Long-term biphasic effects of lithium treatment on phospholipase C-coupled m 3-muscarinic acetylcholine receptors in cultured cerebellar granule cells

Abstract

We have studied the long-term effects of lithium on neuronal morphology and the functional expression of phospholipase C-coupled m 3-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 (⩽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 [ 3 H]N- methyl-scopolamine binding to mAChRs and the levels of mRNAs for m 3-mAChR and c-Fos. The up-regulation of m 3-mAChR mRNA induced by low concentrations of lithium was associated with a down-regulation of m 3-mAChR mRNA and no change in either total RNA or β-actin mRNA. Lithium's effects on m 2- and m 3-mAChR mRNAs were time-dependent, requiring a pretreatment time of ⩾3 days. The biphasic effect was also demonstrated by the binding of [ 3H]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.