The BSE benchmark Sensex was trading 116 points down at 18,643 a shortwhile ago. It fell by over 75 points in opening trade with stocks of oil and gas, IT and banking sectors succumbing to profit-booking. The 30-share index had gained 48.61 points on Thursday.Similarly, the wide-based National Stock Exchange Nifty Index was also trading 35 points down at 5,670 .