The subcommissural organ of the frog Rana perezi is innervated by nerve fibres containing GABA

The innervation of the frog subcommissural organ was studied by light-micro scopic and ultrastructural immunocytochemistry using antisera against serot onin, noradrenaline, dopamine, gamma-aminobutyric acid (GABA), glutamic aci d decarboxylase, different GABA receptor subunits and bovine Reissner's fib re material (AFRU). In the proximity of the organ, serotonin- and noradrena line-containing fibres were rare whereas dopamine-immunoreactive fibres wer e more numerous. Many GABA- and glutamic acid decarboxylase-containing nerv e fibres were found at the basal portion of the ependymal cells of the subc ommissural organ. Under the electron microscope, these GABA-immunolabelled nerve endings appeared to establish axoglandular synapses with secretory ep endymal cells of the subcommissural organ. In addition, the secretory epend ymal cells expressed high amounts of the beta(2)-subunit of the GAB(A), rec eptor. Since GABA-immunoreactive neurons were present in the frog pineal or gan proper and apparently contributed axons to the pineal tract, we suggest that at least part of the GABAergic fibres innervating the frog subcommiss ural organ could originate from the pineal organ.