Current Neurovascular Research, 2009, 6, 259-266 259 1567-2026/09 $55.00+.00 © 2009 Bentham Science Publishers Ltd. Chronic Methylphenidate-Effects Over Circadian Cycle of Young and Adult Rats Submitted to Open-Field and Object Recognition Tests Karin M. Gomes 1 , Renan P. Souza 2 , Samira S. Valvassori 1 , Gislaine Z. Réus 1 , Cecília G. Inácio 1 , Márcio R. Martins 1 , Clarissa M. Comim 1 and João Quevedo *,1 1 Laboratório de Neurociências and Instituto Nacional de Ciência e Tecnologia Translacional em Medicina, Programa de Pós-Graduação em Ciências da Saúde, Unidade Acadêmica de Ciências da Saúde, Universidade do Extremo Sul Catarinense, 88806-000 Criciúma, SC, Brazil 2 Neurogenetics Section, Neuroscience Department, Centre for Addiction and Mental Health, M5T158 Toronto, ON, Canada Abstract: In this study age-, circadian rhythm- and methylphenidate administration- effect on open field habituation and object recognition were analyzed. Young and adult male Wistar rats were treated with saline or methylphenidate 2.0mg/kg for 28 days. Experiments were performed during the light and the dark cycle. Locomotor activity was significantly altered by circadian cycle and methylphenidate treatment during the training session and by drug treatment during the testing session. Exploratory activity was significantly modulated by age during the training session and by age and drug treatment during the testing session. Object recognition memory was altered by cycle at the training session; by age 1.5 h later and by cycle and age 24 h after the training session. These results show that methylphenidate treatment was the major modulator factor on open-field test while cycle and age had an important effect on object recognition experiment. Keywords: Memory, methylphenidate, locomotor activity, aging, rat behavior, circadian rhythm. INTRODUCTION Attention-deficit/hyperactivity disorder is the most com- monly diagnosed neuropsychiatry disorder in childhood, characterized by excessive levels of inattentiveness, impulsi- vity, and hyperactivity. Most of attention-deficit/hyperacti- vity disorder (ADHD) patients benefit from treatment with methylphenidate hydrochloride (MPH), irrespective of the etiology of the disorder, effectively reducing symptoms in up to 70% of children patients (Wilens, 2008). However, recog- nition that ADHD persists into adulthood has led to an increased use of MPD in adult patients (Quinn et al., 2008). Studies in humans suggest that blockade of the dopamine transporter (DAT) produces the therapeutic effect of MPD by increasing concentrations of dopamine in the central nervous system (Volkow et al., 1998, 2002). Pharmacologic properties of MPH have been well characterized in several preclinical studies: MPH has been found to improve spatial, working memory, response inhibition, set shifting, and other prefrontal cognitive functions in children and adults with attention-deficit/hyperactivity disorder, besides the effects on attention and impulsivity (Peterson et al., 2008). However, its mechanism of action is not completely understood (Souza et al., 2008). Biological rhythms, particularly circadian rhythm, are an essential part of the temporal order of living systems and their disruption has been related with some neuropsychiatric *Address correspondence to this author at the Laboratório de Neurociências, PPGCS, UNASAU, Universidade do Extremo Sul Catarinense, 88806-000 Criciúma, SC, Brazil; Fax: #55 48 3443 4817; E-mail: quevedo@unesc.net Received: July 6, 2009, Revised: August 07, 2009, Accepted: August 25, 2009 disorders. Age related changes have been documented in endocrine, metabolic and behavioral parameters in a variety of animal species, including humans. It has been shown that MPH and other stimulants effects are modulated by age and circadian rhythms (Gaytan et al., 1996). One of the most fundamental behaviors an organism exhibits is exploration of its environment. Exploration includes both spatial explora- tion and investigation of specific stimuli and is the sum of motor and locomotor activity, emotional drives (motivation, anxiety, and fear), and cognitive capacity (Barnett, 1958). Simple measures of total activity demonstrate that infant rats are generally more active in the dark than in the light phase of the daily cycle (Norton et al., 1975), have considerable locomotor capacity (Moorcroft et al., 1971) and are capable of rearing (Campbell and Mabry, 1973). As age advances, it may be expected that the animal behavior performance decline gradually (Peng et al., 1994). Measures interpreted as anxiety in rodents include certain uses of the open field (locomotor - crossings - and exploratory - rearings - activity. This method is frequently used in adults but is rarely used in young rats (Smith and Morrell 2007). Object recognition task has been increasingly used in recent years as a model for investigation of the neurobiolo- gical mechanisms of learning and memory, especially recognition memory that confers the ability to discriminate between novel and familiar entities (Winters et al., 2008). Originally developed by Ennaceur and Delacour (1988), this task can study in a relatively “pure” manner the recognition memory without the potential complications of interpretation introduced by, for example, extensive training phases of rule acquisition, or motivational considerations, because no explicit reinforcement or aversive stimulation is used during training (Winters et al., 2008). This is particularly advanta-