Tokyo stocks opened higher Monday on a weaker yen and gains of key U.S. stock indexes that boosted sentiment here. At 9:15 a.m., the 225-issue Nikkei Stock Average rose 121.32 points, or 0.79 percent, from Friday to 15,503.04. The broader Topix index of all First Section issues on the Tokyo Stock Exchange was up 6.59 points, or 0.53 percent, to 1,255. 16.