Alterations of signal transduction in the lesioned entorhinal—hippocampal system: A mini‐review on alzheimer's disease‐related changes and experimental data

Effects of entorhinal cortex lesions on sensory integration and spatial learning

BACKGROUND

The entorhinal cortex provides sensory information to the hippocampus for memory and learning. Damage to the entorhinal cortex is common in patients who experience traumatic brain injury, stroke, and Alzheimer's disease. Entorhinal damage is assumed to interfere with sensory integration; however, substantive knowledge of behavioral patterns is lacking.

OBJECTIVES

To describe specific behavioral deficits associated with entorhinal cortex injury related to special senses identification, sensory integration, and spatial learning.

METHOD

Adult male rats received bilateral entorhinal cortex damage (n = 19) or sham surgery (n = 11) with a subset randomized to participate in special senses identification, exploration, and sensory integration testing. Spatial learning was examined using a water maze.

RESULTS

Lesion and control animals were similar in special senses identification testing. Sensory integration was markedly impaired in lesion animals over 3 days for all integration tasks; however, travel deficit persisted for 4 days. By day 5 sensory integration ability was equal. Lesion animals were significantly impaired across all days of spatial learning for swim time (p = .0001) and directional heading error (p = .03). Control animals exposed to sensory testing demonstrated significantly more efficient learning (p = .005) on swim days 2 and 3 versus control animals not exposed to sensory testing.

CONCLUSIONS

Early and prolonged behavioral changes are evident following entorhinal cortex damage including sensory integration deficits and persistent spatial learning impairment.

A quantitative autoradiographic study of [3H]cAMP binding to cytosolic and particulate protein kinase A in post-mortem brain staged for Alzheimer's disease neurofibrillary changes and amyloid deposits

The cAMP-dependent protein kinase (PKA) has been implicated in the Alzheimer's disease pathology of abnormal tau phosphorylation leading to neurofibrillary tangle (NFT) formation, as well as in amyloid precursor protein alpha-secretase processing. In the present study, we determined whether [3H]cAMP binding to cytosolic and particulate PKA showed any relationship to the extent of Alzheimer's disease pathology at post-mortem. Autoradiographic [3H]cAMP binding to cytosolic and particulate PKA was measured in sections of entorhinal cortex/hippocampal formation from 23 cases that had been staged for Alzheimer's disease-related neurofibrillary changes and amyloid deposits according to Braak and Braak [H. Braak, E. Braak, Neuropathological staging of Alzheimer's-related changes, Acta Neuropathol. 82 (1991) 239-259]. [3H]cAMP binding to cytosolic PKA showed statistically significant reductions in the entorhinal cortex (P<0.01, ANOVA) with respect to neurofibrillary changes. Post-hoc analysis with Fisher's PLSD test showed significant reductions of [3H]cAMP binding to cytosolic PKA at the isocortical stages (V and VI), compared to the non-pathological (O) (by 55%, P<0.01), transentorhinal (I and II) (by 58%, P<0.001) and limbic (III and IV) (by 45%, P<0.05) stages. A significant reduction (by 25%, P<0.05) was also seen in the transentorhinal compared to the limbic stages. [3H]cAMP binding to cytosolic PKA showed no significant alterations with respect to neurofibrillary changes in either the subiculum, CA1-CA4 subfields of the hippocampus or the dentate gyrus. [3H]cAMP binding to cytosolic PKA also showed significant declines in the entorhinal cortex (P<0.01) and subiculum (P<0.05) with respect to staging for amyloid deposits. Post-hoc analysis with Fisher's PLSD test showed significant reductions of [3H]cAMP binding to cytosolic PKA in the entorhinal cortex at amyloid stage C compared to stages O (by 41%, P<0.01) and A (by 38%, P<0.01). In the subiculum, there were significant reductions of [3H]cAMP binding at stages C (by 41%, P<0.01) and B (by 40%, P<0.05), respectively, compared to stage O. [3H]cAMP binding to particulate PKA did not show significant relationships to staging for either neurofibrillary changes or amyloid deposits in either the entorhinal cortex or any of the hippocampal subregions. These findings suggest that whereas [3H]cAMP binding to cytosolic PKA in the entorhinal cortex is reduced with progression of neurofibrillary and amyloid pathology, other hippocampal regions show a preservation of cytosolic and particulate PKA even in late stage pathologies.

Dendritic changes in Alzheimer's disease and factors that may underlie these changes

It seems likely that the Alzheimer disease (AD)-related dendritic changes addressed in this article are induced by two principally different processes. One process is linked to the plastic response associated with deafferentation, that is, long-lasting transneuronally induced regressive changes in dendritic geometry and structure. The other process is associated with severe alterations of the dendritic- and perikaryal cytoskeleton as seen in neurons with the neurofibrillary pathology of AD, that is, the formation of paired helical filaments formed by hyperphosphorylated microtubule-associated protein tau. As the development of dendritic and cytoskeletal abnormalities are at least mediated by alterations in signal transduction, this article also reviews changes in signal pathways in AD. We also discuss transgenic approaches developed to model and understand cytoskeletal abnormalities.

First one in, last one out: the role of gabaergic transmission in generation and degeneration

This paper is the result of discussions between scientists working in widely separated areas, united by an interest in the hippocampus. The discussions focused on the possible role of GABA in the development and maturation of the hippocampus and in neurodegeneration in Alzheimer's disease (AD). GABA neurons are among the first to differentiate in the hippocampus and the properties of GABA neurotransmission in the developing hippocampus are distinct from those in the adult. GABAergic transmission may play a role in the clustering and maturation of GABA receptors, as well as of receptors for other neurotransmitters. The development and maturation of synaptic connections involves changes in the organization of the cytoskeleton, and mechanical force generation is probably required to establish appropriate points of contact. This generation of force may require coupling of specific receptors to the cytoskeleton through specialized proteins. In AD, much of the developmental process is progressively unraveled in the hippocampus, as afferent fibers, most notably from entorhinal excitatory neurons and from basal forebrain cholinergic cells, degenerate. This denervation undoubtedly has consequences for receptor systems, dendritic morphology and the underlying cytoskeleton. GABA neurons remain in the AD hippocampus, and may actually contribute to abnormal firing and degeneration of remaining pyramidal neurons. This attempt to bring together data from different areas of research has allowed the development of a scheme which identifies significant specific gaps in our knowledge, which could be readily filled by focused experimental work.

The organotypic entorhinal-hippocampal complex slice culture of adolescent rats. A model to study transcellular changes in a circuit particularly vulnerable in neurodegenerative disorders

The entorhinal-hippocampal system is severely altered in many neurodegenerative disorders with mnemonic malfunction, e.g. Alzheimer's, Parkinson's and Huntington's disease. The present approach characterizes an organotypic complex slice culture comprising both the entorhinal cortex and the hippocampal formation in order to establish a tool for experimental studies of the entorhinal-hippocampal interaction and its presumed neurodegenerative alterations in vitro. Slices were obtained from rats at about postnatal day 15 and maintained in culture using the interface technique. Thus, also structures known to be developed gradually during the first weeks postnatally are in accord to structures seen in adult rats. After two-three weeks in vitro, slices in the culture dish still revealed the typical morphological features of the entorhinal-hippocampal formation as visible with the dissecting microscope. Biocytin, which is taken up by and transported within living cells, labeled typical cell bodies, dendrites and axons of stellate neurons in layer II and pyramidal cells in layer III when applied to the outer layers of the entorhinal cortex. Small injections of biocytin within the dentate gyrus displayed living granule cells and the maintenance of their projection to the pyramidal cells in CA3, i.e., a typical suprapyramidal plexus of mossy fibers. The presence of axons of entorhinal neurons traveling towards the hippocampus and growth cones traversing the deep layers of the entorhinal cortex indicate that both brain regions are still interacting. Immunocytochemistry for calbindin D-28K revealed labeled neurons in layer II of the entorhinal cortex and dentate granule cells which are known to contain this calcium-binding protein.