Lithium actions on inositol lipid cell signalling.
thesisposted on 19.11.2015, 08:43 by Eleanor D. Kennedy
Lithium (Li+) is the major drug presently prescribed in the treatment of manic depression. Its site of action within the central nervous system is unknown although it has been known for several years to have profound effects upon inositol (poly)phosphate metabolism. The basis of the work presented here is to investigate the effects of Li+ upon the accumulation of labelled and unlabelled inositol (poly)phosphates in muscarinic cholinergically stimulated rat cerebral cortical slices and cultured cell lines. Its effects on the accumulation of CMP-phosphatidic acid (CMP-PA), a precursor to (poly)phosphoinositide lipid resynthesis and upon the inhibition of inositol monophosphatase activity are also described. It has been shown that the presence of Li+ leads to reduced accumulation of, not only Ins(l,3,4,5)P4 but also of Ins(l,4,5)P3, the molecule responsible for the release of intracellular Ca2+. The labelled Ins(l,4,5)P3 and Ins(l,3,4)P3 which accumulate under these conditions were separated in the absence of mg++ using an enzymic preparation from rat cerebral homogenate. This technique allows an effective and accurate separation which circumventss the use of h.p.l.c. It has also been demonstrated that both [3H] InsP1 and [14C]CMP-PA increase with similar EC50 values in the presence of increasing Li+ concentrations and also with increasing time in the presence of Li+. Furthermore, work has indicated that the preincubation of rat cerebral cortical slices with myo-inositol can, at least partially, reverse the accumulation of labelled CMP-PA. However, if the myo-inositol is added subsequent to the Li+ block being established, incomplete reversal is only observed in the presence of very high concentrations of inositol. In addition, similar experiments were conducted to observe the effects of myo-inositol upon the Li+-induced reduced accumulation of both Ins(l,4,5)P3 and Ins(1,3,4,5)P4 as measured by radio-receptor assay. Whilst the presence of 10mM myo-inositol prolonged the lag phase routinely observed between 5 and 10 mins, it could not prevent the fall in accumulation from occurring. Scyllo-inositol, a naturally occurring isomer of myo-inositol, was found to be completely ineffective in reducing the accumulation of [14C]CMP-PA. In other experiments, the effects of Li+ upon the carbachol-stimulated inositol (poly)phosphate fractions were examined in CHO cells transfected with an M1 muscarinic receptor subtype and also in SH-SY5Y cells and the results compared to those obtained in rat cerebral cortical slices. Finally, an assessment of inhibition of the inositol monophosphatase activity was made in a variety of cell free systems. This inhibition was caused using either Li+ or polyclonal antibodies raised to purified inositol monophosphatase. In conclusion, the results obtained verify the profound effects which Li+ can have upon inositol (poly)phosphate metabolism and suggest that whilst the primary intracellular target of Li+ may indeed be the inositol monophosphatase it may also be causing other subtle alterations to the cell functioning. These possibilities and their implications are discussed.