cAMP and intracellular Ca2+ 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 Ca2+ in mouse oocytes and surrounding cumulus cells. On the basis of a reverse transcription-polymerase chain reaction study, 5-HT7 receptor mRNA is expressed in cumulus cells, oocytes, and embryos up to the four-cell stage, and 5-HT2A and 5-HT2B receptor mRNAs are expressed in cumulus cells only, whereas 5-HT2C, 5-HT4, and 5-HT6 receptors are expressed in neither oocytes nor cumulus cells. The addition of 5-HT (10 nM to 10 microM) to isolated metaphase II oocytes had no effect on their internal cAMP or Ca2+ levels, whereas it caused dose-dependent cAMP and Ca2+ 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-HT7 receptors. As measured with cumulus cells preloaded with fura-2/acetoxymethyl ester (AM), the addition of 5-HT also caused dose-dependent Ca2+ increases, which were probably linked to detected 5-HT2A and 5-HT2B receptors. Adding the Ca2+ ionophore ionomycin to cumulus cells resulted in both Ca2+ and cAMP elevations, whereas preincubation of cells with the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA)-AM abolished the 5-HT-induced Ca2+ increase and reduced the cAMP increase, indicating cross-talk between the 5-HT-sensitive Ca2+ 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 Ca2+ signaling, as mediated by 5-HT2A, 5-HT2B, and 5-HT7 receptors.