The Nikkei stock index opened slightly higher Wednesday on upbeat U.S. economic data. In the first 15 minutes of trading, the benchmark 225-issue Nikkei Stock Average rose 7.80 points, or 0.09 percent, from Tuesday to 8,549.73. The broader Topix index of all First Section issues on the Tokyo Stock Exchange was down 0.37 point, or 0.05 percent, to 730. 54.