Olfactory sensitivity, learning and cognition in young adult and aged male Wistar rats Simone Kraemer, Raimund Apfelbach * Department of Zoology, Animal Physiology, University of Tuebingen, Auf der Morgenstelle 28, D-72076 Tuebingen, Germany Received 9 October 2003; received in revised form 17 January 2004; accepted 28 January 2004 Abstract Psychophysical experiments with male Wistar rats, ranging from 2 to more than 25 months old, revealed age-related differences in olfactory sensitivity. The highest sensitivities were found in rats 13 months old, and the lowest sensitivity was found in the group aged 25 months and older. Consequently, we considered the hypotheses that young rats will require less time and less trials than aged conspecifics to learn an olfactory discrimination task and that olfactory cognitive abilities will be reduced in older individuals. Rats were initially trained in an olfactometer using operant techniques to discriminate between the odor ethyl acetate (EA) and clean air. Next, young adult and 28-month- old rats were tested on seven different go/no-go odor discrimination tasks. Aged rats performed as well as young adults did on all tasks and we conclude that, for a variety of odor discrimination problems, aged rats show no deterioration in learning ability. This is the first report on olfactory sensitivity, learning ability and cognition in Wistar rats that have passed the normal life span for this strain. Data show that the inability to learn and cognitive deficits do not necessarily develop with age. D 2004 Elsevier Inc. All rights reserved. Keywords: Olfactory sensitivity; Odor discrimination; Odor memory; Aging; Rat 1. Introduction Senescence is characterized by specific disorders includ- ing decreased resistance to common diseases, poor ability to recover after perturbing events, loss of sensory faculties and decreased learning and memory capacities [32,48]. Evi- dence for senescence in other mammals is difficult to obtain and often conflicting [32,36]: Several studies have demon- strated an impairment of learning and memory in old rats [5,7,15,33], while others have reported unchanged learning ability and little or no deterioration [4,17]. Because most experimental tests of learning require at least some locomotory activity, methodological problems in describing age-related learning and memory differences may have impeded progress in understanding cognitive abilities in aged individuals. Overall physical condition is often compromised in older animals compared with younger conspecifics, and this may complicate the assessment of cognitive abilities in the aged individuals. In our compara- tive study of rats of different ages, we therefore decided to use an experimental procedure that places minimal demands on locomotory activity. For rats, stimulus modality is a critical variable in determining efficient learning [43]. If, for instance, a learn- ing task is based on visual cues, rats may only demonstrate mediocre ability in mastering the problem or in remember- ing what they mastered. However, if the task includes odor stimuli, learning improves markedly, typically producing rapid acquisition that is highly resistant to forgetting [34]. We therefore focused on the olfactory modality in the present study and used olfactometer-based tasks to assess learning and memory. During all the training and testing phases, the animal was allowed to sit, stand or lie in the testing chamber of the olfactometer; only sniffing, licking and head movements were required for the performance of the olfactory tasks. With this approach, the possible confounding effects of physical condition and motor capacity can be largely ex- cluded in the test of cognitive ability. In the present study, we established age-related olfactory sensitivity for the odorant ethyl acetate (EA). Additionally, in two related series of experiments, we tested the hypoth- eses that young rats will learn olfactory discrimination tasks 0031-9384/$ – see front matter D 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.physbeh.2004.01.012 * Corresponding author. Fax: +49-7071-294634. E-mail address: raimund.apfelbach@uni-tuebingen.de (R. Apfelbach). Physiology & Behavior 81 (2004) 435 – 442