Space and Memory (Far) Beyond the Hippocampus: Many Subcortical Structures Also Support Cognitive Mapping and Mnemonic Processing.

Memory research remains focused on just a few brain structures-in particular, the hippocampal formation (the hippocampus and entorhinal cortex). Three key discoveries promote this continued focus: the striking demonstrations of enduring anterograde amnesia after bilateral hippocampal damage; the realization that synapses in the hippocampal formation are plastic e.g., when responding to short bursts of patterned stimulation ("long-term potentiation" or LTP); and the discovery of a panoply of spatially-tuned cells, principally surveyed in the hippocampal formation (place cells coding for position; head-direction cells, providing compass-like information; and grid cells, providing a metric for 3D space). Recent anatomical, behavioral, and electrophysiological work extends this picture to a growing network of subcortical brain structures, including the anterior thalamic nuclei, rostral midline thalamic nuclei, and the claustrum. There are, for example, spatially-tuned cells in all of these regions, including cells with properties similar to place cells of the hippocampus proper. These findings add new perspectives to what had been originally been proposed-but often overlooked-half a century ago: that damage to an extended network of structures connected to the hippocampal formation results in diencephalic amnesia. We suggest these new findings extend spatial signaling in the brain far beyond the hippocampal formation, with profound implications for theories of the neural bases of spatial and mnemonic functions.

Environmental enrichment increases prefrontal EEG power and synchrony with the hippocampus in rats with anterior thalamus lesions.

The anterior thalamic nuclei (ATN) are a major interface between the hippocampus and prefrontal cortex within an extended Papez circuit. Rat models suggest that the deficits caused by ATN damage, which is associated with "diencephalic amnesia", can be ameliorated by environmental enrichment (EE) through unknown mechanisms. We examined whether changes in theta rhythmicity within and between the hippocampus and prefrontal cortex are influenced by EE in rats with ATN lesions. Here, we show that ATN lesions and EE produced essentially opposed functional effects in terms of changes in rhythmicity between two consecutive trials when rats forage for chocolate hail. On the second trial, standard-housed rats with ATN lesions showed: (a) a clear reduction in prefrontal cortex experience-dependent power change in the theta band and in two adjacent bands; (b) little change in the theta band in hippocampal area CA1; and (c) only a modest overall reduction in experience-dependent power change at lower theta frequencies in the dentate gyrus. EE exposure prevented the decrease in prefrontal theta power in rats with ATN lesions, and in fact caused a clear increase in prefrontal cortex power across all bands. While ATN lesions did not reliably affect prefrontal-CA1 or prefrontal-dentate theta coherence, EE increased the coherence between prefrontal cortex and area CA1 in both the sham and ATN groups. Thus, EE increases functional connectivity between prefrontal cortex and hippocampus via pathways that bypass the ATN, and increases behaviorally dependent prefrontal rhythmicity. These EEG effects may contribute to improved learning and memory in the ATN-lesion model of diencephalic amnesia.

The importance of mammillary body efferents for recency memory: towards a better understanding of diencephalic amnesia.

Despite being historically one of the first brain regions linked to memory loss, there remains controversy over the core features of diencephalic amnesia as well as the critical site for amnesia to occur. The mammillary bodies and thalamus appear to be the primary locus of pathology in the cases of diencephalic amnesia, but the picture is complicated by the lack of patients with circumscribed damage. Impaired temporal memory is a consistent neuropsychological finding in Korsakoff syndrome patients, but again, it is unclear whether this deficit is attributable to pathology within the diencephalon or concomitant frontal lobe dysfunction. To address these issues, we used an animal model of diencephalic amnesia and examined the effect of mammillothalamic tract lesions on tests of recency memory. The mammillothalamic tract lesions severely disrupted recency judgements involving multiple items but left intact both recency and familiarity judgements for single items. Subsequently, we used disconnection procedures to assess whether this deficit reflects the indirect involvement of the prefrontal cortex. Crossed-lesion rats, with unilateral lesions of the mammillothalamic tract and medial prefrontal cortex in contralateral hemispheres, were unimpaired on the same recency tests. These results provide the first evidence for the selective importance of mammillary body efferents for recency memory. Moreover, this contribution to recency memory is independent of the prefrontal cortex. More broadly, these findings identify how specific diencephalic structures are vital for key elements of event memory.

Comparable reduction in Zif268 levels and cytochrome oxidase activity in the retrosplenial cortex following mammillothalamic tract lesions.

Damage to the mammillothalamic tract (MTT) produces memory impairments in both humans and rats, yet it is still not clear why this diencephalic pathway is vital for memory. One suggestion is that it is an important route for midbrain inputs to reach a wider cortical and subcortical network that supports memory. Consistent with this idea, MTT lesions produce widespread hypoactivity in distal brain regions as measured by the immediate-early gene, c-fos. To determine whether these findings were selective to c-fos or reflected more general changes in neuronal function, we assessed the effects of MTT lesions on the expression of the immediate-early gene protein, Zif268 and the metabolic marker, cytochrome oxidase, in the retrosplenial cortex and hippocampus. The lesions decreased levels of both activity markers in the superficial and deep layers of the retrosplenial cortex in both its granular and dysgranular subregions. In contrast, no significant changes were observed in the hippocampus, despite the MTT-lesioned animals showing marked impairments on T-maze alternation. These findings are consistent with MTT lesions providing important, indirect inputs for normal retrosplenial cortex functioning. These distal functional changes may contribute to the memory impairments observed after MTT lesions.

Impaired spatial working memory after anterior thalamic lesions: recovery with cerebrolysin and enrichment.

Lesions to the anterior thalamic nuclei (ATN) in rats produce robust spatial memory deficits that reflect their influence as part of an extended hippocampal system. Recovery of spatial working memory after ATN lesions was examined using a 30-day administration of the neurotrophin cerebrolysin and/or an enriched housing environment. As expected, ATN lesions in standard-housed rats given saline produced severely impaired reinforced spatial alternation when compared to standard-housed rats with sham lesions. Both cerebrolysin and enrichment substantially improved this working memory deficit, including accuracy on trials that required attention to distal cues for successful performance. The combination of cerebrolysin and enrichment was more effective than either treatment alone when the delay between successive runs in a trial was increased to 40 s. Compared to the intact rats, ATN lesions in standard-housed groups produced substantial reduction in c-Fos expression in the retrosplenial cortex, which remained low after cerebrolysin and enrichment treatments. Evidence that multiple treatment strategies restore some memory functions in the current lesion model reinforces the prospect for treatments in human diencephalic amnesia.

Functional heterogeneity of the limbic thalamus: From hippocampal to cortical functions.

Today, the idea that the integrity of the limbic thalamus is necessary for normal memory functions is well established. However, if the study of thalamic patients emphasized the anterior and the mediodorsal thalamus as the critical thalamic loci supporting cognitive functions, clinical studies have so far failed to attribute a specific role to each of these regions. In view of these difficulties, we review here the experimental data conducted in rodents harboring specific lesions of each thalamic region. These data clearly indicate a major functional dissociation within the limbic thalamus. The anterior thalamus provides critical support for hippocampal functions due to its cardinal location in the Papez circuit, while the mediodorsal thalamus may signal relevant information in a circuit encompassing the basolateral amygdala and the prefrontal cortex. Interestingly, while clinical studies have suggested that diencephalic pathologies may disconnect the medial temporal lobe from the cortex, experimental studies conducted in rodent show how this may differently affect distinct temporo-thalamo-cortical circuits, sharing the same general organization but supporting dissociable functions.

Effects of thalamic lesions on repeated relearning of a spatial working memory task.

Anterior thalamic (ATN) dysfunction produces memory deficits in rats and humans. The current study shows that, with a substantial delay between post-surgery tests, controls show repeated relearning on a spatial working memory task whereas rats with neurotoxic ATN lesions showed repeated relearning deficits. Rats were pre-trained to criterion, but not over trained, on the spatial task. ATN lesions produced the expected spatial memory and relearning deficits about two weeks post-surgery and again either one or 15 weeks later. Control rats also showed forgetting post-surgery and after a 15 week break, relearning the task on each occasion. Controls with only a 1 week break before their final re-test showed negligible forgetting. Thus, a short break between re-tests replicated previous findings with ATN lesions, but a long break allows repeated comparison of rates of learning from a common starting point in sham and ATN-lesioned animals, providing a useful paradigm for future testing of pro-cognitive treatments.

Rapidly progressive dementia due to neurosarcoidosis / Demência rapidamente progressiva causada por neurosarcoidose

Rapidly progressive dementia (RPD) is typically defined as a cognitive decline progressing to severe impairment in less than 1-2 years, typically within weeks or months. Accurate and prompt diagnosis is important because many conditions causing RPD are treatable. Neurosarcoidosis is often cited as an unusual reversible cause of RPD. METHODS: We report two cases of neurosarcoidosis presenting as RPD. RESULTS: Case 1: A 61-year-old woman developed a RPD associated with visual loss. In seven months she was dependent for self-care. Magnetic resonance imaging (MRI) revealed temporal and suprasellar brain lesions. Treatment with high-dose intravenous prednisolone was associated with partial improvement. Case 2: A 43-year-old woman who was being treated for diabetes insipidus developed a severe episodic amnesia one year after onset of cognitive symptoms. Previous MRI had shown a hypothalamic lesion and she had been treated with oral prednisone and cyclophosphamide. There was reduction of the MRI findings but no improvement in the cognitive deficits. Brain biopsy disclosed noncaseous granulomas and granulomatous angiitis; treatment was changed to high-dose intravenous methylprednisolone, with poor symptomatic response. CONCLUSION: The diagnosis of RPD due to neurosarcoidosis can be challenging when the disease is restricted to the nervous system. In these cases, clinical presentation of RPD associated with neuroendocrine and visual dysfunction, imaging findings showing hypothalamic lesions and, in some cases, brain biopsy, are the key to a correct diagnosis. It is possible that earlier diagnoses and treatment could have led to a better outcome in these patients.

Demência rapidamente progressiva (DRP) é tipicamente definida como um declínio cognitivo que progride para prejuízo funcional severo em menos de 1-2 anos, geralmente em semanas a meses. O diagnóstico rápido e acurado é fundamental, já que muitas condições que levam a DRP são reversíveis. MÉTODOS: Relatamos dois casos de neurosarcoidose que se apresentaram como DRP. RESULTADOS: Caso 1: Uma mulher de 61 anos desenvolveu uma DRP associada a perda de acuidade visual. Em sete meses evoluiu com dependência para auto-cuidado. A ressonância magnética (RM) revelou lesões encefálicas temporais e supraselares. Evoluiu com melhora parcial após tratamento com metilprednisolona intravenosa em altas doses. Caso 2: Uma mulher de 43 anos que estava em tratamento para diabetes insipidus desenvolveu uma amnésia episódica severa um ano após o início dos sintomas cognitivos. A RM anterior mostrava uma lesão hipotalâmica, e ela recebeu tratamento oral com prednisona e ciclofosfamida. Houve redução dos achados da RM, porem sem melhora dos déficits cognitivos. A biópsia cerebral mostrou granulomas não caseosos e angeíte granulomatosa; o tratamento foi modificado para metilprednisolona intravenosa em altas doses, com resposta sintomática pobre. CONCLUSÃO: O diagnóstico de DRP por neurosarcoidose pode ser desafiador quando a doença está restrita ao sistema nervoso central. Nestes casos, a apresentação clínica da DRP associada a disfunção neuroendócrina e visual, exames de imagem com lesões hipotalâmicas e, em alguns casos, a biópsia cerebral são fundamentais para um diagnóstico correto. é possível que o diagnóstico e tratamento precoces poderiam ter trazido melhores resultados nesses pacientes.

Rapidly progressive dementia due to neurosarcoidosis.

Rapidly progressive dementia (RPD) is typically defined as a cognitive decline progressing to severe impairment in less than 1-2 years, typically within weeks or months. Accurate and prompt diagnosis is important because many conditions causing RPD are treatable. Neurosarcoidosis is often cited as an unusual reversible cause of RPD. METHODS: We report two cases of neurosarcoidosis presenting as RPD. RESULTS: Case 1: A 61-year-old woman developed a RPD associated with visual loss. In seven months she was dependent for self-care. Magnetic resonance imaging (MRI) revealed temporal and suprasellar brain lesions. Treatment with high-dose intravenous prednisolone was associated with partial improvement. Case 2: A 43-year-old woman who was being treated for diabetes insipidus developed a severe episodic amnesia one year after onset of cognitive symptoms. Previous MRI had shown a hypothalamic lesion and she had been treated with oral prednisone and cyclophosphamide. There was reduction of the MRI findings but no improvement in the cognitive deficits. Brain biopsy disclosed noncaseous granulomas and granulomatous angiitis; treatment was changed to high-dose intravenous methylprednisolone, with poor symptomatic response. CONCLUSION: The diagnosis of RPD due to neurosarcoidosis can be challenging when the disease is restricted to the nervous system. In these cases, clinical presentation of RPD associated with neuroendocrine and visual dysfunction, imaging findings showing hypothalamic lesions and, in some cases, brain biopsy, are the key to a correct diagnosis. It is possible that earlier diagnoses and treatment could have led to a better outcome in these patients.

Demência rapidamente progressiva (DRP) é tipicamente definida como um declínio cognitivo que progride para prejuízo funcional severo em menos de 1-2 anos, geralmente em semanas a meses. O diagnóstico rápido e acurado é fundamental, já que muitas condições que levam a DRP são reversíveis. MÉTODOS: Relatamos dois casos de neurosarcoidose que se apresentaram como DRP. RESULTADOS: Caso 1: Uma mulher de 61 anos desenvolveu uma DRP associada a perda de acuidade visual. Em sete meses evoluiu com dependência para auto-cuidado. A ressonância magnética (RM) revelou lesões encefálicas temporais e supraselares. Evoluiu com melhora parcial após tratamento com metilprednisolona intravenosa em altas doses. Caso 2: Uma mulher de 43 anos que estava em tratamento para diabetes insipidus desenvolveu uma amnésia episódica severa um ano após o início dos sintomas cognitivos. A RM anterior mostrava uma lesão hipotalâmica, e ela recebeu tratamento oral com prednisona e ciclofosfamida. Houve redução dos achados da RM, porem sem melhora dos déficits cognitivos. A biópsia cerebral mostrou granulomas não caseosos e angeíte granulomatosa; o tratamento foi modificado para metilprednisolona intravenosa em altas doses, com resposta sintomática pobre. CONCLUSÃO: O diagnóstico de DRP por neurosarcoidose pode ser desafiador quando a doença está restrita ao sistema nervoso central. Nestes casos, a apresentação clínica da DRP associada a disfunção neuroendócrina e visual, exames de imagem com lesões hipotalâmicas e, em alguns casos, a biópsia cerebral são fundamentais para um diagnóstico correto. é possível que o diagnóstico e tratamento precoces poderiam ter trazido melhores resultados nesses pacientes.