LONG-TERM BIPHASIC EFFECTS OF LITHIUM TREATMENT ON PHOSPHOLIPASE-C-COUPLED M(3)-MUSCARINIC ACETYLCHOLINE-RECEPTORS IN CULTURED CEREBELLAR GRANULE CELLS

We have studied the long-term effects of lithium on neuronal morphology and the functional expression of phospholipase C-coupled m3-muscarinic acetylcholine receptors (mAChRs) in cerebellar granule cells. There was a biphasic dose-dependent effect on cell morphology following treatment with lithium for 7 days. At low concentrations (less-than-or-equal-to 2 mM), this drug elicited an increase in the number and thickness of connecting nerve fibers, and the size of neuronal aggregates. At high concentrations (5 10 mM), lithium induced a severe deterioration of cell morphology, which ultimately resulted in neuronal death. Carbachol-induced phosphoinositide (PI) turnover was similarly affected by lithium treatment with a significant potentiation at concentrations up to 2 mM and a marked inhibition at doses higher than 5 mM due to lithium-induced neurotoxicity, The biphasic effect on mAChR-mediated PI hydrolysis was associated with corresponding changes in the maximal extent of carbachol-induced inositol phosphate accumulation, and was accompanied by similar changes in [H-3]N-methyl-scopolamine binding to mAChRs and the levels of mRNAs for m3-mAChR and c-Fos. The up-regulation of m3-mAChR mRNA induced by low concentrations of lithium was associated with a down-regulation of m2-mAChR mRNA and no change in either total RNA or beta-actin mRNA. Lithium's effects on m2- and m3-mAChR mRNAs were time-dependent, requiring a pretreatment time of greater-than-or-equal-to 3 days. The biphasic effect was also demonstrated by the binding of [H-3]ouabain to Na+, K+-ATPase, which was shown to be a convenient method for quantifying viable neurons. The neurotoxic effect induced by treatment with high concentrations of lithium was not prevented by known neuroprotective/neurotrophic substances such as 9-amino-tetrahydroacridine or N-methyl-D-aspartate, or the co-presence of excess myo-inositol. Since the neurotrophic influence was induced by concentrations of lithium which overlap the clinical dose range and require long-term treatment, this effect might be relevant to the efficacy of this drug in the treatment of manic-depressive illness.