Functional cerebral activity during regeneration from entorhinal lesions in the rat

Combined actions of Na/K-ATPase, NCX1 and glutamate dependent NMDA receptors in ischemic rat brain penumbra

Instrumental role of Na⁺ and Ca²⁺ influx via Na⁺/K⁺ adenosine triphosphatase (Na⁺/K⁺-ATPase) and Na⁺/Ca²⁺ exchanger 1 (NCX1) is examined in the N-Methyl-D-aspartate (NMDA) receptor-mediated pathogenesis of penumbra after focal cerebral ischemia. An experimental model of 3, 6, and 24 h focal cerebral ischemia by permanent occlusion of middle cerebral artery was developed in rats. The changes in protein expression of Na⁺/K⁺-ATPase and NCX1 as well as functional subunits of NMDA receptor 2A and 2B (NR2A and NR2B) in the penumbra were assessed using by quantitative immunoblottings. The most prominent changes of Na⁺/K⁺-ATPase (78±6%, n=4, ^*P<0.05) and NCX1 (144±2%, n=4, ^*P<0.05) in the penumbra were developed 24 h after focal cerebral ischemia. The expression of NR2A in the penumbra was significantly increased (153±9%, n=4, ^*P<0.05) whereas the expression of NR2B was significantly decreased (37±2%, n=4, ^*P<0.05) as compared with sham-operated controls 3 h after focal cerebral ischemia. However, the expression of NR2A and NR2B in the penumbra was reversed 24 h after focal cerebral ischemia (NR2A: 40±7%; NR2B: 120±16%, n=4, ^*P<0.05). Moreover, the decreased expression of neuronal nuclei (NeuN) in the penumbra was most prominent than that of glial fibrillary acidic protein (GFAP) 24 h after focal cerebral ischemia. These findings imply that intracellular Na⁺ accumulation via decreased Na⁺/K⁺-ATPase exacerbate the Ca²⁺ overload cooperated by the increased NCX1 and NR2B-containing NMDA receptor which may play an important role in the pathogenesis of the penumbra.

Brain glucose hypometabolism after perirhinal lesions in baboons: implications for Alzheimer disease and aging

The authors previously reported that excitotoxic lesions of both the perirhinal and entorhinal cortices in baboons induce remote neocortical and hippocampal hypometabolism reminiscent of that observed in Alzheimer disease (AD), suggesting that disconnection may play a role in AD. Because the cerebral metabolic rate of glucose (CMR ) was preferentially correlated with perirhinal damage, the area first affected by neurofibrillary tangles in both AD and normal aging, the present series of experiments aimed at assessing the specific metabolic effects of perirhinal lesions. Using PET, CMR was measured before surgery and sequentially over the ensuing 10 months. Compared with sham-operated baboons, perirhinal lesions induced significant-albeit late and transient-CMR decreases in several brain regions, which significantly correlated with histologic damage for some of these regions. Among them, the temporal and hippocampal regions are metabolically affected after extensive rhinal lesions, in early AD, and aging, while the prefrontal region is affected in aging only. Furthermore, in contrast to AD and rhinal lesions, the posterior cingulate cortex was spared. Both the progressive but significant metabolic effects and specific hypometabolic pattern after perirhinal lesions were confirmed by direct comparisons with previous data obtained after combined lesions of both rhinal areas. Thus, although perirhinal damage appears in itself insufficient to induce sustained CMR decreases, it may contribute to the hypometabolic profile of both AD and normal aging, most likely with a stronger contribution in the latter.

Neocortical and hippocampal glucose hypometabolism following neurotoxic lesions of the entorhinal and perirhinal cortices in the non-human primate as shown by PET. Implications for Alzheimer's disease

Temporoparietal glucose hypometabolism, neuronal loss in the basal forebrain cholinergic structures and preferential accumulation of neurofibrillary tangles in the rhinal cortex (i.e. in the entorhinal and perirhinal cortices) are three early characteristics of Alzheimer's disease. Based on studies of the effects of neurotoxic lesions in baboons, we previously concluded that damage to the cholinergic structures plays, at best, a marginal role in the association neocortex hypometabolism of Alzheimer's disease. In the present study, we have assessed the remote metabolic effects of bilateral neurotoxic lesions of both entorhinal and perirhinal cortices. Using coronal PET coregistered with MRI, the cerebral metabolic rate for glucose (CMR(glc)) was measured before surgery and sequentially for 2-3 months afterward (around days 30, 45 and 80). Compared with sham-operated baboons, the lesioned animals showed a significant and long-lasting CMR(glc) decline in a small set of brain regions, especially in the inferior parietal, posterior temporal, posterior cingulate and associative occipital cortices, as well as in the posterior hippocampal region, all of which also exhibit glucose hypometabolism in Alzheimer's disease. Remarkably, the degree of CMR(glc) decline in four of these regions significantly correlated with the severity of histologically determined damage in the rhinal cortex, strongly supporting the specificity of the observed metabolic effects. There were also differences between the metabolic pattern observed in the lesioned animals and that classically reported in Alzheimer's disease; for instance, the hypometabolism we found in the stratum has not been reported in early Alzheimer's disease, although this structure can be affected in late stages of the disease and has direct anatomical connections with the rhinal cortex. Nevertheless, this study shows for the first time that the temporoparietal and hippocampal hypometabolism found in Alzheimer's disease may partly result from neuroanatomical disconnection with the rhinal cortex. This, in turn, further strengthens the hypothesis that neuronal damage and dysfunction in the rhinal cortices play a major role in the expression of Alzheimer's disease.

Lesion-induced plasticity of central neurons: sprouting of single fibres in the rat hippocampus after unilateral entorhinal cortex lesion

In response to a central nervous system trauma surviving neurons reorganize their connections and form new synapses that replace those lost by the lesion. A well established in vivo system for the analysis of this lesion-induced plasticity is the reorganization of the fascia dentata following unilateral entorhinal cortex lesions in rats. After general considerations of neuronal reorganization following a central nervous system trauma, this review focuses on the sprouting of single fibres in the rat hippocampus after entorhinal lesion and the molecular factors which may regulate this process. First, the connectivity of the fascia dentata in control animals is reviewed and previously unknown commissural fibers to the outer molecular layer and entorhinal fibres to the inner molecular layer are characterized. Second, sprouting of commissural and crossed entorhinal fibres after entorhinal cortex lesion is described. Single fibres sprout by forming additional collaterals, axonal extensions, boutons, and tangle-like axon formations. It is pointed out that the sprouting after entorhinal lesion mainly involves unlesioned fibre systems terminating within the layer of fibre degeneration and is therefore layer-specific. Third, molecular changes associated with axonal growth and synapse formation are considered. In this context, the role of adhesion molecules, glial cells, and neurotrophic factors for the sprouting process are discussed. Finally, an involvement of sprouting processes in the formation of neuritic plaques in Alzheimer's disease is reviewed and discussed with regard to the axonal tangle-like formations observed after entorhinal cortex lesion.