Can I get a research paper or certificate, where is mentioned that in terrariums dart frogs lose their toxicity?
( I live in Russia, translation crooked)

Can't give you a certificate, but as far as I can tell the general concensus is that the alkaloid toxins these frogs secrete comes from dietary sources in the wild and so - unless deliberately fed to remain toxic - they become harmless in captivity.

Some links to paper abstract that may contain the info you are looking for are below.  You should be able to e-mail the authors and ask for a pdf if you'd like to read the full text.