Differential Effects of Methylphenidate on Locus Coeruleus Neurons and Behavior Across Developmental Stages

The increasing use of Methylphenidate (MPD) among adolescents and adults raises important concerns regarding its developmental and long-term neurophysiological effects. This study examined behavioral and neuronal responses in the Locus Coeruleus (LC)—a critical hub for Norepinephrine (NE) signaling—following administration of various MPD doses in adolescent and adult rats. Four experimental groups were used for each age cohort: saline, 0.6, 2.5, and 10.0 mg/kg MPD. Animals received six consecutive daily MPD injections from Experimental Days (ED) 1 to 6, followed by a threeday washout period and a single rechallenge dose on ED10. A total of 174 adolescent and 157 adult rats were used for concurrent behavioral assessments, while LC neuronal activity was recorded from 409 and 461 neurons, respectively. Significant age-dependent differences emerged in both acute and chronic responses to MPD. As the MPD dose increased, a greater number of rats and LC neurons exhibited altered activity. Notably, some animals displayed behavioral and neuronal sensitization at all MPD doses, whereas others exhibited tolerance. These effects varied significantly between age and MPD groups in both magnitude and incidence. Furthermore, Baseline (BL) LC activity following the six-day MPD regimen and after the washout period (ED10) was markedly altered, particularly in adolescents, indicating withdrawal-related changes. These shifts in BL activity from ED1 to ED10 indicate the presence of withdrawal behavior. This study highlights clear age-dependent differences in response to chronic MPD exposure. The observed patterns of sensitization, tolerance, and withdrawal serve as potential experimental biomarkers for drug dependence, emphasizing the clinical relevance of these findings. Importantly, the results underscore the value of combining electrophysiological and behavioral analyses and reaffirm the central role of the LC and NE signaling in mediating MPD’s effects, with distinct implications for adolescent and adult populations