The Nikkei stock index opened higher Monday as the yen’s decline against the U.S. dollar in early trading lifted exporter issues, but it soon turned top-heavy on profit-taking. In the first 15 minutes of trading, the 225-issue Nikkei Stock Average rose 185.29 points, or 0.46 percent, from Friday to 40,554.73. The broader Topix index was down 5.64 points, or 0.20 percent, at 2,762.98. On the top-tier Prime Market, gainers were led by precision instrument and machinery issues, while mining and insurance issues led decliners. At 9 a.m., the dollar fetched 151.38-41 yen after trading just above 15…