Tokyo stocks opened slightly lower Tuesday as investors locked in gains after a climb that has seen the Nikkei index rise for six consecutive trading days and reach 34-year highs. In the first 15 minutes of trading, the 225-issue Nikkei Stock Average fell 138.54 points, or 0.39 percent, from Monday to 35,763.25. The broader Topix index was down 8.85 points, or 0.35 percent, at 2,515.75. On the top-tier Prime Market, decliners were led by bank, iron and steel and electric appliance issues. At 9 a.m., the U.S. dollar fetched 145.78-81 yen compared with 145.75-85 yen in London at 4 p.m. and 145.4…