Secretion of Bax-Like Protein into Systemic Circulation from the Posterior pituitary in the Rat Hypothalamo-Posterior Pituitary System

摘要

To investigate the possibility of the neuro-secretion of Bax-like protein in the adult rat, we examined its distribution in the hypo-thalamo-posterior pituitary system using light and electron microscopic immuno-histochemistry. We also measured the serum Bax-like protein using an enzyme im-munoassay. Neurons in the paraventricular nucleus (PVN) and supraoptic nucleus of the hypothalamus exhibited intense Bax-like immunoreactivity (Bax-lR). Bax-like immuno-reactive axons projected to the posterior pituitary through the internal layer of the median eminence. By immunoelectron microscopy, Bax-IR was observed in the granulated vesicles and in the membrane of the rough-surfaced endoplasmic reticulum in the neu-ronal perikarya in the medial magnocellular division of PVN. It was also detected in the secretory vesicles in the nerve terminals of the posterior lobe of the pituitary gland. Double-labeling histochemistry revealed that almost all Bax-like immunoreactive substances were present in the oxytocin neurons, and many Bax-like immunoreactive neurons were nitro-oxidergic neurons. The enzyme immunoassay revealed that the average Bax-like protein concentration in the serum was 39.0+-9.6 pmol/ml in the adult male rat. Colchicine treatment significantly increased the densities of Bax-IR in the PVN and decreased the serum Bax-like protein levels. From these results, Bax-like protein is considered to be produced in the hypotha-lamic oxytocin neurons, transported through axopiasmic flow and secreted into systemic circulation at the posterior pituitary as a "neurohormone".