Laura A. Craig

NMDA-receptor blockade by CPP impairs post-training consolidation of a rapidly acquired spatial representation in rat hippocampus

Recent evidence suggests that N‐methyl‐d‐aspartate (NMDA)‐receptor mediated plasticity in hippocampus has a more subtle role in memory‐based behaviours than originally thought. One idea is that NMDA‐based plasticity is essential for the consolidation of post‐training memory but not for the initial encoding or for short‐term memory. To further test this idea we used a three‐phase variant of the hidden goal water maze task. In the first phase, rats were pretrained to an initial location. Next, intense, massed training was done in a 2‐h interval to teach the rats to go to a new location after either an injection of the NMDA receptor antagonist (6)‐3‐(2‐carboxypiperazin‐4‐yl)propyl‐1‐phosphonic acid (CPP) or of vehicle. Finally, under drug‐free conditions 24 h after new location training, a competition test was done between the original and new locations. We find that N‐methyl‐d‐aspartate (NMDA)‐receptor blockade has little or no effect on new location training. In contrast, when tested 24 h later, the strength of the trace for the new location learned during NMDA‐receptor blockade was much weaker compared with the trace for the new location learned after saline injection. Further experiments showed similar effects when NMDA‐receptors were blocked immediately after the new location training, suggesting that this is a memory consolidation effect. Our results therefore reinforce the notion that hippocampal NMDA‐receptors participate in post‐training memory consolidation but are not essential for the processes necessary to learn or retain navigational information in the short term.

Enhanced cell death in hippocampus and emergence of cognitive impairments following a localized mini-stroke in hippocampus if preceded by a previous episode of acute stress.

This series of experiments represents a test of a theory concerning the etiology of age‐related cognitive decline, including Alzheimers disease (AD). The theory suggests that multiple combinations of cofactors produce variants of these disorders. Two factors that have been linked to the etiology of AD, that are of interest to our laboratories, are stress and vascular strokes. The current experiments tested the cofactors theory by evaluating the neuronal and functional effects of localized subthreshold strokes in the hippocampus of different groups of rats. One group experienced episodes of stress prior to stroke induction while the other did not. The results showed that a low dose of endothelin‐1 (ET‐1) injected into the hippocampus of groups of rats that had previously experienced stressful episodes showed enhanced hippocampal cell death and neurodegeneration that did not occur in the rats that did not experience stress prior to stroke induction. The results also showed that the stressed rats given subthreshold ET‐1 injections into the hippocampus showed hippocampal‐based learning and memory deficits that were not present in the non‐stressed group given the same injections. This pattern of results suggests that individuals that are under stress are more vulnerable to insults to the hippocampus that have little effect on an individual that is not stressed. This vulnerability might be due to the actions of stress hormones, like the glucocorticoids, that have been previously shown to endanger hippocampal neurons.

Enhanced cell death and learning deficits after a mini-stroke in aged hippocampus

One view of the etiology of age-related pathology is that a single genetic abnormality or some other single factor causes the disorder. An alternative view is that multiple combinations of factors produce variants of pathology. For example, the occurrence of stroke increases with age and has been linked to neurodegenerative disorders like Alzheimers disease (AD). The current experiments test the hypothesis that a vascular insult and aging are co-factors that contribute to dementia by evaluating the neuronal and functional integrity of the hippocampus following small, localized strokes induced by the potent vasoconstrictor, endothelin-1 (ET-1) in the rat model of hippocampal aging. The neurotoxic effects of a low dose of ET-1 injected into the hippocampus measured by lesion size (volumetrics) and cell death (Fluorojade-B) were amplified in aged rats. The aged rats also showed hippocampal-dependent memory deficits that were not present in young rats. Overall, our pattern of results suggest that the aged hippocampus is more vulnerable to the same insult that has little or no effect on the young hippocampus.

The etiology of age-related dementia is more complicated than we think

Alzheimers disease (AD) is the most common form of age-related dementia (ARD). Most research directed at understanding the causes of AD is focused on the genetic-based pathology associated with the familial form of this disorder. This is important work and significant progress has been made but 85% of all AD patients have the sporadic form of the disorder. This means that a complete understanding of these complex disorders will remain elusive unless alternative approaches are developed. In this paper we want to make two main points. First, we argue that the current diagnostic distinctions between AD and ARD do not accurately reflect the heterogeneity of these disorders. Second, we present an approach to understanding the etiology of these disorders by suggesting that multiple combinations of co-factors produce variants of the sporadic form of AD. Various proof of principle experiments are presented and the mechanistic and treatment implications of this view are discussed.

Emergence of spatial impairment in rats following specific cholinergic depletion of the medial septum combined with chronic stress

A consistent finding in patients suffering from Alzheimers disease is a loss of the cholinergic neurons of the basal forebrain that project to the hippocampus. However, the role this depletion plays in the development of Alzheimers disease remains unclear. The loss of this ascending neurotransmitter system could potentially render hippocampal neurons more susceptible to further insult, such as chronic stress, ultimately resulting in neuronal death and memory loss. We explored this possibility by using the highly specific toxin 192 IgG‐Saporin to destroy the majority of cholinergic activity in the septo‐hippocampal pathway in rats. Following depletion, rats were subjected to 2 weeks of restraint stress. Rats were divided into two groups and were tested either on a hippocampal‐dependent (water maze) task or a hippocampal‐independent task (fear conditioning to tone and context). We showed that cholinergic depletion or stress alone had no effect on the successful performance of either of the tasks. However, rats with a combination of cholinergic depletion and stress were significantly impaired on the water‐maze task. No deficits were apparent in the combined group that was tested on fear conditioning to tone or context, suggesting that this impairment is specific to spatial working memory. These rats had no obvious hippocampal neuronal loss or damage; however, there were likely subtle changes in hippocampal processing that led to the observed deficit on the hippocampal‐dependent task. These findings support our theory that cholinergic depletion of the medial septum increases hippocampal vulnerability to further insults such as stress.

Reduced cholinergic status in hippocampus produces spatial memory deficits when combined with kainic acid induced seizures

Alzheimers disease is the most common form of dementia in North America today. Though many risk factors have been suggested to increase the likelihood of developing this disease, an accurate etiology has yet to be described. One of these risk factors commonly associated with Alzheimers disease is the loss of cholinergic neurons of the medial septum that project to the hippocampus, leading to depletion in cholinergic activity. A second risk factor is the presence of seizures, which can increase the risk of excitotoxic cell death. To examine the interaction between these two common risk factors, we gave rats a focal cholinergic lesion of the medial septum using the specific immunotoxin 192‐IgG Saporin, followed 2 weeks later by a non‐convulsive dose of kainic acid. We then assessed the rats for seizure severity, hippocampal damage and performance on a spatial memory task. The combination of the two factors resulted in a trend towards increased seizure severity in the cholinergic depleted rats, but more importantly, the lesioned rats that had non‐convulsive seizures were significantly impaired on a spatial version of the Morris water maze when compared with either the rats with a cholinergic depletion or non‐convulsive seizure alone. This result could not be explained by seizure severity or the extent of hippocampal damage, suggesting a more subtle interaction between these two risk factors in the development of a hippocampal based memory impairment.

Cholinergic depletion of the medial septum followed by phase shifting does not impair memory or rest-activity rhythms measured under standard light/dark conditions in rats

The robustness of an individuals circadian rhythms has been correlated with the quality of their cognitive aging. This has been observed in both human and non-human animals and circadian rhythms are especially disrupted in patients with Alzheimers disease (AD). It is possible that the circadian disruption observed in AD contributes to the cognitive decline in these patients; however, this has not been conclusively proven. A common observation in AD patients is the loss of basal forebrain cholinergic neurons, some of which project to the suprachiasmatic nucleus (SCN) responsible for maintaining circadian rhythms. We were interested to see if cholinergic depletion increased susceptibility to circadian disruption, and to explore possible interactions between these two factors on measures of learning and memory. We lesioned the cholinergic neurons of the medial septum in rats using the specific immunotoxin 192 IgG Saporin and then disrupted circadian rhythms using a six day phase shifting procedure. We looked at measures of circadian rhythmicity, as well as behaviour on tasks designed to test hippocampal dependent (water maze) or hippocampal independent (fear conditioning) learning and memory. We found no difference between the groups on any of the measures examined suggesting that the cholinergic depletion of the medial septum does not increase susceptibility to circadian disruption, and that this combination of risk factors does not contribute to learning and memory impairments.

Selective lesion of medial septal cholinergic neurons followed by a mini-stroke impairs spatial learning in rats

Reduced levels of hippocampal acetylcholine are a common finding in patients diagnosed with Alzheimer’s disease, but it remains unclear what role this depletion plays in the development of dementia. It is possible that the reduced levels of acetylcholine increases the vulnerability of hippocampal neurons to future insults which could lead to neuronal death and cognitive impairment. One insult that is commonly observed in the demented elderly and often co-exists with Alzheimer’s disease is stroke. In the current experiment, we used the immunotoxin 192 IgG-Saporin to specifically lesion the cholinergic neurons of the medial septum that project to the hippocampus. We then explored the effects of small, localised strokes in the hippocampus on spatial learning and memory. The combination of cholinergic depletion and stroke resulted in significant impairment on the spatial water maze compared to the performance of rats receiving either factor alone. Quantification of hippocampal damage revealed no difference in the overall lesion size of stroke-only or combined (cholinergic depletion and stroke) rats, suggesting that a more subtle mechanism is responsible for the observed impairment. We propose that healthy hippocampal neurons may normally be able to withstand, and compensate for a small ischemic insult. However, in the absence of cholinergic projections from the medial septum, these compensatory processes in the hippocampus may be compromised resulting in the spatial learning impairment reported here. This suggests an association between the cholinergic depletion observed during aging and the potential for functional recovery following stroke.

Targeting inflammatory monocytes in sepsis-associated encephalopathy and long-term cognitive impairment

Sepsis-associated encephalopathy manifesting as delirium is a common problem in critical care medicine. In this study, patients that had delirium due to sepsis had significant cognitive impairments at 12-18 months after hospital discharge when compared with controls and Cambridge Neuropsychological Automated Test Battery-standardized scores in spatial recognition memory, pattern recognition memory, and delayed-matching-to-sample tests but not other cognitive functions. A mouse model of S. pneumoniae pneumonia-induced sepsis, which modeled numerous aspects of the human sepsis-associated multiorgan dysfunction, including encephalopathy, also revealed similar deficits in spatial memory but not new task learning. Both humans and mice had large increases in chemokines for myeloid cell recruitment. Intravital imaging of the brains of septic mice revealed increased neutrophil and CCR2+ inflammatory monocyte recruitment (the latter being far more robust), accompanied by subtle microglial activation. Prevention of CCR2+ inflammatory monocyte recruitment, but not neutrophil recruitment, reduced microglial activation and other signs of neuroinflammation and prevented all signs of cognitive impairment after infection. Therefore, therapeutically targeting CCR2+ inflammatory monocytes at the time of sepsis may provide a novel neuroprotective clinical intervention to prevent the development of persistent cognitive impairments.