Nikkei opened slightly lower Tuesday as selling was triggered by a firmer yen and concern over market overheating. At 9:15 a.m., the 225-issue Nikkei Stock Average was down 24.22 points, or 0.17 percent, from Monday at 14,633.82. The broader Topix index of all First Section issues on the Tokyo Stock Exchange was down 2.10 points, or 0.17 percent, at 1, 214.43.