Russell Martin hit a tie-breaking sacrifice fly in the Pittsburgh eighth, and Pedro Alvarez and Travis Ishikawa each drove in two runs in front of 26,177 well-bundled fans.
Castro smacked a three-run homer in the third and a solo shot in the sixth in this rematch from last week's opening series at PNC Park, which the Pirates took, 2-1. This was the fourth one-run game between the two teams.
Renteria didn't stay for the end. He was ejected in the ninth inning for arguing balls and strikes with home-plate umpire Jeff Nelson. The first manager to use the expanded instant replay on Opening Day, Renteria now is the first Major League manager to be tossed this season.
Back to Castro. He has hit safely in five straight games after going 0-for-9 in the first two games of the season.
"He's just starting to get into a rhythm," Renteria said. "He's got some guys getting on base ahead of him. He's seeing a lot of good at-bats from his teammates. He's making a real good conscientious effort of bearing down and doing his thing.
"You can't expect him to hit two homers every single day, but at least his approaches seem to be working," Renteria said. "He's seeing the field, and it looks like he's gaining some confidence."
The reason for that may be the change in the batting order. Castro batted third in the first two games of the season, hit second in the next three and was in the six-hole for the second game. It's not that Castro can't hit at the top of the order, Renteria said, but if he's batting in the first and making a lot of outs, he could be feeling that he's a "quote-unquote failure."
"Sometimes you just do that to give everybody in the game the ability to sit back and watch everybody work in front of them, that's all," Renteria said of the switch. "He's hit first and second throughout his career. It doesn't mean he's not going to do it again. These are just little things you do."
Former Cubs manager Dale Sveum dropped Castro in the batting order after the shortstop scuffled last season. This year is different, Castro said.
"I want to hit with men in scoring position," Castro said. "I don't care what spot [Renteria] puts me in. He tries to put me in a spot in a good way, not in a bad way, not because I didn't do this, I didn't do that. He trusts me and is trying to move me to bring in more RBIs."
So, hitting sixth is OK?
"You got men on base -- he put me there for a reason, a good reason," Castro said.
Edwin Jackson was making his second straight start against the Pirates, who seemed to have a better scouting report this time. On April 2, Jackson gave up two hits over 5 1/3 innings. This time, Pittsburgh collected six runs on nine hits and four walks over 4 2/3 innings.
"They came out aggressive and I left a lot of balls out over the plate, which is a bad combination," Jackson said. "Our team did a great job coming back and fighting back. At the end of the day, I have to do a better job of getting the ball down and executing pitches, especially when your team comes back and gives you runs."
Starling Marte doubled off Jackson's first pitch of the game, and one out later, Andrew McCutchen walked, with both runners scoring on Alvarez's double. Jackson then hit Martin with a pitch, and Neil Walker followed with an RBI single. Ishikawa added a sacrifice fly to open a 4-0 lead.
But the Cubs answered. Emilio Bonifacio smacked an RBI single with two outs in the Chicago second. In the third, Anthony Rizzo and Luis Valbuena each singled to set up Castro's first home run, a shot into the left-field bleachers, to tie the game at 4.
The Pirates took a 6-4 lead, but Castro made it 6-5 with a leadoff homer in the sixth, driving an 0-2 pitch from Charlie Morton to left. In the seventh, Bonifacio singled for his third hit of the game and 17th this year, moved up on a sacrifice by Ryan Kalish, stole third, and scored on Rizzo's single off lefty Tony Watson to tie the game.
Bonifacio now has five multi-hit games out of seven, and is 14-for-21 against the Pirates.
"I'm just seeing the ball pretty good against them," Bonifacio said. "I want to hit like that against every team."
"We've got to make some adjustments," Pirates manager Clint Hurdle said of Bonifacio. "We've talked about it, just weren't able to do it tonight. We know what we need to do, haven't been able to do it. ... Give him credit because he's barreling up a lot of balls from both sides of the plate."
Pedro Strop walked both Marte and McCutchen with one out in the eighth, and was pulled for James Russell, who walked Alvarez to load the bases before serving up Martin's sacrifice fly.
"Had we just laid down after the first inning and giving up four runs, I would've been extremely disappointed, but we didn't," Renteria said. "We kept chipping away. We have to fight, we have to keep battling every day."