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 (< or = 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 [3H]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 > or = 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.(ABSTRACT TRUNCATED AT 250 WORDS)