Neuroanatomical mapping of juvenile rat brain regions with prominent basal signal in [(35)S]GTPgammaS autoradiography

Activity of muscarinic, galanin and cannabinoid receptors in the prodromal and advanced stages in the triple transgenic mice model of Alzheimer's disease

Neurochemical alterations in Alzheimer's disease (AD) include cholinergic neuronal loss in the nucleus basalis of Meynert (nbM) and a decrease in densities of the M2 muscarinic receptor subtype in areas related to learning and memory. Neuromodulators present in the cholinergic pathways, such as neuropeptides and neurolipids, control these cognitive processes and have become targets of research in order to understand and treat the pathophysiological and clinical stages of the disease. This is the case of the endocannabinoid and galaninergic systems, which have been found to be up-regulated in AD, and could therefore have a neuroprotective role. In the present study, the functional coupling of Gi/o protein-coupled receptors to GalR1, and the CB1 receptor subtype for endocannabinoids were analyzed in the 3xTg-AD mice model of AD. In addition, the activity mediated by Gi/o protein-coupled M2/4 muscarinic receptor subtypes was also analyzed in brain areas involved in anxiety and cognition. Thus, male mice were studied at 4 and 15months of age (prodromal and advanced stages, respectively) and compared to age-matched non-transgenic (NTg) mice (adult and old, respectively). In 4-month-old 3xTg-AD mice, the [(35)S]GTPγS binding stimulated by galanin was significantly increased in the hypothalamus, but a decrease of functional M2/4 receptors was observed in the posterior amygdala. The CB1 cannabinoid receptor activity was up-regulated in the anterior thalamus at that age. In 15-month-old 3xTg-AD mice, muscarinic receptor activity was found to be increased in motor cortex, while CB1 activity was decreased in nbM. No changes were found in GalR1-mediated activity at this age. Our results provide further evidence of the relevance of limbic areas in the prodromal stage of AD, the profile of which is characterized by anxiety. The up-regulation of galaninergic and endocannabinoid systems support the hypothesis of their neuroprotective roles, and these are established prior to the onset of clear clinical cognitive symptoms of the disease.

Increased tonic cannabinoid CB1R activity and brain region-specific desensitization of CB1R Gi/o signaling axis in mice with global genetic knockout of monoacylglycerol lipase

In mammalian brain, monoacylglycerol lipase (MAGL) is the primary enzyme responsible for terminating signaling function of the endocannabinoid 2-arachidonoylglycerol (2-AG). Previous in vivo studies with mice indicate that both genetic and chronic pharmacological inactivation of MAGL result in 8-30-fold increase of 2-AG concentration in the brain, causing desensitization and downregulation of cannabinoid CB1 receptor (CB1R) activity, leading to functional and behavioral tolerance. However, direct evidence for reduced CB1R activity in the brain is lacking. In this study, we used functional autoradiography to assess basal and agonist-stimulated CB1R-dependent Gi/o protein activity in multiple brain regions of MAGL-KO mice in comparison to their wild-type (WT) littermates. In addition, the role of endogenous cannabinoids in basal CB1R signaling was assessed after comprehensive pharmacological blockade of 2-AG hydrolysis by determining the contents of endocannabinoids (eCBs) in WT and MAGL-KO brain tissues by LC/MS/MS technology. To show whether lack of MAGL cause compensatory alterations in the serine hydrolase activity, we compared serine hydrolase pattern of WT and MAGL-KO using activity-based protein profiling. Consistent with studies using chronic pharmacological MAGL inactivation in vivo, we observed a statistically significant decrease of CB1R-Gi/o signaling in most of the studied brain regions. In MAGL-KO brain sections, elevated 2-AG levels were mirrored to heightened basal CB1R-dependent Gi/o-activity, as well as, dampened agonist-evoked responses in several brain regions. The non-selective serine hydrolase inhibitor methylarachidonoylfluorophosphonate (MAFP) was able to significantly elevate 2-AG levels in brain sections of MAGL-KO mice, indicating that additional serine hydrolases possess 2-AG hydrolytic activity in MAGL-KO brain sections.

Anatomical location of LPA1 activation and LPA phospholipid precursors in rodent and human brain

Lysophosphatidic acid (LPA) is a signaling molecule that binds to six known G protein-coupled receptors: LPA1 -LPA6 . LPA evokes several responses in the CNS, including cortical development and folding, growth of the axonal cone and its retraction process. Those cell processes involve survival, migration, adhesion proliferation, differentiation, and myelination. The anatomical localization of LPA1 is incompletely understood, particularly with regard to LPA binding. Therefore, we have used functional [(35) S]GTPγS autoradiography to verify the anatomical distribution of LPA1 binding sites in adult rodent and human brain. The greatest activity was observed in myelinated areas of the white matter such as corpus callosum, internal capsule and cerebellum. MaLPA1 -null mice (a variant of LPA1 -null) lack [(35) S]GTPγS basal binding in white matter areas, where the LPA1 receptor is expressed at high levels, suggesting a relevant role of the activity of this receptor in the most myelinated brain areas. In addition, phospholipid precursors of LPA were localized by MALDI-IMS in both rodent and human brain slices identifying numerous species of phosphatides and phosphatidylcholines. Both phosphatides and phosphatidylcholines species represent potential LPA precursors. The anatomical distribution of these precursors in rodent and human brain may indicate a metabolic relationship between LPA and LPA1 receptors. Lysophosphatidic acid (LPA) is a signaling molecule that binds to six known G protein-coupled receptors (GPCR), LPA1 to LPA6 . LPA evokes several responses in the central nervous system (CNS), including cortical development and folding, growth of the axonal cone and its retraction process. We used functional [(35) S]GTPγS autoradiography to verify the anatomical distribution of LPA1 -binding sites in adult rodent and human brain. The distribution of LPA1 receptors in rat, mouse and human brains show the highest activity in white matter myelinated areas. The basal and LPA-evoked activities are abolished in MaLPA1 -null mice. The phospholipid precursors of LPA are localized by MALDI-IMS. The anatomical distribution of LPA precursors in rodent and human brain suggests a relationship with functional LPA1 receptors.

Lipid phosphate phosphatase inhibitors locally amplify lysophosphatidic acid LPA1 receptor signalling in rat brain cryosections without affecting global LPA degradation

Background

Lysophosphatidic acid (LPA) is a signalling phospholipid with multiple biological functions, mainly mediated through specific G protein-coupled receptors. Aberrant LPA signalling is being increasingly implicated in the pathology of common human diseases, such as arteriosclerosis and cancer. The lifetime of the signalling pool of LPA is controlled by the equilibrium between synthesizing and degradative enzymatic activity. In the current study, we have characterized these enzymatic pathways in rat brain by pharmacologically manipulating the enzymatic machinery required for LPA degradation.

Results

In rat brain cryosections, the lifetime of bioactive LPA was found to be controlled by Mg²⁺-independent, N-ethylmaleimide-insensitive phosphatase activity, attributed to lipid phosphate phosphatases (LPPs). Pharmacological inhibition of this LPP activity amplified LPA₁ receptor signalling, as revealed using functional autoradiography. Although two LPP inhibitors, sodium orthovanadate and propranolol, locally amplified receptor responses, they did not affect global brain LPA phosphatase activity (also attributed to Mg²⁺-independent, N-ethylmaleimide-insensitive phosphatases), as confirmed by P_i determination and by LC/MS/MS. Interestingly, the phosphate analog, aluminium fluoride (AlF_x^-) not only irreversibly inhibited LPP activity thereby potentiating LPA₁ receptor responses, but also totally prevented LPA degradation, however this latter effect was not essential in order to observe AlF_x^--dependent potentiation of receptor signalling.

Conclusions

We conclude that vanadate- and propranolol-sensitive LPP activity locally guards the signalling pool of LPA whereas the majority of brain LPA phosphatase activity is attributed to LPP-like enzymatic activity which, like LPP activity, is sensitive to AlF_x^- but resistant to the LPP inhibitors, vanadate and propranolol.

G protein-coupled receptors in the hypothalamic paraventricular and supraoptic nuclei--serpentine gateways to neuroendocrine homeostasis

G protein-coupled receptors (GPCRs) are the largest family of transmembrane receptors in the mammalian genome. They are activated by a multitude of different ligands that elicit rapid intracellular responses to regulate cell function. Unsurprisingly, a large proportion of therapeutic agents target these receptors. The paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothalamus are important mediators in homeostatic control. Many modulators of PVN/SON activity, including neurotransmitters and hormones act via GPCRs--in fact over 100 non-chemosensory GPCRs have been detected in either the PVN or SON. This review provides a comprehensive summary of the expression of GPCRs within the PVN/SON, including data from recent transcriptomic studies that potentially expand the repertoire of GPCRs that may have functional roles in these hypothalamic nuclei. We also present some aspects of the regulation and known roles of GPCRs in PVN/SON, which are likely complemented by the activity of 'orphan' GPCRs.

Quantification of lysophosphatidic acids in rat brain tissue by liquid chromatography-electrospray tandem mass spectrometry

Lysophosphatidic acid (LPA) is a lipid mediator with multiple biological functions. A highly selective and sensitive liquid chromatography-tandem mass spectrometry (LC/MS/MS) method was developed for the determination of LPAs (16:0 LPA, 18:0 LPA, 18:1 LPA, 20:4 LPA) in rat brain cryosections. After partitioning the LPAs from other lipophilic material present in the tissue with a liquid-liquid extraction, a reversed-phase column and ion pair technique was used for separating analytes with a gradient elution. An internal standard (17:0 LPA) was included in the analysis. Detection and quantification of the LPAs were carried out with a triple quadrupole mass spectrometer using negative electrospray ionization (ESI) and multiple reaction monitoring (MRM). The artificial formation of LPAs from lysophosphatidylcholines during the sample preparation procedure and instrumentation was carefully studied during the method development. The method was validated; acceptable selectivity, accuracy, precision, recovery, and stability were obtained for concentrations within the calibration curve range of 0.02-1.0muM for LPAs. The quantification limit of the assay was 54fmol injected into column for each LPAs. The method was applied to comparative studies of LPA levels in rat brain cryosections after the various chemical pre-treatments of the sections.