Gary C. Stratmann

Topographical distribution of neurochemical changes in Alzheimer's disease

Biochemical indices of cortical nerve cells affected in Alzheimers disease have been proposed (excitatory dicarboxylic amino acid, EDAA, sodium-dependent carrier; phosphate-activated glutaminase activity; serotonin type 2 recognition site; somatostatin-like immunoreactivity). These and the content of EDAAs and two related amino acids, and choline acetyltransferase (ChAT) activity have been measured in up to 13 areas of cerebral cortex and the cerebellar cortex from 16 patients with Alzheimers disease and 17 controls. Reduction of the index of the serotonin recognition site, somatostatin content and another biochemical index of interneurones coincide and indicate a rather unexpected focal loss of such neurones from the parietal lobe. No unequivocal measure of the integrity of pyramidal neurones could be established as the content of no amino acid was reduced, the index of the EDAA carrier showed evidence of change in few brain regions and glutaminase activity was subject to unexplained variability. ChAT activity alone closely paralleled a previous report of the distribution of morphological degeneration. The results are discussed in relation to therapy and positron emission tomography.

Excitatory amino acid-releasing and cholinergic neurones in Alzheimer's disease

Brains of normal controls and patients with primary degenerative dementia were investigated for choline acetyltransferase (ChAT) activity in the frontal, temporal and parietal cortex, hippocampus, amygdala and thalamus. A few patients with Alzheimers disease were unusual as the cholinergic marker was unaffected, except in the amygdala. Other patients with dementia and undiagnosed neurodegenerative disorder had elevated cortical ChAT activity. The interpretations offered are: (a) the syndrome of dementia and Alzheimer pathologic change precedes significant loss of cortical cholinergic innervation; (b) denervation in dementia can occur early in olfactory areas, exemplified here by the amygdala; (c) dementia is associated with the loss of non-cholinergic structure. An indication of structures involved was given by loss of a marker of excitatory amino acid-releasing neurones.

Reduced glycine stimulation of [3H]MK-801 binding in Alzheimer's disease.

The novel N‐methyl‐D‐aspartate recjeptor channel ligand (+)‐[3H]5‐methyl‐J0, l1‐dihydro‐5H‐dibenzo[a, d]‐cyclohepten‐5, 10‐imine maleate ([3H]MK‐801) has been utilized to label this receptor in human brain tissiie. Characteristics of [3H]MK‐801 binding to well‐washeq membranes from 17 control subjects and 16 patients with Alzheimers disease were determined in frontal, parietal, and temporal cerebral cortex and cerebellar cortex. In control tissue the pharmacological specificity of the binding of this substance is entirely consistent with the profile previously reported for rat brain. Binding could be stimulated by the addition of glutamic acid to the incubation medium; addition of glycine produced further enhancement which was not prevented by strychnine. The specificity of the effects of the and other amino acids on the binding was the same as in the rat. In Alzheimers disease significantly less binding Was observed in the frontal cortex under glutamate‐ and glycine‐stimulated conditions. This appears to be associated with a reduced affinity of the site whereas the pharmacological specificity of the site remained unchanged. The effect did not appear to be due to differences in mode of death between Alzheimers disease and control subjects and is unlikely to be related to factors for which the groups were matched. In contrast, binding was not altered in the absence of added amino acids and presence of glutamate alone. These results imply that in the cerebral cortex the agonist site and a site in the cation channel of the receptor are not selectively altered, but that their coupling to a strychnine‐insensitive glycine recognition site is impaired.

NMDA-induced glutamate and aspartate release from rat cortical pyramidal neurones: evidence for modulation by a 5-HT1A antagonist.

1 We have investigated an aspect of the regulation of cortical pyramidal neurone activity. Microdialysis was used to assess whether topical application of drugs (in 10 μl) to fill a burr hole over the frontal cortex, where part of the corticostriatal pathway originates, would change concentrations of the excitatory amino acids glutamate and aspartate in the striatum of the anaesthetized rat. 2 Topical application of N‐methyl‐D‐aspartate (NMDA, 2 and 20 mM) dose‐dependently increased glutamate and aspartate concentrations in the striatum. Coapplication of tetrodotoxin (10 μm) blocked the NMDA‐evoked rise in these amino acids. A calcium‐free medium, perfused through the probe also blocked the rise, indicating that it was due to an exocytotic mechanism in the striatum. 3 It was hypothesized that the rise observed was due to an increase in the activity of the corticostriatal pathway. As 5‐hydroxytryptamine1A (5‐HT1a) receptors are enriched on cell bodies of corticostriatal neurones, a selective 5‐HT1A‐antagonist (WAY 100135) was coapplied with the lower dose of NMDA. Compared to NMDA alone, coapplication of 50 μm WAY 100135 significantly increased glutamate release. This effect was sensitive to tetrodotoxin and calcium‐dependent. Application of 50 μm WAY 100135 alone significantly enhanced glutamate release above baseline; this was also tested at 100 μm (not significant). 4 Compared to NMDA alone, coapplication of WAY 100135 (20 μm) significantly enhanced aspartate release; the mean value was also increased (not significantly) with 50 μm. This rise was calcium‐dependent, but not tetrodotoxin‐sensitive. WAY 100135 (100 μm) reduced NMDA‐induced aspartate release. Application of the drug alone had no effect on basal aspartate release. 5 Coapplication of the 5‐HT1A agonist, 8‐OHDPAT (5 mM) with NMDA did not affect the NMDA‐evoked increase in glutamate and aspartate. 6 Topical application of high potassium (100 mM) to the surface of the cortex did not result in a detectable rise in striatal glutamate or aspartate. 7 Perfusion of WAY 100135 (tested at 50 μm) through the dialysis probe did not affect glutamate or aspartate concentrations. 8 It was concluded that a selective 5‐HT1A‐antagonist can increase the activity of corticostriatal pyramidal neurones. As in Alzheimers disease hypoactivity of pyramidal neurones almost certainly exists, a selective 5‐HT1A‐antagonist may be potentially useful in the treatment of the cognitive symptoms of this disease.

Loss of glycine-dependent radioligand binding to the N-methyl-D-aspartate-phencyclidine receptor complex in patients with Alzheimer's disease.

Well washed membranes have been prepared from samples of cerebral cortex of control subjects and patients with Alzheimers disease, obtained both at post mortem and by neurosurgical procedures earlier in the course of the disease. Binding to these membranes of two radioligands for the N-methyl-D-aspartate-phencyclidine receptor complex has been determined in the presence and absence of glycine. Glycine increased the binding in both control and Alzheimer tissue samples. At one concentration of radioligand, in the presence of glycine there was less binding to post-mortem samples, which Scatchard analysis showed was associated with a 36% loss of sites. In rare neurosurgical samples, there was also a loss of binding of radioligand which suggests that the effect is not due to post-mortem artefacts or epiphenomena. These new results may have implications for the symptomatic and preventative treatment of Alzheimers disease.

Serotonergic Pathology Is Not Widespread in Alzheimer Patients Without Prominent Aggressive Symptoms

Behavioural symptoms of Alzheimers disease, such as aggression, may determine the care patients required. Most postmortem neurochemical studies have been of institutionalized patients and conclusions drawn from these may not be valid for all patients. We have shown that serotonin 2 receptors are not lost from 12 of the 13 areas of cerebral cortex examined in the patients assessed to be free of aggressive symptoms. This has been interpreted as representing the relative preservation of cortical interneurones. In contrast choline acetyltransferase activity was reduced in all areas whereas serotonin content was reduced in only 2 of the 4 areas examined.

Cholinomimetics Increase Glutamate Outflow via an Action on the Corticostriatal Pathway: Implications for Alzheimer's Disease

Abstract: Physostigmine, the acetylcholinesterase inhibitor (0.3 mg/kg, i.m.), increased extracellular glutamate but not aspartate concentrations in the striatum of anaesthetised rats, determined using microdialysis and HPLC. The rise was both tetrodotoxin and calcium dependent. In contrast, neither physostigmine (10 µM) added to the perfusion fluid nor vehicle (injected intramuscularly) affected amino acid concentrations. To obtain evidence that the action of acetylcholine was to modulate positively cortical pyramidal neurone activity via the M1 receptor, the selective M1 agonist PD 142505‐0028 (10 µM) was topically applied to the frontal cortex. Like physostigmine, PD 142505‐0028 rapidly increased glutamate but not aspartate concentrations in the striatum. Moreover, the effect of intramuscular physostigmine was blocked by a topically applied M1 antagonist. These new data add to our hypothesis that cholinomimetics increase pyramidal neurone function.

Characterisation of the Glycine Modulatory Site of the N‐Methyl‐d‐Aspartate Receptor‐Ionophore Complex in Human Brain

Abstract: [3H]Glycine binding and glycine modulation of [3H]MK‐801 binding have been used to study the glycine allosteric site associated with the N‐methyl‐D‐aspartate receptor complex in postmortem human brain. The effect of glycine on [3H]MK‐801 binding appeared sensitive to duration of terminal coma, and possibly postmortem delay. Thirty percent of the binding occurred in a subfraction of brain tissue and did not show enhancement by glycine and glutamic acid. [3H]Glycine binding to a subfraction free from this component was studied and showed high specific binding. KD and Bmax values showed considerable intersubject variability which did not appear to be due to demographic features or to tissue content of amino acids with an affinity for this site. The pharmacological characteristics of binding in this subfraction and a correlation between Bmax values and the maximal enhancement of [3H]MK‐801 binding by glycine are consistent with [3H]glycine binding occurring to an N‐methyl‐D‐aspartate receptor complex associated site. Further support for this is provided by a significantly lower Bmax value for [3H]glycine binding in subjects with Alzheimers disease and reduced glycine enhancement of [3H]MK‐801 binding. However, the effect of perimortem factors makes it difficult to confidently attribute this solely to a disease‐related change in the receptor. The possible role of the glycine allosteric site in the treatment of neuropsychiatric disorders is discussed.

Neurotransmitters and Second Messengers in Aging and Alzheimer's Disease

A substantial loss of cortical cholinergic nerve endings, along with a much more circumscribed cortical degeneration of pyramidal neurons, almost certainly causes glutamatergic hypoactivity in live Alzheimers patients. These selective pathologies are discussed in terms of therapy. An additional effect of some proposed treatments is emerging as there is evidence that processing pathways for β‐amyloid precursor proteins in cortical pyramidal neurons, a target cell for acetylcholine, are affected by neuronal activity.

Postmortem brains reveal similar but not identical amyloid precursor protein-like immunoreactivity in Alzheimer compared with other dementias

Concentrations of amyloid precursor protein (APP)-like immunoreactivity (APPLIR) have been determined by Western blotting in a soluble fraction and two membrane fractions of two areas of brain cortex from patients with Alzheimers disease (AD) and other dementias. There were no significant differences between AD and other cases in species with the Kunitz protease inhibitor domain. However, the total soluble APPLIR was higher in AD and it was hypothesized that this relates to cholinergic hypoactivity.