Galvanic stimulation of the vestibular periphery in guinea pigs during passive whole body rotation and self-generated head movement

Galvanic vestibular stimulation: from basic concepts to clinical applications

Galvanic vestibular stimulation (GVS) plays an important role in the quest to understand sensory signal processing in the vestibular system under normal and pathological conditions. It has become a highly relevant tool to probe neuronal computations and to assist in the differentiation and treatment of vestibular syndromes. Following its accidental discovery, GVS became a diagnostic tool that generates eye movements in the absence of head/body motion. With the possibility to record extracellular and intracellular spikes, GVS became an indispensable method to activate or block the discharge in vestibular nerve fibers by cathodal and anodal currents, respectively. Bernie Cohen, in his attempt to decipher vestibular signal processing, has used this method in a number of hallmark studies that have added to our present knowledge, such as the link between selective electrical stimulation of semicircular canal nerves and the generation of directionally corresponding eye movements. His achievements paved the way for other major milestones including the differential recruitment order of vestibular fibers for cathodal and anodal currents, pronounced discharge adaptation of irregularly firing afferents, potential activation of hair cells, and fiber type-specific activation of central circuits. Previous disputes about the structural substrate for GVS are resolved by integrating knowledge of ion channel-related response dynamics of afferents, fiber type-specific innervation patterns, and central convergence and integration of semicircular canal and otolith signals. On the basis of solid knowledge of the methodology, specific waveforms of GVS are currently used in clinical diagnosis and patient treatment, such as vestibular implants and noisy galvanic stimulation.

Ionic Direct Current Modulation for Combined Inhibition/Excitation of the Vestibular System

OBJECTIVE

Prosthetic electrical stimulation delivered to the vestibular nerve could provide therapy for people suffering from bilateral vestibular dysfunction. Common encoding methods use pulse-frequency modulation (PFM) to stimulate the semicircular canals of the vestibular system. We previously showed that delivery of ionic direct current (iDC) can also modulate the vestibular system. In this study, we compare the dynamic range of head velocity encoding from iDC modulation to that of PFM controls.

METHODS

Gentamicin-treated wild-type chinchillas were implanted with microcatheter tubes that delivered ionic current to the left ear vestibular canals and stimulated with steps of anodic/cathodic iDC or PFM. Evoked vestibulo-ocular reflex eye velocity was used to compare PFM and iDC vestibular modulation.

RESULTS

Cathodic iDC steps effectively elicited eye rotations consistent with an increased firing rate of the implanted semicircular canal afferents. Anodic iDC current steps elicited eye rotations in the opposite direction that, when paired with an adapted cathodic offset, increased the dynamic range of eye rotation velocities in comparison to PFM controls.

CONCLUSION

Our results suggest that iDC modulation can effectively modulate the vestibular system across a functional range of rotation vectors and velocities, with a potential benefit over a PFM stimulation paradigm.

SIGNIFICANCE

In conjunction with a safe dc delivery system, iDC modulation could potentially increase the range of simulated head rotation velocities available to neuroelectric vestibular prostheses.

Effects of subcutaneous alfaxalone alone and in combination with dexmedetomidine and buprenorphine in guinea pigs (Cavia porcellus)

OBJECTIVE

To characterize alfaxalone administered subcutaneously (SC) in guinea pigs, both alone and in combination with dexmedetomidine and buprenorphine.

STUDY DESIGN

Prospective, blinded, crossover study.

ANIMALS

A total of 15 healthy female guinea pigs weighing 400-600 g.

METHODS

Alfaxalone (10, 20 and 40 mg kg^-1) was administered SC to three guinea pigs as a pilot dose-finding study. Alfaxalone (20 mg kg^-1; A₂₀) was selected for comparison against combination protocols of alfaxalone (15 and 20 mg kg^-1) with dexmedetomidine (0.25 mg kg^-1) and buprenorphine (0.05 mg kg^-1; A₁₅DB, A₂₀DB). Each protocol was randomly administered to 12 guinea pigs separated by ≥7 days. Time and quality of induction and recovery, heart rate, respiratory rate, peripheral hemoglobin oxygen saturation, rectal temperature, pedal withdrawal reflex and adverse effects were recorded.

RESULTS

The median time to induction for A₂₀, A₁₅DB and A₂₀DB was 6.8-8.0 minutes with no significant difference between treatments. Mean duration of recumbency for A₂₀ was 73.6 ± 19.6 minutes. Recumbency duration for A₁₅DB and A₂₀DB extended to 90 minutes, at which time dexmedetomidine was antagonized using atipamezole (0.025 mg kg^-1 SC). Physiological variables were within normal limits with the exception of one animal that died 45 minutes following treatment with A₂₀DB. Pedal withdrawal reflex remained intact with all treatments. Minor side effects such as twitching or bruxism occurred sporadically with treatment A₂₀ but not with A₁₅DB and A₂₀DB.

CONCLUSIONS AND CLINICAL RELEVANCE

SC alfaxalone produced uncomplicated sedation that may be recommended for nonpainful procedures that do not require complete immobility. The addition of dexmedetomidine and buprenorphine increased the duration of sedation and immobility, but did not result in general anesthesia. This combination sedation protocol may be useful for nonpainful procedures requiring extended immobility.

Galvanic Vestibular Stimulation: Cellular Substrates and Response Patterns of Neurons in the Vestibulo-Ocular Network

Galvanic vestibular stimulation (GVS) uses modulated currents to evoke neuronal activity in vestibular endorgans in the absence of head motion. GVS is typically used for a characterization of vestibular pathologies; for studies on the vestibular influence of gaze, posture, and locomotion; and for deciphering the sensory-motor transformation underlying these behaviors. At variance with the widespread use of this method, basic aspects such as the activated cellular substrate at the sensory periphery or the comparability to motion-induced neuronal activity patterns are still disputed. Using semi-intact preparations of Xenopus laevis tadpoles, we determined the cellular substrate and the spatiotemporal specificity of GVS-evoked responses and compared sinusoidal GVS-induced activity patterns with motion-induced responses in all neuronal elements along the vestibulo-ocular pathway. As main result, we found that, despite the pharmacological block of glutamatergic hair cell transmission by combined bath-application of NMDA (7-chloro-kynurenic acid) and AMPA (CNQX) receptor blockers, GVS-induced afferent spike activity persisted. However, the amplitude modulation was reduced by ∼30%, suggesting that both hair cells and vestibular afferent fibers are normally recruited by GVS. Systematic alterations of electrode placement with respect to bilateral semicircular canal pairs or alterations of the bipolar stimulus phase timing yielded unique activity patterns in extraocular motor nerves, compatible with a spatially and temporally specific activation of vestibulo-ocular reflexes in distinct planes. Despite the different GVS electrode placement in semi-intact X. laevis preparations and humans and the more global activation of vestibular endorgans by the latter approach, this method is suitable to imitate head/body motion in both circumstances.

SIGNIFICANCE STATEMENT

Galvanic vestibular stimulation is used frequently in clinical practice to test the functionality of the sense of balance. The outcome of the test that relies on the activation of eye movements by electrical stimulation of vestibular organs in the inner ear helps to dissociate vestibular impairments that cause vertigo and imbalance in patients. This study uses an amphibian model to investigate at the cellular level the underlying mechanism on which this method depends. The outcome of this translational research unequivocally revealed the cellular substrate at the vestibular sensory periphery that is activated by electrical currents, as well as the spatiotemporal specificity of the evoked eye movements, thus facilitating the interpretation of clinical test results.

Projection neurons of the vestibulo-sympathetic reflex pathway

Changes in head position and posture are detected by the vestibular system and are normally followed by rapid modifications in blood pressure. These compensatory adjustments, which allow humans to stand up without fainting, are mediated by integration of vestibular system pathways with blood pressure control centers in the ventrolateral medulla. Orthostatic hypotension can reflect altered activity of this neural circuitry. Vestibular sensory input to the vestibulo-sympathetic pathway terminates on cells in the vestibular nuclear complex, which in turn project to brainstem sites involved in the regulation of cardiovascular activity, including the rostral and caudal ventrolateral medullary regions (RVLM and CVLM, respectively). In the present study, sinusoidal galvanic vestibular stimulation was used to activate this pathway, and activated neurons were identified through detection of c-Fos protein. The retrograde tracer Fluoro-Gold was injected into the RVLM or CVLM of these animals, and immunofluorescence studies of vestibular neurons were conducted to visualize c-Fos protein and Fluoro-Gold concomitantly. We observed activated projection neurons of the vestibulo-sympathetic reflex pathway in the caudal half of the spinal, medial, and parvocellular medial vestibular nuclei. Approximately two-thirds of the cells were ipsilateral to Fluoro-Gold injection sites in both the RVLM and CVLM, and the remainder were contralateral. As a group, cells projecting to the RVLM were located slightly rostral to those with terminals in the CVLM. Individual activated projection neurons were multipolar, globular, or fusiform in shape. This study provides the first direct demonstration of the central vestibular neurons that mediate the vestibulo-sympathetic reflex.

Head movements evoked in alert rhesus monkey by vestibular prosthesis stimulation: implications for postural and gaze stabilization

The vestibular system detects motion of the head in space and in turn generates reflexes that are vital for our daily activities. The eye movements produced by the vestibulo-ocular reflex (VOR) play an essential role in stabilizing the visual axis (gaze), while vestibulo-spinal reflexes ensure the maintenance of head and body posture. The neuronal pathways from the vestibular periphery to the cervical spinal cord potentially serve a dual role, since they function to stabilize the head relative to inertial space and could thus contribute to gaze (eye-in-head + head-in-space) and posture stabilization. To date, however, the functional significance of vestibular-neck pathways in alert primates remains a matter of debate. Here we used a vestibular prosthesis to 1) quantify vestibularly-driven head movements in primates, and 2) assess whether these evoked head movements make a significant contribution to gaze as well as postural stabilization. We stimulated electrodes implanted in the horizontal semicircular canal of alert rhesus monkeys, and measured the head and eye movements evoked during a 100 ms time period for which the contribution of longer latency voluntary inputs to the neck would be minimal. Our results show that prosthetic stimulation evoked significant head movements with latencies consistent with known vestibulo-spinal pathways. Furthermore, while the evoked head movements were substantially smaller than the coincidently evoked eye movements, they made a significant contribution to gaze stabilization, complementing the VOR to ensure that the appropriate gaze response is achieved. We speculate that analogous compensatory head movements will be evoked when implanted prosthetic devices are transitioned to human patients.

Multisensory integration in early vestibular processing in mice: the encoding of passive vs. active motion

The mouse has become an important model system for studying the cellular basis of learning and coding of heading by the vestibular system. Here we recorded from single neurons in the vestibular nuclei to understand how vestibular pathways encode self-motion under natural conditions, during which proprioceptive and motor-related signals as well as vestibular inputs provide feedback about an animal's movement through the world. We recorded neuronal responses in alert behaving mice focusing on a group of neurons, termed vestibular-only cells, that are known to control posture and project to higher-order centers. We found that the majority (70%, n = 21/30) of neurons were bimodal, in that they responded robustly to passive stimulation of proprioceptors as well as passive stimulation of the vestibular system. Additionally, the linear summation of a given neuron's vestibular and neck sensitivities predicted well its responses when both stimuli were applied simultaneously. In contrast, neuronal responses were suppressed when the same motion was actively generated, with the one striking exception that the activity of bimodal neurons similarly and robustly encoded head on body position in all conditions. Our results show that proprioceptive and motor-related signals are combined with vestibular information at the first central stage of vestibular processing in mice. We suggest that these results have important implications for understanding the multisensory integration underlying accurate postural control and the neural representation of directional heading in the head direction cell network of mice.