Effects of estrogen on synapsin I distribution in developing hypothalamic neurons

Estradiol (17β-estradiol, E2) plays an essential role in sexual differentiation of the rodent brain. The purpose of the present study was to investigate the effects of E2 on developing hypothalamic neurons by focusing on a presynaptic protein, synapsin I. We applied E2 to cultured hypothalamic cells removed from fetal rats and investigated resultant effects upon synapsin I. Our immunocytochemical study revealed that administration of E2 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 E2 did not increase both protein and mRNA expression levels of synapsin I. Studies with cyclohexamide (CHX), membrane impermeable E2 (E2-BSA), and an estrogen receptor (ER) antagonist ICI 182,780 indicated that E2 affected the synapsin I-area mainly via a non-genomic pathway mediated by membrane ER. Immunoblotting showed that E2 suppressed phosphorylation of synapsin I at residues Ser-9, Ser-553, and Ser-603. On the other hand, E2 did not affect phosphorylation of synapsin I at Ser-62, Ser-67 and Ser-549. The present study suggests that E2 affects localization of synapsin I in hypothalamic neurons by altering site-specific phosphorylation of synapsin I, which is likely mediated by membrane ER.