One of the most interesting substances in this regard is Tianeptine. It is chemically classified as a TCA, but lacks many classical effects of that class, and moreover it displays opioid activity at the MOR with 383-768 Ki.
Hence, it is technically an opioid, and higher dosages (normal dosages are between 10mg to 30mg) such as 100+ mg produce an unmistakable opiod-like buzz.
The antidepressant pharmacodynamics are quite complex and not well understood, the opioid receptor affinity definitely plays a role.
Here's a
link to the Wikipedia article.