Effect of electrical stimulation on phosphoinositide metabolism in rat sciatic nerve in vivo

Effects of muscarinic agonists and depolarizing agents on inositol monophosphate accumulation in the rabbit vagus nerve

The effects of muscarinic agonists and depolarizing agents on inositol phospholipid hydrolysis in the rabbit vagus nerve were assessed by the measurement of [3H]inositol monophosphate production in nerves that had been preincubated with [3H]inositol. After 1 h of drug action, carbachol, oxotremorine, and arecoline increased the inositol monophosphate accumulation, though the maximal increase induced by these agonists differed. Addition of the muscarinic antagonists atropine or pirenzepine shifted the carbachol dose-response curves to the right, without decreasing the carbachol maximal stimulatory effects. The KB for pirenzepine was 35 nM, which is characteristic of muscarinic high-affinity binding sites coupled to phosphoinositide turnover and often associated with the M1 receptor subtype. On the other hand, agents known to depolarize or to increase the intracellular Ca2+ concentration, e.g., elevated extracellular K+, ouabain, Ca2+, and the Ca2+ ionophore A23187, also increased inositol monophosphate accumulation. These effects were not mediated by the release of acetylcholine, as suggested by the fact that they could not be potentiated by the addition of physostigmine nor inhibited by the addition of atropine. The Ca(2+)-channel antagonist Cd2+, also known to inhibit the Na+/Ca2+ exchanger, was able to block the effects of K+ and ouabain, but did not alter those of carbachol. These results suggest that depolarizing agents increase inositol monophosphate accumulation in part through elevation of the intracellular Ca2+ concentration and that muscarinic receptors coupled to phosphoinositide turnover are present along the trunk of the rabbit vagus nerve.

Retrograde axonal transport of locally synthesized phosphoinositides in the rat sciatic nerve

Although autoradiography has demonstrated local incorporation of [3H]inositol into axonal phospholipids after intraneural injection, retrograde axonal transport of phosphatidylinositol has only been demonstrated after injection of lipid precursor into the cell body regions (L4 and L5 dorsal root ganglia) of the sciatic nerve. We now report the retrograde axonal transport of inositol phospholipids synthesized locally in the axons. Following microinjection of myo-[3H]inositol into the rat sciatic nerve (50-55 mm distal to L4 and L5 dorsal root ganglia), a time-dependent accumulation of 3H label occurred in the dorsal root ganglia ipsilateral to the injection site. The ratio of dpm present in the ipsilateral dorsal root ganglia to that in the contralateral dorsal root ganglia was not significantly different from unity between 2 and 8 h following isotope injection but increased to 10-12-fold between 24 and 72 h following precursor injection. By 24 h following precursor injection, the ipsilateral/contralateral ratio of the water-soluble label in the dorsal root ganglia still remained approximately 1.0, whereas the corresponding ratio in the chloroform/methanol-soluble fraction was approximately 20. The time course of appearance of labeled lipids in the ipsilateral dorsal root ganglia after injection of precursor into the nerve at various distances from the dorsal root ganglia indicated a transport rate of at least 5 mm/h. Accumulation of label in the dorsal root ganglia could be prevented by intraneural injection of colchicine or ligation of the sciatic nerve between the dorsal root ganglia and the isotope injection site. These results demonstrate that inositol phospholipids synthesized locally in the sciatic nerve are retrogradely transported back to the nerve cell bodies located in the dorsal root ganglia.

Reduced phosphoinositide concentrations in anterior temporal cortex of Alzheimer-diseased brains

Samples of brain anterior temporal cortex from 17 patients with Alzheimer's disease and 18 age-matched controls have been analysed for myo-inositol and the three phosphoinositides. There was significantly less phosphatidylinositol in the Alzheimer samples (1.36 mumol/g wet weight) than in the controls (2.28 mumol/g). The concentrations of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate were also lower in Alzheimer cortex, but differences from the control group were not statistically significant. Free myo-inositol concentrations were 5.11 mumol/g (Alzheimer) and 4.44 mumol/g (control) and again the difference was not significant. The lack of phosphoinositides in Alzheimer temporal cortex may impair receptor function.

Phospholipid metabolism in mouse sciatic nerve in vivo

To probe the activities of various pathways of lipid metabolism in peripheral nerve, six phospholipid-directed precursors were individually injected into the exposed sciatic nerves of adult mice, and their incorporation into phospholipids and proteins was studied over a 2-week period. Tritiated choline, inositol, ethanolamine, serine, and glycerol were mainly used in phospholipid synthesis; in contrast, methyl-labeled methionine was primarily incorporated into protein. Phosphatidylcholine was the main lipid formed from tritiated choline, glycerol, and methionine precursors. Phosphatidylserine, phosphatidylethanolamine, and phosphatidylinositol were the main lipids formed from serine, ethanolamine, and inositol, respectively. With time there was a shift in label among phospholipids, with higher proportions of choline appearing in sphingomyelin, glycerol in phosphatidylserine, ethanolamine in phosphatidylethanolamine (plasmalogen), and inositol in polyphosphoinositides, especially phosphatidylinositol 4,5-bisphosphate. We suggest that the delay in formation of these phospholipids, which are concentrated in peripheral nerve myelin, may, at least in part, be due to their formation at a site(s) distant from the sites where the bulk of Schwann cell lipids are made. We propose that separating the synthesis of these myelin-destined lipids to near the Schwann cell's plasma membrane would facilitate their concentration in peripheral nerve myelin sheaths. At earlier labeling times, ethanolamine and glycerol were more actively incorporated into phosphatidylcholine and phosphatidylinositol, respectively, than later. The transient labeling of these phospholipids may reflect some unique role in peripheral nerve function.

Technique for injection into the sciatic nerve of the mouse for quantitative in vivo metabolic studies

In this paper we describe a technique for intraneural injections, applicable to mouse peripheral nerves, which, compared with previous techniques, reduces trauma to the nerves and increases the level and reduces the variability of label recovery. Our technique employs glass needles (tip diameter, 50 micron) linked to a peristaltic pump by polyethylene tubing to inject small volumes (in the microliter range) of radiolabeled substrate solutions into mouse sciatic nerves, and allows the recovery of 20.9 +/- 1.9% (mean +/- standard deviation) and 30.5 +/- 4.8% of the injected radioactivity for 2 microliter [3H]acetate and 0.5 microliter of [3H]stearate, respectively.

Regulation of phosphatidylinositol turnover in brain synaptoneurosomes: stimulatory effects of agents that enhance influx of sodium ions

Norepinephrine and carbamoylcholine stimulate accumulation of [3H]inositol phosphates from [3H]inositol-labeled guinea pig cerebral cortical synaptoneurosomes through interaction with alpha 1-adrenergic and muscarinic receptors, respectively. In addition to such agonist, a variety of natural products that affect voltage-dependent sodium channels can markedly stimulate accumulation of [3H]inositol phosphates. These include alkaloids that activate sodium channels, such as batrachotoxin, veratridine, and aconitine; peptide toxins that alter activation or slow inactivation of sodium channels, such as various scorpion toxins from Leiurus, Centruroides, and Tityus species; and agents that cause repetitive firing of sodium channel-dependent action potentials, such as pyrethroids and pumiliotoxin B. Ouabain, and agent that will increase accumulation of internal sodium by inhibition of Na+, K+-ATPase, also stimulates formation of [3H]inositol phosphates, as does monensin, a sodium ionophore. Tetrodotoxin and saxitoxin, specific blockers of voltage-dependent sodium channels, prevent or reduce the stimulatory effects of sodium channel agents and ouabain on phosphatidylinositol turnover, while having lesser or no effect, respectively, on receptor-mediated or monensin-mediated stimulation. Removal of extracellular sodium ions markedly reduces stimulatory effects of sodium channel agents, while removal of extracellular calcium ions with EGTA blocks both receptor-mediated and sodium channel agent-mediated phosphatidylinositol turnover. The results provide evidence for a hitherto unsuspected messenger role for sodium ions in excitable tissue, whereby neuronal activity and the resultant influx of sodium will cause activation of phospholipase systems involved in hydrolysis of phosphatidylinositols, thereby generating two second messengers, the inositol phosphates, which mobilize calcium from internal stores, and the diacylglycerols, which activate protein kinase C.