Axonal Transport of Proteins in the Hypothalamo‐neurohypophysial System of the Rat

Abstract —Axonal transport of proteins in the hypothalamo‐neurohypophysial system of the rat was studied after a local injection of [ 35 S]cysteine in the region of the supraoptic nucleus. The migration of labelled proteins was followed by measuring the specific radioactivity of the proteins in various parts of the hypothalamo‐neurohypophysial tract. Between 2 and 4 h after the isotope injection there was a sharp increase in the protein‐bound specific radioactivity of the posterior pituitary lobe, demonstrating that a transport of 35 S‐labelled proteins had occurred from the supraoptic nucleus to the neurohypophysis. The rate of the transport was 2‐3 mm/h. During the first 24 h after the injection a continuous accumulation of labelled material occurred in the neural lobe. Considerable radioactivity could still be recovered 6 days after the isotope injection. Fractionation of the neurohypophysial proteins by polyacrylamide gel electrophoresis revealed that approximately 90 per cent of the radioactivity of the soluble proteins was recovered in a single protein fraction. Labelling of this fraction was not observed until 2 h after isotope injection. The radioactivity increased markedly up to 4 h. It is suggested that this protein component is involved in the neurohypophysial response to osmotic stress since the protein disappeared from the posterior lobe upon dehydration of the rat.