Olfactory Stimulus Control and the Behavioral Pharmacology of Remembering

Evidence for novelty reward cross-cueing in the odor span task in rats: implications for odor-based reward-motivated tasks

The odor span task (OST) infers working memory capacity (WMC) by requiring rodents to discriminate between previously presented and session-novel odors to obtain a hidden food reward. Here, rats' responses to session-novel odors and food rewards were assessed to determine whether rats use mitigating strategies in the OST. Rats accurately responded to session-novel odors but also reliably responded to the food reward alone and performed at chance when both a session-novel odor and food reward were presented in separate locations. The inclusion of unscented sand in the cups holding the food reward significantly reduced the rats' responses to the food reward alone. Collectively, these results demonstrate the need for rigorous tests of potential mitigating strategies and hold wide implications for rodent odor discrimination-based behavioral tasks.

High-THC Cannabis Smoke Impairs Incidental Memory Capacity in Spontaneous Tests of Novelty Preference for Objects and Odors in Male Rats

Working memory is an executive function that orchestrates the use of limited amounts of information, referred to as working memory capacity, in cognitive functions. Cannabis exposure impairs working memory in humans; however, it is unclear whether Cannabis facilitates or impairs rodent working memory and working memory capacity. The conflicting literature in rodent models may be at least partly because of the use of drug exposure paradigms that do not closely mirror patterns of human Cannabis use. Here, we used an incidental memory capacity paradigm where a novelty preference is assessed after a short delay in spontaneous recognition-based tests. Either object or odor-based stimuli were used in test variations with sets of identical [identical stimuli test (IST)] and different [different stimuli test (DST)] stimuli (three or six) for low-memory and high-memory loads, respectively. Additionally, we developed a human-machine hybrid behavioral quantification approach which supplements stopwatch-based scoring with supervised machine learning-based classification. After validating the spontaneous IST and DST in male rats, 6-item test versions with the hybrid quantification method were used to evaluate the impact of acute exposure to high-Δ9-tetrahydrocannabinol (THC) or high-CBD Cannabis smoke on novelty preference. Under control conditions, male rats showed novelty preference in all test variations. We found that high-THC, but not high-CBD, Cannabis smoke exposure impaired novelty preference for objects under a high-memory load. Odor-based recognition deficits were seen under both low-memory and high-memory loads only following high-THC smoke exposure. Ultimately, these data show that Cannabis smoke exposure impacts incidental memory capacity of male rats in a memory load-dependent, and stimuli-specific manner.

Effects of NMDA antagonist dizocilpine (MK-801) are modulated by the number of distractor stimuli in the rodent odor span task of working memory

The rodent odor span task (OST) uses an incrementing non-matching to sample procedure in which a series of odors is presented and selection of the session-novel odor is reinforced. An OST is frequently used to test the effects of neurobiological variables on memory capacity as the number of odors to remember increases during the course of the session. In this regard, one important finding has been that NMDA receptor antagonists selectively impair OST performance at doses that spare accuracy on control tasks. However, in many versions of the odor span task the number of stimuli to remember is confounded with the number of distractor odors presented to the rat on each trial. The present study compared the effects of the NMDA antagonist dizocilpine when the number of choices was held constant at two (one novel odor-S+ and one previously presented distractor odor-S-) and when the number of choice stimuli was permitted to increase up to 10 (one S+ and 9 S-). Dizocilpine impaired OST accuracy at doses that had no effect on a reference memory control task in both 2-choice and 10-choice conditions; however, the dose-response function was shifted to the left in the 10-choice tests. The impairments produced by dizocilpine were exacerbated as the memory load increased in both 2- and 10-choice conditions. These findings support the hypothesis that NMDA antagonism reduces the number of stimuli that rats can remember accurately, but the interaction between the effective DZP dose and the number of distractors shows that drug effects on OST performances may involve attentional factors in addition to memory capacity. The findings also demonstrate that variations in number of OST distractors can be used to alter sensitivity of the task.

Validation of the human odor span task: effects of nicotine

RATIONALE

Amongst non-smokers, nicotine generally enhances performance on tasks of attention, with limited effect on working memory. In contrast, nicotine has been shown to produce robust enhancements of working memory in non-humans.

OBJECTIVES

To address this gap, the present study investigated the effects of nicotine on the performance of non-smokers on a cognitive battery which included a working memory task reverse-translated from use with rodents (the odor span task, OST). Nicotine has been reported to enhance OST performance in rats and the present study assessed whether this effect generalizes to human performance.

METHODS

Thirty non-smokers were tested on three occasions after consuming either placebo, 2 mg, or 4 mg nicotine gum. On each occasion, participants completed a battery of clinical and experimental tasks of working memory and attention.

RESULTS

Nicotine was associated with dose-dependent enhancements in sustained attention, as evidenced by increased hit accuracy on the rapid visual information processing (RVIP) task. However, nicotine failed to produce main effects on OST performance or on alternative measures of working memory (digit span, spatial span, letter-number sequencing, 2-back) or attention (digits forward, 0-back). Interestingly, enhancement of RVIP performance occurred concomitant to significant reductions in self-reported attention/concentration. Human OST performance was significantly related to N-back performance, and as in rodents, OST accuracy declined with increasing memory load.

CONCLUSIONS

Given the similarity of human and rodent OST performance under baseline conditions and the strong association between OST and visual 0-back accuracy, the OST may be particular useful in the study of conditions characterized by inattention.

Amnestic drugs in the odor span task: Effects of flunitrazepam, zolpidem and scopolamine

The odor span task is an incrementing non-matching-to-sample procedure designed to provide an analysis of working memory capacity in rodents. The procedure takes place in an arena apparatus and rats are exposed to a series of odor stimuli in the form of scented lids with the selection of new stimuli reinforced. This procedure makes it possible to study drug effects as a function of the number of stimuli to remember. In the present study, the non-selective positive allosteric GABA_A receptor modulator flunitrazepam impaired odor span performance at doses that did not affect a control odor discrimination. In contrast, the alpha-1 selective positive GABA_A receptor modulator zolpidem and the cholinergic receptor antagonist scopolamine only impaired odor span at doses that produced more global impairment, including decreased accuracy in the control discrimination and increased response omissions in the both the odor span and control discrimination procedures. Even though the effects of flunitrazepam were selective to odor span performance, they did not depend on the number of stimuli to remember-the same degree of impairment occurred regardless of the memory load. These findings suggest that flunitrazepam interfered selectively with conditional discrimination performance rather than working memory and tentatively suggest that flunitrazepam's selective effects in the odor span task relative to the control odor discrimination are mediated by one or more non-alpha1 GABA_A receptor subtypes.

The chemotherapeutic agent paclitaxel selectively impairs reversal learning while sparing prior learning, new learning and episodic memory

Chemotherapy is widely used to treat patients with systemic cancer. The efficacy of cancer therapies is frequently undermined by adverse side effects that have a negative impact on the quality of life of cancer survivors. Cancer patients who receive chemotherapy often experience chemotherapy-induced cognitive impairment across a variety of domains including memory, learning, and attention. In the current study, the impact of paclitaxel, a taxane derived chemotherapeutic agent, on episodic memory, prior learning, new learning, and reversal learning were evaluated in rats. Neurogenesis was quantified post-treatment in the dentate gyrus of the same rats using immunostaining for 5-Bromo-2'-deoxyuridine (BrdU) and Ki67. Paclitaxel treatment selectively impaired reversal learning while sparing episodic memory, prior learning, and new learning. Furthermore, paclitaxel-treated rats showed decreases in markers of hippocampal cell proliferation, as measured by markers of cell proliferation assessed using immunostaining for Ki67 and BrdU. This work highlights the importance of using multiple measures of learning and memory to identify the pattern of impaired and spared aspects of chemotherapy-induced cognitive impairment.

Behavioral pharmacology of the odor span task: Effects of flunitrazepam, ketamine, methamphetamine and methylphenidate

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 GABA_A 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.