Abstract
α-Conotoxin MII (CtxMII), a peptide toxin from the venom of the predatory cone snail Conus magus, displays an unusual nicotinic pharmacology. Specific binding of a radioiodinated derivative (125I-α-CtxMII) was identified in brain region homogenates and tissue sections. Quantitative autoradiography indicated that 125I-α-CtxMII binding sites have an unique pharmacological profile and distribution in mouse brain, being largely confined to the superficial layers of the superior colliculus, nigrostriatal pathway, optic tract, olivary pretectal, and mediolateral and dorsolateral geniculate nuclei. Expression of α-CtxMII binding sites in the nigrostriatal pathway, combined with evidence for α-CtxMII-sensitivity of nicotine-induced [3H]dopamine release in rodent striatal preparations indicates that 125I-α-CtxMII binding nicotinic acetylcholine receptors are likely to be physiologically important. Unlabeled α-CtxMII potently (Ki < 3 nM) competed for a subset of [3H]epibatidine binding sites in mouse brain homogenates, but weakly (IC50 > 10 μM) interacted with 125I-α-bungarotoxin and (−)-[3H]nicotine binding sites, confirming this compound's novel nicotinic pharmacology. Quantitative autoradiography revealed that α-CtxMII binds with high affinity at a subset of [3H]epibatidine binding sites with relatively low cytisine affinity (“cytisine-resistant” sites), resolving [3H]epibatidine binding into three different populations, each probably corresponding to a receptor subtype. The majority population seems to correspond to that which binds nicotine and cytisine with high affinity (“cytisine-sensitive” sites). Comparison of the cytisine-resistant population's distribution with that of α3 subunit mRNA expression suggests that the fractions both more and less sensitive to α-CtxMII probably contain the α3 subunit, perhaps in combination with different β subunits.
Footnotes
-
Send reprint requests to: Dr. A.C. Collins, Institute for Behavioural Genetics, University of Colorado, Campus Box 447, Boulder, CO 80303-0447.
-
This work was supported by National Institute on Drug Abuse Grants DA12242, DA03194, and DA10156, National Institutes of Mental Health Grant MH53631, and National Institute of General Medical Sciences Grant GM48677. A.C.C. is supported, in part, by Research Scientist Award DA00197 from the National Institute on Drug Abuse.
- Abbreviations:
- nAChR
- nicotinic acetylcholine receptor
- CtxMII
- conotoxin MII
- Bgt
- bungarotoxin
- PEI
- polyethylenimine
- Yo-α-CtxMII
- N-terminal tyrosine-tagged α-conotoxin MII
- TFA
- trifluoroacetic acid
- SSC
- standard saline citrate
- Received October 18, 1999.
- Accepted February 3, 2000.
- The American Society for Pharmacology and Experimental Therapeutics
MolPharm articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|