Electronystagmography in the laboratory rat

The Neural Correlates of Spatial Disorientation in Head Direction Cells

While the brain has evolved robust mechanisms to counter spatial disorientation, their neural underpinnings remain unknown. To explore these underpinnings, we monitored the activity of anterodorsal thalamic head direction (HD) cells in rats while they underwent unidirectional or bidirectional rotation at different speeds and under different conditions (light vs dark, freely-moving vs head-fixed). Under conditions that promoted disorientation, HD cells did not become quiescent but continued to fire, although their firing was no longer direction specific. Peak firing rates, burst frequency, and directionality all decreased linearly with rotation speed, consistent with previous experiments where rats were inverted or climbed walls/ceilings in zero gravity. However, access to visual landmarks spared the stability of preferred firing directions (PFDs), indicating that visual landmarks provide a stabilizing signal to the HD system while vestibular input likely maintains direction-specific firing. In addition, we found evidence that the HD system underestimated angular velocity at the beginning of head-fixed rotations, consistent with the finding that humans often underestimate rotations. When head-fixed rotations in the dark were terminated HD cells fired in bursts that matched the frequency of rotation. This postrotational bursting shared several striking similarities with postrotational "nystagmus" in the vestibulo-ocular system, consistent with the interpretation that the HD system receives input from a vestibular velocity storage mechanism that works to reduce spatial disorientation following rotation. Thus, the brain overcomes spatial disorientation through multisensory integration of different motor-sensory inputs.

Nondestructive and objective assessment of the vestibular function in rodent models: A review

The normal function of the vestibular system is crucial for the sense of balance. The techniques used to assess the vestibular function plays a vital role in the research of the vestibular system. In this article, we have systematically reviewed some popular methods employing vestibular reflexes and vestibular evoked potentials for assessing the vestibular function in rodent models. These vestibular reflexes and vestibular evoked potentials to effective stimuli have been used as nondestructive and objective functional measures. The main types of vestibular reflexes include the vestibulo-ocular reflex (VOR), vestibulocollic reflex (VCR), and vestibulo-sympathetic reflex (VSR). They are all capable of indicating the functions of the semicircular canals and otoliths. However, the VOR assessment is much more prevalently used because of the relatively stereotypical inputoutput relationship and simple motion pattern of the ocular response. In contrast, the complicated motion pattern and small gain of the VCR response, as well as the undesired component possibly contributed from the acceleration receptors outside the labyrinths in the VSR response, restrict the widespread applications of VCR and VSR in the assessment of the vestibular system. The vestibular evoked myogenic potentials (VEMPs) and vestibular sensory evoked potentials (VsEPs) are the two typical evoked potentials that have been also employed for evaluating the vestibular function. Through exploiting different types of the VEMPs, the saccular and utricular functions can be evaluated separately. The sound-induced VEMPs, moreover, are capable of noninvasively assessing the unilateral vestibular function. The VsEPs, via the morphology of their signal waveforms, enable the access to the location-specific information that indicates the functional statuses of different components within the vestibular neural pathway.

Fractionating dead reckoning: role of the compass, odometer, logbook, and home base establishment in spatial orientation

Rats use multiple sources of information to maintain spatial orientation. Although previous work has focused on rats' use of environmental cues, a growing number of studies have demonstrated that rats also use self-movement cues to organize navigation. This review examines the extent that kinematic analysis of naturally occurring behavior has provided insight into processes that mediate dead-reckoning-based navigation. This work supports a role for separate systems in processing self-movement cues that converge on the hippocampus. The compass system is involved in deriving directional information from self-movement cues; whereas, the odometer system is involved in deriving distance information from self-movement cues. The hippocampus functions similar to a logbook in that outward path unique information from the compass and odometer is used to derive the direction and distance of a path to the point at which movement was initiated. Finally, home base establishment may function to reset this system after each excursion and anchor environmental cues to self-movement cues. The combination of natural behaviors and kinematic analysis has proven to be a robust paradigm to investigate the neural basis of spatial orientation.

Chronic recording of the vestibulo-ocular reflex in the restrained rat using a permanently implanted scleral search coil

A technique is described which allows accurate long-term monitoring of eye movements in the rat using permanently implanted scleral search coils. Search coils permanently sutured around the sclera yield vestibulo-ocular reflex (VOR) gain and phase values which are comparable to those reported previously in the literature using acutely implanted coils or electrooculographic electrodes. Considerations related to strain, sex and surgical procedures which permit measurement of responses in the chronically restrained rat are described. VOR gain and phase show a time course to their recovery following the implant surgery, with asymptotic performance typically attained approximately 10 days post-surgically. This technique, with the ability to monitor eye movements over weeks to months, appears ideal for development of rodent models of reflex adaptation which require observation of reflex behavior over extended periods of time. Development of a chronic procedure for monitoring eye movement in rodents is especially important given their initial response to restraint (extensive struggling). Finally, adaptation of this technique to smaller species (e.g., mouse) appears technically feasible which should permit the application of transgenic and knockout techniques to the determination of various vestibular reflex functions requiring long-term monitoring.

Short latency vestibular evoked potentials (VsEPs) to linear acceleration impulses in rats

In this study, short latency (t < 12.7 ms) vestibular evoked potentials (VsEPs) in response to linear acceleration impulses were recorded in 37 rats. A new technique (based on a solenoid) was used for generating linear force impulses that were delivered to the animal's head. The impulse had a maximal peak acceleration of 12 g. During the impulse, the displacement was 50 microns (at 4 g) and the rise time was 1.0 ms. A stimulation rate of 2/s was usually used. The VsEPs (averaged responses to 128 stimulations, digital filter: 300-1500 Hz) were recorded with electrodes on pinna and vertex, and were composed of 4-6 clear waves with mean amplitudes (for a 4 g stimulus) of 1-5 microV. The VsEPs were resistant to white noise masking, and were significantly suppressed (P < 0.05) following bilateral application of a saturated KCl solution to the inner ear, showing that contributions of the auditory and somatosensory systems are negligible. The latency of the response decreased as a power law function of stimulus magnitude, and the amplitude of the first wave increased as a sigmoid function of stimulus magnitude. VsEP responses were still present at the lowest intensities attainable (0.06-0.4 g) and reached saturation at 9 g. The amplitude of the later components was reduced when stimulus rate was elevated to 20/s. These results suggest that VsEPs in response to linear accelerations are similar in their nature to VsEPs in response to angular acceleration impulses that were previously recorded. These VsEPs to linear accelerations are most likely initiated in the otolith organs.

Development of vestibular and auditory function: effects of hypothyroidism and thyroxine replacement therapy on nystagmus and auditory evoked potentials in the pigmented rat

The functional development of semicircular canals and some brainstem structures of the auditory system was followed in parallel with time in control and propylthiouracyl-induced hypothyroid pigmented rats by respective recording of postrotatory nystagmus response and auditory evoked brainstem potentials, with the aim of discovering the timing of permanent alterations of these responses in congenital hypothyroidism. A group of hypothyroid rats which under went thyroxine-replacement therapy from postnatal day 12 onward was also included in our studies to corroborate the involvement of thyroid hormones in these effects. Postrotatory nystagmus and auditory evoked responses were absent in congenital hypothyroid rats. In the thyroxine-replaced group postrotatory nystagmus values showed no differences from the control group from postnatal day 28 onward. Auditory evoked potentials in thyroxine-replaced animals could not be elicited at 30 dB, but by increasing the intensity of stimulus to 70 dB, values of latencies of the four waves composing the response were indistinguishable from controls from postnatal day 39 and thereafter. These results show that hypothyroidism affects both semicircular canal and auditory function, the latter more severely than the former, but that these effects can be prevented when thyroxine replacement treatment is started in early stages of postnatal development.

Development of vestibular function: biochemical, morphological and electronystagmographical assessment in the rat

Glutamate decarboxylase and choline acetyltransferase were measured in homogenated ampullar cristae of rats during development from postnatal day 13 to 60 to determine changes in levels of these enzymes during early postnatal development. Afferent and efferent innervation of the hair cells of the developing cristae were studied using electron microscopy. In parallel, groups of rats, postrotatory nystagmus were used to assess the development of semicircular canal function during the same time interval. The level of glutamate decarboxylase was high on postnatal day 15 and did not change notably over the remaining days to day 60. Activity of choline acetyltransferase was nearly absent at day 15, but reached levels seen in mature animals by day 17, and remained almost unchanged thereafter. In contrast, as revealed by electronmicroscopy, afferent and efferent innervation appeared to be mature by day 8. Postrotatory nystagmus presented the adult-like features from day 19 onward. According to these results, a role for glutamate decarboxylase in afferent transmission is suggested by the parallel development of levels of glutamate decarboxylase and afferent innervation of the ampullary cristae. The finding of a similar time course of development of choline acetyltransferase levels and postrotatory nystagmus suggests that a cholinergic efferent innervation is involved in the onset of vestibular-ocular function.

Orientation of the semicircular canals in rat

The orientation of the rat semicircular canals was determined using one of two techniques. Null point analysis was used to define physiologically the planar equations of the anterior (n = 15) and posterior canals (n = 15); equations for the horizontal canal (n = 19) were determined using an anatomical dissection technique. Canal orientation was defined with respect to stereotaxic coordinate system and, for comparison, relative to head position during freeze (startle) behavior. Results show that ipsilateral canal planes are orthogonal within 4-8 degrees, and pairs of right-left synergistic pairs are essentially co-planar. The horizontal canals are inclined upwards 35 degrees with respect to the horizontal plane, but a head position of 43 degrees nose-down was determined to produce near optimal horizontal canal and minimal vertical canal activation with horizontal rotation. Finally, a loud or unexpected auditory stimulus initiates a freeze (startle) response in rat characterized by an transient followed by a sustained head position lasting several seconds. Transients are complete within 300-400 ms. Thereafter, the head becomes momentarily stabilized in the startle position which averaged 14 +/- 8 degrees (nose-down with respect to horizontal stereotaxic zero) across the population (n = 14). The response habituated only slightly, but the final position was sufficiently variable so as to limit the usefulness of the freeze (startle) position as a reference of semicircular canal position in the rat.

Cochlear and vestibular functions of the rat after obliteration of the endolymphatic sac

A combined morphological and physiological study on the effect of saccus obliteration on the cochlea and the vestibular labyrinth of the rat is presented. Endolymphatic hydrops was successfully induced in 49% of the animals. It was frequently associated with fistulae of the membranous wall and degenerative changes in the organ of Corti and spiral ganglion. In hydropic ears electrocochleography revealed a CAP threshold increase only in those cases where hydrops was associated with loss of hair cells and ganglion cells. In two of these animals also an increased negative SP was present. Static otolith reflexes were found to be normal in all hydropic ears despite severe lesions of the saccule in some of these. Abnormal canal reflexes measured by electronystagmography and cupulometry were established in animals with hydrops of the semicircular canals and in one animal with a fistula of the saccule.

Horizontal optokinetic ocular nystagmus in wildtype (B6CBA+/+) and weaver mutant mice

Horizontal optokinetic nystagmus (OKN) evoked by a random dot pattern moving at a constant speed around the animal was investigated in wildtype mice and "Weaver" mutants (cerebellar impairment) by means of chronically implanted EOG-electrodes. The shape of OKN in the homozygotic Weaver mouse was clearly different from that in normal mice. The OKN in the mutant showed inconstant velocity during the slow phase. Nystagmus frequency of the mutant was significantly below that of normal controls for velocities of 1.4 to 25 degrees.s-1. In one group of normals the mean slow-phase gain was relatively constant for stimulus angular velocities between 1.4 and 15 degrees.s-1 and declined thereafter. In a second group the mean slow-phase gain decreased gradually between stimulus angular velocities from 1.4 to 15 degrees.s-1 and thereafter with a steeper slope. In mutants gain decreases with increasing stimulus velocity over the entire range tested (1.4 to 42 degrees.s-1). Normals and mutants with one eye occluded exhibited strong OKN when the pattern was moved in a temporonasal direction; little response was obtained by stimuli moving in a naso-temporal direction.

Vestibular nerve and nuclei unit responses and eye movement responses to repetitive galvanic stimulation of the labyrinth in the rat

Two-second cathodal current pulses were applied at one-minute intervals at a point external to the round window in the ear of each albino rat subject. Responses were recorded in the vestibular nerve ganglion, the vestibular nuclei (single units), or in the eye movements (search coil recording method) of anaesthetized, decerebrated, or alert rats. The unit responses to the galvanic stimuli were characterized and compared with responses to galvanic and rotational stimuli reported in the literature. The main focus of the study, however, was effects of stimulus repetition. In both the vestibular nerve and vestibular nuclei recordings, the responses of many units were substantially larger or smaller at the end of a 13-pulse stimulus train than at the beginning. In the vestibular nuclei, but not in the nerve, there was a slight bias towards a decrease in response magnitude, with 10/88 units showing decreases great enough to be considered as reflecting an habituation process. In contrast, the eye movement responses showed more consistent response decrements, especially in the alert condition, but also in the other conditions (none of the unit recordings were done in alert rats). It is concluded that some of the modifications underlying habituation of the vestibuloocular reflex probably occur in portions of the neuronal reflex pathways that are downstream from the vestibular nuclei.

The vestibular functions of the manganese-deficient rat

Vestibular functions were examined in 25 animals of the second generation of rats raised on a manganese-deficient diet; 9 animals appeared to be normal and 16 showed behavioural abnormalities, characterized by inability to swim and lack of tonic eye deviations. The nystagmic responses elicited by rotatory stimulation appeared to be normal. Histological studies showed that the only defect was the absence of both utricular and saccular otoconia. These rats' swimming disability was similar to that of bilabyrinthectomized rats in normogravic condition and to that of normal rats immersed in water during weightlessness. It is concluded that the observed anomalies must be attributed solely to the lack of normal otolith function.

Virus-induced central positional nystagmus in mice

Geotropic direction-changing nystagmus in lateral body positions was observed in 4-week-old BALB/c mice after intracerebral injection with a temperature-sensitive mutant of mouse hepatitis virus. The positional nystagmus was detected already 2 days after infection and it lasted half a year at least. The nystagmic responses of the semicircular canals were also evaluated before and after infection. They were unaltered during the disease, which was clinically manifested by general weakness, ataxia and tremor. Histopathological examination 2 weeks after infection revealed demyelination in various parts of the CNS.

Eye and head movements in the pigmented rat

Vestibular, optokinetic, and spontaneous eye and head movements have been examined in the hooded rat. Eye movement range was 18-20 degrees, and frequency of ocular saccades was 5-20/min; there was a weak linkage of eye and head movements and a weak vestibulocollic reflex. Response to optokinetic stimulation with unity gain (eye velocity matches drum velocity) was seen only at velocities below 1 deg/sec; maximal eye velocity evoked by drum velocities of over 20 deg/sec never exceeded 4-6 deg/sec. These motor responses were not altered by head movements: thus, gaze velocity is not improved by optocollic (head movement) responses, and such optocollic activity occurs only when substantial retinal image motion is present.

Physiological abnormalities in experimental allergic encephalomyelitis (EAE): II. Correlation between clinical signs and vestibular hyperreactivity and other signs of brain-stem dysfunction in rats with EAE

12 Lewis rats were inoculated with a guinea pig spinal cord tissue preparation. They developed experimental allergic encephalomyelitis (EAE) after 12-14 days manifested by weight loss, tail flaccidity, ataxia, hind limb paresis or paralysis and urinary incontinence. Concomitantly with EAE, all animals developed vestibular hyperreactivity (VH) of canal and otolith reflexes. Other signs of brain-stem dysfunction were also observed: abducens paralysis, facial weakness, tachypnoe and mydriasis with defective pupillary light reflex. The vestibular and other abnormalities subsided with some delay after recovery from clinical EAE, whilst histological abnormalities were still present in the CNS.

Physiological abnormalities in experimental allergic encephalomyelitis (EAE). I. Vestibular hyperreactivity (VH) in rats with EAE

Twelve Lewis rats were inoculated with a guinea pig spinal cord tissue preparation. They developed experimental allergic encephalomyelitis (EAE) after 12-14 days, manifested by weight loss, tail flaccidity, ataxia, hind limb paresis and incontinence. The CNS lesions are produced in this animal model on the basis of inflammatory demyelination, which provides a useful model for multiple sclerosis. Concomitantly with EAE, all animals developed vestibular hyperreactivity (VH) of otolith and canal reflexes. In surviving animals these reflexes renormalized after full clinical recovery. The major effect on the canal response was an increased duration of postrotatory nystagmus caused by an increase in time constant. These findings are similar in part to those previously reported in patients with multiple sclerosis.

Electronystagmographic findings in rats exposed to styrene or toluene

A previously described experimental model for studying the effect of industrial solvents in the vestibular system of rabbits has been applied to rats. To achieve a constant concentration, the solvent was infused intravenously, dissolved in a lipid emulsion. Arterial blood levels were estimated by gas chromatography. The vestibulo-oculomotor behaviour of rats during repeated rotatory acceleration was investigated by electronystagmography. The effect of two solvents-toluene and styrene-on the rotatory induced nystagmus was examined. Both solvents caused an exaggerated reaction at arterial blood levels above 75 ppm. The investigation indicated that the rat will be suitable species for further electronystagmographic investigation of the influence of organic solvents on the vestibular system, for example in screening studies of the toxicity of these solvents.

The organization of the horizontal semicircular duct, ampulla and utricle in the rat and guinea pig

Microdissections of the labyrinth of the guinea pig and rat were carried out with the labyrinth in place in the skull. This enabled photographs to be taken from a standardized identified orientation so as to show the relation of the horizontal semicircular duct and utricle to the planes of the head, and changes in the horizontal duct and utricle during development. It was also possible to show, for the first time, the relation of the horizontal duct and utricle to major brainstem structures. Photographic evidence is presented of the distribution of dark cells around the horizontal duct and ampulla in the guinea pig and of postnatal developmental changes in dark cells in the rat.