Synaptic regulation of paraventricular arginine vasopressin-containing neurons by neuropeptide Y-containing monoaminergic neurons in rats. Electron-microscopic triple labeling.

Synaptic regulation of arginine vasopressin (AVP)-containing neurons by neuropeptide Y (NPY)-containing monoaminergic neurons was demonstrated in the paraventricular nucleus of the rat hypothalamus. NPY and AVP were immunolabeled in the pre- and the post-embedding procedures, respectively, and monoaminergic fibers were marked by incorporating 5-hydroxydopamine (5-OHDA), a false neurotransmitter. The immunoreaction for NPY was expressed by diaminobenzidine (DAB) chromogen, and that for AVP by gold particles. The DAB chromogen was localized on the surface of the membrane structures, such as vesicles or mitochondria, and on the core of large cored vesicles. Gold particles were located on the core of the secretory granules within the AVP cell bodies and processes. The incorporated 5-OHDA was found as dense cores within small or large vesicular structures. From these data, three types of nerve terminals were discernible: NPY-containing monoaminergic, NPY-containing non-aminergic, and monoaminergic fibers. The AVP cell bodies appeared to have synaptic junctions formed by these nerve terminals as well as by the unlabeled nerve terminals which have small clear vesicles and large cored vesicles. These different types of nerve terminals were frequently observed in a closely apposed position on the same AVP cell bodies. The functional relationships of these three types of neuronal terminals are discussed.