Dendritic reorganisation in the basal forebrain under degenerative conditions and its defects in Alzheimer's disease. III. The basal forebrain compared with other subcortical areas.

The distribution of the reticular neuronal type in the human brain and its involvement in both degeneration and dendritic reorganisation under the conditions of ageing, Korsakoff's disease (KD), Alzheimer's disease (AD), and Parkinson's disease (PD) was comparatively investigated after Golgi impregnation. Reticular neurones are distributed throughout different areas along the brain axis. The cholinergic basal forebrain nuclei, i.e., the basal nucleus of Meynert, the nucleus of the diagonal band, and the medial septal nucleus form the most rostral part of this network of "open nuclei," which is collectively referred to as the "reticular core." Reticular neurones of the following areas were quantitatively investigated by a computer-based three-dimensional analysis: caudate nucleus, globus pallidus, medial septal nucleus, nucleus of the vertical limb of the diagonal band, basal nucleus, medial amygdaloid nucleus, reticular thalamic nucleus, lateral hypothalamic area, subthalamic nucleus, substantia nigra, locus coeruleus, pedunculopontine tegmental nucleus, and raphe magnus nucleus. There are three major findings. First, neurones that were found to be susceptible to degeneration in AD were largely part of the same neuronal populations prone to degeneration during ageing, in KD and PD. Thus, areas could be classified according to their overall degree of vulnerability under the present degenerative conditions as being highly vulnerable (basal forebrain nuclei, caudate nucleus, locus coeruleus), moderately vulnerable (medial amygdaloid nucleus, raphe magnus nucleus, lateral hypothalamic area, substantia nigra, pedunculopontine tegmental nucleus), or marginally vulnerable (globus pallidus, subthalamic nucleus, reticular thalamic nucleus). Second, neuronal populations that are particularly vulnerable to degenerative changes show a high degree of structural plasticity. Third, the degree of this dendritic plasticity is inversely related to the complexity of dendritic arborisation of the neurone. It is concluded that the sparsely ramified reticular type of neurone forms a pool of pluripotent neurones that have retained their plastic capacity throughout life, which makes them vulnerable to a variety of perturbations.