Microtubule-associated protein-2 in the hypothalamo-neurohypophysial system: Low-molecular-weight microtubule-associated protein-2 in pituitary astrocytes

Microtubule-associated protein-2 is the most abundant microtubule-associate d protein in the brain and is responsible for morphogenesis and maintenance of the nervous system. In the present experiments, we have examined the lo calization of microtubule-associated protein-2 in the hypothalamo-neurohypo physial system of the rat using western blots and immunohistochemistry. Two monoclonal antibodies against microtubule-associated protein-2, antibody C and AP20, were used: antibody C recognizes both the high- and low-molecula r-weight isoforms of microtubule-associated protein-2; antibody AP20 specif ically detects high-molecular-weight microtubule-associated protein-2 only. Western blot analysis revealed expression of high-molecular-weight microtu bule-associated protein-2 in the whole brain, hippocampus and whole hypotha lamus. While the supraoptic nucleus expressed only high-molecular-weight mi crotubule-associated protein-2, the adult posterior pituitary predominantly expressed low-molecular-weight microtubule-associated protein-2, which was also seen in the embryonic whole brain. Light microscopic immunohistochemi stry revealed that both antibody C and AP20 intensely stained dendrites of the dendritic and somatic zones in the supraoptic nucleus. Double labeling with antibodies against microtubule-associated protein-2 and oxytocin (or v asopressin) demonstrated that microtubule-associated protein-2 was localize d in dendrites of magnocellular neurons in the supraoptic nucleus. In the p osterior pituitary, however, antibody C stained fine processes and cell bod ies of astrocytes, which were identified by an antibody against glial fibri llary acidic protein. Antibody AP20 also stained fine processes of some ast rocytes in the posterior pituitary, but the intensity of immunoreactivity w ith antibody AP20 was weaker than that with antibody C. This result suggest s that microtubule-associated protein-2 in astrocytes of the posterior pitu itary is predominantly of the low-molecular-weight type. Moreover, western blots revealed low-molecular-weight microtubule-associated protein-2 of the posterior pituitary at a molecular weight slightly higher than embryonical ly expressed low-molecular-weight microtubule-associated protein-2, indicat ing that low-molecular-weight microtubule-associated protein-2 in the poste rior pituitary is possibly the isoform microtubule-associated protein-2d. The present results demonstrate that astrocytes in the posterior pituitary of adult rats still retain the ability to express the immature variant of m icrotubule-associated protein-2, low-molecular-weight microtubule-associate d protein-2, and its expression is probably linked to structural plasticity . (C) 1998 IBRO. Published by Elsevier Science Ltd.