Emma L. Lane and Stephen B. Dunnett (eds.)NeuromethodsAnimal Models of Movement Disorders1Volume I10.1007/978-1-61779-298-4_17© Springer Science+Business Media, LLC 2011
17. Comparing Behavioral Assessment of Sensorimotor Function in Rat and Mouse Models of Parkinson’s Disease and Stroke
(1)
Departments of Psychology and Neurology, University of Cincinnati, Cincinnati, IL, USA
(2)
Department of Psychology, University of Texas at Austin, Austin, TX, USA
Abstract
To maximize the success of any translational research endeavor, sensitive and reliable behavioral outcome measures in valid animal models are essential. A common goal of preclinical studies in both Parkinson’s disease and stroke is to reduce or reverse sensorimotor impairments associated with damage to the striatum and sensorimotor cortex. Here, we describe several behavioral tests of sensorimotor function that have been shown to be sensitive to varying degrees of nigrostriatal and cortical damage in both rats and mice and are highly useful for preclinical studies of potential therapeutics.
Key words
Limb-use asymmetryMotor coordinationSensory neglectVibrissae-elicited limb placingMovement disordersPreclinical testing1 Introduction
In movement disorders such as Parkinson’s disease (PD) and stroke, animal models have been integral to the development of current clinical treatments, rehabilitative therapies, and symptomatic treatments commonly used in patients. For example, administration of 3,4-dihydroxyphenylalanine (l-DOPA) remains the gold standard treatment for PD and was originally shown by Carlsson et al. (1) to reverse reserpine-induced akinesia in animals. Several decades later, Bergman et al. (2) showed that lesions of the subthalamic nucleus in nonhuman primates could reverse motor impairments induced by the toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) leading to the development of deep brain stimulation as a treatment for PD. In stroke, tissue plasminogen activator treatment, currently the only established clinical treatment for stroke, was shown to reduce neurological damage following cerebral embolism in rabbits (3). However, despite the progress made, improved treatments are still needed in both PD and stroke. Sensitive behavioral outcome measures in animal models continue to be essential in the discovery and development of potential novel therapies. In this chapter, we highlight several behavioral tests that are uniquely useful in rat or mouse models of PD and stroke. A key characteristic is that these tests are not only highly sensitive to cell loss, but also can detect small gradations in the extent of damage and treatment effects across a wide range of brain degeneration and dysfunction.
1.1 Overview of Rodent Models of Parkinsonism and Stroke
Toxins that relatively selectively disrupt or destroy nigrostriatal dopaminergic neurons in rodents have been used to model Parkin-sonism for over 40 years (4). The drug reserpine and toxins, 6-hydroxydopamine (6-OHDA) and MPTP, continue to contribute to our understanding of basal ganglia circuitry and voluntary movement (1, 5, 6). More recently, toxins, such as paraquat and rotenone, highlight selective vulnerability of nigrostriatal dopaminergic neurons (7–9). Administration of these toxins results in largely selective loss of nigrostriatal dopaminergic neurons and sensorimotor impairments in rats and mice (1, 5, 6, 10–15). These models tend to have good face validity in that they display impairments in movement initiation, weight shifting, and postural stability that are reversible with l-DOPA similar to Parkinson patients (16–18).
Although most cases of PD are sporadic, the discovery of specific mutations in genes that cause familial forms of PD has led to new animal models of PD. Several genes, including α-synuclein, parkin, DJ-1, UCHL1, Pink1, LRRK2, and ATP13A2, have been implicated in familial PD (19–27). Over the last decade, with the discovery of each mutation, mice with similar gain or loss of function mutations have been developed (28–32). Behavioral tests that can be used in both rats and mouse models would be very helpful in evaluating potential therapeutics in multiple models.
The most commonly studied models of stroke include focal ischemia created by transient or permanent occlusion of the middle cerebral artery. Occlusion can be induced with electrocoagulation and introduction of an embolus, vasoconstricting agents, or photothrombosis (33–35). These models can range in their severity of cortical and striatal damage depending on the time of occlusion and reperfusion and whether the proximal or distal branch of the MCAO is affected. Similarly, the extent and permanency of sensorimotor impairment can vary, especially depending on the degree of striatal damage (36).
Following injury or disease, both humans and animals use compensatory strategies to perform tasks accurately, making it difficult to detect impairments (37–39). Therefore, in animal models, it is important to use tests that minimize deficit masking via motor strategies. Here, we highlight several tests that are effective in detecting varying degrees of sensorimotor injury, but also minimize the influence of compensatory movements (see also http://www.schallertlab.org for relevant movies).
2 Materials and Methods
2.1 Limb-Use Asymmetry During Wall Exploration
The limb-use asymmetry test is a measure of voluntary limb use for weight shifting and maintaining stability during vertical exploration of the walls of an enclosure (36, 40–44). This test is commonly used in the unilateral 6-OHDA rat and mouse models of Parkinsonism and focal ischemia (36, 45). Forelimb use during wall explorative activity can be assessed by videotaping rats in a transparent cylinder (20 cm diameter and 30 cm height) until 20 limb usages occur. A mirror placed behind and slightly to the side of the cylinder at an angle enables the rater to record all forelimb movements, including when the animal is turned away from the camera. The image of the entire cylinder and its unobscured mirror image should fill the video frame without overlapping. The camera should be placed several feet away so that its zoom feature can be employed to ensure nonblurred limb usage. The cylindrical shape encourages vertical exploration of the walls with the forelimbs. Several behaviors are scored to determine the extent of the forelimb-use asymmetry displayed by the animal. These behaviors include independent and simultaneous or rapidly alternating use of the left and/or right forelimb for contacting the wall during a full rear. This test has been shown to be highly sensitive to dopamine cell loss and focal ischemia (16, 36, 43, 46) and is widely used to assess the efficacy of potential treatment interventions (46–52). With unilateral dopamine deficiency or middle cerebral artery occlusion, the animal relies primarily on the ipsilateral forelimb for wall support and lateral movements. In contrast, the contralateral forelimb is rarely used (depending on the extent of cell loss) or is used simultaneously for weight support on the wall or alternating with the ipsilateral forelimb, and even with mild degeneration it is not used independently in successive steps (44, 53).
This test can also be adapted to measure spontaneous movement and limb use in bilateral models of Parkinsonism, including MPTP or rotenone-treated rats and genetic mouse models (9, 54–56). Here, the cylinder for mice (height = 15.5 cm, diameter = 12.7 cm) or rats is placed on a piece of glass with a mirror positioned at an angle beneath the cylinder to allow a clear view of movements along the floor and walls of the cylinder. Videotapes are viewed and rated in slow motion and the number of wall-dependent vs. wall-free rears, forelimb wall exploratory movements, both forelimb and hind limb steps, and time spent grooming can be measured within a designated amount of time. In this version of the cylinder test, MPTP or rotenone-treated rats show decreased rearing (especially wall-free (57)) while several genetic mouse models of Parkin-sonism show reduced stepping, especially with the hind limbs (16, 54–56).
2.2 Ledged Beam Test
The ledged beam test has been used in both rats with a unilateral 6-OHDA nigrostriatal microinfusion or middle cerebral artery occlusion. An adapted version has also been shown to be highly sensitive in the unilateral 6-OHDA mouse and several genetic mouse models of Parkinsonism (30, 52, 54–56). For most adult rats, the beam typically should be 130 cm long and tapered in width from 10 to 1.5 cm. The beam is divided and marked into three 43.3-cm bins of increasing difficulty. The animal’s home cage (including bedding), placed on its side at the end of the beam, is used as a potent reinforcer to encourage repeated traverses. Tapered widths should be adjusted to fit the size of the animals tested (for example, for rats weighing 300–500 g, the beam should taper from a width of 6–1.5 cm keeping the length the same). Large, aged rats require a more generous starting width to accommodate the distance between their hind limbs. The surface of the beam is covered with a rubber mat for optimal purchase, and underhanging ledges placed 2.0 cm below the upper surface of the beam permit foot faults to occur without the animal slipping off the beam or needing to compensate with postural adjustments with the intact limb or tail deviations for balance. That is, the ledges (2 cm wide) provide a support for the animal to use if it cannot keep all limbs on the top of the beam. The extent of ledge use gives an indication of the degree of injury or true recovery of function.
The beam is placed at a 15° angle three feet above the ground, and a mirror is placed on one side of the beam to allow simultaneous viewing of all limbs. Animals are trained to walk from the widest, lowest part of the beam to the narrowest, highest part of the beam, which leads directly into the animal’s home cage. To maximize motivation to traverse the beam, it is important that the animals remain in the home cage for a few moments after each trial rather than being picked up immediately for another trial. During pretraining, each animal is placed at the narrow end of the beam in close proximity to the home cage. The distance from the home cage should be progressively increased until the animal is at the widest part of the beam and can traverse the entire length without lateral movements or assistance from the experimenter. Following training trials (approximately 10–15), rats are videotaped as they walk across the beam. Only steps and faults by the animal during forward movement should be scored. Hind limb and forelimb errors are measured separately by recording (1) the number of times an animal places a limb (fore or hind) on the support ledge (foot faults), (2) the location of the fault on the beam, and (3) the type of fault made. In addition, the total number of steps is recorded, which is an important detail because larger rats cross the beam using fewer steps than smaller rats. Steps and faults are not counted if an animal’s head is oriented left or right. This does not alter lesion vs. sham results, but is done because even sham animals use the ledge when oriented laterally during pauses. The number of times the animal steps onto the ledge instead of walking along the beam surface is measured. Foot faults are divided into two categories: full faults, in which the animal places the entire surface of the limb flat upon the support ledge, and half-faults, where the limb lands in between the beam and ledge surfaces (i.e., lands partially on the beam and partially on the ledge). A full fault is given a score of 1.0, and a half-fault is given a score of 0.5. A step is not scored as a foot fault if the animal’s digits extend over the edge of the surface but the rest of the limb rests wholly on the beam surface. Ipsilateral and contralateral hind limb foot faults are recorded. The scores are averaged across five trials for each animal and the percent ipsilateral faults per step and percent contralateral faults per step are calculated. To derive one score, the percent ipsilateral faults per step is subtracted from the percent contralateral faults per step for each bin. Videotapes are viewed using a camcorder with slow motion and frame-by-frame capabilities and rated by an experimenter blind to all experimental conditions.
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