Dopamine-transporter occupancy after intravenous doses of cocaine and methylphenidate in mice and humans.
OBJECTIVE
Recent studies using positron emission tomography (PET) have established the relationship between an intravenous dose of cocaine and the percentage occupancy of the dopamine transporter in humans, and have documented the requirement of more than 50% occupancy for perception of the "high". The present experiments were conducted to examine dose-occupancy and dose-effect relationships in mice for cocaine and also for methylphenidate, a dopamine uptake blocker used in pediatric psychiatry.


METHODS
Percentage occupancies of the dopamine transporter by cocaine and methylphenidate were estimated after intravenous injection in mice from the displacement of in vivo binding of [(3)H]cocaine from the striatum. Locomotor activity was measured in a photocell apparatus.


RESULTS
The relationship between drug doses (milligrams of hydrochloride salt per kilogram body weight) and percentage occupancy of the dopamine transporter was indistinguishable for cocaine and methylphenidate, and corresponded to about 50% occupancy at 0.25 mg/kg and about 80% at 1 mg/kg. This was similar to the relationship between drug dose and transporter occupancy, previously measured in human and baboons using [(11)C]cocaine or [(11)C]d-threo-methylphenidate and PET. Methylphenidate increased locomotor activity in the mice substantially more than cocaine at the same dose and the same degree of dopamine-transporter receptor occupancy.


CONCLUSIONS
The range of dopamine-transporter occupancy required for behavioral activation in the mice was thus similar to that previously reported for experience of a cocaine- or methylphenidate-induced "high" in human subjects. Our results are consistent with other studies in which both cocaine and methylphenidate were evaluated in animal behavioral assays and were found to have very similar psychopharmacological properties.
Gatley
S J
SJ
Medical Department, Brookhaven National Laboratory, Upton, NY 11973, USA. gatley@bnl.gov
Volkow
N D
ND
Gifford
A N
AN
Fowler
J S
JS
Dewey
S L
SL
Ding
Y S
YS
Logan
J
J
D000818:Animals; D002352:Carrier Proteins / Q000187:drug effects* / Q000378:metabolism; D003042:Cocaine / Q000378:metabolism / Q000494:pharmacology*; D050483:Dopamine Plasma Membrane Transport Proteins; D018765:Dopamine Uptake Inhibitors / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D006801:Humans; D007275:Injections, Intravenous; D008297:Male; D008562:Membrane Glycoproteins; D026901:Membrane Transport Proteins; D008774:Methylphenidate / Q000378:metabolism / Q000494:pharmacology*; D051379:Mice; D009043:Motor Activity / Q000187:drug effects; D009419:Nerve Tissue Proteins; D010215:Papio; D013045:Species Specificity
D002352:Carrier Proteins; D050483:Dopamine Plasma Membrane Transport Proteins; D018765:Dopamine Uptake Inhibitors; D008562:Membrane Glycoproteins; D026901:Membrane Transport Proteins; D009419:Nerve Tissue Proteins; D008774:Methylphenidate; D003042:Cocaine