Immunohistochemical localization of avian pancreatic polypeptide‐like immunoreactivity in the rat hypothalamus

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Abstract
The distribution of avian pancreatic polypeptide‐like (APP) immuno‐reactivity within the rat hypothalamus was investigated with the indirect immunoperoxidase method. APP immun ore active perikarya are found in largest numbers in the retrochiasmatic area, the arcuate nucleus, and the supracommissural portion of the interstitial nucleus of the stria terminalis. Small clusters of immunoreactive neurons are also consistently observed in the ventral aspect of the medial preoptic area and lateral hypothalamic area, immediately dorsolateral to the optic chiasm and tracts. These neurons are apparent in all animals but are more intensely stained and occur in larger numbers following colchicinepretreatment. Other immunoreactive neurons are visible only in colchicine‐treated rats and are scattered throughout the anterior and lateral hypothalamic areas and the supramammillary nucleus. Immunoreactive axons and terminal fields present an extensive and highly characteristic distribution throughout the hypothalamus, which in many instances exhibits differential distribution within specific subfields of hypo‐thalamic nuclei and areas. The heaviest concentrations of APP immuno‐reactive axons are present in the periventricular nucleus throughout the ros‐trocaudal extent of the hypothalamus, the ventrolateral portion of the suprachiasmatic nucleus, the retrochiasmatic area, the parvocellular para‐ventricularnucleus, the ventral supraopticnucleus, theperifornical nucleus, the ventral dorsomedial nucleus, and the arcuate nucleus. Moderate plexuses of immunoreactive fibers are also present in the medial preoptic area, the anterior and lateral hypothalamic areas, the nucleus circularis, the median eminence, and the ventral premammillary area. Other areas, such as the ventromedial nucleus, contain virtually no immunoreactive axons but are encapsulated by a dense plexus of immunoreactive terminals. The distribution of a major component of APP immunoreactive fibers exhibits a marked similarity to that of previously described norepinephrine‐containing hypothalamic afferents. Other groups of APP immunoreactive perikarya and fibers appear to represent components of intrinsic diencephalic systems.