Intracellular cAMP and calcium signaling by serotonin in mouse cumulus-oocyte complexes

cAMP and intracellular Ca(2+) are important second messengers involved in mammalian follicular growth and oocyte meiotic maturation. We investigated the capacity of the neurohormone serotonin (5-hydroxytryptamine, 5-HT) to regulate intracellular cAMP and Ca(2+) in mouse oocytes and surrounding cumulus cells. On the basis of a reverse transcription-polymerase chain reaction study, 5-HT 7 receptor mRNA is expressed in cumulus cells, oocytes, and embryos up to the four-cell stage, and 5-HT(2A) and 5-HT(2B) receptor mRNAs are expressed in cumulus cells only, whereas 5-HT(2C), 5-HT(4), and 5-HT(6) receptors are expressed in neither oocytes nor cumulus cells. The addition of 5-HT (10 nM to 10 mu M) to isolated metaphase II oocytes had no effect on their internal cAMP or Ca(2+) levels, whereas it caused dose-dependent cAMP and Ca(2+) increases in cumulus cells. This cAMP increase in cumulus cells could be mimicked by 5-HT agonists with the following order of potency: 5-HT > 8-hydroxy-2-(di-n-propylamino)tetralin = alpha-methyl-5-HT = 5-carboxamidotryptamine maleate > 2-[1-(4-piperonyl)piperazinyl] benzo-triazole, thereby supporting a preferential involvement of 5- HT 7 receptors. As measured with cumulus cells preloaded with fura-2/acetoxymethyl ester ( AM), the addition of 5- HT also caused dose-dependent Ca(2+) increases, which were probably linked to detected 5-HT(2A) and 5-HT2B receptors. Adding the Ca(2+) ionophore ionomycin to cumulus cells resulted in both Ca(2+) and cAMP elevations, whereas preincubation of cells with the Ca(2+) chelator 1,2-bis(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid (BAPTA)-AM abolished the 5-HT-induced Ca(2+) increase and reduced the cAMP increase, indicating cross-talk between the 5-HT-sensitive Ca(2+) and cAMP pathways. Our results show that 5-HT may be a local regulator in mouse cumulus-oocyte complexes through its actions on cAMP and Ca(2+) signaling, as mediated by 5-HT(2A), 5-HT(2B), and 5- HT(7) receptors.