Variation of pro‐vasopressin processing in parvocellular and magnocellular neurons in the paraventricular nucleus of the hypothalamus: Evidence from the vasopressin‐related glycopeptide copeptin

Arginine vasopressin (AVP) is synthesized in parvocellular‐ and magnocellular neuroendocrine neurons in the paraventricular nucleus (PVN) of the hypothalamus. Whereas magnocellular AVP neurons project primarily to the posterior pituitary, parvocellular AVP neurons project to the median eminence (ME) and to extrahypothalamic areas. The AVP gene encodes pre‐pro‐AVP that comprises the signal peptide, AVP, neurophysin (NPII), and a copeptin glycopeptide. In the present study, we used an N‐terminal copeptin antiserum to examine copeptin expression in magnocellular and parvocellular neurons in the hypothalamus in the mouse, rat, and macaque monkey. Although magnocellular NPII‐expressing neurons exhibited strong N‐terminal copeptin immunoreactivity in all three species, a great majority (~90%) of parvocellular neurons that expressed NPII was devoid of copeptin immunoreactivity in the mouse, and in approximately half (~53%) of them in the rat, whereas in monkey hypothalamus, virtually all NPII‐immunoreactive parvocellular neurons contained strong copeptin immunoreactivity. Immunoelectron microscopy in the mouse clearly showed copeptin‐immunoreactivity co‐localized with NPII‐immunoreactivity in neurosecretory vesicles in the internal layer of the ME and posterior pituitary, but not in the external layer of the ME. Intracerebroventricular administration of a prohormone convertase inhibitor, hexa‐d‐arginine amide resulted in a marked reduction of copeptin‐immunoreactivity in the NPII‐immunoreactive magnocellular PVN neurons in the mouse, suggesting that low protease activity and incomplete processing of pro‐AVP could explain the disproportionally low levels of N‐terminal copeptin expression in rodent AVP (NPII)‐expressing parvocellular neurons. Physiologic and phylogenetic aspects of copeptin expression among neuroendocrine neurons require further exploration