Mutation of the alpha2A-adrenoceptor impairs working memory performance and annuls cognitive enhancement by guanfacine

Abstract

Norepinephrine strengthens the working memory, behavioral inhibition, and attentional functions of the prefrontal cortex through actions at postsynaptic alpha2-adrenoceptors (alpha2-AR). The alpha2-AR agonist guanfacine enhances prefrontal cortical functions in rats, monkeys, and human beings and ameliorates prefrontal cortical deficits in patients with attention deficit hyperactivity disorder. The present study examined the subtype of alpha2-AR underlying these beneficial effects. Because there are no selective alpha2A-AR, alpha2B-AR, or alpha2C-AR agonists or antagonists, genetically altered mice were used to identify the molecular target of the action of guanfacine. Mice with a point mutation of the alpha2A-AR, which serves as a functional knock-out, were compared with wild-type animals and with previously published studies of alpha2C-AR knock-out mice (Tanila et al., 1999). Mice were adapted to handling on a T maze and trained on either a spatial delayed alternation task that is sensitive to prefrontal cortical damage or a spatial discrimination control task with similar motor and motivational demands but no dependence on prefrontal cortex. The effects of guanfacine on performance of the delayed alternation task were assessed in additional groups of wild-type versus alpha2A-AR mutant mice. We observed that functional loss of the alpha2A-AR subtype, unlike knock-out of the alpha2C-AR subtype, weakened performance of the prefrontal cortical task without affecting learning and resulted in loss of the beneficial response to guanfacine. These data demonstrate the importance of alpha2A-AR subtype stimulation for the cognitive functions of the prefrontal cortex and identify the molecular substrate for guanfacine and novel therapeutic interventions.

Fig. 1.

A comparison of the performance of wild-type versus α_2A-AR mutant mice in the time needed to habituate to the test procedures and to learn the cognitive tasks. The graph illustrates the mean ± SEM days to criterion for habituation to handling in the T maze (A), spatial discrimination performance (reference memory, control task) (B), and spatial delayed alternation performance with 0 sec delays (working memory, PFC task) (C). Criterion for each condition is described in Materials and Methods.Solid bars, Results for wild-type (WT) mice; open bars, for α_2A-AR mutant mice (α_2A). α_2A-AR mutant mice took significantly longer to habituate to the testing conditions but subsequently learned both tasks at normal rates.**Significantly different from wild-type animals,p < 0.002, n = 6.

Fig. 2.

Comparison of the abilities of the wild-type versus α_2A-AR mutant mice on the delayed alternation task, a test of working memory, behavioral inhibition, and attention regulation. Results represent the mean ± SEM delay needed to achieve baseline performance of ∼70% correct after the 25th and 50th daily test sessions of the delayed alternation task. Solid bars, Results for wild-type mice; open bars, for α_2A-AR mutant mice. α_2A-AR mutant mice needed significantly lower delays than wild-type controls to perform at 70% correct. *Significantly different from wild-type animals, p < 0.03; **significantly different from wild-type animals, p < 0.00001;n = 12.

Fig. 3.

The effects of saline versus the α₂-AR agonist guanfacine (1.0 mg/kg) in wild-type mice (left) versus α_2A-AR mutant mice (right) performing the delayed alternation task, a test of working memory, behavioral inhibition, and attention regulation. Guanfacine improved performance in the wild-type mice but had no effect in the α_2A-AR mutant animals. Results represent mean ± SEM percentage correct on the delayed alternation task.*Significantly different from saline, p < 0.04; n = 12.