Working memory deficits and altered prefrontal-hippocampal activation in p35 knockout mice with ADHD-like phenotypes: influence of sex and acute monoaminergic treatments.

Attention-Deficit/Hyperactivity Disorder (ADHD) is a prevalent neurodevelopmental condition characterized by persistent deficits in working memory (WM) and executive control. Dysregulation of the Cyclin-dependent kinase 5 (Cdk5)/p35 signaling pathway has been implicated in ADHD pathophysiology due to its impact on neuronal connectivity and dopamine regulation. Using p35 knockout (p35KO) mice-an animal model exhibiting ADHD-like phenotypes-we investigated WM performance, task-related c-Fos immunoreactivity (c-Fos-IR) expression under basal conditions as a function of sex and genotype, as well as WM responses to acute treatment with methylphenidate (MPH) or fluoxetine (FLX), administered alone or in combination. Under basal conditions, p35KO mice exhibited significantly reduced spontaneous alternation in the Y-maze test compared with wild type (WT), whereas recognition memory remained intact. Analysis of c-Fos-IR expression, used as a marker of task-related neuronal activity, revealed region- and genotype-dependent differences, with effects of sex. p35KO animals showed reduced c-Fos-IR expression in prefrontal cortical regions and increased expression in hippocampal regions. In the prefrontal cortex, males generally displayed higher c-Fos-IR expression than females, whereas hippocampal increases were evident in p35KO mice of both sexes, with a female-specific increase in the dentate gyrus. Acute MPH or FLX treatment significantly increased spontaneous alternation in p35KO mice in a genotype-dependent manner, whereas this effect was not observed following combined treatment (MPH + FLX). In WT mice, treatment with MPH, FLX, or MPH + FLX reduced spontaneous alternation; this reduction appeared more pronounced in females, although no significant interaction involving sex was detected. Exploratory activity was increased in p35KO mice independently of treatment. These findings support a role for Cdk5/p35 signaling in the prefrontal-hippocampal activation patterns associated with WM performance and suggest that cognitive and pharmacological responses to monoaminergic treatments are shaped by neurobiological background, with potential sex-related differences, underscoring the importance of considering these factors in preclinical ADHD research.