Adenosine receptor antagonists improve short-term object-recognition ability of spontaneously hypertensive rats: a rodent model of attention-deficit hyperactivity disorder

The strain of spontaneously hypertensive rats (SHR) is considered a genetic model for the study of attention-deficit hyperactivity disorder (ADHD), as it displays hyperactivity, impulsivity and poorly sustained attention. Recently, we have shown the involvement of adenosinergic neuromodulation in the SHR's short-term and long-term memory impairments. In this study, we investigated the performance of male and female SHR in a modified version of the object-recognition task (using objects with different structural complexity) and compared them with Wistar rats, a widely used outbred rat strain for the investigation of learning processes. The suitability of the SHR strain to represent an animal model of ADHD, as far as mnemonic deficits are concerned, was pharmacologically validated by the administration of methylphenidate, the first-choice drug for the treatment of ADHD patients. The role of adenosine A(1) and A(2A) receptors in object discrimination was investigated by the administration of caffeine (nonselective antagonist) or selective adenosine receptor antagonists. Wistar rats discriminated all the objects used (cube vs. pyramid; cube vs. T-shaped object), whereas SHR only discriminated the most structurally distinct pairs of objects (cube vs. pyramid). Pretraining administration of methylphenidate [2 mg/kg, intraperitoneal (i.p.)], caffeine (1-10 mg/kg, i.p.), the selective adenosine receptor antagonists DPCPX (8-cyclopenthyl-1,3-dipropylxanthine; A(1) antagonist, 5 mg/kg, i.p.) and ZM241385 (A(2A) antagonist, 1.0 mg/kg, i.p.), or the association of ineffective doses of DPCPX (3 mg/kg) and ZM241385 (0.5 mg/kg), improved the performance of SHR in the object-recognition task. These findings show that the discriminative learning impairments of SHR can be attenuated by the blockade of either A(1) or A(2A) adenosine receptors, suggesting that adenosinergic antagonists might represent potentially interesting drugs for the treatment of ADHD. Behavioural Pharmacology 20:134-145 (C) 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins.