Extending Wednesday's gain, the Bombay Stock Exchange benchmark index Sensex opened up by over 30 points on cautious buying by funds ahead of the release of industrial production data. But around noon, it slipped back in red and registered a loss of over 80 points.The 30-share BSE Sensex was hovering around 16,760-level, a short while ago, down by 86 points.The wide-based National Stock Exchange index Nifty also started the day in green but took a U-turn and shed 21 points in noon session.