Identification of Lysophosphatidic Acid Receptor 1 in Astroglial Cells as a Target for Glial Cell Line-derived Neurotrophic Factor Expression Induced by Antidepressants

Astrocytic GDNF ameliorates anesthesia and surgery-induced cognitive impairment by promoting hippocampal synaptic plasticity in aged mice

BACKGROUND

Perioperative neurocognitive disorders (PND) are common complications after surgery in older patients. However, the specific mechanism of this condition remains unclear. Glial cell line-derived neurotrophic factor (GDNF) is an important neurotrophin that abundantly expressed throughout the brain. It can enhance synaptic plasticity and alleviate learning and memory impairments. Thus, the purpose of this study was to investigate the role of GDNF in PND and the mechanisms involved.

METHODS

The PND animal model was established by performing left tibial fracture surgery on 18-month-old C57BL/6 mice under sevoflurane anesthesia. Recombinant adeno-associated virus (rAAV)-GDNF or empty vectors were injected bilaterally into the hippocampal CA1 region of aged mice 3 weeks before anesthesia/surgery. The open field and fear conditioning test were used to assess the behavior changes. Golgi staining and electrophysiology were utilized to evaluate the morphological and functional alterations of neuronal synaptic plasticity. Western blot analysis was carried out to measure the proteins expression levels and immunofluorescence staining was performed to probe the cellular localization of GDNF.

RESULTS

Mice with surgery and anesthesia showed a significant decrease in hippocampus-dependent learning and memory, accompanied by a decline in hippocampal synaptic plasticity. Anesthesia/surgery induced a reduction of GDNF, which was colocalized with astrocytes. Overexpression of GDNF in astrocytes could ameliorate the decline in cognitive function by improving hippocampal synaptic plasticity, meanwhile astrocytic GDNF rescued the anesthesia/surgery-induced decrease in GFRα1 and NCAM.

CONCLUSION

The study concludes that astrocytic GDNF may improve anesthesia/surgery-induced cognitive impairment by promoting hippocampal synaptic plasticity in aged mice via the GFRα1/NCAM pathway.

G protein-biased LPAR1 agonism of prototypic antidepressants: Implication in the identification of novel therapeutic target for depression

Prototypic antidepressants, such as tricyclic/tetracyclic antidepressants (TCAs), have multiple pharmacological properties and have been considered to be more effective than newer antidepressants, such as selective serotonin reuptake inhibitors, in treating severe depression. However, the clinical contribution of non-monoaminergic effects of TCAs remains elusive. In this study, we discovered that amitriptyline, a typical TCA, directly binds to the lysophosphatidic acid receptor 1 (LPAR1), a G protein-coupled receptor, and activates downstream G protein signaling, while exerting a little effect on β-arrestin recruitment. This suggests that amitriptyline acts as a G protein-biased agonist of LPAR1. This biased agonism was specific to TCAs and was not observed with other antidepressants. LPAR1 was found to be involved in the behavioral effects of amitriptyline. Notably, long-term infusion of mouse hippocampus with the potent G protein-biased LPAR agonist OMPT, but not the non-biased agonist LPA, induced antidepressant-like behavior, indicating that G protein-biased agonism might be necessary for the antidepressant-like effects. Furthermore, RNA-seq analysis revealed that LPA and OMPT have opposite patterns of gene expression changes in the hippocampus. Pathway analysis indicated that long-term treatment with OMPT activated LPAR1 downstream signaling (Rho and MAPK), whereas LPA suppressed LPAR1 signaling. Our findings provide insights into the mechanisms underlying the non-monoaminergic antidepressant effects of TCAs and identify the G protein-biased agonism of LPAR1 as a promising target for the development of novel antidepressants.

Differential targeting of lysophosphatidic acid LPA1, LPA2, and LPA3 receptor signalling by tricyclic and tetracyclic antidepressants

We previously reported that in different cell types antidepressant drugs activate lysophosphatidic acid (LPA) LPA1 receptor to induce proliferative and prosurvival responses. Here, we further characterize this unique action of antidepressants by examining their effects on two additional LPA receptor family members, LPA2 and LPA3. Human LPA1-3 receptors were stably expressed in HEK-293 cells (HEK-LPA1, -LPA2 and -LPA3 cells) and their functional activity was determined by Western blot and immunofluorescence. LPA effectively stimulated the phosphorylation of extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) in HEK-LPA1, -LPA2, and -LPA3 cells. The tricyclic antidepressants amitriptyline, clomipramine, imipramine and desipramine increased phospho-ERK1/2 levels in HEK-LPA1 and -LPA3 cells but were relatively poor agonists in LPA2-expressing cells. The tetracyclic antidepressants mianserin and mirtazapine were active at all three LPA receptors. When combined with LPA, both amitriptyline and mianserin potentiated Gi/o-mediated phosphorylation of ERK1/2 induced by LPA in HEK-LPA1, -LPA2 and -LPA3 cells, CHO-K1 fibroblasts and HT22 hippocampal neuroblasts. This potentiation was associated with enhanced phosphorylation of CREB and S6 ribosomal protein, two molecular targets of activated ERK1/2. The antidepressants also potentiated LPA-induced Gq/11-mediated phosphorylation of AMP-activated protein kinase in HEK-LPA1 and -LPA3 cells. Conversely, amitriptyline and mianserin were found to inhibit LPA-induced Rho activation in HEK-LPA1 and LPA2 cells. These results indicate that tricyclic and tetracyclic antidepressants can act on LPA1, LPA2 and LPA3 receptor subtypes and exert differential effects on LPA signalling through these receptors.

Downregulation of connexin 43 potentiates amitriptyline-induced brain-derived neurotrophic factor expression in primary astrocytes through lysophosphatidic acid receptor1/3, Src, and extracellular signal-regulated kinase

Connexin 43 (Cx43) expression is decreased in the prefrontal cortex of patients with depression, but its significance is still unknown. Neurotrophic factors, such as brain-derived neurotrophic factor (BDNF), are involved in the effects of antidepressant. However, the relationship between Cx43 expression and induction of brain-derived neurotrophic factor production by antidepressants is unknown. On the basis of our previous study, which showed that adrenergic receptors stimulation results in potentiation of BDNF expression in astrocytes with downregulated Cx43 expression, we investigated the induction of BDNF expression by amitriptyline, a tricyclic antidepressant, in Cx43-knockdown astrocytes. Amitriptyline treatment potentiated BDNF expression in Cx43-knockdown astrocytes compared with those treated with non-targeting small interfering RNA (siRNA). Using a pharmacological approach, we revealed that the potentiating effect of amitriptyline on BDNF expression was mediated by lysophosphatidic acid (LPA) receptor_1/3 (LPA_1/3) stimulation and subsequent activation of Src-extracellular signal-regulated kinase (ERK) signaling. These findings suggest that downregulation of Cx43 in patients with depression might contribute to the therapeutic efficacy of antidepressants rather than the pathogenesis of depression.

Exposure to di-(2-ethylhexyl) phthalate reduces secretion of GDNF via interfering with estrogen pathway and downregulating ERK/c-fos signaling pathway in astrocytes

Di-(2-ethylhexyl) phthalate (DEHP) is a typical endocrine-disrupting chemical (EDC) that can increase the risk of central nervous system disease. This study aimed to investigate the in vitro and in vivo effects of DEHP exposure on GDNF secretion and the underlying mechanisms. Pregnant Wistar rats were randomly assigned into four groups and administered 0, 30, 300, or 750 mg/kg DEHP daily by oral gavage. In addition, primary astrocytes were exposed to mono-(2-ethylhexyl) phthalate (MEHP), the main metabolite of DEHP. Our results showed that DEHP exposure reduced GDNF levels and downregulated the ERK/c-fos signaling pathway in the cerebral cortex of male, but not female, offspring. Moreover, exogenous estrogen could overcome the decreased GDNF levels in astrocytes caused by MEHP exposure. MEHP also decreased p300 levels and downregulated the ERK/c-fos signaling pathway in primary astrocytes. Honokiol restored GDNF levels following MEHP exposure by activating the ERK/c-fos signaling pathway, while the inhibitor U0126 further reduced the GDNF levels. These results suggested that DEHP exposure could interfere with the normal effects of estrogen in the brain and downregulate the ERK/c-fos signaling pathway to decrease the GDNF secretion from astrocytes in the cerebral cortex.

Downregulation of connexin43 potentiates noradrenaline-induced expression of brain-derived neurotrophic factor in primary cultured cortical astrocytes

Decreased expression of brain-derived neurotrophic factor (BDNF) is involved in the pathology of depressive disorders. Astrocytes produce BDNF following antidepressant treatment or stimulation of adrenergic receptors. Connexin43 (Cx43) is mainly expressed in central nervous system astrocytes and its expression is downregulated in patients with major depression. How changes in Cx43 expression affect astrocyte function, including BDNF production, is poorly understood. The current study examined the effect of Cx43 knockdown on BDNF expression in cultured cortical astrocytes after stimulation of adrenergic receptors. The expression of Cx43 in rat primary cultured cortical astrocytes was downregulated with RNA interference. Levels of messenger RNAs (mRNAs) or proteins were measured by real-time PCR and western blotting, respectively. Knockdown of Cx43 potentiated noradrenaline (NA)-induced expression of BDNF mRNA in cultured astrocytes. NA treatment induced proBDNF protein expression in astrocytes transfected with small interfering RNA (siRNA) targeting Cx43, but not with control siRNA. This potentiation was mediated by the Src tyrosine kinase-extracellular signal-regulated kinase (ERK) pathway through stimulation of adrenergic α1 and β receptors. Furthermore, the Gq/11 protein-Src-ERK pathway and the G-protein coupled receptor kinase 2-Src-ERK pathway were involved in α1 and β adrenergic receptor-mediated potentiation of BDNF mRNA expression, respectively. The current studies demonstrate a novel mechanism of BDNF expression in cortical astrocytes mediated by Cx43, in which downregulation of Cx43 increases, through adrenergic receptors, the expression of BDNF. The current findings indicate a potentially novel mechanism of action of antidepressants, via regulation of astrocytic Cx43 expression and subsequent BDNF expression.

The Roles of Lpar1 in Central Nervous System Disorders and Diseases

Lysophosphatidic acid receptor 1 (Lpar1), which is found in almost all human tissues but is most abundant in the brain, can couple to G protein-coupled receptors (GPCRs) and participate in regulating cell proliferation, migration, survival, and apoptosis. Endothelial differentiation gene-2 receptor (Edg2), the protein encoded by the Lpar1 gene, is present on various cell types in the central nervous system (CNS), such as neural stem cells (NSCs), oligodendrocytes, neurons, astrocytes, and microglia. Lpar1 deletion causes neurodevelopmental disorders and CNS diseases, such as brain cancer, neuropsychiatric disorders, demyelination diseases, and neuropathic pain. Here, we summarize the possible roles and mechanisms of Lpar1/Edg2 in CNS disorders and diseases and propose that Lpar1/Edg2 might be a potential therapeutic target for CNS disorders and diseases.

Reduced Cerebrospinal Fluid Levels of Lysophosphatidic Acid Docosahexaenoic Acid in Patients With Major Depressive Disorder and Schizophrenia

BACKGROUND

Lysophosphatidic acid (LPA) is involved in numerous biological processes, including neurodevelopment, chronic inflammation, and immunologic response in the central nervous system. Autotaxin (ATX) is a secreted enzyme that produces LPA from lysophosphatidylcholine (LPC). Previous studies have demonstrated decreased protein levels of ATX in cerebrospinal fluid (CSF) of patients with major depressive disorder (MDD). Based on those studies, the current study investigated the levels of lysophospholipids species including LPA and related metabolic enzymes, in CSF of patients with MDD and schizophrenia (SCZ).

METHODS

The levels of lysophospholipids species and related metabolic enzymes were measured with either liquid chromatography-tandem mass spectrometry or enzyme-linked immunosorbent assay. Japanese patients were diagnosed with DSM-IV-TR. CSF was obtained from age- and sex-matched healthy controls (n = 27) and patients with MDD (n = 26) and SCZ (n = 27).

RESULTS

Of all lysophospholipids species, the levels of LPA 22:6 (LPA - docosahexaenoic acid) were significantly lower in patients with MDD and SCZ than in healthy controls. These levels were negatively correlated with several clinical symptomatic scores of MDD, but not those of SCZ. In addition, the levels of LPA 22:6 were significantly correlated with the levels of LPC 22:6 among all 3 groups. On the other hand, the levels of LPA 22:6 were not correlated with ATX activity in patients with MDD and SCZ.

CONCLUSION

The lower levels of LPA 22:6 in patients with MDD and SCZ suggest an abnormality of LPA 22:6 metabolism. In addition, several depressive symptoms in patients with MDD were significantly associated with the lower levels of LPA 22:6, suggesting an involvement of LPA 22:6 in the pathophysiology of MDD.

Multiregional profiling of the brain transmembrane proteome uncovers novel regulators of depression

Transmembrane proteins play vital roles in mediating synaptic transmission, plasticity, and homeostasis in the brain. However, these proteins, especially the G protein-coupled receptors (GPCRs), are underrepresented in most large-scale proteomic surveys. Here, we present a new proteomic approach aided by deep learning models for comprehensive profiling of transmembrane protein families in multiple mouse brain regions. Our multiregional proteome profiling highlights the considerable discrepancy between messenger RNA and protein distribution, especially for region-enriched GPCRs, and predicts an endogenous GPCR interaction network in the brain. Furthermore, our new approach reveals the transmembrane proteome remodeling landscape in the brain of a mouse depression model, which led to the identification of two previously unknown GPCR regulators of depressive-like behaviors. Our study provides an enabling technology and rich data resource to expand the understanding of transmembrane proteome organization and dynamics in the brain and accelerate the discovery of potential therapeutic targets for depression treatment.

Lysophosphatidic acid induces thrombospondin-1 production in primary cultured rat cortical astrocytes

Lysophosphatidic acid (LPA), a brain membrane-derived lipid mediator, plays important roles including neural development, function, and behavior. In the present study, the effects of LPA on astrocyte-derived synaptogenesis factor thrombospondins (TSPs) production were examined by real-time PCR and western blotting, and the mechanism underlying this event was examined by pharmacological approaches in primary cultured rat cortical astrocytes. Treatment of astrocytes with LPA increased TSP-1 mRNA, and TSP-2 mRNA, but not TSP-4 mRNA expression. TSP-1 protein expression and release were also increased by LPA. LPA-induced TSP-1 production were inhibited by AM966 a LPA₁ receptor antagonist, and Ki16425, LPA_1/3 receptors antagonist, but not by H2L5146303, LPA₂ receptor antagonist. Pertussis toxin, Gi/o inhibitor, but not YM-254890, Gq inhibitor, and NF499, Gs inhibitor, inhibited LPA-induced TSP-1 production, indicating that LPA increases TSP-1 production through Gi/o-coupled LPA₁ and LPA₃ receptors. LPA treatment increased phosphorylation of extracellular signal-regulated kinase (ERK), p38 mitogen-activated protein kinase (MAPK), and c-Jun N-terminal kinase (JNK). LPA-induced TSP-1 mRNA expression was inhibited by U0126, MAPK/ERK kinase (MEK) inhibitor, but not SB202190, p38 MAPK inhibitor, or SP600125, JNK inhibitor. However, LPA-induced TSP-1 protein expression was diminished with inhibition of all three MAPKs, indicating that these signaling molecules are involved in TSP-1 protein production. Treatment with antidepressants, which bind to astrocytic LPA₁ receptors, increased TSP-1 mRNA and protein production. The current findings show that LPA/LPA_1/3 receptors signaling increases TSP-1 production in astrocytes, which could be important in the pathogenesis of affective disorders and could potentially be a target for the treatment of affective disorders.

Differential anatomical and cellular expression of lysophosphatidic acid receptor 1 in adult mouse brain

Lysophosphatidic acid (LPA) is a bioactive phospholipid that acts as an extracellular signaling molecule through six G-protein-coupled receptors: LPA₁-LPA₆. Recent studies have demonstrated that LPA signaling via LPA₁ receptor plays a crucial role in cognition and emotion. However, because of limited availability of reliable antibodies, it is currently difficult to identify the cell types expressing LPA₁ receptor in the brain. The current study explored the cellular distribution pattern of LPA₁ receptor in the brain using the LPA₁ lacZ-knock-in reporter mice. In situ hybridization and immunohistochemistry revealed that LacZ gene expression in these mice reflected the expression of endogenous LPA₁ receptor in the brain. Overall, some brain nuclei contained higher levels of LPA₁ receptor than others. The majority of LPA₁ receptor-expressing cells were Olig2⁺ oligodendrocytes. In addition, ALDH1l1⁺ astrocytes and CD31⁺ vascular endothelial cells also expressed LPA₁ receptor. By contrast, NeuN⁺ neuron and Iba1⁺ microglia expressed little or no LPA₁ receptor. The current neuroanatomical findings will aid in elucidating a role of brain LPA₁ receptor, especially those involved in cognition and emotion.

Japanese Herbal Medicine Ninjinyoeito Mediates Its Orexigenic Properties Partially by Activating Orexin 1 Receptors

Cancer cachexia is highly prevalent in patients with progressive cancer and is characterized by decreased food consumption, and body weight. Japanese herbal medicine Ninjinyoeito (NYT), composed of 12 herbal crude drugs, is prescribed in Asian countries to improve several symptoms such as anorexia and fatigue, which are commonly observed in patients with cancer cachexia. However, the action mechanisms of NYT in improving anorexia or fatigue in patients with cancer are not clear. Therefore, in the present study, we examined the effects of NYT on the activities of several G-protein-coupled receptors (GPCRs), which activate hyperphagia signaling in the central nervous system, using an in vitro assay with the CellKey™ system, which detects the activation of GPCRs as a change in intracellular impedance (ΔZ). NYT increased the ΔZ of human embryonic kidney 293 (HEK293) cells expressing orexin 1 receptor (OX1R) and those expressing neuropeptide Y1 receptor (NPY1R) in a dose-dependent manner. On the contrary, NYT did not significantly increase the ΔZ of HEK293A cells expressing growth hormone secretagogue receptor (GHSR) and those expressing NPY5R. The selective OX1R antagonist SB674042 significantly decreased the NYT-induced increase in ΔZ in OX1R-expressing cells. Contrarily, the selective NPY1R antagonist BIBO3340 failed to inhibit the NPY-induced increase in ΔZ in NPY1R-expressing cells. Additionally, we prepared modified NYT excluding each one of the 12 herbal crude drugs in NYT and investigated the effects on the activity of OX1R. Among the 12 modified NYT formulations, the one without citrus unshiu peel failed to activate OX1R. A screening of each of the 12 herbal crude drugs showed that citrus unshiu peel significantly activated OX1R, which was significantly suppressed by SB674042. These finding suggest that NYT and citrus unshiu peel could increase food intake via activation of orexigenic OX1R-expressing neurons in the hypothalamus. This study provides scientific evidence to support the potential of NYT for cancer patients with anorexia.

Mirtazapine increases glial cell line-derived neurotrophic factor production through lysophosphatidic acid 1 receptor-mediated extracellular signal-regulated kinase signaling in astrocytes

Different classes of antidepressants, such as tricyclic antidepressants, selective serotonin reuptake inhibitor (SSRI), and serotonin and norepinephrine reuptake inhibitor (SNRI), have been shown to increase GDNF production in astrocytes, which could be a key mechanism of the psychotropic effect of antidepressants. The antidepressant mirtazapine is a noradrenaline and specific serotonergic antidepressant (NaSSA) and does not block reuptake of catecholamines and serotonin. The present study examined the effect of mirtazapine on GDNF expression in rat C6 astroglial cells (C6 cells) and rat primary cultured cortical astrocytes (primary astrocytes). Mirtazapine treatment significantly increased GDNF mRNA expression and GDNF release in both C6 cells and primary astrocytes. In primary astrocytes, mirtazapine also increased the expressions of brain-derived neurotrophic factor mRNA. To mimic mirtazapine's putative mechanism of action, cells were treated with either a α₂-adrenoceptor antagonist (yohimbine), 5-HT₂ receptor antagonist (ketanserin), 5-HT₃ receptor antagonist (ondansetron), or a mixture of these--no effect on GDNF mRNA expression was observed. Mirtazapine treatment increased phosphorylation of extracellular signal-regulated kinase (ERK) 1/2, and the mirtazapine-induced GDNF and BDNF expression were blocked by MAPK/ERK kinase (MEK) inhibitor (U0126). Furthermore, the effect of mirtazapine on ERK phosphorylation and expressions of GDNF and BDNF was antagonized by Gi/o inhibitor (pertussis toxin), lysophosphatidic acid-1 (LPA₁) receptor antagonist (AM966), and LPA₁/LPA₃ receptors antagonist (Ki16425). The current findings demonstrate that the NaSSA mirtazapine, similar to other classes of antidepressants, increases GDNF expression through a Gi/o coupled LPA₁ receptor-mediated ERK pathway. The current findings suggest a general mechanism underlying the psychotropic effect antidepressants.

Antidepressant amitriptyline-induced matrix metalloproteinase-9 activation is mediated by Src family tyrosine kinase, which leads to glial cell line-derived neurotrophic factor mRNA expression in rat astroglial cells

Background

Astrocytes have been implicated in the pathophysiology of mood disorders and in the mechanism of the pharmacological effects of antidepressant drugs by the production of neurotrophic/growth factors. Previous studies have identified astrocyte‐expressed Gα_i/o‐coupled lysophosphatidic acid receptor 1 (LPAR1), as being involved in antidepressant‐induced production of glial cell line‐derived neurotrophic factor (GDNF) and matrix metalloproteinase‐9 (MMP‐9) activation, an important step in the production of GNDF. However, the precise mechanism of MMP‐9 activation by antidepressants has yet to be identified, in particular the intracellular signaling pathway between LPAR1/Gα_i/o and MMP‐9.

Methods and Results

Treatment of rat C6 astroglial cells (C6 cells) with amitriptyline increased Src family tyrosine kinase phosphorylation in a time and concentration‐dependent manner. Amitriptyline‐induced GDNF mRNA expression was blocked by Src family tyrosine kinase inhibitors. In addition, inhibiting Src family tyrosine kinase blocked amitriptyline‐induced zymographic MMP‐9 activation in C6 cells. The amitriptyline‐induced zymographic MMP‐9 activity was completely blocked by selective inhibition of Gα_i/o protein and LPAR1. Furthermore, the amitriptyline‐induced Src family tyrosine kinase phosphorylation was blocked by LPAR1, but not MMP‐9 inhibition, indicating that Src family tyrosine kinase involvement is downstream of LPAR1.

Conclusions

The current findings suggest that the pharmacological effect of antidepressant such as amitriptyline is mediated through an intracellular signaling pathway via the LPAR1/Gα_i/o/Src family tyrosine kinase, which leads to MMP‐9 activation and GDNF production.

Treatment of rat C6 astroglial cells (C6 cells) with amitriptyline increased Src family tyrosine kinase phosphorylation in a time‐ and concentration‐dependent manner. The current findings suggest that the pharmacological effect of antidepressant such as amitriptyline is mediated through an intracellular signaling pathway via the LPAR1/Gαi/o/Src family tyrosine kinase, which leads to MMP‐9 activation and glial cell line‐derived neurotrophic factor production.

Reduced Serum and Cerebrospinal Fluid Levels of Autotaxin in Major Depressive Disorder

BACKGROUND

The autotaxin/lysophosphatidic acid axis is involved in diverse biological processes including neurodevelopment, inflammation, and immunological functioning. The lysophosphatidic acid 1 receptor has been implicated in the pathophysiology of major depressive disorder and in the mechanism of action of antidepressants. However, it is unclear whether central or peripheral autotaxin levels are altered in patients with major depressive disorder.

METHODS

Serum autotaxin levels were measured by an enzyme-linked immunosorbent assay in 37 patients with major depressive disorder diagnosed using DSM-IV-TR who underwent electroconvulsive therapy and were compared with those of 47 nondepressed controls matched for age and sex between January 2011 and December 2015. Patient serum levels of autotaxin before and after electroconvulsive therapy were also compared. In a separate sample set, cerebrospinal fluid autotaxin levels were compared between 26 patients with major depressive disorder and 27 nondepressed controls between December 2010 and December 2015. A potential association was examined between autotaxin levels and clinical symptoms assessed with the Hamilton Depression Rating Scale.

RESULTS

Before electroconvulsive therapy, both serum and cerebrospinal fluidautotaxin levels were significantly lower in major depressive disorder patients than in controls (serum: P = .001, cerebrospinal fluid: P = .038). A significantly negative correlation between serum, but not cerebrospinal fluid, autotaxin levels and depressive symptoms was observed (P = .032). After electroconvulsive therapy, a parallel increase in serum autotaxin levels and depressive symptoms improvement was observed (P = .005).

CONCLUSION

The current results suggest that serum autotaxin levels are reduced in a state-dependent manner. The reduction of cerebrospinal fluidautotaxin levels suggests a dysfunction in the autotaxin/lysophosphatidic acid axis in the brains of patients with major depressive disorder.

Neuropeptide oxytocin enhances μ opioid receptor signaling as a positive allosteric modulator

Oxytocin (OT) is a 9-amine neuropeptide that plays an essential role in mammalian labor, lactation, maternal bonding, and social affiliation. OT has been reported to exert an analgesic effect in both humans and animals, and the results of certain animal experiments have shown that the analgesic effect of OT is partially blocked by opioid receptor antagonists. To investigate the relationship between OT and μ opioid receptor (MOR), we evaluated how OT affects MOR in vitro by performing an electrical impedance-based receptor biosensor assay (CellKey™ assay), an intracellular cAMP assay, and a competitive receptor-binding analysis by using cells stably expressing human MOR and OT receptor. In both the CellKey™ assay and the intracellular cAMP assay, OT alone exerted no direct agonistic effect on human MOR, but treatment with 10^-6 M OT markedly enhanced the MOR signaling induced by 10^-6 M endomorphin-1, β-endorphin, morphine, fentanyl, and DAMGO. Moreover, in the competitive receptor-binding assay, 10^-6 M OT did not alter the affinity of endomorphin-1 or morphine for MOR. These results suggest that OT could function as a positive allosteric modulator that regulates the efficacy of MOR signaling, and thus OT might represent a previously unrecognized candidate analgesic agent.

Assessments of cellular melatonin receptor signaling pathways: β-arrestin recruitment, receptor internalization, and impedance variations

Melatonin receptors belong to the family of G-protein coupled receptors. Agonist-induced receptor activation is terminated with the recruitment of β-arrestin, which leads to receptor internalization. Furthermore, agonist binding induces a shift in cellular shape that translates into a change in the electric impedance of the cell. In the present study, we employed engineered cells to study these internalization-related processes in the context of the two melatonin receptors, MT₁ and MT₂. To assess these three receptor internalization-related functions and validate the results, we employed four classical ligands of melatonin receptors: the natural agonist melatonin; the super-agonist 2-iodo-melatonin and the two antagonists luzindole and 4-phenyl-2-propionamidotetralin. The assessments confirmed the nature of the agonistic ligands but showed that 4-phenyl-2-propionamidotetralin, a described antagonist, is a biased partial agonist at MT₂ with poorer affinity for MT₁. The methods are now available to be applied to any receptor system for which multiple signaling pathways must be evaluated for new molecules.

The protein and mRNA expression levels of glial cell line-derived neurotrophic factor in post stroke depression and major depressive disorder

Previous studies have indicated that the level of glial cell line-derived neurotrophic factor (GDNF) may be correlated with stroke and depression. Here, we investigated whether GDNF can be a discriminant indicator for post stroke depression (PSD). 159 participants were divided into four groups: PSD, stroke without depression (Non-PSD), major depressive disorder (MDD) and normal control (NC) group, and the protein and mRNA expression levels of GDNF in serum were measured. The results showed that only MDD group had statistical difference in protein and mRNA levels compared with the other three groups (Bonferroni test, P < 0.05). The results of receiver operating curve (ROC) analysis supported GDNF as general distinguishing models in PSD and MDD groups with the area under the curve (AUC) at 0.797 (P < 0.001) and 0.831 (P < 0.001) respectively. In addition, the Spearman analysis demonstrated that the GDNF protein level negatively correlated with the value of Hamilton depression rating scale (HAMD) in PSD patients (correlation coefficient = -0.328, P = 0.047). Together, these findings suggest the protein and mRNA expression levels of GDNF decreased in patients with depression. GDNF may serve as a potential biomarker for differential diagnosis of PSD from MDD patients.

New MT₂ Melatonin Receptor-Selective Ligands: Agonists and Partial Agonists

The search for melatonin receptor agonists and antagonists specific towards one of the receptor subtypes will extend our understanding of the role of this system in relaying circadian information to the body. A series of compounds derived from a hit compound discovered in a screening process led to powerful agonists specific for one of the isoform of the melatonin receptor namely, MT₂. The compounds are based on a poorly explored skeleton in the molecular pharmacology of melatonin. By changing the steric hindrance of one substituent (i.e., from a hydrogen atom to a tributylstannyl group), we identified a possible partial agonist that could lead to antagonist analogues. The functionalities of these compounds were measured with a series of assays, including the binding of GTPγS, the inhibition of the cyclic AMP production, the β-arrestin recruitment, and the cell shape changes as determined by cellular dielectric spectroscopy (CellKey®). The variations between the compounds are discussed.

Striatal hypodopamine phenotypes found in transgenic mice that overexpress glial cell line-derived neurotrophic factor

Glial cell line-derived neurotrophic factor (GDNF) positively regulates the development and maintenance of in vitro dopaminergic neurons. However, the in vivo influences of GDNF signals on the brain dopamine system are controversial and not fully defined. To address this question, we analyzed dopaminergic phenotypes of the transgenic mice that overexpress GDNF under the control of the glial Gfap promoter. Compared with wild-type, the GDNF transgenic mice contained higher levels of GDNF protein and phosphorylated RET receptors in the brain. However, there were reductions in the levels of tyrosine hydroxylase (TH), dopamine, and its metabolite homovanillic acid in the striatum of transgenic mice. The TH reduction appeared to occur during postnatal development. Immunohistochemistry revealed that striatal TH density was reduced in transgenic mice with no apparent signs of neurodegeneration. In agreement with these neurochemical traits, basal levels of extracellular dopamine and high K+-induced dopamine efflux were decreased in the striatum of transgenic mice. We also explored the influences of GDNF overexpression on lomomotor behavior. GDNF transgenic mice exhibited lower stereotypy and rearing in a novel environment compared with wild-type mice. These results suggest that chronic overexpression of GDNF in brain astrocytes exerts an opposing influence on nigrostriatal dopamine metabolism and neurotransmission.