Revisiting places passed: Sensitization of exploratory activity in rats with hippocampal lesions (original) (raw)
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European Journal of Neuroscience, 2002
The contribution that components of the hippocampal system in the rat make to the modulation of attention or stimulus processing was assessed using several simple behavioural assays: the orienting response (OR) to a novel stimulus, the subsequent habituation and dishabituation of this OR, and the latent inhibition effect that typically results from repeated exposure to a stimulus. Excitotoxic lesions of components of the hippocampal system produce dissociable effects on the OR, habituation and latent inhibition: lesions of the entorhinal cortex have no effect on the OR or changes in the OR during exposure to a stimulus, but disrupt latent inhibition; lesions of the subiculum disrupt the OR but not latent inhibition; and lesions of the hippocampus disrupt the OR and latent inhibition. These effects have important implications for our understanding of habituation and latent inhibition, and the neural mechanisms involved in attentional modulation.
Hippocampus, 2010
Performance in several memory tasks is known to be unaffected by hippocampal damage sustained before learning, but is severely disrupted if the same damage occurs after learning. Memories for preferred locations, or home bases, in exploratory tasks can be formed by rats with hippocampal damage, but it is unknown if the memory for a home base survives hippocampal damage. To examine this question, for 30 min each day for five consecutive days, rats explored a circular open field containing one local cue. By Day 5 the rats preferentially went directly to that location, spent the majority of their time at that location, made rapid direct trips to that location when returning from an excursion and so demonstrated that the location was a home base. Memory for the cued location was examined after a 24 h or 14-day interval with the cue removed. In Experiments 1 and 2, control rats and rats with prior N-methyl-D-aspartic acid hippocampal lesions demonstrated memory of the home base location by making direct trips to that location. In Experiment 3, rats that had first explored the open field and cue and then received hippocampal lesions showed no memory for the cued location. The absence of anterograde impairment vs. the presence of retrograde impairment for memory of a spatial home base confirms a role for the hippocampus in the retention of spatial memory acquired during exploration. V V C 2009 Wiley-Liss, Inc.
Behavioral Neuroscience, 1997
Rats received excitotoxic lesions of different subsystems within the hippocampal systemeither the hippocampus proper (cornu ammonis and dentate gyrus; hippocampal lesions) or the entorhinal cortex-subicular region (entorhinal lesions). Subsequently, their activity in an operant chamber was monitored both before and after footshock had been delivered (Experiment 1). Rats with hippocampal lesions showed enhanced activity before the delivery of footshock and reduced freezing after the delivery of shock. Rats with entorhinal lesions showed control levels of activity before the delivery of footshock and control levels of freezing after the delivery of footshock. Both types of lesion impaired spatial learning in a water maze (Experiment 2). These results suggest that the deficits arising from damage to the hippocampal system in contextual and spatial learning have different origins.
Hippocampus, 2005
The conditioned cue preference (CCP) task was used to study the ability of rats to discriminate between spatial locations. Fooddeprived rats explored an eight-arm radial maze with no food present (pre-exposure). On subsequent days, they were alternately confined in one arm of the maze with food and in another arm with no food (training), followed by a preference test with no food present, to determine if they had learned to discriminate between the two arm locations. No injections were given during the two latter phases. With adjacent radial maze arms, rats given three 10-min pre-exposure sessions and four food-pairing trials exhibited a preference for their food-paired arms; rats not pre-exposed did not exhibit this preference. Rats pre-exposed 30 min after dorsal hippocampus injections of muscimol exhibited the preference. With widely separated maze arms, rats given two training trials with no pre-exposure exhibited a preference for the food-paired arm; rats that were given one pre-exposure session did not. Rats preexposed 30 min after dorsal hippocampus injections of muscimol did not exhibit the preference. The same intrahippocampal muscimol injections that failed to affect the influence of pre-exposure on CCP learning with both arm configurations impaired win-shift performance, a standard test of spatial learning. These findings suggest that a functional dorsal hippocampus is not required for the (incidental or latent) learning that occurs during unreinforced exploration of a novel environment. The information acquired during this activity subsequently produces a latent learning effect if it is used to discriminate between two ambiguous locations (adjacent arms) or a latent inhibition-like effect if it is used to discriminate between two unambiguous locations (separated maze arms). V V
The Effects of Hippocampal Lesions on Response, Direction, and Place Learning in Rats
Behavioral Neuroscience, 2005
Rats with hippocampal or sham lesions were trained to find food on a T maze located at 2 positions. Response rats were required to make a right or left turn. Direction rats were required to go in a consistent direction (east or west). Place rats were required to go to a consistent location, relative to room cues. One place group had distinguishable start points at the 2 maze positions, whereas another place group had start points facing the same side of the room. Controls took longer to solve a place problem than the response and direction problems when the start points were not distinguishable. Rats with hippocampal lesions were not different than controls on the response problem but were impaired on the direction and place problems.
Functional inactivation of the rat hippocampus disrupts avoidance of a moving object
The hippocampus is well known for its critical involvement in spatial memory and information processing. In this study, we examined the effect of bilateral hippocampal inactivation with tetrodotoxin (TTX) in an "enemy avoidance" task. In this paradigm, a rat foraging on a circular platform (82 cm diameter) is trained to avoid a moving robot in 20-min sessions. Whenever the rat is located within 25 cm of the robot's center, it receives a mild electrical foot shock, which may be repeated until the subject makes an escape response to a safe distance. Seventeen young male Long-Evans rats were implanted with cannulae aimed at the dorsal hippocampus 14 d before the start of the training. After 6 d of training, each rat received a bilateral intrahippocampal infusion of TTX (5 ng in 1 μL) 40 min before the training session on day 7. The inactivation severely impaired avoidance of a moving robot (n = 8).
Behavioral Neuroscience, 2007
Dual-process models of recognition memory in animals propose that recognition memory is supported by two independent processes that reflect the operation of distinct brain structures: a familiarity process that operates independently of the hippocampus and a context-dependent (episodic) memory process that is dependent on the hippocampus. A novel variant of an object recognition procedure was used to examine this proposal. Healthy rats showed a preference for exploring a novel object rather than a familiar object: a familiarity-dependent recognition effect. They also showed a preference for exploring a familiar object that was presented in a different spatiotemporal context rather than a familiar object that was presented either in a different spatial or temporal context: a context-dependent form of recognition that is sensitive to "what" object has been presented "where" and "when." Rats with excitotoxic hippocampal lesions showed the familiarity-dependent but not the context-dependent form of recognition. The results provide support for dual-process theories of recognition memory.
Behavioural Brain Research, 2009
Studies show that changes in environmental context alter the spatial firing patterns ('remapping') and increase immediate early gene activation in hippocampal but not subicular neurons. However, such studies rarely report co-occurring behavioural responses. We examined the behavioural effects of habituating rats to a walled open field, and then of changing the environmental context by altering wall patterns and floor colour. These kinds of cue change are known to elicit spatial remapping in hippocampal regions, but not the subiculum. Relative to controls (no cue alterations), alteration of wall patterns elicited significant increases in exploratory locomotion through the open field, while combined alterations of floor colour and wall patterns elicited an even higher increase in exploratory locomotion. In addition, combined alterations (floor colour and wall patterns) significantly increased rearing frequency, and significantly decreased the time the rats spent immobile. These findings are relevant to how changes in environmental context affect neuronal responses in the hippocampal formation, and may aid in the development of novelty-response tasks where novelty resides in open field surfaces.