The Time Course of D1 Agonist Induced Striatonigral ERK1/2 Signaling in a Rat Model of Parkinson’s Disease

Using a rat model of hemiparkinsonism, we examined the time-course of D1 agonist, SKF-38393-induced changes in extracellular signaling regulated kinases 1/2 (ERK1/2) phosphorylation in the striatum and substantia nigra (SN). We unilaterally lesioned the rat median forebrain bundle with 6-hydroxydopamine. Dopaminergic lesioned rats were administered with SKF-38393 and perfused at 15, 30, 60, or 120 minutes after the drug. Immunohistochemical analysis of striatum and SN revealed, as expected, a loss of tyrosine hydroxylase and a decrease of substance P in lesioned rats. SKF-38393 induced a robust increase in phospho-ERK1/2 levels in the lesioned striatum, which peaked at 15 min and substantially declined by 120 min. We report for the first time that similar changes were observed in the SN. The time-dependent ERK 1/2 activation in the striatonigral neurons may play a role in the therapeutic and/or side effects such as dyskinesias related to the dopamine agonist treatment for Parkinson’s disease.

Dopamine and GABA Interaction in Basal Ganglia: GABA-A or GABA-B Receptor Stimulation Attenuates L-DOPA-Induced Striatal and Nigral ERK1/2 Signaling in a Rat Model of Parkinson’s Disease

Parkinson’s disease (PD) is characterized by degeneration of nigrostriatal dopamine (DA) neurons. The primary drug used to treat PD symptoms is L-DOPA, but side effects such as dyskinesias limit its use. Previous findings show that L-DOPA treatment induces extracellular signal-regulated kinase (ERK1/2), a MAP-kinase protein. γ-aminobutyric acid (GABA) is intimately involved in basal ganglia function. Our previous study using a unilaterally lesioned rat model of PD indicated that elevating GABA levels by GABA transaminase inhibitor, aminooxyacetic acid significantly attenuated L-DOPA-induced ERK phosphorylation in the striatum and substantia nigra (SN). The aim of the present study was to assess the role of GABA-A and GABA-B receptor by using a selective agonists, muscimol and baclofen respectively, on L-DOPA-induced ERK phosphorylation in the striatum and SN. Unilaterally 6-OHDA-lesioned rats were prescreened by apomorphine induced rotation test for the extent of DA loss. Lesioned rats were treated with L-DOPA alone or after muscimol or baclofen pretreatment. Appropriate control groups were used. Phospho-ERK levels, tyrosine hydroxylase (to ascertain DA loss) and substance P (an indirect marker for DA loss) levels were assessed by immunohistochemistry using coronal slices at the level of striatum and SN. L-DOPA administration induced a robust increase (>300%) in phospho-ERK1/2 levels in the striatum and SN. Muscimol as well as baclofen pretreatment attenuated the L-DOPA-induced increase in phospho-ERK1/2 levels by >60% in the striatum and SN. Muscimol and baclofen pretreatment also greatly reduced the number of L-DOPA induced phospho-ERK1/2 stained cells in the striatum as well as the contralateral rotational behavior. The present data taken together with our previous study indicate that the L-DOPA induced increase in ERK1/2 is attenuated by GABA via a GABA-A and GABA-B receptor linked mechanism. The study provides further insight into a dopamine-GABA-ERK interaction in the therapeutic and/or side effects of L-DOPA in the basal ganglia.

GABA Attenuates L-DOPA-Induced Striatal and Nigral ERK1/2 Signaling in a Rat Model of Parkinson’s Disease

L-DOPA is the primary drug used to treat Parkinson’s disease (PD) symptoms, but motor side effects limit its long term use. Previous experimental studies show that L-DOPA acts on supersensitive D1 receptors in the basal ganglia to induce extracellular signal-regulated kinases 1 and 2 (ERK1/2), a pair of MAP-kinase proteins that may be involved in induction of motor side effects. Since GABA is known to be intimately involved in basal ganglia function, we investigated whether elevating GABA levels via a GABA-transaminase (GABA-T) inhibitor affects the L-DOPA-induced ERK1/2 phosphorylation in the striatum and substantia nigra (SN) using a rat model of PD. Unilateral dopaminergic lesions of median forebrain bundle neurons were done using the neurotoxin 6-hydroxydopamine. Rats were prescreened for the extent of the lesion by apomorphine-induced rotation test. Lesioned rats were treated with aminooxyacetic acid (AOAA, a GABA-T inhibitor), L-DOPA, or in combination. Immunohistochemistry of tyrosine hydroxylase (TH, a direct indicator of dopaminergic lesion), substance P (SP, an indirect marker that decreases after lesion), and phospho-ERK1/2 was done using slices at the level of striatum and SN. Unilateral dopaminergic lesioned rats, as expected, exhibited >90% TH loss and a modest SP loss in the striatum and SN. L-DOPA alone induced a 343% and 330% increase in phospho-ERK1/2 in the striatum and SN, respectively. We report here a novel finding that pretreatment with AOAA attenuated the L-DOPA induced increase in phospho-ERK1/2 by 62% and 68% in the striatum and SN, respectively, suggesting a DA-GABA-ERK1/2 link in the therapeutic and/or side effects of L-DOPA.