new-variations-for-strategy-set-shifting-in-the-rat

New Variations for Strategy Set-shifting in the Rat

Behavioral flexibility is crucial for survival in changing environments. Broadly defined, behavioral flexibility requires a shift of behavioral strategy ...

Okinawa Institute of Science and Technology

The mammalian constitutive androstane receptor (CAR) is a transcription factor that participates in controlling the expression of xenobiotic metabolizing and transporting genes in response to xenobiotics in an organ-specific manner. In addition to the wild-type CAR (CAR WT) mRNA, mRNAs for five splice variants (SVs) could be detected in the liver of 7-week-old male Wistar rats by RT-PCR using primer pairs covering a full-length mRNA derived from 9 exons; insertion of 18 bp at the 5'-end of intron 8 with or without deletion of 3 bp from the 5'-end of exon 7 (CAR SV1 or SV2), deletion of 4 bp from the 5'-end of exon 8 (CAR SV3), insertion of 195 bp intron 7 (CAR SV4), and insertion of 91 bp intron 6 (CAR SV5). In contrast, only CAR SV5 was detected in lung. Due to the introduction of novel stop codons, all the SVs were considered to code for premature proteins. The liver homogenate gave two protein bands in the vicinity of 37 kDa on Western blotting. They were attributable to CAR WT and SV-complex, respectively, based on their putative molecular weights in descending order. Upon cotransfection with the reporter plasmid, only the cells transfected with the CAR SV4-expression plasmid showed enhanced luciferase activity similar to the WT-transfected cells, for which the further splicing of the remaining intron 7 seemed to be responsible. The transactivation-defective SVs downregulated CAR WT-induced luciferase activity to some extent in the cotransfection experiments.

Expression of constitutive androstane receptor splice variants in rat liver and lung and their functional properties.

One of the main hallmarks of Alzheimer's disease (AD) is senile plaques composed of amyloid β (Aβ). We developed a new edible vaccine: rice expressing GFP-Aβ42. In a previous report, we described the production of anti-Aβ antibodies in B6 mice fed Aβ rice mixed with cholera toxin B subunit (CTB). In this report, we investigated whether Aβ rice had therapeutic effects in the Tg2576 AD model mice. The anti-Aβ antibody titer was increased and levels of intracerebral Aβ (soluble and insoluble) and serum Aβ decreased. Because the value of IgG1/IgG2a in the Aβ feeding group was >1, immunization via Aβ rice may induce a non-inflammatory Th2 reaction. We also found that the Aβ vaccine improved memory, as assessed in a Y-maze test. The number of arm entries in the Y-maze test was lower in the Aβ feeding group than in the control group. These results suggest that the new edible vaccine Aβ rice may have therapeutic effects in AD.

Effect of rice-expressed amyloid β in the Tg2576 Alzheimer's disease transgenic mouse model.

Stepping over obstacles to avoid tripping is an essential component in safe and smooth locomotion. Obstacle avoidance during locomotion is completed in several steps during the approach phase toward the obstacle and stepping over the obstacle. The purpose of this study was to investigate gait modification during the approach phase when stepping over obstacles of different heights in rats. In all four limbs, the toe height when the toe was just above the obstacle increased depending on the obstacle height, leaving a safe margin. However, the horizontal distance between toe and obstacle just prior to stepping over was not influenced by obstacle height. In the fore- and hindlimbs that served as trailing limbs, it was found that the stride length and its related swing phase duration in the final step were significantly shorter than those in both the penultimate step and overground locomotion. These results suggest that adjustment of trailing limb in the final step during the approach phase is important in preparation for the stepping movement over an obstacle.

Gait modification during approach phase when stepping over an obstacle in rats.

Walking smoothly and safely often involves stepping over an obstacle. The purpose of this study was to examine forelimb movements and toe trajectories in stepping over an obstacle during overground locomotion in rats. We performed a kinematic analysis of forelimb movements and measured electromyographic (EMG) activities in the biceps and triceps brachii of the forelimbs. We found that mean toe height just above the obstacle was lower in the leading forelimb than in the trailing forelimb. The toe positions of the leading forelimb at maximal elevation over the obstacle (peak toe position) were closer to the upper edge of the obstacle than those of the trailing forelimb. The linear distance between peak toe position and the upper edge of the obstacle was significantly less in the leading forelimb compared to the trailing forelimb. The peak toe position of the leading forelimb spatially corresponds to the transition point from flexion to extension of the elbow joint. This transition appeared to be controlled mainly by an offset of EMG activity of the elbow flexor, the biceps brachii muscle. In contrast, the trailing forelimb appeared to be controlled by the shoulder and wrist joints. These results suggest that the toe trajectory of the leading forelimb is more accurately regulated than that of the trailing forelimb. In addition, the activities of the elbow flexor may in part contribute to the toe trajectory of the leading forelimb.

Characteristics of leading forelimb movements for obstacle avoidance during locomotion in rats.

Obstacle avoidance during locomotion is essential for safe, smooth locomotion. Physiological studies regarding muscle synergy have shown that the combination of a small number of basic patterns produces the large part of muscle activities during locomotion and the addition of another pattern explains muscle activities for obstacle avoidance. Furthermore, central pattern generators in the spinal cord are thought to manage the timing to produce such basic patterns. In the present study, we investigated sensory-motor coordination for obstacle avoidance by the hindlimbs of the rat using a neuromusculoskeletal model. We constructed the musculoskeletal part of the model based on empirical anatomical data of the rat and the nervous system model based on the aforementioned physiological findings of central pattern generators and muscle synergy. To verify the dynamic simulation by the constructed model, we compared the simulation results with kinematic and electromyographic data measured during actual locomotion in rats. In addition, we incorporated sensory regulation models based on physiological evidence of phase resetting and interlimb coordination and examined their functional roles in stepping over an obstacle during locomotion. Our results show that the phase regulation based on interlimb coordination contributes to stepping over a higher obstacle and that based on phase resetting contributes to quick recovery after stepping over the obstacle. These results suggest the importance of sensory regulation in generating successful obstacle avoidance during locomotion.

Contributions of phase resetting and interlimb coordination to the adaptive control of hindlimb obstacle avoidance during locomotion in rats: a simulation study.

During locomotion, stepping over an obstacle under visual guidance is crucial to continuous safe walking. Studies of the role of the central nervous system in stepping movements have focused on cerebral cortical areas such as the primary motor cortex and posterior parietal cortex. There is speculation that the lateral cerebellum, which has strong anatomical connections with the cerebral cortex, also plays a key role in stepping movements over an obstacle, although this function of the lateral cerebellum has not yet been elucidated. Here we investigated the role of the lateral cerebellum during obstacle avoidance locomotion in rats with a lateral cerebellar lesion. A unilateral lesion in the lateral cerebellum did not affect limb movements during overground locomotion. Importantly, however, the lesioned animals showed overshooting of the toe trajectory specific to the leading forelimb ipsilateral to the lesion when stepping over an obstacle, and the peak toe position, in which the toe is maximally raised during stepping, shifted away from the upper edge of the obstacle. Recordings of EMG activity from elbow flexor and extensor muscles suggested that the overshooting toe trajectory in the ipsilateral leading forelimb possibly resulted from sustained elbow flexion and delayed elbow extension following prolonged activity of the biceps brachii. These results suggest that the lateral cerebellum specifically contributes to generating appropriate toe trajectories in the ipsilateral leading forelimb and to controlling related muscle activities in stepping over an obstacle, especially when accurate control of the distal extremity is achieved under visual guidance.

Lesion in the lateral cerebellum specifically produces overshooting of the toe trajectory in leading forelimb during obstacle avoidance in the rat.

The importance of the cerebellum in control of locomotion is demonstrated by the ataxic gait of cerebellar patients. The intermediate cerebellum contains somatotopical representations for forelimbs and hindlimbs in both anterior and posterior lobes. However, it is not known whether these separate regions have discrete roles in control of limb movements during locomotion. Here we compared the effect of muscimol-induced inactivation of the anterior or posterior intermediate cerebellum on limb movements in walking rats. Inactivation of the anterior intermediate cerebellum had clear effects on limb movements during overground locomotion, resulting in excessive toe elevation and hyperflexion of joints in the swing phase. Inactivation of the posterior region resulted in similar but less pronounced deficits. Postural defects were not present in either group of rats. These findings suggest that the intermediate cerebellum of the anterior lobe has a greater influence on the ability to control limb movements during overground locomotion than the posterior lobe.

Effect of inactivation of the intermediate cerebellum on overground locomotion in the rat: a comparative study of the anterior and posterior lobes.

The ability to change strategies in different contexts is a form of behavioral flexibility that is crucial for adaptive behavior. The striatum has been shown to contribute to certain forms of behavioral flexibility such as reversal learning. Here we report on the contribution of striatal cholinergic interneurons-a key element in the striatal neuronal circuit-to strategy set-shifting in which an attentional shift from one stimulus dimension to another is required. We made lesions of rat cholinergic interneurons in dorsomedial or ventral striatum using a specific immunotoxin and investigated the effects on set-shifting paradigms and on reversal learning. In shifting to a set that required attention to a previously irrelevant cue, lesions of dorsomedial striatum significantly increased the number of perseverative errors. In this condition, the number of never-reinforced errors was significantly decreased in both types of lesions. When shifting to a set that required attention to a novel cue, rats with ventral striatum lesions made more perseverative errors. Neither lesion impaired learning of the initial response strategy nor a subsequent switch to a new strategy when response choice was indicated by a previously relevant cue. Reversal learning was not affected. These results suggest that in set-shifting the striatal cholinergic interneurons play a fundamental role, which is dissociable between dorsomedial and ventral striatum depending on behavioral context. We propose a common mechanism in which cholinergic interneurons inhibit neurons representing the old strategy and enhance plasticity underlying exploration of a new rule.

Sho Aoki

Neurobiology Research Unit

Okinawa Institute of Science and Technology

New Variations for Strategy Set-shifting in the Rat