Comparative Phenotypic Resolution of Spontaneous, D₂-Like and D₁-Like Agonist-Induced Orofacial Movement Topographies in Congenic Mutants with Dopamine D₂ vs. D₃ Receptor "Knockout"

Using a novel system, the role of D₂-like dopamine receptors in distinct topographies of orofacial movement was assessed in mutant mice with congenic D₂ vs. D₃ receptor knockout, and compared with findings in D_1A mutants. Under spontaneous conditions, D₂ mutants evidenced increased vertical jaw movements and unaltered horizontal jaw movements, with reductions in tongue protrusions and incisor chattering; in D₃ mutants, only incisor chattering was reduced. Given previous evidence that D_1A mutants show reduced horizontal but not vertical jaw movements, this indicates that apparent oppositional D ₁-like:D₂-like interactions in the regulation of composited jaw movements may in fact reflect the independent actions of D ₂ receptors to inhibit vertical jaw movements and of D_1A receptors to facilitate horizontal jaw movements. Effects of the D ₂-like agonist RU 24213 to exert greater reduction in horizontal than in vertical jaw movements were not altered prominently in either D ₂ or D₃ mutants. The D₁-like agonists A 68930 and SK&F 83959 induced vertical jaw movements, tongue protrusions, and incisor chattering; induction of tongue protrusions by A 68930 was reduced in D₂ mutants. D₂ receptors exert topographically specific regulation of orofacial movements in a manner distinct from their D _1A counterparts, while D₃ receptors exert only minor regulation of such movements.