"Nathan has told the Bruins he wants to explore free agency. It's not a financial issue, just a decision to move on," Krepelka said.
Horton, 28, registered 13 goals and nine assists for 22 points in 43 games with the Bruins this past season.
The Bruins have already informed forwards Jaromir Jagr and Jay Pandolfo, and defenseman Andrew Ference that they will not be re-signing them.
In 591 career NHL regular-season games with the Florida Panthers and Bruins, Horton has accumulated 198 goals and 204 assists for 402 points.
Drafted third-overall by Florida in 2003, Horton was traded to Boston, along with Gregory Campbell, on June 22, 2010.