Protein kinase G increases AMPA receptor GluR1 phosphorylation at serine 845 after repeated cocaine administration in the rat nucleus accumbens

The Interplay between cGMP and Calcium Signaling in Alzheimer’s Disease

Cyclic guanosine monophosphate (cGMP) is a ubiquitous second messenger and a key molecule in many important signaling cascades in the body and brain, including phototransduction, olfaction, vasodilation, and functional hyperemia. Additionally, cGMP is involved in long-term potentiation (LTP), a cellular correlate of learning and memory, and recent studies have identified the cGMP-increasing drug Sildenafil as a potential risk modifier in Alzheimer’s disease (AD). AD development is accompanied by a net increase in the expression of nitric oxide (NO) synthases but a decreased activity of soluble guanylate cyclases, so the exact sign and extent of AD-mediated imbalance remain unclear. Moreover, human patients and mouse models of the disease present with entangled deregulation of both cGMP and Ca²⁺ signaling, e.g., causing changes in cGMP-mediated Ca²⁺ release from the intracellular stores as well as Ca²⁺-mediated cGMP production. Still, the mechanisms governing such interplay are poorly understood. Here, we review the recent data on mechanisms underlying the brain cGMP signaling and its interconnection with Ca²⁺ signaling. We also discuss the recent evidence stressing the importance of such interplay for normal brain function as well as in Alzheimer’s disease.

Adolescent high-fructose corn syrup consumption leads to dysfunction in adult affective behaviors and mesolimbic proteins in male Sprague-Dawley rats

Adolescence is a critical period of development, during which the brain undergoes rapid maturation. Problematically, adolescents are the top consumers of high fructose corn syrup (HFCS) sweetened beverages and snacks, which may have neurodevelopmental consequences. While HFCS consumption has been linked to an increased likelihood of obesity and other physical health impairments, the link between HFCS and persistent behavioral changes is not yet fully established. The present study aimed to assess whether adolescent HFCS consumption could lead to alterations in adult behaviors and protein expression, following cessation. Adolescent HFCS-exposure contributed to deficits in learning and motivation on an effort-related T-Maze procedure, as well as increased immobility time in the forced swim paradigm during adulthood. Molecularly, protracted decreases in accumbal dopamine D1 and D2 receptors and protein kinase G (PKG), as well as increases in tyrosine hydroxylase and GluA2 receptor subunits, were observed following HFCS-exposure. Taken together, these data suggest that adolescent HFCS-consumption leads to protracted dysfunction in affective behaviors and alterations in accumbal proteins which persist following cessation of HFCS-consumption.

Amygdala levels of the GluA1 subunit of glutamate receptors and its phosphorylation state at serine 845 in the anterior hippocampus are biomarkers of ictal fear but not anxiety

Fear is a conscious state caused by exposure to real or imagined threats that trigger stress responses that affect the body and brain, particularly limbic structures. A sub-group of patients with mesial temporal lobe epilepsy related to hippocampus sclerosis (MTLE-HS) have seizures with fear, which is called ictal fear (IF), due to epileptic activity within the brain defensive survival circuit structures. Synaptic transmission efficacy can be bi-directionally modified through potentiation (long-term potentiation (LTP)) or depression (long-term depression (LTD)) as well as the phosphorylation state of Ser831 and Ser845 sites at the GluA1 subunit of the glutamate AMPA receptors, which has been characterized as a critical event for this synaptic plasticity. In this study, GluA1 levels and the phosphorylation at Ser845 and Ser831 in the amygdala (AMY), anterior hippocampus (aHIP) and middle gyrus of temporal neocortex (CX) were determined with western blots and compared between MTLE-HS patients who were showing (n = 06) or not showing (n = 25) IF. Patients with IF had an 11% decrease of AMY levels of the GluA1 subunit (p = 0.05) and a 21.5% decrease of aHIP levels of P-GluA1-Ser845 (p = 0.009) compared to patients not showing IF. The observed associations were not related to imbalances in the distribution of other concomitant types of aura, demographic, clinical or neurosurgical variables. The lower levels of P-GluA1-Ser845 in the aHIP of patients with IF were not related to changes in the levels of the serine/threonine-protein phosphatase PP1-alpha catalytic subunit or protein kinase A activation. Taken together, the GluA1 subunit levels in AMY and P-GluA1-Ser845 levels in the aHIP show an overall accuracy of 89.3% (specificity 95.5% and sensitivity 66.7%) to predict the presence of IF. AMY levels of the GluA1 subunit and aHIP levels of P-GluA1-Ser845 were not associated with the psychiatric diagnosis and symptoms of patients. Taken together with previous findings in MTLE-HS patients with IF who were evaluated by stereotactic implanted depth electrodes, we speculate our findings are consistent with the hypothesis that AMY is not a centre of fear but together with other sub-cortical and cortical structures integrates the defensive circuit that detect and respond to threats. This is the first report to address neuroplasticity features in human limbic structures connected to the defensive survival circuits, which has implications for the comprehension of highly prevalent psychiatric disorders and symptoms.

Interactions of Glutamatergic Neurotransmission and Brain-Derived Neurotrophic Factor in the Regulation of Behaviors after Nicotine Administration

Nicotine causes tobacco dependence, which may result in fatal respiratory diseases. The striatum is a key structure of forebrain basal nuclei associated with nicotine dependence. In the striatum, glutamate release is increased when α7 nicotinic acetylcholine receptors expressed in the glutamatergic terminals are exposed to nicotine, and over-stimulates glutamate receptors in gamma amino-butyric acid (GABA)ergic neurons. These receptor over-stimulations in turn potentiate GABAergic outputs to forebrain basal nuclei and contribute to the increase in psychomotor behaviors associated with nicotine dependence. In parallel with glutamate increases, nicotine exposure elevates brain-derived neurotrophic factor (BDNF) release through anterograde and retrograde targeting of the synapses of glutamatergic terminals and GABAergic neurons. This article reviews nicotine-exposure induced elevations of glutamatergic neurotransmission, the bidirectional targeting of BDNF in the striatum, and the potential regulatory role played by BDNF in behavioral responses to nicotine exposure.

Single and Repeated Administration of Methylphenidate Modulates Synaptic Plasticity in Opposite Directions via Insertion of AMPA Receptors in Rat Hippocampal Neurons

Methylphenidate (MPH) is widely used in the treatment of Attention Deficit Hyperactivity Disorder. Several lines of evidence support that MPH can modulate learning and memory processes in different ways including improvement and impairment of test performances. A relevant factor in the efficacy of treatment is whether administration is performed once or several times. In this study we demonstrate opposite effects of MPH on performance of preadolescent rats in the Morris Water Maze test. Animals treated with a single dose (1 mg/kg) performed significantly better compared to controls, while in animals treated with repetitive administration at the same concentration performance was reduced. We found that hippocampal LTP in slices from rats treated with a single dose was increased, while LTP from rats treated with repetitive injections of MPH was lower than in controls. Using Western blot of CA1 areas from potentiated slices of rats treated with a single dose we found a significant increase of phosphorylation at Ser845 of GluA1 subunits, associated to an increased insertion of GluA1-containing AMPARs in the plasma membrane. These receptors were functional, because AMPA-dependent EPSCs recorded on CA1 were enhanced, associated to a significant increase in short-term plasticity. In contrast, CA1 samples from rats injected with MPH during six consecutive days, showed a significant decrease in the phosphorylation at Ser845 of GluA1 subunits associated to a lower insertion of GluA1-containing AMPARs. Accordingly, a reduction of the AMPA-mediated EPSCs and short-term plasticity was also observed. Taken together, our results demonstrate that single and repeated doses with MPH can induce opposite effects at behavioral, cellular, and molecular levels. The mechanisms demonstrated here in preadolescent rats are relevant to understand the effects of this psychostimulant in the treatment of ADHD.

Activation of Protein Kinase G After Repeated Cocaine Administration Is Necessary for the Phosphorylation of α-Amino-3-Hydroxy-5-Methyl-4-Isoxazolepropionic Acid Receptor GluA1 at Serine 831 in the Rat Nucleus Accumbens

Phosphorylation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in the striatum plays a crucial role in regulating the receptor-coupled signaling cascades leading to behavioral changes associated with psychostimulant exposure. The present study determined if activation of protein kinase G (PKG) contributes to the phosphorylation of AMPA receptor GluA1 subunit at the position of serine 831 (GluA1-S831) in the rat nucleus accumbens (NAc) after repeated cocaine administration. The results demonstrated that repeated intraperitoneal (i.p.) injections of cocaine (20 mg/kg) once a day for seven consecutive days significantly increased the level of phosphorylated (p)GluA1-S831. This increase was decreased by the intra-NAc infusion of either the cyclic guanosine monophosphate (cGMP) analog, Rp-8-Br-PET-cGMPS (5 nmol/1 μL), or the PKG inhibitor, KT5823 (2 nmol/1 μL). Repeated cocaine administration increased PKG binding activity to GluA1. This increase in GluA1-S831 phosphorylation after repeated cocaine administration was decreased by the intra-NAc infusion of the synthetic peptide (Tat-tagged interfering peptide (Tat-GluA1-i)), that interferes with the binding of PKG to GluA1. Intra-NAc infusion of the interfering peptide also reduced the repeated cocaine-induced increase in locomotor activity. These findings suggest that activated PKG, after repeated exposure to cocaine, binds to AMPA receptor GluA1 and is required for the phosphorylation of S831, contributing to behavioral changes.

Protein Kinases Type II (PKG II) Combined with L-Arginine Significantly Ameliorated Xenograft Tumor Development: Is L-Arginine a Potential Alternative in PKG II Activation?

BACKGROUND The mammalian cyclic guanosine monophosphate (cGMP)-dependent protein kinases type II (PKG II) plays critical physiological or pathological functions in different tissues. However, the biological effects of PKG II are dependent on cGMP. Published data indicated that L-arginine (L-Arg) promoted NO production, NO can activate soluble guanylate cyclase (sGC), and catalyzes guanosine triphosphate (GTP) into cGMP, which suggested L-Arg could activate PKG II. Therefore, the present work was performed to address: (i) whether L-Arg could be a potential alternative in PKG II activation, and (ii) whether L-Arg also contributes to PKG II against cancer. MATERIAL AND METHODS Nude BALB/c mice were inoculated with human MCF-7, HepG2, and SW480 cell lines via subcutaneous (s.c.) injecting. After 7 days of inoculation, Ad-PKG II was injected into the cancer tissues every 4 days, and the next day 10 μmol/mouse L-Arg was administered. Western blotting and immunohistochemistry were used to assess protein expression. RESULTS Our results demonstrated that L-Arg significantly activated PKG II and effectively ameliorated xenograft tumor development through inhibiting cancer growth, angiogenesis, and metastasis, which was partially dependent on blocking of epidermal growth factor receptor (EGFR) activity, as well as downstream signaling pathways such as Erk1/2. CONCLUSIONS Our results provide an exciting new insight: L-Arg is a potential alternative to PKG II activation.

Methylphenidate amplifies long-term potentiation in rat hippocampus CA1 area involving the insertion of AMPA receptors by activation of β-adrenergic and D1/D5 receptors

Methylphenidate (MPH, Ritalin©) is widely used in the treatment of Attention Deficit Hyperactivity Disorder and recently as a drug of abuse. Although the effect of MPH has been studied in brain regions such as striatum and prefrontal cortex (PFC), the hippocampus has received relatively little attention. It is known that MPH increases the TBS-dependent Long Term Potentiation (LTP) in the CA1 area. However, the cellular and molecular mechanisms involved in this process are still unknown. Using field potential recordings and western blot analysis in rat hippocampal slices of young rats, we found that acute application of MPH enhances LTP in CA3-CA1 synapses in a dose-dependent manner with an EC50 of 73.44±6.32 nM. Using specific antagonists and paired-pulse facilitation protocols, we observed that the MPH-dependent increase of LTP involves not only β-adrenergic receptors activation but also post-synaptic D1/D5 dopamine receptors. The inhibition of PKA with PKI, suppressed the facilitation of LTP induced by MPH consistent with an involvement of the adenyl cyclase-cAMP-PKA dependent cascade downstream of the activation of D1/D5 receptors. In addition, samples of CA1 areas taken from slices potentiated with MPH presented an increase in the phosphorylation of the Ser845 residue of the GluA1 subunit of AMPA receptors compared to control slices. This effect was reverted by SCH23390, antagonist of D1/D5 receptors, and PKI. Moreover, we found an increase of surface-associated functional AMPA receptors. We propose that MPH increases TBS-dependent LTP in CA3-CA1 synapses through a polysynaptic mechanism involving activation of β-adrenergic and D1/D5 dopaminergic receptors and promoting the trafficking and insertion of functional AMPA receptors to the plasma membrane.

Identification of new phosphorylation sites of AMPA receptors in the rat hippocampus--A resource for neuroscience research

PURPOSE

AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptors (AMPARs) are glutamate-gated ion channels that mediate the majority of fast excitatory synaptic transmissions in the mammalian brain. A series of phosphorylation sites have been predicted or identified and knowledge on phosphorylations is mandatory for understanding receptor biology and functions.

EXPERIMENTAL DESIGN

Immunoprecipitation from extracted hippocampal rat proteins was carried out using an antibody against the AMPAR GluA1 subunit, followed by identification of GluA1 and binding partners by MS. Bands from SDS-PAGE were picked, peptides were generated by trypsin and chymotrypsin digestion and identified by MS/MS (LTQ Orbitrap Velos).

RESULTS

Using Mascot as a search engine, phosphorylation sites S506, S645, S720, S849, S863, S895, T858, Y228, Y419, and T734 were found on GluA1; S357, S513, S656, S727, T243, T420, T741, Y 143, Y301,Y426 on GluA2; S301, S516, S657, S732, T222, and T746 were observed on GluA3; and S514, S653 was phosphorylated on GluA4.

CONCLUSIONS AND CLINICAL RELEVANCE

A series of additional protein modifications were observed and in particular, tyrosine and tryptophan nitrations on GluA1 were detected that may raise questions on additional regulation mechanisms for AMPARs in addition to phosphorylations. The findings are relevant for interpretation of previous work and design of future studies using AMPAR serving as a resource for neuroscience research and indeed, phosphorylations and PTMs per se would have to be respected when neuropathological and neurological disorders are being studied.

Interactions between N-ethylmaleimide-sensitive factor and GluR2 in the nucleus accumbens contribute to the expression of locomotor sensitization to cocaine

Many studies have reported a withdrawal-dependent increase in synaptic AMPA receptor (AMPAR) levels in the nucleus accumbens (NAc) of cocaine-sensitized rats; however, the exact relationship between the expression of sensitization and altered AMPAR surface expression in the NAc has not yet been investigated. We demonstrated that the expression of behavioral sensitization was negatively controlled by N-ethylmaleimide-sensitive factor (NSF)-GluR2 interactions in the NAc. The upregulation of NSF-GluR2 interactions, which may be resulted by the increase in NSF S-nitrosylation after withdrawal from cocaine, was associated with the changes in the expression of behavioral sensitization. Disruption of NSF-GluR2 interactions in the NAc with a specific peptide, TAT-pep-R845A, increased the locomotor response of rats to cocaine by decreasing GluR2 surface insertion. In contrast, prevention of GluR2-containing AMPARs removal from synapses with Pep2-EVKI attenuated the expression of behavioral sensitization. Similarly, treatment with the nitric oxide donor, S-Nitroso-N-acetyl-DL-penicillamine (SNAP), attenuated the expression of locomotor sensitization by promoting GluR2 surface expression. This effect was mediated by the binding of S-nitrosylated NSF to GluR2, which promoted the surface expression of AMPARs. Noticeably, exogenous injection of SNAP into NAc also attenuated the expression of cocaine-induced conditioned place preference. Thus, these results indicate that increased NSF-GluR2 interactions in the NAc after withdrawal from cocaine attenuated the expression of behavioral sensitization and serves as a negative regulatory mechanism in drug-exposed individuals.