Dopamine D(1A) receptor-deficient mice were assessed in a wide variety of tasks chosen to reflect the diverse roles of this receptor subtype in behavioural regulation. The protocol included examination of exploration and locomotor activity in an open field, a test of sensorimotor orienting, both place and cue learning in the Morris water maze, and assessment of simple associative learning in an olfactory discrimination task. Homozygous mice showed broad-based impairments that were characterized by deficiencies in initiating movement and/or reactivity to external stimuli. Data obtained from flash evoked potentials indicated that these deficits did not reflect gross visual impairments. The partial reduction in D(1A) receptors in the heterozygous mice did not affect performance in most tasks, although circumscribed deficits in some tasks were observed (e.g., failure to develop a reliable spatial bias in the water maze). These findings extend previous behavioural studies of null mutant mice lacking D(1A) receptors and provide additional support for the idea that the D(1A) receptor participates in a wide variety of behavioural functions. The selective impairments of heterozygous mice in a spatial learning task suggest that the hippocampal/cortical dopaminergic system may be uniquely vulnerable to the partial loss of the D(1A) receptor.
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