Early exposure to methylphenidate increases fear responses in an aversive context in adult rats.

ABSTRACT

The effects of methylphenidate hydrochloride (MPH) administration during development on fear acquisition and retention in adulthood were examined using classical fear conditioning. Male Sprague-Dawley rats were administered MPH (2 mg/kg) or saline twice daily from postnatal day (PD) 25 to 39, and were trained and tested on PD 81 and 82. During training, shock unconditioned stimulus (US) presentations were explicitly paired with an auditory conditioned stimulus (CS) or occurred unsignaled in the training context. No effect of MPH treatment was found during fear acquisition when shock was signaled or unsignaled during training, but 24 h retention tests in the training context revealed enhanced fear responses in MPH-treated animals that received unsignaled training. These results support recent reports of enhanced anxiety-like behaviors in adult rats caused by early developmental MPH treatment and highlight the need for further research into the long-term effects of developmental exposure to stimulants aimed at pediatric populations.