Use of immediate-early gene expression to map relationships between limbic structures supporting memory

This thesis explores the influence of brain regions within the "extended hippocampal memory system" on the activity of the retrosplenial cortex in the rat. One of the first goals was to use lesion studies to improve the understanding of the vulnerability of the retrosplenial cortex, especially in the context of diencephalic and temporal lobe amnesia. The second was to assess what are the brain areas within the temporal lobe involved in object recognition and how they interact. These two objectives were made possible by visualising immediate-early gene expression. By combining this technique with lesions, distal effects of different lesions (hippocampus, mammillothalamic tract and fornix) on the activity of the retrosplenial cortex were measured. For object recognition, the immediate-early gene imaging enabled the assessment of normal brain activity in rats associated with behavioural discrimination of novelty. The lesion studies provide information about the specific and common vulnerability of the retrosplenial cortex, as all three distal lesions resulted in a decrease of immediate-early gene activity in the retrosplenial cortex. In addition, these findings unify diencephalic amnesia with temporal amnesia, and emphasize the need to study networks or systems instead of individual structure. The immediate-early gene/object recognition experiment implicated the caudal part of the perirhinal cortex (and Te2) and of the hippocampus in object recognition, and highlighted the importance of mapping brain region relationships within a connected system. Taken together, these experiments provide clear support for the concept of an extended hippocampal memory system, but also show how this system may interact with other structures involved in different forms of memory. The findings underlie the potential afforded by use of immediate-early gene expression techniques in animal studies.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Use of immediate-early gene expression to map relationships between limbic structures supporting memory

This thesis explores the influence of brain regions within the 'extended hippocampal memory system' on the activity of the retrosplenial cortex in the rat. One of the first goals was to use lesion studies to improve the understanding of the vulnerability of the retrosplenial cortex, especially in the context of diencephalic and temporal lobe amnesia. The second was to assess what are the brain areas within the temporal lobe involved in object recognition and how they interact. These two objectives were made possible by visualising immediate-early gene expression. By combining this technique with lesions, distal effects of different lesions (hippocampus, mammillothalamic tract and fornix) on the activity of the retrosplenial cortex were measured. For object recognition, the immediate-early gene imaging enabled the assessment of normal brain activity in rats associated with behavioural discrimination of novelty. The lesion studies provide information about the specific and common vulnerability of the retrosplenial cortex, as all three distal lesions resulted in a decrease of immediate-early gene activity in the retrosplenial cortex. In addition, these findings unify diencephalic amnesia with temporal amnesia, and emphasize the need to study networks or systems instead of individual structure. The immediate-early gene/object recognition experiment implicated the caudal part of the perirhinal cortex (and Te2) and of the hippocampus in object recognition, and highlighted the importance of mapping brain region relationships within a connected system. Taken together, these experiments provide clear support for the concept of an extended hippocampal memory system, but also show how this system may interact with other structures involved in different forms of memory. The findings underlie the potential afforded by use of immediate-early gene expression techniques in animal studies

Lesions of the Rat Perirhinal Cortex Spare the Acquisition of a Complex Configural Visual Discrimination Yet Impair Object Recognition

Rats with perirhinal cortex lesions were sequentially trained in a rectangular water tank on a series of 3 visual discriminations, each between mirror-imaged stimuli. When these same discriminations were tested concurrently, the rats were forced to use a configural strategy to solve the problems effectively. There was no evidence that lesions of the perirhinal cortex disrupted the ability to learn the concurrent configural discrimination task, which required the rats to learn the precise combination of stimulus identity with stimulus placement (“structural” learning). The same rats with perirhinal cortex lesions were also unimpaired on a test of spatial working memory (reinforced T maze alternation), although they were markedly impaired on a new test of spontaneous object recognition. For the recognition test, rats received multiple trials within a single session in which on every trial, they were allowed to explore 2 objects, 1 familiar, the other novel. On the basis of their differential exploration times, rats with perirhinal cortex lesions showed very poor discrimination of the novel objects, thereby confirming the effectiveness of the surgery. The discovery that bilateral lesions of the perirhinal cortex can leave configural (structural) learning seemingly unaffected points to a need to refine those models of perirhinal cortex function that emphasize its role in representing conjunctions of stimulus features

Perirhinal cortex lesions in rats: Novelty detection and sensitivity to interference

Rats with perirhinal cortex lesions received multiple object recognition trials within a continuous session to examine whether they show false memories. Experiment 1 focused on exploration patterns during the first object recognition test postsurgery, in which each trial contained 1 novel and 1 familiar object. The perirhinal cortex lesions reduced time spent exploring novel objects, but did not affect overall time spent exploring the test objects (novel plus familiar). Replications with subsequent cohorts of rats (Experiments 2, 3, 4.1) repeated this pattern of results. When all recognition memory data were combined (Experiments 1–4), giving totals of 44 perirhinal lesion rats and 40 surgical sham controls, the perirhinal cortex lesions caused a marginal reduction in total exploration time. That decrease in time with novel objects was often compensated by increased exploration of familiar objects. Experiment 4 also assessed the impact of proactive interference on recognition memory. Evidence emerged that prior object experience could additionally impair recognition performance in rats with perirhinal cortex lesions. Experiment 5 examined exploration levels when rats were just given pairs of novel objects to explore. Despite their perirhinal cortex lesions, exploration levels were comparable with those of control rats. While the results of Experiment 4 support the notion that perirhinal lesions can increase sensitivity to proactive interference, the overall findings question whether rats lacking a perirhinal cortex typically behave as if novel objects are familiar, that is, show false recognition. Rather, the rats retain a signal of novelty but struggle to discriminate the identity of that signal

Association rules for rat spatial learning: the importance of the hippocampus for binding item identity with item location

Three cohorts of rats with extensive hippocampal lesions received multiple tests to examine the relationships between particular forms of associative learning and an influential account of hippocampal function (the cognitive map hypothesis). Hippocampal lesions spared both the ability to discriminate two different digging media and to discriminate two different room locations in a go/no-go task when each location was approached from a single direction. Hippocampal lesions had, however, differential effects on a more complex task (biconditional discrimination) where the correct response was signaled by the presence or absence of specific cues. For all biconditional tasks, digging in one medium (A) was rewarded in the presence of cue C, while digging in medium B was rewarded in the presences of cue D. Such biconditional tasks are “configural” as no individual cue or element predicts the solution (AC+, AD−, BD+, and BC−). When proximal context cues signaled the correct digging choice, biconditional learning was seemingly unaffected by hippocampal lesions. Severe deficits occurred, however, when the correct digging choice was signaled by distal room cues. Also, impaired was the ability to discriminate two locations when each location was approached from two directions. A task demand that predicted those tasks impaired by hippocampal damage was the need to combine specific cues with their relative spatial positions (“structural learning”). This ability makes it possible to distinguish the same cues set in different spatial arrays. Thus, the hippocampus appears necessary for configural discriminations involving structure, discriminations that potentially underlie the creation of cognitive maps

Hippocampus and neocortex: recognition and spatial memory

Recognition and spatial memory are typically associated with the perirhinal cortex and hippocampal formation, respectively. Solely focusing on these structures for these specific mnemonic functions may, however, be limiting progress in the field. The distinction between these subdivisions of memory is becoming less defined as, for example, hippocampal cells traditionally considered to encode locations also encode place–object associations. There is increasing evidence for the involvement of overlapping networks of brain structures for aspects of both spatial and recognition memory. Future models of spatial and recognition memory will have to extend beyond the hippocampus and perirhinal cortex to incorporate a wider network of cortical and subcortical structures

Hippocampal, retrosplenial, and prefrontal hypoactivity in a model of diencephalic amnesia: Evidence towards an interdependent subcortical-cortical memory network

The medial diencephalon is vital for memory, but it is not known why. The present study tested between the predictions of current hypotheses as to why this region is critical for memory. Lesions were made in the rat mammillothalamic tract, the only diencephalic structure consistently associated with amnesia in humans after ischemia. Decreased activity, as measured by immediate-early gene expression (c-fos), was found in three key sites associated with memory function: the hippocampus, the prefrontal cortex, and the retrosplenial cortex. The specificity of these changes was confirmed by the qualitatively different patterns of immediately-early gene changes seen after amygdala lesions, e.g., hypoactivity in the hippocampus and retrosplenial cortex following mammillothalamic tract lesions but not following amygdala lesions. The mammillothalamic lesion results unify substrates linked to diencephalic and temporal lobe amnesia, and thereby support a new account of diencephalic amnesia that emphasizes multiple dysfunctions across hippocampal, retrosplenial, and prefrontal areas. This account suggests a role for the mammillary bodies that is independent of their hippocampal input

Advances in the behavioural testing and network imaging of rodent recognition memory

Research into object recognition memory has been galvanised by the introduction of spontaneous preference tests for rodents. The standard task, however, contains a number of inherent shortcomings that reduce its power. Particular issues include the problem that individual trials are time consuming, so limiting the total number of trials in any condition. In addition, the spontaneous nature of the behaviour and the variability between test objects add unwanted noise. To combat these issues, the ‘bow-tie maze’ was introduced. Although still based on the spontaneous preference of novel over familiar stimuli, the ability to give multiple trials within a session without handling the rodents, as well as using the same objects as both novel and familiar samples on different trials, overcomes key limitations in the standard task. Giving multiple trials within a single session also creates new opportunities for functional imaging of object recognition memory. A series of studies are described that examine the expression of the immediate-early gene, c-fos. Object recognition memory is associated with increases in perirhinal cortex and area Te2 c-fos activity. When rats explore novel objects the pathway from the perirhinal cortex to lateral entorhinal cortex, and then to the dentate gyrus and CA3, is engaged. In contrast, when familiar objects are explored the pathway from the perirhinal cortex to lateral entorhinal cortex, and then to CA1, takes precedence. The switch to the perforant pathway (novel stimuli) from the temporoammonic pathway (familiar stimuli) may assist the enhanced associative learning promoted by novel stimuli

Contrasting brain activity patterns for item recognition memory and associative recognition memory:Insights from immediate-early gene functional imaging

Recognition memory, the discrimination of a novel from a familiar event, can be classified into item recognition and associative recognition. Item recognition concerns the identification of novel individual stimuli, while associative recognition concerns the detection of novelty that arises when familiar items are reconfigured in a novel manner. Experiments in rodents that have mapped the expression of immediate-early genes, e.g., c-fos, highlight key differences between these two forms of recognition memory. Visual item novelty is consistently linked to increased c-fos activity in just two brain sites, the perirhinal cortex and the adjacent visual association area Te2. Typically there are no hippocampal c-fos changes. In contrast, visual associative recognition is consistently linked to c-fos activity changes in the hippocampus, but not the perirhinal cortex. The lack of a c-fos perirhinal change with associative recognition presumably reflects the fact that the individual items in an array remain familiar, even though their combinations are unique. Those exceptions, when item recognition is associated with hippocampal c-fos changes, occur when rats actively explore novel objects. The increased engagement with objects will involve multisensory stimulus processing and potentially create conditions in which rats can readily learn stimulus attributes such as object location or object order, i.e., attributes involved in associative recognition. Correlations based on levels of immediate-early gene expression in the temporal lobe indicate that actively exploring novel stimuli switches patterns of entorhinal–hippocampal functional connectivity to emphasise direct entorhinal–dentate gyrus processing. These gene activity findings help to distinguish models of medial temporal lobe function

Qualitatively different modes of perirhinal–hippocampal engagement when rats explore novel vs. familiar objects as revealed by c-Fos imaging

Expression of the immediate-early gene c-fos was used to test for different patterns of temporal lobe interactions when rats explore either novel or familiar objects. A new behavioural test of recognition memory was first devised to generate robust levels of novelty discrimination and to provide a matched control condition using familiar objects. Increased c-Fos activity was found in caudal but not rostral portions of the perirhinal cortex (areas 35/36) and in area Te2 in rats showing object recognition, i.e. preferential exploration of novel vs. familiar objects. The findings are presented at a higher anatomical resolution than previous studies of immediate-early gene expression and object novelty and, crucially, provide the first analyses when animals are actively discriminating the novel objects. Novel vs. familiar object comparisons also revealed altered c-Fos patterns in hippocampal subfields, with relative increases in CA3 and CA1 and decreases in the dentate gyrus. These hippocampal changes match those previously reported for the automatic coding of object–spatial associations. Additional analyses of the c-Fos data using structural equation modelling indicated the presence of pathways starting in the caudal perirhinal cortex that display a direction of effects from the entorhinal cortex to the CA1 field (temporo-ammonic) when presented with familiar objects, but switch to the engagement of the direct entorhinal cortex pathway to the dentate gyrus (perforant) with novel object discrimination. This entorhinal switch provides a potential route by which the rhinal cortex can moderate hippocampal processing, with a dynamic change from temporo-ammonic (familiar stimuli) to perforant pathway (novel stimuli) influences