Metabolism of phosphatidylinositol in plasma membranes and synaptosomes of rat cerebral cortex: a comparison between endogenous vs exogenous substrate pools

Tandem regulation of phosphoinositide signaling and acute behavioral effects induced by antidepressant agents in rats

RATIONALE

Antidepressants increase synaptic monoamine concentrations, but the subsequent signaling events that produce the beneficial clinical effects remain unclear. Diverse antidepressants increase CDP-diacylglycerol, a crucial step in phosphoinositide signaling. Serotonin 5HT(2) receptors, implicated in depression or the actions of some antidepressants, signal through phosphoinositide hydrolysis. Thus, cross talk between antidepressant-induced CDP-diacylglycerol and 5HT(2) signaling could contribute to the antidepressant mechanism.

OBJECTIVE

The objective of the study was to test the hypotheses that antidepressants enhance net signaling via 5HT(2) receptors by augmenting the supply of phosphoinositide substrates and that this action contributes to the behavioral effects of the drugs.

MATERIALS AND METHODS

Brain slices pre-labeled with [(3)H]inositol in the presence of various antidepressant concentrations were washed and incubated with the 5HT(2) agonist, alpha-methylserotonin, followed by measuring phosphoinositide synthesis and inositol phosphate accumulation. Further, rats administered antidepressants after pretreatment with neomycin to inhibit metabolic utilization of phosphoinositides were behaviorally evaluated in the forced swim test.

RESULTS

Diverse antidepressants significantly enhanced phosphoinositide synthesis. While alpha-methylserotonin increased inositol phosphate accumulation, this effect was significantly accentuated in hippocampal or cortical tissues pre-incubated in the presence of imipramine, desipramine, fluoxetine, paroxetine, or maprotiline. Drug-induced behavioral antidepressant effects were reversed by neomycin pretreatment, whereas neomycin alone did not alter basal immobility times.

CONCLUSIONS

Antidepressants probably exert tandem neurochemical effects by increasing synaptic monoamine concentrations and by producing phosphoinositides used in 5HT(2) receptor signaling. This combination of actions may constitute the mechanism of at least the acute behavioral effects of the drugs and could implicate aberrant neurolipid signaling in the pathophysiology of depression.

Stimulation of group II phospholipase A2 mRNA expression and release in an immortalized astrocyte cell line (DITNC) by LPS, TNF alpha, and IL-1 beta. Interactive effects

Astrocytes are immunoactive cells in brain and have been implicated in the defense mechanism in response to external injury. Previous studies using cultured glial cells indicated the ability of astrocytes to respond to bacteria endotoxin and cytokines, resulting in the release of phospholipase A2. In this study, we examined the interactive effects of lipopolysaccharides (LPS), interleukin 1 beta (IL-1 beta) and tumor necrosis factor (TNF alpha) to stimulate phospholipase A2 (PLA2) in an immortalized astrocyte cell line (DITNC) with many properties of type I astrocytes. Northern blot analysis using oligonucleotide probes derived from the cDNA encoding the rat spleen group II PLA2 indicated the ability of DITNC cells to respond to all three factors in the induction of gene expression and the release of PLA2. After an initial lag time of 2 h, PLA2 release was proportional to time, reaching a plateau by 12 h. This event occurred at a time period preceding any signs of cell death. Cycloheximide at 1.25 microM completely inhibited cytokine-induced PLA2 release. When suboptimal amounts of TNF alpha were added to the DITNC culture together with IL-1 beta or LPS, a synergistic increase in the induction of PLA2 release could be observed. On the other hand, combination of IL-1 beta and LPS resulted only in an additive increase in PLA2 release. Antibodies to IL-1 beta and TNF alpha completely neutralized the effects of these two agents on PLA2 release. However, neither antibody was able to inhibit the PLA2 release induced by LPS, suggesting that the effect of LPS was not complicated by the release of IL-1 beta or TNF alpha. Taken together, results show that the immortalized astrocyte cell line (DITNC) can be used for studies to elucidate the molecular mechanism underlying the cytokine signaling cascade and subsequent induction of PLA2 synthesis.

Involvement of a pertussis toxin-sensitive G protein-coupled phospholipase A2 in agonist-stimulated arachidonic acid release in membranes isolated from bovine iris sphincter smooth muscle

We have shown that in bovine iris sphincter membranes G proteins are involved in coupling muscarinic-, PGF2 alpha-, endothelin- and platelet-activating factor receptors to the activation of phospholipase A2 and the release of arachidonic acid. GTP gamma S and GTP gamma S plus carbachol stimulated arachidonic acid release in the membranes in a dose- and time-dependent manner. Nucleotide stimulation was specific to GTP gamma S, since GDP, GDP beta S and ATP had no effect. The stimulatory effect of GTP gamma S plus carbachol was blocked by atropine and it required the presence of physiological concentrations of Ca2-. AIF4-, which bypasses the receptor and directly activates the G protein, induced arachidonic acid liberation in the intact iris sphincter, but was ineffective in the membranes. Addition of GTP gamma S plus carbachol to sphincter muscle membranes prelabeled with [3H]inositol or 3H-arachidonic acid resulted in the formation of lysophosphatidylinositol and the liberation of arachidonic acid, thus suggesting the involvement of phospholipase A2. In vitro treatment of the iris membranes with pertussis toxic inhibited arachidonic acid release by the agonists. This is in contrast to the pertussis toxin-insensitive G protein that activates phospholipase C in this tissue (22). These data demonstrate that in the iris sphincter a G protein is involved in the step between receptor activation and the activation of phospholipase A2, and that arachidonic acid release in this tissue is mediated by a pertussis-toxin-sensitive G protein-coupled phospholipase A2. Thus, GTP can regulate arachidonic acid release and its subsequent conversion into eicosanoids by stimulating its formation.

Phosphatidylinositol availability and polyphosphoinositide synthesis in pancreatic islet cell membranes

Polyphosphoinositide synthesis in isolated islets of the rat was determined by the phosphorylation of endogenous phosphatidylinositol (PtdIns) by PtdIns kinase and [gamma-32P]ATP to form [32P]-phosphatidylinositol 4-phosphate (PtdInsP) in cell homogenates. Glucose stimulation of intact islets resulted in a time- and concentration-dependent reduction in PtdInsP synthesis. Similarly, the stimulation of intact islets with carbachol (CCh), cholecystokinin (CCK-8S), or tolbutamide for 15 min reduced PtdInsP production in a concentration-dependent manner. The effects of glucose, tolbutamide and CCh were reversible. PtdInsP hydrolysis did not account for the reduction in PtdInsP recovery. The addition of exogenous PtdIns to the PtdIns kinase assay significantly increased basal PtdInsP levels. In addition, exogenous PtdIns completely reversed the inhibitory effects of glucose and increased PtdIns kinase activity in homogenates of glucose-stimulated islets to levels found in control homogenate with PtdIns. Exogenous PtdIns also increased PtdIns kinase activity in CCK-8S-treated islets, although exogenous PtdIns did not overcome the tolbutamide-induced inhibition of PtdIns kinase. The Vmax of PtdIns kinase in homogenates of islets treated with tolbutamide was reduced significantly, although glucose did not affect the Vmax. In addition, the Km values for ATP and PtdIns were not altered by exposure of the islets to cell stimuli. The results suggest that the level of PtdIns in islet cell membranes is rate limiting for PtdInsP synthesis, and that tolbutamide is a noncompetitive inhibitor of PtdIns kinase.