A non-neuronal cholinergic system of the ovarian follicle.

We have recently provided evidence that acetylcholine (ACh) is a non-neuronal intraovarian signalling molecule, produced by granulosa cells (GCs) and which appears to act as signalling factor in the growing follicle. The ACh biosynthesis enzyme, choline-acetyltransferase (ChAT), is expressed only in growing, antral follicles in rodent and primate species. This restriction to follicle stages, which depend on the activity of follicle-stimulating hormone (FSH), may suggest that ACh could be an as yet unknown local mediator of FSH actions. In respect of ACh actions, our ongoing studies indicate that they may be exerted via different muscarinic ACh-receptors (MR) in GCs, but also in oocytes in an overlapping fashion. To elucidate functional details we have studied cultured human GCs isolated from preovulatory follicles. Activation of MRs increases intracellular calcium and, e.g., induces the master transcription factor egr-1, implying involvement in cell differentiation events. ACh agonists also activate a calcium-activated potassium channel (BK(Ca)) resulting in membrane hyperpolarization, which allows activation of other voltage-dependent ion channels. Experimental modulation of the chain of these events causes altered steroidogenesis, implying a crucial role of ACh in endocrine functions. Further ACh actions include phosphorylation of the gap junction molecule connexin 43 and disruption of intercellular communication between GCs. This may allow strongly coupled GCs to escape from the functional syncytium of the follicle in order to initiate proliferation. Proliferation is indeed strongly increased in cultured human GCs when treated with cholinergic agents. The repertoire of ACh/MR actions is far from being fully appreciated and may include epigenetic regulation in healthy growing follicles. Although many aspects of the ovarian cholinergic system, including, for instance, influence of follicular ACh on the MR-bearing oocyte, remain to be examined. The present data pinpoint ACh as an emerging, unique intraovarian signalling molecule.