Effects of estrogen on synapsin I distribution in developing hypothalamic neurons.

Abstract Estradiol (17β-estradiol, E 2 ) plays an essential role in sexual differentiation of the rodent brain. The purpose of the present study was to investigate the effects of E 2 on developing hypothalamic neurons by focusing on a presynaptic protein, synapsin I. We applied E 2 to cultured hypothalamic cells removed from fetal rats and investigated resultant effects upon synapsin I. Our immunocytochemical study revealed that administration of E 2 increased the dendritic area (‘MAP2-area’) and synaptic area detected as dot-like staining of synapsin I (‘synapsin I-area’). However, immunoblotting and real-time PCR showed that E 2 did not increase both protein and mRNA expression levels of synapsin I. Studies with cyclohexamide (CHX), membrane impermeable E 2 (E 2 -BSA), and an estrogen receptor (ER) antagonist ICI 182,780 indicated that E 2 affected the synapsin I-area mainly via a non-genomic pathway mediated by membrane ER. Immunoblotting showed that E 2 suppressed phosphorylation of synapsin I at residues Ser-9, Ser-553, and Ser-603. On the other hand, E 2 did not affect phosphorylation of synapsin I at Ser-62, Ser-67 and Ser-549. The present study suggests that E 2 affects localization of synapsin I in hypothalamic neurons by altering site-specific phosphorylation of synapsin I, which is likely mediated by membrane ER.