[Neurotransmitter anomalies in Alzheimer's disease]

Neurochemical and histochemical techniques for characterization of neurotransmitters and their receptors in normal and pathological human brain have modified our understanding of Alzheimer's disease. From a hopeless clinico-pathological entity, it has become one of the models of possible physiopathological relations between neurotransmitter anomalies and dementia processes. Three types of neuromediator (or neuro-modulator): cholinergic, aminergic and peptidergic, appear to be affected to different degrees in 5 electively involved anatomical systems: cholinergic innominocorticoamygdalian and septohippocampic systems, noradrenergic ceruleocortical system, serotoninergic pontocortical system and cortical somatostatin and substance P systems. Critical analysis of neurochemical data shows that the biochemical nosology of Alzheimer's disease is confronted by the difficulty of constituting homogeneous series of both normal and pathological cases. Difficulties are increased when an attempt is made to establish correlations between neurotransmitter deficits and lesions or the demential process. This is the result of several factors: individual variability, difficulty in selecting valid controls, time elapsed before post-mortem sampling, imperfect understanding of the progressive topographical course of both cortical and subcortical lesions, which only now are being studied systematically and in a quantifiable manner, and finally the frequent absence of distinction between early and late forms of the disease. Truly senile forms should probably be distinguished, some authors believing them to be a particular type of aging process, from the presenile forms with more extensive biochemical changes. The constant presence of cholinergic symptoms show these to be fundamental features of the dementia but lesions of noradrenergic and serotoninergic systems also probably play an important role, and this triad is found in other dementia processes: trisomy 21, Parkinson's disease with dementia. Numerous questions concerning the neurotransmitter disorders in Alzheimer's disease remain unanswered: to what extent are the lesions due to neurone destruction? What occurs to the receptors? What factors condition severity of the disease? May the latter be directly correlated with the severity of the global dementia process or rather with the constitutive elements of the syndrome such as memory or attention disorders? What are the consequences of the peptidergic lesions? What is the sequential relation between lesions of corticopetal and intracortical afferent systems and what are their respective physiopathological significance?(ABSTRACT TRUNCATED AT 400 WORDS)