Cholinergic stimulation of polyphosphoinositide metabolism in brain in vivo

In vivo stimulation of phosphoinositide metabolism in the brainstem of rats following osmotic stress

We have investigated the effects of osmotic stress on the activity of inositol phospholipid turnover in the central nervous system of rats. Intracerebroventricular injection of [3H]myo-inositol was used for metabolic labelling of brain phosphoinositides in vivo. The levels of radiolabelled inositol lipids increased in a time-dependent manner in several areas of the brain, reaching a maximum 24 h after the injection. Treatment with LiCl 20 h after the administration of tritiated myo-inositol did not modify the levels of inositol lipids, but resulted in a dose-dependent accumulation of inositol phosphates. In rats treated with 10 mEq/kg LiCl, intraperitoneal injections of hypertonic saline (1.50 M NaCl) resulted in the stimulation of phosphoinositide turnover in the brainstem, but not in any of the other regions of the brain studied. This response was not prevented by unilateral cervical vagotomy, but was significantly lower in vasopressin-deficient Brattleboro rats. Brain phosphoinositide metabolism was not stimulated by acute blood volume depletion. We conclude that osmotic stress, but not acute hypovolemia, results in vasopressin-dependent activation of brainstem neurons by stimulating inositol phospholipid metabolism. In addition, metabolic labelling in vivo followed by treatment with LiCl provides a useful approach for assessing the physiological significance of the activation of polyphosphoinositide metabolism in the central nervous system in vivo. Our study provides evidence for a functional role of this second messenger system in the brainstem.

Role of inositol trisphosphate as a second messenger in signal transduction processes: an essay

This essay attempts to summarize some of the best evidence for the role of inositol trisphosphate as a second messenger in signal transduction processes. The following aspects are addressed in the essay: (a) The synthesis of inositol trisphosphate and other inositol lipids, (b) Receptor-phosphatidylinositol bisphosphate phospholipase C coupling and the N-ras protooncogene, (c) Inositol trisphosphate and intracellular calcium, (d) Cell growth and oncogenes, (e) Receptors linked to the phosphatidylinositol cycle, (f) Phototransduction and (g) Interactions between inositol trisphosphate and other second messengers.

Effects of calcium ionophore A23187 and calcium antagonists on 32Pi incorporation into polyphosphoinositides of rat cortical synaptosomes

The role of Ca2+ on 32Pi incorporation into polyphosphoinositides (PPI) of rat cortical synaptosomes was studied. Stimulation of muscarinic receptor by carbachol (1 mM) resulted in a decrease in 32Pi incorporation into phosphatidylinositol-4,5-bisphophaphate (TPI) and phosphatidylinositol-4-phosphate (DPI), and an increase in 32Pi incorporation into phosphatidylinositol (PI) and phosphatidic acid (PA), whereas no significant effect on other membrane phospholipids was found. This response could be blocked by atropine (1 microM). The stimulatory effect of carbachol required Ca2+ in the medium; the presence of 0.5 mM EGTA blocked the effect of carbachol on PPI turnover completely. Calcium ionophore A23187, at 1 microM, had a similar effect on PPI turnover by carbachol (1 mM). At higher concentrations (10-100 microM) of A23187, the PPI turnover rate was much enhanced. Depolarization of the membrane by high potassium (60 mM) in the presence of calcium resulted in an enhanced PPI turnover, which was similar to the results of the carbachol (1 mM) effect but to a lesser extent. Calcium antagonists, diltiazem and trifluoperazine, at 10 microM could block the carbachol effect on 32Pi incorporation into PPI in this preparation. Our results suggest that the enhancement of PPI turnover in rat cortical synaptosomes by carbachol, calcium ionophore or high potassium requires Ca2+, and it can be blocked by compounds which interfere with the availability of this ion, such as EGTA or calcium antagonists.

Effects of pre-weaning undernutrition and post-weaning rehabilitation on polyphosphoinositide pools in rat brain regions

In order to assess the effects of undernutrition during the pre-weaning period on polyphosphoinositide (PolyPI) pools in rat cerebral cortex, brain stem, and cerebellum, dams were fed 5% (L-) or 22% (L+) protein diets from birth to weaning and the pups were used at this age for analyses. To examine rehabilitation post-weaning, L- and L+ pups were fed 22% protein diets (P+) for an additional six week period. Rats were decapitated and the dissection begun either immediately ("0 min" samples) or 10 min later (10 min samples). Body and tissue weights, and cerebroside levels were determined in addition ot PolyPI concentrations. In brain the extent of disappearance of PolyPI during the 10 min post-mortem period paralleled the content of gray matter: cerebral cortex greater than cerebellum greater than brain stem in all groups regardless of diet. Levels of PtdIns4P and PtdIns4,5P2 were decreased by 40% and 70% respectively in cerebral cortex of L- "0 min" samples. Deficits of both lipids in brain stem and cerebellum were 40-50%. In the L- 10 min samples, deficits were 20-30% in all three regions as compared with L+ 10 min levels, indicating the presence of a portion of both lipids affected only moderately by nutritional insufficiency. The effects on this relatively inert pool, much of it localized in myelin, were reversed on nutritional rehabilitation. The PolyPI pool lost post-mortem in L+ brain regions was practically absent in L- brain regions and was not restored in L-P+ animals. Thus, this study indicates that a metabolically labile pool, primarily located in gray matter structures, is more sensitive to nutritional deprivation during the pre-weaning period than the more stable pool. The precise role and function of these pools remain to be determined.

Lipids of nervous tissue: composition and metabolism

As indicated in the Introduction, the many significant developments in the recent past in our knowledge of the lipids of the nervous system have been collated in this article. That there is a sustained interest in this field is evident from the rather long bibliography which is itself selective. Obviously, it is not possible to summarize a review in which the chemistry, distribution and metabolism of a great variety of lipids have been discussed. However, from the progress of research, some general conclusions may be drawn. The period of discovery of new lipids in the nervous system appears to be over. All the major lipid components have been discovered and a great deal is now known about their structure and metabolism. Analytical data on the lipid composition of the CNS are available for a number of species and such data on the major areas of the brain are also at hand but information on the various subregions is meagre. Such investigations may yet provide clues to the role of lipids in brain function. Compared to CNS, information on PNS is less adequate. Further research on PNS would be worthwhile as it is amenable for experimental manipulation and complex mechanisms such as myelination can be investigated in this tissue. There are reports correlating lipid constituents with the increased complexity in the organization of the nervous system during evolution. This line of investigation may prove useful. The basic aim of research on the lipids of the nervous tissue is to unravel their functional significance. Most of the hydrophobic moieties of the nervous tissue lipids are comprised of very long chain, highly unsaturated and in some cases hydroxylated residues, and recent studies have shown that each lipid class contains characteristic molecular species. Their contribution to the properties of neural membranes such as excitability remains to be elucidated. Similarly, a large proportion of the phospholipid molecules in the myelin membrane are ethanolamine plasmalogens and their importance in this membrane is not known. It is firmly established that phosphatidylinositol and possibly polyphosphoinositides are involved with events at the synapse during impulse propagation, but their precise role in molecular terms is not clear. Gangliosides, with their structural complexity and amphipathic nature, have been implicated in a number of biological events which include cellular recognition and acting as adjuncts at receptor sites. More recently, growth promoting and neuritogenic functions have been ascribed to gangliosides. These interesting properties of gangliosides wIll undoubtedly attract greater attention in the future.(ABSTRACT TRUNCATED AT 400 WORDS)

Muscarinic-receptor stimulation enhances polyphosphoinositide breakdown in guinea-pig ileum smooth muscle

Muscarinic-receptor stimulation by 0.1 mM-carbachol in longitudinal muscle of the guinea-pig ileum increases the incorporation of [3H]inositol into inositol-containing phospholipid. This effect was blocked by 16 microM-atropine. After 60 min incubation, carbachol increased the accumulation of total inositol phosphates 20-fold in the presence of 10 mM-Li+. Less than 20% of the total inositol phosphate corresponded to inositol 1-phosphate by ion-exchange chromatography, whereas of the remainder about two-thirds corresponded to inositol bisphosphate and one third to inositol trisphosphate. It is concluded that stimulation of muscarinic receptors in guinea-pig ileum enhances breakdown of polyphosphoinositides, suggesting that this may be a primary event associated with Ca2+ mobilization in the guinea-pig ileum.

Inositol phospholipid hydrolysis in rat cerebral cortical slices: I. Receptor characterisation

Characterisation of receptor-mediated breakdown of inositol phospholipids in rat cortical slices has been performed using a direct assay which involves prelabelling with [3H]inositol. When slices were preincubated with [3H]inositol, lithium was found to greatly amplify the capacity of receptor agonists such as carbachol, noradrenaline, and 5-hydroxytryptamine to increase the amount of radioactivity appearing in the inositol phosphates. Using a large variety of agonists and antagonists it could be shown that cholinergic muscarinic, alpha 1-adrenoceptor, and histamine H1 receptors appear to be linked to inositol phospholipid breakdown in cortex. The large responses produced by receptor agonists allowed a clear discrimination between full and partial agonists as well as quantitative analysis of competitive antagonists for each receptor. Whereas carbachol and acetylcholine (in the presence of a cholinesterase inhibitor) were full agonists, oxotremorine and arecoline were only partial agonists. Very low concentrations of atropine shifted the carbachol dose-response curve to the right and allowed inhibition constants for the antagonist to be easily calculated. The nicotinic antagonist, mecamylamine, was ineffective. Noradrenaline adrenaline were full agonists at alpha 1-adrenoceptors, but phenylephrine and probably methoxamine were partial agonists. Prazosin, but not yohimbine, potently and competitively antagonised the noradrenaline inositol phospholipid response. Mepyramine but not cimetidine competitively antagonised the histamine response. These data provide strong confirmation for the potentiating effect of lithium on neurotransmitter inositol phospholipid breakdown and emphasise the ease with which functional responses at a number of cortical receptors can be characterised.

A rapid stimulation of phosphatidylinositol metabolism in rabbit leukocytes by pseudomonal leukocidin

The time course experiments of 32Pi-labelling and breakdown of phospholipids in rabbit leukocytes exposed to leukocidin from Pseudomonas aeruginosa suggested that the initial action of this toxin was to stimulate phosphatidic acid production, presumably by causing a rapid metabolic change of phosphatidylinositol (PI response) correlating with phosphatidylinositol-specific phospholipase C and 1,2-diacylglycerol kinase. It appears that a rapid formation of phosphatidic acid and degradation of polyphosphoinositides in leukocytes treated with the toxin might be related a Ca2+-movement from extra- and intracellular spaces, resulting in the activation of Ca2+-dependent enzymes involved in the leukocidic process.

The effect of isobutylmethylxanthine on the photoresponse and phosphorylation of phosphatidylinositol in octopus retina

The effect of isobutylmethylxanthine on the photoresponse and the phosphorylation of phosphatidylinositol was studied. The recovery of photoresponse after a bright flash was greatly altered by the addition of isobutylmethylxanthine. Phosphatidylinositol phosphorylation, assessed by the incorporation of label from [gamma-32P]ATP, was influenced by isobutylmethylxanthine. Incorporation of 32P was decreased into diphosphoinositide and triphosphoinositide, and increased into phosphatidic acid. These changes were interpreted by introducing a parameter, phi, calcium charge of the plasma membrane.