Cholinergic projections to the anterior thalamic nuclei in the rat: a combined retrograde tracing and choline acetyl transferase immunohistochemical study

Retrograde transport of horseradish peroxidase (HRP) was combined with choline acetyltransferase (ChAT) immunohistochemistry to study cholinergic projections to the anterior thalamic nuclei in the rat. Small iontophoretic injections of HRP placed into different subdivisions of the anterior thalamic nuclear complex resulted in distinct patterns of retrograde labelling in two major cholinergic cell groups of the mesopontine tegmentum, the laterodorsal tegmental nucleus (LDTg), in which a majority of the labelled cells was located, and the pedunculopontine tegmental nucleus (PPT). After injections into the posterior subdivision of the anteroventral thalamic nucleus (AVp), double-labelled neurons were present predominantly in the ipsilateral LDTg while a smaller number was found in the PPT. In the ipsilateral LDTg, 60–70% of ChAT-positive neurons were HRP-labelled, and 90–95% of the HRP-labelled neurons were ChAT-positive. In the contralateral LDTg, 30–40% of ChAT-positive neurons were HRP-labelled. After injections in the medial subdivision of the anteroventral thalamic nucleus (AVm), the pattern of labelling in LDTg was similar to that detected after injections in the AVp. The number of double-labelled neurons in the LDTg and PPT was much lower after injections into AVm than after injections into AVp. When injections were confined to the anterodorsal thalamic nucleus (AD), no HRP-labelled cells were present in the LDTg or PPT. These results show that the LDTg and PPT are the sources of the cholinergic input to the rat anterior thalamus. The major projection from LDTg and PPT is to the AVp, whereas there is a lighter cholinergic projection to the AVm. The AD does not receive a projection from cholinergic cells in the mesopontine tegmentum.Abbreviations AChE Acetylcholinestrase - AD anterodorsal thalamic · nucleus - AM anteromedial thalamic nucleus - ATN anterior thalamic nuclei - AVm medial subdivision of the anteroventral thalamic nucleus - AVp posterior subdivision of the anteroventral thalamic nucleus - CG central gray - ChAT choline acetyltransferase - DAB diaminobenzidine tetrahydrochloride - DR dorsal raphé nucleus - f fornix - HRP horseradish peroxidase - ic internal capsule - LD laterodorsal thalamic nucleus - LDTg laterodorsal tegmental nucleus - ml medial lemniscus - mlf medial longitudinal fasciculus - mt mamillothalamic tract - NRS normal rabbit serum - PPT pedunculopontine tegmental nucleus - PT paratenial thalamic nucleus - Re reuniens thalamic nucleus - Rh rhomboid thalamic nucleus - RPn raphé pontis nucleus - scp superior cerebellar peduncle - sm stria medullaris thalami - st stria terminalis - TAAB glutaraldehyde - TRN thalamic reticular nucleus - VL ventrolateral thalamic nucleus - VM ventromedial thalamic nucleus - xscp decussation of superior cerebellar peduncle