Rats use hippocampus to recognize positions of objects located in an inaccessible space.

Rat hippocampus plays a crucial role in many spatial tasks, including recognition of position of objects, which can be approached and explored. Whether hippocampus is also necessary for recognizing positions of objects located in an inaccessible part of the environment remains unclear. To address this question, we conditioned rats to press a lever when an object displayed on a distant computer screen was in a particular position ("reward position") and not to press the lever when the object was in other positions ("nonreward positions"). After the rats had reached an asymptotic performance, the role of the dorsal hippocampus was assessed by blocking its activity with muscimol. The rats without functional dorsal hippocampus did not discriminate the reward position from the nonreward positions. Then the same rats were trained to discriminate light and dark conditions. The hippocampal inactivation did not disrupt the ability to discriminate these two conditions. It indicated that the inactivation itself had no major effect on the operant behavior and its control by visual stimuli. We conclude that rats use dorsal hippocampus for recognizing positions of objects located in an inaccessible part of the environment.

Real-space path integration is impaired in Alzheimer's disease and mild cognitive impairment.

INTRODUCTION: Path integration (PI) is an important component of spatial navigation that integrates self-motion cues to allow the subject to return to a starting point. PI depends on the structures affected early in the course of Alzheimer's disease (AD) such as the medial temporal lobe and the parietal cortex. OBJECTIVES: To assess whether PI is impaired in patients with mild AD and amnestic mild cognitive impairment (aMCI) and to investigate the role of the hippocampus, entorhinal and inferior parietal cortex in this association. METHODS: 27 patients with aMCI, 14 with mild AD and 18 controls completed eight trials of Arena Path Integration Task. The task required subjects with a mask covering their eyes to follow an enclosed triangle pathway through two previously seen places: start-place1-place2-start. Brains were scanned at 1.5T MRI and respective volumes and thicknesses were derived using FreeSurfer algorithm. RESULTS: Controlling for age, education, gender and Mini-Mental State Examination score the aMCI and AD subjects were impaired in PI accuracy on the pathway endpoint (p=0.042 and p=0.013) compared to controls. Hippocampal volume and thickness of entorhinal and parietal cortices explained separately 36-45% of the differences in PI accuracy between controls and aMCI and 28-31% of the differences between controls and AD subjects. CONCLUSIONS: PI is affected in aMCI and AD, possibly as a function of neurodegeneration in the medial temporal lobe structures and the parietal cortex. PI assessment (as a part of spatial navigation testing) may be useful for identification of patients with incipient AD.

Effect of block of α-1-adrenoceptors on overall motor activity but not on spatial cognition in the object-position recognition task.

Prazosin, an alpha(1)-adrenoceptor antagonist, is well known for its depressant effect on motivation and motor activity, while it has no effect on retention of spatial behavior in several tasks, e.g. in the Morris water maze and radial arm maze. The role of alpha(1)-adrenoceptors in operant tasks with stimulus-controlled behavior has not yet been tested. The present study investigated the effect of prazosin on the modulation of overall motor activity and on cognitive performance in a spatial operant task called object-position recognition task, where operant behavior (lever pressing) was controlled by spatial stimuli displayed on a computer screen. This task has been previously showed to be hippocampal-dependent. Pre-test injection of prazosin at the dose of 3 mg/kg decreased the responding rate, while it did not affect the recognition of object's position. In conclusion, we validated the new cognitive test with a drug with known pharmacological effects on behavior and confirmed the depressant effect of prazosin on motor activity and no effect on retrieval of spatial memory in the hippocampal-dependent operant task.