Sex, diurnal variation and retention interval differently affect performance of marmoset monkeys in a recognition memory task for object location.

In the spontaneous object-location (SOL) task, the ability to recognize where stimuli were located in a past encounter is assessed. Even if widely used in rodents, several aspects can affect task performance. It is thus important to assess potentially intervening variables in the new monkey SOL task. Here we assessed whether sex (male vs. females), circadian time (morning vs. afternoon) and retention interval (24 vs. 48 h) affect the performance of adult marmosets in this task. Two identical stimuli were initially explored on a 10-min sample trial. Thereafter, preferential exploration of the displaced vs. the stationary object was analyzed on a 10-min test trial. Both sexes similarly explored the displaced object quicker and longer than the stationary item after a 24 h inter-trial interval. This response pattern was also seen when males were similarly assessed in the morning or afternoon. However, males tested after 48 h explored both objects equally and after a similar latency, yet spent more time where the displaced item had been previously located. Task performance was not related to object exploration during encoding and general activity remained constant. Therefore, sex and diurnal variation in circadian time had no effect on this version of the task, whereas place memory naturally decayed after 24 h. Our results underscore the importance of alternative task parameters to evaluate memory decay. Given the widespread use of spontaneous recognition tasks, a more ecologically-inclined task in monkeys will help translate animal data to clinical research.

Differential modulatory effects of cocaine on marmoset monkey recognition memory.

Acute and repeated exposure to cocaine alters the cognitive performance of humans and animals. How each administration schedule affects the same memory task has yet to be properly established in nonhuman primates. Therefore, we assessed the performance of marmoset monkeys in a spontaneous object-location (SOL) recognition memory task after acute and repeated exposure to cocaine (COC; 5mg/kg, ip). Two identical neutral stimuli were explored on the 10-min sample trial, after which preferential exploration of the displaced vs the stationary object was analyzed on the 10-min test trial. For the acute treatment, cocaine was given immediately after the sample presentation, and spatial recognition was then tested after a 24-h interval. For the repeated exposure schedule, daily cocaine injections were given on 7 consecutive days. After a 7-day drug-free period, the SOL task was carried out with a 10-min intertrial interval. When given acutely postsample, COC improved the marmosets' recognition memory, whereas it had a detrimental effect after the repeated exposure. Thus, depending on the administration schedule, COC exerted opposing effects on the marmosets' ability to recognize spatial changes. This agrees with recent studies in rodents and the recognition impairment seen in human addicts. Further studies related to the effects of cocaine's acute×prior drug history on the same cognitive domain are warranted.

Scopolamine Induces Deficits in Spontaneous Object-Location Recognition and Fear-Learning in Marmoset Monkeys.

The non-selective muscarinic receptor antagonist scopolamine (SCP) induces memory deficits in both animals and humans. However, few studies have assessed the effects of amnesic agents on memory functions of marmosets - a small-bodied neotropical primate that is becoming increasingly used as a translational model for several neuropathologies. Here we assessed the effects of an acute SCP administration (0.03 mg/kg, sc) on the behavior of adult marmoset monkeys in two tasks. In the spontaneous object-location (SOL) recognition task, two identical neutral stimuli were explored on the sample trial, after which preferential exploration of the displaced versus the stationary object was analyzed on the test trial. In the fear-motivated behavior (FMB) procedure, the same subjects were submitted to an initial baseline trial, followed by an exposure period to a snake model and lastly a post-exposure trial. All trials and inter-trial intervals lasted 10 min for both tests. Results showed that on the SOL test trial, the saline group explored the displaced object significantly longer than its identical stationary counterpart, whereas SCP-treated marmosets explored both objects equivalently. In the FMB test, the saline group - but not the SCP-treated animals - spent significantly less time where the stimulus had been specifically encountered and more time being vigilant of their surroundings, compared to pre-exposure levels. Drug-related effects on general activity, overall exploration (SOL task) and behavioral response to the aversive stimulus (FMB task) were not observed. SCP thus impaired the marmosets' short-term ability to detect changes associated with the spatial location of ethologically irrelevant (SOL task) and relevant stimuli (FMB task). Similar results have been reported in other animal species. Marmosets may thus help reduce the translational gap between pre-clinical studies and memory-associated human pathologies.