Decrease in adrenergic axon sprouting in the senescent rat.

When the septal area in young adult rats is denervated by a lesion of the fimbria-fornix, adrenergic fibers proliferate within the denervated area. The same operation performed on aged animals gives rise to a qualitatively similar but quantitatively less pronounced response. This reduction in reactive growth may reflect a decreased capacity of the aged brain to remodel its circuitry and restore lost function.

Caspase-3 activity is reduced after spinal cord injury in mice lacking dynorphin: differential effects on glia and neurons.

Dynorphins are endogenous opioid peptide products of the prodynorphin gene. An extensive literature suggests that dynorphins have deleterious effects on CNS injury outcome. We thus examined whether a deficiency of dynorphin would protect against tissue damage after spinal cord injury (SCI), and if individual cell types would be specifically affected. Wild-type and prodynorphin(-/-) mice received a moderate contusion injury at 10th thoracic vertebrae (T10). Caspase-3 activity at the injury site was significantly decreased in tissue homogenates from prodynorphin(-/-) mice after 4 h. We examined frozen sections at 4 h post-injury by immunostaining for active caspase-3. At 3-4 mm rostral or caudal to the injury, >90% of all neurons, astrocytes and oligodendrocytes expressed active caspase-3 in both wild-type and knockout mice. At 6-7 mm, there were fewer caspase-3(+) oligodendrocytes and astrocytes than at 3-4 mm. Importantly, caspase-3 activation was significantly lower in prodynorphin(-/-) oligodendrocytes and astrocytes, as compared with wild-type mice. In contrast, while caspase-3 expression in neurons also declined with further distance from the injury, there was no effect of genotype. Radioimmunoassay showed that dynorphin A(1-17) was regionally increased in wild-type injured versus sham-injured tissues, although levels of the prodynorphin processing product Arg(6)-Leu-enkephalin were unchanged. Our results indicate that dynorphin peptides affect the extent of post-injury caspase-3 activation, and that glia are especially sensitive to these effects. By promoting caspase-3 activation, dynorphin peptides likely increase the probability of glial apoptosis after SCI. While normally beneficial, our findings suggest that prodynorphin or its peptide products become maladaptive following SCI and contribute to secondary injury.

Synaptogenesis in the hippocampal CA1 field following traumatic brain injury.

Traumatic brain injury (TBI) results in both acute and chronic disruption of cognitive ability that may be mediated through a disruption of hippocampal circuitry. Experimental models of TBI have demonstrated that cortical contusion injuries can result in the loss of specific neurons in the CA3 subfield of the ipsilateral hippocampus, resulting in partial loss of afferents to the CA1 subfield. Numerous studies have documented the ability of the central nervous system to compensate for deafferentation by initiating a plasticity response capable of restoring lost synaptic contacts. The present study was designed to examine the time course of loss and replacement of synaptic contacts in stratum radiatum dendritic field of CA1. Young adult rats were subjected to a lateral cortical contusion injury and assayed for total synaptic numbers using unbiased stereology coupled with transmission electron microscopy. Injured animals demonstrated a 60% loss of synapses in CA1 at 2 days post-injury, followed by a reinnervation process that was apparent as early as 10 days post-injury. By 60 days post-injury, total synaptic numbers had approached pre-injury levels but were still significantly lower. Some animals were behaviorally tested for spatial memory in a Morris Water Maze at 15 and 30 days post-injury. While there was some improvement in spatial memory, injured animals continued to demonstrate a significant deficit in acquisition. These results show that the hippocampus ipsilateral to the cortical contusion is capable of a significant plasticity response but that synapse replacement in this area does not necessarily result in significant improvement in spatial learning.

Hippocampal plasticity during the progression of Alzheimer's disease.

Neuroplasticity involves molecular and structural changes in central nervous system (CNS) throughout life. The concept of neural organization allows for remodeling as a compensatory mechanism to the early pathobiology of Alzheimer's disease (AD) in an attempt to maintain brain function and cognition during the onset of dementia. The hippocampus, a crucial component of the medial temporal lobe memory circuit, is affected early in AD and displays synaptic and intraneuronal molecular remodeling against a pathological background of extracellular amyloid-beta (Aß) deposition and intracellular neurofibrillary tangle (NFT) formation in the early stages of AD. Here we discuss human clinical pathological findings supporting the concept that the hippocampus is capable of neural plasticity during mild cognitive impairment (MCI), a prodromal stage of AD and early stage AD.

Synaptic density in the inner molecular layer of the hippocampal dentate gyrus in Alzheimer disease.

We examined the inner molecular layer (IML) of the hippocampal dentate gyrus for possible changes in synaptic density. Material was obtained from 9 individuals with Alzheimer disease (AD) and compared to samples obtained from 10 age-matched, postmortem-matched neurologically normal controls, employing standard ultrastructural techniques. Statistical analyses demonstrated a significant decline in synaptic numbers between controls and AD subjects. This decline was accompanied by a significant increase in apposition length and resulted in a significant correlation with the synaptic density. As the number of synapses declined, the apposition length increased. Assessment was also made of the granule cells density and the analyses showed a significant decline in the synapse to granule cell ratio in the AD group. This decline in the density of synaptic contacts in the IML reflects a more widespread decline in plasticity in AD and may be related to the memory problems associated with the disease.

Temporal sequence of plaque formation in the cerebral cortex of non-demented individuals.

One of the hallmarks of Alzheimer's disease is the presence of argyrophilic plaques (arg-P) accompanying dementia and other forms of cognitive alterations. In the present investigation 195 non-demented, cognitively normal patients were grouped according to the presence or absence of critical coronary artery disease (cCAD), defined as a 75% or greater stenosis of one of the epicardial arteries. None of the subjects had significant cerebral vascular disease. The parahippocampal gyrus (PHG) and frontal pole were analyzed for the presence of arg-P, A4 deposition, ALZ-50 immunoreactive (IR) neurons and neuropil threads (NT). Individuals with cCAD have a significantly greater incidence of plaques than non-heart disease (non-HD) subjects. Every cCAD subject had ALZ-50 IR neurons in the PHG and a greater incidence of NT as compared to the non-HD subjects. Every subject with plaques also had IR neurons and NT in the PHG. Based on the presumption that early neurodegeneration labeled by ALZ-50 antibody and amyloid deposition are in some way linked, then the sequence of plaque formation is initiated by the presence of ALZ-50 IR neurons followed in order by NT, A4 deposition and diffuse form arg-P.

Strain comparison of synaptic density in hippocampal CA1 of aged rats.

Synaptic density in hippocampal CA1 stratum radiatum was studied in two different strains of rat at 3 and 24-28 months of age. Neither Fischer 344 nor Sprague-Dawley rodents showed any age-related loss of synapses. These data support the contention that synapse loss with age is not a generalized phenomenon in the mammalian CNS.

Quantitative assessment of cortical synaptic density in Alzheimer's disease.

Significant progress has been made over the last decade in delineating the neuropathological and neurochemical changes in the brains of patients with Alzheimer's disease (AD). Less well studied are the actual synaptic connections of affected areas of the brain, such as the cerebral cortex. Because the final common pathway for neurotransmission involves synaptic integrity, we quantitatively assessed synaptic number and synapse size in lamina III and V of human frontal cortex (Brodmann area 9) in patients with AD and age-matched controls. Samples were also matched for postmortem interval, and artifacts associated with postmortem change were eliminated. We found a significant decrease in synaptic number per unit volume in both lamina, more marked in lamina III (-42%) than V (-29%). In both normal controls and AD brains, there was a negative correlation between synapse number and synapse size as indexed by the length of the postsynaptic density (PSD); cortical samples with fewer synapses had larger synapse size. This appeared to be a compensatory response, rather than a selective loss of small synapses, since the total amount of synaptic contact area per unit volume did not decline in lamina V (despite a 29% loss of synapses); in lamina III it was reduced 11% despite a 42% loss of synapses. The loss of synapses in AD is widespread and significant in frontal cortex; there is observable compensation by enlargement of synaptic size. This compensatory effort is overcome by the continuing loss of synapses in areas most affected by the degeneration.

Quantitation of synaptic density in the septal nuclei of young and aged Fischer 344 rats.

Synaptic density in the medial and lateral septal nuclei was examined in 3 and 24-28 months of age Fischer 344 rats. The lateral nucleus had a higher synaptic density than the medial region in both age groups. There were no statistically significant differences in synapse density in either region as a function of age, but the data suggested a subpopulation of aged animals which did show an age-related decline in synaptic density in the lateral, but not medial area of the septum. These data indicate that sample size may be an important variable in assessing possible age-related differences in synaptic density, since a broad range of values, some significantly below the range of young animals, exists in the aged brain.

Quantitative assessment of possible age-related change in synaptic numbers in the human frontal cortex.

To investigate possible age-associated changes in human synaptic connectivity, superior-middle frontal cortex (Brodmann area 9) was evaluated with ultrastructural techniques. Short post mortem autopsy tissue was obtained from 37 cognitive normal individuals ranging in age from 20 to 89 years. A minimum of five subjects represented each decade of life. Synaptic volume density (Nv) was quantified in lamina III and V of the superior-middle frontal cortex employing the physical disector. The stereological assessment demonstrated maintenance of Nv in both lamina III and V of the frontal cortex. The lack of synaptic decline in the frontal cortex in neurologically normal individuals older than 65 years lends support to the idea that many stereotypic views of age-related changes in the CNS do not apply to all brain regions. It also suggests that synaptic loss observed in pathological conditions such as Alzheimer's disease, may be the result of the disease process and not a consequence of normal aging.

Strain differences and laminar localization of structural neurochemical changes in aging rat.

To obtain comparisons of age-related microchemical changes in cerebral cortex of two commonly employed rat strains (Fischer 344 and Sprague-Dawley), neurochemical assays of substances regarded as quantitative indices of structural entities in brain were performed. These included DNA as a marker for cells, lipid sialoganglioside as an index of neuronal membrane mass, and galactocerebroside as an index of myelin. Fischer 344 rats were studied at 3-4 months (young), 14-16 months (middle age) and 25-28 months (old). Sprague-Dawleys were examined at 3-6 months (young), 14-17 months (middle age) and 25-28 months (old). Significant differences in the time courses of changes occurred; Fischer rats increased their brain weight at each aging point, while Sprague-Dawley rats reached stable brain weights by 4 months of age. Neither strain had a significant change in cell packing density of somatosensory cortex as measured by DNA. However, total ganglioside sialic acid declined in both strains, occurring by middle age in the Fischer and not until senescence in the Sprague-Dawley cortex. Cerebroside galactose increased in the Fischer between young and middle age, and was not further elevated in the older group. The Sprague-Dawley had its major increase in this marker between the middle aged and senescent groups. Intralaminar assays of these same markers in young and old Fisher 344 rats again indicated that DNA did not change, and that sialoganglioside was lost from all layers of the cortex in equal amounts. However, the increase in galactocerebroside resulted entirely from increases in the lower lamina of somatosensory cortex (lamina IV and below), suggesting on-going myelination of afferent and efferent axons. The time course of lipid membrane alteration is strain-dependent and selective as to cortical laminar localization. The findings are discussed in reference to human aging change in the same neurochemical indices.

Cortical senile plaques in coronary artery disease, aging and Alzheimer's disease.

Mild alterations in cognitive function are present in normal aging and severe cognitive alterations are a hallmark of Alzheimer's disease (AD). The cognitive change in AD has been correlated to the characteristic pathologic lesions in the brain, senile plaques (SP) and neurofibrillary tangles. Senile plaques are the most consistent correlative marker in AD. We present preliminary data indicating that abundant SP are found in the brains of nondemented patients dying with or as a result of critical coronary artery disease (cCAD) compared to nonheart disease (non-HD) subjects; 15 of 20 cCAD patients contained SP and only two of 16 non-HD patients contained SP.

Reactive synaptogenesis in hippocampal area CA1 of aged and young adult rats.

Selective lesions that result in a partial loss of neuronal input appear to signal residual, undamaged inputs to sprout and replace synaptic connections that have been lost. Previous investigations have compared this process of reactive synaptogenesis in young and old animals in the hippocampal dentate gyrus and have demonstrated that the aged brain has a diminished capacity for reinnervation following massive denervation of a target area. This investigation has focused on the lesion-induced plasticity of an adjacent area of the hippocampal formation, area CA1 of regio superior, in young adult and aged rats. Young adult aged Fischer 344 rates were subjected to a unilateral, intraventricular injection of kainic acid that selectively destroyed the CA3-CA4 hippocampal pyramidal neurons. Following a 2-day interoperative interval, the rats sustained an ipsilateral transection of the fimbria-fornix. Animals were killed at 4, 10, 30, and 60 days following the second transection and processed for electron microscopic analysis. Photographic montages were constructed of area CA1 extending from the alveus to the hippocampal fissure. The density of synapses, both intact and degenerating, was determined and analyzed as a function of age, days postlesion, and zone of analysis. Synaptic density decreased 30-40% contralaterally and 60-70% ipsilaterally in both aged and young adult rats. While both age groups restored synaptic density to preoperative levels, aged subjects required significantly more time. Aged rats appeared to be retarded in the initial phases of synaptic replacement. The delay in the aged animals' reactive response was not due to any differences in degeneration clearance between the age groups.

Lesion-induced synaptogenesis in the dentate gyrus of aged rats: II. Demonstration of an impaired degeneration clearing response.

Previously we reported that a delayed onset in the reinnervation of the outer two-thirds of the dentate molecular layer occurred in aged rats after an entorhinal lesion. Several factors associated with formation of new synaptic contacts and removal of degenerative debris may affect the reinnervation process. In this study the appearance and removal of degeneration was analyzed and evaluated with respect to the delayed reinnervation process in aged rats. After a complete lesion of the entorhinal cortex, 85-90% of the input to the outer two-thirds of the ipsilateral molecular layer is lost. Electron-dense and electron-lucent degeneration are present throughout the outer two-thirds of the denervated molecular layer. In both aged and young adult rats, the electron-lucent degeneration disappears by 10 days postlesion. The predominant electron dense degeneration, however, is removed at a different rate by young adult and aged rats. Young adults demonstrate a biphasic degeneration removal process, with almost half of this degeneration rapidly lost by 10 days postlesion, and nearly all by 60 days postlesion. Aged animals in contrast, have lost only 16% of the dense degeneration at 10 days postlesion, with about 30% of the degeneration remaining at 60 days postlesion. The impaired removal of the degeneration from the denervated zone appears to be reciprocally related to the reinnervation response in both age groups and may be related to the normal astrocyte hypertrophy and elevated corticosteroid levels in aged rats.

Lesion-induced synaptogenesis in the dentate gyrus of aged rats: I. Loss and reacquisition of normal synaptic density.

Quantitative electron microscopy was used to examine the ability of aged (2-year-old) and young adult (90-day-old) rats to replace those synapses lost (85-90%) in the outer two-thirds of the molecular layer of the dentate gyrus after a complete unilateral lesion of the entorhinal cortex. In aged rats the synaptic density is significantly lower than that of young adults at 10 days postlesion. Synaptic replacement begins between 2 and 4 days postlesion in young adults, whereas there is a delay until after 10 days postlesion in aged rats. Once synapse replacement begins in aged rats, the rate of synapse reappearance is about equal that of young adults. Thus the initial 10 days postlesion appears critical to growth of responding afferents and reformation of synaptic contacts. Analysis of synapses in terms of noncomplex and complex synaptic types shows that the noncomplex type accounts for the significant synaptic density difference between the two age groups. Replacement of complex synapses is nearly indistinguishable between age groups and is complete by 60 days postlesion. In contrast the initial replacement rate of noncomplex synapses in aged rats is much slower than young adults, though the control synaptic density is achieved by the end of the time course.

Compensatory synapse growth in aged animals after neuronal death.

The capacity of neurons to grow new synapses following partial denervation has been studied in the brain of aged rats and compared to that of younger animals. Lesion induced synapse formation is reduced in aged rats in the hippocampus and septum, two brain areas which show particularly robust growth responses in younger animals. The rate of growth as well as the final magnitude of the response is diminished in aged animals. A possible mechanism for the decreased growth response in aged animals is discussed in light of current models of reactive synaptogenesis. The loss of a compensatory growth response in the aged animal may be one of the factors which contribute to decreased brain plasticity and the slower and poorer recovery from brain damage following injury.

Laminar organization of cholinergic circuits in human frontal cortex in Alzheimer's disease and aging.

Cholinergic enzyme activity (choline acetyltransferase, CAT; acetylcholinesterase, AChE) and muscarinic cholinergic receptor density were measured in frontal cortex (Brodmann's area 9) of normal patients over the life span and in brains of patients with Alzheimer's disease (AD). CAT, but not AChE activity, declined with normal aging. Significant loss of CAT and AChE activity occurred in the AD brains, but later onset AD was associated with less severe loss of frontal cortex CAT activity. The majority of normal CAT activity resided in lamina I, II, and upper lamina III; CAT loss in AD resulted in large losses from all depths, most notably the upper cortical layers. AChE did not precisely correspond to the localization of CAT; loss of AChE in AD was consistent across all six laminae. No differences were seen in muscarinic cholinergic receptor binding between AD and age-matched controls; the distribution of binding was equal in all layers of normal frontal cortex, and no laminar differences were detected in distribution of cholinergic receptors between normal and AD samples.

Volumetric atrophy of the amygdala in Alzheimer's disease: quantitative serial reconstruction.

The present study quantitatively assessed volumes of the amygdala and its subnuclei in autopsied cases of advanced Alzheimer's disease (AD) for comparison with age-matched controls. Amygdalar nuclei showed significant atrophy in AD with the exception of the paralaminar portion of the basal nucleus. The magnocellular regions of the amygdala showed proportionately greater size reductions as a fraction of total amygdala volume than did other areas. Computerized reconstruction of the amygdala provided three-dimensional views of a variety of structural alterations accompanying the volumetric declines with AD. The apparent selective vulnerability of the magnocellular amygdalar areas coincides with the loss of large nerve cells in AD.

Chronic nicotine treatment attenuates alpha 7 nicotinic receptor deficits following traumatic brain injury.

Traumatic brain injury (TBI) often causes a persistent and debilitating impairment of cognitive function. Although the neurochemical basis for TBI-induced cognitive dysfunction is not well characterized, some studies suggest prominent involvement of the CNS cholinergic system. Previous studies from our laboratories have shown that alpha 7* nicotinic cholinergic receptors (nAChrs) are especially vulnerable to the pathophysiological effects of TBI. Hippocampal and cortical alpha-[(125)I]-bungarotoxin (BTX) expression of alpha 7* nAChrs is significantly decreased in many brain regions following TBI and this reduction persists for at least 3 weeks following injury. In the present study we evaluated whether chronic nicotine infusion could attenuate TBI-induced deficits in alpha 7* nAChr expression. Male Sprague-Dawley rats were sham-operated, or subjected to mild or moderate unilateral cortical contusion injury. Immediately following brain injury, osmotic mini-pumps that delivered chronic saline or nicotine (0.125 or 0.25 mg/kg/h) were implanted. The animals were euthanatized and the brains prepared for nAChr quantitative autoradiography, 7 days following surgery. Brain injury caused significant decreases in BTX binding in several regions of the hippocampus. TBI-induced deficits in alpha 7* nAChr density were reversed in four of the six hippocampal brain regions evaluated following chronic nicotine administration. If TBI-induced deficits in alpha 7* nAChr expression play a role in post-injury cognitive impairment, pharmacological treatments which restore nAChr binding to control levels may be therapeutically useful.

Chronic intermittent nicotine administration attenuates traumatic brain injury-induced cognitive dysfunction.

Traumatic brain injury (TBI) initiates immediate and secondary neuropathological cascades that can result in persistent neurological dysfunction. Previous studies from our laboratory have shown that experimental rat brain injury causes a rapid and persistent decrease in CNS alpha7* nicotinic cholinergic receptor (nAChr) expression. The purpose of this study was to investigate whether intermittent nicotine injections could improve cognitive performance in the Morris water maze (MWM) following experimental brain injury. Adult male rats were anesthetized and subjected to a 1.5 mm controlled cortical impact (CCI) injury of the somatosensory cortex. Animals received twice daily i.p. nicotine injections for 11 days prior to CCI, 11 days following CCI or during both pre- and post-surgical intervals. MWM training was initiated 12 days post-injury. In the training phase of cognitive testing, twice-daily nicotine treatment following injury attenuated trauma-induced deficits in the distance traveled to reach the escape platform. This group of animals also had improvements in several measures of the probe test, including time spent, distance traveled and total entries into the target quadrant. TBI caused significant deficits in alpha7* nAChr expression in several regions of the hippocampus and cerebral cortex, which were largely unaffected by intermittent nicotine treatment. However, nicotine treatment up-regulated [(3)H]-epibatidine binding to non-alpha7* nAChrs, attenuating TBI-induced deficits in receptor expression in several brain regions evaluated. These results suggest that nicotine is efficacious at attenuating CCI-induced cognitive deficits in a manner independent of changes in alpha7* nAChr expression, perhaps via up-regulation of non-alpha7* nAChrs.