Interesting, thanks, I wish I had the full paper to see binding data...the kappa agonism must be quite slight, because at least subjectively kappa agonists have a very recognizable dysphoric effects profile for me! I've taken Rhynchophylline in its pure form and the effects were quite unremarkable, even at high doses...
The most unpleasant aspects for me are the anhedonia, lethargy, and total loss of motivation. I get this from other opioids too, but Kratom carries it's own signature. The withdrawal is also surprisingly nasty for how mild the effects are, comparatively. I get a week to ten days of opioid like withdrawals, but with much more anxiety, and then
several weeks of symptoms that are different than opioid PAWS....more antidepressant like in nature. I haven't made it more than 60 days, so I can't speak as to how long it takes for me to get to baseline...
I have kicked heroin, methadone, etc and maybe I'm just getting old but the Kratom withdrawals are proving more insidious than the aforementioned. I've taken Kratom off and on for over ten years, but this last stretch has been for 2.5, as I mentioned. I go through 1.5-2 kilos a month. The only real positive I get is relief of withdrawal symptoms for a few hours....a burst of relief, then more grey for a few hours, then withdrawal...rinse, wash, repeat...
I recently took Ibogaine, which was amazing, and I lasted two weeks. It did a number on the opioid aspect of withdrawals, but not the secondary phase. When I quit for two months at at the end of 2013 I was getting close to getting over that phase, feeling good, but one flawed decision led me right back into it. Same with the recent Ibogaine induced break. It's a bitch. I'm only about a week back into it, but the addiction and dependance is back full force. I am going to have a go at it again in a few days...this time assisted only by loperamide and exercise, as well as some some cognitive and social support. I'm over it...it's not exactly a gorilla on my back, but a bush baby with 12" long claws!
People often scoff at Kratom addiction....but I have heartily abused and been addicted to just about every class of drugs there is the last 20 years ( seriously...i was a "professional"

) and Kratom has, is, one of the harder ones to shake. I believe this is due to both a combination of its effects, withdrawal, and it's accessibility and "acceptability". It is a great plant but is no joke when abused.
I have done plenty of rounds with extracts like FST, UEI, and gold reserve, but my experiences the past great while have been with plain leaf only.
You've got the opioidergic effects, NMDA antagonism, adrenergic effects, calcium channel blockade, etc etc etc and the cocktail is proving a mighty one to overcome despite the gentle profile...
Good luck man. If my next attempt does not pay off I will likely switch to a straight Mu agonist and divide my withdrawal into two parts....the opioidergic one and the nastier mind fuckery one!