Simultaneous immunogold labeling of GABAergic terminals and vasopressin-containing neurons in the rat paraventricular nucleus.

The GABAergic innervation of vasopressin-containing cells in the magnocellular part of the paraventricular nucleus was studied at the electron-microscope level using antibodies against GABA and vasopressin. The detection of both GABA and vasopressin on the same ultrathin section, performed with a double-labeling immunogold method, revealed GABAergic terminals in symmetrical synaptic contact with vasopressin-containing neurons. These GABAergic terminals displayed mitochondria, clear synaptic vesicles and varying numbers of electron-dense vesicles. Vasopressin-immunoreactivity was associated with neurosecretory granules, whereas GABA-immunoreactivity was found above mitochondria, clear synaptic vesicles and some electron-dense vesicles. This study, demonstrating the extensive participation of GABA in the innervation of magnocellular vasopressin-secreting neurons, suggests that this inhibitory neurotransmitter regulates vasopressin secretion at the level of the paraventricular nucleus.