Metabotropic glutamate 5 receptor blockade may attenuate cocaine self-administration by decreasing brain reward function in rats.
BACKGROUND
Evidence is accumulating that metabotropic glutamate 5 (mGlu5) receptors play an important role in regulating the reinforcing actions of drugs of abuse.


OBJECTIVE
We examined the effects of the mGlu5 receptor antagonist MPEP on cocaine consumption and cocaine-enhanced brain reward function in rats.


METHODS
Cocaine consumption was measured in rats with 1 h (short-access; ShA) or 6 h (long-access; LgA) daily access to intravenous cocaine (0.25 mg/infusion) self-administration. Cocaine-enhanced brain reward function was measured by cocaine-induced lowering of intracranial self-stimulation (ICSS) thresholds.


RESULTS
Cocaine consumption remained stable and unaltered over successive self-administration sessions in ShA rats. In contrast, cocaine consumption progressively "escalated" in LgA rats. MPEP (1--9 mg/kg) dose-dependently decreased responding similarly in ShA and LgA rats. These data suggest that mGlu5 receptors regulate the reinforcing properties of cocaine, and that this action of mGlu5 receptors is independent of the escalation in consumption associated with extended access to cocaine self-administration. MPEP doses (1--9 mg/kg) that decreased cocaine self-administration elevated brain reward thresholds to a similar degree in cocaine- and vehicle-treated rats, indicating that MPEP induced a negative affective state. The additive effects of MPEP and cocaine on thresholds resulted in attenuation of the magnitude of cocaine-induced (10 mg/kg) lowering of ICSS thresholds.


CONCLUSIONS
Overall, mGlu5 receptors appear to play an important role in regulating cocaine consumption, and also in regulating brain reward function. Further, it is likely that blockade of mGlu5 receptors may attenuate cocaine consumption, at least in part, by decreasing the baseline activity of brain reward circuitries.
Kenny
Paul J
PJ
Department of Neuropharmacology, CVN-7, The Scripps Research Institute, La Jolla, CA 92037, USA.
Boutrel
Benjamin
B
Gasparini
Fabrizio
F
Koob
George F
GF
Markou
Athina
A
D000818:Animals; D001921:Brain / Q000187:drug effects*; D002755:Choice Behavior; D003042:Cocaine / Q000008:administration & dosage*; D003213:Conditioning, Psychological / Q000187:drug effects; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology*; D008297:Male; D011725:Pyridines / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D064529:Receptor, Metabotropic Glutamate 5; D018094:Receptors, Metabotropic Glutamate / Q000037:antagonists & inhibitors*; D012201:Reward; D012646:Self Administration; D012653:Self Stimulation
D018691:Excitatory Amino Acid Antagonists; D011725:Pyridines; D064529:Receptor, Metabotropic Glutamate 5; D018094:Receptors, Metabotropic Glutamate; C121465:6-methyl-2-(phenylethynyl)pyridine; D003042:Cocaine