RAPID MORPHOLOGICAL-CHANGES IN SUPRAOPTIC NUCLEUS AND POSTERIOR PITUITARY INDUCED BY A SINGLE HYPERTONIC SALINE INJECTION

Neurosecretory neurons of the supraoptic nucleus (SON) which manufacture, transport, and secrete neuropeptide hormones are activated by administration of hypertonic NaCl. Male rats were given single intraperitoneal injections of isotonic (0.15 M) or hypertonic (1.5 M) NaCl solution and sacrificed 5 h after injection. Electron micrographs were compared to determine morphological differences in the SON and in the neural lobe in the two conditions. In the SON, significant decreases were found in the extent of glial contact with the magnocellular neuroendocrine cell (MNC) membrane: the amount of nerve terminal contact and amount of apposition of MNC membrane with other MNC cell bodies or dendrites increased. The number of multiple synapses (one terminal contacting two or more cells) per 100-mu-m of somatic membrane in the 1.5 M NaCl condition was double that found in the 0.15 M NaCl condition. Size of nucleoli, overall cell body size, and amount of Golgi apparatus were significantly larger in the animals injected with 1.5 M NaCl. The neural lobes of animals receiving 1.5 M NaCl injections showed increased neural contact with the basal lamina and decreased cytoplasmic enclosure of axon terminals. The large and rapid changes induced by this stimulus suggest that it is an extremely effective activator of morphological plasticity in the hypothalmo-neurohypophysial system.