Alzheimer's disease: changes in hippocampal N-methyl-D-aspartate, quisqualate, neurotensin, adenosine, benzodiazepine, serotonin and opioid receptors--an autoradiographic study

Laminar distribution of the multiple opioid receptors in the human cerebral cortex

Quantitative autoradiographic assessment of cerebral cortical laminar distribution of mu, delta and kappa opioid receptors was carried out in coronal sections of five post-mortem human brains obtained at autopsy. The cortical areas studied were: cingulate, frontal, insular, parietal, parahippocampal, temporal, occipitotemporal, occipital and striate area. In general, the laminar patterns of distribution for the three types of receptors are distinctive. Peak levels of delta opioid binding are in laminae I, II, and IIIa. mu-Receptors are located in lamina III followed by I and II in cingulate, frontal, insular and parietal cortices and lamina IV in temporal and occiptotemporal cortices. kappa-Receptors are found concentrated in laminae V and VI. The patterns of opioid binding in cortical laminae showed remarkable consistency in all five brains examined. In contrast to other cortical areas, the parahippocampal gyrus, at the level of the amygdaloid formation, demonstrated peak kappa receptor density in laminae I, II and III. mu-Opioid binding was undetectable in the lateral occipital cortex and in the striate area.

Ionotropic glutamate receptor subtypes in the aged memory-impaired and unimpaired Long-Evans rat

The comparative quantitative autoradiographic distribution of ionotropic glutamate receptor subtypes were investigated in young adults (six months) and aged (24-25 months) cognitively impaired and unimpaired male Long-Evans rats. Aged rats were behaviorally characterized as either cognitively impaired or unimpaired based upon their performances in the Morris water maze task compared to the young adult controls. The status of the N-methyl-D-aspartate, [125I]dizocilpine maleate, [3H]kainate and amino-3-hydroxy-5-methylisoxasole-4-propionate (AMPA, [3H]AMPA) receptor binding sites were then established in these three subgroups of animals as a function of their cognitive performance in the Morris water maze task. The apparent densities of both N-methyl-D-aspartate/[125I]dizocilpine maleate and kainate binding sites were significantly decreased in various regions of the aged rat brain. Marked losses in [125I]dizocilpine maleate binding sites were observed in outer laminae of the frontal, parietal and temporal cortices, and the stratum radiatum of the CA3 subfield of the hippocampus. Interestingly, losses in [125I]dizocilpine maleate binding sites were generally most evident in the cognitively unimpaired aged subgroup, suggesting a possible inverse relationship between losses of this receptor subtype and cognitive performances in the Morris water maze task. The levels of [3H]kainate binding were most significantly diminished in various cortical and hippocampal areas as well as the striatum and septal nuclei of both groups of aged rats. In contrast, the apparent density of [3H]AMPA binding was increased in most hippocampal subfields and the superficial laminae of the occipital cortex of the cognitively impaired vs young adult rats. Changes in [3H]AMPA labeling failed to reach significance in the unimpaired cohort. Taken together, these results show that while losses in [3H]kainate binding were similar in both subgroups of aged rats, differences were seen with respect to cognitive status for both [125I]dizocilpine maleate/N-methyl-D-aspartate and [3H]AMPA binding sites. Decreases in [125I]dizocilpine maleate binding sites were mostly restricted to cortical areas of cognitively unimpaired rats, while increases in the AMPA binding subtype were seen in the memory-impaired subgroup. It would thus appear that changes in N-methyl-D-aspartate and AMPA receptor subtypes may be more critical than alterations in kainate binding sites for the emergence of the functional deficits seen in the aged cognitively impaired rat.

A role for immediate-early transcription factors in learning and memory

This article summarizes recent studies from the long-term potentiation (LTP), long-term depression (LTD), and behavioral learning literature, indicating that immediate-early genes (IEGs) may play an important role in learning and memory. The LTP studies suggest that synaptic modifications occurring during NMDA-receptor-mediated hippocampal LTP and LTD are stabilized by the protein products of the krox family of IEGs (as well as by brain-derived neurotrophic factor, BDNF). Activation of muscarinic receptors also induces members of the krox as well as the fos and jun family (jun-B but not c-jun) IEGs in hippocampal neurons and this action may be involved in the facilitatory effects of muscarinic receptor activation on both hippocampal LTP and learning. The possible role of IEGs in the learning-enhancing effects of cholinergically mediated hippocampal theta is also discussed. Finally, I review a number of recent studies showing IEG expression in brain neurons after behavioral learning. Together these results suggest some role for select IEGs (e.g., Krox 24) in learning and memory, although definitive studies using antisense DNA technology are required to establish any causal links. In particular, IEGs may be critical components of the signal transduction cascade that links NMDA and muscarinic receptors to the neuronal genome and ultimately to the generation of permanent modifications in neuronal biochemistry that provides the substrate for learning.

Loss of A1 adenosine receptors in human temporal lobe epilepsy

Using quantitative receptor autoradiographic methods we have examined A1 adenosine receptors, adenosine uptake sites, benzodiazepine receptors, NMDA, AMPA, and kainic acid receptors in temporal lobes removed from patients suffering from complex partial seizures and in normal control post-mortem temporal cortex. Binding to A1 adenosine receptors and NMDA receptors was reduced in epileptic temporal cortex, while the other neurochemical parameters were unchanged. The reason for this A1 receptor loss is unclear as it occurred in both idiopathic and symptomatic cases and thus may be a consequence rather than an initial cause of seizures. However, because adenosine is a powerful anticonvulsant substance, loss of anticonvulsant A1 receptors may contribute to the human epileptic condition. It is also possible that the observed differences in A1 binding are due to autopsy vs. biopsy changes in the levels of A1 adenosine receptors.

Localisation of the adenosine uptake site in the human brain: a comparison with the distribution of adenosine A1 receptors

Using quantitative receptor autoradiography we investigated the distribution of the adenosine uptake site labelled with [3H]NBTI in post-mortem human brain and compared its distribution with that of the A1 adenosine receptor labelled with [3H]CHA. The highest levels of [3H]NBTI binding were found in the cortex and striatum, with moderate levels in the hippocampus, globus pallidus, cerebellum and some midbrain and spinal cord nuclei. The distribution of A1 receptors and this adenosine uptake site differed in the hippocampus where A1 receptors were highest in CA1 but the uptake site was low in CA1 and higher in the molecular layer of the dentate gyrus. These results define the anatomical distribution of the high affinity NBTI sensitive adenosine uptake site in the normal human brain.

The neurochemistry of Alzheimer type, vascular type and mixed type dementias compared

We present the results of a meta-analysis of neurochemical changes in human post mortem brains of Alzheimer type (AD), vascular type (VD) and mixed type (MF) dementias, and matched controls based on 275 articles published between January 1980 and February 1994. Severity of degeneration between the different neurochemical systems is as follows, although ranking is difficult with regard to limited numbers of investigations in some neurochemical systems: Cholinergic system > serotonergic system > excitatory amino acids > GABAergic system > energy metabolism > NA > oxidative stress parameters > neuropeptides > DA. But, within a neurochemical system, degeneration is not evenly distributed. Spared parameters, e.g. muscarinic receptors and MAO-B, allow to make some suggestions for future therapeutic strategies.

AMPA-selective glutamate receptor subtype immunoreactivity in the aged human hippocampal formation

It has been hypothesized that, in Alzheimer's disease, glutamate-mediated excitotoxicity contributes to the degeneration of selected populations of neurons. In the present study, immunocytochemical techniques were used to determine the distribution and anatomical features of GluR1- and GluR2/3-immunolabeled cell bodies and processes within the hippocampal formation of normal (i.e., no pathology) elderly humans. The results of this study provide an essential baseline with which to compare the expression and distribution of glutamate receptor subunits within the brains of patients with Alzheimer's disease. With respect to GluR1 immunoreactivity, the molecular layer of the dentate gyrus displays the most intense immunolabeling of any hippocampal structure. Contributing to this intense labeling are apical dendrites that arise from neurons within the adjacent granule cell layer. Interestingly, GluR1-labeled neurons account for a relatively small percentage of the total number of neurons as revealed by Nissl staining in the granule cell layer. In contrast, GluR2/3-labeled neurons are densely distributed throughout the granule cell layer, yet they provide relatively few processes to the adjacent molecular layer compared to GluR1-positive processes. GluR1 labeling is also prominent within the CA fields of Ammon's horn, with CA2 > CA3 > CA1 > or = CA4. Most prominent within the CA fields are the labeled dendrites of pyramidal neurons. In many instances, apical dendrites can be traced into the adjacent stratum radiatum, where they impart a deep striated appearance to this region of the hippocampus. Robust GluR2/3 labeling is also observed within the pyramidal layer of Ammon's horn, with an order of staining intensity similar to that observed for GluR1. However, unlike GluR1 labeling, which is localized predominantly along dendrites, GluR2/3 labeling is observed primarily in association with cell bodies. Collectively, these data suggest that the molecular composition of the AMPA receptor complex may differ between the dendrite and soma of granule and pyramidal neurons within the hippocampal formation, so functionally we may predict that these two regions of the neuron would respond differently following glutamate receptor stimulation.

Regional differences in the interaction of the excitotoxins domoate and L-beta-oxalyl-amino-alanine with [3H]kainate binding sites in human hippocampus

The excitotoxic amino acid domoate causes anterograde amnesia and memory deficits while the excitotoxin L-beta-oxalyl-amino-alanine (L-BOAA) is considered the causative agent of the motoneurone disorder, neurolathyrism. Employing quantitative autoradiography we investigated the potency of domoate and L-BOAA to inhibit [3H]kainate binding in human hippocampus. Domoate inhibited binding of [3H]kainate with inhibition constants between 5.8 +/- 2.8 nM (deep layers of gyrus parahippocampalis) and 200.9 +/- 247.8 nM (CA1 region of hippocampus). It was about a thousandfold more potent than L-BOAA with inhibition constants between 2.1 +/- 0.5 microM (superficial layers of gyrus parahippocampalis) and 51.0 +/- 41.9 microM (CA2/3 region of hippocampus). Interestingly, L-BOAA showed lowest affinity to [3H]kainate binding sites in those regions in which domoate showed highest affinity (e.g. CA2/3) and vice versa (e.g. CA1). These data further support the notion that the neurological symptoms observed after domoate intoxication are due to an excitotoxic action at kainate receptors and provide evidence for heterogeneity of kainate receptors in human hippocampus.

Autoradiographic characterization of neurotensin receptors in the entorhinal cortex of schizophrenic patients and control subjects

Neurotensin, an endogenous peptide and putative neurotransmitter, exhibits a wide range of interactions with dopaminergic neurons and displays some actions akin to neuroleptics. Moreover, neurotensin receptors are abundant in specific layers of the entorhinal cortex where cytoarchitectural abnormalities have been reported in schizophrenia. We therefore examined the entorhinal cortex from postmortem specimens of five control patients and six schizophrenic patients for alterations in neurotensin receptor quantitation and distribution using receptor autoradiography. Specific 125I- neurotensin binding was concentrated in layer II cell clusters, with a 40% reduction in binding in the schizophrenic group (p < 0.05). Moderate binding was observed in both cohorts in deep layers V/VI, with negligible binding in the hippocampus. There was no statistical difference in quantitative neurotensin binding in other lamina of the entorhinal cortex of schizophrenics compared with controls. The characteristic laminar pattern of binding did not differ between cohorts. The reduction in neurotensin binding in schizophrenics is consistent with an increasing number of reports of structural abnormalities in the medial temporal lobe of schizophrenics in general and the entorhinal cortex in particular. Further studies are required to examine the evidence for neuroanatomic and neurochemical pathology in the entorhinal cortex.

AMPA-selective glutamate receptor subtype immunoreactivity in the hippocampal formation of patients with Alzheimer's disease

Immunocytochemical techniques were employed in order to examine the distribution and relative intensity of the AMPA receptor subunits GluR1 and GluR2/3 within the hippocampal formation of normal controls and Alzheimer's disease (AD) cases. Throughout our investigation we examined cases exhibiting a wide range of pathologic severity, thus allowing us to correlate our immunohistochemical data with the extent of pathology. Specifically, we investigated the distribution of these receptor subunits in hippocampal sectors that are particularly vulnerable to AD pathology (i.e., CA1 and subiculum) and compared these findings with those obtained following examination of sectors that are generally resistant to pathologic change (i.e., CA2/3, dentate gyrus). Within vulnerable sectors we observed a variable loss of GluR1 and GluR2/3 immunolabeling. The degree to which these proteins were reduced appeared to correlate with the extent of neurofibrillary pathology and cell loss. Despite the loss of labeled cells, the intensity of immunolabeling within the remaining neurons was comparable with, and in many instances even greater than, that observed in control cases. Within resistant sectors, the distribution of immunoreactive elements was comparable in both case groups yet the intensity of immunolabeling was markedly increased in AD cases, particularly in the molecular layer of the dentate gyrus and in the stratum lucidum of CA3 (i.e., the termination zones of perforant pathway and mossy fibers). In addition, within AD cases dramatic increases were observed within the supragranular and polymorphic layer of the dentate gyrus (i.e., the terminal zones of sprouting mossy fiber collaterals). The increase in GluR1 and GluR2/3 immunolabeling is hypothesized to occur in response to the deafferentation of selected glutamatergic pathways. Moreover, our data support that hippocampal plasticity is preserved, even in severe AD cases, and suggest a critical role for AMPA receptor subunits in this plasticity and in maintaining hippocampal functioning.

Cannabinoid receptor binding and messenger RNA expression in human brain: an in vitro receptor autoradiography and in situ hybridization histochemistry study of normal aged and Alzheimer's brains

The distribution and density of cannabinoid receptor binding and messenger RNA expression in aged human brain were examined in several forebrain and basal ganglia structures. In vitro binding of [3H]CP-55,940, a synthetic cannabinoid, was examined by autoradiography in fresh frozen brain sections from normal aged humans (n = 3), patients who died with Alzheimer's disease (n = 5) and patients who died with other forms of cortical pathology (n = 5). In the structures examined--hippocampal formation, neocortex, basal ganglia and parts of the brainstem--receptor binding showed a characteristic pattern of high densities in the dentate gyrus molecular layer, globus pallidus and substantia nigra pars reticulata, moderate densities in the hippocampus, neocortex, amygdala and striatum, and low densities in the white matter and brainstem. In situ hybridization histochemistry of human cannabinoid receptor, a ribonucleotide probe for the human cannabinoid receptor messenger RNA, showed a pattern of extremely dense transcript levels in subpopulations of cells in the hippocampus and cortex, moderate levels in hippocampal pyramidal neurons and neurons of the striatum, amygdala and hypothalamus, and no signal over dentate gyrus granule cells and most of the cells of the thalamus and upper brainstem, including the substantia nigra. In Alzheimer's brains, compared to normal brains, [3H]CP-55,940 binding was reduced by 37-45% in all of the subfields of the hippocampal formation and by 49% in the caudate. Lesser reductions (20-24%) occurred in the substantia nigra and globus pallidus, internal segment. Other neocortical and basal ganglia structures were not different from control levels. Levels of messenger RNA expression did not differ between Alzheimer's and control brains, but there were regionally discrete statistically significant losses of the intensely expressing cells in the hippocampus. The reductions in binding did not correlate with or localize to areas showing histopathology, estimated either on the basis of overall tissue quality or silver staining of neuritic plaques and neurofibrillary tangles. Reduced [3H]55,940 binding was associated with increasing age and with other forms of cortical pathology, suggesting that receptor losses are related to the generalized aging and/or disease process and are not selectively associated with the pathology characteristic of Alzheimer's disease, nor with overall decrements in levels of cannabinoid receptor gene expression.

AMPA/kainate receptor gene expression in normal and Alzheimer's disease hippocampus

Alzheimer's disease is a progressive dementia characterized by pronounced degeneration of certain populations of neurons in the hippocampus and cerebral cortex of the brain. One theory is that glutamate receptor-mediated toxicity plays a role in cell loss associated with Alzheimer's disease. We used in situ hybridization to examine GluR1, GluR2, and GluR3 messengerRNAs (encoding alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid/kainate receptor subunits) in sections of autopsy samples of Alzheimer's disease brains and age-, sex-, and post-mortem delay-matched brains from non-demented (control) subjects. GluR1 and GluR2 exhibited a heterogeneous distribution in control brain. GluR1 was expressed in granule cells of the dentate gyrus, in pyramidal cells of the CA1 and CA3 hippocampal subfields and in neurons of the subiculum and entorhinal cortex. GluR2 mRNA was at high density in the dentate gyrus and in CA3, but was at low density in CA1, subiculum, and entorhinal cortex. GluR3 hybridization was at very low levels but selectively localized to the dentate gyrus and CA3. In cerebellum, GluR1 was found in granule and Purkinje cell layers. In sections from Alzheimer's disease brain, a high degree of intersubject variability was observed: some samples showed markedly reduced GluR1 mRNA levels in dentate gyrus, CA1 and CA3 relative to controls; others showed no changes. Microscopic observation of emulsion-dipped sections revealed that the reduction of GluR1 seen in the dentate gyrus and CA3 of some Alzheimer's disease subjects was not due to cell loss.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuropathobiology of senile dementia and mechanism of action of nootropic drugs

Recent advances in neuroscience and molecular neurochemistry have substantially increased the knowledge of the neuropathobiology of senile dementia and Alzheimer's disease. On the basis of various hypotheses concerning degenerative processes in aging brains, new therapeutic strategies have been developed, including nootropic drugs with different mechanisms of action and heterogenous chemical structures. Mutual relationships exist between neuroscientific research and nootropic drug development. To date, such areas of research and drug development have involved deficits of brain neurotransmission (cholinergic, monoaminergic, peptidergic), free radical-induced damage, disturbances of calcium homeostasis and excitatory amino acid function, and deposition of amyloid protein.

AMPA-selective glutamate receptor subtype immunoreactivity in the entorhinal cortex of non-demented elderly and patients with Alzheimer's disease

The present work employed immunocytochemical techniques and examined the distribution and cytological features of the AMPA receptor subunits, GluR2/3 and GluR1 within the entorhinal cortex of non-demented elderly (NC), patients with neuropathological and clinical verification of Alzheimer's disease (AD) and patients without a clinical history of dementia yet exhibiting sufficient quantities of senile plaques to meet neuropathological criteria of Alzheimer's disease (HPND). In NC cases, GluR2/3-immunolabeled neurons were abundantly distributed throughout layers II, III, V and VI of the entorhinal cortex. In contrast, GluR1-positive cells were comparatively sparse in number and largely restricted to layers V and VI. In AD, GluR2/3- and GluR1-labeled neurons were markedly reduced. Similarly, adjacent Nissl-stained tissue sections revealed substantial cell loss in the entorhinal cortex thus providing a reasonable explanation for the loss of these receptor subunits. Importantly, a dramatic loss of GluR2/3- and GluR1-immunolabeled neurons is also observed in the HPND cases, although examination of Nissl-stained tissue sections reveals little if any evidence of cell loss. The latter data suggest that a 'down-regulation' of these receptor subunits occurs prior to the actual loss of these cells. Furthermore, we hypothesize that the decrease of specific AMPA receptor subunits may influence neuronal vulnerability via a mechanism involving increased intracellular calcium and the destabilization of intracellular calcium homeostasis.

Excitotoxins L-beta-oxalyl-amino-alanine (L-BOAA) and 3,4,6-trihydroxyphenylalanine (6-OH-DOPA) inhibit [3H] alpha-amino-3- hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) binding in human hippocampus

Excitotoxins L-beta-oxalyl-amino-alanine (L-BOAA) and 3,4,6-trihydroxyphenylalanine (6-OH-DOPA) have been investigated with regard to their potency to inhibit [3H] alpha-amino-3-hydroxy-5-methyl-4- isoxazole-propionic acid (AMPA) binding in human hippocampus in a quantitative autoradiographic study. With dissociation constants (KD) of [3H]AMPA binding and inhibition concentrations (IC50) of L-BOAA, 6-OH-DOPA and L-glutamate obtained from saturation and displacement experiments inhibition constants (Ki) for the inhibition of [3H]AMPA binding in individual hippocampal subregions could be calculated. They were between 5.2 +/- 2.9 and 35.1 +/- 39.9 microM for L-BOAA and 39.1 +/- 26.8 and 59.4 +/- 44.1 microM for 6-OH-DOPA. L-BOAA was equally potent as the endogenous agonist L-glutamate with Ki's between 13.1 +/- 3.9 and 21.4 +/- 12.1 microM (n = 4, mean +/- S.D.). Limbic system symptoms like cognitive deficits, mood disturbances and vivid dreams observed in patients with the motor neuron disease neurolathyrism may thus well be mediated by agonistic action of L-BOAA at AMPA glutamate receptors in hippocampus.

kappa-1 Opioid receptors of the temporal cortex are preserved in Alzheimer's disease

The binding of [3H]-U-69593 and [3H]-CI-977 to kappa-1 opioid receptors has been examined in the temporal cortex of postmortem brains from patients with Alzheimer's disease and age-matched controls using quantitative autoradiography. There was no significant difference between Alzheimer and control subjects in the level of [3H]-U-69593 and [3H]-CI-977 binding, but ChAT activity was markedly reduced (by 73% compared to controls). These results are not consistent with a presynaptic localisation of kappa-1 receptors on cholinergic terminals in human temporal cortex.

Oxidative stress: free radical production in neural degeneration

It is not yet established whether oxidative stress is a major cause of cell death or simply a consequence of an unknown pathogenetic factor. Concerning chronic diseases, as Parkinson's and Alzheimer's disease are assumed to be, it is possible that a gradual impairment of cellular defense mechanisms leads to cell damage because of toxic substances being increasingly formed during normal cellular metabolism. This point of view brings into consideration the possibility that, besides exogenous factors, the pathogenetic process of neurodegeration is triggered by endogenous mechanisms, either by an endogenous toxin or by inherited metabolic disorders, which become progressively more evident with aging. In the following review, we focus on the oxidative stress theory of neurodegeneration, on excitotoxin-induced cell damage and on impairment of mitochondrial function as three major noxae being the most likely causes of cell death either independently or in connection with each other. First, having discussed clinical, pathophysiological, pathological and biochemical features of movement and cognitive disorders, we discuss the common features of these biochemical theories of neurodegeneration separately. Second, we attempt to evaluate possible biochemical links between them and third, we discuss experimental findings that confirm or rule out the involvement of any of these theories in neurodegeneration. Finally, we report some therapeutic strategies evolved from each of these theories.

Excitatory amino acid AMPA receptor mRNA localization in several regions of normal and neurological disease affected human brain. An in situ hybridization histochemistry study

In situ hybridization histochemistry was used to localize the mRNAs coding for four alpha-aminoisoxazole propionic acid-sensitive glutamate receptor subunits in human brain (age range 51-95 years, postmortem delay 4.5-10 h). High levels of the B receptor subunit mRNA were present in all the studied regions, followed by the A-subunit and the C-subunit. Only very low levels of the D-subunit mRNA were detected. In hippocampus, the mRNA coding for the B-subunits of the glutamate receptor was observed in granule cells of dentate gyrus and in the pyramidal cells of Ammon's horn. In cortex, the highest levels of glutamate receptor subunit mRNAs were found in layer I and layers III-IV of entorhinal and temporal cortex, although significant levels were also observed in the other cell layers. A differential distribution was seen in cerebellum where the A-subunit mRNA is expressed mainly by Purkinje cells, while the B-subunit mRNA is present in the internal granule cell layer. These results correlate well with previous data from autoradiographic studies on the localization of excitatory amino acid binding sites in human brain and pinpoint the cells where these receptors are synthesized. In situ hybridization in the hippocampus of patients affected by Alzheimer's disease (age range 77-82 years, postmortem delay 19-25.5 h) revealed a decrease on the content of the mRNAs coding for these excitatory amino acid receptors, while an increase was detected in surgically dissected epileptic human hippocampi. These results corroborate and extend the previous data from in vitro autoradiography and suggest alteration of the excitatory amino acid disfunction during these neurodegenerative processes.

Autoradiographic characterization of 125I-neurotensin binding sites in human entorhinal cortex

The laminar and rostro-caudal distribution of 125I-neurotensin binding sites is described in human entorhinal cortex using quantitative autoradiography. Specific binding was most prominent over the cell clusters of layer II of the entorhinal cortex throughout its rostro-caudal extent. Dense binding was also observed in the adjacent presubiculum and cortical amygdaloid transition area, whereas minimal binding was detected in the hippocampus and dentate gyrus. 125I-Neurotensin may serve as a selective probe for neurotensin receptor alterations and layer II-specific cytoarchitectural disturbances in the entorhinal cortex in neuropsychiatric diseases associated with abnormalities of the mesial temporal lobe.

Modulation of AMPA receptor function in relation to glutamatergic abnormalities in Alzheimer's disease

Abnormalities in the excitatory glutamate neurotransmitter system appear to be a prominent factor in Alzheimer's disease (AD). Whereas hypoactivity of this system is observed in some areas of Alzheimer brains, hyperactivity may play a role in the degenerative processes in other brain areas. This apparently paradoxical situation makes therapeutic intervention in the glutamatergic system in AD difficult and demands the development of unique therapeutic approaches. The involvement of the (RS)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) subtype of glutamate receptors in AD brain dysfunctions seems to be important, and compounds showing different modulatory activity at AMPA receptors are discussed in relation to therapeutic possibilities. Compounds enhancing excitatory activity at AMPA receptors may have beneficial effects on the learning and memory deficits observed in AD, whereas agents showing antagonistic or partial agonist profiles may block or delay the progressive neurodegeneration, which is a key phenomenon in AD. In vitro experiments with compounds capable of enhancing AMPA receptor activity have been performed. Such compounds without excitatory activity on their own may not show the excitotoxic properties characteristic of glutamate agonists. Another possibility for therapeutic intervention is the use of a partial agonist. The concept of "functional partial agonism" at the AMPA receptors is described with a specific example.

Opioid receptor density changes in Alzheimer amygdala and putamen

Since opioids can influence the release of acetylcholine, substance P and a number of other neurotransmitters that have been implicated in the pathogenesis of Alzheimer's disease (AD), it is of interest to assess opioid receptor levels in AD. We have examined mu, delta and kappa opioid receptor binding parameters, binding sensitivity to a GTP analog and distribution in amygdala, frontal cortex and putamen of AD brain. Control brains were matched according to age, sex, post-mortem interval and storage time. Kd values and GTP analog binding sensitivity did not differ in AD and control brains. Bmax values for mu ([3H]DAMGE) sites also appeared unaffected by in vitro binding assays. In contrast, kappa ([3H]U69593) and delta ([3H]DSLET) opioid receptor levels, were significantly changed. In AD amygdala kappa Bmax values increased from control levels of 123 +/- 12 to 168 +/- 13 fmol/mg protein, whereas densities of kappa and delta sites were decreased from 94 +/- 8 to 48 +/- 8 and 102 +/- 3.6 to 69 +/- 8.5 fmol/mg protein, respectively, in putamen. Autoradiography revealed corresponding differences in the distribution of kappa opioid receptors. The findings indicate that the kappa binding site, which is quantitatively the major opioid receptor class in human brain, undergoes marked changes in AD amygdala and putamen.

NMDA and AMPA receptors in transgenic mice expressing human beta-amyloid protein

The human beta-amyloid protein may play an important, possibly primary, role in the pathogenesis of Alzheimer's disease (AD), and it appears to potentiate the susceptibility of neurons to excitotoxicity. AD is associated with alterations in the N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) subtypes of glutamate receptors, and it has been suggested that excitotoxicity may play a role in neuronal damage in AD. In this study, we have used quantitative receptor autoradiography to examine NMDA and AMPA receptors in transgenic mice that contain the gene for the carboxyl-terminal 100 amino acids of the human amyloid precursor protein, beginning with the beta-amyloid region, which is under the control of the JC viral early region promoter. Reverse transcriptase-polymerase chain reaction confirmed that the brains of transgenic mice expressed beta-amyloid mRNA and that control mice did not. NMDA receptors, assessed with [3H]MK-801, were unchanged in the transgenic compared with the control mice. In the transgenic mice, there were no significant changes in [3H]AMPA receptor binding compared with controls. This study represents the first attempt to evaluate in transgenic mice the in vivo interaction between beta-amyloid expression and excitatory amino acid receptors.

Loss of sigma binding sites in the CA1 area of the anterior hippocampus in Alzheimer's disease correlates with CA1 pyramidal cell loss

The densities of [3H]1,3-di-o-tolylguanidine ([3H]DTG) binding to sigma binding sites in the CA1 stratum pyramidale region in 7 hippocampi affected by Alzheimer's disease, were compared with densities in 7 normal hippocampi. There was an average reduction of 26% in [3H]DTG binding in this area, which was correlated with an average 29% pyramidal cell loss in the same region. These results are consistent with experiments in animals indicating that sigma binding sites are preferentially associated with the somata of large cells.

Neural Network Modeling of Memory Deterioration in Alzheimer's Disease

The clinical course of Alzheimer's disease (AD) is generally characterized by progressive gradual deterioration, although large clinical variability exists. Motivated by the recent quantitative reports of synaptic changes in AD, we use a neural network model to investigate how the interplay between synaptic deletion and compensation determines the pattern of memory deterioration, a clinical hallmark of AD. Within the model we show that the deterioration of memory retrieval due to synaptic deletion can be much delayed by multiplying all the remaining synaptic weights by a common factor, which keeps the average input to each neuron at the same level. This parallels the experimental observation that the total synaptic area per unit volume (TSA) is initially preserved when synaptic deletion occurs. By using different dependencies of the compensatory factor on the amount of synaptic deletion one can define various compensation strategies, which can account for the observed variation in the severity and progression rate of AD.

Differential alterations in adenosine A1 and kappa 1 opioid receptors in the striatum in Alzheimer's disease

The alterations in Alzheimer's disease (AD) of two binding sites in the striatum suggested to have a presynaptic localisation have been investigated by quantitative ligand binding autoradiography. Adenosine A1 binding sites labelled with [3H]cyclohexyladenosine (CHA) and kappa 1 opioid binding sites labelled with [3H]U-69593 were studied in adjacent sections of the striatum obtained postmortem from 10 patients with AD and 9 matched controls. In AD, there was a significant reduction of [3H]CHA binding sites in the caudate nucleus (control = 88 +/- 4; AD = 56 +/- 6 pmol/g tissue; mean +/- S.E.M.) and putamen (control = 83 +/- 4; AD = 58 +/- 7 pmol/g). In control subjects, highest levels of [3H]U-69593 binding were localised to patches within the caudate nucleus (9.66 +/- 0.58 pmol/g) with lower levels in the matrix (5.54 +/- 0.48 pmol/g). There was no alteration in [3H]U-69593 binding sites in either the caudate nucleus (patches and matrix) or putamen of AD patients. The activity of choline acetyltransferase (ChAT), determined in the same tissue samples used for autoradiographic analysis, was significantly reduced in AD (control = 124 +/- 11; AD = 64 +/- 14 nmol/h/mg protein). There was a positive correlation between ChAT activity and [3H]CHA binding (r = 0.769), but not [3H]U-69593 binding (r = 0.197). The results indicate that a marked loss of adenosine A1 receptors occurs in the striatum of AD with no loss of kappa 1 opioid receptors, and that the loss of A1 receptors parallels the loss of choline acetyltransferase activity.

NMDA, AMPA, and benzodiazepine binding site changes in Alzheimer's disease visual cortex

Quantitative receptor autoradiography was used to measure the laminar distribution of [3H]glycine and [3H]glutamate binding to the N-methyl-D-aspartate (NMDA) receptor complex, [3H]D,L-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) binding to the AMPA receptor, and [3H]flunitrazepam binding to the benzodiazepine (BDZ) receptor in three areas of visual cortex in control and Alzheimer's disease (AD) postmortem human brains (primary or striate visual cortex, visual association cortex, and higher-order visual association cortex, corresponding to Brodmann Areas 17, 18, and 21, respectively). In Area 17, binding to the NMDA, AMPA, and BDZ receptors was not significantly altered in the AD brains (except in layer VI for [3H]glycine and layer III for [3H]AMPA, where binding was reduced in the AD brains). Ligand binding to the two EAA receptors in Area 18 was, however, significantly reduced in the AD brains (layers I through III for [3H]glycine and layers III through VI for [3H]AMPA). In Area 21, binding to both the NMDA and BDZ receptors but not to the AMPA receptor, was significantly reduced in almost all laminae of the AD brains (layers I through VI for [3H]glycine and layers I through V for [3H]flunitrazepam). This hierarchical pattern of laminar binding loss with increasing complexity of association visual cortices is consistent with the increasing numbers of neurofibrillary tangles found in those areas, implicating NMDA and BDZ receptor bearing cells in AD neuropathology. AMPA receptor losses do not parallel the pathology, suggesting that AMPA receptors are not directly correlated with the pathology.

Alterations in [3H]-kainate receptor binding in the hippocampal formation of aged Long-Evans rats

The present study used in vitro autoradiography to examine the density of [3H]-kainate (KA) binding in subregions of the hippocampal formation and certain cortical areas in young (7-8 months) and aged (27-29 months) Long-Evans rats. In addition, the topography of KA binding in the dentate molecular layer was examined for evidence of reactive reorganization in the aged brain. This investigation of age-related changes in [3H]-KA binding included correlations with the animals' spatial learning performance in a Morris water maze. The results showed an age-related decrease in the density of [3H]-KA binding in several regions of the hippocampal formation (CA3, CA1, hilus) and within related cortical areas (subicular complex, entorhinal cortex, perirhinal cortex). In addition, an expanded zone of KA binding in the molecular layer of the dentate gyrus was observed in the aged group. This expansion of KA binding may reflect sprouting due to a loss of perforant path input to the dentate. The results of additional correlational analyses, however, indicated that these changes in the density and topography of [3H]-KA binding were not strongly correlated with a decline in place learning ability.

Production and characterization of a specific 5-HT2 receptor antibody

A synthetic peptide was used to generate antibodies against the rat serotonin-2 (5-HT2) receptor. The peptide corresponds to a unique sequence from the N-terminal extracellular portion of the receptor protein (antibody = Ab 5HT2-N). This peptide was chosen based on its theoretical antigenic index and for specificity to the 5-HT2 receptor. In dot blot analysis, antisera detected 2 ng-2 micrograms of synthetic peptide at dilutions of 1/200-1/20,000. COS-7 cells transiently transfected with a eukaryotic expression vector containing the 5-HT2 cDNA displayed intense immunoreactivity with crude and affinity-purified Ab 5HT2-N. In contrast, no immunoreactivity was seen in control experiments when: (1) non-transfected or vector transfected COS-7 cells were used; (2) pre-immune sera was substituted for primary antisera; (3) primary antisera was omitted; or (4) antiserum was pre-adsorbed to 10 microM synthetic peptide. Immunohistochemical analysis of sections of perfused rat brain revealed intense immunolabelling of a subset of neurons in regions of the ventral forebrain, dorsal hippocampus, striatum, cerebral cortex, and laterodorsal tegmental nucleus (LDT). An especially dense band of small cells was seen in layer 2 of pyriform cortex. There was a very high concentration of labelled cells in the laterodorsal tegmental nucleus. In situ hybridization histochemistry with a 5-HT2 antisense cRNA riboprobe showed a pattern of hybridization in forebrain similar to the pattern of immunolabelling with Ab 5HT2-N. Western blot analysis of proteins extracted from the LDT revealed a single protein species reacting with the antibody. This reactivity is not present in the pre-immune sera and is blocked by the synthetic antigen.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunocytochemical localization and description of neurons expressing serotonin2 receptors in the rat brain

Serotonin2 receptors have been implicated in a variety of behavioral and physiological processes, as well as a number of neuropsychiatric disorders. To specify the brain regions and specific cell types possessing serotonin2 receptors, we conducted an immunocytochemical study of the rat brain using a polyclonal serotonin2 receptor antibody. Perfusion-fixed rat brain sections were processed for immunocytochemistry and reactivity was visualized using an immunoperoxidase reaction. Numerous small, round neurons were heavily labeled in the granular and periglomerular regions of the olfactory bulb. Heavy labeling of medium-sized multipolar and bipolar neurons was also seen in olfactory regions of the ventral forebrain, including the anterior olfactory nucleus and olfactory tubercle. Other regions of the basal forebrain exhibiting high levels of immunoreactivity were the nucleus accumbens, ventral pallidum, Islands of Calleja, fundus striatum and endopyriform nucleus. Immunoreactive neurons were also seen in the lateral amygdala. A dense band of small, round cells was stained in layer 2 of pyriform cortex. In neocortex, a very sparse and even distribution of bipolar and multipolar neurons was seen throughout layers II-VI. A much more faintly labeled population of oval cells was observed in the deep layer of retrosplenial and posterior cingulate cortex, and in the granular layer of somatosensory frontoparietal cortex. A moderate number of medium bipolar and multipolar cells were scattered throughout the neostriatum, and a moderate number of pyramidal and pyramidal-like cells were seen in the CA fields of the hippocampus. Diencephalic areas showing immunolabeling included the medial habenula and anterior pretectal nucleus, with less labeling in the ventral lateral geniculate. In the hindbrain, two dense populations of large multipolar cells were heavily labeled in the pedunculopontine and laterodorsal tegmental nuclei, with lesser labeling in the periaqueductal gray, superior colliculus, spinal trigeminal nucleus and nucleus of the solitary tract. Based on the distribution, localization and morphology of immunoreactive neurons in these regions, we hypothesize that subpopulations of serotonin2 containing cells may be GABAergic interneurons or cholinergic neurons. Further, the observed distribution suggests that the physiological effects of serotonin acting through serotonin2 receptors are mediated by a relatively small number of cells in the brain. These observations may have strong functional implications for the pharmacological treatment of certain neuropsychiatric disorders.

Glucocorticoid receptor gene expression is unaltered in hippocampal neurons in Alzheimer's disease

Excessive glucocorticoid levels increase the metabolic vulnerability of hippocampal neurons to a wide variety of insults. Since glucocorticoid hypersecretion occurs in Alzheimer's-type dementia it has been proposed that a primary reduction in hippocampal glucocorticoid receptor expression leads to failure of feedback, hypercortisolemia and hence further neuronal loss. However, we have recently found that lesions of the cholinergic innervation of the hippocampus--known to be severely affected in Alzheimer's disease--increase corticosteroid receptor gene expression in the rat hippocampus. We have now examined both glucocorticoid (GR) and mineralocorticoid (MR) receptor gene expression in individual neurons in human postmortem hippocampus, using in situ hybridization histochemistry in 5 patients with Alzheimer's disease (81 +/- 3 years) and 7 controls (81 +/- 7 years) without neurological disease. The distribution and intensity of MR and GR mRNA expression in the hippocampus of Alzheimer's disease were similar to that in control tissue, with high expression in dentate gyrus and CA2-4, but significantly lower expression in CA1. In a separate group of patients with Alzheimer's disease we found significantly increased 24 h integrated plasma cortisol levels (59% greater than age-matched controls) and reduced cortisol-binding globulin (21% lower). These data do not suggest a primary deficiency of biosynthesis of hippocampal corticosteroid receptors in Alzheimer's disease. The maintenance of hippocampal GR and MR gene expression, in the face of an increased glucocorticoid feedback signal, may reflect loss of the cholinergic innervation.

Reduced density of adenosine A1 receptors and preserved coupling of adenosine A1 receptors to G proteins in Alzheimer hippocampus: a quantitative autoradiographic study

Binding to adenosine A1 receptors and the status of their coupling to G proteins were studied in the hippocampus and parahippocampal gyrus of Alzheimer individuals and age-matched controls. The binding to A1 receptors was compared with binding to the N-methyl-D-aspartate receptor complex channel-associated sites (labeled with (+)-[3H]5-methyl-10,11-dihydro-5H- dibenzo[a,d]cyclohepten-5,10-imine maleate). In vitro quantitative autoradiography demonstrated a similar anatomical distribution of A1 receptors labeled either with an agonist ((-)-[3H]phenylisopropyladenosine) or antagonist ([3H]8-cyclopentyl-1,3-dipropylxanthine) in the brains of elderly controls. In Alzheimer patients, significant decreases in the density of both agonist and antagonist binding sites were found in the molecular layer of the dentate gyrus. Decreased A1 agonist binding was also observed in the CA1 stratum oriens and outer layers of the parahippocampal gyrus, while reduced antagonist binding was found in the subiculum and CA3 region. Reduced density of the N-methyl-D-aspartate receptor channel sites was found in the CA1 region and parahippocampal gyrus. The reductions in binding to adenosine A1 and N-methyl-D-aspartate receptors were due to a decrease in the density of binding sites (Bmax), and not changes in receptor affinity (KD). In both elderly control and Alzheimer subjects, GTP substantially reduced the density of A1 agonist binding sites with a concomitant increase in the KD values, whereas antagonist binding was unaffected by GTP. The results suggest that adenosine A1 receptor agonists and antagonists recognize overlapping populations of binding sites. Reduced density of A1 receptors in the molecular layer of the dentate gyrus most probably reflects damage of the perforant path input in Alzheimer's disease, while altered binding in the CA1 and CA3 regions is probably due to loss of intrinsic neurons. Similar effects of GTP on binding to A1 receptors in control and Alzheimer individuals suggest lack of alterations in coupling of A1 receptors to G proteins in Alzheimer's disease, thus supporting the notion of normal receptor coupling to their effector systems in Alzheimer's disease.

Entorhinal-hippocampal connections: a speculative view of their function

On the basis of neuroanatomical studies, the entorhinal cortex (EC) has long been regarded as a relay station that provides the major source of afferent input to the hippocampus. The perforant path input to the dentate gyrus from layer II has traditionally been regarded as the major pathway by which information is transferred. However, electrophysiological studies are now indicating that other elements of the perforant path that project directly to CA1 and CA3 are more important than thought previously, and that the properties of different neuronal elements in the EC may determine the way in which information is passed on to and processed by the hippocampus. This article summarizes some of the properties of synaptic transmission in the EC and speculates on how frequency-dependent changes in transmission may be involved in the pre- and post-processing of hippocampal information by the EC.

Effects of age on L-glutamate-induced depolarization in three hippocampal subfields

The effects of aging on the translation of L-glutamate-induced depolarization into hippocampal neuronal firing frequency were studied in vitro. L-glutamate was iontophoretically-applied to the somatic region of extracellularly recorded single units. In none of the three principal hippocampal subfields (fascia dentata, CA3, and CA1) were there any effects of age on neuronal sensitivity to L-glutamate. Because there are pronounced, region-specific age effects on AMPA sensitivity (3), these results are in agreement with the conclusions of other investigators that the depolarization caused by exogenously applied L-glutamate probably exerts its effects through nonsynaptic mechanisms. These mechanisms, however, which lead to powerful depolarization and action potentials in hippocampal cells, are unaffected by age.

Potential use of DPCPX as probe for in vivo localization of brain A1 adenosine receptors

The suitability of (3H)DPCPX (8-cyclopentyl-1,3-dipropylxanthine), a xanthine derivative, as an vivo probe for labelling adenosine A1 receptors was studied in rats. [3H]DPCPX (nM) penetrated largely into the brain (0.8% of the injected dose per gram of brain tissue 5 min after injection). Brain concentrations stayed at a plateau level from 5 to 15 min after the injection. The distribution in the different brain regions was heterogeneous with the highest amount of [3H]DPCPX in cerebellum and hippocampus and the lowest concentrations in hypothalamus and brain stem. Displacement (45-70% of total radioactivity) was obtained by the injection of 250 nM of cold DPCPX or cyclopentylxanthine, an analog of DPCPX. The ex vivo autoradiographic distribution of [3H]DPCPX was similar to the in vitro autoradiographic distribution of tritiated A1 adenosine receptor ligand as [3H]CHA. These results suggest the potential use of DPCPX for further in vivo investigation of A1 adenosine receptors with techniques such as positron emission tomography.

NMDA receptors assessed by autoradiography with [3H]L-689,560 are present but not enriched on corticofugal-projecting pyramidal neurones

Experimental lesions followed by binding of [3H]4-trans-2-carboxy-5,7-dichloro-4-phenylamino-carbonylamino-1,2 ,3,4- tetrahydroquinoline ([3H]L-689,560, a novel ligand that binds to the glycine modulatory site), [3H]glycine and [3H]glutamate (N-methyl-D-aspartate (NMDA) sensitive) to cryostat sections and quantitative autoradiography were used to investigate the cellular localization of the NMDA receptor complex in the neocortex of the rat. The lesions were produced by intrastriatal injections of either volkensin (2 and 6 ng) or ricin (10 ng): both are suicide transport agents but only the former is retrogradely transported in the CNS. The binding of [3H]L-689,560 was significantly reduced in rats receiving 2 or 6 ng volkensin in deep cortical layers of Fr1/Fr2 ipsilateral to the striatal lesion. Similar reductions were also seen in [3H]glycine and [3H]glutamate binding, but only in rats receiving 6 ng volkensin. Quantitative histological analysis had previously revealed a loss of large infragranular pyramidal neurones with sparing of both interneurones and supragranular pyramidal neurones. There were no significant reductions in binding of any ligand in the superficial layers. In cortical areas Par1/Par2, [3H]L-689,560 was also significantly reduced in deep layers but only in rats receiving 6 ng volkensin. Binding was also reduced in the superficial layers by contrast to Fr1/Fr2. [3H]Glycine and [3H]glutamate binding were unaffected in this area. Binding of [3H]L-689,560 was unaffected in any area following intrastriatal ricin injection. The present study indicates that the NMDA receptor complex is present on pyramidal cells forming the corticofugal pathways. This is discussed in terms of the 5-HT1A receptor which is enriched on these cells.

Effects of combined acetylcholinesterase inhibition and serotonergic receptor blockade on age-associated memory impairments in rats

We recently reported that post-training administration of serotonergic receptor antagonists attenuated the inhibitory-avoidance memory deficits normally exhibited by aged rats. In the present study, we determined whether a subeffective dose of the serotonergic type-2 receptor antagonist, ketanserin, would augment the facilitative effects produced by the acetylcholinesterase inhibitor, physostigmine, on memory in aged rats using the same task. The drugs were injected intraperitoneally alone, or in combination, immediately following training. Retention testing occurred 24 hours following training. A dose-dependent enhancement of memory was demonstrated as a result of the two treatment conditions (physostigmine 0.01-10.0 micrograms/kg, ketanserin 1.0 mg/kg + physostigmine 0.001-0.01 micrograms/kg). The facilitation of memory produced by the combined treatment was observed at doses well below those required to produce a similar effect when each drug was administered alone. The results provide additional evidence for an interaction between the cholinergic and serotonergic neurotransmitter systems in learning and memory, and may have important implications in the treatment of age-related memory impairments.

Region-specific age effects on AMPA sensitivity: electrophysiological evidence for loss of synaptic contacts in hippocampal field CA1

The effects of aging on the responsiveness of hippocampal neurons to iontophoretic application of L-glutamate and AMPA were studied in vitro. There were no effects of age on neuronal responses to L-glutamate; however, CA1 pyramidal cells of old rats, but not granule cells in the fascia dentata, showed both a smaller reduction in extracellularly-recorded synaptic responses following application of AMPA (presumably mediated by depolarization), and smaller extracellular "DC" fields (measured by subtracting the DC potentials at the dendrite and soma following AMPA application in the dendrites). To examine the cellular bases of this age-related alteration in AMPA sensitivity, two additional electrophysiological approaches were used: (1) measurement of the amplitude ratios of extracellular EPSP and fiber potential components of the Schaffer collateral-CA1 response; (2) measurement of intracellularly recorded unitary EPSPs and quantal analysis of their fluctuations. The interpretations that would be placed on four hypothetical possible outcomes of such experiments are outlined and assessed in relation to the experimental data. The pattern of results obtained in the present experiments supports the following conclusions: In old rats, individual Schaffer collateral synapses do not appear to have altered AMPA receptor properties, as neither the mean size of the unitary synaptic response nor the apparent quantal size differs between age groups; however, the data do support the conclusion that there are fewer synapses per Schaffer collateral branch in old versus young CA1 pyramidal cells.

N-methyl-D-aspartate receptor complex in the hippocampus of elderly, normal individuals and those with Alzheimer's disease

The various ligand binding sites of the N-methyl-D-aspartate receptor complex in the hippocampal formation and parahippocampal gyrus of Alzheimer's disease patients and age-matched normal individuals were examined using quantitative autoradiography. The hippocampus and parahippocampal gyrus of the normal elderly brain exhibited virtually identical distributions of L-[3H]glutamate, [3H]5-methyl-10,11-dihydro-5H- dibenzo[a,d]cyclohepten-5,10-iminemaleate ([3H]MK-801), [3H][(+/-)2-carboxypiperazine-4-yl]propyl-1-phosphonic acid ([3H]CPP) and strychnine-insensitive [3H]glycine binding sites (r greater than 0.87) suggesting that binding occurred to different domains of the same receptor macromolecule. The binding of [3H]MK-801 to channel-associated phencyclidine sites appeared to be most severely impaired in Alzheimer's disease, especially at the anterior hippocampal level. When the data were averaged and the means for Alzheimer's disease and control group compared, a 34% decrease (P less than 0.01) in [3H]MK-801 binding was identified in the CA1 stratum pyramidale and a smaller decrease was found in the dentate gyrus molecular layer, parahippocampal gyrus and subiculum. The CA1 region exhibited a similar 35% reduction (P less than 0.05) in L-[3H]glutamate binding to N-methyl-D-aspartate-sensitive sites. This decrease most probably reflected a decline in receptor density. Binding of [3H]CPP to antagonist-preferring sites and [3H]glycine to glycine modulatory sites did not change significantly. However, a marked intersubject variability in N-methyl-D-aspartate receptor binding was observed in control and Alzheimer's disease groups. This variability was not related to age, sex or post mortem delay. Some Alzheimer's disease patients showed markedly reduced receptor binding levels, while others showed no changes or even increased binding. The loss of N-methyl-D-aspartate-sensitive sites did not correlate with a loss of neurons in the CA1 region (r = 0.286). Similarly, no correlation between the level of binding to N-methyl-D-aspartate-sensitive sites and the density of neuritic plaques and neurofibrillary tangles was found. Intersubject variability in N-methyl-D-aspartate receptor responses in the Alzheimer's disease group may partially explain conflicting reports in the literature on the N-methyl-D-aspartate receptor changes in Alzheimer's disease, and imply that caution should be exercised before making any generalizations about receptor changes in Alzheimer's disease based on mean values only. The analysis of the individual Alzheimer's disease cases may also be valuable in determining the mechanism(s) underlying the disease.

In vitro and in vivo characterization of the NMDA receptor-linked strychnine-insensitive glycine site

Modulation of the NMDA receptor by the strychnine-insensitive glycine site was studied both in vitro and in vivo. In vitro the glycinergic stimulation of [3H]MK801 binding was measured in three different rat forebrain membrane preparations. An increased association rate of [3H]MK801 in the presence of glycine was observed. The binding of the radioligand was also enhanced by D-serine, whereas L-serine was less potent. The concentration-effect curves were shifted to the right by the glycine antagonist 7-chlorokynurenic acid (7CKA). In vivo modulation of the N-methyl-D-aspartate (NMDA) receptor was studied using NMDA induced convulsions in 7 day old rats. The NMDA effect was blocked by (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten,5,10-imine maleate (MK801) and D-(-)-2-amino-5-phosphono-pentanoic acid (AP5). The effect of a submaximal dose of NMDA was dose-dependently potentiated by 1-10 mg/kg D-serine, whereas higher doses of L-serine were needed to obtain a similar effect. 7CKA did not affect NMDA-induced convulsions but reduced the D-serine potentiation of NMDA responses. This study illustrates the ability of the strychnine-insensitive glycine site to modulate the NMDA receptor function both in vitro and in vivo.

Excitatory amino acids and memory: evidence from research on Alzheimer's disease and behavioral pharmacology

The excitatory amino acid transmitter (EAA) system is believed to play a crucial role in a variety of physiological processes related to neuronal plasticity. Substantial neurophysiological evidence suggests that, under normal physiological conditions. EAAs may be involved in mechanisms underlying learning and memory. However, overactivity of this system produces excitotoxic damage to neurons which is believed to be an etiological factor in various neurological conditions, such as epilepsy, and stroke-induced impairments. The fact that EAAs have been implicated in both, normal cognitive function and in degenerative neurological conditions suggests that they may contribute to the etiology of Alzheimer's disease (AD), because AD is characterized by memory deficits and specific histopathological signs of neuronal damage. This paper summarizes information regarding 1) the involvement of EAAs in Alzheimer's disease and 2) results from psychopharmacological studies of EAAs in laboratory animal models of learning. Investigations of the pathophysiology of AD indicate that glutamatergic deficits are associated with this syndrome. However, there is controversy concerning the nature of this defect. As a result, it is unclear whether it is a consequence of excitotoxic changes produced by glutamatergic overactivity or result from a decrease in glutamatergic function. Evidence from behavioral studies is consistent with the conclusion that EAAs may be involved in the acquisition of conditioned responses. However, in parallel with the clinical findings, learning impairments have been produced by treatments which either increase or decrease activity within this transmitter system. Therefore, although present results suggest that the EAAs play a role in cognition and in clinical syndromes in which such function is compromised, the specific nature of that role needs to be elucidated by future research.

Selective reduction of [125I]apamin binding sites in Alzheimer hippocampus: a quantitative autoradiographic study

[125I]Apamin binding sites were examined using quantitative autoradiography in the hippocampus of 9 patients with Alzheimer's disease and 8 age-matched controls. Within the hippocampal formation from control subjects, [125I]apamin binding sites were highly concentrated in the subiculum and CA1. In Alzheimer's disease there was a marked and discrete loss of [125I]apamin binding sites in the subiculum (control = 1.10 +/- 0.10 pmol/g; Alzheimer = 0.71 +/- 0.09 pmol/g) and CA1 (control = 1.41 +/- 0.09 pmol/g; Alzheimer = 0.85 +/- 0.11 pmol/g; values are mean +/- S.E.M.). This reduction of [125I]apamin binding sites in the subiculum correlated with cell density but not neuritic plaque density. These results indicate that an anatomically discrete loss of Ca(2+)-dependent K+ channels within the hippocampal formation occurs in Alzheimer's disease.

Endogenous opiates: 1990

This paper, an examination of works published during 1990, is thirteenth in a series of our annual reviews of the research involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence, eating; drinking; gastrointestinal, renal, and hepatic functions; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; locomotor activity; sex, pregnancy, development, and aging; immunological responses; and other behavior.