Hypo...
O.K. I have at least 80 on mitragynine itself and several for each of the remaining indoles as well as the oxindoles. I have only found my mitra notebook so I will offer 5 good ones that should get you started. all except one are not available [in complete form] on the net so I cannot offer urls [on them. I will however offer the complete reference so that you can track them down. if I had a scanner I'd upload them but I don't [so I won't].
1] "Evaluation of Analgesic Induced by Mitragynine, Morphine, and Paracetamol of Mice" Idid, S.Z., Saad, L.B., Yaacob, H., Shahimi, M.M. [I especially like this paper because of the parallels it draws with other analgesics. They performed acetic acid writhings, as well as hot and cold tail flicks. They discovered [among other things] 200 mg./kg. of mitragynine equalled 5 mg./kg. of morphine reduced writhings. I won't bore with you with the figures but they're there. I do have a url.:
www.arbec.com./my/pdf/may-4.pdf Of course it is only available there in pdf form but that should be of no consequence].
2]"Studies of the Synthesis and Opioid Agonistic Activities of Mitragynine-Related Indole Alkaloids: Discovery of Opioid Agonists Structurally Different from Other Opioid Ligands" Takayama, et al. J. Med. Chem. 4/02 45[9] pp.1949-1956. [This was a study out of Chiba U. in Chiba, Japan. Chiba has done alot of work with this alkaloid over the last decade or so. you might want to check out their database
].
3]" Suppressive Effect of Mitragynine on the 5-methoxy-N,N-dimethyltryptamine-Induced Head Twitch Response in Mice" Matsumoto, et al. Pharmacol Biochem Behav [1997] 57[1-2] pp.319-323 [You are in luck with this one because Erowid has this one in their reference database. It is reference# 6343. I usually don't put much stock in Erowid but sometimes they are amzing. I wish I had found this in their database before buying this paper].
4]" Opioid Receptor Agonist Differences of Mitragynine Pseudoindoxyl in Comparison with Mitragynine Derived from the Thai Medicinal Plant Mitragyna Speciosa" Yamamoto, et al. Gen Pharmacol [7/99] 33[1] pp.73-81 [This is another Chiba article...check that database! This paper should be of great interest to you as it compares the the pseudo with the natural].
5] This is the mother of all mitra studies. The original paper by Zacharias, Rosenstein, and Jeffrey who actually were the first to define the alkaloid in 64. " The Properties of Mitragynine, an Alkaloid of the Leaf of the Plant Mitragyna Speciosa" [You can find a copy of this paper [and others by the same group] in Acta Cryst [1965] 18 pp.1039-1043. This is a journal of the "Foundation of Crystallography"].
If you still need more let me know. Sorry it took so long.
I just tried the hyper link above and haven't found it working...The url is good though.