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 A1 and A2A 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; A1 antagonist, 5 mg/kg, i.p.) and ZM241385 (A2A 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 A1 or A2A adenosine receptors, suggesting that adenosinergic antagonists might represent potentially interesting drugs for the treatment of ADHD.

Chronic caffeine treatment during prepubertal period confers long-term cognitive benefits in adult spontaneously hypertensive rats (SHR), an animal model of attention deficit hyperactivity disorder (ADHD).

The spontaneously hypertensive rat (SHR) is frequently used as an experimental model for the study of attention deficit hyperactivity disorder (ADHD) since it displays behavioural and neurochemical features of ADHD. Increasing evidence suggests that caffeine might represent an important therapeutic tool for the treatment of ADHD and we recently demonstrated that the acute administration of caffeine improves several learning and memory impairments in adult SHR rats. Here we further evaluated the potential of caffeine in ADHD therapy. Female Wistar (WIS) and SHR rats were treated with caffeine (3mg/kg, i.p.) or methylphenidate (MPD, 2mg/kg, i.p.) for 14 consecutive days during the prepubertal period (post-natal days 25-38) and they were tested later in adulthood in the object-recognition task. WIS rats discriminated all the objects used, whereas SHR were not able to discriminate pairs of objects with subtle structural differences. Chronic treatment with caffeine or MPD improved the object-recognition deficits in SHR rats. Surprisingly, these treatments impaired the short-term object-recognition ability in adult WIS rats. The present drug effects are independent of changes in locomotor activity, arterial blood pressure and body weight in both rat strains. These findings suggest that chronic caffeine treatment during prepubertal period confers long-term cognitive benefits in discriminative learning impairments of SHR, suggesting caffeine as an alternative therapeutic strategy for the early management of ADHD symptoms. Nevertheless, our results also emphasize the importance of a correct diagnosis and the caution in the use of stimulant drugs such as caffeine and MPD during neurodevelopment since they can disrupt discriminative learning in non-ADHD phenotypes.