Chronic Δ9-Tetrahydrocannabinol Exposure Induces a Sensitization of Dopamine D2/3 Receptors in the Mesoaccumbens and Nigrostriatal Systems

Delta(9)-Tetrahydrocannabinol (THC), through its action on cannabinoid type-1 receptor (CB1R), is known to activate dopamine (DA) neurotransmission. Functional evidence of a direct antagonistic interaction between CB1R and DA D-2-receptors (D2R) suggests that D2R may be an important target for the modulation of DA neurotransmission by THC. The current study evaluated, in rodents, the effects of chronic exposure to THC (1 mg/kg/day; 21 days) on D2R and D3R availabilities using the D2R-prefering antagonist and the D3R-preferring agonist radiotracers [F-18] fallypride and [H-3]-(+)-PHNO, respectively. At 24 h after the last THC dose, D2R and D3R densities were significantly increased in midbrain. In caudate/ putamen (CPu), THC exposure was associated with increased densities of D2R with no change in D2R mRNA expression, whereas in nucleus accumbens (NAcc) both D3R binding and mRNA levels were upregulated. These receptor changes, which were completely reversed in CPu but only partially reversed in NAcc and midbrain at 1 week after THC cessation, correlated with an increased functionality of D2/3R in vivo, based on findings of increased locomotor suppressive effect of a presynaptic dose and enhanced locomotor activation produced by a postsynaptic dose of quinpirole. Concomitantly, the observations of a decreased gene expression of tyrosine hydroxylase in midbrain together with a blunted psychomotor response to amphetamine concurred to indicate a diminished presynaptic DA function following THC. These findings indicate that the early period following THC treatment cessation is associated with altered presynaptic D2/3R controlling DA synthesis and release in midbrain, with the concurrent development of postsynaptic D2/3R supersensitivity in NAcc and CPu. Such D2/3R neuroadaptations may contribute to the reinforcing and habit-forming properties of THC. Neuropsychopharmacology (2012) 37, 2355-2367; doi:10.1038/npp.2012.91; published online 13 June 2012