The Nikkei stock index dropped Tuesday morning as investors locked in gains after it closed at an all-time high above the 40,000 mark the previous day. The 225-issue Nikkei Stock Average fell 168.62 points, or 0.42 percent, from Monday to 39,940.61. The broader Topix index was up 3.81 points, or 0.14 percent, at 2,710.09. The U.S. dollar stayed around the mid-150 yen zone, finding some support after a Federal Reserve official suggested Monday that the central bank is not rushing to reduce interest rates given the strength of the U.S. economy, dealers said. At noon, the dollar fetched 150.49-50…