[3H]PtdIns hydrolysis in postmortem human brain membranes is mediated by the G-proteins Gq/11 and phospholipase C-beta

Location-dependent role of phospholipase C signaling in the brain: Physiology and pathology

Phosphoinositide-specific phospholipases C (PI-PLCs) are a class of enzymes involved in the phosphatidylinositol metabolism, which is implicated in the activation of several signaling pathways and which controls several cellular processes. The scientific community has long accepted the existence of a nuclear phosphoinositide (PI) metabolism, independent from the cytoplasmic one, critical in nuclear function control. Indeed, nuclear PIs are involved in many activities, such as cell cycle regulation, cell proliferation, cell differentiation, membrane transport, gene expression and cytoskeletal dynamics. There are several types of PIs and enzymes implicated in brain activities and among these enzymes, PI-PLCs contribute to a specific and complex network in the developing nervous system. Moreover, considering the abundant presence of PI-PLCβ1, PI-PLCγ1 and PI-PLCβ4 in the brain, a specific role for each PLC subtype has been suggested in the control of neuronal activity, which is important for synapse function, development and other mechanisms. The focus of this review is to describe the latest research about the involvement of PI-PLC signaling in the nervous system, both physiologically and in pathological conditions. Indeed, PI-PLC signaling imbalance seems to be also linked to several brain disorders including epilepsy, movement and behavior disorders, neurodegenerative diseases and, in addition, some PI-PLC subtypes could become potential novel signature genes for high-grade gliomas.

Serotonergic Psychedelics: Experimental Approaches for Assessing Mechanisms of Action

Recent, well-controlled - albeit small-scale - clinical trials show that serotonergic psychedelics, including psilocybin and lysergic acid diethylamide, possess great promise for treating psychiatric disorders, including treatment-resistant depression. Additionally, fresh results from a deluge of clinical neuroimaging studies are unveiling the dynamic effects of serotonergic psychedelics on functional activity within, and connectivity across, discrete neural systems. These observations have led to testable hypotheses regarding neural processing mechanisms that contribute to psychedelic effects and therapeutic benefits. Despite these advances and a plethora of preclinical and clinical observations supporting a central role for brain serotonin 5-HT2A receptors in producing serotonergic psychedelic effects, lingering and new questions about mechanisms abound. These chiefly pertain to molecular neuropharmacology. This chapter is devoted to illuminating and discussing such questions in the context of preclinical experimental approaches for studying mechanisms of action of serotonergic psychedelics, classic and new.

Phosphoinositide pathway and the signal transduction network in neural development

The development of the nervous system is under the strict control of a number of signal transduction pathways, often interconnected. Among them, the phosphoinositide (PI) pathway and the related phospholipase C (PI-PLC) family of enzymes have been attracting much attention. Besides their well-known role in the regulation of intracellular calcium levels, PI-PLC enzymes interact with a number of molecules belonging to further signal transduction pathways, contributing to a specific and complex network in the developing nervous system. In this review, the connections of PI signalling with further transduction pathways acting during neural development are discussed, with special regard to the role of the PI-PLC family of enzymes.

Deletion of PLCB1 gene in schizophrenia-affected patients

A prevalence of 1% in the general population and approximately 50% concordance rate in monozygotic twins was reported for schizophrenia, suggesting that genetic predisposition affecting neurodevelopmental processes might combine with environmental risk factors. A multitude of pathways seems to be involved in the aetiology and/or pathogenesis of schizophrenia, including dopaminergic, serotoninergic, muscarinic and glutamatergic signalling. The phosphoinositide signal transduction system and related phosphoinositide-specific phospholipase C (PI-PLC) enzymes seem to represent a point of convergence in these networking pathways during the development of selected brain regions. The existence of a susceptibility locus on the short arm of chromosome 20 moved us to analyse PLCB1, the gene codifying for PI-PLC β1 enzyme, which maps on 20p12. By using interphase fluorescent in situ hybridization methodology, we found deletions of PLCB1 in orbito-frontal cortex samples of schizophrenia-affected patients.

Levels of G-protein alpha q/11 subunits and of phospholipase C-beta(1-4), -gamma, and -delta1 isoforms in postmortem human brain caudate and cortical membranes: potential functional implications

The levels of expression of G-protein alpha(q/11) (Galpha(q/11)) subunits and PLC-beta(1-4), -gamma, and -delta(1) isoforms were quantified by Western blot analysis in order to establish their contribution to the patterns of PLC functioning reported here. Quantitative measurements of the levels of Galpha(q/11) subunits in each region were obtained by comparison with known amounts of Escherichia coli expressed recombinant Galpha(q) subunits. Quantitative analysis indicated that Galpha(q/11) subunits are abundant polypeptides in human brain, with values ranging from about 1200 ng/mg in cerebral cortex to close to 900 ng/mg of membrane protein in caudate. In cerebral cortical membranes, the PLC-beta(1) isoform was more abundant than in caudate membranes. The highest levels of PLC-beta(2) expression were detected in caudate membranes. PLC-beta(3) was little expressed, and there were no significant differences in the relative values between both brain regions. Finally, the levels of the PLC-beta(4) isoform were significantly lower in caudate than in cortical membranes. It is concluded that although most of these data represent relative, not absolute, measures of protein levels within these regions, they contribute nonetheless to the significant differences observed in signaling capacities through the PLC system in both human brain regions.

Neurotransmission- and cellular stress-related gene expression associated with prepulse inhibition in mice

Prepulse inhibition (PPI) is a cross-species measure of sensorimotor gating. PPI deficits have been associated with a number of neuropsychiatric disorders, including schizophrenia. Differential PPI has been demonstrated also across various inbred mouse strains; however, the molecular mechanisms underlying these differences in sensorimotor gating remain unclear. Here, we sought to identify gene expression in the medial prefrontal cortex (mPFC) of mice associated with PPI using a laser microdissection and microarray analysis-based approach. C57BL/6 mouse substrains were used for the study as they have dramatically different PPI. Transcriptional analysis of closely related substrains was predicted to reduce the detection of genetic variation incidental to the phenotype. Microarray analysis comparing the mPFC of C57BL/6J to C57BL/6NHsd mice revealed neurotransmission- and cellular stress-related transcriptional responses associated with lower PPI. Down-regulation of metabotropic glutamate receptor 5, phospholipase C, and inositol monophosphatase 1 gene expression suggest altered phosphoinositide signaling, while decreased expression of a gamma-amino-butyric acid (GABA)A receptor subunit implies changes in GABAergic signaling. Genes involved in neuronal excitation and protection were also differentially expressed, including up-regulation of five immediate early genes and anti-apoptotic/survival factors as Bcl2-associated athanogene 3 and brain-derived neurotrophic factor. These data support previous findings of genetic influences on PPI, and provide novel insights into the molecular mechanisms regulating sensorimotor gating.

Stimulation of 5-HT(2) receptors in prefrontal pyramidal neurons inhibits Ca(v)1.2 L type Ca(2+) currents via a PLCbeta/IP3/calcineurin signaling cascade

There is growing evidence linking alterations in serotonergic signaling in the prefrontal cortex to the etiology of schizophrenia. Prefrontal pyramidal neurons are richly innervated by serotonergic fibers and express high levels of serotonergic 5-HT(2)-class receptors. It is unclear, however, how activation of these receptors modulates cellular activity. To help fill this gap, whole cell voltage-clamp and single-cell RT-PCR studies of acutely isolated layer V-VI prefrontal pyramidal neurons were undertaken. The vast majority (>80%) of these neurons had detectable levels of 5-HT(2A) or 5-HT(2C) receptor mRNA. Bath application of 5-HT(2) agonists inhibited voltage-dependent Ca(2+) channel currents. L-type Ca(2+) channels were a particularly prominent target of this signaling pathway. The L-type channel modulation was blocked by disruption of G(alphaq) signaling or by inhibition of phospholipase Cbeta. Antagonism of intracellular inositol trisphosphate signaling, chelation of intracellular Ca(2+), or depletion of intracellular Ca(2+) stores also blocked this modulation. Inhibition of the Ca(2+)-dependent phosphatase calcineurin prevented receptor-mediated modulation of L-type currents. Last, the 5-HT(2) receptor modulation was robustly expressed in neurons from Ca(v)1.3 knockout mice. These findings argue that 5-HT(2) receptors couple through G(alphaq) proteins to trigger a phospholipase Cbeta/inositol trisphosphate signaling cascade resulting in the mobilization of intracellular Ca(2+), activation of calcineurin, and inhibition of Ca(v)1.2 L-type Ca(2+) currents. This modulation and its blockade by atypical neuroleptics could have wide-ranging effects on synaptic integration and long-term gene expression in deep-layer prefrontal pyramidal neurons.

Reduced phospholipase C-beta activity and isoform expression in the cerebellum of TS65Dn mouse: a model of Down syndrome

Agonist- and guanine-nucleotide-stimulated phospholipase C-beta (PLC) activity was characterized in crude plasma membrane preparations from cerebral cortex, hippocampus and cerebellum of Ts65Dn mice, a model for Down syndrome, and their control littermates. The levels of expression of PLC-beta((1-4)) isoforms and G-protein alpha(q/11) subunits were also quantified by Western blot analysis to establish their contribution to the patterns of PLC functioning. PLC activity regulated by G-proteins and muscarinic and 5-HT(2) receptors presented a regional distribution in both control and Ts65Dn mice. In both groups of mice, the intensity of PLC responses to maximal activation by calcium followed the sequence cerebellum > cortex > hippocampus. Both basal and maximal PLC activities, however, were significantly lower in cerebellar membranes of Ts65Dn than in control mice. This difference was mostly revealed in crude plasma membranes prepared from cerebellum at the level of G-protein-dependent-PLC activity because the concentration-response curve to GTPgammaS showed a reduction of the maximal effect in Ts65Dn mice, with no change in sensitivity (EC(50)). Western blot analysis showed a heterogeneous distribution of PLC-beta((1-4)) isoforms in both groups of mice. The levels of PLC-beta4 isoform, however, were significantly lower in the cerebellum of Ts65Dn than in control mice. We conclude that the cerebellum of Ts65Dn mice has severe deficiencies in PLC activity stimulated by guanine nucleotides, which are specifically related to a lower level of expression of the PLC-beta4 isoform, a fact that may account for the neurological phenotype observed in this murine model of Down syndrome.

Regulation of phospholipase Cbeta activity by muscarinic acetylcholine and 5-HT(2) receptors in crude and synaptosomal membranes from human cerebral cortex

Stimulation of phospholipase Cbeta by receptor agonists and G proteins has been characterized in crude cerebral membrane preparations, but little is known about their presynaptic localizations and little information is currently available for human brain tissue. The characteristics of phosphoplipase C transmembrane signaling were studied in crude and synaptosomal plasma membranes from postmortem human prefrontal cortex by measuring the hydrolysis of exogenous [(3)H]phosphatidylinositol4,5bisphosphate(PIP(2)) and the immunoreactive levels of phospholipase C (PLC) and G(alphaq/11) proteins. Regulation of PLC activity by Ca(2+) and the 5-HT(2) receptor agonist 5-methyltryptamine, but not by guanosine 5'-O-[3-thiotriphosphate] and the muscarinic acetylcholine receptor agonist carbachol were different between crude and synaptosomal membranes. KCl (20 mM) stimulation was absent in both preparations. Levels of G(alphaq/11)-protein subunits differed between preparations. The functional inhibition carried out with pirenzepine in crude membranes in order to reverse the carbachol-induced PLC stimulation indicates the existence of a component (53%) of the response that is activated by the M(1) muscarinic acetylcholine receptor subtype, and another component (47%) probably mediated by the M(3) muscarinic acetylcholine receptor subtype. In synaptosomal plasma membranes an increased inhibition of carbachol-induced PLC activation through M(1) was found. The PLC activation by 5-methyltryptamine (ketanserin-sensitive in crude membranes) was absent in synaptosomal plasma membranes suggesting the lack of activity mediated by 5-HT(2)-serotonin receptors.

Increased expression of Galpha(q/11) and of phospholipase-Cbeta1/4 in differentiated human NT2-N neurons: enhancement of phosphoinositide hydrolysis

The CNS is enriched in phosphoinositide-specific phospholipase C (PLC) and in the G proteins linked to its activation. Although the regional distributions of these signaling components within the brain have been determined, neither their cell type-specific localizations (i.e., neuronal versus glial) nor the functional significance of their high expression has been definitively established. In this study, we have examined the expression of phosphoinositide signaling proteins in human NT2-N cells, a well characterized model system for CNS neurons. Retinoic acid-mediated differentiation of NT2 precursor cells to the neuronal phenotype resulted in five- to 15-fold increases in the expression of PLC-beta1, PLC-beta4, and Galpha(q/11) (the prime G protein activator of these isozymes). In contrast, the expression of PLC-beta3 and PLC-gamma1 was markedly reduced following neuronal differentiation. Similar alterations in cell morphology and in the expression of PLC-beta1, PLC-beta3, and Galpha(q/11) expression were observed when NT2 cells were differentiated with berberine, a compound structurally unrelated to retinoic acid. NT2-N neurons exhibited a significantly higher rate of phosphoinositide hydrolysis than NT2 precursor cells in response to direct activation of either G proteins or PLC. These results indicate that neuronal differentiation of NT2 cells is associated with dramatic changes in the expression of proteins of the phosphoinositide signaling system and that, accordingly, differentiated NT2-N neurons possess an increased ability to hydrolyze inositol lipids.

Opposite changes in phosphoinositide-specific phospholipase C immunoreactivity in the left prefrontal and superior temporal cortex of patients with chronic schizophrenia

BACKGROUND

Abnormalities in types of neurotransmitter signaling that are coupled with phosphoinositide-specific phospholipase C (PLC) have previously been reported in brains from patients with schizophrenia. PLC, a main component of this pathway, may be affected in schizophrenia.

METHODS

We immunoquantified PLC beta 1, gamma 1 and delta 1 in the left prefrontal cortex and superior temporal cortex, nucleus accumbens and amygdala, and in the right superior temporal cortex of postmortem brains obtained from a total of 19 patients with schizophrenia and a total of 27 controls.

RESULTS

PLC beta 1 immunoreactivities were increased in the particulate fraction from the prefrontal cortex (by 64%), although they were decreased in the particulate fraction from the left superior temporal cortex (by 28%), as compared with the values in controls. There was no difference in PLC beta 1 immunoreactivities in the nucleus accumbens, the amygdala or the right superior temporal cortex between schizophrenic patients and controls. PLC gamma 1 and delta 1 immunoreactivities did not differ between the two groups in any of the regions studied.

CONCLUSIONS

Changes in PLC beta 1 immunoreactivities in the prefrontal and superior temporal cortex of patients with schizophrenia may reflect abnormalities in neurotransmissions via receptors that are coupled with the Gq alpha-PLC beta 1 cascade.

Effects of lithium on receptor-mediated activation of G proteins in rat brain cortical membranes

The underlying molecular mechanism of action of lithium in the treatment of manic-depressive illness is not clear. The effect of chronic lithium on GTP-binding and toxin-mediated ADP-ribosylation of specific G proteins in brain cortical membranes was examined. Incubation of cortical membranes with 5-HT increased [35S]GTPgammaS binding to Galphas, Galphai, Galphao and Galphaq proteins. Six weeks but not 1 week of lithium treatment reduced the increases in [35S]GTPgammaS binding to Galphas, Galphai and Galphao which are produced by 5-HT by 75-85%, whereas 5-HT stimulated [35S]GTPgammaS binding to Galphaq was reduced by 38%. No changes in membrane levels of Galpha and Gbeta proteins were noted in lithium-treated rats. Pertussis toxin (PTX)-mediated ADP-ribosylation of Galphai/o was increased by 60% in cortical membranes of chronically treated rats. Lithium treatment did not affect cholera toxin-mediated ribosylation of Galphas. Increases in [35S]GTPgammaS binding to Galpha proteins evoked by 5-HT were also inhibited by 0.5-2.0 mM lithium chloride added in vitro to the assay mixture. Rubidium and cesium did not change 5-HT-stimulated G protein activation. ADP-ribosylation of Galphai/o catalyzed by PTX was not changed by in vitro LiCl. The inhibitions of 5-HT-stimulated increases in [35S]GTPgammaS-binding to Galphas and Galphaq were completely suppressed by 2.4 mM MgCl2 this concentration of MgCl2 inhibited the effect of lithium on Galphai and Galphao by 50%. Similar findings were also noted when [alpha-32P]GTP was used in the binding assay. The results suggest that lithium interferes with receptor-G protein coupling via a Mg2+-sensitive mechanism. This action of the drug is more effective for Gs, Gi and Go than for Gq and may result from its interference with the recycling of trimeric G proteins.

Alzheimer's disease: relationship between muscarinic cholinergic receptors, beta-amyloid and tau proteins

Senile dementia is one of the most important health problems in developed countries. The main disease causing dementia is Alzheimer's disease that is characterized by the progressive deterioration of the cholinergic system, beta-amyloid production and deposition, and neurofibrillary tangle formation. Most of the reviewed data, along with data from experiments performed in our laboratory, suggest that there are no changes in the number of muscarinic receptors between Alzheimer and control brains, although the receptors expressed in Alzheimer's disease brains can be anomalous in their function. The muscarinic receptor-G-protein interaction also seems to be impaired in Alzheimer's disease compared with control brains, as well as the G-protein system, with an important decrease in the function of the Gq/11, the most important G-protein stimulating phosphoinositide hydrolysis in human brain; in addition, the second messenger system is also impaired, with a decrease in the synthesis of phosphoinositides and in the number of IP3 receptors. Muscarinic cholinergic receptors are also linked to beta-amyloid production, stimulation of the M1 subtype with agonists results in the processing of the beta-amyloid precursor protein to non-amyloidogenic products and administration of a fraction of the beta-amyloid (beta-amyloid 25-35) to rats, results in a decrease in the number of muscarinic receptors in brain. M1 agonists also decrease the phosphorylation of tau proteins, playing again a modulatory role in the pathogenesis of Alzheimer's disease. The existence of a link between beta-amyloid and tau proteins also has been reported; treatment of hippocampal neurones with beta-amyloid, or the 25-35 residue fragment, resulted in an increase in tau protein phosphorylation. The particular contribution of muscarinic receptors, beta-amyloid and tau proteins in the pathogenesis of Alzheimer's disease remains still unclear. Probably Alzheimer's disease could be due to a progressive degeneration in the relationship between the three components covered in this review.

Increased G alpha q/11 immunoreactivity in postmortem occipital cortex from patients with bipolar affective disorder

As disturbances in guanine nucleotide binding (G) protein-coupled phosphoinositide second messenger systems have been implicated in bipolar disorder, we examined whether the abundance of G alpha q/11 and phospholipase C (PLC)-beta 1 two key transducing proteins in this signaling pathway, are altered in this disorder. Compared with the controls, immunoreactive levels of G alpha q/11 were significantly elevated by 62% (p = .047) in occipital cortex of bipolar subjects. A similar increase (52%) in the PLC-beta 1 immunolabeling was also found in the occipital cortex of the bipolar subjects, but only reached marginal statistical significance (p = .07). In contrast, frontal and temporal cortex G alpha q/11 or PLC-beta 1 immunolabeling did not differ between bipolar and control subjects. Cerebral cortical immunoreactive levels of G beta 1 or G beta 2, included as a negative control, were not different between comparison groups. These findings support and extend earlier observations suggesting that disturbances in G protein-coupled second messenger signaling pathways may play an important role in the pathophysiology of bipolar affective disorder.

Cholinergic activation of phosphoinositide signaling is impaired in Alzheimer's disease brain

The function of the phosphoinositide signal transduction system was compared in membranes from Alzheimer's disease (AD) and control postmortem brain. [3H]Phosphatidylinositol hydrolysis was concentration-dependently stimulated by GTP[S] and this was 40% lower than controls in AD prefrontal cortical membranes. Carbachol induced a response greater than that of GTP[S] alone, and this response was impaired in AD by 45%. Differential analysis of the receptor-coupled and G-protein contributions to the responses indicated that the G-protein deficit in AD had a predominant influence on the lowered responses to cholinergic agonists. Similar deficits were observed in AD in the responses to five additional cholinergic agonists, including acetylcholine with three different acetylcholinesterase inhibitors. Deficits in stimulated phosphoinositide hydrolysis were regionally selective and these deficits did not correlate directly with reductions in choline acetyltransferase activity in AD tissues. These data demonstrate that in AD there is a brain region-selective, large impairment of cholinergic agonist-induced signal transduction mediated by the phosphoinositide system, which we speculate may impact on amyloid precursor protein processing.

Platelet imidazoline receptors and regulatory G proteins in patients with major depression

The newly discovered imidazoline receptors have been found to be upregulated in patients with major depression (platelet 45 kDa and 35 kDa proteins) and in suicide victims (brain 45 kDa protein). The signalling pathways coupled to these receptors are not known however. The aim of this study was to quantify, in platelets of depressed patients, the density of various G proteins to assess possible associations with the abundance of imidazoline proteins. There were positive correlations between the immunoreactivities of 45 kDa imidazoline receptors and those of G alpha q/11 (r = 0.64, n = 19, p < 0.005), G alpha i2 (r = 0.46, n = 22, p < 0.05) and G beta (r = 0.62, n = 18, p < 0.01) proteins. The relationship with regulatory G alpha q/11 proteins suggests that this 45 kDa protein (putative I1 imidazoline receptor) may couple to phosphoinositide pathway in platelets. This finding might be of relevance in understanding the functional implications of the abnormal higher expression of imidazoline receptors (45 kDa protein) in the pathogenesis of major depression.

Stimulation of phospholipase C activity by norepinephrine, t-ACPD and bombesin in LA-N-2 cells

The release of [3H]inositol phosphates from myo-[3H]inositol-prelabeled LA-N-2 cells was measured in the presence of beta-adrenoceptor, metabotropic glutamate and bombesin agonists. Norepinephrine and isoproterenol increased the formation of [3H]inositol phosphates in a dose-dependent manner, with an EC50 of 100 microM for norepinephrine and an EC50 of 5 microM for isoproterenol. These stimulations were abolished by propranolol, a beta-adrenoceptor antagonist, with an IC50 in the range of 50-55 microM for both norepinephrine and isoproterenol. The stimulation of [3H]inositol phosphate appearance occurred with varying concentrations of trans-1-amino-1,3-cyclopentanedicarboxylic acid (t-ACPD), a metabotropic glutamate receptor agonist. This release of [3H] inositol phosphates was blunted by its antagonist, 2-amino-3-phosphonopropionic acid (AP-3). Bombesin and neuromedin-B, a bombesin-like peptide, also increased the appearance of [3H]inositol phosphates. This was blunted by the antagonist [Tyr4, D-Phe12] bombesin. The appearance of [3H]inositol phosphates stimulated by t-ACPD was coupled through a cholera toxin-sensitive G-protein and the bombesin-stimulated appearance of [3H]inositol phosphates was coupled through a pertussis toxin-sensitive G-protein. The norepinephrine-stimulated appearance of [3H]inositol phosphates was toxin insensitive. The stimulation of the [3H]inositol phosphate appearance by these three agonists was protein kinase and Ca2+ independent.

Alterations in phosphoinositide signaling and G-protein levels in depressed suicide brain

The function of the phosphoinositide signal transduction system and the levels of heterotrimeric G-protein alpha-subunits were examined in postmortem prefrontal cortex regions (8/9) and region (10) from suicide victims with major depression and matched control subjects without psychiatric illness. The hydrolysis of [3H]phosphatidylinositol (PI) stimulated by phospholipase C, GTP-gamma-S, NaF, and neurotransmitter receptor agonists was measured in membrane preparations from both groups. Phospholipase C-beta activity was similar in depressed suicide and control subjects in the two regions of prefrontal cortex. In prefrontal cortex (10), but not in (8/9), the GTP-gamma-S concentration-dependent stimulation of [3H]PI hydrolysis was significantly lower (30%) in the depressed suicide group compared to the control group. Receptor-coupled, G-protein-mediated [3H]PI hydrolysis induced with carbachol, histamine, trans-1-aminocyclopentyl-1, 3-dicarboxylic acid (ACPD, a glutamatergic metabotropic receptor agonist), serotonin, or 2-methylthio-adenosine triphosphate (2mATP, a purinergic receptor agonist) in the presence of GTP-gamma-S stimulated equivalent responses in the two groups of subjects in each brain region. In prefrontal cortex (10) there was a 68% increase in the level of the 45 kDa subtype of G alpha s and in prefrontal cortex (8/9) there was a significant decrease (21%) in the level of G alpha i2 in the depressed suicide group compared to the control group. Levels of other heterotrimeric G-protein alpha-subunits (G alpha q/11, G alpha i1, and G alpha o) were not different in depressed suicide and control subjects in either brain region. Moreover, there were no differences in the levels of phospholipase C-beta or protein kinase C-alpha in the two groups of subjects in either brain region examined. These results demonstrate that in the prefrontal cortex of suicide victims with major depression compared to normal control subjects there is a region-specific alteration of G-protein-induced activation of the phosphoinositide signal transduction system and in the levels of G-protein alpha-subunits involved in cyclic AMP synthesis. These findings provide direct evidence in human brain that these two important signal transduction systems are altered in suicide subjects with major depression.

ATP-activated cation conductance in a Xenopus renal epithelial cell line

1. Using a whole-cell voltage-clamp technique and fura-2 fluorescence measurements, the actions of extracellular adenosine 5'-triphosphate (ATP) in single cells of an epithelial cell line (A6), were investigated. 2. ATP (0.1-1000 microM) induced currents in cells held under voltage clamp. The sequence of purinergic agonist potency in activating the currents (2-methylthio ATP > adenosine 5'-diphosphate (ADP) > ATP > alpha, beta-methylene ATP) was consistent with that of P2y receptors. 3. Reversal potentials (Erev) of the currents under various ionic conditions suggest that potassium channels and non-selective cation channels were responsible for the ATP-activated conductance, which was permeable to calcium. 4. ATP activated the currents in a calcium-free extracellular solution. In the presence of extracellular calcium, the currents were completely inhibited with 10 mM EGTA in the pipette. 5. ATP (10 microM) increased the intracellular calcium concentration ([Ca2+]i) whether cells were bathed in a solution containing calcium or not. 6. These results indicate that ATP evoked a calcium-dependent cation conductance, permeable to calcium, through P2y receptors by releasing calcium from intracellular stores in A6 cells.

Age-related loss of cholinergic-muscarinic coupling to PLC: comparison with changes in brain regional PLC subtypes mRNA distribution

Activation of phospholipase C (PLC) coupled to phosphoinositide (PtdIns) hydrolysis occurs through one of the two pathways. One of the major pathways for the neurotransmitter signaling involves phosphoinositide (PtdIns) specific and G-protein dependent PLC-beta, which stimulates the formation of inositol triphosphate (IP3) and inositol tetraphosphate (IP4). Another pathway through the stimulation of calcium influx can directly activate all of the PLC isozymes. At least three isozymes of PLC have been characterized in the brain; PLC-A (alpha), PLC-I (beta) and PLC-II (gamma), which are shown to be localized differentially in brain regions. Muscarinic-cholinergic signals are mediated in large part through the hydrolysis of PtdIns by PLC. To investigate changes in muscarinic coupling to PLC during aging, we examined carbachol stimulated and calcium stimulated PtdIns hydrolysis in cerebral cortical membranes in young, middle aged and old rats. In order to determine whether PtdIns hydrolysis changes correspond to PLC isozyme expression in these animals, we examined three subtypes of PLC mRNA expression in brain sections of young and old rats using in situ hybridization technique. Our study indicated decreased carbachol-induced PLC activity in the cerebral cortex and, in contrast, increased PLC-beta mRNA in the frontal cortex and superficial cortical layer of aged rats. PLC-alpha mRNA was decreased in hippocampal regions of older rats. These studies suggest that during aging there is an uncoupling of muscarinic stimulated PtdIns hydrolysis, which is accompanied by an increased PLC-beta mRNA and decreased PLC-alpha mRNA that may represent compensatory changes in PLC expression.

Phosphoinositide hydrolysis, G alpha q, phospholipase C, and protein kinase C in post mortem human brain: effects of post mortem interval, subject age, and Alzheimer's disease

Influences of post mortem time interval, subject age and Alzheimer's disease were investigated on several components of the phosphoinositide second messenger system, including stimulation of [3H]phosphatidylinositol hydrolysis by GTP[S] and several receptor agonists and the levels of Galphaq, beta, delta and gamma subtypes of phospholipase C, and five protein kinase C isoforms, in membranes prepared from post mortem human prefrontal cortex. Most of these components were stable with post mortem delays in the range of 5-21 h, but decreases of Galphaq and the alpha and xi protein kinase C subtypes were detected. Within the subject age range of 19-100 years, G-protein- and agonist-induced [3H]phosphatidylinositol hydrolysis decreased, as did levels of Galphaq, but the levels of phospholipase C and protein kinase C subtypes were generally unchanged. In Alzheimer's disease, compared with age- and post mortem interval-matched controls, there was a decrease in [3H]phosphatidylinositol hydrolysis stimulated by G-proteins and by several receptor agonists, but the levels of Galphaq and most of the phospholipase C and protein kinase C isoforms were unaffected. The greatest deficits, which were >50%, occurred with GTP[S]- and carbachol-induced [3H]phosphatidylinositol hydrolysis, indicating that this G-protein function and the response to cholinergic stimulation are significantly impaired in Alzheimer's disease. In summary a comprehensive assessment of several components of the phosphoinositide second messenger system was made in post mortem human brain. Most elements were stable within the post mortem interval range of 5-21 h, lending validity to measurements using these tissues. Significant age-related reductions in several components were identified, indicating loss of responses with increasing age. Most importantly, severe reductions in responses to several stimuli were found in Alzheimer's disease brain, deficits in signal transduction which may contribute to impaired cognition and to the limited therapeutic responses to drugs, such as those used to activate cholinergic receptors coupled with the phosphoinositide system.