[Role of actin microfilaments in depression of acetylcholine-induced current in Helix lucorum neurons on cellular analogue of habituation]

Toxins that impair the function of actin microfilaments in cytoskeleton, cytochalasin B (disrupts microfilaments by inhibiting actin polymerization) and phalloidin (binds polymeric F-actin, stabilizing it and interfering with the function of actin-rich structures) reduce the depression of acetylcholine-induced inward current in Helix lucorum command neurons of defensive behavior during rhythmical local acetylcholine applications to soma (cellular analogue of habituation). These results and mathematical simulation allow us to suggest that the depression of cholinosensitivity of extrasynaptic membrane zones in command neurons on the cellular analogue of habituation is associated with the involvement of actin microfilaments in reduction of the number of membrane cholinoreceptors.

PPARγ agonist improves neuronal insulin receptor function in hippocampus and brain mitochondria function in rats with insulin resistance induced by long term high-fat diets

We previously demonstrated that a high-fat diet (HFD) consumption can cause not only peripheral insulin resistance, but also neuronal insulin resistance. Moreover, the consumption of an HFD has been shown to cause mitochondrial dysfunction in both the skeletal muscle and liver. Rosiglitazone, a peroxizome proliferator-activated receptor-γ ligand, is a drug used to treat type 2 diabetes mellitus. Recent studies suggested that rosiglitazone can improve learning and memory in both human and animal models. However, the effects of rosiglitazone on neuronal insulin resistance and brain mitochondria after the HFD consumption have not yet been investigated. Therefore, we tested the hypothesis that rosiglitazone improves neuronal insulin resistance caused by a HFD via attenuating the dysfunction of neuronal insulin receptors and brain mitochondria. Rosiglitazone (5 mg/kg · d) was given for 14 d to rats that were fed with either a HFD or normal diet for 12 wk. After the 14(th) week, all animals were euthanized, and their brains were removed and examined for insulin-induced long-term depression, neuronal insulin signaling, and brain mitochondrial function. We found that rosiglitazone significantly improved peripheral insulin resistance and insulin-induced long-term depression and increased neuronal Akt/PKB-ser phosphorylation in response to insulin. Furthermore, rosiglitazone prevented brain mitochondrial conformational changes and attenuated brain mitochondrial swelling, brain mitochondrial membrane potential changes, and brain mitochondrial ROS production. Our data suggest that neuronal insulin resistance and the impairment of brain mitochondria caused by a 12-wk HFD consumption can be reversed by rosiglitazone.

[The fundaments of neuronal plasticity]

Plasticity of the nervous system is determined by the modification of efficacy ofsynaptic transmission: long-term potentiation and long-term depression. Different modern technical approaches such as: registration of ionic currents in single neuron, molecular-genetic analysis, neurovisualization, and others reveal the molecular mechanisms of synaptic plasticity. The understanding of these mechanisms, in its turn, stimulates the development of methods of pharmacological correction of different forms of brain pathology such as Alzheimer disease, parkinsonism, alcoholism, aging and others.

Roles of p75(NTR), long-term depression, and cholinergic transmission in anxiety and acute stress coping

BACKGROUND

Stress is causally associated with anxiety. Although the underlying cellular mechanisms are not well understood, the basal forebrain cholinergic neurons have been implicated in stress response. p75(NTR) is a panneurotrophin receptor expressed almost exclusively in basal forebrain cholinergic neurons in adult brain. This study investigated whether and how p75(NTR), via regulation of the cholinergic system and hippocampal synaptic plasticity, influences stress-related behaviors.

METHODS

We used a combination of slice electrophysiology, behavioral analyses, pharmacology, in vivo microdialysis, and neuronal activity mapping to assess the role of p75(NTR) in mood and stress-related behaviors and its underlying cellular and molecular mechanisms.

RESULTS

We show that acute stress enables hippocampal long-term depression (LTD) in adult wild-type mice but not in mice lacking p75(NTR). The p75(NTR) mutant mice also exhibit two distinct behavioral impairments: baseline anxiety-like behavior and a deficit in coping with and recovering from stressful situations. Blockade of stress-enabled LTD with a GluA2-derived peptide impaired stress recovery without affecting baseline anxiety. Pharmacological manipulations of cholinergic transmission mimicked the p75(NTR) perturbation in both baseline anxiety and responses to acute stress. Finally, we show evidence of misregulated cholinergic signaling in animals with p75(NTR) deletion.

CONCLUSIONS

Our results suggest that loss of p75(NTR) leads to changes in hippocampal cholinergic signaling, which may be involved in regulation of stress-enabled hippocampal LTD and in modulating behaviors related to stress and anxiety.

An anticholinergic reverses motor control and corticostriatal LTD deficits in Dyt1 ΔGAG knock-in mice

DYT1 early-onset generalized torsion dystonia is an inherited movement disorder associated with mutations in DYT1 that codes for torsinA protein. The most common mutation seen in this gene is a trinucleotide deletion of GAG. We previously reported a motor control deficit on a beam-walking task in our Dyt1 ΔGAG knock-in heterozygous mice. In this report we show the reversal of this motor deficit with the anticholinergic trihexyphenidyl (THP), a drug commonly used to treat movement problems in dystonia patients. THP also restored the reduced corticostriatal long-term depression (LTD) observed in these mice. Corticostriatal LTD has long been known to be dependent on D2 receptor activation. In this mouse model, striatal D2 receptors were expressed at lower quantities in comparison to wild-type mice. Furthermore, the mice were also partially resistant to FPL64176, an agonist of L-type calcium channels that have been previously reported to cause severe dystonic-like symptoms in wild-type mice. Our findings collectively suggest that altered communication between cholinergic interneurons and medium spiny neurons is responsible for the LTD deficit and that this synaptic plasticity modification may be involved in the striatal motor control abnormalities in our mouse model of DYT1 dystonia.

The facilitative effects of bilobalide, a unique constituent of Ginkgo biloba, on synaptic transmission and plasticity in hippocampal subfields

Bilobalide, a unique constituent of Ginkgo biloba, has been reported to potentiate population spikes in hippocampal CA1 pyramidal cells and to protect the brain against cell death. In this study, the effects of bilobalide on synaptic transmission and its plasticity in rat hippocampal subfields were electrophysiologically investigated. Bilobalide (50 μM) significantly potentiated the input-output relationship at Schaffer collateral (SC)-CA1 synapses but not at medial perforant path (MPP)-dentate gyrus (DG), lateral perforant path (LPP)-DG, or mossy fiber (MF)-CA3 synapses. Facilitative effects of bilobalide on synaptic plasticity were only observed at MPP-DG synapses, in which the induction of long-term depression was blocked in the presence of bilobalide. However, no effect on synaptic plasticity was observed at SC-CA1 synapses. These results suggest that bilobalide has differential effects on synaptic efficacy in each hippocampal subfield.

NMDA receptor subunit composition determines the polarity of leptin-induced synaptic plasticity

Leptin is a hormone that crosses the blood-brain barrier and regulates numerous CNS functions. The hippocampus in particular is an important site for leptin action. Indeed, leptin markedly influences excitatory synaptic transmission and synaptic plasticity in this brain region. Recent studies indicate that leptin modulation of hippocampal excitatory synaptic transmission is age-dependent however the cellular basis for this is unclear. Here we show that early in development leptin evokes a transient (P11-18) or persistent (P5-8) depression of synaptic transmission, whereas leptin evokes a long lasting increase (LTP) in synaptic strength in adulthood. The synaptic depressions induced by leptin required activation of NMDA receptor GluN2B subunits and the ERK signalling cascade. Conversely, leptin-induced LTP in adult was mediated by GluN2A subunits and involved PI 3-kinase dependent signalling. In addition, low-frequency stimulus (LFS)-evoked LTD occluded the persistent effects of leptin at P5-8 and vice versa. Similarly, synaptically-induced LTP occluded the persistent increase in synaptic transmission induced by leptin, indicating that similar expression mechanisms underlie leptin-induced LTD and LFS-induced LTD at P5-8, and leptin-induced LTP and HFS-induced LTP in adult. These findings have important implications for the role of leptin in hippocampal synaptic function during early neuronal development and in aging.

[Chronic effects of oligomeric Aβ(1-42) on hippocampal synaptic plasticity in vivo]

Synaptic plasticity, including long-term potentiation (LTP) and long-term depression (LTD), is widely considered as one of the major mechanisms underlying learning and memory. This study explored hippocampal synaptic plasticity and spatial memory formation of an Alzheimer's disease (AD) rat model established by intrahippocampal injection of oligomeric Aβ(1-42). Twenty four Sprague-Dawley rats at 2.5 months of age were randomly divided into AD and control groups, and were bilaterally injected with 5 μg oligomeric Aβ(1-42) or normal saline into dentate gyrus (DG) of hippocampus. Morris water maze test was used to observe the capability of learning and memory of two groups, 30 d after injection. To investigate the variations of paired-pulse facilitation (PPF) and range of synaptic plasticity, field potentials were recorded in the DG of the dorsal hippocampus by stimulating the perforant path (PP). The results showed that oligomeric Aβ(1-42) obviously impaired spatial memory formation in rats (P < 0.05). Furthermore, oligomeric Aβ(1-42) reduced the PPF ratio (P < 0.05) and hippocampal LTP formation (P < 0.05), while facilitated the hippocampal LTD formation (P < 0.05). These data suggest that chronic Aβ aggregation impairs synaptic plasticity of hippocampal PP-DG pathway, which may be involved in the spatial memory deficit in AD rats.

Presynaptic induction and expression of timing-dependent long-term depression demonstrated by compartment-specific photorelease of a use-dependent NMDA receptor antagonist

NMDA receptors are important for synaptic plasticity, including long-term potentiation (LTP) and long-term depression (LTD). To help investigate the precise location of the NMDA receptors that are required for different types of synaptic plasticity, we synthesized a caged form of the use-dependent NMDA receptor antagonist MK801, which we loaded into individual neurons in vitro, followed by compartment-specific uncaging. We used this method to investigate timing-dependent plasticity at layer 4-layer 2/3 synapses of mouse barrel cortex. Somatodendritic photorelease of MK801 in the postsynaptic neuron produced a use-dependent block of synaptic NMDA receptor-mediated currents and prevented the induction of LTP. Compartment-specific photorelease of MK801 in the presynaptic neuron showed that axonal, but not somatodendritic, presynaptic NMDA receptors are required for induction of LTD. The rate of use-dependent block of postsynaptic NMDA receptor current was slower following induction of LTD, consistent with a presynaptic locus of expression. Thus, this new caged compound has demonstrated the axonal location of NMDA receptors required for induction and the presynaptic locus of expression of LTD at layer 4-layer 2/3 synapses.

The role of growth retardation in lasting effects of neonatal dexamethasone treatment on hippocampal synaptic function

BACKGROUND

Dexamethasone (DEX), a synthetic glucocorticoid, is commonly used to prevent or lessen the morbidity of chronic lung disease in preterm infants. However, evidence is now increasing that this clinical practice negatively affects somatic growth and may result in long-lasting neurodevelopmental deficits. We therefore hypothesized that supporting normal somatic growth may overcome the lasting adverse effects of neonatal DEX treatment on hippocampal function.

METHODOLOGY/PRINCIPAL FINDINGS

To test this hypothesis, we developed a rat model using a schedule of tapering doses of DEX similar to that used in premature infants and examined whether the lasting influence of neonatal DEX treatment on hippocampal synaptic plasticity and memory performance are correlated with the deficits in somatic growth. We confirmed that neonatal DEX treatment switched the direction of synaptic plasticity in hippocampal CA1 region, favoring low-frequency stimulation- and group I metabotropic glutamate receptor agonist (S)-3,5,-dihydroxyphenylglycine-induced long-term depression (LTD), and opposing the induction of long-term potentiation (LTP) by high-frequency stimulation in the adolescent period. The effects of DEX on LTP and LTD were correlated with an increase in the autophosphorylation of Ca(2+)/calmodulin-dependent protein kinase II at threonine-286 and a decrease in the protein phosphatase 1 expression. Neonatal DEX treatment resulted in a disruption of memory retention subjected to object recognition task and passive avoidance learning. The adverse effects of neonatal DEX treatment on hippocampal synaptic plasticity and memory performance of the animals from litters culled to 4 pups were significantly less than those for the 8-pup litters. However, there was no significant difference in maternal care between groups.

CONCLUSION/SIGNIFICANCE

Our results demonstrate that growth retardation plays a crucial role in DEX-induced long-lasting influence of hippocampal function. Our findings suggest that therapeutic strategies designed to support normal development and somatic growth may exert beneficial effects to reduce lasting adverse effects following neonatal DEX treatment.

[Effect of harmaline of the complex spike waveform and depression time in cerebellar Purkinje cell discharge in rat postnatal ontogenesis]

Functional relationship between waveform of complex spike (CS) and depression time of simple spike (SS) in discharge of cerebellar Purkinje cells was studied after their activation with afferent climbing fiber at different terms of postnatal ontogenesis in norm and after treatment with harmaline. The experiments were carried out on three age groups of Wistar rats: rat pups (2 weeks), the adult (4-6 months), and the old animals (22-26 months). It was established that the CS duration in norm was approximately equal in rat pups, adult, and old animals, whereas it markedly decreased from the young to the old animals during the SS depression in the Purkinje cell discharge. Frequency of small action potential (sAP) and their number in the Purkinje cell discharge were approximately equal in young rat pups and adult animals, while in old animals these parameters were higher, on average, by 30%. After administration of harmaline, all CS parameters in rat pups and old animals increased in parallel with the depression time elongation. In adult rats, harmaline did not produce statistically significant changes of the mean values of CS parameters, but an increase of the simple spike depression time was observed. The obtained results allow concluding that the CS waveform and the simple spike depression time in norm are functionally coupled and change with age. The effect of harmaline on the CS waveforms as well as on interrelation of the CS duration and the SS depression time in the Purkinje cell discharge was more pronounced at the early and the late stages of Wistar rat postnatal ontogenesis.

The mGluR2/3 agonist LY379268 blocks the effects of GLT-1 upregulation on prepulse inhibition of the startle reflex in adult rats

The main glutamate transporter GLT-1 is responsible for clearing synaptically released glutamate from the extracellular space and contributes to the shaping of glutamatergic transmission. Recently, it has been shown that ceftriaxone (CEF)-induced GLT-1 upregulation is associated with an impairment of the prepulse inhibition (PPI) of the startle reflex, a simple form of information processing that is reduced in schizophrenia, and determines a strong reduction in hippocampal metabotropic glutamate receptor (mGluR)2/3-dependent long-term depression. In this study, we tested the hypothesis that administration of the mGluR2/3 agonist LY379268 blocks the effect of GLT-1 upregulation on PPI of the startle. We showed that administration of LY379268 (1 mg/kg) prevented PPI alterations associated with GLT-1 upregulation, suggesting that CEF-induced PPI impairment was mGluR2/3 dependent. In addition, we showed that CEF-induced GLT-1 upregulaton did not alter the expression of mGluR2/3, and also that it occurred at sites of mGluR2/3 expression. These results indicate a novel mechanism by which GLT-1 upregulation modulates PPI of the startle.

Distinct forms of Gq-receptor-dependent plasticity of excitatory transmission in the BNST are differentially affected by stress

Long-term depression (LTD) is an important synaptic mechanism for limiting excitatory influence over circuits subserving cognitive and emotional behavior. A major means of LTD induction is through the recruitment of signaling via G(q)-linked receptors activated by norepinephrine (NE), acetylcholine, and glutamate. Receptors from these transmitter families have been proposed to converge on a common postsynaptic LTD maintenance mechanism, such that hetero- and homosynaptic induction produce similar alterations in glutamate synapse efficacy. We report that in the dorsolateral and ventrolateral bed nucleus of the stria terminalis (BNST), recruitment of G(q)-linked receptors by glutamate or NE initiates mechanistically distinct forms of postsynaptically maintained LTD and these LTDs are differentially regulated by stress exposure. In particular, we show that although both mGluR5- and alpha(1)-adrenergic receptor (AR)-dependent LTDs involve postsynaptic endocytosis, the alpha(1)-AR-initiated LTD exclusively involves modulation of signaling through calcium-permeable AMPA receptors. Further, alpha(1)-AR- but not mGluR5- dependent LTD is disrupted by restraint stress. alpha(1)-AR LTD is also impaired in mice chronically exposed to ethanol. These data thus suggest that in the BNST, NE- and glutamate-activated G(q)-linked signaling pathways differentially tune glutamate synapse efficacy in response to stress.

[Nonuniform distribution and contribution of the P- and P/Q-type calcium channels to short-term inhibitory synaptic transmission in cultured hippocampal neurons]

In the present study, we investigated the sensitivity of GABAergic short-term plasticity to the selective P- and P/Q-type calcium channels blocker omega-agatoxin-IVA. To block the P-type channels we used 30 nM of this toxin and 200 nM of the toxin was used to block the P/Q channel types. The evoked inhibitory postsynaptic currents (eIPSC) were studied using patch-clamp technique in whole-cell configuration in postsynaptic neuron and local extracellular stimulation of single presynaptic axon by rectangular pulse. The present data show that the contribution of P- and P/Q-types channels to GABAergic synaptic transmission in cultured hippocampal neurons are 30% and 45%, respectively. It was shown that the mediate contribution of the P- and P/Q-types channels to the amplitudes of eIPSC is different to every discovered neuron. It means that distribution of these channels is non-uniform. To study the short-term plasticity of inhibitory synaptic transmission, axons of presynaptic neurons were paired-pulse stimulated with the interpulse interval of 150 ms. Neurons demonstrated both the depression and facilitation. The application of 30 nM and 200 nM of the blocker decreased the depression and increased facilitation to 8% and 11%, respectively. In addition, we found that the mediate contribution of the P- and P/Q-types channels to realization of synaptic transmission after the second stimuli is 4% less compared to that after the first one. Therefore, blocking of both P- and P/Q-types calcium channels can change the efficiency of synaptic transmission. In this instance it facilitates realization of the transmission via decreased depression or increased facilitation. These results confirm that the P- and P/Q-types calcium channels are involved in regulation of the short-term inhibitory synaptic plasticity in cultured hippocampal neurons.

Acute ethanol treatment prevents endocannabinoid-mediated long-lasting disinhibition of striatal output

Recent research has suggested that the neuronal circuit adaptations elicited by drugs of abuse share common features with traditional learning models, and that drugs of abuse cause long-term changes in behavior by altering synaptic function and plasticity. Especially, the endocannabinoid (eCB) system appears to be involved in the neuronal circuitry regulating ethanol (EtOH) preference in rodent. The aim of this study was to evaluate if acute EtOH exposure could modulate eCB-mediated plasticity in the dorsolateral striatum. Our data show that EtOH (20-50 mM) prevents eCB-mediated long-lasting disinhibition (DLL) of striatal output induced by a single stimulation train delivered at 5 Hz for 60 s, and reduces long-term depression (LTD) induced by low-frequency stimulation at inhibitory synapses. Acute EtOH-treatment also prevents DLL induced by the L-type calcium channel activator 2,5-dimethyl-4-[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylic acid methylester (FPL64176; 500 nM), or by the cannabinoid 1 receptor (CB(1)R) agonist WIN55,212-2 (300 nM), indicating that EtOH affects eCB-signaling at a stage that is downstream from eCB production and release. Importantly, high-frequency stimulation, or a higher concentration of WIN55,212-2 (1 muM), induces EtOH-insensitive depression of striatal output, suggesting that EtOH affects CB(1)R-mediated signaling in a synapse-specific manner. Maintaining the balance between excitation and inhibition is vital for neuronal networks, and EtOH-mediated modulation of eCB-signaling might thus affect the stability and the fine-tuning of neuronal circuits in the striatum. Our data suggest that changes in eCB-signaling could be involved in the physiological response to acute alcohol intoxication.

Two forms of long-term depression in a polysynaptic pathway in the leech CNS: one NMDA receptor-dependent and the other cannabinoid-dependent

Although long-term depression (LTD) is a well-studied form of synaptic plasticity, it is clear that multiple cellular mechanisms are involved in its induction. In the leech, LTD is observed in a polysynaptic connection between touch mechanosensory neurons (T cells) and the S interneuron following low frequency stimulation. LTD elicited by 450 s low frequency stimulation was blocked by N-methyl-D-aspartic acid (NMDA) receptor antagonists. However, LTD elicited by 900 s low frequency stimulation was insensitive to NMDA receptor antagonists and was instead dependent on cannabinoid signaling. This LTD was blocked by both a cannabinoid receptor antagonist and by inhibition of diacylglycerol lipase, which is necessary for the synthesis of the cannabinoid transmitter 2-arachidonyl glycerol (2-AG). Bath application of 2-AG or the cannabinoid receptor agonist CP55 940 also induced LTD at this synapse. These results indicate that two forms of LTD coexist at the leech T-to-S polysynaptic pathway: one that is NMDA receptor-dependent and another that is cannabinoid-dependent and that activation of either form of LTD is dependent on the level of activity in this circuit.

Soluble oligomers of amyloid Beta protein facilitate hippocampal long-term depression by disrupting neuronal glutamate uptake

In Alzheimer's disease (AD), the impairment of declarative memory coincides with the accumulation of extracellular amyloid-beta protein (Abeta) and intraneuronal tau aggregates. Dementia severity correlates with decreased synapse density in hippocampus and cortex. Although numerous studies show that soluble Abeta oligomers inhibit hippocampal long-term potentiation, their role in long-term synaptic depression (LTD) remains unclear. Here, we report that soluble Abeta oligomers from several sources (synthetic, cell culture, human brain extracts) facilitated electrically evoked LTD in the CA1 region. Abeta-enhanced LTD was mediated by mGluR or NMDAR activity. Both forms of LTD were prevented by an extracellular glutamate scavenger system. Abeta-facilitated LTD was mimicked by the glutamate reuptake inhibitor TBOA, including a shared dependence on extracellular calcium levels and activation of PP2B and GSK-3 signaling. In accord, synaptic glutamate uptake was significantly decreased by soluble Abeta. We conclude that soluble Abeta oligomers perturb synaptic plasticity by altering glutamate recycling at the synapse and promoting synapse depression.

TRPV1 channels control synaptic plasticity in the developing superior colliculus

Long-term depression (LTD) in the rodent superior colliculus (SC) is regarded as a model of synaptic refinement because it can be induced during development but not in adults. We investigated the role of transient receptor potential vanilloid type-1 (TRPV1) channels in this type of synaptic plasticity. Experiments were carried out in pigmented mice aged between postnatal day 8 (P8) and 42 (P42) and in adult mice. Retinal axons to the SC were labelled by injection of cholera toxin-beta (CTbeta) into the eye. Immunohistochemical staining for CTbeta, TRPV1 and markers of glutamatergic and GABAergic cells and fibres (VGLUT1 and VGAT or GAD65, respectively) was performed by using multiple immunofluorescence. This showed that both glutamatergic retinal afferents to, and some GABAergic neurones in, the superficial SC are TRPV1 positive in juvenile but not adult mice. Field potential recordings were made from the superficial grey layer in parasagittal SC slices, and LTD (76 +/- 8% of control responses) was induced with a 50 Hz, 20 s tetanus. Activation of TRPV1 with resiniferatoxin also reduced field potential amplitude to 84 +/- 8% of control values. Blockade of TRPV1 with the selective antagonist 5'-iodo-resiniferatoxin prevented the induction of LTD (98 +/- 4% of control values), but did not cause its reversal if LTD was already established. N-acylphosphatidylethanolamine-specific phospholipase D and 12-lipoxygenase, two proposed endovanilloid biosynthesizing enzymes, were co-expressed with TRPV1 in the SC at P14 and P28. These results suggest that TRPV1 modulates retinocollicular responses in the developing SC and is activated during tetanic stimulation by endovanilloid ligands to participate in the induction of LTD.

The ubiquitin-proteasome system is necessary for long-term synaptic depression in Aplysia

The neuropeptide Phe-Met-Arg-Phe-NH(2) (FMRFa) can induce transcription-dependent long-term synaptic depression (LTD) in Aplysia sensorimotor synapses. We investigated the role of the ubiquitin-proteasome system and the regulation of one of its components, ubiquitin C-terminal hydrolase (ap-uch), in LTD. LTD was sensitive to presynaptic inhibition of the proteasome and was associated with upregulation of ap-uch mRNA and protein. This upregulation appeared to be mediated by CREB2, which is generally regarded as a transcription repressor. Binding of CREB2 to the promoter region of ap-uch was accompanied by histone hyperacetylation, suggesting that CREB2 cannot only inhibit but also promote gene expression. CREB2 was phosphorylated after FMRFa, and blocking phospho-CREB2 blocked LTD. In addition to changes in the expression of ap-uch, the synaptic vesicle-associated protein synapsin was downregulated in LTD in a proteasome-dependent manner. These results suggest that proteasome-mediated protein degradation is engaged in LTD and that CREB2 may act as a transcription activator under certain conditions.

Loss of muscarinic autoreceptor function impairs long-term depression but not long-term potentiation in the striatum

Muscarinic autoreceptors regulate cholinergic tone in the striatum. We investigated the functional consequences of genetic deletion of striatal muscarinic autoreceptors by means of electrophysiological recordings from either medium spiny neurons (MSNs) or cholinergic interneurons (ChIs) in slices from single M(4) or double M(2)/M(4) muscarinic acetylcholine receptor (mAChR) knock-out (-/-) mice. In control ChIs, the muscarinic agonist oxotremorine (300 nM) produced a self-inhibitory outward current that was mostly reduced in M(4)(-/-) and abolished in M(2)/M(4)(-/-) mice, suggesting an involvement of both M(2) and M(4) autoreceptors. In MSNs from both M(4)(-/-) and M(2)/M(4)(-/-) mice, muscarine caused a membrane depolarization that was prevented by the M(1) receptor-preferring antagonist pirenzepine (100 nM), suggesting that M(1) receptor function was unaltered. Acetylcholine has been involved in striatal long-term potentiation (LTP) or long-term depression (LTD) induction. Loss of muscarinic autoreceptor function is predicted to affect synaptic plasticity by modifying striatal cholinergic tone. Indeed, high-frequency stimulation of glutamatergic afferents failed to induce LTD in MSNs from both M(4)(-/-) and M(2)/M(4)(-/-) mice, as well as in wild-type mice pretreated with the M(2)/M(4) antagonist AF-DX384 (11-[[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5,1 1-dihydro-6H-pyrido[2,3b][1,4] benzodiazepin-6-one). Interestingly, LTD could be restored by either pirenzepine (100 nM) or hemicholinium-3 (10 microM), a depletor of endogenous ACh. Conversely, LTP induction did not show any difference among the three mouse strains and was prevented by pirenzepine. These results demonstrate that M(2)/M(4) muscarinic autoreceptors regulate ACh release from striatal ChIs. As a consequence, endogenous ACh drives the polarity of bidirectional synaptic plasticity.

D-serine augments NMDA-NR2B receptor-dependent hippocampal long-term depression and spatial reversal learning

The contributions of hippocampal long-term depression (LTD) to explicit learning and memory are poorly understood. Electrophysiological and behavioral studies examined the effects of modulating NMDA receptor-dependent LTD on spatial learning in the Morris water maze (MWM). The NMDA receptor co-agonist D-serine substantially enhanced NR2B-dependent LTD, but not long-term potentiation (LTP) or depotentiation, in hippocampal slices from adult wild type mice. Exogenous D-serine did not alter MWM acquisition, but substantially enhanced subsequent reversal learning of a novel target location and performance in a delayed-matching-to-place task. Conversely, an NR2B antagonist disrupted reversal learning and promoted perseveration. Endogenous synaptic D-serine likely saturates during LTP induction because exogenous D-serine rescued deficient LTP and MWM acquisition in Grin1(D481N) mutant mice having a lower D-serine affinity. Thus, D-serine may enhance a form of hippocampal NR2B-dependent LTD that contributes to spatial reversal learning. By enhancing this form of synaptic plasticity, D-serine could improve cognitive flexibility in psychiatric disorders characterized by perseveration of aberrant ideation or behaviors.

Antidepressant activity of fosinopril, ramipril and losartan, but not of lisinopril in depressive paradigms of albino rats and mice

Fosinopril, ramipril and losartan significantly decreased the duration (sec) of immobility in forced swim test and were comparable to amitriptyline. The duration of immobility were significantly decreased in fosinopril, ramipril and losartan in the tail suspension test and were comparable to amitriptyline. Only losartan significantly increased the rearing number of entries, time spent (sec) in open arm and in light area in comparison to control animals. Fosinopril and ramipril and not lisinopril showed significant antidepressant activity while losartan showed a significant antidepressant and anxiolytic activity. Present findings suggest that these drugs could be better antihypertensives in hypertensive patients with comorbidity like depression or anxiety.

Group S8A serine proteases, including a novel enzyme cadeprin, induce long-lasting, metabotropic glutamate receptor-dependent synaptic depression in rat hippocampal slices

Long-term potentiation and long-term depression (LTD) are forms of synaptic plasticity in the central nervous system. We now report that a group of chymotrypsin-like serine proteases, especially members of the S8A subfamily, induce LTD of evoked potentials in rat hippocampal slices. The proteolytic activity of these enzymes is required for the induction of LTD, as serine protease inhibitors prevent the effect. The depression is partly mediated by the suppression of transmitter release from glutamatergic terminals but also involves an elevation of action potential threshold with no change of post-synaptic membrane potential or input resistance. We have also isolated a novel and more potent related enzyme, cadeprin, from Aspergillus. The LTD produced by all of these proteases is not dependent on receptors for several transmitter systems, including N-methyl-d-aspartate or adenosine receptors, but is prevented by blocking group I metabotropic glutamate receptors. The activity of cadeprin, subtilisin and other S8A serine proteases may shed light on the mechanisms of LTD and a related endogenous molecule could have a physiological or pathological role as a modulator of synaptic plasticity in the mammalian hippocampus.

Synaptic accumulation of PSD-95 and synaptic function regulated by phosphorylation of serine-295 of PSD-95

The scaffold protein PSD-95 promotes the maturation and strengthening of excitatory synapses, functions that require proper localization of PSD-95 in the postsynaptic density (PSD). Here we report that phosphorylation of ser-295 enhances the synaptic accumulation of PSD-95 and the ability of PSD-95 to recruit surface AMPA receptors and potentiate excitatory postsynaptic currents. We present evidence that a Rac1-JNK1 signaling pathway mediates ser-295 phosphorylation and regulates synaptic content of PSD-95. Ser-295 phosphorylation is suppressed by chronic elevation, and increased by chronic silencing, of synaptic activity. Rapid dephosphorylation of ser-295 occurs in response to NMDA treatment that causes chemical long-term depression (LTD). Overexpression of a phosphomimicking mutant (S295D) of PSD-95 inhibited NMDA-induced AMPA receptor internalization and blocked the induction of LTD. The data suggest that synaptic strength can be regulated by phosphorylation-dephosphorylation of ser-295 of PSD-95 and that synaptic depression requires the dephosphorylation of ser-295.