Erasing initial gains, the Sensex at the Bombay Stock Exchange was trading 23 points in the negative territory at 18,405 in late morning deals, when reports last came in. The 30-share index, which had gained almost 275 points in the previous two sessions, rose by 85 points in opening trade today to 18,513.In a similar fashion, the wide-based Nifty of the National Stock Exchange was trading 14 points down to 5,593.