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The basal forebrain (BF) comprises morphologically and functionally heterogeneous cell populations, including cholinergic and non-cholinergic corticopetal neurons that are implicated in sleep-wake modulation, learning, memory and attention. Several studies suggest that glutamate may be among inputs affecting cholinergic corticopetal neurons but such inputs have not been demonstrated unequivocally. We examined glutamatergic axon terminals in the sublenticular substantia innominata in rats using double-immunolabeling for vesicular glutamate transporters (Vglut1 and Vglut2) and choline acetyltransferase (ChAT) at the electron microscopic level. In a total surface area of 30,000 microm(2), we classified the pre- and postsynaptic elements of 813 synaptic boutons. Vglut1 and Vglut2 boutons synapsed with cholinergic dendrites, and occasionally Vglut2 axon terminals also synapsed with cholinergic cell bodies. Vglut1 terminals formed synapses with unlabeled dendrites and spines with equal frequency, while Vglut2 boutons were mainly in synaptic contact with unlabeled dendritic shafts and occasionally with unlabeled spines. In general, Vglut1 boutons contacted more distal dendritic compartments than Vglut2 boutons. About 21% of all synaptic boutons (n=347) detected in tissue that was stained for Vglut1 and ChAT were positive for Vglut1, and 14% of the Vglut1 synapses were made on cholinergic profiles. From separate cases stained for Vglut2 and ChAT, 35% of all synaptic boutons (n=466) were positive for Vglut2, and 23% of the Vglut2 synapses were made on cholinergic profiles. On average, Vglut1 boutons were significantly smaller than Vglut2 synaptic boutons. The Vglut2 boutons that synapsed cholinergic profiles tended to be larger than the Vglut2 boutons that contacted unlabeled, non-cholinergic postsynaptic profiles. The presence of two different subtypes of Vgluts, the size differences of the Vglut synaptic boutons, and their preference for different postsynaptic targets suggest that the action of glutamate on BF neurons is complex and may arise from multiple afferent sources.

An electron microscopic analysis of the nucleus basalis in the macaque monkey was carried out following the immunohistochemical labeling of choline acetyltransferase, either by itself or in conjunction with glutamate decarboxylase or tyrosine hydroxylase. Cholinergic axon varicosities were frequently encountered, and formed large, usually asymmetric, synapses on both choline acetyltransferase-immunopositive and -immunonegative dendrites of nucleus basalis neurons. Catecholaminergic (tyrosine hydroxylase-immunoreactive) axon varicosities formed synapses which in most cases were classified as asymmetric, and glutamate decarboxylase-immunoreactive (GABAergic) axons formed clearly symmetric synapses, each on to choline acetyltransferase-immunopositive or -immunonegative dendrites. These findings indicate that cholinergic cells in the nucleus basalis of the monkey, also known as Ch4 neurons, receive numerous synaptic inputs from cholinergic, catecholaminergic and GABAergic axons.

The ventral forebrain, including the ventral striatum, the ventral pallidum and the substantia innominata, is an important region involved in the functions of the basal ganglia and the limbic system, as well as the magnocellular corticopetal neurons of the nucleus basalis of Meynert. Although previous studies have shown that this region is richly innervated by GABAergic fibers, little is known with respect to the relative densities of GABAergic to non-GABAergic axon terminals in this region. To address this issue, we have developed a specific rabbit antiserum to GABA and used a postembedding immunocytochemical reaction to analyse the distribution of GABA-like immunoreactive axon terminals in the rat ventral striatum, ventral pallidum and substantia innominata. Of all axon terminals that form identifiable synapses within single ultrathin sections taken from these regions, 11.6% in the ventral striatum, 85.5% in the ventral pallidum and 64.8% in the substantia innominata were GABAergic. Differences were also found in the distribution patterns of these terminals with respect to the size of their synaptic target dendrites. These findings are consistent with previous findings that a majority of inputs to the ventral striatum are excitatory, and that a majority of inputs to the ventral pallidum are inhibitory. Our results provide a first approximation of the anatomical substrate for the physiology and pharmacology of GABA actions in the ventral forebrain region. These results also show that GABA may play an important role in the substantia innominata, where both the cholinergic and the non-cholinergic magnocellular corticopetal neurons reside within a neuropil innervated by many different non-cholinergic fibers.

In order to study changes in neuronal activity in the nucleus basalis of Meynert in aging and Alzheimer's disease, we applied a polyclonal antibody directed against the Golgi apparatus on formalin-fixed, paraffin-embedded material. Subsequently, an image analysis system was used to measure the size of the Golgi apparatus in (i) all nucleus basalis neurons and also separately in (ii) the remaining large cells (perikaryonal diameter > 30 microns). A significant reduction of 49% in the size of the Golgi apparatus was found in the entire population of nucleus basalis neurons in Alzheimer's disease. Furthermore, although there was no significant decrease in the size of the persisting large neurons in the nucleus basalis of Meynert, a significantly decreased size of the Golgi apparatus was found in these neurons in Alzheimer's disease. These results suggest that the overall activity of nucleus basalis neurons is severely decreased in Alzheimer's disease. Furthermore, these data support the idea that atrophy and decreased activity are the main phenomena in the nucleus basalis in Alzheimer's disease; they also indicate that the size of the Golgi apparatus is a sensitive parameter to follow this process.

The amygdala is severely and consistently affected by pathology in Alzheimer's disease. The distribution of Thioflavin S-stained neurofibrillary tangles and neuritic plaques was examined in the various nuclei that form the amygdala in 20 cases of clinically diagnosed Alzheimer's disease and five non-demented control cases. Large numbers of neurofibrillary tangles and neuritic plaques were observed in the accessory basal and cortical nuclei and the cortical transition area, while there was lesser involvement of the mediobasal nucleus. The medial, lateral, laterobasal and central nuclei were relatively spared. The distribution of neurofibrillary tangles and neuritic plaques was compared with neuroanatomic connections known from non-human primate experimental studies. This comparison suggests that (1) nuclei receiving and giving rise to hippocampal projections are consistently affected by neuropathological alterations in Alzheimer's disease; (2) the nuclei which receive strong cholinergic projections from the nucleus basalis of Meynert (e.g. laterobasal nucleus) have, in general, relatively few neuritic plaques; and (3) nuclei which receive olfactory projections are not uniformly affected, the cortical nucleus being heavily affected but the medial nucleus consistently spared.