Noisy galvanic vestibular stimulation promotes GABA release in the substantia nigra and improves locomotion in hemiparkinsonian rats

Dopamine related disorders usually respond to dopaminergic drugs, but not all symptoms are equally responsive. In Parkinson’s disease (PD) in particular, axial symptoms resulting in impaired gait and postural control are difficult to treat. Stochastic vestibular stimulation (SVS) has been put forward as a method to improve CNS function in dopamine related disorders, but the mechanisms of action are not well understood. This thesis aimed to investigate the effects of SVS on neuronal brain activity and to evaluate the possible enhancing effect of SVS on motor control in PD and on cognitive functions and motor learning in Attention deficit hyperactivity disorder (ADHD). Behavioural tests were conducted in the 6-OHDA rat model of PD using the accelerating Rotarod and the Montoya skilled reach test to evaluate the effect of SVS on motor control. The effect of SVS on brain activity was assessed using in vivo microdialysis and immunohistochemistry. We evaluated the effect of SVS on postural control and Parkinsonism in patients with PD and the effect of SVS on cognitive function in people with ADHD. The behavioural animal studies indicate that SVS may have an enhancing effect on locomotion, but not skilled forepaw function. SVS increased GABA transmission in the ipsilesional substantia nigra (SN) and may have a rebalancing effect on dysfunctional brain activity. SVS increased c-Fos activity more than levodopa and saline in the vestibular nucleus of all animals. c-Fos expression was also higher in this region in the 6-OHDA lesioned than in shamlesioned animals, supporting the theory that SVS may have larger effects in the dopamine depleted brain. SVS increased c-Fos expression in the habenula nucleus substantially more than levodopa did. Furthermore, SVS and levodopa had similar effects on many brain regions, including the striatum, where saline had no effect. The clinical studies revealed improvement of postural control in PD during SVS. There was a trend towards reduced Parkinsonism during SVS when off levodopa. No substantial effects were found on cognitive performance in ADHD. In PD, SVS may improve motor control by inhibiting the overactive SN, possibly through a non-dopaminergic modulatory pathway involving increased neurotransmission in the habenula nucleus. SVS could be trialled in larger studies to evaluate long-term effects on treatment resistant axial symptoms associated with PD

Absolute concentrations of GABA and dopamine in the ipsi- and contralesionalSN of hemilesioned 6-OHDA animals.

<p>Panel A–B shows the GABA concentrations and panel C–D the simultaneous dopamine (DA) concentrations following stochastic vestibular stimulation (SVS) and L-DOPA treatment (nM, mean±SEM). Panel A and C are measurements from day 1 and B and D from day 2. NNC 711 (30 µM) was present throughout the experiment and left in the microdialysis tube that was re-sealed over night. SVS treatment is indicated by a horizontal bar and the L-DOPA injection by an arrow.</p

Microdialysate concentrations of GABA during stochastic vestibular stimulation (SVS) in intact animals and untreated intact controls.

<p>Percent of baseline, mean±SEM is shown. The GAT-inhibitor NNC 711 was included in the dialysates throughout the experiment. The SVS period is indicated by a horizontal bar. A. Nigral GABA concentration in SVS treated intact animals (n = 9) and untreated controls (n = 6). P-values from Bonferroni corrected post hoc tests following two-way ANOVA for timepoints T = 90–T = 150 minutes, indicating significant interaction between treatment and time F(2,26) = 3.53, p = 0.044. B. GABA concentrations in the PPN, VM and striatum of intact animals following SVS.</p

Rotarod performance after 6-OHDA or sham-hemilesions.

<p>Δt represents the change in time on rod (s, mean±SEM) compared to baseline performance on the day of the experiment. Stochastic vestibular stimulation (SVS) or no stimulation (No SVS) was administered in a counterbalanced order. In the following week a repeat experiment was performed with 6-OHDA hemilesioned animals and the effect of L-DOPA treatment or a vehicle injection (NaCl) was evaluated in counterbalanced order. P-value for paired t-test.</p

Relative changes in GABA concentrations in the ipsi- and contralesional substantia nigra of hemilesioned 6-OHDA animals following L-DOPA treatment.

<p>The microdialysate concentrations of GABA (percent of baseline, mean±SEM, n = 5–7) in the bilateral substantiae nigrae (SN) of 6-OHDA hemilesioned rats following an i.p. injection of L-DOPA are compared to GABA concentrations in the SN of untreated control animals (n = 6). Two-way repeated measure ANOVA for t = 90–t = 150 with L-DOPA treatment and time as main factors indicated a significant effect of treatment F(2,30) = 4.68, p = 0.026. * p<0.05 in post hoc Bonferroni corrected t-tests comparing ipsilesional SN in treated animals to control SN.</p

Schematic illustration of the experimental design for the Rotarod testing procedure.

<p>The black arrows represent training episodes for each animal. Rod performance during stochastic vestibular stimulation (SVS) or no SVS was compared to the baseline measurement (BL, 4×10 min before either condition) on the day of the experiment. The stimulated condition (30 min SVS in cage followed by 4×10 min testing during SVS) or non stimulated condition (30 min rest in cage followed by 4×10 min testing during no SVS) was carried out in a counterbalanced order after the BL measurements. L-DOPA testing was carried out in the beginning of the following week in counterbalanced order; BL testing followed by either an L-DOPA or NaCl injection, rest in cage for 30 min followed by testing for 4×10 min.</p

Vestibular pathways that may influence basal ganglia transmission.

<p>A) SVS is expected to activate multiple pathways from the vestibular nuclear complex (VN). Of particular interest may be pathways that connect the cerebellum and the basal ganglia over the subthalamic nucleus and thalamus <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0029308#pone.0029308-Bostan1" target="_blank">[48]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0029308#pone.0029308-Hoshi1" target="_blank">[49]</a>. We chose to sample the SN, the striatum, the PPN and the VM for dopamine and amino acid concentrations before, during and for 60 minutes after stochastic vestibular stimulation in unlesioned animals. Panel B indicates the activity of the direct and indirect loop projections to the SN in normal intact rats and after nigral dopamine cell degeneration. Loss of nigrostriatal dopamine neurons lead to hyperexcitation of SN <i>pars reticulata</i> neurons, which can be counteracted by increased GABA release following L-DOPA treatment. SVS also increases nigral GABA concentrations, but the pathways involved in this effect remains to be elucidated. RF: reticular formation, SNc: Substantia nigra <i>pars compacta</i>, SNr: Substantia nigra <i>pars reticulata</i>, STN: subthalamic nucleus, SVS: stochastic vestibular stimulation, Thal: thalamus.</p

Maximum stimulation amplitudes (mA, mean±SEM) in the different experiments.

<p>Stimulation amplitudes were determined by subjecting the animals to a sinusoid current that was increased until head rocking was just visible. During stochastic stimulation the amplitude never exceeded ± this amplitude. A different current delivery system was used for the microdialysis experiments in unlesioned animals. 6-OHDA 6-hydroxy-dopamine, PPN pedunculopontine nucleus, ST striatum, SN substanta nigra, VM ventromedial thalamus.</p