Distribution of AMPA receptor subunits in the nucleus basalis of Meynert in aged humans: implications for selective neuronal degeneration

Alzheimer' s disease, oxidative stress and gammahydroxybutyrate

Although the cause of Alzheimer's disease is unknown, oxidative stress, energy depletion, excitotoxicity and vascular endothelial pathology are all considered to play a part in its pathogenesis. In reaction to these adverse events, the Alzheimer brain appears to deploy a highly conserved biological response to tissue stress. Oxidative metabolism is turned down, the expression of antioxidative enzymes is increased and intermediary metabolism is shifted in the direction of the pentose phosphate shunt to promote reductive detoxification, repair and biosynthesis. Gathering evidence suggests that the release of beta-amyloid and the formation of neurofibrillary tangles, the two hallmarks of Alzheimer's disease, are components of this protective response. Gammahydroxybutyrate (GHB), an endogenous short chain fatty acid, may be able to buttress this response. GHB can reduce glucose utilization, shift intermediary metabolism in the direction the pentose phosphate shunt and generate NADPH, a key cofactor in the activity of many antioxidative and reductive enzymes. GHB has been shown to spare cerebral energy utilization, block excitotoxicity and maintain vascular integrity in the face of impaired perfusion. Most important, GHB has repeatedly been shown to prevent the tissue damaging effects of oxidative stress. It may therefore be possible to utilize GHB to strengthen the brain's innate defences against the pathological processes operating in the Alzheimer brain and, in this way, stem the advance of Alzheimer's disease.

Cholinergic and glutamatergic drugs in Alzheimer's disease therapy

The pathology and clinical symptoms of Alzheimer's disease are well known and include plaques, tangles, cell loss and dysfunction. The target of current treatments is to improve neuronal dysfunction and produce symptomatic benefits based on a clear understanding of neurotransmitter biochemistry. The purpose of this review is to examine the scientific background to currently available treatments, discuss the clinical experience of employing these drugs in Alzheimer's disease patients and review the socioeconomic influences on their use in the future.

Human cholinergic basal forebrain: chemoanatomy and neurologic dysfunction

The human cholinergic basal forebrain (CBF) is comprised of magnocellular hyperchromic neurons within the septal/diagonal band complex and nucleus basalis (NB) of Meynert. CBF neurons provide the major cholinergic innervation to the hippocampus, amygdala and neocortex. They play a role in cognition and attentional behaviors, and are dysfunctional in Alzheimer's disease (AD). The human CBF displays a continuum of large cells that contain various cholinergic markers, nerve growth factor (NGF) and its cognate receptors, calbindin, glutamate receptors, and the estrogen receptors, ERalpha and ERbeta. Admixed with these cholinergic neuronal phenotypes are smaller interneurons containing the m2 muscarinic acetylcholine receptor (mAChRs), NADPH-diaphorase, GABA, calcium binding proteins and several inhibitory neuropeptides including galanin (GAL), which is over expressed in AD. Studies using human autopsy material indicate an age-related dissociation of calbindin and the glutamate receptor GluR2 within CBF neurons, suggesting that these molecules act synergistically to induce excitotoxic cell death during aging, and possibly during AD. Choline acetyltrasnferease (ChAT) activity and CBF neuron number is preserved in the cholinergic basocortical system and up regulated in the septohippocampal system during prodromal as compared with end stage AD. In contrast, the number of CBF neurons containing NGF receptors is reduced early in the disease process suggesting a phenotypic silence and not a frank loss of neurons. In end stage AD, there is a selective reduction in trkA mRNA but not p75(NTR) in single CBF cells suggesting a neurotrophic defect throughout the progression of AD. These observations indicate the complexity of the chemoanatomy of the human CBF and suggest that multiple factors play different roles in its dysfunction in aging and AD.

Glutamatergic systems in Alzheimer's disease

Glutamate is the major transmitter of the brain and is involved in all aspects of cognitive function since it is the transmitter of cortical and hippocampal pyramidal neurones. Furthermore, glutamate and glutamate receptors are involved in long-term potentiation, a process believed to underlie learning and memory. Histological studies indicate loss of pyramidal neurones and their synapses in Alzheimer's disease (AD), this together with biochemical evidence suggests presynaptic (and postsynaptic) glutamatergic hypoactivity. This represents a 'double blow' as the activity of glutamatergic neurones is heavily influenced by the cholinergic system, which is also dysfunctional in AD. The clinical relevance of these changes is emphasised because glutamatergic and cholinergic dysfunction are strong correlates of cognitive decline in AD. The mechanism by which glutamatergic (and cholinergic) cells die is likely to be a combination of necrosis and apoptosis caused by a range of factors which include tangle formation and the effects of too much and too little glutamatergic neurotransmission.

Loss of calbindin-D28k from aging human cholinergic basal forebrain: relation to neuronal loss

Cholinergic neurons of the basal forebrain (BFCN) are selectively vulnerable in neurodegenerative disorders of the elderly, particularly in Alzheimer's disease (AD). We investigated age-related changes in the BFCN that may serve as a substrate for this vulnerability. We report a substantial and selective age-related loss of the calcium binding protein calbindin-D(28K) (CB) from the human BFCN. Unbiased stereological estimation indicated that, in individuals under age 65 years, 72% of the choline acetyltransferase (ChAT)-positive BFCN contained CB immunoreactivity. In individuals over age 65 years, only 28% of the BFCN contained CB immunoreactivity, a dramatic loss of 61%. Similar results were obtained using neuronal counts from matching single- or double-stained sections in a larger cohort. The loss of CB immunoreactivity was neurochemically specific. No age-related changes were observed in the number of ChAT- or low-affinity nerve growth factor receptor (p75(NTR))-immunoreactive profiles. The loss of CB was greatest in very old individuals, in whom a small loss of BFCN was observed. Furthermore, the loss of CB displayed the same pattern as the loss of BFCN in AD and was more substantial in the posterior compared with the anterior BFCN sector, suggesting a role for CB in the selective vulnerability of BFCN in AD. The depletion of CB from the BFCN is likely to deprive these neurons of the capacity to buffer high levels of intracellular Ca(2+) and thus to leave them vulnerable to pathological processes, such as those in neurodegenerative disorders, which can cause increased intracellular Ca(2+), thus leading to their degeneration.

AMPA-type glutamate receptor subunits 2/3 in the human trigeminal sensory ganglion and subnucleus caudalis from prenatal ages to adulthood

Using immunohistochemistry, the occurrence and distribution of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) glutamate receptor subunits GluR2/3 is shown in the human trigeminal ganglion and subnucleus caudalis from 20 weeks of gestation to adulthood. In the trigeminal ganglion a subpopulation of GluR2/3-like immunoreactive (LI) primary sensory neurons occurred at all examined ages, amounting to about 20% of all ganglion cells in the earliest pre-term newborn and in the adult, to about 30% at 24 and 32 weeks of gestation, and peaking to about 40% in the neonate. At all ages examined, GluR2/3-LI neurons were heterogeneous in size, although in the adult most of the labeled perikarya were large-sized, with a mean cell diameter above 35 microm. In the trigeminal subnucleus caudalis, positive elements could be first detected at 30 weeks of gestation and persisted at all other examined ages. At pre- and perinatal ages, the immunoreactivity was restricted to neuronal perikarya in the superficial layers and in the marginal zone of the nucleus. In the adult tissue, the subnucleus caudalis harbored a loose meshwork of varicose thread- and dot-like elements in the superficial layers and numerous immunoreactive neurons, distributed in lamina I, substantia gelatinosa, and in the superficial zone of the magnocellular region.

Age-related changes in GluR2 and NMDAR1 glutamate receptor subunit protein immunoreactivity in corticocortically projecting neurons in macaque and patas monkeys

A distinct subpopulation of neurons forming long corticocortical projections in the association neocortex is highly vulnerable to the degenerative process in Alzheimer's disease. However, the degree to which age-related molecular and morphologic alterations of identifiable neuronal populations reflects early cellular degeneration leading to functional deficits has not yet been fully investigated in the aging brain. We performed an immunohistochemical analysis of neurons forming short and long corticocortical projections in young and old monkeys using antibodies to the GluR2 and NMDAR1 glutamate receptor subunit proteins. Projection neurons differed in their expression of these receptor subunits, as GluR2 was less prevalent than NMDAR1 among retrogradely labeled neurons. Long and short corticocortical pathways in old animals demonstrated a considerable decrease in the proportions of projection neurons containing GluR2 and NMDAR1, an observation that was particularly consistent in the case of GluR2. No age-related differences were observed in distribution of neurofilament protein in either type of projection neurons. These data suggest that cortical neurons furnishing long and short corticocortical projections display consistent neurochemical changes during aging and that a differential decrease in cellular expression of glutamate receptor subunit proteins occurs. The fact that in aging these neurons have lower levels of GluR2 than in young individuals, but comparatively higher levels of NMDAR1 than GluR2, may render them prone to calcium-mediated excitotoxicity, which in humans may be related to the selective vulnerability of such neurons during the course of Alzheimer's disease. Also, it is apparent that age-related neuronal changes are quite subtle and involve subcellular components of the cortical circuits rather than major morphologic alterations.

Synaptic and non-synaptic AMPA receptors permeable to calcium

For a long time, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA) receptors permeable to calcium have been considered to be either non-existent or as "atypical". There is now ample evidence that these receptors exist in numerous regions of the nervous system and in many neuronal as well as non-neuronal cell populations. This evidence has been accumulated by several methods, including electrophysiological recording, calcium imaging and cobalt-loading. Functional AMPA receptors permeable to calcium are already expressed at very early stages of embryonic development, well before the onset of synaptogenesis. They are probably involved in the paracrine signaling necessary for construction of the nervous system before becoming involved in synaptic transmission. In immature cells, cyclothiazide strongly increases the steady-state level of responses not only to AMPA, but also to kainate. Ingestion, during pregnancy, of food or drug substances that can cross the placental barrier and act upon the embryonic receptors may constitute a risk for normal development. In the adult nervous system, synaptic as well as non-synaptic (paracrine) AMPA receptors permeable to calcium are probably widely expressed in both glial and neuronal cells. They may also participate in controlling some aspects related to adult neurogenesis, in particular the migration of newly formed neurons.

Age-related loss of the AMPA receptor subunits GluR2/3 in the human nucleus basalis of Meynert

Magnocellular cholinergic neurons in the basal forebrain have long been recognized as vulnerable to the pathology of Alzheimer's disease. Despite numerous anatomical, pharmacological, behavioral, and physiological investigations of these neurons the cellular mechanism that underlines their selective vulnerability remains unclear. As part of an ongoing investigation into the molecular mechanism(s) underlying neuronal vulnerability in Alzheimer's disease and normal aging, we employed immunocytochemical techniques and examined the cellular localization of the alpha-amino-3-hydroxy-5-methyl-4-isoaxolepropionate (AMPA) glutamate receptor subunits GluR1 and GluR2/3 in the basal forebrain of eight nondemented elderly human subjects (66-102 years). For each case we observed GluR1-positive magnocellular cells darkly labeled within all main divisions of the basal forebrain (Ch1-Ch4). Double-labeling immunohistochemical techniques confirmed that the overwhelming majority (94%) of these neurons were also positive for the p75NGFr antibody, thus substantiating the cholinergic nature of these neurons. In contrast, GluR2/3 immunolabeling upon magnocellular neurons was relatively faint or nonexistent. The latter observations were most apparent in cases of advanced age and in the posterior part of the nucleus basalis of Meynert (NBM) (i.e., Ch4). In contrast, in adjacent structures (e.g., globus pallidus), a number of robustly labeled GluR2/3-positive cells were observed. In addition to the eight elderly subjects, we examined GluR1 and GluR2/3 immunostaining in the NBM of five younger cases, 5, 33, 36, 47, and 48 years of age. Although practical considerations limited our observations to the Ch4 region, we observed both GluR1 and GluR2/3 labeling upon NBM neurons in this latter region. On average, the distribution of labeled cells and intensity of immunoreaction were comparable between GluR1 and GluR2/3. The presence of GluR2/3- and GluR1-labeled neurons in the Ch4 region of younger cases but primarily GluR1 in cases of advanced age suggests an age-related decrease in GluR2/3. Functionally, the loss of GluR2 from the AMPA receptor complex results in ion channels highly permeable to Ca(2+). These alterations in cation permeability of the AMPA receptor together with the occurrence of a number of other intrinsic and extrinsic events (i.e., decrease Ca(2+)-binding protein) likely contribute to the vulnerability of these neurons in aging and in AD.

Distribution of glutamate receptor subunit NMDAR1 in the hippocampus of normal elderly and patients with Alzheimer's disease

Immunocytochemical techniques were employed to study the distribution and cytological features of NMDAR1-immunoreactive elements in the human hippocampal formation. Subjects with Alzheimer's disease (AD), presenting with a wide range of neuropathology and classified into six Braak stage (I-VI), and nondemented age-matched controls were examined. In control cases, the most intense NMDAR1 immunoreactivity was observed within the soma and dendrites of granule cells in the dentate gyrus and pyramidal neurons in Ammon's horn. Whereas small variations in the pattern of immunoreactivity were noted in control cases, AD subjects were characterized with intersubject variability which in most instances correlated with neuropathologic severity. For example, AD cases, particularly those with mild/modest pathology (Braak I-III), were indistinguishable from controls in the overall pattern of immunolabeling. In contrast, in those more severe AD cases (Braak IV-VI) the intensity of immunolabeling within the CA fields was greater than observed in controls and those with mild AD pathology. In addition, in pathologically severe cases numerous NMDAR1-positive pyramidal neurons were characterized by unique morphologic features including long and often tortuous apical dendrites. These latter findings were most prevalent in the CA1 region and subiculum. In contrast to the marked increase in immunolabeling in the CA fields, in the dentate gyrus we observed a reduction in NMDAR1 labeling particularly within the outer molecular layer (i.e., termination zone of the perforant pathway). This latter region was also the site of a number of NMDAR1-labeled plaques. Notably, the overall pattern of NMDAR1 immunoreactivity is distinct from that observed with antibodies against AMPA receptor subunits and suggests a differential role of various inotropic glutamate receptors in hippocampal plasticity in AD.

Immunohistochemical study of GABA(A) receptor beta2/3 subunits in the hippocampal formation of aged brains with Alzheimer-related neuropathologic changes

In AD, it is hypothesized that one factor contributing to the vulnerability of neurons is a delicate balance of excitatory and inhibitory inputs. To examine this hypothesis we have initiated a number of studies examining the role of the excitatory neurotransmitter glutamate and the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) in the neurodegeneration of AD. As an initial investigation into the GABAergic system in AD, we employed immunocytochemical techniques and examined the distribution and density of the GABAA receptor subunits beta2/3 within the hippocampus of 13 subjects with a clinical diagnosis of AD and 6 nondemented elderly subjects. Collectively, these 19 subjects presented with a broad range of pathologic severity (i.e., Braak stages I-VI). Density measurements of nine hippocampal regions demonstrated highest levels of beta2/3 immunolabeling in the inner molecular layer of the dentate gyrus > CA1 > CA2, while the lowest levels were found in the granular layer of the dentate gyrus < or = CA4 < CA3 field. Despite these regional variations no significant difference in the mean density of beta2/3 immunolabeling was observed when comparing the pathologically mild (stages I and II), moderate (stages III and IV), and severe (stages V and VI) groups. These data suggest that in the hippocampus receptor subunits associated with GABAergic neurotransmission are relatively maintained even until the terminal stages of the disease.

Distribution of AMPA receptor subunit mRNAs in the human basal ganglia: an in situ hybridization study

The distribution of AMPA receptor subunit mRNAs (spliced flip and flop variants of GluR-A to GluR-D) in the human post-mortem striatum, nucleus accumbens, globus pallidus and basal nucleus of Meynert was determined by in situ hybridization histochemistry. In the striatum and nucleus accumbens, for each subunit, the mRNA for the flop variant was more enriched than that for the corresponding flip variant. The GluR-C(flop) mRNA was most abundant, followed by the GluR-A(flop) mRNA. Transcripts for flop forms were evenly distributed in these regions, whereas those for flip forms showed a dorsoventral increasing gradient of the hybridization signals. The signals in these areas were found to originate mainly from medium-sized neurons. In the globus pallidus, mRNAs encoding GluR-A(flop) and GluR-C(flop) were also abundantly expressed. The basal nucleus of Meynert was enriched for mRNAs of flop forms. In conclusion, AMPA receptors in these areas of the human basal ganglia appeared to be mainly composed of flop variants, especially GluR-A(flop) and GluR-C(flop). However, the finding that flip transcripts were more abundant in the nucleus accumbens than in the striatum implies differences in functions of AMPA receptors between the two regions.