Ultrastructural localization of neuropeptide Y and galanin immunoreactivity in the paraventricular nucleus of the hypothalamus in the rat.

Preembedding immunoperoxidase staining methods were used to allow ultrastructural localization of neuropeptide Y (NPY) and galanin immunoreactivity (IR) in the paraventricular nucleus (PVH) of the rat hypothalamus. NPY-IR was localized exclusively in axons and axon terminals which could be grouped into three types: (1) symmetric axo-somatic contacts predominantly with parvocellular neurons, many of which displayed neurosecretory specializations, (2) predominantly asymmetric contacts onto larger dendritic processes, including some of magnocellular neurosecretory neurons, and (3) predominantly symmetric contacts with small dendritic and spine-like profiles. Galanin-IR terminals displayed a more limited distribution and formed both symmetric and asymmetric contacts with parvocellular neurons, and primarily asymmetric contacts with larger dendritic shafts. Numerous dendritic and somatic profiles, including those of some magnocellular neurosecretory neurons, were lightly stained for galanin IR. These results establish that NPY and galanin IR afferents form a variety of conventional synaptic contacts in the PVH. The two peptidergic terminal types differed with respect to the frequency of their interaction with various postsynaptic targets and/or their distribution upon them. Both peptidergic inputs arise at least in part from brainstem catecholaminergic neurons, and the relationship of the present results to the fine structure of catecholaminergic terminals is discussed.