The effects of benzamide analogues on cocaine self-administration in rhesus monkeys.
BACKGROUND
Based on the differential distribution of dopamine (DA) D(3) receptors in mesolimbic regions relative to nigrostriatal regions, the hypothesis was that D(3)-selective antagonists (i.e., higher affinity at D(3)- than D(2)-receptors) would be more potent than D(2)-selective antagonists at decreasing total cocaine intake relative to disrupting rates of responding.


OBJECTIVE
To evaluate the effects of acute administration of seven DA antagonists with varying affinities for D(2) and D(3) receptors in monkeys self-administering cocaine.


METHODS
Rhesus monkeys were trained to self-administer intravenous cocaine (0.01-0.3 mg/kg per injection) under a fixed-interval (FI) 5-min schedule during daily 4-h sessions. The use of a FI schedule allowed for independent assessment of rate effects and changes in reinforcement frequency as a consequence of drug pretreatments. The compounds examined, in order of D(3) binding affinity, were: 2,3-dimethoxy-N-(9-p-fluorobenzyl)-azabicyclo[3.3. 1]nonan-3beta-yl benzamide (MABN) = eticlopride = 5-bromo- 2, 3-dimethoxy-N-[1-(4-fluorobenzyl)piperidin-4-yl]benz-amide (BBP) > spiperone > fluoroclebopride (FCP) > 2, 3-dimethoxy-N-(p-fluorobenzyl)piperdin-4-yl benzamide (MBP) > haloperidol.


RESULTS
In the absence of any pretreatments, cocaine-maintained responding varied as a function of dose and was characterized as an inverted U-shaped function, while cocaine intake increased in a dose-related fashion. When the dose of cocaine that maintained peak rates was available, all DA antagonists decreased response rates and cocaine intake in a dose- dependent manner. Increases in cocaine dose attenuated the effects of the DA antagonists, resulting in rightward shifts of the cocaine dose-response curves. Based on the ratio of behavioral potency at decreasing response rates relative to intake (ED(50) rate/ED(50) intake) when the highest cocaine dose was available, the order of potency and ED(50) ratio values were: MABN (2.5) > eticlopride (1. 63) > BBP = spiperone (1.5) > FCP (1.35) > MBP = haloperidol (0.89). This order parallels each compound's affinity at D(3) receptors (r(2)=0.84) to a greater degree than D(2) receptor affinity (r(2)=0. 34).


CONCLUSIONS
These results, using a FI schedule of cocaine self-administration, suggest that D(3) receptor antagonists are more likely to selectively decrease intake relative to response rates than D(2) receptor antagonists.
Nader
M A
MA
Center for the Neurobiological Investigation of Drug Abuse, Department of Physiology and Pharmacology, Wake Forest University School of Medicine, Medical Center Boulevard, Winston-Salem, NC 27157-1083, USA. mnader@wfubmc.edu
Green
K L
KL
Luedtke
R R
RR
Mach
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RH
D000818:Animals; D001549:Benzamides / Q000494:pharmacology*; D003042:Cocaine / Q000008:administration & dosage*; D018492:Dopamine Antagonists / Q000378:metabolism / Q000494:pharmacology*; D018765:Dopamine Uptake Inhibitors / Q000008:administration & dosage*; D008253:Macaca mulatta; D008297:Male; D017448:Receptors, Dopamine D2 / Q000187:drug effects*; D050637:Receptors, Dopamine D3; D012055:Reinforcement Schedule; D012646:Self Administration
D001549:Benzamides; D018492:Dopamine Antagonists; D018765:Dopamine Uptake Inhibitors; D017448:Receptors, Dopamine D2; D050637:Receptors, Dopamine D3; C037689:benzamide; D003042:Cocaine