Elevated Prolactin during Pregnancy Drives a Phenotypic Switch in Mouse Hypothalamic Dopaminergic Neurons

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Orexinergic Input to Dopaminergic Neurons of the Human Ventral Tegmental Area

The mesolimbic reward pathway arising from dopaminergic (DA) neurons of the ventral tegmental area (VTA) has been strongly implicated in reward processing and drug abuse. In rodents, behaviors associated with this projection are profoundly influenced by an orexinergic input from the lateral hypothalamus to the VTA. Because the existence and significance of an analogous orexigenic regulatory mechanism acting in the human VTA have been elusive, here we addressed the possibility that orexinergic neurons provide direct input to DA neurons of the human VTA. Dual-label immunohistochemistry was used and orexinergic projections to the VTA and to DA neurons of the neighboring substantia nigra (SN) were analyzed comparatively in adult male humans and rats. Orexin B-immunoreactive (IR) axons apposed to tyrosine hydroxylase (TH)-IR DA and to non-DA neurons were scarce in the VTA and SN of both species. In the VTA, 15.0±2.8% of TH-IR perikarya in humans and 3.2±0.3% in rats received orexin B-IR afferent contacts. On average, 0.24±0.05 and 0.05±0.005 orexinergic appositions per TH-IR perikaryon were detected in humans and rats, respectively. The majority (86–88%) of randomly encountered orexinergic contacts targeted the dendritic compartment of DA neurons. Finally, DA neurons of the SN also received orexinergic innervation in both species. Based on the observation of five times heavier orexinergic input to TH-IR neurons of the human, compared with the rat, VTA, we propose that orexinergic mechanism acting in the VTA may play just as important roles in reward processing and drug abuse in humans, as already established well in rodents.     

Lactate Modulates the Activity of Primary Cortical Neurons through a Receptor-Mediated Pathway

Lactate is increasingly described as an energy substrate of the brain. Beside this still debated metabolic role, lactate may have other effects on brain cells. Here, we describe lactate as a neuromodulator, able to influence the activity of cortical neurons. Neuronal excitability of mouse primary neurons was monitored by calcium imaging. When applied in conjunction with glucose, lactate induced a decrease in the spontaneous calcium spiking frequency of neurons. The effect was reversible and concentration dependent (IC₅₀ ∼4.2 mM). To test whether lactate effects are dependent on energy metabolism, we applied the closely related substrate pyruvate (5 mM) or switched to different glucose concentrations (0.5 or 10 mM). None of these conditions reproduced the effect of lactate. Recently, a G_i protein-coupled receptor for lactate called HCA1 has been introduced. To test if this receptor is implicated in the observed lactate sensitivity, we incubated cells with pertussis toxin (PTX) an inhibitor of G_i-protein. PTX prevented the decrease of neuronal activity by L-lactate. Moreover 3,5-dyhydroxybenzoic acid, a specific agonist of the HCA1 receptor, mimicked the action of lactate. This study indicates that lactate operates a negative feedback on neuronal activity by a receptor-mediated mechanism, independent from its intracellular metabolism.     

NTS Catecholamine Neurons Mediate Hypoglycemic Hunger via Medial Hypothalamic Feeding Pathways

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Characterization of a Dopaminergic Receptor that Modulates Adrenomedullary Catecholamine Release

Nicotine evokes the release of catecholamines from bovine adrenal glands perfused with oxygenated Krebs-bicarbonate solution. Two 2-min pulses of 5 microM nicotine, at 40-min intervals (S1 and S2), gave net catecholamine outputs of 45.2 +/- 3.6 and 29.1 +/- 3.5 micrograms/8 min, respectively. Apomorphine (1 or 10 microM) markedly inhibited catecholamine release during S2 to 9.1 +/- 2.2 and 0.5 micrograms/8 min, respectively. Haloperidol (0.5 microM) reversed the inhibitory effects of apomorphine. Haloperidol alone enhanced catecholamine release induced by nicotine to 67.9 +/- 7.9 micrograms/8 min. [3H]Spiperone binds to adrenomedullary membranes with a KD of 0.24 nM and a Bmax of 117 fmol/mg of protein. Whereas spiperone and haloperidol potently displaced such binding, 3,4-dihydroxyphenylethylamine (dopamine) and sulpiride were poorer displacers, and SCH23390, prazosin, phenoxybenzamine, propranolol, BAY-K-8644, and nitrendipine did not displace [3H]spiperone bound. These data strongly suggest that, as in the cat, the bovine adrenal medulla chromaffin cell contains a dopaminergic receptor that modulates the catecholamine secretory process triggered by stimulation of the nicotinic cholinoceptor. Such a receptor seems to be of the D2 type and might be involved in a sympatho-adrenal cooperative mechanism contributing to the maintenance of cardiovascular homeostasis during stressful situations as well as to the pathogenesis of hypertension. If so, selective dopaminergic agonists might prove clinically useful in the treatment of hypertension.     

Chemicals Possessing a Neurotrophin-Like Activity on Dopaminergic Neurons in Primary Culture

Background

Neurotrophic factors have been shown to possess strong neuroprotective and neurorestaurative properties in Parkinson''s disease patients. However the issues to control their delivery into the interest areas of the brain and their surgical administration linked to their unability to cross the blood brain barrier are many drawbacks responsible of undesirable side effects limiting their clinical use. A strategy implying the use of neurotrophic small molecules could provide an interesting alternative avoiding neurotrophin administration and side effects. In an attempt to develop drugs mimicking neurotrophic factors, we have designed and synthesized low molecular weight molecules that exhibit neuroprotective and neuritogenic potential for dopaminergic neurons.

Principal Findings

A cell-based screening of an in-house quinoline-derived compound collection led to the characterization of compounds exhibiting both activities in the nanomolar range on mesencephalic dopaminergic neurons in spontaneous or 1-methyl-4-phenylpyridinium (MPP⁺)-induced neurodegeneration. This study provides evidence that rescued neurons possess a functional dopamine transporter and underlines the involvement of the extracellular signal-regulated kinase 1/2 signaling pathway in these processes.

Conclusion

Cell-based screening led to the discovery of a potent neurotrophic compound possessing expected physico-chemical properties for blood brain barrier penetration as a serious candidate for therapeutic use in Parkinson disease.     

Wnt-5a Ligand Modulates Mitochondrial Fission-Fusion in Rat Hippocampal Neurons

The Wnt signaling pathway plays an important role in developmental processes, including embryonic patterning, cell specification, and cell polarity. Wnt components participate in the development of the central nervous system, and growing evidence indicates that this pathway also regulates the function of the adult nervous system. In this study, we report that Wnt-5a, a noncanonical Wnt ligand, is a potent activator of mitochondrial dynamics and induces acute fission and fusion events in the mitochondria of rat hippocampal neurons. The effect of Wnt-5a was inhibited in the presence of sFRP, a Wnt scavenger. Similarly, the canonical Wnt-3a ligand had no effect on mitochondrial fission-fusion events, suggesting that this effect is specific for Wnt-5a alone. We also show that the Wnt-5a effects on mitochondrial dynamics occur with an increase in both intracellular and mitochondrial calcium (Ca²⁺), which was correlated with an increased phosphorylation of Drp1(Ser-616) and a decrease of Ser-637 phosphorylation, both indicators of mitochondrial dynamics. Electron microscope analysis of hippocampal tissues in the CA1 region showed an increase in the number of mitochondria present in the postsynaptic region, and this finding correlated with a change in mitochondrial morphology. We conclude that Wnt-5a/Ca²⁺ signaling regulates the mitochondrial fission-fusion process in hippocampal neurons, a feature that might help to further understand the role of Wnt-related pathologies, including neurodegenerative diseases associated with mitochondrial dysfunction, and represents a potentially important link between impaired metabolic function and degenerative disorders.     

Maitotoxin Induces Phosphoinositide Turnover and Modulates Glutamatergic and Muscarinic Cholinergic Receptor Function in Cultured Cerebellar Neurons

Maitotoxin (MTX) stimulated inositol phosphate (IP) formation in primary cultures of rat cerebellar granule cells. MTX-induced IP production was dependent on extracellular Ca2+ but independent of extracellular Na+. The stimulation of IP formation elicited by MTX was unaffected by pretreatment of cells with phorbol dibutyrate, pertussis toxin, and a variety of Ca2+ entry blockers, such as nimodipine, nisoldipine, Co2+, and Mn2+. The presence of MTX markedly attenuated IP production induced by carbachol and glutamate, with no apparent effect on the responses to norepinephrine (NE), histamine, 5-hydroxytryptamine (5-HT), and endothelin-1. The inhibition of the carbachol- and glutamate-induced responses by MTX was dose dependent with IC50 values of 1.2 and 0.5 ng/ml, respectively. Pretreatment of cells with a lower concentration of MTX (0.3 ng/ml) also attenuated carbachol- and glutamate-induced IP formation, in a time-dependent manner, with a decrease observed after 30 min prestimulation, but failed to affect NE-, histamine-, 5-HT-, endothelin-1, and sarafotoxin S6b-induced responses. Thus, MTX elicited a marked Ca2(+)-dependent phosphoinositide (PI) turnover in cerebellar granule cells and selectively inhibited carbachol- and glutamate-induced PI hydrolysis. Possible mechanisms underlying these selective modulations are discussed.     

Orexin A/Hypocretin Modulates Leptin Receptor-Mediated Signaling by Allosteric Modulations Mediated by the Ghrelin GHS-R1A Receptor in Hypothalamic Neurons

The hypothalamus is a key integrator of nutrient-seeking signals in the form of hormones and metabolites originated in both the central nervous system and the periphery. The main autocrine and paracrine target of orexinergic-related hormones such as leptin, orexin/hypocretin, and ghrelin are neuropeptide Y neurons located in the arcuate nucleus of the hypothalamus. The aim of this study was to investigate the expression and the molecular and functional relationships between leptin, orexin/hypocretin and ghrelin receptors. Biophysical studies in a heterologous system showed physical interactions between them, with potential formation of heterotrimeric complexes. Functional assays showed robust allosteric interactions particularly different when the three receptors are expressed together. Further biochemical and pharmacological assays provided evidence of heterotrimer functional expression in primary cultures of hypothalamic neurons. These findings constitute evidence of close relationships in the action of the three hormones already starting at the receptor level in hypothalamic cells.     

Dopamine Receptor 1 Modulates the Discharge Activities of Inspiratory and Biphasic Expiratory Neurons via cAMP-Dependent Pathways

Dopamine receptor 1 (D₁R) plays an essential role in regulating respiratory activity in mammals, however, little is known about how this receptor acts to modulate the basic respiratory rhythmogenesis. Here, by simultaneously recording the discharge activities of biphasic expiratory (biphasic E) neurons/inspiratory (I) neurons and the XII nerve rootlets from brainstem slices, we found that the application of D₁R agonist cis-(±)-1-(aminomethyl)-3,4-dihydro-3-phenyl-1H-2-benzopyran-5,6-diolhydrochloride (A68930, 5 μM), or forskolin, an intracellular cAMP-increasing agent, substantially decreased respiratory cycle and expiratory time of both types of neurons, and elevated the integral amplitude and frequency of XII nerve rootlets discharge. These changes were reversed by subsequent application of their antagonists SCH-23390 and Rp-Adenosine 3′,5′-cyclic monophosphorothioate triethylammonium salt hydrate (Rp-cAMPS), respectively. Importantly, after pretreatment with Rp-cAMPS, the effects of A68930 in both types of neurons were blocked, suggestive of a cAMP-dependent action of A68930. Thus, the current study indicates that D₁R may modulate basic breathing rhythmogenesis via cAMP-dependent mechanisms.     

Tripartite Motif Ligases Catalyze Polyubiquitin Chain Formation through a Cooperative Allosteric Mechanism

Ligation of polyubiquitin chains to proteins is a fundamental post-translational modification, often resulting in targeted degradation of conjugated proteins. Attachment of polyubiquitin chains requires the activities of an E1 activating enzyme, an E2 carrier protein, and an E3 ligase. The mechanism by which polyubiquitin chains are formed remains largely speculative, especially for RING-based ligases. The tripartite motif (TRIM) superfamily of ligases functions in many cellular processes including innate immunity, cellular localization, development and differentiation, signaling, and cancer progression. The present results show that TRIM ligases catalyze polyubiquitin chain formation in the absence of substrate, the rates of which can be used as a functional readout of enzyme function. Initial rate studies under biochemically defined conditions show that TRIM32 and TRIM25 are specific for the Ubc5 family of E2-conjugating proteins and, along with TRIM5α, exhibit cooperative kinetics with respect to Ubc5 concentration, with submicromolar [S]_0.5 and Hill coefficients of 3–5, suggesting they possess multiple binding sites for their cognate E2-ubiquitin thioester. Mutation studies reveal a second, non-canonical binding site encompassing the C-terminal Ubc5α-helix. Polyubiquitin chain formation requires TRIM subunit oligomerization through the conserved coiled-coil domain, but can be partially replaced by fusing the catalytic domain to GST to promote dimerization. Other results suggest that TRIM32 assembles polyubiquitin chains as a Ubc5-linked thioester intermediate. These results represent the first detailed mechanistic study of TRIM ligase activity and provide a functional context for oligomerization observed in the superfamily.     

Glucose Regulates Hypothalamic Long-chain Fatty Acid Metabolism via AMP-activated Kinase (AMPK) in Neurons and Astrocytes

Hypothalamic controls of energy balance rely on the detection of circulating nutrients such as glucose and long-chain fatty acids (LCFA) by the mediobasal hypothalamus (MBH). LCFA metabolism in the MBH plays a key role in the control of food intake and glucose homeostasis, yet it is not known if glucose regulates LCFA oxidation and esterification in the MBH and, if so, which hypothalamic cell type(s) and intracellular signaling mechanisms are involved. The aim of this study was to determine the impact of glucose on LCFA metabolism, assess the role of AMP-activated Kinase (AMPK), and to establish if changes in LCFA metabolism and its regulation by glucose vary as a function of the kind of LCFA, cell type, and brain region. We show that glucose inhibits palmitate oxidation via AMPK in hypothalamic neuronal cell lines, primary hypothalamic astrocyte cultures, and MBH slices ex vivo but not in cortical astrocytes and slice preparations. In contrast, oleate oxidation was not affected by glucose or AMPK inhibition in MBH slices. In addition, our results show that glucose increases palmitate, but not oleate, esterification into neutral lipids in neurons and MBH slices but not in hypothalamic astrocytes. These findings reveal for the first time the metabolic fate of different LCFA in the MBH, demonstrate AMPK-dependent glucose regulation of LCFA oxidation in both astrocytes and neurons, and establish metabolic coupling of glucose and LCFA as a distinguishing feature of hypothalamic nuclei critical for the control of energy balance.     

Leptin Acts via Leptin Receptor-Expressing Lateral Hypothalamic Neurons to Modulate the Mesolimbic Dopamine System and Suppress Feeding

The lateral hypothalamic area (LHA) acts in concert with the ventral tegmental area (VTA) and other components of the mesolimbic dopamine (DA) system to control motivation, including the incentive to feed. The anorexigenic hormone leptin modulates the mesolimbic DA system, although the mechanisms underlying this control have remained incompletely understood. We show that leptin directly regulates a population of leptin receptor (LepRb)-expressing inhibitory neurons in the LHA and that leptin action via these LHA LepRb neurons decreases feeding and body weight. Furthermore, these LHA LepRb neurons innervate the VTA, and leptin action on these neurons restores VTA expression of the rate-limiting enzyme in DA production along with mesolimbic DA content in leptin-deficient animals. Thus, these findings reveal that LHA LepRb neurons link anorexic leptin action to the mesolimbic DA system.     

Ceramide Sphingolipid Signaling Mediates Tumor Necrosis Factor (TNF)-Dependent Toxicity via Caspase Signaling in Dopaminergic Neurons

ABSTRACT: BACKGROUND: Dopaminergic (DA) neurons in the ventral midbrain selectively degenerate in Parkinsons disease (PD) in part because their oxidative environment in the substantia nigra (SN) may render them vulnerable to neuroinflammatory stimuli. Chronic inhibition of soluble Tumor Necrosis Factor (TNF) with dominant-negative TNF inhibitors protects DA neurons in rat models of parkinsonism, yet the molecular mechanisms and pathway(s) that mediate TNF toxicity remain(s) to be clearly identified. Here we investigated the contribution of ceramide sphingolipid signaling in TNF-dependent toxicity. RESULTS: Ceramide dose-dependently reduced the viability of DA neuroblastoma cells and primary DA neurons and pharmacological inhibition of sphingomyelinases (SMases) with three different inhibitors during TNF treatment afforded significant neuroprotection by attenuating increased endoplasmic reticulum (ER) stress, loss of mitochondrial membrane potential, caspase-3 activation and decreases in Akt phosphorylation. Using lipidomics mass spectrometry we confirmed that TNF treatment not only promotes generation of ceramide, but also leads to accumulation of several atypical deoxy-sphingoid bases (DSBs). Exposure of DA neuroblastoma cells to atypical DSBs in the micromolar range reduced cell viability and inhibited neurite outgrowth and branching in primary DA neurons, suggesting that TNFinduced de novo synthesis of atypical DSBs may be a secondary mechanism involved in mediating its neurotoxicity in DA neurons. CONCLUSIONS: We conclude that TNF/TNFR1-dependent activation of SMases generates ceramide and sphingolipid species that promote degeneration and caspase-dependent cell death of DA neurons. Ceramide and atypical DSBs may represent novel drug targets for development of neuroprotective strategies that can delay or attenuate the progressive loss of nigral DA neurons in patients with PD.     

Postprandial Hyperglycemia Stimulates Neuroglial Plasticity in Hypothalamic POMC Neurons after a Balanced Meal

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