MORPHOLOGICAL PLASTICITY THAT OCCURS IN THE NEUROHYPOPHYSIS FOLLOWING ACTIVATION OF THE MAGNOCELLULAR NEUROSECRETORY-SYSTEM CAN BE MIMICKED INVITRO BY BETA-ADRENERGIC STIMULATION

The osmotic stress to rats of replacing drinking water with 2% NaCl for four days (salt loading) led to dramatic ultrastructural morphological changes in the neural lobe of the pituitary, including a decrease in the ratio of glial to neurosecretory terminal coverage of the pericapillary basal lamina. The decrease in the proportion of the basal lamina covered by pituicytes, the specialized astrocytic glial cells of the neural lobe, was due to a decrease in the number of pituicyte processes reaching the pericapillary spaces. The concomitant increase in the proportion of neuronal coverage was due to the combination of an increase in the length of individual nerve terminals and a change in the number of terminals. Similar structural changes to those seen in vivo were produced by incubation of the isolated neural lobe with the beta-adrenergic agonist isoprenaline. A decrease in the ratio of glial to neuronal coverage of the basal lamina was achieved within 1 h and could be blocked by inclusion of the beta-adrenergic antagonist propranolol. It is proposed that the morphological plasticity is explained by the active movement of pituicyte cytoplasm.