The staircase test of skilled reaching in mice

Abstract

The “staircase” test has become established for measurement of side-specific deficits in coordinated paw reaching in rats, and has been shown to reveal impairments on the contralateral side following unilateral lesions in a wide range of motor structures of the brain. As mice become more widely used in behavioural neuroscience, we have scaled down the staircase reaching test for application to this latter species. We here validate the test in C57BL/6J mice by (a) establishing the optimal dimensions of the apparatus, (b) comparing the effects of test parameters including sex, test duration, levels of deprivation and alternative reward pellets, and (c) demonstrating contralateral deficits after aspirative lesions of the motor cortex. Differences between mice and rats in normal performance of the task are noted. The staircase test provides a simple objective test of skilled motor function that allows measurement of lateralised effects without unduly constraining the animal, and which may prove as useful for mice as has previously been demonstrated in rats.

Introduction

Mice are increasingly used to study genetic factors in health and disease based on transgenic and targeted knock-out and knock-in strategies 26, 45. However, most behavioural analyses have been developed in rats, and sophisticated behavioural assessment for mice, whether based on comprehensive screening of neurological deficits or targeted assessment of specific functions, has until recently been lacking. This situation is now being rapidly addressed using different strategies of neurological screening to characterise new strains, mutations and transgenes 12, 46. Moreover, more detailed analyses, for example using operant techniques, can discriminate specific aspects of complex function 27, 33.

Motor deficits are a common feature of neurological mutants and transgenic mice, and so have to be evaluated prior to any other level of analysis 3, 7, 30, 39. Most motor testing for mice has been based on descriptions of behaviour in open fields 10, 13, 30 or on coordination and balance on tasks such as the rotarod and raised beam 7, 29, 31. However, observation of mice confirms that they, as in other rodent species, have considerable manipulative ability, most obviously seen when climbing or when picking up pieces of food to investigate or eat.

In order to test manipulative skills, a variety of tasks have been developed for rats based on reaching for food, configuring the apparatus so as to require the animals to make explicit reach and grasp movements to pick up and retrieve food pellets. One of these, the “staircase” test, was introduced for rats in 1990 by Montoya and colleagues 1, 35, 36 and has subsequently been shown to be sensitive to detect unilateral lesions in motor cortex [36], striatum [20], nigrostriatal bundle 2, 5, 52, subthalamic nucleus [25], pedunculopontine tegmental nucleus [15], or ischaemia 22, 32, and the restorative effects of neuroprotection 4, 14, 18, 47 or neuronal grafts 2, 21, 35, 37, 38. We have therefore sought to scale down the rat staircase apparatus to be applicable to mice. We here provide a first report of the viability of this test for mice, we establish the importance of various test parameters such as session length, level of deprivation, and composition of the food pellets used as rewards, and we validate the test by confirming a selective contralateral reaching deficit after unilateral lesion of the motor cortex.