LITHIUM STIMULATES THE 1ST MEIOTIC DIVISION IN MOUSE OOCYTES

Cumulus enclosed oocytes, cumulus enclosed oocytes denuded of their cu mulus and cumulus free oocytes from 21 day old unstimulated mice were cultured for 18 hours in control medium supplemented with lithium chlo ride, dbcAMP and forskolin at various concentrations. In control mediu m 66% of the cumulus enclosed oocytes, 93% of the denuded oocytes, and 94% of the cumulus free oocytes resumed meiosis (germinal vesicle bre akdown), whereas the levels of polar body formation were 27%, 12% and 39%, respectively. In the presence of lithium significantly more cumul us enclosed oocytes and cumulus free oocytes resumed meiosis and forme d a polar body, whereas lithium had no effect on the denuded oocytes. Forskolin and dbcAMP stimulated resumption of meiosis and cumulus expa nsion in the cumulus enclosed oocytes and inhibited resumption of meio sis in the cumulus free oocytes. Lithium more or less eliminated this inhibitory effect of both forskolin and dbcAMP in the cumulus free ooc ytes. The results indicate (i) that activation of the cAMP second mess enger path in the cumulus cells induces them to synthesize a meiosis i nducing substance(s) which stimulates the oocyte to resume meiosis, an d (ii). that other second messenger systems than the cAMP pathway, e.g . the phosphatidylinositol cycle, are involved in resumption of meiosi s and polar body formation. We conclude that lithium enhances the capa bility of mouse oocytes for resumption of meiosis and polar body forma tion.