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.

Research indicates that norepinephrine enhances the working memory functions of the prefrontal cortex (PFC) through actions at post-synaptic, alpha-2A adrenoceptors.The current study examined the effects of the alpha-2A adrenoreceptor agonist, guanfacine (0.7 mg/kg, i.m.), compared to saline on SPECT measures of regional cerebral blood flow (rCBF) Evidence from a variety of disciplines indicates that norepinephrine (NE) has a critical beneficial influence on the working memory functions of the prefrontal cortex (PFC) through its actions at post-synaptic, alpha-2A adrenergic receptors (reviewed in Arnsten et al. 1996). In monkeys, the ability to perform spatial working memory tasks has been closely linked to the dorsolateral PFC (dlPFC) surrounding the principal sulcus, especially the caudal two thirds of the principal sulcal region (Goldman and Rosvold 1970;Goldman-Rakic 1987).Alpha-2 adrenergic agonists such as clonidine, guanfacine, and medetomidine can improve spatial working memory performance in monkeys (Arnsten et al. 1988), rats (Carlson et al. 1992), and humans (Coull et al. 1995;Jakala et al. 1999), but have little effect or impair tasks dependent on posterior cortices or hippocampus (reviewed in Arnsten 1998). Dose response curves are con- N EUROPSYCHOPHARMACOLOGY 2000 -VOL . 23 , NO . 3 Guanfacine Increases PFC Blood Flow in Monkeys 241 sistent with beneficial actions at post-synaptic alpha-2 receptors, as the drugs become more potent and more efficacious in animals and patients with NE depletion (Arnsten and Goldman-Rakic 1985;Cai et al. 1993;Franowicz and Arnsten 1999;McEntee and Mair 1990). However, very high doses of guanfacine have recently been found to significantly improve spatial working memory performance in normal adult monkeys as well . It is likely that the alpha-2A receptor subtype underlies guanfacine's beneficial effects on working memory function (Arnsten et al. 1988;Tanila et al. 1999), and guanfacine is currently the most selective alpha-2A agonist available (Uhlen and Wikberg 1991).Several findings suggest that alpha-2 agonists improve, while alpha-2 antagonists impair, working memory function through direct actions in the PFC. For example, infusion of the alpha-2 antagonist, yohimbine, into the monkey PFC produces a delay-related impairment in working memory, whereas alpha-1 and beta adrenergic antagonists are without effect (Li and Mei 1994). Similarly, electrophysiological studies show that iontophoretic application of yohimbine onto PFC neurons suppresses delay-related activity, the cellular measure of working memory Sawaguchi 1998). Conversely, iontophoretic or systemic administration of clonidine increases delay-related firing, and this enhancement is reversed by iontophoretic application of yohimbine . Similarly, infusion of the alpha-2 agonist medetomidine, into aged rat PFC improves spatial working memory performance (Tanila et al. 1996).Very few studies have taken advantage of imaging technologies to examine NE alpha-2 mechanisms in the cortex. Most human studies ha...

In aged monkeys with naturally occurring catecholamine depletion, alpha-2 adrenergic agonists such as guanfacine have repeatedly been shown to improve dorsolateral prefrontal cortical function, as assessed by the spatial delayed response task. Both low (0.0001-0.001 mg/kg) and high (0.5 mg/kg) but not intermediate (0.01-0.05 mg/kg) doses of guanfacine improve spatial working memory performance in aged animals. However, it is not known whether guanfacine would similarly improve performance in young animals. In the present study, the effects of guanfacine on delayed response performance were characterized in seven young adult rhesus monkeys. Low doses of guanfacine (0.0001-0.01 mg/kg) had no effect on task performance, while high doses of guanfacine (0.1-0.7 mg/kg) significantly improved task performance. The highest doses produced mild sedation that was independent of drug effects on delayed response. The most effective dose of guanfacine was challenged with the alpha-2 antagonist idazoxan (0.1 mg/kg). This dose of idazoxan had no effect on task performance when given alone. Consistent with an alpha-2 mechanism, idazoxan significantly decreased delayed response performance in guanfacine-treated animals. These results support the hypothesis that delayed response performance in young intact animals can be improved through actions at alpha-2 adrenergic receptors.

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The identical pattern of dose-dependent reversal of cognitive improvement and hypotension indicates that, in non-human primates, the same receptor subtype mediates both effects. Previous evidence suggests that the most likely candidate is the alpha-2A receptor subtype.