Sources of presumptive glutamatergic/aspartatergic afferents to the magnocellular basal forebrain in the rat

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Abstract
The distribution of presumptive glutamatergic and/or aspartatergic neurons retrogradely labeled following injections of [3H]-D-aspartate into the magnocellular basal forebrain of the rat was compared with the distribution of neurons labeled by comparable injections of the nonspecific retrograde axonal tracer wheat germ agglutinin conjugated to horseradish peroxidase. Cells retrogradely labeled by wheat germ agglutinin-horseradish peroxidase were found in a wide range of limbic and limbic-related structures in the forebrain and brainstem. In the telencephalon, labeled neurons were seen in the orbital, medial prefrontal, and agranular insular cortical areas, the amygdaloid complex, and the hippocampal formation. Labeled cells were also seen in the olfactory cortex, the lateral septum, the ventral striatopallidal region, and the magnocellular basal forebrain itself. In the diencephalon, neurons were labeled in the midline nuclear complex of the thalamus, the lateral habenular nucleus, and the hypothalamus. In the brainstem, labeled cells were found bilaterally in the ventral midbrain, the central gray, the reticular formation, the parabrachial nuclei, the raphe nuclei, the laterodorsal tegmental nucleus, and the locus coeruleus. A significant fraction of the afferents to the magnocellular basal forebrain appear to be glutamatergic and/or aspartatergic. Only a few of the regions labeled with wheat germ agglutinin-horseradish peroxidase were not also labeled with [3H]-D-aspartate in the comparable experiments. Most prominent among the non-glutamatergic/aspartatergic projections were those from fields CA1 and CA3 of the hippocampus, the hilus of the dentate gyrus, the dorsal subiculum, the tuberomammillary nucleus, and the ventral pallidum. In addition, most of the lateral hypothalamic and brainstem projections to the magnocellular basal forebrain were not significantly labeled with [3H]-D-aspartate. In addition to these inputs, a commissural projection from the region of the contralateral nucleus of the horizontal limb of the diagonal band was confirmed with both wheat germ agglutinin-horseradish peroxidase and the anterograde axonal tracer Phaseolus vulgaris leucoagglutinin. This projection did not label with [3H]-D-aspartate or [3H]-GABA, suggesting that it is not glutamatergic/aspartatergic or GABAergic. Furthermore, double labeling experiments with the fluorescent retrograde tracer True Blue and antibodies against choline acetyltransferase indicate that the projection is not cholinergic.

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