The Drug Mianserin and 6-Aza-Mianserin(Mirtazapine appear devoid of clinically relevant agonist activity. The effect you mentioned occurs with virtually all tested SSR(N)I antidepressants and has helped lend creedence to an opioidergic theory of depression(though this re-regulation/NT increase happens in many other neurotransmitter sys.)
Apparently Mianserin and 6-Aza-Mianserin mediate their opioidergic analgesia via mu and kappa 3.
For contrast/comparison Venlafaxine mediated the above effect through mu, kappa 1, kappa 3 and delta-opioid receptor subtypes.
To further lend credibility to the SSRI - opioid theory while low-high doses of naltrexone antagonize the analgesia ULD Naloxone potentiates it. As well tolerance develops to the analgesia of SSRIs which ULDNX abolishes.
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From the literature Mianserin and Mirtazapine produce increased analgesia when combined with standard SSRI's. The combo also produces greater and more sustained activation of the 5 HT1a receptor- critical in Antidepressant effect, and anxiolysis.
Also for those that don't like the side effects of the various SSRI's (anxiety, mood, sexual, nausea or otherwise) Mirtazapine / Mianserin will generally abolish them by blocking the 5HT2a and 5HT3 receptor where they are mediated.
--There have been a few isolated reports of Tianeptine abuse but for the most part this is a long term thing...if it works and you can wait it's cool to wake up each day knowing you'll feel better than well, sure you can't pop 5 more and feel even better but hey...