J. Caston

Salicylate Induces Tinnitus through Activation of Cochlear NMDA Receptors

Salicylate, the active component of aspirin, is known to induce tinnitus. However, the site and the mechanism of generation of tinnitus induced by salicylate remains unclear. Here, we developed a behavioral procedure to measure tinnitus in rats. The behavioral model was based on an active avoidance paradigm in which rats had to display a motor task (i.e., to jump on a climbing pole when hearing a sound). Giving salicylate led to a decrease in the percentage of correct responses (score) and a drastic increase in the number of false positive responses (i.e., animals execute the motor task during a silent period). Presentation of the sound at a constant perceptive level prevents decrease of the score, leading to the proposal that score is related to hearing performance. In contrast, the increase of false positive responses remained unchanged. In fact, animals behaved as if they hear a sound, indicating that they are experiencing tinnitus. Mefenamate in place of salicylate also increased the number of false positive responses, suggesting that salicylate-induced tinnitus is related to an inhibition of cyclooxygenase. One physiological basis of salicylate ototoxicity is likely to originate from altered arachidonic acid metabolism. Because arachidonic acid potentiates NMDA receptor currents, we tested the involvement of cochlear NMDA receptors in the occurrence of tinnitus. Application of NMDA antagonists into the perilymphatic fluids of the cochlea blocked the increase in pole-jumping behavior induced by salicylate, suggesting that salicylate induces tinnitus through activation of cochlear NMDA receptors.

Rearing environmental enrichment in two inbred strains of mice: 1. Effects on emotional reactivity.

The effects of an enriched rearing environment on two types of anxiety-like behavior (designated “trait” and “state” anxiety) and on spontaneous activity were investigated in two inbred strains of mice, BALB/c (C) and C57BL/6(B6). Subjects were socially reared from birth to 56 days of age under enriched or standard rearing conditions. The enriched environment consisted of an assembly of plastic boxes in which a various number of objects (running wheels, pieces of plastic, etc.) offered the possibility of multiple activities. The subjects were subsequently tested in three situations: a spontaneous activity recorder, an elevated plus-maze test (a model of state anxiety), and a free exploration test (a model of trait anxiety). No group differences could be found in spontaneous activity. Environmental enrichment, however, decreased the level of both types of anxiety-like behavior in the C strain. In contrast, the level of trait anxiety of the B6 mice was not modified. The results were discussed in relation to possible CNS modifications, especially in the limbic system.

Effects of early midline cerebellar lesion on cognitive and emotional functions in the rat.

Midline lesion of the cerebellum was performed in young 10-day-old DA/HAN strained (pigmented) rats. Once adults, the lesioned animals were subjected to a series of behavioral tests and their performances were compared with those of control (nonlesioned) rats. Compared with controls, the spontaneous motor activity of the lesioned rats was higher, they showed persevering behavior and did not pay attention to environmental distractors. In anxiety and social discrimination tests, disinhibition tendencies were obvious, which suggested that the animals were less dependent on the context. These abnormalities were most likely due to early midline lesion of the cerebellum and not to a deficit in maternal care before weaning, since the dams took care of the lesioned and control pups similarly. From these results, it can be concluded that the cerebellar vermis is involved in motor control, attentional capabilities and emotional behavior. Given that the lesioned rats observed in this study presented obvious autistic-like symptoms, and since a number of autistic subjects have cerebellar deficits and, particularly, a hypoplasia of vermal lobules, our results may strengthen the idea that the cerebellar vermis is involved in autism, as already suggested in the guinea pig (Caston J, et al. Eur J Neurosci 1998;10:2677-2684).

Acute Stress in Pregnant Rats: Effects on Growth Rate, Learning, and Memory Capabilities of the Offspring

Growth rate of the offspring of female rats stressed by the presence of a cat at the 10th or the 19th gestational day was lower than that of controls whereas footshocks administered at the same periods did not significantly influence growth rate of the young. Whatever the nature of the stress and the time when it was administered to the mother, the death rate of the young rats was much greater than that in controls. When adult, the offspring of stressed mothers exhibited learning and memory impairments in a delayed alternation task as well as in passive avoidance conditioning. Alteration of these cognitive functions is interpreted in terms of subtle dysfunctions in the development of the nervous system through modifications of the hormonal components of the mothers, particularly eventual alterations of the nervous system biochemistry of the offspring.

Chronic stress in pregnant rats: effects on growth rate, anxiety and memory capabilities of the offspring.

Female rats were repeatedly stressed for 10 periods of 15 min by the presence of a cat, at the 10th (S10) or the 19th (S19) gestational day. The litter from stressed females often contained a majority of males or a majority of females, especially in the S19 group. The death of pups was dramatically high in the S19 group and, compared with controls, growth of the surviving animals was slower. When adult, their long-term memory was altered and they exhibited an aversive behavior relative to wide areas. Moreover, cognitive alterations were revealed by the low level of exploration and the inability to rapidly process the relevant environmental cues. These deficits resemble those of psychiatric patients who had been submitted to pre-natal stress.

Effects of midline and lateral cerebellar lesions on motor coordination and spatial orientation

Rats were lesioned in the midline cerebellum, comprising the vermis and fastigial nucleus, or the lateral cerebellum, comprising the cerebellar hemispheres and dentate nucleus, and evaluated in a series of motor and non-motor learning tests. Rats with midline lesions had difficulty in maintaining their equilibrium on a bridge and were slower before turning upward and traversed less squares on an inclined grid. They were not impaired for muscle strength when suspended from a horizontal wire. Rats with lateral lesions had milder deficits on the bridge and were not affected in the other two tests. In the Morris water maze test, rats with lateral lesions were deficient in spatial orientation, whereas rats with midline lesions were deficient in visuomotor coordination. Lateral lesions had no effects on visual discrimination learning. These results illustrate the differential influence of midline as opposed to lateral cerebellar regions on both motor and non-motor behaviors. Fastigial nucleus lesions decreased the time spent in equilibrium and latencies before falling on the bridge and the distance travelled along the inclined grid but had no effect on muscle strength when suspended from the horizontal string. Quadrant entries and escape latencies were higher in rats with fastigial lesions during the hidden platform condition of the Morris water maze but not during the visible platform condition. It is concluded that fastigial-lesioned rats are impaired in equilibrium and spatial orientation but with repeated trials learn to improve their performances.

Motor skills and motor learning in Lurcher mutant mice during aging.

Motor learning abilities on the rotorod and motor skills (muscular strength, motor coordination, static and dynamic equilibrium) were investigated in three-, nine-, 15- and 21-month-old Lurcher and control mice. Animals were subjected to motor training on the rotorod before being subjected to motor skills tests. The results showed that control mice exhibited decrease of muscular strength and specific equilibrium impairments in static conditions with age, but were still able to learn the motor task on the rotorod even in old age. These results suggest that, in control mice, efficiency of the reactive mechanisms, which are sustained by the lower transcerebellar loop (cerebello-rubro-olivo-cerebellar loop), decreased with age, while the efficiency of the proactive adjustments, which are sustained by the upper transcerebellar loop (cerebello-thalamo-cortico-ponto-cerebellar loop), did not. In spite of their motor deficits, Lurcher mutants were able to learn the motor task at three months, but exhibited severe motor learning deficits as soon as nine months. Such a deficit seems to be associated with dynamic equilibrium impairments, which also appeared at nine months in these mutants. By two months of age, degeneration of the cerebellar cortex and the olivocerebellar pathway in Lurcher mice has disrupted both lower and upper transcerebellar loops. Disruption of the lower loop could well explain precocious static equilibrium deficits. However, in spite of disruption of the upper loop, motor learning and dynamic equilibrium were preserved in young mutant mice, suggesting that either deep cerebellar nuclei and/or other motor structures involved in proactive mechanisms needed to maintain dynamic equilibrium and to learn motor tasks, such as the striatopallidal system, are sufficient. The fact that, in Lurcher mutant mice, motor learning decreased by the age of nine months suggests that the above-mentioned structures are less efficient, likely due to degeneration resulting from precocious and focused neurodegeneration of the cerebellar cortex. From this behavioral approach of motor skills and motor learning during aging in Lurcher mutant mice, we postulated the differential involvement of two transcerebellar systems in equilibrium maintenance and motor learning. Moreover, in these mutants, we showed that motor learning abilities decreased with age, suggesting that the precocious degeneration of the cerebellar Purkinje cells had long-term effects on motor structures which are not primarily affected. Thus, from these results, Lurcher mutant mice therefore appear to be a good model to study the pathological evolution of progressive neurodegeneration in the central nervous system during aging.

Differential roles of cerebellar cortex and deep cerebellar nuclei in the learning of the equilibrium behavior : studies in intact and cerebellectomized lurcher mutant mice

Three- to 6-month-old lurcher mutant mice (+/lc), which exhibit a massive loss of neurons in the cerebellar cortex and in the inferior olivary nucleus but whose deep cerebellar nuclei are essentially intact, were trained daily, for 9 days, to maintain their equilibrium upon a rota rod rotating at 20 or 30 revolutions per minute (rpm). Their scores were measured and their behavior upon the rotating rod quantified in comparison to those of matched control (+/+) mice. Lurcher mice were able to learn to maintain their equilibrium efficiently when rotated at 20 rpm but were not when rotated at 30 rpm. After cerebellectomy, the equilibrium capabilities of the animals were much altered, especially in +/lc. These results show that the deep cerebellar nuclei are sufficient for motor learning, provided the task is not too difficult (20 rpm), but that the cerebellar cortex is required when the task is more difficult (30 rpm). Therefore, it can be concluded that the adaptive motor capabilities of lurcher mice are less developed than those of control animals.

Role of the cerebellum in exploration behavior

Compared to +/+ mice, Lurcher (+/Lc) mutant mice whose cerebellar cortex is lacking almost all Purkinje cells and granule cells, exhibit a low level of exploration; this deficit is not due to a low level of activity but to both a decreased motivation to explore a novel environment and to spatial deficits. The characteristics of exploration in cerebellectomized +/+ and +/Lc mice suggest that the cerebellum is involved not only in cognitive but also in motivational processes.

Role of the inferior olivary complex in motor skills and motor learning in the adult rat

The inferior olivary complex of adult rats was chemically destroyed using intraperitoneal injection of 3-acetylpyridine. Animals were submitted to different motor tasks: hanging test, equilibrium test and motor co-ordination test. The different scores show that 3-acetylpyridine-treated rats had motor co-ordination and static equilibrium deficiencies, whereas their rod suspension capabilities were intact. Animals were also trained on an unrotated rod or on a rod rotating at 5, 10 or 20 r.p.m. 3-Acetylpyridine-treated rats were able to maintain their equilibrium on the unrotated rod and at 5 r.p.m. Moreover, after motor training at 5 r.p.m., rats were able to improve their motor skills and reached the same score as controls. Despite their good motor skills, animals were unable to maintain their equilibrium when rotated at 10 and 20 r.p.m. These results suggest that the inferior olivary complex is needed for motor learning involving the temporal organization of movement.