Photochemical, photophysical and redox properties of novel fulgimide derivatives with attached 2,2 '-bipyridine (bpy) and [M(bpy)(3)](2+) (M = Ru and Os) moieties

Fulgimides monosubstituted with [M(bpy)(3)](2+) (M = Ru, Os; bpy = 2,2'-bipyridine) chromophore units and with a single bpy group were synthesized and investigated as components of conceivable dinuclear photochromic switches of luminescence. The E-, Z- and closed-ring (C) photoisomer forms of the bpy-bound fulgimide were successfully separated by semi-preparative HPLC. The same procedure failed, however, in the case of the [M(bpy)(3)](2+)-substituted fulgimides. Energy transfer from the excited photochromic unit to the metal-bpy centre competes with the fulgimide cyclization, reducing the photocyclization quantum yields by approximately one order of magnitude compared to the non-complexed fulgimide-bpy ligand (phi(EC) = 0.17, phi(EZ) = 0.071, phi(ZE) = 0.15 at lambda(exc) = 334 nm). The cycloreversion of the fulgimide-bpy ligand is less efficient (phi(CE) = 0.047 at lambda(exc) = 520 nm). The intensity of the (MLCT)-M-3-based luminescence of the metal-bpy chromophore (in MeCN, phi(deaer) = 6.6 x 10(-2) and tau(deaer) = 1.09 mu s for Ru; phi(deaer) = 6.7 x 10(-3) and tau(deaer) = 62 ns for Os) is not affected by the fulgimide photoconversion. These results and supporting spectro-electrochemical data reveal that the lowest triplet excited states of the photochromic fulgimide moiety in all its E-, Z- and closed-ring forms lie above the lowest 3MLCT levels of the attached ruthenium and osmium chromophores. The actual components are therefore unlikely to form a triad acting as functional switch of energy transfer from [Ru(bpy)(3)](2+) to [Os(bpy)(3)](2+) through the photochromic fulgimide bridge.

The neuroprotective potential of flavonoids: a multiplicity of effects

Flavonoids exert a multiplicity of neuroprotective actions within the brain, including a potential to protect neurons against injury induced by neurotoxins, an ability to suppress neuroinflammation, and the potential to promote memory, learning and cognitive function. These effects appear to be underpinned by two common processes. Firstly, they interact with critical protein and lipid kinase signalling cascades in the brain leading to an inhibition of apoptosis triggered by neurotoxic species and to a promotion of neuronal survival and synaptic plasticity. Secondly, they induce beneficial effects on the vascular system leading to changes in cerebrovascular blood flow capable of causing angiogenesis, neurogenesis and changes in neuronal morphology. Through these mechanisms, the consumption of flavonoid-rich foods throughout life holds the potential to limit neurodegeneration and to prevent or reverse age-dependent loses in cognitive performance. The intense interest in the development of drugs capable of enhancing brain function means that flavonoids may represent important precursor molecules in the quest to develop of a new generation of brain enhancing drugs.

Flavonoids and cognition: The molecular mechanisms underlying their behavioural effects

Evidence Suggests that a group of phytochemicals known as flavonoids are highly effective in reversing age-related declines in neuro-cognitive performance through their ability to interact with the cellular and molecular architecture of the brain responsible for memory and by reducing neuronal loss due to neurodegenerative Processes. In particular, they may increase the number of, and strength of, connections between neurons, via their specific interactions with the ERK and Akt signalling pathways, leading to an increase in neurotrophins Such as BDNF. Concurrently, their effects on the peripheral and Cerebral vascular system may also lead to enhancements in cognitive performance through increased brain blood flow and an ability to initiate neurogenesis in the hippocampus. Finally, they have also been shown to reduce neuronal damage and losses induced by various neurotoxic species and neuroinflammation. Together, these processes act to maintain the number and quality of synaptic connections in the brain. a factor known to be essential for efficient LTP, synaptic plasticity and ultimately the efficient working of memory. (C) 2009 Elsevier Inc. All rights reserved.

The impact of flavonoids on memory: physiological and molecular considerations

Emerging evidence suggests that a group of dietary-derived phytochemicals known as flavonoids are able to induce improvements in memory acquisition, consolidation, storage and retrieval. These low molecular weight polyphenols are widespread in the human diet, are absorbed to only a limited degree and localise in the brain at low concentration. However, they have been found to be highly effective in reversing age-related declines in memory via their ability to interact with the cellular and molecular architecture of the brain responsible for memory. These interactions include an ability to activate signalling pathways, critical in controlling synaptic plasticity, and a potential to induce vascular effects capable of causing new nerve cell growth in the hippocampus. Their ability to activate the extracellular signal-regulated kinase (ERK1/2) and the protein kinase B (PKB/Akt) signalling pathways, leading to the activation of the cAMP response element-binding protein (CREB), a transcription factor responsible for increasing the expression of a number of neurotrophins important in de. ning memory, will be discussed. How these effects lead to improvements in memory through induction of synapse growth and connectivity, increases in dendritic spine density and the functional integration of old and new neurons will be illustrated. The overall goal of this critical review is to emphasize future areas of investigation as well as to highlight these dietary agents as promising candidates for the design of memory-enhancing drugs with relevance to normal and pathological brain ageing (161 references).

Food for thought: the role of dietary flavonoids in enhancing human memory, learning and neuro-cognitive performance

Emerging evidence suggests that dietary-derived flavonoids have the potential to improve human memory and neuro-cognitive performance via their ability to protect vulnerable neurons, enhance existing neuronal function and stimulate neuronal regeneration. Long-term potentiation (LTP) is widely considered to be one of the major mechanisms underlying memory acquisition, consolidation and storage in the brain and is known to be controlled at the molecular level by the activation of a number of neuronal signalling pathways. These pathways include the phosphatidylinositol-3 kinase/protein kinase B/Akt (Akt), protein kinase C, protein kinase A, Ca-calmodulin kinase and mitogen-activated protein kinase pathways. Growing evidence suggests that flavonoids exert effects on LTP, and consequently memory and cognitive performance, through their interactions with these signalling pathways. Of particular interest is the ability of flavonoids to activate the extracellular signal-regulated kinase and the Akt signalling pathways leading to the activation of the cAMP-response element-binding protein, a transcription factor responsible for increasing the expression of a number of neurotrophins important in LTP and long-term memory. One such neurotrophin is brain-derived neurotrophic factor, which is known to be crucial in controlling synapse growth, in promoting an increase in dendritic spine density and in enhancing synaptic receptor density. The present review explores the potential of flavonoids and their metabolite forms to promote memory and learning through their interactions with neuronal signalling pathways pivotal in controlling LTP and memory in human subjects.

The interactions of flavonoids within neuronal signalling pathways

Emerging evidence suggests that dietary phytochemicals, in particular flavonoids, may exert beneficial effects in the central nervous system by protecting neurons against stress-induced injury, by suppressing neuroinflammation and by promoting neurocognitive performance, through changes in synaptic plasticity. It is likely that flavonoids exert such effects in neurons, through selective actions on different components within a number of protein kinase and lipid kinase signalling cascades, such as phosphatidylinositol-3 kinase (PI3K)/Akt, protein kinase C and mitogen-activated protein kinase. This review details the potential inhibitory or stimulatory actions of flavonoids within these pathways, and describes how such interactions are likely to affect cellular function through changes in the activation state of target molecules and/or by modulating gene expression. Although, precise sites of action are presently unknown, their abilities to: (1) bind to ATP binding sites on enzymes and receptors; (2) modulate the activity of kinases directly; (3) affect the function of important phosphatases; (4) preserve neuronal Ca2+ homeostasis; and (5) modulate signalling cascades lying downstream of kinases, are explored. Future research directions are outlined in relation to their precise site(s) of action within the signalling pathways and the sequence of events that allow them to regulate neuronal function in the central nervous system.

(-)Epicatechin stimulates ERK-dependent cyclic AMP response element activity and up-regulates GluR2 in cortical neurons

Emerging evidence suggests that the cellular actions of flavonoids relate not simply to their antioxidant potential but also to the modulation of protein kinase signalling pathways. We investigated in primary cortical neurons, the ability of the flavan-3-ol, (-)epicatechin, and its human metabolites at physiologically relevant concentrations, to stimulate phosphorylation of the transcription factor cAMP-response element binding protein (CREB), a regulator of neuronal viability and synaptic plasticity. (-)Epicatechin at 100-300 nmol/L stimulated a rapid, extracellular signal-regulated kinase (ERK)- and PI3K-dependent, increase in CREB phosphorylation. At micromolar concentrations, stimulation was no longer apparent and at the highest concentration tested (30 mu mol/L) (-)epicatechin was inhibitory. (-)Epicatechin also stimulated ERK and Akt phosphorylation with similar bell-shaped concentration-response characteristics. The human metabolite 3 '-O-methyl-(-)epicatechin was as effective as (-)epicatechin at stimulating ERK phosphorylation, but (-)epicatechin glucuronide was inactive. (-)Epicatechin and 3 '-O-methyl-(-)epicatechin treatments (100 nmol/L) increased CRE-luciferase activity in cortical neurons in a partially ERK-dependent manner, suggesting the potential to increase CREB-mediated gene expression. mRNA levels of the glutamate receptor subunit GluR2 increased by 60%, measured 18 h after a 15 min exposure to (-)epicatechin and this translated into an increase in GluR2 protein. Thus, (-)epicatechin has the potential to increase CREB-regulated gene expression and increase GluR2 levels and thus modulate neurotransmission, plasticity and synaptogenesis.

The impact of flavonoids on spatial memory in rodents: from behaviour to underlying hippocampal mechanisms

Emerging evidence suggests that a group of dietary-derived phytochemicals known as flavonoids are able to induce improvements in memory, learning and cognition. Flavonoids have been shown to modulate critical neuronal signalling pathways involved in processes of memory, and therefore are likely to affect synaptic plasticity and long-term potentiation mechanisms, widely considered to provide a basis for memory. Animal dietary supplementation studies have further shown that flavonoid-rich foods are able to reverse age-related spatial memory and spatial learning impairments. A more accurate understanding of how a particular spatial memory task works and of which aspects of memory and learning can be assessed in each case, are necessary for a correct interpretation of data relating to diet-cognition experiments. Further understanding of how specific behavioural tasks relate to the functioning of hippocampal circuitry during learning processes might be also elucidative of the specific observed memory improvements. The overall goal of this review is to give an overview of how the hippocampal circuitry operates as a memory system during behavioural tasks, which we believe will provide a new insight into the underlying mechanisms of the action of flavonoids on cognition.

G protein ss gamma subunits mediate presynaptic inhibition of transmitter release from rat superior cervical ganglion neurones in culture

The activation of presynaptic G protein-coupled receptors (GPCRs) is widely reported to inhibit transmitter release; however, the lack of accessibility of many presynaptic terminals has limited direct analysis of signalling mediators. We studied GPCR-mediated inhibition of fast cholinergic transmission between superior cervical ganglion neurones (SCGNs) in culture. The adrenoceptor agonist noradrenaline (NA) caused a dose-related reduction in evoked excitatory postsynaptic potentials (EPSPs). NA-induced EPSP decrease was accompanied by effects on the presynaptic action potential (AP), reducing AP duration and amplitude of the after-hyperpolarization (AHP), without affecting the pre- and postsynaptic membrane potential. All effects of NA were blocked by yohimbine and synaptic transmission was reduced by clonidine, consistent with an action at presynaptic alpha 2-adrenoceptors. NA-induced inhibition of transmission was sensitive to pre-incubation of SCGNs with pertussis toxin (PTX), implicating the involvement of G alpha(i)/(o)beta y subunits. Expression of G alpha transducin, an agent which sequesters G protein beta gamma (G beta y) subunits, in the presynaptic neurone caused a time-dependent attenuation of NA-induced inhibition. Injection of purified G beta gamma subunits into the presynaptic neurone inhibited transmission, and also reduced the AHP amplitude. Furthermore, NA-induced inhibition was occluded by pre-injection of G beta gamma subunits. The Ca2+ channel blocker Cd2+ mimicked NA effects on transmitter release. Cd2+, NA and G beta gamma subunits also inhibited somatic Ca2+ current. In contrast to effects on AP-evoked transmitter release, NA had no clear action on AP-independent EPSPs induced by hypertonic solutions. These results demonstrate that G beta gamma subunits functionally mediate inhibition of transmitter release by alpha 2-adrenoceptors and represent important regulators of synaptic transmission at mammalian presynaptic terminals.

The phytocannabinoid Δ9-tetrahydrocannabivarin modulates inhibitory neurotransmission in the cerebellum

Background and purpose: The phytocannabinoid Delta(9)-tetrahydrocannabivarin (Delta(9)-THCV) has been reported to exhibit a diverse pharmacology; here, we investigate functional effects of Delta(9)-THCV, extracted from Cannabis sativa, using electrophysiological techniques to define its mechanism of action in the CNS. Experimental approach: Effects of Delta(9)-THCV and synthetic cannabinoid agents on inhibitory neurotransmission at interneurone-Purkinje cell (IN-PC) synapses were correlated with effects on spontaneous PC output using single-cell and multi-electrode array (MEA) electrophysiological recordings respectively, in mouse cerebellar brain slices in vitro. Key results: The cannabinoid receptor agonist WIN 55,212-2 (WIN55) decreased miniature inhibitory postsynaptic current (mIPSC) frequency at IN-PC synapses. WIN55-induced inhibition was reversed by Delta(9)-THCV, and also by the CB1 receptor antagonist AM251; Delta(9)-THCV or AM251 acted to increase mIPSC frequency beyond basal values. When applied alone, Delta(9)-THCV, AM251 or rimonabant increased mIPSC frequency. Pre-incubation with Delta(9)-THCV blocked WIN55-induced inhibition. In MEA recordings, WIN55 increased PC spike firing rate; Delta(9)-THCV and AM251 acted in the opposite direction to decrease spike firing. The effects of Delta(9)-THCV and WIN55 were attenuated by the GABA(A) receptor antagonist bicuculline methiodide. Conclusions and implications: We show for the first time that Delta(9)-THCV acts as a functional CB1 receptor antagonist in the CNS to modulate inhibitory neurotransmission at IN-PC synapses and spontaneous PC output. Delta(9)-THCV- and AM251-induced increases in mIPSC frequency beyond basal levels were consistent with basal CB1 receptor activity. WIN55-induced increases in PC spike firing rate were consistent with synaptic disinhibition; whilst Delta(9)-THCV-and AM251-induced decreases in spike firing suggest a mechanism of PC inhibition.

Evidence that GABA rho subunits contribute to functional ionotropic GABA receptors in mouse cerebellar Purkinje cells

Ionotropic gamma-amino butyric acid (GABA) receptors composed of heterogeneous molecular subunits are major mediators of inhibitory responses in the adult CNS. Here, we describe a novel ionotropic GABA receptor in mouse cerebellar Purkinje cells (PCs) using agents reported to have increased affinity for rho subunit-containing GABA(C) over other GABA receptors. Exogenous application of the GABA(C)-preferring agonist cis-4-aminocrotonic acid (CACA) evoked whole-cell currents in PCs, whilst equimolar concentrations of GABA evoked larger currents. CACA-evoked currents had a greater sensitivity to the selective GABA(C) antagonist (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA) than GABA-evoked currents. Focal application of agonists produced a differential response profile; CACA-evoked currents displayed a much more pronounced attenuation with increasing distance from the PC soma, displayed a slower time-to-peak and exhibited less desensitization than GABA-evoked currents. However, CACA-evoked currents were also completely blocked by bicuculline, a selective agent for GABA(A) receptors. Thus, we describe a population of ionotropic GABA receptors with a mixed GABA(A)/GABA(C) pharmacology. TPMPA reduced inhibitory synaptic transmission at interneurone-Purkinje cell (IN-PC) synapses, causing clear reductions in miniature inhibitory postsynaptic current (mIPSC) amplitude and frequency. Combined application of NO-711 (a selective GABA transporter subtype 1 (GAT-1) antagonist) and SNAP-5114 (a GAT-(2)/3/4 antagonist) induced a tonic GABA conductance in PCs; however, TPMPA had no effect on this current. Immunohistochemical studies suggest that rho subunits are expressed predominantly in PC soma and proximal dendritic compartments with a lower level of expression in more distal dendrites; this selective immunoreactivity contrasted with a more uniform distribution of GABA(A) alpha 1 subunits in PCs. Finally, co-immunoprecipitation studies suggest that rho subunits can form complexes with GABA(A) receptor alpha 1 subunits in the cerebellar cortex. Overall, these data suggest that rho subunits contribute to functional ionotropic receptors that mediate a component of phasic inhibitory GABAergic transmission at IN-PC synapses in the cerebellum.

Agonist-dependent internalization of D2 receptors: Imaging quantification by confocal microscopy

In positron emission tomography and single photon emission computed tomography studies using D2 dopamine (DA) receptor radiotracers, a decrease in radiotracer binding potential (BP) is usually interpreted in terms of increased competition with synaptic DA. However, some data suggest that this signal may also reflect agonist (DA)-induced increases in D2 receptor (D2R) internalization, a process which would presumably also decrease the population of receptors available for binding to hydrophilic radioligands. To advance interpretation of alterations in D2 radiotracer BP, direct methods of assessment of D2R internalization are required. Here, we describe a confocal microscopy-based approach for the quantification of agonist-dependent receptor internalization. The method relies upon double-labeling of the receptors with antibodies directed against intracellular as well as extracellular epitopes. Following agonist stimulation, DA D2R internalization was quantified by differentiating, in optical cell sections, the signal due to the staining of the extracellular from intracellular epitopes of D2Rs. Receptor internalization was increased in the presence of the D2 agonists DA and bromocriptine, but not the D1 agonist SKF38393. Pretreatment with either the D2 antagonist sulpiride, or inhibitors of internalization (phenylarsine oxide and high molarity sucrose), blocked D2-agonist induced receptor internalization, thus validating this method in vitro. This approach therefore provides a direct and streamlined methodology for investigating the pharmacological and mechanistic aspects of D2R internalization, and should inform the interpretation of results from in vivo receptor imaging studies.

Inverse problems in dynamic cognitive modeling

Inverse problems for dynamical system models of cognitive processes comprise the determination of synaptic weight matrices or kernel functions for neural networks or neural/dynamic field models, respectively. We introduce dynamic cognitive modeling as a three tier top-down approach where cognitive processes are first described as algorithms that operate on complex symbolic data structures. Second, symbolic expressions and operations are represented by states and transformations in abstract vector spaces. Third, prescribed trajectories through representation space are implemented in neurodynamical systems. We discuss the Amari equation for a neural/dynamic field theory as a special case and show that the kernel construction problem is particularly ill-posed. We suggest a Tikhonov-Hebbian learning method as regularization technique and demonstrate its validity and robustness for basic examples of cognitive computations.

The 'glial' glutamate transporter, EAAT2 (Glt-1) accounts for high affinity glutamate uptake into adult rodent nerve endings

The excitatory amino acid transporters (EAAT) removes neurotransmitters glutamate and aspartate from the synaptic cleft. Most CNS glutamate uptake is mediated by EAAT2 into glia, though nerve terminals show evidence for uptake, through an unknown transporter. Reverse-transcriptase PCR identified the expression of EAAT1, EAAT2, EAAT3 and EAAT4 mRNAs in primary cultures of mouse cortical or striatal neurones. We have used synaptosomes and glial plasmalemmal vesicles (GPV) from adult mouse and rat CNS to identify the nerve terminal transporter. Western blotting showed detectable levels of the transporters EAAT1 (GLAST) and EAAT2 (Glt-1) in both synaptosomes and GPVs. Uptake of [3H]D-aspartate or [3H]L-glutamate into these preparations revealed sodium-dependent uptake in GPV and synaptosomes which was inhibited by a range of EAAT blockers: dihydrokainate, serine-o-sulfate, l-trans-2,4-pyrrolidine dicarboxylate (PDC) (+/-)-threo-3-methylglutamate and (2S,4R )-4-methylglutamate. The IC50 values found for these compounds suggested functional expression of the 'glial, transporter, EAAT2 in nerve terminals. Additionally blockade of the majority EAAT2 uptake sites with 100 micro m dihydrokainate, failed to unmask any functional non-EAAT2 uptake sites. The data presented in this study indicate that EAAT2 is the predominant nerve terminal glutamate transporter in the adult rodent CNS.

Ischaemia differentially regulates GABAB receptor subunits in organotypic hippocampal slice cultures

Reduced synaptic inhibition due to dysfunction of ionotropic GABAA receptors has been proposed as one factor in cerebral ischaemia-induced excitotoxic cell death. However, the participation of the inhibitory metabotropic GABAB receptors in these pathological processes has not been extensively investigated. We used oxygen–glucose deprivation (OGD) and NMDA-induced excitotoxicity as models to investigate whether ischaemia-like challenges alter the protein levels of GABAB1 and GABAB2 receptor subunits in rat organotypic hippocampal slice cultures. Twenty-four hours after the insult both OGD and NMDA produced a marked decrease in the total levels of GABAB2 (75%), while there was no significant change in the levels of GABAB1 after OGD, but an increase after NMDA treatment (100%). The GABAB receptor agonist baclofen (100 μM) was neuroprotective following OGD or NMDA treatment if added before or during the insult. GABAB receptors comprise heterodimers of GABAB1 and GABAB2 subunits and our results suggest that the separate subunits are independently regulated in response to extreme neuronal stress. However, because GABAB2 is required for functional surface expression, down-regulation of this subunit removes an important inhibitory feedback mechanism under pathological conditions.