Lithium Modulation of Phosphoinositide Signaling System in Rat Cortex: Selective Effect on Phorbol Ester Binding

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Abstract
Recent work indicates that the therapeutic action of lithium may be mediated through perturbation of postreceptor second messenger systems. To elucidate further the postreceptor cellular sites of action(s) of lithium, the effect of chronic lithium treatment on various components of the receptor-activated phosphoinositide pathway was investigated. We found that chronic administration of lithium (0.2% LiCl, 21 days) to adult male rats did not significantly affect phosphoinositide hydrolysis in cerebral cortical slices induced by carbachol (1 mM) or NaF (10 mM). Nor did the same treatment alter the carbachol (1 mM) potentiation of guanosine 5'-(gamma-thio)triphosphate (30 microM) stimulation of phosphoinositide hydrolysis (an index of receptor/G protein coupling) in cortical membranes. Immunoblotting studies revealed no changes in the levels of G alpha q/11 immunoreactivity in the cortex after chronic lithium treatment. The levels of protein kinase C, as revealed by specific binding of [3H]phorbol dibutyrate ([3H]PDBu), were significantly reduced in the cytosolic fraction and increased in the particulate fraction of rat cortex after chronic lithium, whereas the KD of [3H]PDBu binding remained relatively constant. A small and insignificant decrease in the density of [3H]inositol 1,4,5-trisphosphate binding was also found in the cortex. The above data suggest that chronic lithium treatment affects neither the muscarinic cholinergic-linked phosphoinositide turnover nor the putative G protein alpha subunit (G alpha q/11) responsible for phospholipase C activation. However, a possible translocation and activation of protein kinase C activity may be significant in the therapeutic effect of this mood-stabilizing agent.