Repetitive methylphenidate administration modulates the diurnal behavioral activity pattern of adult female SD rats.

Diurnal rhythms influence many of the physiological processes that act to maintain homeostasis of the body in response to different environmental changes. Thus, disturbances in diurnal rhythms can lead to various physiological complications. Repeated exposure to psychostimulants may cause long-term effects by disturbing diurnal rhythms. The aim of the present study is to use the open field assay to determine whether repeated exposure to the psychostimulant methylphenidate (MPD) changes diurnal locomotor activity patterns of female adult Sprague-Dawley (SD) rats. As much as 31 female adult SD rats were divided into four groups. On experimental day (ED) 1, all groups were given an injection of saline. On ED 2-7, animals were injected once a day with either saline, or 0.6 mg/kg MPD, or 2.5 mg/kg MPD, or 10 mg/kg MPD depending on the group. On ED 8-10, no injections were given (washout period). On ED 11, animals were treated as they were on ED 2-7. Locomotor movements were recorded using a computerized animal activity monitoring system. The horizontal activity (HA), total distance traveled (TDT), and number of stereotypies (NOS) were analyzed by cosine curve statistical analysis (CCSA) test. The HA and TDT diurnal rhythm activity patterns of ED 2, 7, 8, and 11 were significantly different (p < 0.05) from the control recording of ED 1 according to the CCSA test. The observation obtained in this study suggests that repeated administration of MPD (all doses tested) is able to change diurnal locomotor patterns, which indicates that chronic MPD treatment exerts long-term effects.