Serotonin drives striatal synaptic plasticity in a sex-related manner

Introduction: Plasticity at corticostriatal synapses is a key substrate for a variety of brain functions - including motor control, learning and reward processing - and is often disrupted in disease conditions. Despite intense research pointing toward a dynamic interplay between glutamate, dopamine (DA), and serotonin (5-HT) neurotransmission, their precise circuit and synaptic mechanisms regulating their role in striatal plasticity are still unclear. Here, we analyze the role of serotonergic raphe-striatal innervation in the regulation of DA-dependent corticostriatal plasticity. Methods: Mice (males and females, 2-6 months of age) were housed in standard plexiglass cages at constant temperature (22 +/- 1 degrees C) and maintained on a 12/12 h light/dark cycle with food and demineralized water ad libitum. In the present study, we used a knock-in mouse line in which the green fluorescent protein reporter gene (GFP) replaced the I Tph2 exon (Tph2(GFP) mice), allowing selective expression of GFP in the whole 5-HT system, highlighting both somata and neuritis of serotonergic neurons. Heterozygous, Tph2(+/)(GFP), mice were intercrossed to obtain experimental cohorts, which included Wild-type (Tph2(+/+)), Heterozygous (Tph2(+/)(GFP)), and Mutant serotonin-depleted (Tph2(GFP)(/)(GFP)) animals. Results: Using male and female mice, carrying on different Tph2 gene dosages, we show that Tph2 gene modulation results in sex-specific corticostriatal abnormalities, encompassing the abnormal amplitude of spontaneous glutamatergic transmission and the loss of Long Term Potentiation (LTP) in Tph2(GFP/GFP) mice of both sexes, while this form of plasticity is normally expressed in control mice (Tph2(+/+)). Once LTP is induced, only the Tph2(+/GFP) female mice present a loss of synaptic depotentiation. Conclusion: We showed a relevant role of the interaction between dopaminergic and serotonergic systems in controlling striatal synaptic plasticity. Overall, our data unveil that 5-HT plays a primary role in regulating DA-dependent corticostriatal plasticity in a sex-related manner and propose altered 5-HT levels as a critical determinant of disease-associated plasticity defects.