An immunoelectron microscopic study of peptide substances in the rat neurohypophysis

The ultrastructural localization of peptide substances, such as vasopressin (VP), substance P (SP), enkephalin (ENK) and somatostatin (SRIF), and the distribution of peptidergic nerves in rat neurohypophysis were studied by immunoelectron microscopy. The results show that VP, SP, ENK and SRIF immunoreactive products are located in large granular vesicles (110-150 nm in diameter), at the periphery of microvesicles and on the outer membrane of mitochondria. The VP-, SP-, ENK- and SRIF- containing nerve fibers are distributed at the periphery of capillaries and in the vicinity of pituicytes. VP- and ENK-positive nerve terminals form axo-axonic synapses with negative terminals. VP- and ENK-terminals are pre- or postsynaptic elements. Axo-axonic synapses formed by two SRIF- positive terminals were also found. In addition, ENK- and SP-positive terminals with pituicytes form synaptoid structures respectively. The authors have discovered VP-positive pituicytes for the first time.