Chronic osmotic stimulation reduces vasopressin but not synaptophysin content in rat neurohypophysis

The content of synaptophysin, a vesicular integral membrane protein of neurons and endocrine cells, and that of vasopressin was measured in neurohypophyses of rats during chronic osmotic stimulation. The animals received 2% NaCl in their drinking water for up to 4 days. Synaptophysin content of neurohypophyses was determined using quantitative immunoblotting, vasopressin content was measured by radioimmunoassay. Salt loading caused a decrease in the content of vasopressin to about 15% of that of control animals, whether expressed per neurohypophysis or relative to the total tissue protein. In contrast, no change was found in the synaptophysin content. Taken together with published evidence of changes in the relative numbers of the hormone-containing neurosecretory granules (NSGs) and the microvesicles (MVs) under the conditions of chronic osmotic stimulation, these results strongly indicate the surface density of synaptophysin on NSGs to be significantly lower than its surface density on MVs.