In our contemporary western society drugs are the answer.
What does a rat have do be depressed about? Being locked up in a cage perhaps? How does one tell? Are there shrinks for rats, is the wife a bitch? Is the taxman after you? Maybe that giant in the white labcoat has something to do with it.
I admit that I am quite often depressed but I think that encounters with smiling idiots has a lot to do with it.
All of his little rat friends dying of cancer long before their time from getting 1000 times the normal dose of saccharine.