The integration of neural information by a passive kinetic stimulus and galvanic vestibular stimulation in the lateral vestibular nucleus

Galvanic vestibular stimulation down-regulated NMDA receptors in vestibular nucleus of PD model

Parkinsonian symptoms relief by electrical stimulation is constructed by modulating neural network activity, and Galvanic vestibular stimulation (GVS) is known to affect the neural activity for motor control by activating the vestibular afferents. However, its underlying mechanism is still elusive. Due to the tight link from the peripheral vestibular organ to vestibular nucleus (VN), the effect by GVS was investigated to understand the neural mechanism. Using Sprague Dawley (SD) rats, behavioral response, extracellular neural recording, and immunohistochemistry in VN were conducted before and after the construction of Parkinson's disease (PD) model. Animals' locomotion was tested using rota-rod, and single extracellular neuronal activity was recorded in VN. The immunohistochemistry detected AMPA and NMDA receptors in VN to assess the effects by different amounts of electrical charge (0.018, 0.09, and 0.18 coulombs) as well as normal and PD with no GVS. All PD models showed the motor impairment, and the loss of TH⁺ neurons in medial forebrain bundle (mfb) and striatum was observed. Sixty-five neuronal extracellular activities (32 canal & 33 otolith) were recorded, but no significant difference in the resting firing rates and the kinetic responding gain were found in the PD models. On the other hand, the numbers of AMPA and NMDA receptors increased after the construction of PD model, and the effect by GVS was significantly evident in the change of NMDA receptors (p < 0.018). In conclusion, the increased glutamate receptors in PD models were down-regulated by GVS, and the plastic modulation mainly occurred through NMDA receptor in VN.

Neural Interruption by Unilateral Labyrinthectomy Biases the Directional Preference of Otolith-Related Vestibular Neurons

Background: The directional preference of otolith-related vestibular neurons elucidates the neuroanatomical link of labyrinths, but few direct experimental data have been provided. Methods: The directional preference of otolith-related vestibular neurons was measured in the vestibular nucleus using chemically induced unilateral labyrinthectomy (UL). For the model evaluation, static and dynamic behavioral tests as well as a histological test were performed. Extracellular neural activity was recorded for the neuronal responses to the horizontal head rotation and the linear head translation. Results: Seventy-seven neuronal activities were recorded, and the total population was divided into three groups: left UL (20), sham (35), and right UL (22). Based on directional preference, two sub-groups were again classified as contra- and ipsi-preferred neurons. There was no significance in the number of those sub-groups (contra-, 15/35, 43%; ipsi-, 20/35, 57%) in the sham (p = 0.155). However, more ipsi-preferred neurons (19/22, 86%) were observed after right UL (p = 6.056 × 10⁻⁵), while left UL caused more contra-preferred neurons (13/20, 65%) (p = 0.058). In particular, the convergent neurons mainly led this biased difference (ipsi-, 100% after right UL and contra-, 89% after left UL) (p < 0.002). Conclusions: The directional preference of the neurons depended on the side of the lesion, and its dominance was mainly led by the convergent neurons.

Postural control in paw distance after labyrinthectomy-induced vestibular imbalance

Balance control is accomplished by the anatomical link which provides the neural information for the coordination of skeletal muscles. However, there are few experimental proofs to directly show the neuroanatomical connection. Here, we examined the behavioral alterations by constructing an animal model with chemically induced unilateral labyrinthectomy (UL). In the experiment using rats (26 for UL, 14 for volume cavity, 355-498 g, male), the models were initially evaluated by the rota-rod (RR) test (21/26, 80.8%) and ocular displacement (23/26, 88.5%). The duration on the rolling rod decreased from 234.71 ± 64.25 s (4th trial before UL) to 11.81 ± 17.94 s (1st trial after UL). Also, the ocular skewed deviation (OSD) was observed in the model with left (5.79 ± 3.06°) and right lesion (3.74 ± 2.69°). Paw distance (PW) was separated as the front (FPW) and the hind side (HPW), and the relative changes of HPW (1.71 ± 1.20 cm) was larger than those of FPW (1.39 ± 1.06 cm), providing a statistical significance (p = 1.51 × 10^-4, t test). Moreover, the results of the RR tests matched to those of the changing rates (18/21, 85.7%), and the changes (16/18, 88.9%) were dominantly observed in HPW (in FPW, 2/18, 11.1%). Current results indicated that the UL directly affected the changes in HPW more than those in FPW. In conclusion, the missing neural information from the peripheral vestibular system caused the abnormal posture in HPW, and the postural instability might reduce the performance during the voluntary movement shown in the RR test, identifying the relation between the walking imbalance and the unstable posture in PW. Graphical abstract.

Repeated Galvanic Vestibular Stimulation Modified the Neuronal Potential in the Vestibular Nucleus

Vestibular nucleus (VN) and cerebellar flocculus are known as the core candidates for the neuroplasticity of vestibular system. However, it has been still elusive how to induce the artificial neuroplasticity, especially caused by an electrical stimulation, and assess the neuronal information related with the plasticity. To understand the electrically induced neuroplasticity, the neuronal potentials in VN responding to the repeated electrical stimuli were examined. Galvanic vestibular stimulation (GVS) was applied to excite the neurons in VN, and their activities were measured by an extracellular neural recording technique. Thirty-eight neuronal responses (17 for the regular and 21 for irregular neurons) were recorded and examined the potentials before and after stimulation. Two-third of the population (63.2%, 24/38) modified the potentials under the GVS repetition before stimulation (p = 0.037), and more than half of the population (21/38, 55.3%) changed the potentials after stimulation (p = 0.209). On the other hand, the plasticity-related neuronal modulation was hardly observed in the temporal responses of the neurons. The modification of the active glutamate receptors was also investigated to see if the repeated stimulation changed the number of both types of glutamate receptors, and the results showed that AMPA and NMDA receptors decreased after the repeated stimuli by 28.32 and 16.09%, respectively, implying the modification in the neuronal amplitudes.

Dominant parameter of galvanic vestibular stimulation for the non-associative learning processes

Electrical stimulus is one of the common stimulating methods, and Galvanic vestibular stimulation (GVS) is the oldest form as an electrical stimulation. Nevertheless, GVS is still considered as a secondary stimulating tool for the medical purposes. Even though some unarguable findings have made using GVS, its use has been limited because of its ambiguity as an input source. For better understanding, many previous studies mainly focused on its functional effects, like the ocular reflexes. However, its fundamental effects on the neural activities are still elusive, such as the dominant influences by different parameters of GVS. Here we compared the effects on the neuronal responses by applying two different parameters, strength and rate, of GVS. To assess the dominance on the neuronal responses to these parameters, we designed three independent stimuli. Those stimuli were multiply applied to obtain the responding slopes based on the mechanism of non-associative learning processes, and the effects on the neurons were calculated as an inner angle between two responding slopes. Out of 23 neurons, 15 (65.2%) units were affected more by the strength with a statistical significance (p = 0.047). The ranges of the inner angles also implied the strength (- 3.354°~2.063°) mainly modulated by the neuronal responses comparing with those by the rate (- 2.001°~1.975°). The dominance of the parameters was closely related with the neuronal sensitivity to stimulation (SE) (p = 0.018), while there were few relations with the neuronal regularity, directional preference (DP), and the physiological response (PR) (p > 0.059). Thus, the neural information related with the dominance was delivered by the irregular neurons, and these types of neurons should be the targets for the stimulation. Graphical abstract.