Persistent behavioral sensitization to chronic L-DOPA requires A2A adenosine receptors

Abstract

To investigate the role of A(2A) adenosine receptors in adaptive responses to chronic intermittent dopamine receptor stimulation, we compared the behavioral sensitization elicited by repeated l-DOPA treatment in hemiparkinsonian wild-type (WT) and A(2A) adenosine receptor knock-out (A(2A) KO) mice. Although the unilateral nigrostriatal lesion produced by intrastriatal injection of 6-hydroxydopamine was indistinguishable between WT and A(2A) KO mice, they developed strikingly different patterns of behavioral sensitization after daily treatment with low doses of l-DOPA for 3 weeks. WT mice initially displayed modest contralateral rotational responses and then developed progressively greater responses that reached a maximum within 1 week and persisted for the duration of the treatment. In contrast, any rotational behavioral sensitization in A(2A) KO mice was transient and completely reversed within 2 weeks. Similarly, the time to reach the peak rotation was progressively shortened in WT mice but remained unchanged in A(2A) KO mice. Furthermore, daily l-DOPA treatment produced gradually sensitized grooming in WT mice but failed to induce any sensitized grooming in A(2A) KO mice. Finally, repeated l-DOPA treatment reversed the 6-OHDA-induced reduction of striatal dynorphin mRNA in WT but not A(2A) KO mice, raising the possibility that the A(2A) receptor may contribute to l-DOPA-induced behavioral sensitization by facilitating adaptations within the dynorphin-expressing striatonigral pathway. Together these results demonstrate that the A(2A) receptor plays a critical role in the development and particularly the persistence of behavioral sensitization to repeated l-DOPA treatment. Furthermore, they raise the possibility that the maladaptive dyskinetic responses to chronic l-DOPA treatment in Parkinson's disease may be attenuated by A(2A) receptor inactivation.

Fig. 1.

Effect of A_2A receptor deficiency on dose-dependent l-DOPA-induced behavioral sensitization in hemiparkinsonian mice. WT (black squares) and A_2A KO (gray circles) mice were treated with benserazide (2 mg/kg, i.p.) plus l-DOPA at intraperitoneal doses of 1.0 mg/kg (A;n = 10 per each genotype), 1.8 mg/kg (B;n = 10 WT mice andn = 12 KO mice), or 2.5 mg/kg (C;n = 10 per each genotype) once a day for 20 d. Contralateral rotational behavior was evaluated for the 1 hr test period immediately after the administration of l-DOPA on the indicated days. Data are expressed as mean ± SEM of the net contralateral rotations (contralateral minus ipsilateral turns). ForB, F_(8,160) = 2.92, *p < 0.01 compared with the corresponding KO value (split-plot ANOVA followed by Fisher's LSD comparison test); forC, F_(8,135)=2.32, *p < 0.05 compared with the corresponding KO value (split-plot ANOVA followed by Fisher's LSD comparison test).

Fig. 2.

Peak rotational response to l-DOPA occurs progressively earlier in hemiparkinsonian WT but not A_2A KO mice. A shows the initial, midway, and final time courses for turning induced by the daily intraperitoneal administration of 1.8 mg/kg l-DOPA in WT and A_2A KO mice, i.e., on the 1st (shaded circles), 11th (filled triangles), and 20th (shaded squares) day of 20 consecutive dailyl-DOPA treatments. B (replotted from the 10 min time points in A) depicts the progressive increase in contralateral rotations for the initial 10 min afterl-DOPA administration from the 1st to the 11th to the 20th day in WT mice (F_(2,40)=7.32; *p < 0.01; split-plot ANOVA followed by Fisher's LSD test) but not in A_2A KO mice.

Fig. 3.

Sensitization of DOPA-induced grooming behavior is attenuated in mice lacking adenosine A_2A receptors. Grooming activity, defined as maximal grooming score recorded over the 1 hr testing period, was determined in WT (black squares) and A_2A KO (gray circles) animals receiving an intraperitoneal dose of 1.0 mg/kg (A) or 1.8 mg/kg (B)l-DOPA. Data are expressed as mean ± SEM of 10–12 animals. *p < 0.05; compared with the corresponding KO value; Kruskal–Wallis test followed by Mann–WhitneyU test.

Fig. 4.

Daily l-DOPA reverses the reduction in dynorphin expression induced by 6-OHDA in WT but not A_2A KO mice. The dynorphin mRNA levels were determined by in situ hybridization histochemistry in mice unilaterally lesioned with 6-OHDA, followed by daily treatment with l-DOPA (1.8 or 2.5 mg/kg) or vehicle for 21 d. Dynorphin mRNA levels [optical density (OD)] were quantified at the level of midstriatum and expressed as a percentage of the contralateral side (unlesioned striatum). Chronic treatment with l-DOPA reverses the 6-OHDA-induced reduction in dynorphin mRNA to normal levels in WT but not A_2A KO mice (*p < 0.05; Student'st test compared with the corresponding WT group). Thenumbers inside the bars indicate the animal numbers for each group.