Neurosecretory pathways in the mallard duck (Anas platyrhynchos) brain: localization by aldehyde fuchsin and immunoperoxidase techniques for neurophysin (NP) and gonadotrophin releasing hormone (Gn-RH).

Localization studies of the hypothalamohypophysial and tuberoinfundibular neurosecretory systems were performed in the adult male mallard duck with an immunoperoxidase techinque for the demonstration of neurophysin (NP) and gonado-tropin-releasing hormone (Gn-RH) and with aldehyde fuchsin for the staining of neuosecretory material (NSM). A comparison was made between the distribution of NSM stained with aldehyde fuchsin and NP seen by immunocytochemistry. The magnocellular perikarya of the supraoptic (SON) and paraventricular (PVN) nuclei, the zona externa of the anterior median eminence (ME), the fiber layer of both the anterior and posterior ME, and small neurons in the tractus quintofrontalis were stained by both the immunoperoxidase method for NP and by the aldehyde fuchsin stain. In contrast, the parvocellular neurons of the PVN, extra-hypothalamic neurosecretory fibers dorsal to the anterior commissure in the septal region and tanycytes lining the ventral 1/3 of the third ventricle at the level of the anterior ME, were stained only by the immunocytochemical procedure for NP. These observations indicate that immunocytochemistry is more sensitive than aldehyde fuchsin staining for detecting low concentrations of NP in cells and tissues, but the two techniques produce comparable results where the concentration of the NP is relatively high. Two populations of beaded axons containing Gn-RH were distributed throughout the zone externa of both the anterior and posterior ME. One group of fibers paralleled the hypothalamo-hypophysial neurosecretory tract whereas the other was distributed in the contact zone of the ME. Immunoreactive Gn-RH was found in the cytoplasm of a sparse population of cell bodies in the dorsolateral portion of the arcuate nucleus as well as in the axons that project from this nucleus ventrally towards the ME.