Secretory Function of Neurons of the Suprachiasmatic Nucleus in the White Mouse and Rat

Abstract

The suprachiasmatic nucleus (SCN) of adult male and female white mice and rats has been studied light- and electron-microscopically. Four types of neurons have been recognized in the SCN. Type-I neurons predominate in the SCN. They are very small, have a round and folded nucleus, one or two nucleoli, and a thin rim of cytoplasm. The latter contains numerous loosely arranged polysomes, free ribosomes, single narrow and short canaliculi of the rough endoplasmic reticulum (RER), poorly developed Golgi complex (GC) , solitary lysosomes, and very rare small granules (d =65–110 nm) resembling secretory granules. Type-II neurons are few and generally similar to those of type I. Unlike type-I neurons, however, those of type II contain a great number of larger secretory granules (d=72–192 nm). Images of forming granules are frequent in the GC. Accumulations of the neurons are often seen near the capillaries, which are of the brain type. The neurons occur in female rats especially after opening of the vagina. Type-Ill neurons are rare and are seen in the periphery of the SCN. They are small, poor in cytoplasm, and contain few polysomes. Canaliculi of the RER are longer and more branched compared with those of type-I and -II neurons. The size and structure of the granules are similar to those of type-I neurons, the number of granules is about equal in both types. Type-IV neurons are rare and are observed mainly in the periphery of the SCN. They are markedly larger than those of types I-III. All their organelles are prominent. Canaliculi of the RER are concentrated in the periphery, and GC, mitochondria, and lysosomes are in the central zone of the cytoplasm. Small granules (d =65–110 nm) are rare. Single cilia, basal bodies, rootlets, and synaptic boutons are often observed in the neurons of all four types. Oligodendrocytes and protoplasmic astrocytes are seen among the neurons. The SCN is poor in blood capillaries. Type-I and -II neurons are thought to be one functional type, producing a neurohumoral substance that is active in the regulation of reproduction.