Positive allosteric modulation of the human cannabinoid (CB) receptor by RTI-371, a selective inhibitor of the dopamine transporter.
OBJECTIVE
In our search for an indirect dopamine agonist as therapy for cocaine addiction, several selective inhibitors of the dopamine transporter (DAT), which are 3-phenyltropane analogues, were assayed for their effect on locomotor activity in mice. Interestingly, several of the compounds showed a poor correlation between stimulation of locomotion and DAT inhibition. One of the compounds, 3beta-(4-methylphenyl)-2beta-[3-(4-chlorophenyl)isoxazol-5-yl]tropane (RTI-371), was shown to cross the blood-brain barrier, by binding studies in vivo, and block cocaine-induced locomotor stimulation. As poor pharmacokinetics could not explain the behavioural effects of RTI-371, this compound was screened through our functional assays for activity at other CNS receptors. Initial screening identified RTI-371 as a positive allosteric modulator of the human CB(1) (hCB(1)) receptor.


METHODS
The effect of RTI-371 and other DAT-selective inhibitors on CP55940-stimulated calcium mobilization was characterized in a calcium mobilization-based functional assay for the hCB(1) receptor. Selected compounds were also characterized in a similar assay for human mu opioid receptor activation to assess the specificity of their effects.


RESULTS
RTI-371 and several other DAT-selective inhibitors with atypical actions on locomotor behaviour increased the efficacy of CP55940 in a concentration-dependent manner.


CONCLUSIONS
These results suggest that the lack of correlation between the DAT-binding affinity and locomotor stimulation of RTI-371 could be due at least in part to its activity as a positive modulator of the hCB(1) receptor.
Navarro
Hernán A
HA
RTI International, Research Triangle Park, NC 27709-2194, USA. han@rti.org
Howard
James L
JL
Pollard
Gerald T
GT
Carroll
F Ivy
FI
D000494:Allosteric Regulation; D000818:Animals; D016466:CHO Cells; D002118:Calcium / Q000378:metabolism; D006224:Cricetinae; D003412:Cricetulus; D003511:Cyclohexanols / Q000494:pharmacology; D050483:Dopamine Plasma Membrane Transport Proteins / Q000037:antagonists & inhibitors*; D006801:Humans; D007555:Isoxazoles / Q000494:pharmacology*; D008297:Male; D051379:Mice; D009043:Motor Activity / Q000187:drug effects; D043884:Receptor, Cannabinoid, CB1 / Q000502:physiology*; D017450:Receptors, Opioid, mu / Q000819:agonists; D014326:Tropanes / Q000494:pharmacology*
C541713:3beta-(4-methylphenyl)-2beta-(3-(4-chlorophenyl)isoxazol-5-yl)tropane; D003511:Cyclohexanols; D050483:Dopamine Plasma Membrane Transport Proteins; D007555:Isoxazoles; D043884:Receptor, Cannabinoid, CB1; D017450:Receptors, Opioid, mu; D014326:Tropanes; C054649:3-(2-hydroxy-4-(1,1-dimethylheptyl)phenyl)-4-(3-hydroxypropyl)cyclohexanol; D002118:Calcium