Neil R. Sims

Biochemical assessment of serotonergic and cholinergic dysfunction and cerebral atrophy in Alzheimer's disease.

Abstract: Markers of serotonin synapses in entire temporal lobe and frontal and temporal neocortex were examined for changes in Alzheimers disease by use of both neurosurgical and autopsy samples. Uptake of [3H]sero‐tonin, binding of [3H]imipramine, and content of indola‐mines were all significantly reduced, indicating that serotonin nerve terminals are affected. Binding of [3H]serotonin was also reduced, whereas that of [3H]qui‐nuclidinyl benzilate, [3H]muscimol, and [3H]dihydroal‐prenolol were unaltered. When the Alzheimers samples were subdivided according to age, the reduction in [3H]serotonin binding was a feature of only autopsy samples from younger patients. In contrast, presynaptic cholinergic activity was reduced in all groups of Alzheimers samples, including neurosurgical specimens. Five markers, thought to reflect cerebral atrophy, cytoplasm, nerve cell membrane, and neuronal perikarya were measured in the entire temporal lobe. In Alzheimers disease the reductions (mean 25%, range 20–35%) were thought to be too large to be due only to loss of structures associated with the presumed cholinergic perikarya in the basal forebrain and monoamine neurones in the brain stem.

Presynaptic Cholinergic Dysfunction in Patients with Dementia

Abstract: Indices of presynaptic cholinergic nerve endings were assayed in neocortical biopsy samples from patients with presenile dementia. For those patients in whom Alzheimers disease was histologically confirmed, [14C]acetylcholine synthesis, choline acetyltransferase activity and choline uptake were all found to be markedly reduced (at least 40%) below mean control values. The changes occurred in samples from both the frontal and temporal lobes and for [14C]acetylcholine synthesis the decrease was similar under conditions of high and low neuronal activity (as assessed by incubations in 31 mM and 5 mM K+ respectively). Samples from other demented patients, in whom the histological features of Alzheimers disease were not detected, produced values for all three biochemical parameters which were similar to controls. For the total group of patients with presenile dementia there were correlations between values for the three markers of presynaptic cholinergic nerve endings suggestive of a loss of functional activity at these sites in Alzheimers disease.

Rapid Isolation of Metabolically Active Mitochondria from Rat Brain and Subregions Using Percoll Density Gradient Centrifugation

Two procedures are described for isolating free (nonsynaptosomal) mitochondria from rat brain. Both procedures employ a discontinuous Percoll gradient and yield well coupled mitochondria which exhibit high rates of respiratory activity and contain little residual contamination by synaptosomes or myelin. The procedures are considerably more rapid than methods described previously for the isolation of brain mitochondria and do not require an ultracentrifuge or swing‐out rotor. The first method separates mitochondria by gradient centrifugation from a P2 (crude mitochondrial) fraction and is likely to be widely applicable for studies in which at least 500 mg of tissue are available as starting material. In the second method, the unfractionated homogenate is subjected directly to gradient centrifugation. This method requires the preparation of more gradients (per gram of tissue) than the first method and yields a subcellular fraction with slightly more synaptosomal contamination. However, this second procedure is more rapid, requires less manipulation of the tissue, and is suitable for obtaining mitochondria with well preserved metabolic characteristics from subregions of single rat brains.

Mitochondrial contributions to tissue damage in stroke.

Tissue infarction, involving death of essentially all cells within a part of the brain, is a common pathology resulting from stroke and an important determinant of the long-term consequences of this disorder. The cell death that leads to infarct formation is likely to be the result of multiple interacting pathological processes. A range of factors, including the severity of the ischemic insult and whether this is permanent or reversed, determine which mechanisms predominate. Although evaluating mitochondrial properties in intact brain is difficult, evidence for several potentially deleterious responses to cerebral ischemia or post-ischemic reperfusion have been obtained from investigations using animal models of stroke. Marked changes in ATP and related energy metabolites develop quickly in response to occlusion of a cerebral artery, as expected from limitations in the delivery of oxygen and glucose. However, these alterations are often only partially reversed on reperfusion despite improved substrate delivery. Ischemia-induced decreases in the mitochondrial capacity for respiratory activity probably contribute to the ongoing impairment of energy metabolism during reperfusion and possibly also to the magnitude of changes seen during ischemia. Conditions during reperfusion are likely to be conducive to the induction of the permeability transition in mitochondria. There are as yet no well-characterized techniques to identify this change in the intact brain. However, the protective effects of some agents that block formation of the transition pore are consistent with both the induction of the permeability transition during early recirculation and a role for this in the development of tissue damage. Release of cytochrome c into the cytoplasm of cells has been observed with both permanent and reversed ischemia and could trigger the death of some cells by apoptosis, a process which probably contributes to the expansion of the ischemic lesion. Mitochondria are also likely to contribute to the widely-accepted role of nitric oxide in the development of ischemic damage. These organelles are a probable target for the deleterious effects of this substance and can also act as a source of superoxide for reaction with the nitric oxide to produce the damaging species, peroxynitrite. Further characterization of these mitochondrial responses should help to elucidate the mechanisms of cell death due to cerebral ischemia and possibly point to novel sites for therapeutic interventions in stroke.

Cortical Pyramidal Neurone Loss May Cause Glutamatergic Hypoactivity and Cognitive Impairment in Alzheimer's Disease: Investigative and Therapeutic Perspectives

In the 1960s it became generally accepted that the cognitive impairment associated with old age was due to disorders with specific histological features rather than being an inevitable part of the aging process (see, e.g., Corsellis, 1962). Furthermore, two disorders appeared to account for the majority of cases of dementia amongst the elderly, one characterised by prominent disease of the cerebral vasculature and one with histological features similar to those described in a patient in her fifties by Alois Alzheimer early in the century. Alzheimer’s disease (AD) was therefore recognised as a major cause ofdementia, rather than a rare neurodegenerative disease giving rise to presenile dementia. This observation, coupled with the identification of the neurochemical pathology underlying Parkinson’s disease and the success of L-DOPA treatment following its introduction in 1968, set the scene for the systematic biochemical study ofdementia in old age with the hope of producing similarly dramatic treatments. The demonstration of substantial cholinergic abnormalities in the brains of patients with AD suggested a basis for such rational pharmacological treatments. However, cases have been reported that raise some doubts as to the validity of the view of AD as a primary disorder of the cholinergic system (Bowen et al., 1977). One subset of patients with dementia had typical neuropathological findings of AD, yet their cortical choline acetyltransferase (ChAT) activity was not selectively reduced (Palmer et al., 1986). Other demented patients with AD had normal numbers of cholinergic neurones in the nucleus basalis of Meynert (Perry et al., 1982; Pearson et al., 1983). A reduction in numbers of basal forebrain neurones and cortical ChAT specific activity of a magnitude similar to that seen in moderate to severe AD occurs in another neurodegenerative condition, olivopontocerebellar atrophy, yet cognitive impairment in this condition is not prominent (Kish et al., 1988). It appears likely that the neocortical cholinergic deficit in AD can explain only a part of the entire clinical syndrome. Since 1982 this group (Bowen, 1983) has focused much attention on the intrinsic neurones of the cerebral cortex. An extensive body of literature describes effects on learning and memory in humans exerted by lesions of the cerebral cortex and the hippocampus (Dudai, 1989). Experimental studies in animals have also sought to define the role of these structures in cognition. Lashley (as reviewed by Dudai, 1989) used conditioned rats and monkeys to perform various tasks, mechanically damaged the neocortex either before or after training, and then measured the effect of the lesions on acquisition and retention. He found that the amount of reduction in learning was dependent on the amount of neocortical tissue removed and, also, that the more complex the task, the greater the effect of the removal of neocortex. Studies have been extended to include the hippocampus and have also increased in subtlety by using excitotoxins, with analogous changes in behaviour (Francis et al., 1992~). The excitatory amino acids (EAA), glutamic (Glu) and aspartic acid, are the proposed transmitters of the cortical pyramidal cells and have been the subject of detailed studies in recent years. There is now strong evidence for an excitotoxic role of these amino acids in the pathogenesis of cerebral ischaemia (German0 et al.. 1987: Park et al., 1988: Sheardown et al., 1990).

Presynaptic Serotonergic Dysfunction in Patients with Alzheimer's Disease

Abstract: Indices of presynaptic serotonergic nerve endings were assayed in neocortical biopsy samples from patients with histologically verified Alzheimers disease. The concentrations of 5‐hydroxytryptamine (serotonin) and 5‐hydroxyindoleacetic acid, serotonin uptake, and K+‐stimulated release of endogenous serotonin were all found to be reduced below control values. Changes occurred in samples from both the frontal and temporal lobes, but they were most severe (at least a 55% reduction) in the temporal lobe. This is indicative of substantial serotonergic denervation. Values for serotonergic markers in Alzheimers disease samples did not show correlations with rating of the severity of dementia, indices of cholinergic innervation, or senile plaque and cortical pyramidal neurone loss. However, neuronbrillary tangle count and an index of glucose oxidation (both probably reflecting pyramidal cells) correlated with the concentration of 5‐hydroxyindoleacetic acid.

Biochemical Evidence of Selective Nerve Cell Changes in the Normal Ageing Human and Rat Brain

Abstract: Atrophy with ageing of human whole brain, entire temporal lobe, and caudate nucleus was assessed in autopsy specimens, by biochemical techniques. Only the caudate nucleus showed changes. Markers for several neurotransmitter systems were also examined for changes with age. In neocortex and temporal lobe of human brain, small decreases were detected in markers of cholinergic nerve terminals, whereas a large decrease (79%) occurred in the caudate nucleus. Findings were similar in striatum from 3–33‐month‐old rats. No change occurred in binding of [3H]quinuclidinyl benzilate by human samples. Markers of serotonergic terminals were also unchanged in human and rat brain. By contrast, binding of [3H]lysergic acid diethylamide and [3H]serotonin was decreased (32–81%) in human neocortex and temporal lobe, but not in caudate nucleus. A 43% loss of a marker of γ‐aminobutyrate terminals occurred in human neocortex, while [3H]muscimol binding increased (179%). No changes were detected in markers of catecholamine synapses in temporal lobe or rat striatum. Hence, with human ageing there appears to be a loss of markers of γ‐aminobutyrate neurones intrinsic to neocortex and acetylcholine cells intrinsic to the caudate nucleus, as well as a change in postsynaptic serotonin receptors in neocortex. These losses are accompanied by relative preservation of markers of ascending projections from basal forebrain and brain stem.

Isolation of mitochondria from rat brain using Percoll density gradient centrifugation

We have developed procedures that combine differential centrifugation and discontinuous Percoll density gradient centrifugation to isolate mitochondria from rat forebrains and brain subregions. The use of Percoll density gradient centrifugation is central to obtaining preparations that contain little contamination with synaptosomes and myelin. Protocols are presented for three variations of this procedure that differ in their suitability for dealing with large or small samples, in the proportion of total mitochondria isolated and in the total preparation time. One variation uses digitonin to disrupt synaptosomes before mitochondrial isolation. This method is well suited for preparing mitochondria from small tissue samples, but the isolated organelles are not appropriate for all studies. Each of the procedures produces mitochondria that are well coupled and exhibit high rates of respiratory activity. The procedures require an initial setup time of 45–75 min and between 1 and 3 h for the mitochondrial isolation.

Metabolic Processes in Alzheimer's Disease: Adenine Nucleotide Content and Production of 14CO2 from [U‐14C]Glucose In Vitro in Human Neocortex

Abstract: Samples of neocortex removed at diagnostic craniotomy from patients with Alzheimers disease and incubated in vitro showed an increased production of 14CO2 from [U‐14C]glucose compared with neurosurgical controls. This was a feature of incubations in the presence of both 5 mM K+ (142% control) and 31 mM K + (126%). Specific labelling of the amino acid pool was unaltered, suggesting that the apparent increase of CO2 production was not merely a reflection of changes in dilution of the radiolabel from glucose. The content of adenine nucleotides was significantly less than control values in the tissue from patients with Alzheimers disease after in vitro incubations but the adenylate energy charge was unchanged, indicating that normal energy metabolism was not grossly impaired in these preparations.

Selective Impairment of Respiration in Mitochondria Isolated from Brain Subregions Following Transient Forebrain Ischemia in the Rat

Abstract: Using Percoll density gradient centrifugation, free (nonsynaptosomal) mitochondria were isolated from the dorsal‐lateral striatum and paramedian neocortex of rats during complete forebrain ischemia and reperfusion. Mitochondria prepared from either region after 30 min of ischemia showed decreased state 3 (ADP and substrate present) and uncoupled respiration rates (19–45% reductions) with pyruvate plus malate as substrates, whereas state 4 respiration (no ADP present) was preserved. At 6 h of recirculation, state 3 and uncoupled respiration rates for mitochondria from the paramedian neocortex (a region resistant to ischemic damage) were similar to or even increased compared with control values. By contrast, in mitochondria from the dorsal‐lateral striatum (a region containing neurons susceptible to global ischemia), decreases in state 3 and uncoupled respiration rates (25 and 30% less than control values) were again observed after 6 h of recirculation. With succinate as respiratory substrate, however, no significant differences from control values were found in either region at this time point. By 24 h of recirculation, respiratory activity with either pyruvate plus malate or succinate was greatly reduced in samples from the dorsal‐lateral striatum, probably reflecting complete loss of function in some organelles. In contrast with these marked changes in free mitochondria, the respiratory properties of synaptosomal mitochondria, assessed from measurements in unfractionated homogenates, were unchanged from controls in the dorsal‐lateral striatum at each of the time points studied, but showed reductions (19–22%) during ischemia and after 24 h of recirculation in the paramedian neocortex. The results from this study provide evidence that reductions in the function of free mitochondria, apparently involving restriction of electron flow through complex I of the electron transport chain, develop selectively in an ischemia‐susceptible region at times coincident with initial histological evidence of neuronal damage.