CONICET | Buscador de Institutos y Recursos Humanos

The dopamine D2 receptor (D2R) is expressed postsynaptically in most dopamine (DA) target areas where it participates in the extrapyramidal control of locomotor activity, spatio-temporal organization of goal-oriented behaviors and the reinforcing properties of natural rewards. Also, D2Rs are present in DA neurons where they act as autoreceptors controlling cell firing and DA release. Selective in vivo blockade or stimulation of D2 autoreceptors has been hampered by the fact that active compounds on these receptors also interact with those located on postsynaptic non-DA neurons. To circumvent this difficulty, we created mutant mice lacking D2 autoreceptors by cell-specific conditional gene targeting. Homozygous mutant mice carrying loxP sites flanking Drd2 exon 2 (Drd2flox/flox), overtly indistinguishable from Drd2+/+ mice, were crossed with knockin mice expressing Cre from the dopamine transporter gene Dat+/IresCre. An in situ hybridization analysis performed on compound Drd2flox/flox. Dat+/IresCre mice (autoDrd2-/-) showed a total loss of Drd2 expression within midbrain dopaminergic neurons while retaining expression in forebrain postsynaptic neurons and pituitary cells. DA release in the dorsal striatum of autoDrd2-/- mice showed a significant increase (3.13 ± 0.23 μM) in comparison to Drd2flox/flox control mice (1.97 ± 0.15 μM), as measured by fast cyclic voltammetry on brain slices. The D2R agonist quinpirole inhibited electrically stimulated DA release in Drd2flox/flox mice with an IC50 of 19 ± 0.3 nM whereas this drug had no effect in autoDrd2-/- mice. Basal DA clearance was identical in brain slices of autoDrd2-/- and Drd2flox/flox mice. In addition, the significant changes in DA uptake inhibition elicited by the DA uptake blockers methylphenidate (3 μM) or cocaine (10 μM) did not differ between genotypes. At the behavioral level, autoDrd2-/- mice displayed increased locomotor activity and were insensitive to low doses of the D2R agonist quinpirole. Motor coordination on a rotarod, approach/avoidance behavior on an elevated plus maze and conditioned place preference for cocaine were normal in autoDrd2-/- mice. Interestingly, autoDrd2-/- mice showed increased motivation for food reward in a progressive ratio operant test. Altogether, our results indicate that loss of presynaptic D2 autoreceptors enhances DA release without changing DA uptake and, as a consequence, alters locomotor and motivational behaviors.