RESUMEN
<p><b>OBJECTIVE</b>To explore the effects of λ-cyhalothrin on hippocampus by interfering with estrogen.</p><p><b>METHODS</b>The healthy female ICR mice of postnatal 28 days were random divided into 12 groups, 4 of those were sham-operation include control, λ-cyhalothrin (LCT, 3.0 µg/g), Letrozole (Let, 1.0 µg/g), and LCT (3.0 µg/g)+Let (1.0 µg/g); and the last 8 were ovariectomized include OVX, Estradiol (E2, 10.0 µg/g), LCT, Let, E2+LCT, E2+Let, LCT+Let, E2+LCT+Let. 10 mice in every group received drugs by intraperitoneal injection for 2 days. Then half of every group initiate the ethological test (open field test and Morris water maze) 24 h later. The last half animals were sacrificed to made frozen section for immunofluorescent assay of postsynaptic density protein 95 (PSD95).</p><p><b>RESULTS</b>In ethological test, campared with Sham, OVX can lengthen incubation period in the first grid and to get on the platform (P < 0.05); campared with OVX, OVX+E2 can increase the total numbers of through grid and shorten the incubation period to get on the platform (P < 0.05); campared with OVX+E2, OVX+E2+LCT can reduce the number of grid and standing, lengthen incubation period to the platform (P < 0.05); campared with Sham, Sham+LCT can lengthen incubation period to the platform of Sham mice (P < 0.05), but campared with OVX, OVX+LCT can shoten incubation period in the first grid and to get on the platform in OVX mice (P < 0.05); campared with Sham+Let, Sham+LCT+Let can lengthen incubation period in the first grid, reduce the the number of grid and standing (P < 0.05). In the Immunohistochemical fluorescence experiment we find that, campared with Sham, OVX can reduce the expression of PSD95 in CA1,CA3 and DG (P < 0.05); however campared with OVX, E2 or LCT can both inhibit the effect of OVX (P < 0.05); campared with Sham, Sham+LCT can reduce the expression of PSD95, the same result when OVX+E2+LCT campared with OVX+E2 (P < 0.05); campared with OVX+E2+Let, OVX+E2+LCT+Let can reduce the expression of PSD95 in CA3 (P < 0.05); campared with OVX+Let, OVX+LCT+Let can increase the expression of PSD95 in DG (P < 0.05).</p><p><b>CONCLUSIONS</b>When few estrogen exist in the body, LCT can show estrogen-like action to promote hippocampal synaptic development; but when circulating estrogen exist, LCT can inhibit synaptic development by interfering estrogen.</p>