Age sensitivity of behavioral tests and brain substrates of normal aging in mice

Abstract

Knowledge of age sensitivity, the capacity of a behavioral test to reliably detect age-related changes, has utility in the design of experiments to elucidate processes of normal aging. We review the application of these tests in studies of normal aging and compare and contrast the age sensitivity of the Barnes maze, eyeblink classical conditioning, fear conditioning, Morris water maze, and rotorod. These tests have all been implemented to assess normal age-related changes in learning and memory in rodents, which generalize in many cases to age-related changes in learning and memory in all mammals, including humans. Behavioral assessments are a valuable means to measure functional outcomes of neuroscientific studies of aging. Highlighted in this review are the attributes and limitations of these measures in mice in the context of age sensitivity and processes of brain aging. Attributes of these tests include reliability and validity as assessments of learning and memory, well-defined neural substrates, and sensitivity to neural and pharmacological manipulations and disruptions. These tests engage the hippocampus and/or the cerebellum, two structures centrally involved in learning and memory that undergo functional and anatomical changes in normal aging. A test that is less well represented in studies of normal aging, the context pre-exposure facilitation effect (CPFE) in fear conditioning, is described as a method to increase sensitivity of contextual fear conditioning to changes in the hippocampus. Recommendations for increasing the age sensitivity of all measures of normal aging in mice are included, as well as a discussion of the potential of the under-studied CPFE to advance understanding of subtle hippocampus-mediated phenomena.

Keywords: Barnes maze; Morris water maze; cerebellum; context pre-exposure facilitation effect; eyeblink classical conditioning; fear conditioning; hippocampus; rotorod.

Figure 1

The MWM is a test of spatial learning and memory. Through successive placements into the pool of water, the animal learns to swim to the location of the platform with the aid of distal, extra-maze cues placed around the apparatus. A cued condition where the hidden platform is made visible serves as a control for sensory or motor impairment.

Figure 2

The Barnes maze is a test of spatial memory. The apparatus is a large circular platform with holes spaced evenly around the periphery. The animal is placed in the middle of the platform and is motivated to escape the flat, open surface (necessity of escape is accentuated by bright flood lights shined on the platform). Using spatial cues placed around the platform, the animal learns the location of the single hole, which contains a darkened escape tunnel.

Figure 3

The rotorod is a motor task that assesses coordination and learning. It involves a rotating beam on which the animal is placed. The animal must move forward in a coordinated manner to avoid falling.

Figure 4

Eyeblink classical conditioning is a form of associative learning in which a previously neutral stimulus such as a tone or light (CS) is presented briefly before a blink-inducing unconditioned stimulus (US), such as a puff of air to the eye or a shock to the muscles around the eye. In the standard delay paradigm, the CS and US overlap, and coterminate. After repeated pairings, the animal learns to associate the stimuli and begins to blink to the CS before the onset of the US – a behavior termed conditioned response (CR).

Figure 5

Fear conditioning is a form of associative learning in which a previously neutral conditioned stimulus (CS), such as a tone, is paired with an aversive unconditioned stimulus (US), usually a foot shock. Through this pairing, the CS comes to elicit responses characteristically associated with the US.