D1-like dopamine receptor-mediated function in congenic mutants with D1 vs. D5 receptor "knockout". 
Current understanding of the functional roles of individual dopamine D1-like [D1, D5] and D2-like [D2L/s, D3, D4] receptor subtypes remains incomplete.  In particular, the lack of pharmacological agonists and antagonists able to distinguish between D1 and D5 receptors means that any differential roles in the regulation of behavior are poorly understood.  Mutant mice with targeted gene deletion ("knockout") of individual dopamine receptor subtypes offer an important alternative approach to resolving these functional roles.  In congenic D1 mutants examined ethologically, progressive increases in specific topographies of behavior over wildtypes were considerably greater than those in D1 mutants on a mixed genetic background; D1 knockout appears to influence the neuronal substrate(s) of habituation to disrupt sculpture of the changing topography of behavior from initial exploration through to quiescence.  Similarly, the D1 receptor appears to regulate specific topographies of orofacial movement in the mouse as these are "sculpted" in a time-dependent manner.  Although the well-recognized role of the D1-like family in regulating several aspects of behavioral topography has been assumed to involve primarily D1 receptors, this presumption may require modification to accommodate a subtle but not negligible role for their D5 counterparts as evidenced in the phenotype of congenic D5 mutants.