Neurochemical Correlates of Dementia

NEURODEGENERATION, Vol. 5, pp 403–407 (1996)

Neurochemical Correlates of Dementia Andrew W. Procter Department of Psychiatry, Manchester Royal Infirmary, Manchester Studies of the neurochemical pathology of AD have indicated that early in the course of the disease, abnormalities of relatively few neurotransmitters are obvious. The most reliable and consistent changes are those seen in the cholinergic innervation of the cortex and the cortical pyramidal neurones. However, by the time of death there is usually considerable involvement of other neurones. © 1996 Academic Press Limited

Key words: Alzheimer’s disease, pyramidal cells, glutamate, serotonin, acetylcholine

Lesions of many neurotransmitter systems have been described in AD, but only those of the cholinergic, glutamatergic and serotonergic neurones appear to meet these criteria (see Procter, 1996; Francis et al., 1993), and will be discussed here.

THE MAJOR MOTIVATION for the neurochemical study of dementia has been the hope that effective treatments might thereby be developed. In the short term it is drugs which affect neurotransmitter functions which provide the greatest hope for effective symptomatic treatment. Neurotransmitter function in AD has generally been examined by the studies of brain tissue obtained post mortem. These must be conducted in such a way as to take account of potential artefacts, epiphenomena, and demographic factors such as age, sex and drug treatment. The mode of the patient’s death is of particular note and indicated by measures such as the duration of terminal coma (Harrison et al., 1991), although the pH value of brain tissue homogenates (Yates et al., 1990; Kingsbury et al., 1995) may be a valid index. AD is a progressive disorder but post mortem studies almost invariably examine patients dying at an advanced stage of disease after many years. In general, post mortem studies reveal more extensive and severe neurochemical pathology than is found in biopsy material. The premise on which the interpretation of studies is based is that if any particular neurochemical finding is to be considered of primary importance in AD, then it should meet two key criteria: firstly that it be present in the early stages of the disease and secondly that the magnitude of that change be correlated with the severity of some clinical or pathological hallmark of the disorder.

The neocortical cholinergic system Demonstrations of substantial losses of the enzyme for the synthesis of acetylcholine, choline acetyltransferase (ChAT) from the brains of patients with AD post mortem (Bowen et al., 1976; Davies & Maloney, 1976; Perry et al., 1977a,b; Davies, 1979) and ante mortem (Bowen et al., 1982) has stimulated much subsequent research of this neurotransmitter and established a cholinergic deficit as one of the most prominent features of AD. There is usually considerable loss of the neurones which give rise to the cortical cholinergic innervation, the neurones of the nucleus basalis of Meynert (nbM). The magnitude of this cholinergic dysfunction is correlated with both the severity of the cognitive impairment (Perry et al., 1978; Neary et al., 1986; Palmer et al., 1987), and of senile plaque formation and loss of cortical pyramidal neurones (Mountjoy et al., 1984; Neary et al., 1986). Doubts have been raised as to the validity of the view that the dementia of AD is due primarily to a disorder of the cholinergic system. Patients have been reported with the typical neuropathological features, yet cortical ChAT activity was not selectively reduced (Palmer et al., 1986) and other elderly patients had minimal loss of cholinergic neurones from the nbM (Pearson et al., 1983; Perry et al., 1982). Similar cortical

Correspondence to: Dr Andrew W. Procter, Rawnsley Building, Manchester Royal Infirmary, Oxford Road, Manchester M13 9BX © 1996 Academic Press Limited 1055-8330/96/040403 1 5 $25.00/0

403

404

A.W. Procter

cholinergic deficits occur in olivopontocerebellar atrophy, yet cognitive impairment in this condition is not prominent (Kish et al., 1988). Thus the neocortical cholinergic deficit probably only explains a part of the cognitive decline in AD, as has been suggested by neuropsychological studies of the effects of cholinergic antagonists (e.g. Kopelmant & Corn, 1988). Moreover the effects of tacrine appear to be to enhance tests of attention rather than memory function per se (Sahakian & Coull, 1993).The cortical cholinergic deficit while widespread and often severe can only explain a proportion of this syndrome, and it is deficits of the other neurotransmitters which may explain the other features.

Table 1. Loss of serotonin uptake sites in behaviourally disordered Alzheimer patients Area Behaviour

Behaviour disorder: Behaviour disorder:

Frontal cortex: Depression Overactivity Aggression Temporal cortex: Depression Overactivity Aggression

ABSENT

PRESENT

107 6 18 (8) 83 6 11 (17) 83 6 14 (13)

57 6 7 (12)** 42 6 10 (3) 66 6 9 (7)

55 6 4 (8) 44 6 4 (17) 41 6 4 (13)

33 6 4 (12)** 24 6 3 (3)* 39 6 8 (7)

Values are mean 6 SEM (n) of Bmax for [3H] Paroxetine binding (fmol/mg). Significant differences between groups indicated: P , 0.001, ANOVA with plaque grading as covariate; P , 0.05 (t-test). Data are taken from (Chen et al., 1996).

Serotonin The content of 5-HT and its major metabolite in many areas of the neocortex of AD subjects may be reducedpost mortem (Cross et al., 1983; Gottfries et al., 1983; Arai et al., 1984; Reinikainen et al., 1988), and neurofibrillary degeneration and neuronal loss in the raphe nucleus has been reported (Palmer et al., 1988). However, this is by no means a consistent finding, and even in AD brains at autopsy half of the cortical areas may have no selective reduction of presynaptic 5-HT activity. Many of these studies have been based on predominantly hospitalized patients and this discrepancy between studies may be explained by the inadvertent selection of cases for which institutional care had been necessary because of behavioural symptoms. The study of patients assessed retrospectively indicates that patients judged to be aggressive during life had more severe loss of both 5-HT concentration and post synaptic receptors (Palmer et al., 1988; Procter et al., 1992), and aggressive behaviour has been considered a feature of more advanced disease (Procter et al., 1992) compatible with the view that loss of 5-HT2 receptors is a feature of more extensive pathology found towards the end stage of the disorder. The assessment of such behavioural symptoms retrospectively is potentially unreliable, but when patients were assessed prospectively the loss of the structural marker of cortical serotonergic pathology, [3H] paroxetine binding, from the neocortex was most prominent in those subjects with symptoms of depression in life (Chen et al., 1996; Table 1). In an attempt to examine serotonin neurones in an epidemiologically representative sample of AD patients, Chen and colleagues (1996) examined structural and functional markers of innervation in two

cortical areas from 20 consecutive subjects in a community based study of AD. Loss of 40% of immunoreactive cells occurred from the dorsal raphe nucleus (DRN), yet loss of presynaptic uptake sites ([3H] paroxetine binding) of comparable magnitude was confined to the temporal cortex. Individual DRN neurones project diffusely to several areas of brain yet no significant loss of [3H] paroxetine binding was found in frontal cortex, suggesting some plasticity (i.e. sprouting of remaining serotonergic innervation in a region less affected by the pathological process of AD). Concentrations of 5-HT were unchanged in either area. There was evidence for increased turnover of 5-HT in surviving terminals as the ratio of 5-HIAA to 5-HT was significantly increased in both areas. Thus it appears that only at a late stage of the disease is prominent structural pathology of the serotonergic neurones found in the cerebral cortex, while earlier on, increased turnover of transmitter within remaining serotonergic terminals may effectively maintain serotonergic activity.

Cortical pyramidal neurones Several independent studies indicate that severity of dementia correlates with degeneration of corticocortical pyramidal neurones in association areas. Positron emission tomography shows that temporal and parietal lobes of AD patients are major and early sites of pathological changes. Neuropsychological test scores correlate with both this PET data (Cutler et al., 1985; Miller et al., 1987) and the pyramidal cell and synapse counts in layer III (Neary et al., 1986; DeKosky & Scheff,

405

Neurochemical correlates of dementia

1990). Thus, cognitive symptoms seem to be due to shrinkage or loss of pyramidal neurones from parietotemporal areas. Results obtained using a variety of techniques are consistent with glutamate (GLU) being the major neurotransmitter of pyramidal neurones, yet neurochemical studies of the key parameters of glutamatergic neurotransmission in the neocortex have failed to demonstrate such profound or extensive alterations as these morphological investigations (see Francis et al., 1993). In part this may be because late in the disease tissue atrophy may confound the interpretation of biochemical data expressed relative to unit mass without taking into account the reduction in volume of the brain structure. However, many parameters of glutamate neurotransmission appear sensitive to perimortem epiphenomena (Procter et al., 1991; Procter et al., 1988). A valid and reliable measure of glutamate nerve terminals appears to be the sodium dependent uptake of D-[3H] asparate into preparations containing synaptosomes, prepared without prior freezing of the tissue (Procter et al., 1988; Procter et al., 1994). This is reduced in many areas of neocortex in AD compared to other types of dementia (Procter et al., 1988; Procter

et al., 1994; Table 2). However this technique does not distinguish between the effects of glia, which also have GLU transport systems. Molecular characterization of the excitatory amino acid transporter (EAAT) proteins is now possible (Danbolt et al., 1990; Arriza et al., 1994) and in the rat distinct cDNAs encoding structurally related EAATs have been identified, and those expressed in glia appear distinct from that in neurones (Shashidharan & Plaitakis, 1993; Arriza et al., 1994; Shashidharan et al., 1994a,b). Human sequences analogous to those expressed in other animal neurones provide a potential index of glutamatergic cell bodies, free of the effect of glia. Studies of the EAAT proteins and the mRNA for them may clarify the situation in the disease states.

Conclusions It is generally assumed that the neurochemical study of post mortem brain tissue reveals information of relevance to the understanding of that condition in life. However, many factors need to be considered for this to be a valid assumption. It is probably true to say that

Table 2. Sodium dependent D-[3H] Aspartate uptake (pmol/min/mg protein) into brain tissue from patients with Alzheimer’s disease and non-Alzheimer dementia Non-Alzheimer dementia

Alzheimer’s disease

Alzheimer’s disease as % non-Alzheimer

Frontal lobe superior lobulea

14.4 6 2.1 (4)

8.4 6 1.3 (8)

58*

Temporal lobe superior gyrus inferior gyrusa parahippocampal gyrus amygdala

14.2 6 2.3 (4) 24.8 6 5.4 (4) 23.9 6 13.0 (4) 24.1 6 10.1 (4)

7.2 6 0.9 (6) 7.9 6 1.1 (9) 11.3 6 2.4 (5) 11.0 6 2.4 (8)

51* 32** 47 46

Parietal lobe superior lobule inferior lobule

15.1 6 3.3 (4) 13.1 6 3.0 (4)

7.9 6 1.7 (8) 6.5 6 0.9 (6)

52* 50*

Occipital lobe lateral gyrus

14.4 6 2.3 (4)

7.0 6 1.0 (9)

49**

Cingulate gyrus anterior posterior

14.0 6 1.8 (4) 19.1 6 4.5 (4)

9.2 6 1.6 (6) 9.0 6 1.6 (5)

66 47

Cerebelllum cortex

21.6 6 3.8 (3)

18.4 6 5.4 (5)

85

Patients and methods are described in (Procter et al., 1994). Non-Alzheimer subjects included patients with Pick’s disease, multisystem degeneration, vascular dementia and depressive pseudodementia. Uptake values are mean 6 SEM (n); significant differences between Alzheimer’s disease and non-Alzheimer dementia is indicated *P , 0 05, **P , 0.005 (t-test if appropriate, or Mann-Whitney test as necessary). aData for superior frontal and inferior temporal cortex is from (Procter et al., 1994).

406

A.W. Procter

the rigorous demonstration of reliability and validity which is normally expected in many other areas of psychiatry are rarely applied to biological measures. When these are applied, studies of the neurochemical pathology of AD have indicated that early in the course of the disease abnormalities of relatively few neurotransmitters are obvious. This is in contrast to the situation late in the disease, which is usually examined in post mortem tissue. Thus the most reliable and consistent changes are those seen in the cholinergic innervation of the cortex and the cortical pyramidal neurones. However by the time of death, there is usually considerable involvement of other neurones.

Acknowledgements I am indebted to Prof David Bowen for his help, advice and encouragement during the ten years I spent in his laboratory. The work described in this paper is a result of the efforts of David and the members of his laboratory as well as innumerable collaborators over that period.

References Arai H, Kosaka K, Iizuka R ( 1984) Changes of biogenic amines and their metabolites in postmortem brains from patients with Alzheimer-type dementia. J Neurochem 43:388–393 Arriza JL, Fairman WA, Wadiche JI, Murdoch GH, Kavanaugh MP, Amara SG (1994) Functional comparisons of three glutamate transporter subtypes cloned from human motor cortex. J Neurosci 14:5559–5569 Bowen DM, Smith CB, White P, Davison AN (1976) Neurotransmitter-related enzymes and indices of hypoxia in senile dementia and other abiotrophies. Brain 99:459–496 Bowen DM, Benton JS, Spillane JA, Smith CC, Allen SJ (1982) Choline acetyltransferase activity and histopathology of frontal neocortex from biopsies of demented patients. J Neurol Sci 57:191–202 Chen CPL-H, Alder JT, Bowen DM, Esiri MM, McDonald B, Hope T, Jobst KA, Francis PT (1996) Presynaptic serotonergic markers in community-acquired cases of Alzheimer’s disease: correlations with depression and neuroleptic medication. J Neurochem 66:1592–1598 Cross AJ, Crow TJ, Johnson JA, Joseph MH, Perry EK, Perry RH, Blessed G, Tomlinson BE (1983) Monoamine metabolism in senile dementia of Alzheimer type. J Neurol Sci 60:383–392 Cutler NR, Haxby JV, Duara R, Grady CL, Kay AD, Kessler RM, Sundram M, Rapoport S (1985) Clinical history, brain metabolism, and neuropsychological function in Alzheimer’s disease. Ann Neurol 18:298–309 Danbolt NC, Pines G, Kanner BI (1990) Purification and reconstitution of the sodium- and potassium-coupled glutamate transport glycoprotein from rat brain. Biochemistry 29:6734–6740 Davies P (1979) Neurotransmitter-related enzymes in senile dementia of the Alzheimer type. Brain Res 171:319–327 Davies P, Maloney AJF (1976) Selective loss of central cholinergic neurons in Alzheimer’s disease. Lancet ii:1403 DeKosky S, Scheff SW (1990) Synapse loss in frontal cortex biopsies in Alzheimer’s disease: correlation with cognitive severity. Ann Neurol 27:457–464

Francis PT, Sims NR, Procter AW, Bowen DM (1993) Cortical pyramidal neurone loss may cause glutamatergic hypoactivity and cognitive impairment in Alzheimer’s Disease: investigative and therapeutic perspectives. J Neurochem 60:1589–1603 Gottfries CG, Adolfsson R, Aquilonius SM, Carlsson A, Eckernas SA, Nordberg A, Oreland L, Svennerholm L, Wiberg A, Winblad B (1983) Biochemical changes in dementia disorders of Alzheimer type (AD/SDAT). Neurobiol Aging 4:261–271 Harrison PJ, Procter AW, Barton AJL, Lowe SL, Bertolucci PHF, Bowen DM, Pearson RCA (1991) Terminal coma affects messenger RNA detection in post mortem human brain tissue. Mol Brain Res 9:161–164 Kingsbury AE, Foster OJF, Nisbet AP, Cairns N, Bray L, Eve DJ, Lees AJ, Marsden CD (1995) Tissue pH as an indicator of mRNA preservation in human post mortem brain. Mol Brain Res 28:311–318 Kish SJ, Munir E-A, Schut L, Leach L, Oscar-Berman M, Freedman M (1988) Cognitive deficits in olivopontocerebellar atrophy: implications for the cholinergic hypothesis of Alzheimer’s Dementia. Ann Neurol 24:200–206 Kopelman MD, Corn TH (1988) Cholinergic ‘blockade’ as a model for cholinergic depletion. Brain 111:1079–1110 Miller JD, De Leon MJ, Ferris SH, Kluger A, George AE, Reisberg B, Sachs HJ, Wolf AR (1987) Abnormal temporal lobe response in Alzheimer’s disease during cognitive processing as measured by 11C-2-deoxy-D-glucose and PET. J Cere Blood Flow Metab 7:248–251 Mountjoy CQ, Rossor MN, Iversen LL, Roth M (1984) Correlation of cortical cholinergic and GABA deficits with quantitative neuropathological findings in senile dementia. Brain 107:507–518 Neary D, Snowden JS, Mann DMA, Bowen DM, Sims NR, Northen B, Yates PO, Davison AN (1986) Alzheimer’s disease: a correlative study. J Neurol Neurosurg Psychiatry 49:229–237 Palmer AM, Procter AW, Stratmann GC, Bowen DM (1986) Excitatory amino acid-releasing and cholinergic neurones in Alzheimer’s Disease. Neurosci Lett 66:199–204 Palmer AM, Francis PT, Bowen DM, Benton JS, Neary D, Mann DM, Snowden JS (1987) Catecholaminergic neurones assessed antemortem in Alzheimer’s disease. Brain Res 414:365–375 Palmer AM, Stratmann GC, Procter AW, Bowen DM (1988) Possible neurotransmitter basis of behavioral changes in Alzheimer’s disease. Ann Neurol 23:616–620 Pearson RCA, Sofroniew MV, Cuello AC, Powell TPS, Eckenstein F, Esiri MM, Wilcock GK (1983) Persistence of cholinergic neurones in the basal nucleus in a brain with senile dementia of the Alzheimer’s type demonstrated by immunohistochemical staining for choline acetyltransferase. Brain Res 289:375–379 Perry EK, Gibson PH, Blessed G, Perry RH, Tomlinson BE (1977a) Neurotransmitter enzyme abnormalities in senile dementia. Choline acetyltransferase and glutamic acid decarboxlyase activities in necropsy brain tissue. J Neurol Sci 34:247–265 Perry EK, Perry RH, Blessed G, Tomlinson BE (1977b) Necropsy evidence of central cholinergic deficits in senile dementia. Lancet i: 189 Perry EK, Tomlinson BE, Blessed G, Bergmann K, Gibson PH, Perry RH (1978) Correlation of cholinergic abnormalities with senile plaques and mental test scores in senile dementia. Br Med J 2:1457–1459 Perry RH, Candy JM, Perry EK, Irving D, Blessed G, Fairbairn AF, Tomlinson BE (1982) Extensive loss of choline acetyltransferase activity is not reflected by neuronal loss in the nucleus of Meynert in Alzheimer’s Disease. Neurosci Lett 33:311–315 Procter AW, Palmer AM, Francis PT, Lowe SL, Neary D, Murphy E, Doshi R, Bowen DM (1988) Evidence of glutamatergic denervation and possible abnormal metabolism in Alzheimer’s disease. J Neurochem 50:790–802 Procter AW, Stratmann GC, Francis PT, Lowe SL, Bertolucci PHF, Bowen DM (1991) Characterisation of the glycine modulatory

Neurochemical correlates of dementia site of the N-methyl-D-aspartate receptor-ionophore complex in human brain. J Neurochem 56:299–310 Procter AW, Francis PT, Stratmann GC, Bowen DM (1992) Serotonergic pathology is not widespread in Alzheimer patients without prominent aggressive symptoms. Neurochem Res 17:917–922 Procter AW, Francis PT, Holmes C, Webster M-T, Qume M, Stratmann GC, Doshi R, Mann DMA, Harrison PJ, Pearson RCA, Bowen DM (1994) Beta-amyloid precursor protein isoforms show correlations with neurones but not with glia of demented subjects. Acta Neuropathol 88:545–552 Procter AW ( 1996) Neurochemical pathology of neurodegenerative conditions in old age. In: Jacoby R, Oppenheimer C (eds) Textbook of Psychiatry in the Elderly. Oxford University Press, Oxford Reinikainen KT, Paljarvi L, Huuskonen M, Soininen H, Laasko M, Reikkinen PJ (1988) A post mortem study of noradrenergic serotonergic and GABAergic neurons in Alzheimer’s disease. J Neurol Sci 84:101–116

407 Sahakian BJ, Coull JT (1993) Tetrahydroaminoacridine (THA) in Alzheimer ’s disease: an assessment of attentional and mnemonic function using CANTAB. Acta Neurol Scand Suppl 149:29–35 Shashidharan P, Huntley GW, Meyer T, Morrison JH, Plaitakis A (1994a) Neuron-specific human glutamate transporter: molecular cloning, characterisation and expression in human brain. Brain Res 662:245–250 Shashidharan P, Wittenberg I, Plaitakis A (1 994b) Molecular cloning of human brain glutamate/aspartate transporter II. Biochim Biophys Acta 1191:393–396 Shashidharan P, Plaitakis A (1993) Cloning and characterisation of a glutamate transporter cDNA from human cerebellum. Biochim Biophys Acta 1216:161–164 Yates CM, Butterworth J, Tennant MC, Gordon A (1990) Enzyme activities in relation to pH and lactate in post mortem brain in Alzheimer type and other dementias. J Neurochem 55: 1624–1630