Serotonergic Regulation of Synaptic Dopamine Levels Mitigates L-DOPA-Induced Dyskinesia in a Mouse Model of Parkinson's Disease.

Background: The serotonin (5-HT) system can manipulate the processing of exogenous L-DOPA in the DA-denervated striatum, resulting in the modulation of L-DOPA-induced dyskinesia (LID). Objective: To characterize the effects of the serotonin precursor 5-hydroxy-tryptophan (5-HTP) or the serotonin transporter (SERT) inhibitor, Citalopram on L-DOPA-induced behavior, neurochemical signals, and underlying protein expressions in an animal model of Parkinson's disease. Methods: MitoPark (MP) mice at 20 weeks of age, subjected to a 14-day administration of L-DOPA/Carbidopa, displayed dyskinesia, referred to as LID. Subsequent investigations explored the effects of 5-HT-modifying agents, such as 5-HTP and Citalopram, on abnormal involuntary movements (AIMs), locomotor activity, neurochemical signals, serotonin transporter activity, and protein expression in the DA-denervated striatum of LID MP mice. Results: 5-HTP exhibited duration-dependent suppressive effects on developing and established LID, especially related to abnormal limb movements observed in L-DOPA-primed MP mice. However, Citalopram, predominantly suppressed abnormal axial movement induced by L-DOPA in LID MP mice. We demonstrated that 5-HTP could decrease L-DOPA-upregulation of DA turnover rates while concurrently upregulating 5-HT metabolism. Additionally, 5-HTP was shown to reduce the expressions of p-ERK and p-DARPP-32 in the striatum of LID MP mice. The effect of Citalopram in alleviating LID development may be attributed to downregulation of SERT activity in the dorsal striatum of LID MP mice. Conclusions: While both single injection of 5-HTP and Citalopram effectively mitigated the development of LID, the difference in mitigation of AIM subtypes may be linked to the unique effects of these two serotonergic agents on L-DOPA-derived DA and 5-HT metabolism.