Localization of amino acids, neuropeptides and cholinergic markers in neurons of the septum-diagonal band complex projecting to the retrosplenial granular cortex of the rat

Retrograde labelling was combined with immunohistochemistry to localize neu rons containing choline acetyltransferase, gamma-aminobutyric acid (GABA), glutamate, leu-enkephalin, neurotensin, and substance P-like immunoreactivi ty in the projection pathways from the septum-diagonal band complex to the retrosplenial granular cortex in the rat. Injections of horseradish peroxid ase conjugated to subunit B of cholera toxin (CT-HRP) into the retrosplenia l granular cortex resulted in retrogradely labelled neurons in the ipsilate ral nuclei of the diagonal band of Broca, especially in the horizonatal nuc leus of the diagonal band, and small numbers of CT-HRP-labelled neurons wer e also found in the medial septal nucleus. In the horizontal and vertical n uclei of the diagonal band of Broca, 90-95% of CT-HRP-labelled neurons (35- 45 per section) were immunoreactive for choline acetyltransferase and small numbers of retrogradely labelled neurons 12 to 4-5 per section) were also immunoreactive for GABA, glutamate, neurotensin, leu-enkephalin, or substan ce P, In the medial septal nucleus approximately 75-80% of the retrogradely labelled neurons (8-10 per section) were immunoreactive for choline acetyl transferase and up to 25% of the CT-HRP labelled neurons (1-3 per section) in the medial septal nucleus also displayed GABA-, glutamate-, neurotensin- , leu-enkephalin-, or substance P-immunoreactivity. These results suggest t hat the complexity of the neurotransmitter(s)/neuromodulator(s) of septum-d iagonal band complex projections to the retrosplenial granular cortex shoul d be taken into account when considering the mechanisms of cortical activat ion. (C) 2000 Elsevier Science Inc.