Norepinephrine activates extracellular-regulated kinase in cortical neurons

ERK signaling pathway regulates sleep duration through activity-induced gene expression during wakefulness

Wakefulness is accompanied by experience-dependent synaptic plasticity and an increase in activity-regulated gene transcription. Wake-induced genes are certainly markers of neuronal activity and may also directly regulate the duration of and need for sleep. We stimulated murine cortical cultures with the neuromodulatory signals that are known to control wakefulness in the brain and found that norepinephrine alone or a mixture of these neuromodulators induced activity-regulated gene transcription. Pharmacological inhibition of the various signaling pathways involved in the regulation of gene expression indicated that the extracellular signal-regulated kinase (ERK) pathway is the principal one mediating the effects of waking neuromodulators on gene expression. In mice, ERK phosphorylation in the cortex increased and decreased with wakefulness and sleep. Whole-body or cortical neuron-specific deletion of Erk1 or Erk2 significantly increased the duration of wakefulness in mice, and pharmacological inhibition of ERK phosphorylation decreased sleep duration and increased the duration of wakefulness bouts. Thus, this signaling pathway, which is highly conserved from Drosophila to mammals, is a key pathway that links waking experience-induced neuronal gene expression to sleep duration and quality.

Brimonidine prevents axonal and somatic degeneration of retinal ganglion cell neurons

BACKGROUND

Brimonidine is a common drug for lowering ocular pressure and may directly protect retinal ganglion cells in glaucoma. The disease involves early loss of retinal ganglion cell transport to brain targets followed by axonal and somatic degeneration. We examined whether brimonidine preserves ganglion cell axonal transport and abates degeneration in rats with elevated ocular pressure induced by laser cauterization of the episcleral veins.

RESULTS

Ocular pressure was elevated unilaterally by 90% for a period of 8 weeks post- cauterization. During this time, brimonidine (1mg/kg/day) or vehicle (phosphate-buffered saline) was delivered systemically and continuously via subcutaneous pump. Animals received bilateral intravitreal injections of fluorescent cholera toxin subunit β (CTB) two days before sacrifice to assess anterograde transport. In retinas from the vehicle group, elevated pressure induced a 44% decrease in the fraction of ganglion cells with intact uptake of CTB and a 14-42% reduction in the number of immuno-labelled ganglion cell bodies, with the worst loss occurring nasally. Elevated pressure also caused a 33% loss of ganglion cell axons in vehicle optic nerves and a 70% decrease in CTB transport to the superior colliculus. Each of these components of ganglion cell degeneration was either prevented or significantly reduced in the brimonidine treatment group.

CONCLUSIONS

Continuous and systemic treatment with brimonidine by subcutaneous injection significantly improved retinal ganglion cell survival with exposure to elevated ocular pressure. This effect was most striking in the nasal region of the retina. Brimonidine treatment also preserved ganglion cell axon morphology, sampling density and total number in the optic nerve with elevated pressure. Consistent with improved outcome in the optic projection, brimonidine also significantly reduced the deficits in axonal transport to the superior colliculus associated with elevated ocular pressure. As transport deficits to and from retinal ganglion cell projection targets in the brain are relevant to the progression of glaucoma, the ability of brimonidine to preserve optic nerve axons and active transport suggests its neuroprotective effects are relevant not only at the cell body, but throughout the entire optic projection.

Nitric oxide signaling participates in norepinephrine-induced activity of neuronal intracellular survival pathways

Much evidence has gathered that nitric oxide (NO) signaling, via cGMP-dependent mechanisms, may activate pro-survival pathways in hippocampal neurons and inhibit apoptosis. Past research has revealed that the enhancement of monoaminergic neurotransmission via exercise or treatment with antidepressant medications leads to an enhanced expression of brain-derived neurotrophic factor (BDNF). In isolated hippocampal neurons, norepinephrine (NE) application also increases the immunoreactivity of BDNF and several pro-survival signaling molecules. The data herein support the possibility that NO signaling plays an important role in enhancing neurotrophin expression and activation of the pro-survival phosphatidylinositol 3' kinase (PI-3K) pathway stimulated by NE. In isolated hippocampal neurons, the NO donor, sodium nitroprusside, increases BDNF, PI-3K, and phospho-ERK1 immunoreactivity. Specific inhibitors of the NO system suggest that NE-induced increases in hippocampal BDNF and the PI-3K pathway, but not stimulation of the MAPK pathway, depend upon NO signaling. In addition, inhibiting cGMP suggest that the effects of NE on BDNF immunoreactivity and Akt phosphorylation are also cGMP-dependent. Finally, the application of l-NAME to hippocampal neurons increases cell death. This is the first study of its kind demonstrating the involvement of NE-induced pro-survival signaling in three distinct signaling pathways: PI-3K, MAPK, and NO/cGMP. Possible mechanisms are discussed in light of the results.

Noradrenaline enhances the expression of the neuronal monocarboxylate transporter MCT2 by translational activation via stimulation of PI3K/Akt and the mTOR/S6K pathway

Monocarboxylate transporter 2 (MCT2) expression is up-regulated by noradrenaline (NA) in cultured cortical neurons via a putative but undetermined translational mechanism. Western blot analysis showed that p44/p42 mitogen-activated protein kinase (MAPK) was rapidly and strongly phosphorylated by NA treatment. NA also rapidly induced serine/threonine protein kinase from AKT virus (Akt) phosphorylation but to a lesser extent than p44/p42 MAPK. However, Akt activation persisted over a longer period. Similarly, NA induced a rapid and persistent phosphorylation of mammalian target of rapamycin (mTOR), a kinase implicated in the regulation of translation in the central nervous system. Consistent with activation of the mTOR/S6 kinase pathway, phosphorylation of the ribosomal S6 protein, a component of the translation machinery, could be observed upon treatment with NA. In parallel, it was found that the NA-induced increase in MCT2 protein was almost completely blocked by LY294002 (phosphoinositide 3-kinase inhibitor) as well as by rapamycin (mTOR inhibitor), while mitogen-activated protein kinase kinase and p38 MAPK inhibitors had much smaller effects. Taken together, these data reveal that NA induces an increase in neuronal MCT2 protein expression by a mechanism involving stimulation of phosphoinositide 3-kinase/Akt and translational activation via the mTOR/S6 kinase pathway. Moreover, considering the role of NA in synaptic plasticity, alterations in MCT2 expression as described in this study might represent an adaptation to face energy demands associated with enhanced synaptic transmission.

Stimulation of catecholamine synthesis via activation of p44/42 MAPK in cultured bovine adrenal medullary cells by milnacipran

Milnacipran is a serotonin noradrenaline reuptake inhibitor (SNRI) and is used clinically as an antidepressant. We report here the effect of milnacipran on catecholamine synthesis in cultured bovine adrenal medullary cells. Incubation of adrenal medullary cells with milnacipran (300 ng/ml, 1,065 nM) for 20 min resulted in a significant increase in 14C-catecholamine synthesis from [14C]tyrosine, but not from [14C]DOPA, whereas the selective serotonin reuptake inhibitors (SSRIs), paroxetine (300 ng/ml, 800 nM) and fluvoxamine (300 ng/ml, 691 nM), had little effect. Milnacipran, but not paroxetine or fluvoxamine, increased the activity of tyrosine hydroxylase, the rate-limiting step of catecholamine biosynthesis, in a concentration-dependent manner (100-300 ng/ml, 355-1,065 nM). U0126 (1 microM), an inhibitor of p44/42 mitogen-activated protein kinase (MAPK) kinase, abolished the stimulatory effects of milnacipran on tyrosine hydroxylase activity. Furthermore, incubation of cells with milnacipran (30-100 ng/ml) for 5 min activated p44/42 MAPK, whereas paroxetine and fluvoxamine did not. The present findings suggest that milnacipran activates tyrosine hydroxylase and then stimulates catecholamine synthesis through a p44/42 MAPK-dependent pathway in cultured bovine adrenal medullary cells.

Norepinephrine induces BDNF and activates the PI-3K and MAPK cascades in embryonic hippocampal neurons

Both antidepressant treatment and physical exercise have been shown to increase circulating levels of norepinephine (NE) and hippocampal brain-derived neurotrophic factor (BDNF). Increases in BDNF have been shown to be associated with enhanced dendritic arborization and neuronal survival, which forms the theoretical basis of the Neurotrophin Hypothesis of antidepressant action. Using isolated embryonic hippocampal neurons and immunoblotting, we show that application of NE increases BDNF and phosphorylated Trk, and that these increases can be prevented by ERK and PI-3K inhibitors. In addition, NE-induced increases in phospho-ERK2 and PI-3K were each suppressed by a PI-3K and MAPK inhibitor, respectively. Furthermore, phosphorylation of cAMP-response element binding (CREB) protein was also increased by NE and brought down to baseline levels by MAPK and PI-3K inhibitors. And finally, because both the MAPK and PI-3K inhibitors suppress phosphorylation of both TrkB (upstream) and CREB (downstream), these results indicate that NE-induced BDNF expression follows a cyclic pathway, reminiscent of a positive feedback loop. The results of this study provide an in vitro model of the intracellular signaling mechanisms activated by NE, via ligand-G-protein-coupled receptor (GPCR)-to-BDNF-RTK transactivation, that is putatively thought to occur in vivo as a result of excitatory neural activity.

Imatinib inhibits spontaneous rhythmic contractions of human uterus and intestine

The interstitial cells of Cajal (ICC) in the digestive tract and ICC-like cells in extradigestive organs express the c-kit tyrosine-kinase receptor, and have been implicated as pacemakers of smooth muscle spontaneous activity. We used imatinib mesylate (Glivec) to investigate whether c-kit activity of Cajal-like cells in human myometrium is involved in spontaneous rhythmic contractions of human uterine smooth muscle, taking intestinal smooth muscle as a reference tissue. We show that imatinib concentration-dependently inhibited the myogenic contractions of human myometrium in the organ bath, while it significantly affected noradrenaline or K(+)-induced contractions only at concentrations exceeding 50 muM. An inhibitory antibody directed against the extracellular domain of the platelet derived growth factor receptor (PDGFR), another target of imatinib that is expressed by the uterine muscle cells themselves, failed to affect myogenic contractions. These results suggest that Cajal-type cells of human myometrium, as well as ICC of intestinal smooth muscle, participate in myogenic contractile mechanisms, via a novel ligand-independent c-kit/CD117 tyrosine-kinase signaling.

Neuro-modulation, aminergic neuro-disinhibition and neuro-degeneration. Draft of a comprehensive theory for Alzheimer disease

A comprehensive theory for Alzheimer disease (AD) which can provide a clue to the neuronal selective vulnerability (pathoklisis) is still missing. Based upon evidence from the current literature, the present work is aimed at proposing such a theory, namely the 'aminergic disinhibition theory' of AD. It includes data-based hypotheses as to the pathoklisis, mechanisms of neuro-degeneration and dementia as well as the aetiology of the disease. Alzheimer disease is regarded as a disorder of neural input modulation caused by the degeneration of four modulatory amine transmitter (MAT) systems, namely the serotoninergic, the noradrenergic, the histaminergic, and the cholinergic systems with ascending projections. MATs modulate cognitive processing including arousal, attention, and synaptic plasticity in learning and memory, not only through direct, mostly inhibitory impact on principal neurones but also partially through interaction with local networks of GABA-ergic inter-neurones. The distribution and magnitude of the pathology in AD roughly correlate with the distribution and magnitude of MAT modulation: Regions more densely innervated by ascending MAT projections are, as a rule, more severely affected than areas receiving less MAT innervation. Because the global effect of MATs in the forebrain is inhibition, the degeneration of four MAT systems, some related peptidergic systems and a secondary alleviation of the GABA-ergic transmission means a fundamental loss of inhibitory impact in the neuronal circuitry resulting in neuronal (aminergic) disinhibition. Clearly, the basic mechanism promoting neuronal death in AD is thought to be a chronic disturbance of the inhibition-excitation balance to the advantage of excitation. Chronic over-excitation is conceived to result in Ca2+ dependent cellular excito-toxicity leading to neuro-degeneration including amyloid-beta production and NFT formation. Disinhibited neurons will degenerate while less excited (relatively over-inhibited) neurones will survive. Because the decline of aminergic transmission in AD is likely to start at the receptor level, it is hypothesized that early impairment by a molecular 'hit' to an MAT receptor (or a group of receptors) initiates a pathogenetic cascade that develops in an avalanche-like manner. Based on experimental evidence from the literature, the 'hit' might be the attachment of a targeted pathogen like a small roaming amino acid sequence to the receptor(s), e.g., the serotoninergic 5-HT2A-R. Referential sequence analysis could be a means to identify such a small pathogen hidden in a large receptor molecule.

Electroconvulsive seizures increase the expression of MAP kinase phosphatases in limbic regions of rat brain

The mitogen-activated protein (MAP) kinase cascades regulate a variety of cellular activities, including cell growth, proliferation, and apoptosis, and are reported to play a role in the actions of antidepressant treatment. There are a number of different classes of protein phosphatases that could influence the MAP kinase cascade. One of these, the MAP kinase phosphatase (MKP) family, is known to play a key role in dephosphorylation of activated MAP kinase. In the present study, we analyzed the expression of the MKP1, MKP2, and MKP3 isoforms in rat brain after electroconvulsive seizure (ECS), considered the most effective treatment for depression. In situ hybridization analysis demonstrates that ECS differentially regulates the expression of the MKP isoforms. Expression of MKP1 mRNA is robustly increased by acute ECS in the major cell layers of the hippocampus, including the dentate gyrus granule cell layer and the CA1 and CA3 pyramidal cell layers. In contrast, MKP2 is induced mainly in the dentate gyrus and MKP3 is preferentially increased in the CA1 and CA3 cell layers. In the prefrontal cortex, all three MKP isoforms are upregulated by acute ECS administration. Chronic ECS resulted in a similar pattern of induction for each of the MKP subtypes, demonstrating that there is little or no desensitization of the response to repeated ECS. The induction of MKP expression serves as negative feedback control for the MAP kinase cascades. Upregulation of MKP expression could dampen the actions of ECS, indicating that blockade of the MKPs could enhance the actions of antidepressant treatment.

Gene-environment interplay in neurogenesis and neurodegeneration

Factors associated with predisposition and vulnerability to neurodegenerative disorders may be described usefully within the context of gene-environment interplay. There are many identified genetic determinants for so-called genetic disorders, and it is possible to duplicate many elements of recognized human neurodegenerative disorders in either knock-in or knock-out mice. However, there are similarly, many identifiable environmental influences on outcomes of the genetic defects; and the course of a progressive neurodegenerative disorder can be greatly modified by environmental elements. Constituent cellular defense mechanisms responsive to the challenge of increased reactive oxygen species represent only one crossroad whereby environment can influence genetic predisposition. In this paper we highlight some of the major neurodegenerative disorders and discuss possible links of gene-environment interplay. The process of adult neurogenesis in brain is also presented as an additional element that influences gene-environment interplay. And the so-called priming processes (i.e., production of receptor supersensitization by repeated drug dosing), is introduced as yet another process that influences how genes and environment ultimately and co-dependently govern behavioral ontogeny and outcome. In studies attributing the influence of genetic alteration on behavioral phenotypy, it is essential to carefully control environmental influences.

Inhibition of Mitogen-Activated Protein Kinase-Extracellular Signal-Regulated Kinase Disrupts Latent Inhibition of Cued Fear Conditioning in C57BL/6 Mice

The mitogen-activated protein kinase (MAPK)-extracellular signal-regulated kinase (ERK) cascade has been implicated in a variety of associative conditioning tasks. However, the role of the MAPK-ERK cascades in modulating conditioning is less clear. The authors examined the effect of the potent and selective MAPK-ERK inhibitor SL327 on latent inhibition of cued fear conditioning. The results demonstrate that 50 mg/kg and 100 mg/kg SL327 disrupt latent inhibition of cued fear conditioning. These data provide evidence for an essential role of the MAPK-ERK cascade in tasks that modulate the strength of associative conditioning. The results are discussed in relation to the molecular mechanisms that support latent inhibition of cued fear conditioning.