Pallidal origin of GABA release within the substantia nigra pars reticulata during high-frequency stimulation of the subthalamic nucleus

Abstract

High-frequency stimulation of the subthalamic nucleus (HFS-STN) is an effective treatment for alleviating the motor symptoms of parkinsonian patients. However, the neurochemical basis of its effects remains unknown. We showed previously that 1 h of HFS-STN in normal rats increases extracellular glutamate (Glu) level in the output nuclei of the STN, the globus pallidus (GP), and the substantia nigra pars reticulata (SNr), consistent with an increase in the activity of STN neurons. HFS-STN also increases GABA levels in the SNr, but the origin of this increase is unclear. We investigated the effectiveness of HFS-STN for improving Parkinson's disease symptoms, using intracerebral microdialysis to determine the extracellular Glu and GABA levels of the GP and SNr in response to HFS-STN in anesthetized hemiparkinsonian rats [6-hydroxydopamine lesion of the substantia nigra pars compacta (SNc)]. Basal levels of Glu and GABA in the GP and SNr were significantly higher in hemiparkinsonian than in intact rats. HFS-STN did not affect extracellular Glu level in the SNr of hemiparkinsonian rats but doubled the level of GABA. Ibotenic acid lesion of the GP abolished the increase in GABA levels in the SNr induced by HFS-STN in SNc-lesioned rats. These results provide neurochemical confirmation of the hyperactivity of the STN after dopaminergic denervation and suggest that the therapeutic effects of HFS-STN may result partly from the stimulation of pallidonigral fibers, thereby revealing a potential role for pallidal GABA in the inhibition of basal ganglial output structures during HFS-STN.

Figure 1.

Photographs of TH-immunostained coronal rat-brain sections at the nigral (A) and striatal (B) levels in 6-OHDA-lesioned rats. Note the DA cell loss in the SNc (A) and the DA terminal loss in the striatum (B) on the lesioned side. CPu, Caudate-putamen nucleus; VTA, ventral tegmental area. Scale bar, 1 mm.

Figure 2.

Photographs of cresyl violet-stained coronal rat-brain sections at pallidal (B-D), nigral (F), and subthalamic (H) levels and schematic diagrams (A, E, and G) adapted from the stereotaxic atlas of Paxinos and Watson (1982). B, F, H, Tissue sections at pallidal (B), nigral (F), and subthalamic (H) levels in hemiparkinsonian rats. Note the adequate implantation of microdialysis probes in the GP (B) and SNr (F) and the adequate localization of the stimulation electrode within the STN (H). H, Asterisk indicates stimulation point. Scale bar, 1 mm. C, D, Coronal sections at a higher magnification at the pallidal level (corresponding to the area delimited by a square in A) in GP-lesioned animals on the intact (C) or lesioned (D) side. Note the loss of neuronal cell bodies and the gliosis in the lesioned GP (D) after the two microinjections of 0.5 ml of ibotenic acid solution (1 mg/ml). Scale bar, 40 μm. CPu, Caudate-putamen nucleus; Hi, hippocampus; LV, lateral ventricle.

Figure 3.

Basal levels of extracellular nigral and pallidal glutamate (A) and GABA (B) in controls and SNc-lesioned, GP-lesioned, and GP plus SNc-lesioned rats. ^*p < 0.05 compared with controls; ^#p < 0.05, comparison between SNc-lesioned and GP plus SNc-lesioned rats; ^▿p < 0.05, comparison between GP-lesioned and GP plus SNc-lesioned rats. Values presented for the control group correspond to those obtained in a previous study (Windels et al., 2000). Error bars represent SEM.

Figure 4.

Extracellular glutamate levels determined at 8 min intervals in the SNr (A) and GP (B) ipsilateral or contralateral to the stimulation in 6-0HDA SNc-lesioned rats. The prestimulation period (PRE-STIM; fractions 1-8) gave eight dialysates, whereas the stimulation period (STIM; fractions 9-16) and poststimulation period (POST-STIM; fractions 17-36) gave eight and 20 dialysates, respectively. The mean ± SEM of the eight dialysates collected before HFS-STN was used as the baseline. Results are expressed as a percentage of variation of this baseline value. Each percentage represents the mean variations ± SEM calculated from six animals. Note that Glu levels were not affected significantly by HFS-STN on either side in the GP and SNr. Error bars represent SEM.

Figure 5.

Extracellular GABA levels determined at 8 min intervals in the SNr (A) and GP (B) ipsilateral or contralateral to stimulation in 6-OHDA SNc-lesioned rats (B). The prestimulation period (PRE-STIM; fractions 1-8) gave eight dialysates, whereas the stimulation period (STIM; fractions 9-16) and poststimulation period (POST-STIM; fractions 17-36) gave eight and 20 dialysates, respectively. The mean ± SEM of the eight dialysates collected before HFS-STN was used as the baseline. Results are expressed as a percentage of variation of this baseline value. Each percentage represents the mean variations ± SEM calculated from six animals. The data were analyzed using a one-way ANOVA test with repeated measures over time followed by a Dunnett's or Games-Howell test (n = 6 in each group). For SNr, F_(2,24) = 21.68 and p < 0.05. Note the increase in extracellular GABA level induced by HFS-STN in the SNr ipsilateral to the stimulated side, with no significant change in GP on either side. ^*p < 0.05 compared with baseline values. Error bars represent SEM.

Figure 6.

A, Extracellular Glu levels determined at 8 min intervals in the SNr ipsilateral to stimulation in ibotenic acid GP-lesioned rats. The prestimulation period (PRE-STIM; fractions 1-8) gave eight dialysates, whereas the stimulation period (STIM; fractions 9-16) and poststimulation period (POST-STIM; fractions 17-30) consisted of eight and 14 dialysates, respectively. The mean ± SEM of the eight dialysates collected before HFS-STN was used as the baseline. Results are expressed as a percentage of variation of this baseline value. Each percentage represents the mean variations ± SEM calculated from five animals. The data were analyzed using a one-way ANOVA test with repeated measures over time followed by a Dunnett's or Games-Howell test (n = 5). For SNr, F_(2,18) = 23.456 and p < 0.05. Note the increase in extracellular Glu level induced by HFS-STN on the ipsilateral, stimulated side. ^*p < 0.05 compared with baseline values. B, C, Extracellular Glu (B) and GABA (C) levels, determined at 8 min intervals in the SNr ipsilateral to stimulation in GP plus SNc-lesioned rats. The prestimulation period (fractions 1-8) gave eight dialysates, where as the stimulation (fractions 9-16) and poststimulation (fractions 17-36) periods gave eight and 20 dialysates, respectively. The mean ± SEM of the eight dialysates collected before HFS-STN was used as the baseline. Results are expressed as a percentage of variation of this baseline value. Each percentage represents the mean variations ± SEM calculated from five animals. Note that HFS-STN did not affect extracellular Glu and GABA levels on the ipsilateral, stimulated side. Error bars represent SEM.