5-Hydroxytryptophan Reduces Levodopa-Induced Dyskinesia via Regulating AKT/mTOR/S6K and CREB/ΔFosB Signals in a Mouse Model of Parkinson's Disease

Abstract

Long-term administration of levodopa (L-DOPA) to patients with Parkinson's disease (PD) commonly results in involuntary dyskinetic movements, as is known for L-DOPA-induced dyskinesia (LID). 5-Hydroxytryptophan (5-HTP) has recently been shown to alleviate LID; however, no biochemical alterations to aberrant excitatory conditions have been revealed yet. In the present study, we aimed to confirm its anti-dyskinetic effect and to discover the unknown molecular mechanisms of action of 5-HTP in LID. We made an LID-induced mouse model through chronic L-DOPA treatment to 6-hydroxydopamine-induced hemi-parkinsonian mice and then administered 5-HTP 60 mg/kg for 15 days orally to LID-induced mice. In addition, we performed behavioral tests and analyzed the histological alterations in the lesioned part of the striatum (ST). Our results showed that 5-HTP significantly suppressed all types of dyskinetic movements (axial, limb, orolingual and locomotive) and its effects were similar to those of amantadine, the only approved drug by Food and Drug Administration. Moreover, 5-HTP did not affect the efficacy of L-DOPA on PD motor manifestations. From a molecular perspective, 5-HTP treatment significantly decreased phosphorylated CREB and ΔFosB expression, commonly known as downstream factors, increased in LID conditions. Furthermore, we found that the effects of 5-HTP were not mediated by dopamine1 receptor (D1)/DARPP32/ERK signaling, but regulated by AKT/mTOR/S6K signaling, which showed different mechanisms with amantadine in the denervated ST. Taken together, 5-HTP alleviates LID by regulating the hyperactivated striatal AKT/mTOR/S6K and CREB/ΔFosB signaling.

Keywords: 5-Hydroxytryptophan; Levodopa; Levodopa-induced dyskinesia; Parkinson’s disease; Serotonin.

Fig. 1

Anti-dyskinetic effect of 5-HTP on LID. Experimental schedule (A), AIMs score during 180 min as soon as the start of L-DOPA administration (B). Sum of the 4 AIMs subtypes scores (axial, limb, orolingual and locomotive) during 180min (C). AUC of total AIMs score graph over time (D). Data were analyzed by two-way ANOVA, followed by Dunnett’s post hoc test (B) or one-way ANOVA, followed by Dunnett’s post hoc test (C, D). ^###p<0.001 compared with the PD group; ***p<0.001 compared with the LID group. Values were presented as means ± of SEM. PD: 6-OHDA-lesioned mouse (n=6), LID: 6-OHDA-lesioned mouse treated with vehicle+L-DOPA (n=6), 5-HTP: 6-OHDA-lesioned mouse treated with 5-HTP+L-DOPA (n=6), AMAN: 6-OHDA-lesioned mouse treated with AMAN+L-DOPA (n=5).

Fig. 2

Effect of 5-HTP on AIMs of each subtype (axial; A, limb; B, orolingual; C and locomotive; D). Data were analyzed by one-way ANOVA, followed by Dunnett’s post hoc test. ^###p<0.001 and ^#p<0.05 compared with the PD group; ***p<0.001, **p<0.01 and *p<0.05 compared with the LID group. Values were presented as means ± of SEM. PD: 6-OHDA-lesioned mouse (n=6), LID: 6-OHDA-lesioned mouse treated with vehicle+L-DOPA (n=6), 5-HTP: 6-OHDA-lesioned mouse treated with 5-HTP+L-DOPA (n=6), AMAN: 6-OHDA-lesioned mouse treated with AMAN+L-DOPA (n=5).

Fig. 3

Effect of 5-HTP on the L-DOPA motor efficacy. The 5-HTP administration was done 1 h before L-DOPA treatment. After 1 h of L-DOPA treatment, the test was performed. Data were analyzed by one-way ANOVA, followed by Dunnett’s post hoc test. ^##p<0.01 compared with the SHAM group and *p<0.05 compared with the PD group. Values were presented as means ± of SEM. SHAM: vehicle-lesioned mouse (n=7), PD: 6-OHDA-lesioned mouse (n=6), LID: 6-OHDA-lesioned mouse treated with vehicle+L-DOPA (n=6), 5-HTP: 6-OHDA-lesioned mouse treated with 5-HTP+L-DOPA (n=6), AMAN: 6-OHDA-lesioned mouse treated with AMAN+L-DOPA (n=5).

Fig. 4

Molecular mechanisms of 5-HTP on the phosphorylated CREB, ΔFosB and D1/DARPP32/ERK signaling in the ST. Expressions of P-CREB^S133/CREB (A), ΔFosB/GAPDH (B), P-DARPP32^T34/DARPP32 (C) and P-ERK^1/2/GADPH (D) in the lesioned ST were quantified as compared to the SHAM group. Data were analyzed by one-way ANOVA, followed by Dunnett’s post hoc test. ^#p<0.05 compared with the PD group; **p<0.01 and *p<0.05 compared with the LID group. Values were presented as means ± of SEM. SHAM: vehicle-lesioned mouse (n=7), PD: 6-OHDA-lesioned mouse (n=6), LID: 6-OHDA-lesioned mouse treated with vehicle+L-DOPA (n=6), 5-HTP: 6-OHDA-lesioned mouse treated with 5-HTP+L-DOPA (n=6), AMAN: 6-OHDA-lesioned mouse treated with AMAN+L-DOPA (n=5).

Fig. 5

Molecular mechanisms of 5-HTP on the AKT/mTOR/S6K signaling in the ST. Expressions of P-AKT^S473/AKT (A), P-mTOR^S2481/mTOR (B) and P-S6K^T389/AKT (C) in the lesioned ST were quantified as compared to the SHAM group. Data were analyzed by one-way ANOVA, followed by Dunnett’s post hoc test. ^##p<0.01 and ^#p<0.05 compared with the PD group; **p<0.01 and *p<0.05 compared with the LID group. Values were presented as means ± of SEM. SHAM: vehicle-lesioned mouse (n=7), PD: 6-OHDA-lesioned mouse (n=6), LID: 6-OHDA-lesioned mouse treated with vehicle+L-DOPA (n=6), 5-HTP: 6-OHDA-lesioned mouse treated with 5-HTP+L-DOPA (n=6), AMAN: 6-OHDA-lesioned mouse treated with AMAN+L-DOPA (n=5).

Fig. 6

Scatter plots of correlation between biomolecular expression by 5-HTP and severity of dyskinesia. The P-CREB/CREB (A), ΔFosB/GAPDH (B), P-AKT/AKT (C), P-mTOR/mTOR (D) and P-S6K/S6K (E) expressions were used for correlation analysis. Data were analyzed by Pearson’s correlation coefficient.