Enkephalin-immunoreactive neurons in the guinea-pig hypothalamus.

An ultrastructural study of enkephalin-immunoreactive perikarya forming a hypothalamic nucleus, the magnocellular dorsal nucleus (MDN), was carried out in colchicine-treated guinea pigs. These large cells usually contain a well-developed rough endoplasmic reticulum, a prominent Golgi complex and neurosecretory granules approximately 100 nm in diameter. These granules are not numerous and sometimes appear preferentially located in the periphery of the cytoplasm. Using the PAP method on ultrathin sections, the immunostaining obtained with anti-enkephalin antisera is essentially confined to the granules but also to some ribosomes linked to membranes of the rough endoplasmic reticulum. These images suggest that the cells of the MDN synthetize the enkephalin-related peptide. Around the perikarya many nerve terminals are observed and many of them make axosomatic synaptic contacts. The exact nature of these nerve endings as well as the projection of these enkephalin-containing cells remain to be established.