by promoting synaptogenesis and neurotransmission at glutamatergic terminals. Most importantly, the same intervention also prompted the recovery of hippocampal adult neurogenesis and synaptic plasticity and restored cognitive performance in trisomic mice. Additionally, the expression of brain-derived neurotrophic factor (BDNF) was markedly decreased in the hippocampus of patients with DS. Since the positive effect of exercise was paralleled by increased BDNF expression in trisomic mice, we investigated the effectiveness of a BDNF-mimetic treatment with 7,8-dihydroxyflavone at alleviating intellectual disabilities in the DS model. Pharmacological stimulation of BDNF signaling rescued synaptic plasticity and memory deficits in Ts65Dn mice. Based on our findings, Ts65Dn mice benefit from interventions aimed at promoting brain plasticity, and we provide evidence that BDNF signaling represents a potentially new pharmacological target for treatments aimed at rescuing cognitive disabilities in patients with DS.

Ras Activity Tunes the Period and Modulates the Entrainment of the Suprachiasmatic Clock

The small GTPase Ras is a universal eukaryotic cytoplasmic membrane-anchored protein, which regulates diverse downstream signal transduction pathways that play an important role in the proper functioning of neurons. Ras activity is a central regulator of structural and functional synaptic plasticity in the adult nervous system, where it channels neuronal responses to various extracellular cues allowing the organism to adapt to complex environmental stimuli. The suprachiasmatic nucleus (SCN) is the principle pacemaker of the circadian clock, and the circadian and photic regulation of Ras activity in the SCN is an important modulator of the clockwork. We have generated transgenic mouse expressing constitutively active V12-H-Ras selectively in neurons via a synapsin I promoter (synRas mice), which serves as a suitable model to study the role of neuronal Ras signaling. Modulation of Ras activity affects ERK1,2/CREB signaling and glycogen synthase kinase-3 beta expression in the SCN, which in turn modify the photoentrainment of the clock and the fine tuning the circadian period length. The main focus of this review is to offer an overview of the function of Ras signaling in the circadian rhythm and its potential role in learning and memory consolidation.

Circadian Rhythms in Plasma Brain-derived Neurotrophic Factor Differ in Men and Women

The measurement of circulating levels of brain-derived neurotrophic factor (BDNF) has been proposed to be a marker of disease and an indicator of recovery. Thus, knowing the temporal pattern and influence of potential circadian rhythms is important. Although several studies have measured BDNF at different times of day, no studies have done so while controlling for potential masking influences such as sleep and activity. Further, no previous study has examined circadian rhythms within individuals. We examined circadian rhythms in plasma BDNF while minimizing masking from behavioral and environmental factors using a 30-h constant routine (CR) protocol. In a sample of 39 healthy adults, we found significant circadian rhythms in 75% of women and 52% of men. The timing of the acrophase of the BDNF rhythm, however, was unrelated to clock time in women, while it was related to clock time in men. These results indicate that the use of single-sample measures of plasma BDNF as a marker of disease will be unreliable, especially in women. Repeated plasma BDNF samples over a 24-h period within individuals would be needed to reveal abnormalities related to disease states.

Rhythmic Bdnf and TrkB expression patterns in the prefrontal cortex are lost in aged rats

Aging brain undergoes several changes leading to a decline in cognitive functions. Memory and learning-related genes such as Creb, Bdnf and its receptor TrkB, are expressed in different brain regions including prefrontal cortex. Those genes' proteins regulate a wide range of functions such as synaptic plasticity and long-term potentiation. In this work, our objectives were: 1) to investigate whether Creb1, Bdnf and TrkB genes display endogenous circadian expression rhythms, in the prefrontal cortex of rats maintained under constant darkness conditions; 2) to study the synchronization of those temporal patterns to the local cellular clock and 3) to evaluate the aging consequences on both cognition-related genes and activating clock transcription factor, BMAL1, rhythms. A bioinformatics analysis revealed clock-responsive (E-box) sites in regulatory regions of Creb1, Bdnf and TrkB genes. Additionally, cAMP response elements (CRE) were found in Bdnf and TrkB promoters. We observed those key cognition-related factors expression oscillates in the rat prefrontal cortex. Creb1 and TrkB mRNAs display a circadian rhythm with their highest levels occurring at the second half of the 24h period. Interestingly, the cosinor analysis revealed a 12-h rhythm of Bdnf transcript levels, with peaks occurring at the second half of the subjective day and night, respectively. As expected, the BMAL1 rhythm's acrophase precedes Creb1 and first Bdnf expression peaks. Noteworthy, Creb1, Bdnf and TrkB expression rhythms are lost in the prefrontal cortex of aged rats, probably, as consequence of the loss of BMAL1 protein circadian rhythm and altered function of the local cellular clock.

Chronic Light Exposure in the Middle of the Night Disturbs the Circadian System and Emotional Regulation

In mammals, the circadian system is composed of a principal circadian oscillator located in the suprachiasmatic nucleus (SCN) and a number of subordinate oscillators in extra-SCN brain regions and peripheral tissues/organs. However, how the time-keeping functions of this multiple oscillator circuit are affected by aberrant lighting environments remains largely unknown. In the present study, we investigated the effects of chronic light exposure in the middle of the night on the circadian system by comparing the mice housed in a 12:4:4:4-h L:DLD condition with the controls in 12:12-h L:D condition. Daily rhythms in locomotor activity were analyzed and the expression patterns of protein products of clock genes Period1 and Period2 (PER1 and PER2) were examined in the SCN and extra-SCN brain regions, including the dorsal striatum, hippocampus, paraventricular nucleus (PVN), and basolateral amygdala (BLA). Following 2 weeks of housing in the L:DLD condition, animals showed disturbed daily rhythms in locomotor activity and lacked daily rhythms of PER1 and PER2 in the SCN. In the extra-SCN brain regions, the PER1 and PER2 rhythms were affected in a region-specific pattern, such that they were relatively undisturbed in the striatum and hippocampus, phase-shifted in the BLA, and abolished in the PVN. In addition, mice in the L:DLD condition showed increased anxiety-like behaviors and reduced brain-derived neurotropic factor messenger RNA expression in the hippocampus, amygdala, and medial prefrontal cortex, which are brain regions that are involved in emotional regulation. These results indicate that nighttime light exposure leads to circadian disturbances not only by abolishing the circadian rhythms in the SCN but also by inducing misalignment among brain oscillators and negatively affects emotional processing. These observations serve to identify risks associated with decisions regarding lifestyle in our modern society.

Circadian modulation of memory and plasticity gene products in a diurnal species

Cognition is modulated by circadian rhythms, in both nocturnal and diurnal species. Rhythms of clock gene expression occur in brain regions that are outside the master circadian oscillator of the suprachiasmatic nucleus and that control cognitive functions, perhaps by regulating the expression neural-plasticity genes such as brain derived neurotrophic factor (BDNF) and its high affinity receptor, tyrosine kinase B (TrkB). In the diurnal grass rat (Arvicanthis niloticus), the hippocampus shows rhythms of clock genes that are 180° out of phase with those of nocturnal rodents. Here, we examined the hypothesis that this reversal extends to the optimal phase for learning a hippocampal-dependent task and to the phase of hippocampal rhythms in BDNF/TrkB expression. We used the Morris water maze (MWM) to test for time of day differences in reference memory and monitored daily patterns of hippocampal BDNF/TrkB expression in grass rats. Grass rats showed superior long-term retention of the MWM, when the training and testing occurred during the day as compared to the night, at a time when nocturnal laboratory rats show superior retention; acquisition of the MWM was not affected by time of day. BDNF/TrkB expression was rhythmic in the hippocampus of grass rats, and the phase of the rhythms was reversed compared to that of nocturnal rodents. Our findings provide correlational evidence for the claim that the circadian regulation of cognition may involve rhythms of BDNF/TrkB expression in the hippocampus and that their phase may contribute to species differences in the optimal phase for learning.

Spontaneous sleep-wake cycle and sleep deprivation differently induce Bdnf1, Bdnf4 and Bdnf9a DNA methylation and transcripts levels in the basal forebrain and frontal cortex in rats

Brain-derived neurotrophic factor (Bdnf) regulates neuronal plasticity, slow wave activity and sleep homeostasis. Environmental stimuli control Bdnf expression through epigenetic mechanisms, but there are no data on epigenetic regulation of Bdnf by sleep or sleep deprivation. Here we investigated whether 5-methylcytosine (5mC) DNA modification at Bdnf promoters p1, p4 and p9 influences Bdnf1, Bdnf4 and Bdnf9a expression during the normal inactive phase or after sleep deprivation (SD) (3, 6 and 12 h, end-times being ZT3, ZT6 and ZT12) in rats in two brain areas involved in sleep regulation, the basal forebrain and cortex. We found a daytime variation in cortical Bdnf expression: Bdnf1 expression was highest at ZT6 and Bdnf4 lowest at ZT12. Such variation was not observed in the basal forebrain. Also Bdnf p1 and p9 methylation levels differed only in the cortex, while Bdnf p4 methylation did not vary in either area. Factorial analysis revealed that sleep deprivation significantly induced Bdnf1 and Bdnf4 with the similar pattern for Bdnf9a in both basal forebrain and cortex; 12 h of sleep deprivation decreased 5mC levels at the cortical Bdnf p4 and p9. Regression analysis between the 5mC promoter levels and the corresponding Bdnf transcript expression revealed significant negative correlations for the basal forebrain Bdnf1 and cortical Bdnf9a transcripts in only non-deprived rats, while these correlations were lost after sleep deprivation. Our results suggest that Bdnf transcription during the light phase of undisturbed sleep-wake cycle but not after SD is regulated at least partially by brain site-specific DNA methylation.

Neurotrophins: transcription and translation

Neurotrophins are powerful molecules. Small quantities of these secreted proteins exert robust effects on neuronal survival, synapse stabilization, and synaptic function. Key functions of the neurotrophins rely on these proteins being expressed at the right time and in the right place. This is especially true for BDNF, stimulus-inducible expression of which serves as an essential step in the transduction of a broad variety of extracellular stimuli into neuronal plasticity of physiologically relevant brain regions. Here we review the transcriptional and translational mechanisms that control neurotrophin expression with a particular focus on the activity-dependent regulation of BDNF.

Hippocampal CLOCK protein participates in the persistence of depressive-like behavior induced by chronic unpredictable stress

RATIONALE

Circadian disturbances are strongly linked with major depression. The circadian proteins CLOCK and BMAL1 are abundantly expressed but function differently in the suprachiasmatic nucleus (SCN) and hippocampus. However, their roles in depressive-like behavior are still poorly understood.

OBJECTIVES

To investigate the alterations of CLOCK and BMAL1 in the SCN and hippocampus in rats subjected to chronic unpredictable stress (CUS) and to explore the relationship of circadian protein and the depressive-like behavior.

RESULTS

Together with depressive-like behavior induced by CUS, CLOCK and BMAL1 in the SC were inhibited during the light period, and the peak expression of CLOCK in the hippocampus was shifted from the dark to light period. BMAL1 expression in the hippocampus was not significantly changed. Two weeks after the termination of CUS, abnormalities of CLOCK in the CA1 and CA3 endured, with unchanged depressive-like behavior, but the expression of CLOCK and BMAL1 in the SCN recovered to control levels. Knockdown of the Clock gene in CA1 induced depressive-like behavior in normal rats. CLOCK in the SCN and hippocampus may participate in the development of depressive-like behavior. However, CLOCK in the hippocampus but not SCN was involved in the long-lasting effects of CUS on depressive-like behavior. BMAL1 in the hippocampus appeared to be unrelated to the effects of CUS on depressive-like behavior.

CONCLUSION

CLOCK protein in the hippocampus but not SCN play an important role in the long-lasting depressive-like behavior induced by CUS. These findings suggest a novel therapeutic target in the development of new antidepressants focusing on the regulation of circadian rhythm.

Death by a thousand cuts in Alzheimer's disease: hypoxia--the prodrome

A wide range of clinical consequences may be associated with obstructive sleep apnea (OSA) including systemic hypertension, cardiovascular disease, pulmonary hypertension, congestive heart failure, cerebrovascular disease, glucose intolerance, impotence, gastroesophageal reflux, and obesity, to name a few. Despite this, 82 % of men and 93 % of women with OSA remain undiagnosed. OSA affects many body systems, and induces major alterations in metabolic, autonomic, and cerebral functions. Typically, OSA is characterized by recurrent chronic intermittent hypoxia (CIH), hypercapnia, hypoventilation, sleep fragmentation, peripheral and central inflammation, cerebral hypoperfusion, and cerebral glucose hypometabolism. Upregulation of oxidative stress in OSA plays an important pathogenic role in the milieu of hypoxia-induced cerebral and cardiovascular dysfunctions. Strong evidence underscores that cerebral amyloidogenesis and tau phosphorylation--two cardinal features of Alzheimer's disease (AD), are triggered by hypoxia. Mice subjected to hypoxic conditions unambiguously demonstrated upregulation in cerebral amyloid plaque formation and tau phosphorylation, as well as memory deficit. Hypoxia triggers neuronal degeneration and axonal dysfunction in both cortex and brainstem. Consequently, neurocognitive impairment in apneic/hypoxic patients is attributable to a complex interplay between CIH and stimulation of several pathological trajectories. The framework presented here helps delineate the emergence and progression of cognitive decline, and may yield insight into AD neuropathogenesis. The global impact of CIH should provide a strong rationale for treating OSA and snoring clinically, in order to ameliorate neurocognitive impairment in aged/AD patients.

Effects of sex and chronic neonatal nicotine treatment on Na²⁺/K⁺/Cl⁻ co-transporter 1, K⁺/Cl⁻ co-transporter 2, brain-derived neurotrophic factor, NMDA receptor subunit 2A and NMDA receptor subunit 2B mRNA expression in the postnatal rat hippocampus

Chronic exposure to nicotine during the first postnatal week in rats, a developmental period that corresponds to the third trimester of human gestation, results in sexually dimorphic long-term functional defects in the adult hippocampus. One potential cause could be the sex-specific differences in the maturation of GABA(A) receptor-mediated responses from excitatory to inhibitory, which depends on the expression of the Na(2+)/K(+)/Cl(-) co-transporter 1 (NKCC1) and the K(+)/Cl(-) co-transporter 2 (KCC2). In the rat hippocampus, this switch occurs during the first and second postnatal week in females and males, respectively, and is regulated by nicotinic receptor activation. Excitatory GABAergic signaling can increase brain-derived neurotrophic factor (BDNF) expression, which might exacerbate sex differences by impacting synaptogenesis. We hypothesized that chronic neonatal nicotine (CNN) exposure differentially regulates the expression of these co-transporters and BDNF in males and females. We use quantitative isotopic in situ hybridization to examine the expression of mRNAs for NKCC1, KCC2, BDNF, and NMDA receptor subunit 2A (NR2A) and NMDA receptor subunit 2B (NR2B) in the postnatal day (P) 5 and 8 rat hippocampi in both sexes that were either control-treated or with 6mg/kg/day nicotine in milk formula (CNN) via gastric intubation starting at P1. In line with prolonged GABAergic excitation, we found that at P5 males had significantly higher mRNA expression of NKCC1 and BDNF than females. CNN treatment resulted in a significant increase in KCC2 and BDNF mRNA expression in male but not female hippocampus (p<0.05). Males also had higher expression of NR2A and lower expression of NR2B at P5 compared to females (p<0.05). At P8, there were neither sex nor treatment effects on mRNA expression, indicating the end of a critical period for sensitivity to nicotine. These results suggest that differential maturation of GABA(A)R-mediated responses result in sex-specific sensitivity to nicotine during early postnatal development, potentially explaining the differential long-term effects of CNN on hippocampal function.

Daily patterns of clock and cognition-related factors are modified in the hippocampus of vitamin A-deficient rats

The circadian expression of clock and clock-controlled cognition-related genes in the hippocampus would be essential to achieve an optimal daily cognitive performance. There is some evidence that retinoid nuclear receptors (RARs and RXRs) can regulate circadian gene expression in different tissues. In this study, Holtzman male rats from control and vitamin A-deficient groups were sacrificed throughout a 24-h period and hippocampus samples were isolated every 4 or 5 h. RARα and RXRβ expression level was quantified and daily expression patterns of clock BMAL1, PER1, RORα, and REVERB genes, RORα and REVERB proteins, as well as temporal expression of cognition-related RC3 and BDNF genes were determined in the hippocampus of the two groups of rats. Our results show significant daily variations of BMAL1, PER1, RORα, and REVERB genes, RORα and REVERB proteins and, consequently, daily oscillating expression of RC3 and BDNF genes in the rat hippocampus. Vitamin A deficiency reduced RXRβ mRNA level as well as the amplitude of PER1, REVERB gene, and REVERB protein rhythms, and phase-shifted the daily peaks of BMAL1 and RORα mRNA, RORα protein, and RC3 and BDNF mRNA levels. Thus, nutritional factors, such as vitamin A and its derivatives the retinoids, might modulate daily patterns of BDNF and RC3 expression in the hippocampus, and they could be essential to maintain an optimal daily performance at molecular level in this learning-and-memory-related brain area.

Enhanced hippocampal BDNF/TrkB signaling in response to fear conditioning in an animal model of posttraumatic stress disorder

Because the majority of patients with posttraumatic stress disorder (PTSD) exhibit long-lasting traumatic fear memory, we hypothesize that enhanced fear memory consolidation is closely involved in the pathophysiology of PTSD. Brain-derived neurotrophic factor (BDNF) and its receptor, tyrosine kinase receptor B (TrkB), are crucial for hippocampal-dependent learning and memory. In particular, differential induction of BDNF gene transcripts mediated by histone acetylation plays a role in the consolidation of fear memory. In the present study, total and exon-specific mRNA and protein levels of BDNF and TrkB in the hippocampus after contextual fear conditioning (FC) were compared between rats subjected to single prolonged stress (SPS) and sham treatment. In addition, we examined the degree of histone acetylation at the promoter of each exon of the BDNF gene by chromatin immunoprecipitation (ChIP). We previously demonstrated a significant increase in contextual freezing in SPS rats. In the present study, SPS rats also showed increased total BDNF mRNA (including exons I, IV) and BDNF protein levels in the hippocampus after FC, accompanied by increased acetylation of histone H3 and H4 at the promoter of exon I and IV relative to sham-treated rats. Furthermore, the TrkB protein levels in the hippocampus of SPS rats were significantly higher than those in sham rats. These findings suggest that the enhanced levels of BDNF as well as TrkB along with epigenetic regulation of the BDNF gene during fear memory consolidation is, at least in part, associated with long-lasting fear memory in patients with PTSD.

Epigenetic regulation of BDNF gene in response to stress

Neuronal plasticity induced by changes in synaptic morphology and function is well known to play a pivotal role in leaning and memory as well as adaptation to stress. It is suggested that these plastic changes are due to orchestration of alterations in gene expression in the brain. Recent advances in molecular biology have provided evidence that epigenetic mechanisms, such as DNA methylation and histone modification, are crucial to gene transcription in the mammalian brain. Our research group has recently investigated the involvement of histone actylation at the promoter of the brain-derived neurotrophic factor (BDNF) gene in stress-induced reduction in BDNF, as well as in fear conditioning-induced enhancement of BDNF, in the rat hippocampus. The results of the stress study demonstrated that single-immobilization stress significantly reduced the levels of total, exon I, and exon IV BDNF mRNA, and also significantly reduced acetylation levels of histone H3, but not H4, at the promoter of exons I, IV, and VI. The results of the fear conditioning study showed that footshock stress significantly increased the levels of total, exon I, and exon IV BDNF mRNA, with significantly increased acetylation levels of both histone H3 and H4, at the promoter of exons I and IV, followed by enhanced freezing to fear-context exposure. These findings suggest that changes in BDNF transcription in the rat hippocampus in response to stressful stimuli are, at least in part, regulated by histone acetylation status.

Temporal dynamics of mouse hippocampal clock gene expression support memory processing

Hippocampal plasticity and mnemonic processing exhibit a striking time-of-day dependence and likely implicate a temporally structured replay of memory traces. Molecular mechanisms fulfilling the requirements of sensing time and capturing time-related information are coded in dynamics of so-called clock genes and their protein products, first discovered and described in the hypothalamic suprachiasmatic nucleus. Using real-time PCR and immunohistochemical analyses, we show that in wildtype mice core clock components (mPer1/PER1, mPer2/PER2, mCry1/CRY1, mCry2/CRY2, mClock/CLOCK, mBmal1/BMAL1) are expressed in neurons of all subregions of the hippocampus in a time-locked fashion over a 24-h (diurnal) day/night cycle. Temporal profiling of these transcriptional regulators reveals distinct and parallel peaks, at times when memory traces are usually formed and/or consolidated. The coordinated rhythmic expression of hippocampal clock gene expression is greatly disordered in mice deficient for the clock gene mPer1, a key player implicated in both, maintenance and adaptative plasticity of circadian clocks. Moreover, Per1-knockout animals are severely handicapped in a hippocampus-dependent long-term spatial learning paradigm. We propose that the dynamics of hippocampal clock gene expression imprint a temporal structure on memory processing and shape at the same time the efficacy of behavioral learning.

Changes in alternative brain-derived neurotrophic factor transcript expression in the developing human prefrontal cortex

In this study, we determined when and through which promoter brain-derived neurotrophic factor (BDNF) transcription is regulated during the protracted period of human frontal cortex development. Using quantitative real-time polymerase chain reaction, we examined the expression of the four most abundant alternative 5' exons of the BDNF gene (exons I, II, IV, and VI) in RNA extracted from the prefrontal cortex. We found that expression of transcripts I-IX and VI-IX was highest during infancy, whereas that of transcript II-IX was lowest just after birth, slowly increasing to reach a peak in toddlers. Transcript IV-IX was significantly upregulated within the first year of life, and was maintained at this level until school age. Quantification of BDNF protein revealed that levels followed a similar developmental pattern as transcript IV-IX. In situ hybridization of mRNA in cortical sections showed the highest expression in layers V and VI for all four BDNF transcripts, whereas moderate expression was observed in layers II and III. Interestingly, although low expression of BDNF was observed in cortical layer IV, this BDNF mRNA low-zone decreased in prominence with age and showed an increase in neuronal mRNA localization. In summary, our findings show that dynamic regulation of BDNF expression occurs through differential use of alternative promoters during the development of the human prefrontal cortex, particularly in the younger age groups, when the prefrontal cortex is more plastic.

Hemispheric differences in basilar dendrites and spines of pyramidal neurons in the rat prelimbic cortex: activity- and stress-induced changes

Pyramidal neurons of the rat medial prefrontal cortex have been shown to react to chronic stress by retracting their apical dendrites and by spine loss. We extended these findings by focusing on the basilar dendritic tree of layer III pyramidal neurons in both hemispheres of the rat prelimbic cortex. Animals were subjected to daily restraint stress for 1 week (6 h/day), during either the resting or the activity period. The morphology of basilar dendrites and spines of Golgi-Cox-stained neurons in the left and right hemispheres was digitally reconstructed and analyzed. We observed the following: (i) there was an inherent hemispheric asymmetry in control rats during the resting period: the number of spines on proximal dendrites was higher in the left than in the right hemisphere; (ii) basal dendrites in controls displayed a diurnal variation: there was more dendritic material during the resting period than in the activity period; (iii) chronic stress reduced the length of basal dendrites in only the right prelimbic cortex; (iv) chronic stress reduced spine density on proximal basal dendrites; (v) restraint stress during the activity period had more pronounced effects on the physiological stress parameters than restraint stress during the resting period. Our results show dynamic hemisphere-dependent structural changes in pyramidal neurons of the rat prelimbic cortex that are tightly linked to periods of resting and activity. These morphological alterations reflect the capacity of the neurons to react to external stimuli and mirror presumptive changes in neuronal communication.

Single immobilization stress differentially alters the expression profile of transcripts of the brain-derived neurotrophic factor (BDNF) gene and histone acetylation at its promoters in the rat hippocampus

Decreased levels of brain-derived neurotrophic factor (BDNF) in the hippocampus are implicated in the pathophysiology of major depression, although the mechanism has yet to be characterized. Epigenetic studies revealed that DNA methylation and histone modifications at the promoter of exons of the BDNF gene are the pivotal factors in the regulation of BDNF transcription. Histone acetylation regulates gene transcription through chromatin remodelling. We examined the influence of a single immobilization stress (SIS) at 2 h and 24 h afterwards on the levels of total BDNF mRNA with each exon mRNA by quantitative real-time PCR, acetylated histone at the promoters of the BDNF gene by chromatin immunoprecipitation followed by real-time PCR, and BDNF protein by ELISA in the rat hippocampus. SIS significantly decreased the levels of total BDNF mRNA with significantly reduced levels of exons I and IV mRNA followed by a significant reduction in BDNF protein 4 h after SIS. Significant decreases in the levels of acetylated histone H3, but not H4, were found at the promoters of exons I, IV, and VI. In contrast, no marked changes in the levels of either acetylated histone or BDNF mRNA and protein were found 24 h after SIS. This study demonstrated the involvement of histone acetylation in the regulation of BDNF transcription by SIS, and the plastic change in histone acetylation after SIS. These findings suggest that stress affects BDNF gene transcription via epigenetic regulation, and glucocorticoid may be involved in this regulation.

Brain-derived neurotropic factor and neurogenesis in the adult rat dentate gyrus: interactions with corticosterone

Flattening the diurnal corticosterone rhythm prevented the stimulating action of L-NAME (a nitric oxide synthase, NOS, inhibitor) on progenitor cell proliferation in the dentate gyrus in Lister-Hooded adult male rats. The increased expression of brain-derived neurotrophic factor (BDNF) and trkB mRNA in the dentate gyrus which otherwise occurred after L-NAME was also prevented by clamping the corticoid rhythm in adrenalectomized rats, but was restored by daily additional injections of corticosterone (which replicates the diurnal rhythm). Unilateral infusions of BDNF into the lateral ventricle increased proliferation in the dentate gyrus on the side of the infusion, but this was not observed following implantation of subcutaneous corticosterone, which flattened the diurnal corticosterone rhythm. 5HT1A mRNA in the dentate gyrus was increased on both sides of the brain by unilateral BDNF infusions, but this was also prevented by subcutaneous corticosterone pellets. These results show that the diurnal rhythm of corticosterone regulates the stimulating action of NOS inhibitors on BDNF as well as on neurogenesis in the dentate gyrus, and that BDNF becomes ineffective on both proliferation rates and 5HT1A expression in the absence of a rhythm in corticosterone. This, together with our previous findings, suggests that corticoid rhythms permit both serotonin and NO access to BDNF, and the latter to regulate progenitor cell activity.

A phase advance of the light-dark cycle stimulates production of BDNF, but not of other neurotrophins, in the adult rat cerebral cortex: association with the activation of CREB

Circadian variation in the expression of brain-derived neurotrophic factor (BDNF) indicates that BDNF is involved in the regulation of diurnal rhythms in a variety of biological processes. However, it is still unclear which brain regions alter their BDNF levels in response to external light input. Therefore, in selected brain regions of adult male rats, we investigated diurnal variation, as well as the effects of a single eight-hour phase advance of the light-dark cycle, on the levels of BDNF and of other neurotrophins. The cerebellum, hippocampus and cerebral cortex containing visual cortex (VCX) showed diurnal variation in BDNF protein levels and the VCX also in NT-3 levels. In the VCX and the region containing the entorhinal cortex and amygdala (ECX), BDNF protein levels were increased 12 h after the phase advance, while BDNF mRNA levels were increased significantly in the VCX and slightly in the ECX after 4 h. After one week, however, BDNF protein levels were reduced in eight brain regions out of 13 examined. BDNF levels in the ECX and VCX were significantly different between light rearing and dark rearing, while a hypothyroid status did not produce an effect. Cyclic AMP responsive element-binding protein (CREB), a transcription factor for BDNF, was greatly activated by the phase advance in the ECX and VCX, suggesting the existence of CREB-mediated pathways of BDNF synthesis that are responsive to external light input.

Association study of theta EEG asymmetry and brain-derived neurotrophic factor gene variants in childhood-onset mood disorder

BACKGROUND

Childhood-onset mood disorders (COMD) include various serious, disabling psychiatric conditions that are heterogeneous in presentation and etiology. Because intermediate phenotypes may help to identify genetic contributors to COMD, we tested for an association between variants in the brain-derived neurotrophic factor (BDNF) gene and theta EEG asymmetry, both of which have been independently implicated in affective disorders.

METHODS

Theta EEG asymmetry measures were calculated for a total of 191 individuals with COMD and 93 controls, who were also genotyped at seven BDNF single-nucleotide polymorphism (SNPs), two intergenic flanking SNPs, and one SNP in the lin-7 homolog C (Caenorhabditis elegans) (LIN7C) gene.

RESULTS

Adjusting for sex and ethnicity in linear models of asymmetry scores at ten brain regions, significant genotype and genotype-by-ethnicity interactions were observed for marker Val66Met in two parietal (P3/4 and P7/8) regions in the depressed group only.

CONCLUSIONS

Our results suggest that the functional Val66Met polymorphism affects theta EEG asymmetry in parietal brain regions specifically in individuals with COMD.

Differential regulation of neurotrophins and serotonergic function in mice with genetically reduced glucocorticoid receptor expression

The neurotrophin and serotonin (5-HT) hypotheses of depression were studied in a mouse model of reduced glucocorticoid receptor (GR) function (GR(+/-) mice), which recently has been proven as a murine model of predisposition for depressive behaviour under stressful conditions. In this model we studied diurnal changes in neurotrophins and serotonergic function in candidate brain regions mediating depressive behaviour. Morning and evening levels of brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) were analyzed in representative brain regions of GR(+/-) and wildtype mice. The diurnal variation of hippocampal BDNF in wildtypes with higher levels in the morning was absent in GR(+/-) mice. Hypothalamus and parietal cortex displayed enhanced BDNF levels in GR(+/-) mice. In the frontal cortex, striatum and hypothalamus NGF increased from morning to evening in both genotypes, with an exaggeration in GR(+/-) mice. The diurnal variation of 5-HT levels and turnover did not differ significantly between genotypes. It was only in the hypothalamus that the evening level of 5-HIAA was lower in GR(+/-) mice than in wildtype mice. In conclusion, the present data indicate a contribution of altered BDNF and NGF protein levels to the predisposition for depressive behaviour in the GR(+/-) mouse model of depression, but argue against an eminent role of the serotonergic system.

Learned helplessness is independent of levels of brain-derived neurotrophic factor in the hippocampus

Reduced levels of brain-derived neurotrophic factor (BDNF) in the hippocampus have been implicated in human affective disorders and behavioral stress responses. The current studies examined the role of BDNF in the behavioral consequences of inescapable stress, or learned helplessness. Inescapable stress decreased BDNF mRNA and protein in the hippocampus of sedentary rats. Rats allowed voluntary access to running wheels for either 3 or 6 weeks prior to exposure to stress were protected against stress-induced reductions of hippocampal BDNF protein. The observed prevention of stress-induced deceases in BDNF, however, occurred in a time course inconsistent with the prevention of learned helplessness by wheel running, which is evident following 6 weeks, but not 3 weeks, of wheel running. BDNF suppression in physically active rats was produced by administering a single injection of the selective serotonin reuptake inhibitor fluoxetine (10 mg/kg) just prior to stress. Despite reduced levels of hippocampal BDNF mRNA following stress, physically active rats given the combination of fluoxetine and stress remained resistant against learned helplessness. Sedentary rats given both fluoxetine and stress still demonstrated typical learned helplessness behaviors. Fluoxetine by itself reduced BDNF mRNA in sedentary rats only, but did not affect freezing or escape learning 24 h later. Finally, bilateral injections of BDNF (1 mug) into the dentate gyrus prior to stress prevented stress-induced reductions of hippocampal BDNF but did not prevent learned helplessness in sedentary rats. These data indicate that learned helplessness behaviors are independent of the presence or absence of hippocampal BDNF because blocking inescapable stress-induced BDNF suppression does not always prevent learned helplessness, and learned helplessness does not always occur in the presence of reduced BDNF. Results also suggest that the prevention of stress-induced hippocampal BDNF suppression is not necessary for the protective effect of wheel running against learned helplessness.

Role for the Clock gene in bipolar disorder

Nearly all patients with bipolar disorder have severely disrupted circadian rhythms. Treatment with mood stabilizers can restore these daily rhythms, and this is correlated with patient recovery. However, it is still uncertain whether clock abnormalities are the cause of bipolar disorder or if these rhythm disruptions are secondary to alterations in other circuits. Furthermore, the mechanism by which the circadian clock might influence mood is still unclear. With cloning and characterization of the circadian genes and recent advances in molecular biology, we are starting to understand this strong association between circadian rhythms and bipolar disorder. Recent human genetic and mouse behavioral studies indicate that the Clock gene is particularly relevant in the mood disruptions associated with this disorder. Furthermore, it appears that Clock expression outside of the central pacemaker of the suprachiasmatic nucleus (SCN) is involved in mood regulation. In this chapter, the evidence linking circadian rhythms, the Clock gene, and bipolar disorder is discussed, along with the possible biology that underlies this connection.

Semax, an analog of ACTH(4–10) with cognitive effects, regulates BDNF and trkB expression in the rat hippocampus

The heptapeptide Semax (Met-Glu-His-Phe-Pro-Gly-Pro) is an analog of the adrenocorticotropin fragment (4-10) which after intranasal application has profound effects on learning and exerts marked neuroprotective activities. Here, we found that a single application of Semax (50 microg/kg body weight) results in a maximal 1.4-fold increase of BDNF protein levels accompanying with 1.6-fold increase of trkB tyrosine phosporylation levels, and a 3-fold and a 2-fold increase of exon III BDNF and trkB mRNA levels, respectively, in the rat hippocampus. Semax-treated animals showed a distinct increase in the number of conditioned avoidance reactions. We suggest that Semax affects cognitive brain functions by modulating the expression and the activation of the hippocampal BDNF/trkB system.

Age-related disruptions of circadian rhythm and memory in the senescence-accelerated mouse (SAMP8)

Common complaints of the elderly involve impaired cognitive abilities, such as loss of memory and inability to attend. Although much research has been devoted to these cognitive impairments, other factors such as disrupted sleep patterns and increased daytime drowsiness may contribute indirectly to impaired cognitive abilities. Disrupted sleep-wake cycles may be the result of age-related changes to the internal (circadian) clock. In this article, we review recent research on aging and circadian rhythms with a focus on the senescence-accelerated mouse (SAM) as a model of aging. We explore some of the neurobiological mechanisms that appear to be responsible for our aging clock, and consider implications of this work for age-related changes in cognition.

The exercise-induced expression of BDNF within the hippocampus varies across life-span

Voluntary exercise increases hippocampal brain-derived neurotrophic factor (BDNF) expression in young animals. In this investigation we examined the induction of BDNF protein in the hippocampus of young (2 months), late middle-aged (15 months) and old (24 months) animals over 4 weeks of exercise. Average running distances decreased with age, with the old animals also maintaining a constant level of activity over time, whereas the other groups tended to increase their average running distance. All animals demonstrated a biphasic profile of BDNF protein induction, with a significant (P<0.05) increase after 1 week of exercise followed by a decrease to near sedentary levels at 2 weeks. After this, BDNF protein levels increased significantly (P<0.05), as compared to baseline, primarily only in the young animals. In whole hippocampal homogenates, only particular BDNF mRNA exons were significantly (P<0.05) changed as a result of exercise, with the largest induction occurring in young animals. BDNF protein induction may, therefore, not be directly correlated with significant mRNA changes. Exercise may represent a therapeutic tool for disorders which involve a decrease in BDNF.

Differential regulation of multiple brain-derived neurotrophic factor transcripts in the postnatal and adult rat hippocampus during development, and in response to kainate administration

Brain-derived neurotrophic factor (BDNF) is expressed at high levels in the hippocampus, where it has been implicated in physiological functions such as the modulation of synaptic strength as well as in the pathophysiology of epileptic seizures. BDNF expression is highly regulated and the BDNF gene can generate multiple transcript isoforms by alternate splicing of four 5' exons (exons I-IV) to one 3' exon (exon V). To gain insight into the regulation of different BDNF transcripts in specific hippocampal subfields during postnatal development, exon-specific riboprobes were used. Our data shows that BDNF exon I and exon II mRNAs are regulated in hippocampal subfields during postnatal development, in contrast to BDNF exon III and exon IV mRNA, which remain relatively stable through this period. Further, exons I and II show distinct temporal patterns of expression in the hippocampus: BDNF I mRNA peaks in adulthood in contrast to BDNF II mRNA which peaks at postnatal day 14 (P14). Finally, we have addressed whether kainate treatment in postnatal pups and adults regulates BDNF through the recruitment of the same, or distinct, BDNF promoters. Our data indicates that kainate-induced seizures induce strikingly different expression of distinct BDNF transcripts, both in magnitude as well as spatial patterns in the hippocampal subfields, of pups as compared to adults. These results suggest that kainate-mediated seizures differentially recruit BDNF promoters in the developing postnatal hippocampus in contrast to the adult hippocampus to achieve a hippocampal subfield specific regulation of exon-specific BDNF mRNAs.

Exercise-induced gene expression changes in the rat spinal cord

There is growing evidence that exercise benefits recovery of neuromuscular function from spinal cord injury (SCI). However, the effect of exercise on gene expression in the spinal cord is poorly understood. We used oligonucleotide microarrays to compare thoracic and lumbar regions of spinal cord of either exercising (voluntary wheel running for 21 days) or sedentary rats. The expression data were filtered using statistical tests for significance, and K-means clustering was then used to segregate lists of significantly changed genes into sets based upon expression patterns across all experimental groups. Levels of brain-derived neurotrophic factor (BDNF) protein were also measured after voluntary exercise, across different regions of the spinal cord. BDNF mRNA increased with voluntary exercise, as has been previously shown for other forms of exercise, contributed to by increases in both exon I and exon III. The exercise-induced gene expression changes identified by microarray analysis are consistent with increases in pathways promoting neuronal health, signaling, remodeling, cellular transport, and development of oligodendrocytes. Taken together these data suggest cellular pathways through which exercise may promote recovery in the SCI population.

Differential regulation of brain-derived neurotrophic factor transcripts during the consolidation of fear learning

Brain-derived neurotrophic factor (BDNF) has been implicated as a molecular mediator of learning and memory. The BDNF gene contains four differentially regulated promoters that generate four distinct mRNA transcripts, each containing a unique noncoding 5'-exon and a common 3'-coding exon. This study describes novel evidence for the differential usage of alternative BDNF promoters and 5'-exons during the consolidation of learning. We found a selective increase in BDNF transcripts containing exons I and III in the amygdala 2 h following fear conditioning, while mRNA levels of BDNF exons II and IV remained unchanged. These results provide the first evidence of differential splicing and/or differential BDNF promoter usage in response to a behaviorally relevant learning paradigm.

The timecourse of induction of brain-derived neurotrophic factor mRNA and protein in the rat hippocampus following voluntary exercise

In this study we examined the timecourse of induction of brain-derived neurotrophic factor (BDNF) mRNA and protein after 1, 3, 5, 7, 14 and 28 days of exercise in the rat. To measure the expression of mRNA for individual BDNF exons we utilized a semi-quantitative RT-PCR technique, while BDNF protein was assessed using commercial ELISA kits. We demonstrated that the distance run by animals increased significantly (P<0.05) after 4 weeks. BDNF protein was significantly (P<0.05) increased after 4 weeks of exercise, while the mRNA for individual BDNF exons increased significantly (P<0.05) over the timecourse (exon I after 1 and 28 days and exons II and V after 28 days). The Morris water maze was then utilized to demonstrate that 3 weeks of prior exercise enhanced the rate of learning on this task. Exercise, therefore, was shown to modulate BDNF induction in a time-dependent manner, and this may translate to improvements in neurotrophin-mediated tasks within the CNS.

Circadian variations in expression of the trkB receptor in adult rat hippocampus

The expression of brain-derived neurotrophic factor (BDNF) in the central nervous system (CNS) and the expression of its high-affinity trkB receptor on neuron surfaces are known to depend on neuron activity. The expression of BDNF (mRNA and protein) and trkB mRNA shows circadian oscillations in rat hippocampal homogenates. We investigated circadian variations in trkB expression in specific areas of the adult rat hippocampal formation by immunohistochemistry. In sets of two experiments performed in the spring, 39 2-month-old male Wistar rats were accustomed to a 12-h light-12-h dark cycle for 2 weeks. Three animals were then sacrificed every 4 h. Forty-micrometer-thick coronal sections of hippocampal formation were obtained and processed for trkB immunohistochemistry. Cell staining intensity was assessed by image analysis of different hippocampal areas on five sections per animal. Circadian rhythmicity was evaluated by the cosinor method. Statistically significant circadian variations in trkB expression were found in dentate gyrus, entorhinal cortex, and the CA3 and hilar regions of the hippocampus, with highest expression during the first half of the dark (activity) period. These findings suggest a relationship between trkB expression and the physiological neuronal activation of wakefulness. TrkB receptor expression in the hippocampal regions studied was continuous and changes were gradual over the 24-h cycle, suggesting that more complex regulatory mechanisms also intervened.

Exercise, antidepressant treatment, and BDNF mRNA expression in the aging brain

Principal mental disorders affecting the geriatric population include dementia and depression. A lack of trophic support is thought to contribute to the pathology of these disorders. Physical activity and antidepressant treatment increase the expression of brain-derived neurotrophic factor (BDNF) in the young rat hippocampus. Herein, we investigated the responsiveness of the aging rat hippocampus to antidepressant treatment and voluntary exercise. In situ hybridization revealed that, in young animals, exercise, antidepressant treatment, or their combination elevated BDNF mRNA levels in several hippocampal regions, most notably in the CA3, CA4, and dentate gyrus (DG). This effect was rapid (detectable at 2 days) and sustainable to 20 days. In aged (22-month-old) rats, hippocampal responsiveness to antidepressant treatment and exercise was also rapid and sustainable, but evident mostly in the CA1 and CA2. Daily swimming also revealed that small amounts of activity led to marked elevations in hippocampal BDNF mRNA. The differences in regional patterns of BDNF mRNA elevations between young and aged animals observed with running were maintained with this different exercise modality. Our results indicate that the aged brain is responsive to exercise and antidepressant treatment, and changes in regional response patterns may reflect shifts in hippocampal physiology during the lifespan.

Spatial learning is delayed and brain-derived neurotrophic factor mRNA expression inhibited by administration of MK-801 in rats

Brain-derived neurotrophic factor (BDNF) is involved in activity-dependent plasticity and interacts with the neurotransmitter glutamate. Glutamate N-methl-D-aspartate (NMDA) receptor activation increases BDNF expression, while BDNF facilitates NMDA activity, with both involved in spatial learning. Administration of the NMDA receptor antagonist MK-801 can impair this leaning. The interaction between NMDA and BDNF in learning is examined in this study. Adult male Sprague-Dawley rats received either i.p. MK-801 or saline and were trained to locate a submerged water maze platform. Sedentary and activity yoked groups were included for biochemical comparisons. Control rats quickly learned the platform location while MK-801-treated rats learned at a significantly slower rate (P < 0.0001). In situ hybridization for hippocampal BDNF mRNA indicated significant increases in the yoked and learning groups. However, MK-801 attenuated the BDNF mRNA increase in the learning and activity-yoked conditions (P < 0.05). Administration of MK-801 to the sedentary group did not alter baseline mRNA levels. These data suggest that BDNF expression is important for NMDA-dependent learning and memory. Interestingly, learning still occurs across trials independent of the NMDA and BDNF interaction. Increases in BDNF and NMDA activity may be significant components in learning and memory, and modulation of these systems may be beneficial for developing strategies to improve cognitive function.

Functional implications of seizure-induced neurogenesis

The neurobiological doctrine governing the concept of neurogenesis has undergone a revolution in the past few years. What was once considered dubious is now well accepted: new neurons are born in the adult brain. Science fiction is quickly becoming a reality as scientists discover ways to convert skin, bone, or blood cells into neurons. In the epilepsy arena, widespread interest has developed because of the evidence that neurogenesis increases after seizures, trauma, and other insults or injuries that alter seizure susceptibility. This review discusses some of the initial studies in this field, and their often surprising functional implications. The emphasis will be on the granule cells of hippocampus, because they are perhaps more relevant to epilepsy than other areas in which neurogenesis occurs throughout life, the olfactory bulb and subventricular zone. In particular, the following questions will be addressed: 1. Do granule cells that are born in the adult brain become functional, and what are the limits of their function? Do they behave homogeneously? Results from our own laboratory have focused on cells that become established outside the normal boundaries of the granule cell layer, forming a group of "ectopic" granule cells in the hilar region. 2. Is increased neurogenesis beneficial, or might it actually exacerbate seizures? Evidence is presented that supports the hypothesis that new granule cells may not necessarily act to ameliorate seizures, and might even contribute to them. Furthermore, cognitive deficits following seizures might in part be due to new circuits that develop between new cells and the host brain. 3. How do the new cells interact with the host brain? Several changes occur in the dentate gyrus after seizures, and increased neurogenesis is only one of many. What is the interdependence of this multitude of changes, if any? 4. Is neurogenesis increased after seizures in man? Research suggests that the data from human epileptics are actually inconsistent with the studies in animal models of epilepsy, because there is little evidence of increased neurogenesis in epileptic tissue resected from intractable epileptics. Yet neurogenesis has been shown to occur in humans throughout adult life. What might be the reasons for these seemingly disparate results?

The influence of specific noradrenergic and serotonergic lesions on the expression of hippocampal brain-derived neurotrophic factor transcripts following voluntary physical activity

Previous studies have shown that hippocampal brain-derived neurotrophic factor (BDNF) mRNA levels are significantly increased in rats allowed free access to exercise wheels and/or administered antidepressant medications. Enhancement of BDNF may be crucial for the clinical effect of antidepressant interventions. Since increased function of the noradrenergic and/or serotonergic systems is thought to be an important initial mechanism of antidepressant medications, we sought to test the hypothesis that noradrenergic or serotonergic function is essential for the increased BDNF transcription occurring with exercise. In addition, individual transcript variants of BDNF were examined, as evidence exists they are differentially regulated by discrete interventions, and are expressed in distinct sub-regions of the hippocampus. The neurotransmitter system-specific neurotoxins p-chloroamphetamine (serotonergic) and N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (noradrenergic) were administered to rats prior to commencing voluntary wheel-running activity. In situ hybridization experiments revealed an absence of exercise-induced full-length BDNF mRNA elevations in the hippocampi of noradrenergic-lesioned rats. In addition, the striking elevation of the exon I transcript in the dentate gyrus was removed with this noradrenergic lesion. In contrast, other transcript variants (exons II and III) were elevated in several hippocampal regions as a result of this lesion. In serotonin-lesioned rats, the significant increases in full-length BDNF, exon I and exon II mRNA levels were sustained without alteration (with the exception of exon IV in the cornus ammonis subregion 4, CA4). Overall, these results indicate that an intact noradrenergic system may be crucial for the observed ability of exercise to enhance full-length and exon I hippocampal BDNF mRNA expression. In addition, these results suggest that the promoter linked to exon I may provide a major regulatory point for BDNF mRNA expression in the dentate gyrus. Elevations of other exons, such as II and III, may require the activation of separate neurotransmitter systems and intracellular pathways.

Noradrenergic and serotonergic blockade inhibits BDNF mRNA activation following exercise and antidepressant

Antidepressants and physical exercise have been shown to increase the transcription of hippocampal brain-derived neurotrophic factor (BDNF). Much evidence regarding the initial actions of antidepressant medications as well as exercise leads to the hypothesis that noradrenergic (NE) and/or serotonergic (5-HT) activation is a key element in the BDNF transcriptional elevation common to both interventions. Currently, we used short-term beta-adrenergic, 5-HT(1A), or 5-HT(2A/C) receptor blockade to characterize the influence of NE and 5-HT systems on BDNF transcription during physical exercise and antidepressant treatment. In situ hybridization revealed that beta-adrenergic blockade significantly blunted the BDNF mRNA elevations due to exercise, and also inhibited the modest elevations in the CA3 and dentate gyrus following short-term treatment with tranylcypromine. In contrast, 5-HT(2A/C) blockade only minimally altered exercise-induced BDNF mRNA levels, but inhibited up-regulation of BDNF transcription via tranylcypromine. Finally, 5-HT(1A) blockade did not inhibit exercise-induced BDNF mRNA elevations, but significantly enhanced levels above those achieved with exercise alone in the CA4. These results suggest that NE activation via beta-adrenergic receptors may be essential for both exercise and antidepressant-induced BDNF regulation. 5-HT(1A) and 5-HT(2A/C) activation, on the other hand, appear to be most important for antidepressant-induced BDNF regulation, but may also participate significantly in exercise-induced regulation in the CA4.

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.

Brain-immune interactions in sleep

This chapter discusses various levels of interactions between the brain and the immune system in sleep. Sleep-wake behavior and the architecture of sleep are influenced by microbial products and cytokines. On the other hand, sleep processes, and perhaps also specific sleep states, appear to promote the production and/or release of certain cytokines. The effects of immune factors such as endotoxin and cytokines on sleep reveal species specificity and usually strong dependence on parameters such as substance concentration, time relative to administration or infection with microbial products, and phase relation to sleep and/or the light-dark cycle. For instance, endotoxin increased SWS and EEG SWA in humans only at very low concentrations, whereas higher concentrations increased sleep stage 2 only, but not SWS. In animals, increases in NREM sleep and SWA were more consistent over a wide range of endotoxin doses. Also, administration of pro-inflammatory cytokines such as IL-6 and IFN-alpha in humans acutely disturbed sleep while in rats such cytokines enhanced SWS and sleep. Overall, the findings in humans indicate that strong nonspecific immune responses are acutely linked to an arousing effect. Although subjects feel subjectively tired, their sleep flattens. However, some observations indicate a delayed enhancing effect on sleep which could be related to the induction of secondary, perhaps T-cell-related factors. This would also fit with results in animals in which the T-cell-derived cytokine IL-2 enhanced sleep while cytokines with immunosuppressive functions like IL-4 and L-10 suppressed sleep. The most straightforward similarity in the cascade of events inducing sleep in both animals and humans is the enhancing effect of GHRH on SWS, and possibly the involvement of the pro-inflammatory cytokine systems of IL-1 beta and TNF-alpha. The precise mechanisms through which administered cytokines influence the central nervous system sleep processes are still unclear, although extensive research has identified the involvement of various molecular intermediates, neuropeptides, and neurotransmitters (cp. Fig. 5, Section III.B). Cytokines are not only released and found in peripheral blood mononuclear cells, but also in peripheral nerves and the brain (e.g., Hansen and Krueger, 1997; März et al., 1998). Cytokines are thereby able to influence the central nervous system sleep processes through different routes. In addition, neuronal and glial sources have been reported for various cytokines as well as for their soluble receptors (e.g., Kubota et al., 2001a). Links between the immune and endocrine systems represent a further important route through which cytokines influence sleep and, vice versa, sleep-associated processes, including variations in neurotransmitter and neuronal activity may influence cytokine levels. The ability of sleep to enhance the release and/or production of certain cytokines was also discussed. Most consistent results were found for IL-2, which may indicate a sleep-associated increase in activity of the specific immune system. Furthermore, in humans the primary response to antigens following viral challenge is enhanced by sleep. In animals results are less consistent and have focused on the secondary response. The sleep-associated modulation in cytokine levels may be mediated by endocrine parameters. Patterns of endocrine activity during sleep are probably essential for the enhancement of IL-2 and T-cell diurnal functions seen in humans: Whereas prolactin and GH release stimulate Th1-derived cytokines such as IL-2, cortisol which is decreased during the beginning of nocturnal sleep inhibits Th1-derived cytokines. The immunological function of neurotrophins, in particular NGF and BDNF, has received great interest. Effects of sleep and sleep deprivation on this cytokine family are particularly relevant in view of the effects these endogenous neurotrophins can have not only on specific immune functions and the development of immunological memories, but also on synaptic reorganization and neuronal memory formation.

Environmental influences on functional outcome after a cortical infarct in the rat

The effect of postoperative housing conditions on functional outcome and brain-derived neurotrophic factor (BDNF) gene expression was evaluated 1 month after a distal ligation of the right middle cerebral artery (MCA) in spontaneously hypertensive rats. Two days postoperatively the rats were randomized into four groups; individually housed with no equipment (deprived group), individually housed with free access to a connected running wheel (running group), housed together in a large cage with no equipment (social group) or in the same size of cage furnished with bars, chains and various things to manipulate (enriched group). The enriched rats had significantly higher scores when crossing a rotating horizontal rod than deprived and running rats. The social group performed significantly better than the deprived group. The BDNF gene expression in the ipsi- and contralateral cortex, thalamus, hippocampus and cerebellum did not significantly differ between the groups. The weight of the adrenal glands was significantly increased in running rats suggesting that postischemic running may be stressful. We conclude that the beneficial effect of postischemic environmental enrichment is likely to be a combination of social and various physical activities, and that BDNF gene expression 1 month after a cortical infarct did not correlate with functional outcome.