He said prosecutors had ordered Barrak held for 10 days pending investigation and transferred him to the central prison.
Kuwait allows more free speech than some of its neighbours, but the constitution says the emir is "immune and inviolable".
Insulting him carries a penalty of up to five years in jail.
Three other former lawmakers were earlier detained on similar charges, questioned and freed on bail pending trial, scheduled to start on Nov. 13.
When the emir ordered changes to electoral rules, the opposition announced a boycott of the election and organised one of the biggest protests in Kuwait's recent history, bringing tens of thousands onto the streets.
At least 29 people and 11 policemen were wounded at the Oct. 21 demonstration, which security forces tried to break up with teargas and stun grenades. Police also detained more than 15 people, most of whom were later freed, activists said.
The government has since banned gatherings of more than 20 people and pushed ahead with preparations for the election.
The Information Minister told a news conference that candidate registration began on Wednesday under new rules that allow each voter to pick one candidate instead of four, as previously.
The minister said that under the changes, which also include setting up an election commission, each candidate will get a slot to discuss his or her platform on state television.
Kuwait has a more open political system than other Gulf states, allowing some parliamentary scrutiny over government decisions. But the emir retains most levers of power, including appointing the prime minister and dissolving parliament.
Kuwaiti lawyer and opposition activist Mohammed Abdulqader al-Jassem said Saudi Arabia and the United Arab Emirates, which jointly intervened last year to help Bahrain quell unrest led by its Shi'ite majority, should stay out of Kuwait's affairs.
"If Kuwait's 'democracy' failed to move to your countries over the past five decades, the movement for change, marches and demonstrations and protests may be easier, and it will not take a long time," he wrote on his website in a message to Saudi King Abdullah and UAE Crown Prince Sheikh Mohammed bin Zayed.
His posting won a swift rebuke from the Foreign Ministry, which called it "unacceptable interference" in the affairs of the two neighbours and said it went against Kuwait's interests.