* TSX falls 35.96 points, or 0.29 percent, to 12,337.81 * Eight of 10 main sectors decline * Aeterna Zentaris shares soar 20 percent By John Tilak TORONTO, Dec 28 (Reuters) - Canada's main stock index slipped on Friday, led by financial and energy stocks, as uncertainty surrounding U.S. budget discussions dampened investor sentiment. President Barack Obama and lawmakers were launching a last-chance round of budget talks days before a New Year's deadline to reach a deal to avoid tax hikes and spending cuts that could send the country into a recession. "The volumes are quite low, and if you combine that with the short-term uncertainty of the 'fiscal cliff' in the United States it's a real recipe for a downward market," said Arthur Salzer, chief executive of Northland Wealth Management. "It's something that's going to be resolved sooner or later. There's no way around it. But politically it might look good on the president and on the Congress if they did something over the weekend before the New Year," he said. The Toronto Stock Exchange's S&P/TSX composite index was down 35.96 points, or 0.29 percent, at 12,337.81. Eight of the 10 main sectors of the index were in the red. The financial sector, the index's biggest, was down 0.4 percent. Bank of Nova Scotia was down 0.5 percent at C$57.63, and Royal Bank of Canada dropped 0.2 percent to C$59.92. Energy stocks slipped 0.3 percent, tracking a decline in oil prices. Suncor Energy Inc fell 0.7 percent to C$32.38, playing the biggest role of any single stock in leading the market lower. In company news, Aeterna Zentaris Inc said it was granted a U.S. FDA assessment for an upcoming cancer trial. The drugmaker's shares rose 20 percent to C$2.59.