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

Abstract

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.

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

“…Animals were kept as previously described [22]. Female Sprague-Dawley rats (Harlan Laboratories, Inc.) weighing 245 -320 g were maintained on a 12 h light/12h dark cycle at 22˚C ± 2˚C and 50% ± 10% humidity with ad libitum access to Wayne Lab Box chow and water.…”

“…Lesions of the nigrostriatal DA pathway were made in the right median forebrain bundle (MFB) as previously described [22] by infusion of 9 µg of free-base 6-OHDA neurotoxin (Research Biochemicals, Inc.) in ascorbic acid (4 μl, 0.1% in saline, Sigma-Aldrich, Inc.) using keta-mine HCl/xylazine HCl solution (80 mg/kg, i.p., Sigma-Aldrich, Inc.) for anesthesia. The stereotaxic coordinates of the MFB relative to the bregma were A: 4.4.…”

“…The extent of the unilateral 6-OHDA lesion was assessed by apomorphine-induced rotation test as previously described [22]. Two weeks post-lesion, an apomorphineinduced rotation test (apomorphine HCl, 0.1 mg/kg, i.p., Sigma-Aldrich, Inc.) was applied.…”

“…The protocol for immunohistochemistry is as previously described [22]. In brief, animals were transcardially perfused with 10% sucrose and paraformaldehyde solution (Sigma-Aldrich, Inc.).…”

“…Although altered GABA transmission is well documented in PD, very little is known about the role of GABA receptors in DA-mediated effects as far downstream as ERK1/2. Our recent study [22] showed that elevating GABA levels by aminooxyacetic acid (AOAA) attenuated the L-DOPA induced induction of ERK1/2 in the striatum and SN. The present study employed a pharmacological approach to test whether stimulation of GABA-A or GABA-B receptors, by using selective agonists such as muscimol or baclofen, would modify the L-DOPA-induced rotational response and ERK1/2 phosphorylation in the striatum and SN in a unilaterally lesioned rat model of PD.…”