Righty Jeff Karstens takes the mound for the third time since a two-month stretch on the sidelines tonight, when the first-place Pittsburgh Pirates host the Houston Astros in the finale of a four-game series at PNC Park.

The Pirates took over sole possession of the top spot in the National League's Central Division on Wednesday, when Pedro Alvarez and Michael McKenry each drove in a pair of runs in a 6-4 win.

Clint Barmes knocked in the other run, while Garrett Jones added two hits and scored twice for the Pirates, who won their seventh game in eight tries and now lead Cincinnati by a game after the Reds fell, 4-1, in Los Angeles.

Kevin Correia (5-6) allowed six hits and three runs over six innings for the win.

J.D. Martinez homered and Brian Bogusevic posted two hits, an RBI and a run scored for the Astros, who have dropped seven in a row overall and nine straight on the road.

Dallas Keuchel (1-1) was charged in defeat for seven hits and four runs over 5 2/3 frames.

Astros slugger Carlos Lee was pulled from the game in the seventh inning, and after the contest he was traded to the Miami Marlins. The Astros acquired infielder Matt Dominguez and left-handed pitcher Rob Rasmussen in exchange for the 36-year-old Lee and cash considerations.

Karstens, who won nine times in 30 appearances last season, was 0-1 in his initial three starts of 2012 before a prolonged bout with right shoulder inflammation.

He returned for an 8-3 loss at Philadelphia on June 25, then earned win No. 1 with seven innings of four-hit, two-run ball in a 7-3 defeat of St. Louis on June 30.

In eight career meetings with Houston, the Texas Tech product is 4-2 with a 3.33 earned run average across 46 innings.

The Astros send righty Bud Norris to the mound for his second start since his own stretch on the disabled list.

Norris, a sixth-round draft pick in 2006, was 5-4 through his initial 13 starts before a sprained right knee shelved him after a 6-3 loss at San Francisco on June 12.

He returned on June 29 and fell to .500 on the season after allowing four runs on seven hits in six innings of a 4-0 loss to the Chicago Cubs at Wrigley Field.

Norris is 0-4 in his last five starts after beginning the season 5-1.

He defeated the Pirates, 1-0, with six innings of three-hit ball in Pittsburgh on May 11 and is 4-4 in nine career starts against them.

The Pirates won two of three from the Astros in a May series at PNC Park and 11 of 18 games between the teams last season. Houston last won the series in 2010, taking 11 of 15 games.