The destination of the aged, nonreleasable neurohypophyseal peptides stored in the neural lobe is associated to the remodeling of the neurosecretory axon

Abstract

The present investigation was designed to investigate the fate of the large pool of neurohypophyseal hormones that is never released into the blood. Normal Sprague-Dawley and taiep mutant rats were investigated under normal water balance, after dehydration and after dehydration-rehydration. Lectin histochemistry and light- and electron-microscopic immunocytochemistry using antibodies against vasopressin, oxytocin, and neurophysins used at low (1:1,000) and high (1:15,000) dilutions allowed to distinguish (1) recently packed immature granules, as those located in the perikaryon; (2) mature; and (3) aged granules. The distribution of these granules within the different domains of the neurosecretory axons located in the neural lobe, namely, undilated segments, swellings, terminals, and Herring bodies, and the response of these compartments to dehydration and dehydration-rehydration allowed to roughly follow the routing of the granules through such axonal domains. It is suggested that granules may move backward and forward between the terminals and the swellings. At variance, aged granules located in Herring body are retained in this compartment and would finally become degraded. Herring bodies displayed distinct lectin binding and immunocytochemical properties, allowing to distinguish them from axonal swellings. After a dehydration-rehydration cycle, immunocytochemistry and electron microscopy revealed that Herring bodies were no longer present in the neural lobe and that several terminals had degenerated. It is concluded that (1) the neurophysin axons may undergo remodeling under appropriate stimuli and (2) Herring bodies are a specialized and plastic domain of the magnocellular neurosecretory neuron involved in the disposal of aged neurosecretory granules. No differences were detected at the neural lobe level between normal and mutant rats subjected to the same experimental conditions.