Xin Yun Lu

Leptin: a potential novel antidepressant.

Leptin, a hormone secreted from adipose tissue, was originally discovered to regulate body weight. The localization of the leptin receptor in limbic structures suggests a potential role for leptin in emotional processes. Here, we show that rats exposed to chronic unpredictable stress and chronic social defeat exhibit low leptin levels in plasma. Systemic leptin treatment reversed the hedonic-like deficit induced by chronic unpredictable stress and improved behavioral despair dose-dependently in the forced swim test (FST), a model widely used for screening potential antidepressant efficacy. The behavioral effects of leptin in the FST were accompanied by increased neuronal activation in limbic structures, particularly in the hippocampus. Intrahippocampal infusion of leptin produced a similar antidepressant-like effect in the FST as its systemic administration. By contrast, infusion of leptin into the hypothalamus decreased body weight but had no effect on FST behavior. These findings suggest that: (i) impaired leptin production and secretion may contribute to chronic stress-induced depression-like phenotypes, (ii) the hippocampus is a brain site mediating leptins antidepressant-like activity, and (iii) elevating leptin signaling in brain may represent a novel approach for the treatment of depressive disorders.

Differential Distribution and Regulation of OX1 and OX2 Orexin/Hypocretin Receptor Messenger RNA in the Brain upon Fasting

To further understand the functions of the orexin/hypocretin system, we examined the expression and regulation of the orexin/hypocretin receptor (OX1R and OX2R) mRNA in the brain by using quantitative in situ hybridization. Expression of OX1R and OX2R mRNA exhibited distinct distribution patterns. Within the hypothalamus, expression for the OX1R mRNA was largely restricted in the ventromedial (VMH) and dorsomedial hypothalamic nuclei, while high levels of OX2R mRNA were contained in the paraventricular nucleus, VMH, and arcuate nucleus as well as in mammilary nuclei. In the amygdala, OX1R mRNA was expressed throughout the amygdaloid complex with robust labeling in the medial nucleus, while OX2R mRNA was only present in the posterior cortical nucleus of amygdala. High levels of OX2R mRNA were also observed in the ventral tegmental area. Moreover, both OX1R and OX2R mRNA were observed in the hippocampus, some thalamic nuclei, and subthalamic nuclei. Furthermore, we analyzed the effect of fasting on levels of OX1R and OX2R mRNA in the hypothalamic and amygdaloid subregions. After 20 h of fasting, levels of OX1R mRNA were significantly increased in the VMH and the medial division of amygdala. An initial decrease (14 h) and a subsequent increase (20 h) in OX1R mRNA levels after fasting were observed in the dorsomedial hypothalamic nucleus and lateral division of amygdala. Levels of OX2R mRNA were augmented in the arcuate nucleus, but remained unchanged in the dorsomedial hypothalamic nucleus, paraventricular hypothalamic nucleus, and amygdala following fasting. The time-dependent and region-specific regulatory patterns of OX1R and OX2R suggest that they may participate in distinct neural circuits under the condition of food deprivation.

Leptin Increases Adult Hippocampal Neurogenesis in Vivo and in Vitro

Leptin, an adipose-derived hormone, has been implicated in several physiological processes involving the hippocampus. However, the role of leptin in adult hippocampal neurogenesis remains unknown. Here we show that leptin regulates neurogenesis in the dentate gyrus of adult mice as well as in cultured adult hippocampal progenitor cells. Chronic administration of leptin to adult mice increased cell proliferation without significant effects on the differentiation and the survival of newly proliferated cells in the dentate gyrus. The expression of the long form leptin receptor, LepRb, was detected in hippocampal progenitor cells by reverse transcription-PCR and immunohistochemistry. Leptin treatment also increased proliferation of cultured adult hippocampal progenitor cells. Analysis of signal transduction pathways revealed that leptin stimulated phosphorylation of Akt and STAT3 but not ERK1/2. Furthermore, pre-treating the cells with specific inhibitors of Akt or STAT3 attenuated leptin-induced cell proliferation in a dose-dependent manner. Taken together, our results support a role for leptin in adult hippocampal neurogenesis and suggest the involvement of the Akt and STAT3 signaling pathways in mediating the actions of leptin on neurogenesis.

Neuroadaptations in ionotropic and metabotropic glutamate receptor mRNA produced by cocaine treatment.

Abstract : The expression of glutamate receptor/subunit mRNAs was examined 3 weeks after discontinuing 1 week of daily injections of saline or cocaine. The level of mRNA for GluR1‐4, NMDAR1, and mGluR5 receptors was measured with in situ hybridization and RT‐PCR. In nucleus accumbens, acute cocaine treatment significantly reduced the mRNA level for GluR3, GluR4, and NMDAR1 subunits, whereas repeated cocaine reduced the level for GluR3 mRNA. Acute cocaine treatment also reduced the NMDAR1 mRNA level in dorsolateral striatum and ventral tegmental area. In prefrontal cortex, repeated cocaine treatment significantly increased the level of GluR2 mRNA. The GluR2 mRNA level was not changed by acute or repeated cocaine in any other brain regions examined. Repeated cocaine treatment also significantly increased mGluR5 mRNA levels in nucleus accumbens shell and dorsolateral striatum. Functional properties of the ionotropic glutamate receptors are determined by subunit composition. In addition, metabotropic glutamate receptors can modulate synaptic transmission and the response to stimulation of ionotropic receptors. Thus, the observed changes in levels of AMPA and NMDA receptor subunits and the mGluR5 metabotropic receptor may alter excitatory neurotransmission in the mesocorticolimbic dopamine system, which could play a significant role in the enduring biochemical and behavioral effects of cocaine.

A biochemical function for attractin in agouti-induced pigmentation and obesity.

Agouti protein, a paracrine signaling molecule normally limited to skin, is ectopically expressed in lethal yellow (Ay) mice, and causes obesity by mimicking agouti-related protein (Agrp), found primarily in the hypothalamus. Mouse attractin (Atrn) is a widely expressed transmembrane protein whose loss of function in mahogany (Atrnmg-3J/ Atrnmg-3J) mutant mice blocks the pleiotropic effects of Ay. Here we demonstrate in transgenic, biochemical and genetic-interaction experiments that attractin is a low-affinity receptor for agouti protein, but not Agrp, in vitro and in vivo. Additional histopathologic abnormalities in Atrnmg-3J/Atrn mg-3J mice and cross-species genomic comparisons indicate that Atrn has multiple functions distinct from both a physiologic and an evolutionary perspective.

Leptin restores adult hippocampal neurogenesis in a chronic unpredictable stress model of depression and reverses glucocorticoid-induced inhibition of GSK-3β/β-catenin signaling

Stress and glucocorticoid stress hormones inhibit neurogenesis, whereas antidepressants increase neurogenesis and block stress-induced decrease in neurogenesis. Our previous studies have shown that leptin, an adipocyte-derived hormone with antidepressant-like properties, promotes baseline neurogenesis in the adult hippocampus. This study aimed to determine whether leptin is able to restore suppression of neurogenesis in a rat chronic unpredictable stress (CUS) model of depression. Chronic treatment with leptin reversed the CUS-induced reduction of hippocampal neurogenesis and depression-like behaviors. Leptin treatment elicited a delayed long-lasting antidepressant-like effect in the forced swim behavioral despair test, and this effect was blocked by ablation of neurogenesis with X-irradiation. The functional isoform of the leptin receptor, LepRb, and the glucocorticoid receptor (GR) were colocalized in hippocampal neural stem/progenitor cells in vivo and in vitro. Leptin treatment reversed the GR agonist dexamethasone (DEX)-induced reduction of proliferation of cultured neural stem/progenitor cells from adult hippocampus. Further mechanistic analysis revealed that leptin and DEX converged on glycogen synthase kinase-3β (GSK-3β) and β-catenin. While DEX decreased Ser9 phosphorylation and increased Tyr216 phosphorylation of GSK-3β, leptin increased Ser9 phosphorylation and attenuated the effects of DEX at both Ser9 and Tyr216 phosphorylation sites of GSK-3β. Moreover, leptin increased total level and nuclear translocation of β-catenin, a primary substrate of GSK-3β and a key regulator in controlling hippocampal neural progenitor cell proliferation, and reversed the inhibitory effects of DEX on β-catenin. Taken together, our results suggest that adult neurogenesis is involved in the delayed long-lasting antidepressant-like behavioral effects of leptin, and leptin treatment counteracts chronic stress and glucocorticoid-induced suppression of hippocampal neurogenesis via activating the GSK-3β/β-catenin signaling pathway.

Leptin promotes adult hippocampal neurogenesis in vivo and in vitro

Leptin, an adipose-derived hormone, has been implicated in several physiological processes involving the hippocampus. However, the role of leptin in adult hippocampal neurogenesis remains unknown. Here we show that leptin regulates neurogenesis in the dentate gyrus of adult mice as well as in cultured adult hippocampal progenitor cells. Chronic administration of leptin to adult mice increased cell proliferation without significant effects on the differentiation and the survival of newly proliferated cells in the dentate gyrus. The expression of the long form leptin receptor, LepRb, was detected in hippocampal progenitor cells by reverse transcription-PCR and immunohistochemistry. Leptin treatment also increased proliferation of cultured adult hippocampal progenitor cells. Analysis of signal transduction pathways revealed that leptin stimulated phosphorylation of Akt and STAT3 but not ERK1/2. Furthermore, pre-treating the cells with specific inhibitors of Akt or STAT3 attenuated leptin-induced cell proliferation in a dose-dependent manner. Taken together, our results support a role for leptin in adult hippocampal neurogenesis and suggest the involvement of the Akt and STAT3 signaling pathways in mediating the actions of leptin on neurogenesis.

Expression of D1 receptor mRNA in projections from the forebrain to the ventral tegmental area

In situ hybridization was combined with Fluoro‐Gold retrograde labeling to determine if cells projecting from the forebrain to the ventral tegmental area (VTA) express D1 receptor mRNA. Cell counts were made in the prefrontal cortex, shell of the nucleus accumbens, and ventral pallidum to estimate the percentage of neurons projecting to the VTA that express D1 receptor mRNA. Retrogradely labeled cells were observed in the infralimbic and prelimbic regions of the prefrontal cortex, and up to 37% of the retrogradely labeled cells expressed D1 receptor mRNA. Double‐labeled cells constituted up to 89% of retrogradely labeled neurons in the rostral shell and up to 68% in the caudal shell of the nucleus accumbens. The number of retrogradely labeled cells in the ventral pallidum that were double‐labeled ranged from 13% in the rostral to less than 10% in the caudal portions. These data provide anatomical support for a role of D1 receptors in the reciprocal innervation between the forebrain and VTA. Synapse 25:205–214, 1997.

Relation between the Hypothalamic-Pituitary-Thyroid (HPT) Axis and the Hypothalamic-Pituitary-Adrenal (HPA) Axis during Repeated Stress

Previous work has indicated that acute and repeated stress can alter thyroid hormone secretion. Corticosterone, the end product of hypothalamic-pituitary-adrenal (HPA) axis activation and strongly regulated by stress, has been suggested to play a role in hypothalamic-pituitary-thyroid (HPT) axis regulation. In the current study, we sought to further characterize HPT axis activity after repeated exposure to inescapable foot-shock stress (FS), and to examine changes in proposed regulators of the HPT axis, including plasma corticosterone and hypothalamic arcuate nucleus agouti-related protein (AGRP) mRNA levels. Adult male Sprague-Dawley rats were subjected to one daily session of inescapable FS for 14 days. Plasma corticosterone levels were determined during and after the stress on days 1 and 14. Animals were killed on day 15, and trunk blood and brains were collected for measurement of hormone and mRNA levels. Repeated exposure to FS led to a significant decrease in serum levels of 3,5,3′-triiodothyronine (T3) and 3,5,3′,5′-tetraiodothyronine (T4). Stress-induced plasma corticosterone levels were not altered by repeated exposure to the stress. Despite the decrease in peripheral hormone levels, thyrotropin-releasing hormone (TRH) mRNA levels within the paraventricular nucleus of the hypothalamus were not altered by the stress paradigm. Arcuate nucleus AGRP mRNA levels were significantly increased in the animals exposed to repeated FS. Additionally, we noted significant correlations between stress-induced plasma corticosterone levels and components of the HPT axis, including TRH mRNA levels and free T4 levels. Additionally, there was a significant correlation between AGRP mRNA levels and total T3 levels. Changes in body weight were also correlated with peripheral corticosterone and TRH mRNA levels. These results suggest that repeated exposure to mild-electric foot-shock causes a decrease in peripheral thyroid hormone levels, and that components of the HPA axis and hypothalamic AGRP may be involved in stress regulation of the HPT.

Adiponectin Stimulates Proliferation of Adult Hippocampal Neural Stem/Progenitor Cells through Activation of p38 Mitogen-activated Protein Kinase (p38MAPK)/Glycogen Synthase Kinase 3β (GSK-3β)/β-Catenin Signaling Cascade

Background: The role of adiponectin in adult hippocampal neural stem/progenitor cell regulation is unknown. Results: Adiponectin stimulates proliferation of hippocampal neural stem/progenitor cells in vitro and activates the p38MAPK/GSK-3β/β-catenin signaling pathway. Conclusion: Adult hippocampal neurogenesis is positively regulated by adiponectin. Significance: Adiponectin may alleviate neuropsychiatric disorders that involve impaired neurogenesis. Adiponectin is the most abundant adipokine secreted from adipocytes. Accumulating evidence suggests that the physiological roles of adiponectin go beyond its metabolic effects. In the present study, we demonstrate that adiponectin receptors 1 and 2 (AdipoR1 and AdipoR2) are expressed in adult hippocampal neural stem/progenitor cells (hNSCs). Adiponectin treatment increases proliferation of cultured adult hNSCs in a dose- and time-dependent manner, whereas apoptosis and differentiation of adult hNSCs into neuronal or glial lineage were not affected. Adiponectin activates AMP-activated protein kinase and p38 mitogen-activated protein kinase (p38MAPK) signaling pathways in adult hNSCs. Pretreatment with the p38MAPK inhibitor SB203580, but not the AMP-activated protein kinase inhibitor Compound C, attenuates adiponectin-induced cell proliferation. Moreover, adiponectin induces phosphorylation of Ser-389, a key inhibitory site of glycogen synthase kinase 3β (GSK-3β), and this effect can be blocked by inhibition of p38MAPK with SB203580. Levels of total and nuclear β-catenin, the primary substrate of GSK-3β, were increased by adiponectin treatment. These results indicate that adiponectin stimulates proliferation of adult hNSCs, via acting on GSK-3β to promote nuclear accumulation of β-catenin. Thus, our studies uncover a novel role for adiponectin signaling in regulating proliferation of adult neural stem cells.