Chronic fluoxetine modulates perineuronal nets and inhibitory neuronal function in relation to behavioral outcomes

Chronic fluoxetine administration has been reported to enhance neural plasticity in the adult brain, but the underlying structural correlates remain incompletely understood. In particular, the impact of fluoxetine on aggrecan-positive perineuronal nets (PNNs)—critical regulators of plasticity—is largely unknown. We investigated whether chronic fluoxetine treatment (20 mg/kg/day, i.p., 21 days) alters behavior and PNN expression in adult male C57BL/6 N mice. Behavioral assessments included grip strength, hot plate, light/dark transition, elevated plus-maze, open field, Y-maze, social interaction, tail suspension, Porsolt forced swim, and passive avoidance tests. To evaluate structural plasticity, we performed immunohistochemical analyses of parvalbumin (PV)-positive neurons and PNNs using aggrecan-specific antibodies (Cat-315, AB1031) in the primary somatosensory cortex and hippocampus. Fluoxetine-treated mice exhibited increased exploration and reduced anxiety-like behavior in light/dark transition and elevated plus-maze tests, with no significant changes in grip strength or nociception. They also showed increased locomotor activity (distance and entries) in the Y-maze, but did not significantly alter spontaneous alternation performance, and increased social interaction, the latter possibly reflecting abnormal social behavior. No significant differences were observed in depression-like behavior as assessed by tail suspension and forced swim tests. Passive avoidance performance was impaired, suggesting a decline in cognitive function. Immunohistochemical analysis revealed a significant reduction in aggrecan-positive PNN density, particularly in layer 4 of the primary somatosensory cortex and in the CA1 and CA3 regions of the hippocampus. Our results indicate that chronic fluoxetine administration induces behavioral and structural changes indicative of reactivated neuroplasticity. Importantly, these changes are associated with a region- and layer-specific reduction of aggrecan-positive PNNs, underscoring their critical role in modulating adult cortical plasticity.