Tokyo stocks opened lower Wednesday, weighed down by declines on Wall Street the previous day and selling of export-related issues due to the yen firming against the U.S. dollar. In the first 15 minutes of trading, the 225-issue Nikkei Stock Average fell 197.37 points, or 0.49 percent, from Tuesday to 39,900.26. The broader Topix index was down 3.58 points, or 0.13 percent, at 2,716.35. On the top-tier Prime Market, decliners were led by precision instrument, machinery and electric appliance issues. At 9 a.m., the dollar fetched 150.02-05 yen compared with 150.00-10 yen in New York and 150.41-…