- Inositol is a precursor in the second messenger phosphatidyl-inositol (PI) cycle. Berridge et al.(1982) suggested that lithium (Li) affects PI metabolism by inhibiting inositol monophosphatase and thereby reducing inositol levels. Berridge suggested that such inhibition could be a ratelimiting step for phosphatidyl-inositol biphosphate (PIP2) re- synthesis. Since oral dietary inositol enters most tissues, but does not readily cross the blood±brain barrier, Berridge suggested, such ratelimiting function of lithium might be specifically relevant to brain tissue. However, Sherman et al.(1985) reported chronic Li reduction of inositol level in peripheral adrenal tissue, and other peripheral tissues such as kidney might also exhibit such Li-induced reduction of inositol level.