Evidence for genetically mediated dysfunction of the central dopaminergic system in the stargazer rat

Ethovision™ analysis of open field behaviour in rats following bilateral vestibular loss

Bilateral vestibular loss (BVL) causes a unique behavioural syndrome in rodents, with symptoms such as locomotor hyperactivity and changes in exploratory behaviour. Many of these symptoms appear to be indirect consequences of the loss of vestibular reflex function and are difficult to explain. Although such symptoms have been reported before, there have been few systematic studies of the effects of BVL using automated digital tracking systems in which many behavioural symptoms can be measured simultaneously with high precision. In this study, data were obtained from rats with BVL induced by intratympanic sodium arsanilate injections (n = 7) or sham injections (n = 8) and their behaviour in the open field was measured at 3 days and 23 days post-injection using Ethovision™ tracking software. BVL rats demonstrated reduced thigmotaxis, with more time spent in the central zones. Twenty-three days post-injection, BVL animals showed increased locomotor activity in the open field. The increase in activity was also reflected in the number of transitions between each zone of the field. In addition to increased activity, BVL animals showed increased whole body rotations following lesions. Using linear discriminant analysis (LDA) and random forest classification (RFC), we were able to show that the indirect behavioural effects of BVL, excluding direct measurement of vestibular reflex function, could correctly predict whether animals had received a BVL with a high degree of accuracy at both day 3 and day 23 post-BVL (83% and 100% for LDA, and 100% and 100% for RFC, respectively). RFC has been similarly successful in classifying other hyperactivity syndromes such as attention deficit hyperactivity disorder. These results suggest that BVL results in a unique behavioural signature that can identify vestibular loss in rats even without direct vestibular reflex measurements.

Impaired tilt perception in Parkinson's disease: a central vestibular integration failure

Introduction

Impaired balance control is a hallmark symptom in Parkinson’s disease (PD). Altered sensory-motor integration contributes to the deficiency. We aimed to determine whether impaired vestibular signal processing added to the disorder. We exposed patients (N = 11; 68±6y) and age-matched healthy subjects (hS: N = 19; 65±11y) on a motion platform in complete darkness to two consecutive forward tilt movements (12 series; N = 24; overall 288 trials) and asked them to indicate which tilt was perceived larger. By combing tilt movements with translations we manipulated vestibular sensory input in order to investigate whether putative impairment resulted from a deficiency of the sensory organs (semicircular canals in ‘single-SCC-cue-condition’, otoliths in ‘single-OT-cue-condition’) themselves or to a sensory integration failure (‘multi-cue-condition’).

Results

Tilt discrimination in the multi-cue-condition was inferior in patients compared to hS (p = 0.02). No significant differences between the two groups were found for both single-cue-conditions. Comparison of multi-cue-condition with a prediction resulting from the combination of both single-cue-conditions by optimal observer theory revealed that patients (p = 0.04), in contrast to hS, failed to efficiently combine SCC and OT information to improve tilt perception.

Conclusion

We found that PD patients distinguished forward tilts less precise than hS, suggesting impaired vestibular perception. Tilt discrimination in patients, moreover, did not improve as much as in hS in conditions where both SCC and OT information was available compared to conditions where only SCC or OT cues were activated. The latter provides evidence that tilt misperception in PD most likely results from an integration failure of vestibular signals.

The D₂ dopamine receptor and locomotor hyperactivity following bilateral vestibular deafferentation in the rat

Rats and mice with bilateral vestibular loss exhibit dramatic locomotor hyperactivity and circling behaviours, which to date cannot be explained. Dysfunction of the striatal dopaminergic system is responsible for a number of known movement disorders and the D(2) dopamine receptor is known to be implicated. Therefore, it is possible that changes in striatal function are responsible for locomotor hyperactivity and circling following bilateral vestibular lesions. The aim of this study was to investigate the effects of the D(2) receptor antagonist, eticlopride (0.02, 0.04 and 0.06mg/kg; s.c.), on locomotor behaviour in rats at 5 months following bilateral vestibular deafferentation (BVD), using an open field maze. The levels of the D(2) receptor protein in the striatum were measured at 1 and 6 months post-BVD using western blotting. BVD rats exhibited locomotor hyperactivity and circling, which eticlopride did not eliminate. However, BVD rats did exhibit a decreased response to the inhibitory effect of eticlopride compared to sham controls at the 0.02 mg/kg dose. There were no changes in the amount of the D(2) receptor in the striatum at 1 or 6 months post-BVD; however, D(2) receptor levels were significantly higher on the right side than the left in both sham and BVD animals. These results suggest that locomotor hyperactivity and circling behaviours following BVD are not due simply to changes in D(2) receptor protein expression in the striatum and that other neurophysiological changes in the brain account for these behaviours following BVD.

Comparative analysis of anxiety-like behaviors and sensorimotor functions in two rat mutants, ci2 and ci3, with lateralized rotational behavior

There is increasing evidence that developmental anomalies of cerebral asymmetry are involved in the etiology of psychiatric disorders, including schizophrenia, depression and anxiety. Thus, rodents with abnormal cerebral lateralization are interesting tools to study the association between such anomalies and behavioral dysfunction. The most studied indicator of cerebral asymmetry in the rat is that of circling or rotational behavior. We have recently described two rat mutants, ci2 and ci3, in which lateralized rotational behavior occurs either spontaneously or in response to external stimuli, such as new environment or handling. While cochlear and vestibular defects are found in ci2 rats, ci3 rats do not exhibit any inner ear abnormalities. The abnormal motor response to external stimuli raised the possibility that the circling rat mutants may be more likely to express anxiety-related behavior in tests of emotionality. In the present study, we characterized anxiety-related behaviors of ci2 and ci3 rats in the open field, elevated plus-maze and light/dark exploration test. Furthermore, sensorimotor functions of these rats were evaluated by the rotarod, accelerod and wire hang tests. Heterozygous (ci2/+) littermates or rats of the respective background strains (LEW, BH.7A) were used as controls. In contrast to our expectation, both mutants demonstrated less anxiety-related behavior than controls in tests of emotionality. Ci3 rats exhibited normal sensorimotor functions, whereas marked impairment was observed in ci2 rats, which is most likely a consequence of the vestibular dysfunction in these animals. The acoustic startle response (ASR) and prepulse inhibition of ASR did not differ between ci3 rats and controls. The reduced emotionality of the mutant rats indicated by the present experiments may not be specifically linked to anxiety per se, but is maybe more reflective of impulsivity or the inability to normally perceive or process potentially threatening situations. Based on previous findings of dysfunctions of the central dopamine system in ci2 and ci3 mutant rats, we assume that alterations in dopaminergic activity are involved in the maladaptive behavior observed in the present study.

Auditory and vestibular defects and behavioral alterations after neonatal administration of streptomycin to Lewis rats: Similarities and differences to the circling (ci2/ci2) Lewis rat mutant

The clinical usefulness of aminoglycoside antibiotics is limited by their ototoxicity. In rodents, damage to the inner ear is often associated with rotational behavior and locomotor hyperactivity reminiscent of such behaviors resulting from an imbalance of forebrain dopamine systems. Based on previous observations in the circling (ci2/ci2) Lewis (LEW) rat mutant, a spontaneous mutation leading to hair cell loss, deafness, impairment of vestibular functions, lateralized circling, hyperactivity and alterations in the nigrostriatal dopamine system, we have recently hypothesized that vestibular defects during postnatal development, independent of whether induced or inherited, lead to secondary changes in the dopaminergic system within the basal ganglia, which would be a likely explanation for the typical behavioral phenotype seen in such models. In the present study, we directly compared the phenotype induced by streptomycin in LEW rats with that of the ci2 LEW rat mutant. For this purpose, we treated neonatal LEW rats over 3 weeks by streptomycin, which induced bilateral degeneration of cochlear and vestibular hair cells. Following this treatment period, the behavioral syndrome of the streptomycin-treated animals, including the lateralized rotational behavior, was almost indistinguishable from that of ci2 mutant rats. However, in contrast to the ci2 mutant rat, all alterations, except the hearing loss, were only transient, disappearing between 7 and 24 weeks following treatment. In conclusion, in line with our hypothesis, vestibular defects induced in normal LEW rats led to the same phenotypic behavior as the inherited vestibular defect of ci2 mutant rats. However, with increasing time for recovery, adaptation to the vestibular impairment developed in streptomycin-treated rats, while all deficits persisted in the mutant animals. At least in part, the transient nature of the abnormal behaviors resulting from treatment with streptomycin could be explained by adaptation to the vestibular impairment by the use of visual cues, which is not possible in ci2 rats because of progressive retinal degeneration in these mutants. Although further experiments are needed to prove this hypothesis, the present study shows that direct comparisons between these two models serve to understand the mechanisms underlying the complex behavioral phenotype in rodents with vestibular defects and how these defects are compensated.

Defective balancing ability and hyperactivity in the CLX (circling behavior linked to the X-chromosome) mutant rat

We have reported that the recently described circling behavior rat (CLX) is a hereditary mutant controlled by a single sex-linked recessive gene (gene symbol: clx). This mutant shows intermittent circle walking and/or running and head tossing with the neck twisted. The abnormal behavior begins to appear around weaning and continues throughout life. In the present study, behavioral tests were performed during the suckling and post-weaning periods and when the rats reached maturity, and the following peculiar abnormalities were revealed: (1) in the righting reflex test, the CLX young show a tendency to take a longer time to revert to normal posture; (2) in the negative geotaxis test, they had difficulty moving upward at 12 days of age; (3) in the air righting reflex test, they frequently fell on their backs or shoulders even after weaning; (4) almost none of the CLX rats showed nystagmus, which is invariably observed in normal rats after rotating stimulation, at 20 weeks of age; and (5) they showed hyperactivity in the open field test at the age of 5 or 6 weeks and a higher degree of locomotor activity in the home cage at the age of 7 and 15 weeks. These results suggest that CLX mutant rats may have some defect in vestibular function (balance sense) or abnormalities in an area of the central nervous system responsible for posture control, e.g., in the dopaminergic or GABAergic neurons.

New behavioral mutant rat exhibiting circling behavior (clx) controlled by a sex-linked recessive gene

A male rat showing intermittent circling behavior was discovered among the Jcl:Wistar rats in our laboratories, and among its backcross offspring individuals showing the same behavior were found. The abnormalities in these animals were characterized by intermittent circling behavior (walking and/or running in circles) and head tossing with the neck twisted. No abnormalities were observed in fertility, delivery or pup mortality. The results of mating experiments indicated that the circling behavior phenotype is controlled by a single sex-linked recessive gene, and the mutant was named "circling behavior linked to X-chromosome (gene symbol: clx)." This circling behavior mutant is considered to be different from the previously reported mutants, the behavior in all of which has been found to be autosomally inherited. Sib-mating is continuing to produce an inbred strain with this newly discovered circling behavior mutant gene.

Spontaneous circling behavior and dopamine neuron loss in a genetically hypothyroid mouse

The genetically hypothyroid mouse, Tshr(hyt), has a single point mutation resulting in a defective thyroid-stimulating hormone receptor, and therefore a non-functional thyroid gland. This is an autosomal recessive disorder and affected mice have been reported to have a number of somatic and behavioral deficits. This study reports a pronounced, spontaneous, asymmetrical circling behavior in the Tshr(hyt) mouse. The spontaneous circling behavior appeared in about 25% of the homozygous animals, in both males and females. The circling usually appeared by postnatal day 35 and continued throughout the lifespan of the animal. The circling was in one direction only, either clockwise or counterclockwise, with the directional preference being almost absolute. A stereological analysis of tyrosine hydroxylase immunoreactive neurons in the substantia nigra and adjacent ventral tegmental area of circling homozygous mice, non-circling homozygous mice and heterozygous mice revealed that the circlers had significantly fewer (40% reduction) midbrain dopamine neurons than those animals that did not circle. There was not an association between the direction of the circling and an asymmetry in the number of dopamine neurons in the midbrains of these mice. There was no difference in the number of dopamine neurons in the midbrain of the homozygous non-circlers and the heterozygous mice. These studies indicate that about 25% of genetically hypothyroid mice demonstrated a spontaneous, perseverative, unilateral circling behavior that was associated with a significant reduction in the number of their midbrain dopamine neurons. Thus congenitally hypothyroid mice are at risk for a reduction in the number of nigral dopamine neurons and an associated repetitive movement disorder.

Auditory and vestibular defects in the circling (ci2) rat mutant

The circling rat is an autosomal recessive mutant (homozygous ci2/ci2) that displays lateralized circling behaviour, locomotor hyperactivity, ataxia and stereotypic head-movement. These abnormal behaviours occur in phases or bursts either spontaneously or in response to stress. Heterozygous (ci2/+) littermates display normal spontaneous behaviours. We have previously found that ci2/ci2 rats of both genders have a lower tissue content of dopamine in the striatum ipsilateral to the preferred direction of rotation, indicating that the rats turn away from the brain hemisphere with higher striatal dopaminergic activity. In view of the similarities of the motor syndrome of the ci2/ci2 mutant rat to that of mouse deafness mutants, the present study evaluated the hearing ability of the circling rat mutant by recording brainstem auditory-evoked potentials. To test for vestibular dysfunction, a swimming test was conducted. Histological methods were used to examine the cochlear and vestibular parts of the inner ear and the cochlear and vestibular brainstem nuclei for defects. The absence of auditory-evoked potentials demonstrated a complete hearing loss in the adult ci2/ci2 mutant rat, whereas heterozygous littermates exhibited auditory-evoked potentials with thresholds resembling those of other laboratory strains. Furthermore, the mutant rats were unable to swim. Histological analysis of the inner ear of adult mutants revealed virtually complete loss of the cochlear neuroepithelium, while no such hair cell degeneration was seen in the vestibular parts of the inner ear. However, part of the vestibular hair cells showed protrusions into the endolymphatic space, suggesting alterations in the cytoskeletal architecture. The histological findings in mutant circling rats strongly indicate that the hearing loss of the mutants is of the sensory neural type, the most prevalent type of hearing loss. In the cochlear nuclei of the brain stem of mutant rats, neurons exhibited an abnormal shape, reduced size and increased density compared to controls. In contrast, no abnormal neuronal morphology was seen in the vestibular nuclei, but a significantly reduced neuronal density was found in the medial vestibular nucleus. Abnormal vestibular function would be a likely explanation for the disturbed balance of mutant rats as exemplified by the ataxia and the inability to swim, whereas the previous data on these rats strongly indicate an involvement of the basal ganglia in the abnormal circling behaviour. The genetic defect in the mutant rats, thus, results in a clinical syndrome with features also seen in human genetic disorders with deafness and hyperkinesia, making the ci2/ci2 rat an excellent model for investigating both cochlear/vestibular dysfunction and hyperkinetic movement disorders.