Rationale A number of tasks are used to assess working memory in rodents, but the Odor Span task (OST) is unique in studying performance as a function of the number of stimuli to remember. Objectives The purpose of the present study was to better characterize the behavioral pharmacology of the OST by exploring the effects of several amnestic agents including an NMDA antagonist (dizocilpine), a positive GABA-A modulator (chlordiazepoxide), an anticholinergic compound (scopolamine) and as a negative control, an opiate receptor agonist (morphine). Methods Rats were trained to perform on the OST which is a non-match-to-sample procedure with an incrementing number of sample odors to remember as the session progresses. Trials with a simple odor discrimination task (SD) were interspersed to provide a control for effects unrelated to memory load. Results All four drugs disrupted performances on the OST task in a dose-dependent fashion, but only the NMDA antagonist dizocilpine produced impairments that were clearly dependent on the number of stimuli to remember. Dizocilpine impaired OST performance at a dose (0.1 mg/kg) that did not affect SD and that impairment depended on memory-load. Chlordiazepoxide (3.0 mg/kg) also produced amnestic effects that were manifest by shorter memory spans and runs of correct responding. In contrast, morphine and scopolamine impaired OST accuracy only at doses that also disrupted SD (18.0 and 0.3 mg/kg, respectively). Conclusions These results provide evidence of NMDA and benzodiazepine modulation of working memory as assessed by the OST.

The Odor Span Task is an incrementing non-matching-to-sample procedure that permits the study of behavior under the control of multiple stimuli. Rats are exposed to a series of odor stimuli and selection of new stimuli is reinforced. Successful performance thus requires remembering which stimuli have previously been presented during a given session. This procedure has been frequently used in neurobiological studies as a rodent model of working memory; however, only a few studies have examined the effects of drugs on performance in this task. The present experiments explored the behavioral pharmacology of a modified version of the Odor Span Task by determining the effects of stimulant drugs methylphenidate and methamphetamine, NMDA antagonist ketamine, and positive GABAA modulator flunitrazepam. All four drugs produced dose-dependent impairment of performances on the Odor Span Task, but for methylphenidate and methamphetamine, these occurred only at doses that had similar effects on performance of a simple odor discrimination. Generally, these disruptions were based on omission of responding at the effective doses. The effects of ketamine and flunitrazepam were more selective in some rats. That is, some rats tested under flunitrazepam and ketamine showed decreases in accuracy on the Odor Span Task at doses that did not affect simple discrimination performance. These selective effects indicate disruption of within-session stimulus control. Overall, these findings support the potential of the Odor Span Task as a baseline for the behavioral pharmacological analysis of remembering.

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Transitive inference (TI) is considered to be higher order concept learning that requires animals to be able to learn a hierarchy and infer untrained relations among members of the hierarchy. TI has been demonstrated in a variety of species, including rats in which TI emerges when odor stimuli are used (Davis, 1992; Dusek & Eichenbaum, 1997; Jordan, 2009). Rats in these studies were initially trained on four overlapping simple odor discriminations AB+, BC+, CD+, and DE+. Choice of D given the presentation of the untrained pair 'BD' is said to demonstrate transitive inference. These successful TI studies all involved manual presentation of scented sand or scented lids as stimuli. In the present study a systematic replication of Davis (1992) was conducted with seven rats, using an automated olfactometer apparatus. Rats showed strong discriminative performances on trained baseline pairs; however, TI was not consistently observed. Additional stimulus sets were trained to determine whether multiple exemplar training would enhance emergence of TI. With repeated testing, four of the rats did show overall responding consistent with TI. While TI was not observed as clearly as in the manual procedures, the efficiency and objectivity of computerized assessment make this procedure an attractive option. Thus, we are exploring procedural variations to determine training necessary for the consistent emergence of TI.