Recently, a set of proteins involved in the docking and fusion machinery of
secretory organelles has been identified in anterior pituitary cells. In t
his study we analyzed, by Western blotting and immunocytochemistry, the exp
ression of several proteins involved in exocytosis after long-term administ
ration of 17 beta-estradiol (E-2) in Fischer 344 rats. No differences were
observed in the amount of synaptosomal-associated protein of 25 kDa, synapt
obrevin 2, syntaxin 1, synaptotagmin I and Rab3a in total brain homogenates
from treated rats after E-2 administration. In striking contrast, the leve
ls of all of these exocytotic proteins, including cellubrevin, were notably
decreased in pituitary glands of E-2-treated rats. In addition, no differe
nces were observed in the in vitro basal and 8-Br-cAMP-induced prolactin (P
RL) release between pituitary cells from control and E-2-treated rats, wher
eas TRH-induced PRL release in anterior pituitary cells from E-2-treated an
imals was higher than in control donors. In conclusion, this study shows th
at protein components of the exocytotic machinery are specifically down-reg
ulated in the pituitary gland of E-2-treated Fischer 344 rats.