Perspectives on object-recognition memory following hippocampal damage: lessons from studies in rats

Persistent impairment of gait performances and working memory after bilateral common carotid artery occlusion in the adult Wistar rat

BACKGROUND

The clinical and pathophysiological effects of a chronic reduction of cerebral blood flow in humans are not completely known. We investigated whether rats subjected to bilateral common carotid artery occlusion (bCCA-o) developed focal neurological deficits, gait dysfunction, and working memory alterations.

METHODS

Eighteen male Wistar rats were subjected to bCCA-o, 13 were sham-operated. We assessed sensorimotor functions, gait on a 60 cm-long elevated bridge, and working memory (object recognition and Y maze tests) before and 30, 60, and 90 days after surgery. Histological analysis was performed in a subgroup of 10 rats.

RESULTS

No rat showed sensorimotor alterations after surgery. Although gait performances of both bCCA-o and sham-operated rats declined over time, the differences reached statistical significance only for the bCCA-o group (mean+/-SE: 26.8+/-5.0; 22.4+/-4.9; 24.5+/-5.5 cm at 30, 60, and 90 days, respectively) in comparison with baseline (52.9+/-5.2 cm; P<0.05). At 60 and 90 days, bCCA-o rats in comparison with sham-operated rats showed decreased performances on object recognition (discrimination index: 0.15+/-0.03 vs. 0.29+/-0.05 at 60 days and 0.10+/-0.04 vs. 0.41+/-0.07 at 90 days; P<0.05) and on Y maze test (alternating rats: 9.9 vs. 85.7% at 60 days and 16.6 vs. 100% at 90 days; P<0.01). In none of the animals were cerebral infarcts detected. Selective neuronal necrosis was observed in the cortex and hippocampus of both bCCA-o and sham-operated rats without any obvious difference.

CONCLUSIONS

bCCA-o in the Wistar rat induces persistent and progressive gait and working memory impairment without producing sensorimotor deficit or cerebral infarcts. This model may help to elucidate some physiopathological aspects of neurological impairment associated with states of cerebral chronic ischemia.

Critical role of the hippocampus in memory for sequences of events

Recent models of hippocampal function emphasize the potential role of this brain structure in encoding and retrieving sequences of events that compose episodic memories. Here we show that hippocampal lesions produce a severe and selective impairment in the capacity of rats to remember the sequential ordering of a series of odors, despite an intact capacity to recognize odors that recently occurred. These findings support the hypothesis that hippocampal networks mediate associations between sequential events that constitute elements of an episodic memory.

Hippocampal damage and exploratory preferences in rats: memory for objects, places, and contexts

Rats have a natural tendency to spend more time exploring novel objects than familiar objects, and this preference can be used as an index of object recognition. Rats also show an exploratory preference for objects in locations where they have not previously encountered objects (an index of place memory) and for familiar objects in contexts different from those in which the objects were originally encountered (an index of context memory). In this experiment, rats with cytotoxic lesions of the hippocampal formation were tested on all three versions of the novelty-preference paradigm, with a 5-min retention interval between the familiarization and test phases. Rats with sham lesions displayed a novelty preference on all three trial types, whereas the rats with hippocampal lesions displayed a novelty preference on Object trials but did not discriminate between the objects on Place trials or Context trials. The findings indicate that hippocampal damage impairs memory for contextual or spatial aspects of an experience, whereas memory for objects that were part of the same experience are left relatively intact.

cGMP, but not cAMP, in rat hippocampus is involved in early stages of object memory consolidation

The present study investigates the role of cGMP and cAMP on the memory performance in the object recognition task in rats. The analogue 8-Br-GMP or 8-Br-cAMP was administered bilaterally into the hippocampus (0, 1, 3 and 10 microg in 0.5 microl saline/site) immediately after the exposure to two identical objects. After 24 h, saline-treated animals spent equal times exploring a new and the familiar object demonstrating that they did not recognize the familiar one. However, a dose-dependent improvement in object recognition was found after injection of 8-Br-cGMP. In contrast, 8-Br-cAMP did not improve the memory performance at the doses tested. These results indicate that hippocampal cGMP but not cAMP is involved in early stages of consolidation of object memory.