Overall dietary energy intake, particularly the consumption of simple sugars such as fructose, has been increasing steadily in Western societies, but the effects of such diets on the brain are poorly understood. Here we used functional and structural assays to characterize the effects of excessive caloric intake on the hippocampus, a brain region important for learning and memory. Rats fed a high-fat, high-glucose diet supplemented with high-fructose corn syrup showed alterations in energy and lipid metabolism similar to clinical diabetes, with elevated fasting glucose and increased cholesterol and triglycerides. Rats maintained on this diet for eight months exhibited impaired spatial learning ability, reduced hippocampal dendritic spine density, and reduced LTP at Schaffer collateral -CA1 synapses. These changes occurred concurrently with reductions in levels of brain-derived neurotrophic factor in the hippocampus. We conclude that a high energy diet reduces hippocampal synaptic plasticity and impairs cognitive function, possibly through BDNF-mediated effects on dendritic spines. Keywordsfructose; hippocampus; long-term potentiation; obesity; diabetes; BDNF; high-fat diet Dietary energy intake has increased steadily in Western societies during the past 50 years resulting in increased obesity, diabetes and cardiovascular disease (Everitt et al., 2006). Simple sugars and saturated fats are believed to be major components of the Western diet that promote obesity and insulin resistance (Gross et al., 2004). Data from clinical, epidemiological and animal studies have suggested that excessive energy intake adversely affects the brain, particularly during aging. Studies suggest that individuals with a high energy intake are at increased risk of Alzheimer's disease (Luschsinger et al., 2002). Animal studies have shown that high-calorie diets impair the structure and function of the hippocampus, a brain region critical for learning and memory (Farr et al., 2008;Greenwood and Winocur, 1990;Kanoski et al., 2007;Molteni et al., 2002;Winocur and Greenwood, 1999;Wu et al., 2004). The adverse effects of high calorie diets on learning and memory have been associated with impaired hippocampal synaptic plasticity and neurogenesis (Farr et al., 2008;Lindqvist et al., 2006), suggesting that the hippocampus may be particularly sensitive to changes in dietary energy intake. In the present study we fed rats a diet high in saturated fats and simple sugars, and supplemented their water with high-fructose corn syrup. This diet increased fasting blood glucose levels and serum cholesterol and triglycerides. Additionally, we found that the diet impairs hippocampusdependent learning, synaptic plasticity, and dendritic spine density. These adverse effects on brain function were associated with reduced levels of BDNF in the hippocampus and suggest that "Western" diets impair synaptic function and cognition by a mechanism involving reductions in BDNF and atrophy of dendritic spines. Detailed description of the methods and procedures us...

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Protein kinase cascades likely play a critical role in the signaling events that underlie synaptic plasticity and memory. The extracellular signal-regulated kinase (ERK) cascade is suited well for such a role because its targets include regulators of gene expression. Here we report that the ERK cascade is recruited during long-term depression (LTD) of synaptic strength in area CA1 of the adult hippocampus in vivo and selectively impacts on phosphorylation of the nuclear transcription factor Elk-1. Using a combination of in vivo electrophysiology, biochemistry, pharmacology, and immunohistochemistry, we found the following: (1) ERK phosphorylation, including phosphorylation of nuclear ERK, and ERK phosphotransferase activity are increased markedly, albeit transiently, after the induction of NMDA receptor-dependent LTD at the commissural input to area CA1 pyramidal cells in the hippocampus of anesthetized adult rats; (2) LTD-inducing paired-pulse stimulation fails to produce lasting LTD in the presence of the ERK kinase inhibitor SL327, which suggests that ERK activation is necessary for the persistence of LTD; and (3) ERK activation during LTD results in increased phosphorylation of Elk-1 but not of the transcription factor cAMP response element-binding protein. Our findings indicate that the ERK cascade transduces signals from the synapse to the nucleus during LTD in hippocampal area CA1 in vivo, as it does during long-term potentiation in area CA1, but that the pattern of coupling of the ERK cascade to transcriptional regulators differs between the two forms of synaptic plasticity.

Activation of extracellular signal-regulated kinase (ERK) has been shown to be necessary for NMDA receptor-dependent long-term potentiation (LTP). We studied the role of ERK in three forms of NMDA receptor-independent LTP: LTP induced by very high-frequency stimulation (200 Hz-LTP), LTP induced by the K(+) channel blocker tetraethylammonium (TEA) (TEA-LTP), and mossy fiber (MF) LTP (MF-LTP). We found that ERK was activated in area CA1 after the induction of both 200 Hz-LTP and TEA-LTP and that this activation required the influx of Ca(2+) through voltage-gated Ca(2+) channels. Inhibition of the ERK signaling cascade with either PD 098059 or U0126 prevented the induction of both 200 Hz-LTP and TEA-LTP in area CA1. In contrast, neither PD 098059 nor U0126 prevented MF-LTP in area CA3 induced by either brief or long trains of high-frequency stimulation. U0126 also did not prevent forskolin-induced potentiation in area CA3. However, incubation of slices with forskolin, an activator of the cAMP-dependent protein kinase (PKA) cascade, did result in increases in active ERK and cAMP response element-binding protein (CREB) phosphorylation in area CA3. The forskolin-induced increase in active ERK was inhibited by U0126, whereas the increase in CREB phosphorylation was not, which suggests that in area CA3 the PKA cascade is not coupled to CREB phosphorylation via ERK. Overall, our observations indicate that activation of the ERK signaling cascade is necessary for NMDA receptor-independent LTP in area CA1 but not in area CA3 and suggest a divergence in the signaling cascades underlying NMDA receptor-independent LTP in these hippocampal subregions.

There is growing evidence that activation of either protein kinases or protein phosphatases determines the type of plasticity observed after different patterns of hippocampal stimulation. Because activation of the extracellular signal-regulated kinase (ERK) has been shown to be necessary for long-term potentiation, we investigated the regulation of ERK in long-term depression (LTD) in the adult hippocampus in vivo. We found that ERK immunoreactivity was decreased following the induction of LTD and that this decrease required NMDA receptor activation. The LTD-associated decrease in ERK immunoreactivity could be simulated in vitro via incubation of either purified ERK2 or hippocampal homogenates with either protein phosphatase 1 or protein phosphatase 2A. The protein phosphatase-dependent decrease in ERK immunoreactivity was inhibited by microcystin. Intrahippocampal administration of the protein phosphatase inhibitor okadaic acid blocked the LTD-associated decrease in ERK2, but not ERK1, immunoreactivity. Collectively, these data demonstrate that protein phosphatases can decrease ERK immunoreactivity and that such a decrease occurs with ERK2 during LTD. These observations provide the first demonstration of a biochemical alteration of ERK in LTD. Key Words: HippocampusLong-term depression-Protein phosphatase 1-Protein phosphatase 2A-Extracellular signal-regulated kinase-Plasticity. J. Neurochem. 74, 192-198 (2000).It has been hypothesized that experience-dependent changes at the neuronal level are supported by changes in synaptic strength (Hebb, 1949;Kandel and Schwartz, 1982;Lynch and Baudry, 1984;Bliss and Collingridge, 1993;Chen and Tonegawa, 1997). There have been numerous studies exploring multiple signaling cascades that underlie synaptic plasticity. Previous studies have shown that an increase in synaptic strength, i.e., longterm potentiation, is accompanied by an increase in the activity of various protein kinases (for a review, see Roberson et al., 1996) and a decrease in basal protein phosphatase activity (Blitzer et al., 1998). In contrast, a decrease in synaptic strength, i.e., long-term depression (LTD), is dependent on protein phosphatases (Mulkey et al., 1993(Mulkey et al., , 1994 and is accompanied by an increase in the activity of serine/threonine protein phosphatases (Thiels et al., 1998).We are interested in determining whether two serine/ threonine protein phosphatases, protein phosphatase 1 (PP1) and protein phosphatase 2A (PP2A), regulate protein kinases in synaptic plasticity. Of particular interest is the extracellular signal-regulated kinase (ERK). ERK activity has been shown to be increased following the induction of long-term potentiation and to be necessary for the expression of longterm potentiation (English and Sweatt, 1997). In addition, ERK activity has been shown to increase with and be necessary for hippocampal-dependent associative-fear conditioning (Atkins et al., 1998) and spatial learning (Blum et al., 1999).ERK is activated by mitogen-activated protein kinase kinas...

J. Neurochem. (2010) 114, 430–439. Abstract Proteins that control the excitability of neurons, including voltage‐dependent ion channels and neurotransmitter receptors, reside in a membrane lipid environment that includes sphingomyelin, but the influence of the metabolism of this lipid on excitability is unknown. Sphingomyelin in the plasma membrane can be cleaved by neutral sphingomyelinases (nSMase) to generate ceramides and sphingosine‐1‐phosphate (S1P) which have been shown to play a variety of roles in cellular signaling processes. We found that application of nSMase to hippocampal slices results in a selective enhancement in the population spike amplitude, resulting in fEPSP‐PS potentiation of the CA3‐CA1 schaeffer collateral synapse. Single cell recordings showed that nSMase activity increases action potential frequency in CA1 neurons in a reversible manner. Additional current clamp recordings showed that nSMase reduces the slow after‐hyperpolarization after a burst of action potentials. Mass spectrometry‐based measurements demonstrated that nSMase activity induces a rapid increase in the levels of ceramides and S1P in cells in hippocampal slices. The ability of nSMase to increase CA1 neuron excitability was blocked by an inhibitor of sphingosine kinase, the enzyme that converts ceramide to S1P. Moreover, direct intracellular application of S1P to CA1 neurons increased action potential firing. Our findings suggest roles for sphingomyelin metabolism and S1P in the positive regulation of the excitability of hippocampal neurons.

Tomosyn, a syntaxin-binding protein, is known to inhibit vesicle priming and synaptic transmission via interference with the formation of SNARE complexes. Using a lentiviral vector, we specifically overexpressed tomosyn1 in hippocampal dentate gyrus neurons in adult mice. Mice were then subjected to spatial learning and memory tasks and electrophysiological measurements from hippocampal slices. Tomosyn1-overexpression significantly impaired hippocampus-dependent spatial memory while tested in the Morris water maze. Further, tomosyn1-overexpressing mice utilize swimming strategies of lesser cognitive ability in the Morris water maze compared with control mice. Electrophysiological measurements at mossy fiber-CA3 synapses revealed impaired paired-pulse facilitation in the mossy fiber of tomosyn1-overexpressing mice. This study provides evidence for novel roles for tomosyn1 in hippocampus-dependent spatial learning and memory, potentially via decreased synaptic transmission in mossy fiber-CA3 synapses. Moreover, it provides new insight regarding the role of the hippocampal dentate gyrus and mossy fiber-CA3 synapses in swimming strategy preference, and in learning and memory.

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