By Mary Wisniewski

CHICAGO (Reuters) - Rep. Jesse Jackson Jr. is being treated for bipolar disorder at Mayo Clinic in Rochester, Minnesota, the clinic confirmed Monday.

The Chicago Democrat and son of civil rights leader and former presidential candidate Jesse Jackson "is responding well to the treatment and regaining his strength," Mayo Clinic said in a statement.

The clinic said Jackson was being treated for Bipolar II depression, a condition that affects the parts of the brain controlling emotion, thought and drive.

Millions of people have bipolar disorder, sometimes called manic depression. It is marked by highs and lows of mood, and can be treated by medication and psychological counseling, according to the Mayo Clinic's web site.

"He and his family remain grateful for support and prayers offered and received on his behalf," the clinic said.

Jackson underwent gastric bypass surgery in 2004, which can change how the body absorbs food, liquids, vitamins, nutrients and medications.

Jackson, 47, announced in late June that he had taken a leave from office two weeks earlier for treatment of what was then described as exhaustion.

Amid mounting political pressure to disclose more about his medical condition, Jackson issued a statement on July 5 that his problems were more serious than previously believed and that he needed extended in-patient treatment for unspecified "physical and emotional ailments."

On July 11, his physician said the congressman was receiving intensive care for a "mood disorder" and was expected to make a full recovery.

Jackson has been the subject of a congressional ethics committee probe over an alleged bribe offered to former Illinois Governor Rod Blagojevich by a Jackson supporter in 2008.

The offer was intended to entice Blagojevich into appointing Jackson to President Barack Obama's vacant U.S. Senate seat. Jackson has admitted to lobbying for the seat but has denied knowing about any money offered to Blagojevich, who was convicted of public corruption charges and is in prison.

(Reporting By Mary Wisniewski; Editing by Doina Chiacu)