Although progesterone is the maturation inducer in amphibians, it has been demonstrated that in Bufo arenarum oocytes resumed meiosis with no need of exogenous hormonal stimulus if derived of their enveloping, follicle cells. This phenomenon, called spontaneous maturation, is quite rare in amphibians. In B. arenarum, spontaneous maturation took place only in oocytes obtained during the reproductive period (spring-summer). During this period the oocytes also demonstrated a respiratory activity characteristic of mature oocytes. Interestingly, full-grown B. arenarum oocytes always responded to progesterone regardless of the season in which they were obtained and of their respiratory activity. The disposition of oocytes competent or not competent to mature spontaneously provides a useful system for the study of molecular mechanisms involved in the maturation process. The data presented here indicate that the activation of protein kinase C (PKC) induces germinal vesicle breakdown (GVBD) in denuded oocytes unable to mature spontaneously (winter oocytes) and is involved in the in vitro spontaneous maturation of B. arenarum full-grown oocytes. The inhibition of PKC by 1-(5-isoquinolynyl-sulphonyl-2-methyl-piperazine (H-7) impeded spontaneous maturation in a dose-dependent manner, thus supporting the participation of the PKC pathway during this process. Interestingly phorbol 12-myristate-13-acetate (PMA)-induced GVBD is inhibited by the incubation of the oocytes in dibutyryl cAMP (dbcAMP), indicating that both pathways, PKC and protein kinase A (PKA), are related at a certain point. However, spontaneous GVBD is less sensitive than PMA-induced GVBD to dbcAMP. This fact would support the suggestion that in spontaneous GVBD mechanisms different from activation of PKC are at work.

It has been demonstrated in Bufo arenarum that fully grown oocytes are capable of meiotic resumption in the absence of a hormonal stimulus if they are deprived of their follicular envelopes. This event, called spontaneous maturation, only takes place in oocytes collected during the reproductive period, which have a metabolically mature cytoplasm.

In Bufo arenarum, progesterone acts on the oocyte surface and causes modifications in the activities of important enzymes, such as a decrease in the activity of adenylate cyclase (AC) and the activation of phospholipase C (PLC). PLC activation leads to the formation of diacylglycerol (DAG) and inositol triphosphate (IP3), second messengers that activate protein kinase C (PKC) and cause an increase in intracellular Ca2+. Recent data obtained from Bufo arenarum show that progesterone-induced maturation causes significant modifications in the level and composition of neutral lipids and phospholipids of whole fully grown ovarian oocytes and of enriched fractions in the plasma membrane. In amphibians, the luteinizing hormone (LH) is responsible for meiosis resumption through the induction of progesterone production by follicular cells.

The aim of this work was to study the importance of gap junctions in the spontaneous and LH-induced maturation in Bufo arenarum oocytes. During the reproductive period, Bufo arenarum oocytes are capable of undergoing spontaneous maturation in a similar way to mammalian oocytes while, during the non-reproductive period, they exhibit the behaviour that is characteristic of amphibian oocytes, requiring progesterone stimulation for meiotic resumption (incapable oocytes).

This different ability to mature spontaneously is coincident with differences in the amount and composition of the phospholipids in the oocyte membranes. Capable oocytes exhibit in their membranes higher quantities of phospholipids than incapable oocytes, especially of PC and PI, which are precursors of second messengers such as DAG and IP3.

The uncoupling of the gap junctions with 1-octanol or halothane fails to induce maturation in follicles from the non-reproductive period, whose oocytes are incapable of maturing spontaneously. However, if the treatment is performed during the reproductive period, with oocytes capable of undergoing spontaneous maturation, meiosis resumption occurs in high percentages, similar to those obtained by manual defolliculation.

Interestingly, results show that LH is capable of inducing GVBD in both incapable oocytes and in oocytes capable of maturing spontaneously as long as follicle cells are present, which would imply the need for a communication pathway between the oocyte and the follicle cells. This possibility was analysed by combining LH treatment with uncoupling agents such as 1-octanol or halothane. Results show that maturation induction with LH requires a cell–cell coupling, as the uncoupling of the gap junctions decreases GVBD percentages. Experiments with LH in the presence of heparin, BAPTA/AM and theophylline suggest that the hormone could induce GVBD by means of the passage of IP3 or Ca2+ through the gap junctions, which would increase the Ca2+ level in the oocyte cytoplasm and activate phosphodiesterase (PDE), thus contributing to the decrease in cAMP levels and allowing meiosis resumption.