US Stocks in Focus (AMIE, TSLA, RBY, VRUS, MTOR, KMX, LVS)

The companies whose shares are actively trading in Thursday's morning session include Ambassadors International, Tesla Motors, Rubicon Minerals, Meritor and CarMax.

Gainers:

Shares of Ambassadors International, Inc. (NASDAQ: AMIE) soared as much as 34 percent on Thursday before its quarterly earnings announcement. The company provides travel and cruise services and operates Windstar Sail Cruises.

Tesla Motors (NASDAQ: TSLA) surged about 17 percent to $27.81 after Morgan Stanley upgraded the stock to "overweight" with a $70 price target. The price target is thrice its closing stock price on March 30.

Rubicon Minerals Corp. (NYSE-AMEX: RBY) shares were up 16 percent after the miner released amended estimates for Pheonix Gold project in Ontario.

Shares of Pharmasset, Inc. (NASDASQ: VRUS) were up 9 percent after a study showed that its experimental hepatitis C drug was effective when added to standard therapy.

Losers

Shares of Meritor, Inc. (NYSE: MTOR) plunged 13 percent after the company said it plans to close its Trailer Axle business in Europe on July 31, affecting 171 jobs. The company expects to take charges in the range of $17 million to $23 million over the next year.

CarMax (NYSE: KMX) stock fell 7 percent to $32.10 despite strong quarterly results. For the fourth quarter, the company earned $89.5 million or 39 cents a share, higher than $75.4 million, or 33 cents a share last year. Net sales and operating revenues increased 23 percent to $2.25 billion.

Shares of Las Vegas Sands Corp. (NYSE:LVS) after its Chinese subsidiary by Hong Kong regulator over alleged breaches of the provisions of the Hong Kong Securities and Futures Ordinance.

Shares of Berkshire Hathway (NYSE: BRK-A) (NYSE: BRK-B) were down about 2 percent after David Sokol, who is rumored to succeed when Warren Buffet retires, surprisingly resigned.

"I had not asked for his resignation, and it came as a surprise to me. Twice before, most recently two or so years ago, Dave had talked to me of resigning. In each case he had given me the same reasons that he laid out in his Monday letter. Both times, I and other Board members persuaded him to stay. Berkshire is far more valuable today because we were successful in those efforts," Buffett said in a statement.

Sokol said that he had been interested in leaving Berkshire because he would like to start his own investment firm, Sokol told CNBC.